Ibtissem Guefrachi

Camille Verly

Eva Kondorosi

Benoît Alunni

Peter Mergaert

Role of the bacterial BacA ABC-transporter in chronic infection of nodule cells by Rhizobium bacteria

Keywords: Amplified Fragment-Length Polymorphism AMPs, Antimicrobial peptides APC, Anaphase Promoting Complex ARA, Acetylene Reduction Assay BAP, Cytokinin 6-benzylaminopurine BCIP CCD, Charge-Coupled Device CCRs, Crypt-specific Cysteine-Rich peptides DAPI, DNA fluorescent stain 4',6-diamidino-2-phenylindole DIC, Differential Interference Contrast Fluorescent In Situ Hybridization FITC, Fluorescein isothiocyanate GRPs, Glycine Rich Peptides IRLC, Inverted Repeat-lacking clade Laser-Capture Microdissection Liquid Chromatography coupled to tandem Mass Spectrometry Small Nodulin Acidic RNA-binding Protein SNARPs, Small Nodulin Acidic RNA-binding Proteins SPC, Signal Peptidase Complex

Ce travail de thèse porte sur la thématique des interactions symbiotiques entre les plantes Légumineuses et les bactéries telluriques fixatrices d'azotes appelées rhizobia. Plus particulièrement, je me suis intéressée aux mécanismes qui président la différenciation cellulaire des bactéries au sein des organes symbiotiques formés par la plante.

Contexte général de l'étude

L'azote est un élément fondamental dans la composition de la matière vivante.

 grâce à l'utilisation d'une métalloenzyme : la nitrogénase. Parmi ces organismes diazotrophes, certaines bactéries Gram négatives collectivement appelées rhizobium, ou des bactéries filamenteuses Gram positives du genre Frankia forment des endosymbioses avec respectivement des Légumineuses ou des non Légumineuses du genre Parasponia et des plantes actinorhiziennes (Franche et al., 2008). Cette symbiose aboutit, dans tous ces cas, à la formation d'un organe spécialisé, la nodosité, qui se forme sur les racines de la plante hôte (dans certains cas aussi sur la tige). Cet organe est le siège d'une fixation de l'azote atmosphérique par les bactéries au bénéfice de la plante. Au cours du développement de la nodosité, les bactéries pénètrent un poil absorbant et sont acheminées via un cordon d'infection vers les cellules symbiotiques, où elles sont relarguées par un processus d'endocytose. Les bactéries se retrouvent entourées d'une membrane dérivée de la membrane plasmique végétale et forment des symbiosomes, où elles se différencient en bactéroïdes fixateurs d'azote (Oldroyd et Downie, 2008 ; Desbrosse et Stougaard, 2011). Ces bactéries différenciées ont une physiologie qui est radicalement différente de celle des rhizobia vivant dans la rhizosphère et qui est adaptée au mode de vie symbiotique et à la fixation de l'azote.

La plupart des équipes de recherches travaillant sur la symbiose rhizobium-Légumineuse s'intéressent aux étapes précoces de l'interaction. Ainsi au cours des

Résumé

décennies, les bases moléculaires de la reconnaissance mutuelle entre les partenaires symbiotiques ont été élucidées [START_REF] Stacey | Effects of endogenous salicylic acid on nodulation in the model legumes Lotus japonicus and Medicago truncatula[END_REF]. En conditions de carence azotée, les plantes produisent dans leurs exsudats racinaires des flavonoïdes qui sont reconnus par la bactérie et induisent la production et la sécrétion de lipochitooligosaccharides, les facteurs Nod, qui sont à leur tour reconnus par la plante. La perception des facteurs Nod initie à la fois l'organogenèse de la nodosité et l'infection bactérienne des tissus végétaux [START_REF] Oldroyd | The rules of engagement in the legume-rhizobial symbiosis[END_REF].

A contrario, notre équipe a concentré ses efforts de recherche depuis une quinzaine d'années sur des étapes tardives de l'interaction symbiotique et plus particulièrement sur la différenciation des cellules végétales et bactériennes permettant la mise en place du processus de fixation d'azote. Ainsi les cellules végétales qui sont infectées par les rhizobia (plusieurs milliers de bactéries par cellule), appelées cellules symbiotiques, subissent de profondes modifications pour permettre la présence de nombreux symbiosomes fixateurs d'azote dans leur cytoplasme. Durant leur différentiation, ces cellules sortent du cycle cellulaire classique (G1-S-G2-M) pour le convertir en un cycle d'endoréplication (ou endocycle), c'est-à-dire un cycle cellulaire simplifié ne comprenant que deux phases, une interphase G et une phase S (pour Synthèse d'ADN); les phases G2 et M (pour Mitose) ont disparu. La cellule ne se divise pas par la suite en deux cellules filles, ce qui provoque la polyploïdisation de la cellule. Ces endoréplications successives conduisent à augmenter la ploïdie de 2C (cellule diploïde de méristème) à 64C (cellules symbiotiques matures polyploïdes; C étant le contenu en génome haploïde). La polyploïdisation permet d'accroître le volume cellulaire ainsi que l'activité métabolique des cellules hôtes. Un des acteurs clefs de ces endoréduplications successives est la protéine CCS52A dont l'action est la dégradation des cyclines mitotiques avant qu'elles ne puissent provoquer l'entrée en phase M du cycle cellulaire. Cela provoque l'arrêt de la division cellulaire mais n'affecte en rien la réplication de l'ADN, il en résulte des cellules endorépliquées, polyploïdes [START_REF] Cebolla | The mitotic inhibitor ccs52 is required for endoreduplication and ploidy-dependent cell enlargement in plants[END_REF][START_REF] Vinardell | Endoreduplication mediated by the APC activator CCS52A is required for symbiotic cell differentiation in Medicago truncatula nodules[END_REF][START_REF] Tarayre | Two classes of the CDh1-type activators of the anaphase-promoting complex in plants: novel functional domains and distinct regulation[END_REF]Fulöp et al., 2005).

De façon similaire, les bactéries qui sont internalisées dans les cellules végétales se différencient en bactéroïdes fixateurs d'azote. L'équipe a montré que les bactéries pouvaient prendre plusieurs morphotypes différents (U, pour « unmodified » et E pour « elongated ») en fonction de l'hôte végétal considéré, indiquant que la plante gouverne la différenciation bactérienne [START_REF] Mergaert | Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis[END_REF]. Les bactéroïdes de morphotype E (par exemple chez Medicago, Pisum, Vicia) subissent plusieurs modifications cellulaires et morphologiques comme un allongement cellulaire couplé à une forte endoréplication du génome (les bactéroïdes deviennent polyploïdes) contribuant ainsi à une augmentation importante de la taille des cellules, ainsi qu'une perméabilité membranaire accrue rendant les bactéroïdes incapables de reprendre un cycle de division normal lorsque l'on remet ces bactéroïdes en culture. Les bactéroïdes de morphotype E subissent donc une différenciation terminale ou irréversible, c'est-à-dire qu'ils perdent toute capacité reproductrice [START_REF] Mergaert | Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis[END_REF][START_REF] Van De Velde | Plant peptides govern terminal differentiation of bacteria in symbiosis[END_REF].

Cette différenciation terminale des bactéroïdes est typiquement mise en place par toutes les Légumineuses du clade IRLC (pour Inverted Repeat-Lacking Clade) mais n'est pas conservée dans d'autres clades de Légumineuses. Dans ce second cas (par exemple chez Phaseolus, Vigna, Lotus, Glycine), les bactéries conservent comme dans l'état libre leur morphologie (morphotype U), leur capacité de division dans les symbiosomes et une viabilité importante. Les bactéroïdes de morphotype U subissent donc une différenciation réversible, sans altération du cycle cellulaire ni de perméabilisation de la membrane.

Le processus de différenciation cellulaire et morphologique des bactéroïdes en morphotype E chez M. truncatula est contrôlé en partie par une large famille de peptides antimicrobiens (environ 600 gènes chez M. truncatula) produits par la plante et spécifiquement exprimée dans les nodosités, appelée NCR (pour Nodule-specific Cysteine Rich) (Van de Velde et al., 2010). Ces peptides NCR possèdent un peptide signal et des motifs conservés en cystéine. Ils sont adressés aux bactéroïdes et leur adressage est en partie contrôlé par une « signal peptidase complexe », spécifique des nodosités dont DNF1 est une sous unité (dnf1 pour defective in nitrogen fixation 1 est un mutant de Medicago truncatula qui forme des nodosités non fonctionnels) [START_REF] Wang | A nodule-specific protein secretory pathway required for nitrogen-fixing symbiosis[END_REF]. L'inactivation de dnf1 bloque l'adressage aux symbiosomes des peptides NCR qui restent bloqués dans le réticulum endoplasmique et les bactéroïdes restent indifférenciés (Van de Velde et al., 2010). Localisée au niveau du réticulum endoplasmique, cette « signal peptidase complexe » assurerait le clivage du peptide signal des NCR permettant ensuite leur transport via trafic vésiculaire à la membrane péribactéroïdienne et donc aux symbiosomes [START_REF] Van De Velde | Plant peptides govern terminal differentiation of bacteria in symbiosis[END_REF][START_REF] Wang | A nodule-specific protein secretory pathway required for nitrogen-fixing symbiosis[END_REF].

Les peptides NCRs ressemblent à des défensines, des peptides antimicrobiens concourant à l'immunité innée, système immunitaire présent chez tous les Eucaryotes. Ainsi, les peptides NCRs, tout comme les défensines, perméabilisent les membranes bactériennes et présentent une activité antimicrobienne in vitro, tuant des bactéries et ayant un spectre d'activité large [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF][START_REF] Van De Velde | Plant peptides govern terminal differentiation of bacteria in symbiosis[END_REF].

Résumé

Ainsi, un élément clef dans la différenciation des bactéroïdes est la protéine bactérienne BacA, un transporteur membranaire initialement découvert chez S. meliloti comme requis pour la différenciation des bactéries en bactéroïdes suite à leur relargage intracellulaire [START_REF] Glazebrook | A Rhizobium meliloti homolog of the Escherichia coli peptide-antibiotic transport protein SbmA is essential for bacteroid development[END_REF]. BacA confère une résistance contre l'activité antimicrobienne des peptides NCRs [START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF]. Un mutant du gène bacA chez S. meliloti induit la formation de nodosités non-fonctionnelles chez Medicago, dans lesquelles les bactéries ne se différencient pas en bactéroïdes et meurent instantanément après avoir été confrontées aux peptides NCRs produits par les cellules hôtes [START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF].

Le gène bacA de S. meliloti 1021 code une protéine de 381 acides aminés. Selon les prédictions bioinformatiques, cette protéine qui possède 8 domaines transmembranaires, serait semblable à des transporteurs membranaires de type ABC [START_REF] Glazebrook | A Rhizobium meliloti homolog of the Escherichia coli peptide-antibiotic transport protein SbmA is essential for bacteroid development[END_REF]. Les transporteurs ABC permettent soit l'import ou l'export d'une grande variété de substrats à travers les membranes biologiques. La structure canonique d'un transporteur ABC est composée de quatre domaines : deux domaines hydrophobes transmembranaires et deux domaines liant et hydrolysant l'ATP. La protéine BacA ne possède cependant pas la cassette ATPase.

En culture, la mutation bacA augmente la résistance de S. meliloti aux peptides antimicrobiens ayant une cible intracellulaire, tels que la bléomycine, la microcine B17, et Bac7, un peptide eucaryote riche en proline [START_REF] Ichige | Genetic analysis of the Rhizobium meliloti bacA gene: functional interchangeability with the Escherichia coli sbmA gene and phenotypes of mutants[END_REF][START_REF] Marlow | Essential role for the BacA protein in the uptake of a truncated eukaryotic peptide in Sinorhizobium meliloti[END_REF]. Ce phénotype suggère que l'un des rôles de BacA serait d'internaliser les peptides antimicrobiens. L'étude du mutant bacA de S. meliloti révèle aussi une forte sensibilité aux agents inducteurs de stress membranaire tels que le SDS, l'éthanol et le déoxycholate [START_REF] Ferguson | Deficiency of a Sinorhizobium meliloti BacA mutant in alfalfa symbiosis correlates with alteration of the cell envelope[END_REF]. Chez S. meliloti, BacA est également impliquée dans la synthèse de lipide A, un composant essentiel des lipopolysaccharides qui constituent la membrane externe des bactéries à gram négatif [START_REF] Ferguson | Deficiency of a Sinorhizobium meliloti BacA mutant in alfalfa symbiosis correlates with alteration of the cell envelope[END_REF][START_REF] Ferguson | Similarity to peroxisomal-membrane protein family reveals that Sinorhizobium and Brucella BacA affect lipid-A fatty acids[END_REF]. Des analyses en similarité de séquences ont suggéré que BacA serait un transporteur d'acides gras à longue chaine qui une fois dans le périplasme assurerait la maturation des lipides A. Cette maturation permettrait de masquer les LPS pour la cellule hôte. En effet, le lipide A est un puissant antigène chez les Mammifères, ce qui faciliterait la persistance de la bactérie prolongeant ainsi l'infection [START_REF] Ferguson | Similarity to peroxisomal-membrane protein family reveals that Sinorhizobium and Brucella BacA affect lipid-A fatty acids[END_REF]. La perte de BacA s'accompagne d'une baisse des contenus en VLCFA (very-long-chain fatty acids) des lipides A, expliquant la défection dans l'intégrité membranaire [START_REF] Ferguson | Similarity to peroxisomal-membrane protein family reveals that Sinorhizobium and Brucella BacA affect lipid-A fatty acids[END_REF]. Cependant, ces modifications des lipides A ne sont pas indispensables à la réalisation d'une infection chronique [START_REF] Ferguson | Importance of unusually modified lipid A in Sinorhizobium stress resistance and legume symbiosis[END_REF] indiquant qu'il s'agît probablement de la perte de la fonction de transport de peptides qui est Résumé la cause de l'incapacité à se maintenir dans les nodosités.

Dans les nodosités, la protéine BacA de S. meliloti semble avoir un rôle prépondérant dans la protection des bactéries contre l'activité antimicrobienne des NCRs, permettant ainsi le maintien des bactéries après l'infection. Il est probable que la fonction de BacA soit de procéder à l'internalisation des NCRs afin de les enlever de leur site de toxicité, la membrane.

En accord avec ce modèle, il a été constaté que le gène bacA chez les rhizobia n'est nécessaire pour la symbiose que pour la nodulation des Légumineuses produisant des NCRs [START_REF] Maruya | The bacA Gene Homolog, mlr7400, in Mesorhizobium loti MAFF303099 is dispensable for symbiosis with Lotus japonicus but partially capable of supporting the symbiotic function of bacA in Sinorhizobium meliloti[END_REF][START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF].

De nombreuses études ont apportés des résultats similaires chez d'autres espèces, comme Mycobacterium tuberculosis [START_REF] Domenech | BacA, an ABC transporter involved in maintenance of chronic murine infections with Mycobacterium tuberculosis[END_REF], Salmonella enterica [START_REF] Pränting | Mechanism and fitness costs of PR-39 resistance in Salmonella entericas erovar Typhimurium LT2[END_REF], Mesorhizobium huakuii [START_REF] Tan | BacA is indispensable for successful Mesorhizobium-Astragalus symbiosis[END_REF], Brucella abortus [START_REF] Levier | Similar requirements of a plant symbiont and a mammalian pathogen for prolonged intracellular survival[END_REF], chez lesquelles des homologues de BacA ont été caractérisés. Ces études mettent en exergue le rôle principal de BacA dans l'internalisation de peptides antimicrobiens, et donc la résistance face à ces derniers.

Premier volet de la thèse : L'expression des gènes NCRs chez Medicago

C'est dans le contexte décrit ici en haut que j'ai débuté ma thèse. Dans un premier volet de mon travail de thèse, je me suis intéressée à l'étude de la spécificité de l'expression des gènes NCR chez la Légumineuse M. truncatula en tirant profit de la base de données MtGEA (Medicago truncatula Gene Expression Atlas), qui regroupe des profils transcriptomiques obtenus par hybridation de puces à ADN Affymetrix (50 900 sondes). Cette base de données regroupe des expériences réalisées sur différents organes (feuilles, racines, fleurs, fruits, nodosités…) et en réponse à de nombreux stimuli (hormones, stress biotiques et abiotiques, interactions symbiotiques…). Ainsi, nous avons pu analyser l'expression de 334 Ce travail débouche sur de nombreuses pistes de recherche. Ainsi il serait intéressant d'identifier les bases moléculaires de cette régulation génique. Ce contrôle très strict est-il uniquement dû à l'action de répresseurs/activateurs transcriptionnels ? Ou est-ce qu'un contrôle épigénétique est impliqué, possiblement par le biais de la méthylation de l'ADN ou selon les marques d'histones présentes dans les régions génomiques portant des gènes NCR ? L'endoréplication des cellules végétales était une étape majeure dans l'activation des gènes de nodulation. La plupart des gènes nodosité-spécifiques ne sont pas exprimés dans les autres tissus. Ces gènes sont donc maintenus dans un état inactif pendant tout le développement végétal et l'inactivation transcriptionnelle est levée pendant le développement des nodosités. Cette régulation pourrait dépendre des processus épigénétiques résultant des modifications de la structure chromatinienne (modifications post-traductionnelle des histones et méthylation de l'ADN) pendant la formation des nodosités. Au laboratoire, nous avons proposé que l'endoréplication, pendant la différenciation cellulaire dans les nodosités, fasse partie des mécanismes qui lèvent l'inactivation transcriptionnelle des gènes spécifiques des nodosités, ceci résultant de modifications des codes épigénétiques au niveau de la chromatine.

Résumé

Afin de vérifier le rôle de l'endoréplication dans l'activation du transcriptome, une approche intéressante, actuellement poursuivie au laboratoire, consiste à trier par cytométrie en flux des noyaux, en fonction de leur contenu nucléaire pour ensuite faire chez chacun de ces types nucléaires l'analyse du transcriptome. Cette analyse est couplée avec celle de la structure de la chromatine (méthylation d'ADN et modifications des histones) au niveau des loci génomiques portant les gènes spécifiques des nodosités.

Deuxième volet de la thèse : La différenciation des bactéroïdes chez

Aeschynomene

Les mécanismes de contrôle par la plante sur les rhizobia intracellulaires demeurent à ce jour peu connus et le seul modèle étudié, au début de ce travail de thèse, restait l'interaction entre M. truncatula et S. meliloti. Les études sur ce modèle ont notamment permis de montrer que la plante exerce un contrôle sur la bactérie via les peptides NCRs (Van de Velde et al., 2010). Cependant, ce mécanisme a été supposé spécifique au clade IRLC, un clade de Légumineuses induisant des morphotypes E et absente chez des Légumineuses induisant des morphotypes U [START_REF] Alunni | Genomic organization and evolutionary insights on GRP and NCR genes, two large nodule-specific gene families in Medicago truncatula[END_REF] car la présence de NCR n'a pas été à ce jour révélée chez d'autres espèces de Légumineuses. On peut donc s'interroger sur l'existence possible d'autres clades de Légumineuses dans lesquels les bactéroïdes se différencient ? Et si oui, est ce que la différenciation repose sur les mêmes mécanismes que ceux observés chez les IRLC auxquels appartient

Medicago?

Je me suis donc intéressée à la symbiose de certaines Légumineuses tropicales du genre Aeschynomene appartenant au clade des Dalbergoïdes. Les Aeschynomene sont étroitement liées à l'arachide, deuxième culture de Légumineuse cultivée dans le monde et donc ce modèle est très pertinent pour l'agronomie. Les Aeschynomene se caractérisent par des propriétés symbiotiques très originales. Tout d'abord, la nodulation peut avoir lieu aussi bien sur les racines que sur les tiges conférant à ces plantes un fort potentiel de fixation d'azote [START_REF] Giraud | Effect of Bradyrhizobium photosynthesis on stem nodulation of Aeschynomene sensitiva[END_REF][START_REF] Giraud | Nitrogen-fixing symbiosis between photosynthetic bacteria and legumes[END_REF]. De façon surprenante, certaines espèces d'Aeschynomene ne nécessitent pas la synthèse de facteur Nod par la bactérie pour l'induction de la nodulation [START_REF] Giraud | Legumes symbioses: Absence of Nod genes in photosynthetic Bradyrhizobia[END_REF], alors que ces molécules étaient jusqu'alors considérées comme des signaux ayant un rôle clé et essentiel dans la mise en place de la nodulation chez tous les légumineuses. De plus, les symbiotes, appartenant au genre Bradyrhizobium, ont également des caractéristiques inhabituelles pour des rhizobia, comme leur capacité de Résumé photosynthèse et de fixation d'azote en culture.

Le genre d'Aeschynomene apparaît également comme un modèle de choix pour l'étude des mécanismes de différenciation des bactéroïdes. En effet, il peut être observé chez les Aeschynomene des différences morphologiques drastiques pour la même bactérie selon l'espèce de la plante hôte. Par exemple, les bactéroïdes de la souche Bradyrhizobium sp.

ORS285, microsymbionte d'Aeschynomene présentent chez Aeschynomene afraspera une morphologie en bâtonnet allongé. Le morphotype allongé (morphotype E) observé chez A. afraspera ressemble à celui de S. meliloti en symbiose avec M. truncatula. [START_REF] Bonaldi | Nodulation of Aeschynomene afraspera and A. indica by photosynthetic Bradyrhizobium Sp. strain ORS285: the nod-dependent versus the nod-independent symbiotic interaction[END_REF]. Récemment, un nouveau morphotype de bactéroïde a été identifié chez Aeschynomene indica. Ces bactéroïdes sont sphériques (morphotype S) et peuvent être induits par la souche ORS285.

De plus, la souche Bradyrhizobium japonicum USDA110 (une souche nonphotosynthétique et non capable de fixer l'azote en état libre mais seulement en condition de symbiose), le microsymbionte de soja, est capable de noduler efficacement A. afraspera [START_REF] Renier | Photosynthetic Bradyrhizobium Sp. strain ORS285 synthesizes 2-O-methylfucosylated lipochitooligosaccharides for nod gene-dependent interaction with Aeschynomene plants[END_REF].

Le deuxième objectif de ma thèse a donc consisté à comprendre les mécanismes de la différenciation bactérienne au sein des nodosités chez les Aeschynomene.

! Différenciation terminale des bactéroïdes chez Aeschynomene

Le processus de différenciation des bactéries lors de la symbiose avec les Légumineuses peut être plus ou moins drastique. Dans le cas de la symbiose entre M. truncatula et S. meliloti, cette différenciation est dite 'terminale'. Le fait que la même bactérie (la souche ORS285) puisse avoir deux morphotypes différents en fonction de la plante infectée souligne que cette différenciation est sous le contrôle de la plante. C'est sur la validation de cette hypothèse que nous avons collaboré avec l'équipe d'Eric Giraud pour caractériser en premier lieu l'état de différenciation des bactéroïdes dans les nodosités induites par la souche Bradyrhizobium sp. ORS285 et de décrypter les facteurs de la plante qui induisent cette différenciation.

Le travail a pu démontrer que les bactéroïdes aussi bien allongés que sphériques présentent un niveau d'endoréplication quatre à huit fois supérieur à celui des bactéries en vie libre (7C chez A. afraspera et 16C chez A. indica). Une perte d'intégrité de la membrane et un taux de viabilité extrêmement faible ont également été observés. En dehors de leur morphologie, les bactéroïdes présentent donc des propriétés très similaires chez les deux plantes mais également semblables à celles de S. meliloti en symbiose avec M. truncatula.

Elles sont à différenciation terminale et polyploïdes. Mon objectif étant d'étudier le rôle de ce gène au cours de l'interaction symbiotique Aeschynomene-Bradyrhizobium : En particulier je me suis intéressée à la différenciation et l'endoreplication des bactéroïdes, à la survie in planta ainsi qu'au réponse face aux peptides antimicrobiens. Cette nouvelle étude a constitué le coeur de ma thèse et sera présentée sous forme de deux publications (qui sont prêts à être soumise) dont je suis le premier auteur (chapitre III des résultats).

Ces ressemblances soulèvent une question

Lors du crible sur plante d'une banque de mutants insertionnels de la bactérie Bradyrhizobium sp. ORS278 (insertion aléatoire du transposon mini-Tn5), un mutant dans un gène bclA formant des nodosités non-fonctionnelles chez A. indica a été identifié [START_REF] Bonaldi | Large-scale transposon mutagenesis of photosynthetic Bradyrhizobium sp. strain ORS278 reveals new genetic loci putatively important for nod-independent symbiosis with Aeschynomene indica[END_REF]. A cause du spectre d'hôte limité de cette bactérie, le mutant bclA dans la souche ORS278 ne permet d'étudier le rôle de ce gène que chez A. evenia ou A. indica formant ainsi des bactéroïdes du morphotype S. Afin d'étendre cette étude aux morphotypes E et U, la construction par génétique inverse de mutants de délétion du gène bclA chez ces deux souches de Bradyrhizobium ORS285 et USDA110 a été réalisée. Cela nous a permis d'étudier l'implication du gène bclA dans la survie des bactéroïdes au sein des nodosités, et de déterminer son rôle dans la résistance aux peptides antimicrobiens.

Pour répondre à ces questions, la capacité du mutant bclA, de chaque Bradyrhizobium étudié, à infecter et à se maintenir dans les nodosités de soja et des Aeschynomene, a été étudiée. En parallèle, des tests de sensibilité aux peptides antimicrobiens NCR, à la bléomycine et à Bac7, ont été réalisés afin d'évaluer le rôle de BclA dans la réponse de la bactérie aux divers peptides antimicrobiens.

Pour ce faire, les phénotypes symbiotiques du mutant ORS285∆bclA (la structure des nodosités, la différenciation des bactéroïdes et la fonctionnalité de nodosités) ont été analysés sur les plantes hôtes A. indica et A. evenia (bactéroïdes du morphotype S) et A. afraspera (bactéroïdes du morphotype E). Dans les différents systèmes symbiotiques testés, des nodosités non-fonctionnelles ont été formées. Cependant, les cellules de ces nodosités sont infectées normalement par les bactéries, mais celles-ci ne se différencient pas en bactéroïdes allongées ou sphériques et restent inchangées et meurent ensuite rapidement. Cet état de nondifférenciation a été confirmé par des mesures de la ploïdie des bactéroïdes, elle est de l'ordre de 3C chez A. indica et A. afraspera. Il semble alors qu'il y a une corrélation entre la présence du gène bclA et la formation des bactéroïdes polyploïdes. Par ailleurs, on a pu démontrer que le gène bclA de Bradyrhizobium peut complémenter le phénotype symbiotique du gène bacA chez S. meliloti avec une restauration de la différenciation des bactéroïdes dans les nodosités de Medicago, bien que cette complémentation n'est que partielle et insuffisante pour restaurer la capacité de la souche à fixer l'azote.

Le gène bacA chez Sinorhizobium fredii, Mesorhizobium loti ou Rhizobium leguminosarum bv. phaseoli n'est pas nécessaire pour la formation de bactéroïdes du morphotype U [START_REF] Karunakaran | BacA is essential for bacteroid development in nodules of galegoid, but not phaseoloid, legumes[END_REF][START_REF] Maruya | The bacA Gene Homolog, mlr7400, in Mesorhizobium loti MAFF303099 is dispensable for symbiosis with Lotus japonicus but partially capable of supporting the symbiotic function of bacA in Sinorhizobium meliloti[END_REF][START_REF] Wildermuth | Modulation of host nuclear ploidy: a common plant biotroph mechanism[END_REF][START_REF] Ardissone | Role of BacA in lipopolysaccharide synthesis, peptide transport, and nodulation by Rhizobium sp. strain NGR234[END_REF]. [START_REF] Ferguson | Similarity to peroxisomal-membrane protein family reveals that Sinorhizobium and Brucella BacA affect lipid-A fatty acids[END_REF]. BclA est un transporteur de type ABC qui diffère de ses homologues BacA (S. meliloti) et SbmA (E. coli) par la présence d'un domaine ATPase, absent chez ces derniers.

Il a été démontré, chez S. meliloti et E. coli que le gène bacA/sbmA affecte leur sensibilité à des peptides antimicrobiens qui agissent sur l'enveloppe bactérienne telles que les défensines, à d'autres peptides antimicrobiens qui agissent à l'intérieur des cellules tels que la bléomycine (un antibiotique qui inhibe la transcription et la réplication de l'ADN) et Bac7 (peptide riche en proline, se lie aux ribosomes et inhibe la traduction). L'internalisation de la bléomycine est en majorité dépendante du mécanisme lié à BacA [START_REF] Wehmeier | Internalization of a thiazole-modified peptide in Sinorhizobium meliloti occurs by BacA-dependent and independent mechanisms[END_REF]. La résistance à la bléomycine observée chez les mutants bacA s'explique alors par l'absence du transporteur BacA, qui ne l'internalise plus, limitant sa toxicité. Les orthologues de bacA chez E. coli et S. meliloti sont requis pour l'internalisation de Bac7 [START_REF] Mattiuzzo | Role of the Escherichia coli SbmA in the antimicrobial activity of proline-rich peptides[END_REF][START_REF] Marlow | Essential role for the BacA protein in the uptake of a truncated eukaryotic peptide in Sinorhizobium meliloti[END_REF], ce qui provoque une sensibilité à ce peptide. C'est pourquoi, lorsque les gènes bacA de ces bactéries sont mutés, il y a une diminution de la sensibilité à Bac7.

Résumé

Donc la question se pose si BclA est capable de transporter la bleomycine, Bac7 comme ses homologues BacA et SbmA. Ainsi, des tests de sensibilité in vitro ont été effectués et ont montré que la présence de BclA augmente significativement la sensibilité des bactéries à ces agents antimicrobiens confirmant que BclA peut importer ces peptides. En plus, nous avons montré que BclA augmente la résistance à l'activité antimicrobienne des NCRs comme la protéine BacA. Finalement, nous avons utilisé un peptide NCR modifié avec un groupe fluorescent, combiné avec une analyse par cytométrie en flux pour montrer que BclA ainsi que BacA peut promouvoir l'import de ce NCR démontrant ainsi que BclA et BacA sont des transporteurs de peptides NCRs.

Conclusions générales

Ainsi des résultats originaux ont été obtenus sur le mode de différenciation des bactéroïdes chez les Aeschynomene. Leurs bactéroïdes sont allongés ou sphériques selon la plante hôte, ils présentent une différenciation terminale et sont polyploïdes comme les bactéroïdes des Légumineuses appartenant aux IRLC. Les analyses transcriptomiques, hybridation in situ et protéomiques [START_REF] Czernic | Convergent evolution of endosymbiont differentiation in Dalbergoid and IRLC legumes mediated by nodule-specific cysteine-rich peptides[END_REF] ont démontré que les cellules symbiotiques dans les nodosités d'Aeschynomene produisent des peptides NCR, qui sont transportés vers les bactéroïdes. Le blocage du transport des NCR dans la voie de sécrétion par ARNi (extinction de l'orthologue de DNF1) inhibe la différenciation des bactéroïdes. Ces résultats suggèrent que la différenciation des bactéroïdes dans le clade des Dalbergoïdes, qui a probablement évolué indépendamment de la différenciation des bactéroïdes dans le clade des IRLC, repose sur des mécanismes très similaires [START_REF] Czernic | Convergent evolution of endosymbiont differentiation in Dalbergoid and IRLC legumes mediated by nodule-specific cysteine-rich peptides[END_REF].

Les connaissances acquises au cours des dernières années sur le rôle des NCR dans la différenciation des bactéroïdes chez M. truncatula/S. meliloti offre de nombreuses perspectives de travail sur les NCRs identifiés chez les Aeschynomene. De plus, la technique de transformation par hairy root permet d'envisager une approche par gain de fonction [START_REF] Bonaldi | Large-scale transposon mutagenesis of photosynthetic Bradyrhizobium sp. strain ORS278 reveals new genetic loci putatively important for nod-independent symbiosis with Aeschynomene indica[END_REF] Ces mécanismes de différenciation des bactéries symbiotiques ne sont étonnement pas limités aux couples Légumineuses-rhizobia. En effet, certains insectes, comme le puceron Acyrthosiphon pisum, effectuent aussi une symbiose avec des bactéries qui se différencient en adoptant un morphotype S. Des peptides antimicrobiens produits par l'insecte, et possédant des motifs conservés en cystéine ainsi qu'un signal peptide ont été identifiés et nommés BCR pour Bacteriocyte-specific Cysteine Rich peptides [START_REF] Shigenobu | Aphids evolved novel secreted proteins for symbiosis with bacterial endosymbiont[END_REF]. Par ailleurs, chez un charançon ravageur de céréales (Sitophilus zeamais), des peptides antimicrobiens ont été montrés comme induisant la différenciation de bactéries symbiotiques en morphotype E [START_REF] Login | Antimicrobial peptides keep insect endosymbionts under control[END_REF]. Ces mécanismes de différenciation, imposés par un hôte eucaryote à un symbionte procaryote par l'intermédiaire de peptides antimicrobiens dérivés de l'immunité innée, sont apparus indépendamment à plusieurs reprises chez les plantes et les animaux, suggérant que ce type de stratégie confère un avantage à l'hôte eucaryote.

Les NCR sont des peptides cationiques qui assurent une activité inhibitrice de la croissance microbienne et plusieurs études montrent que les peptides cationiques sont des effecteurs de réponses immunitaires innées. Ceci suggère que les NCR pourraient être des candidats prometteurs pour la mise en place de stratégies thérapeutiques innovantes.

L'étude de Sinorhizobium meliloti et de ses hôtes végétaux est un sujet de recherche relativement bien développé. En effet, outre le fait que S. meliloti est le symbionte de la luzerne, Medicago sativa, et de la Légumineuse modèle Medicago truncatula, c'est aussi une α--protéobactérie très proche de bactéries pathogènes comme Agrobacterium et autres. L'étude du processus d'infection de M. truncatula par S. meliloti pourrait fournir des gènes candidats pour l'étude des mécanismes pathogènes de ces dernières et de développer de nouvelles stratégies thérapeutiques.

Outre cette connaissance fondamentale des gènes et acteurs moléculaires impliqués dans le mécanisme de différenciation, il demeure important de comprendre si ce processus confère un véritable avantage fonctionnel à la plante. Une étude récente suggère que ce processus de différenciation morphologique ne serait pas un caractère ancestral mais serait apparu à plusieurs reprises de façon indépendante dans la famille des Légumineuses (Oono et al., 2010). Une autre étude qui compare l'efficacité symbiotique de différents couples rhizobium-Légumineuse dans lesquels la même bactérie subit ou non une différenciation Résumé montre une meilleure performance symbiotique pour les bactéroïdes différenciés morphologiquement (Oono et Denison, 2010). Ces études suggèrent ainsi que ce processus de différenciation morphologique constituerait un avantage évolutif pour le partenaire végétal.

Différentes hypothèses ont été avancées pour expliquer cette amélioration des performances symbiotiques : i) l'endoréplication du génome de la bactérie permettrait d'assurer une activité métabolique supérieure comme cela est observé chez les plantes et ainsi une plus forte activité de la nitrogénase [START_REF] Kondorosi | Plant cell-size control: growing by ploidy?[END_REF], ii) la fragilisation des parois bactériennes permettrait à la plante de récupérer plus efficacement les nutriments investis dans le développement de la biomasse bactérienne lors de la sénescence de la nodosité [START_REF] Vasse | Correlation between ultrastructural differentiation of bacteroids and nitrogen fixation in alfalfa nodules[END_REF] Les raisons de cet avantage ne sont pas connues, mais nous pouvons supposer que la forme sphérique des bactéroïdes pourrait permettre, compte tenu de l'encombrement minimal assuré par cette forme, un meilleur remplissage des cellules végétales et ainsi d'améliorer l'efficacité de la fixation d'azote des nodosités [START_REF] Wadisirisuk | Importance of bacteroid number in nodules and effective nodule mass to dinitrogen fixation by cowpeas[END_REF]. En accord avec cette hypothèse, des mesures du nombre de bactéroïdes (comptage sur cellule de Malassez) par unité de biomasse nodulaire montre que la densité en bactéroïdes est 25% supérieure dans

Table of contents

List of publications and communications

1. Journal papers 1.1. Guefrachi, I., Pierre, O., Bourge, M., Timchenko, T., Alunni, B., Barrière, Q., Czernic, P., Villaécija-Aguilar, J.A., Verly, C., Fardoux, J., Mars, M., Kondorosi, E., Giraud, E., and Mergaert, P. (2015a). Bradyrhizobium BclA is a NCR peptide transporter required for bacterial differentiation in symbiosis with Aeschynomene. Molecular Plant-Microbe Interactions (Chapter III; In press). 1.2. Guefrachi, I., Pierre, O., Crespo, J.C., Timchenko, T., Alunni, B., Mars, M., Giraud, E., Vinardell, J.M., and Mergaert, P. (2015b). The Bradyrhizobium japonicum BclA NCR transporter is not required for symbiosis with soybean (Chapter III; In preparation). 1.3. Czernic, P., Gully, D., Cartieaux, F., Moulin, L., Guefrachi, I., Patrel, D., [START_REF] Czernic | Convergent evolution of endosymbiont differentiation in Dalbergoid and IRLC legumes mediated by nodule-specific cysteine-rich peptides[END_REF]. Convergent evolution of endosymbiont differentiation in Dalbergoid and IRLC legumes mediated by nodulespecific cysteine-rich peptides (Chapter II; Submitted). 1.4. Guefrachi, I., Nagymihaly, M., Pislariu, C.I., Van de Velde, W., Ratet, P., Mars, M., Udvardi, M.K., [START_REF] Guefrachi | Extreme specificity of NCR gene expression in Medicago truncatula[END_REF]. Extreme specificity of NCR gene expression in Medicago truncatula. BMC Genomics. 15: 712 (Chapter I).

Book Chapter

2.1. Guefrachi, I., Verly, C., [START_REF] Guefrachi | Bradyrhizobium BclA is a NCR peptide transporter required for bacterial differentiation in symbiosis with Aeschynomene Haag[END_REF]. Role of the bacterial BacA ABC-transporter in chronic infection of nodule cells by Rhizobium bacteria.

In: "Biological Nitrogen Fixation", F.J. de Bruijn (ed.). Wiley-Blackwell (In press).

List of figures

List of tables

Nitrogen fixation: background and ecological and agronomic stakes

The element Nitrogen (N), or "Azote" meaning in the French language "without life" as the chimist Antoine Lavoisier called it about 200 years ago, has proved to be anything but lifeless, since it is a component of food, poisons, fertilizers, and explosives. And above all, nitrogen is one of the six basic elements found in living organisms, next to hydrogen, oxygen, carbon, phosphorous and calcium. The element N is highly abundant on the earth's surface where it is found almost entirely as nitrogen gas (N 2 ) in the atmosphere. N 2 is chemically extremely stable (because of the triple bond between the two N atoms) but biologically unusable for most organisms (breaking the triple bond needed for the building of new molecules is extremely difficult and energy costing). Living organisms prefer reduced nitrogen, ammonium (NH 4 + ), or oxidized nitrogen, nitrate (NO 3 -), for incorporation in biomolecules. Therefore, usable nitrogen is limited for biological production in the biosphere as well as in agriculture.

The chemical transformations of the element N on earth into its many oxidation states are called the nitrogen cycle. The major transformation processes of N are nitrogen fixation, nitrification, denitrification, anammox, and ammonification (Fig. 1). These processes are highly dependent on the activities of a diverse assemblage of microorganisms, bacteria, archaea, and fungi, although physical processes such as lightning also contribute to some extent to the nitrogen cycle and produce ammonium from N 2 . Among these transformation processes, nitrogen fixation plays a particularly important role for life on earth because it is the process that introduces N into the biosphere and compensates for the loss of N resulting from denitrification and anammox (Fig. 1). Without nitrogen fixation, the biosphere would be depleted from N and thus the process is critical for the existence of all life on earth.

The natural balance in the global nitrogen cycle is today seriously perturbed by human activity. The human impact on the global nitrogen cycle via the industrial production of nitrogenous fertilizer with the Haber-Bosch process is close to the level of the natural input (130x10 12 g of N per year costing 7x10 10 $ -a cost that has tripled in less than a decade and requiring 1 to 2 % of the world's annual energy supply). The negative consequences of these human nitrogen additions are alarming and management of the nitrogen cycle is urgently needed [START_REF] Gruber | An Earth-system perspective of the global nitrogen cycle[END_REF]. The drawbacks of man-made chemical nitrogen reduction are the pollution of surface-water by nitrates and an important impact on the greenhouse effect, which is related to the high energy cost for production, transportation and application of fertilizer and the emission of the powerful greenhouse gas nitrous oxide or N 2 O by fertilized soils.

Figure 1. The global nitrogen cycle

The major transformations in the nitrogen cycle are shown. These transformations are for the largest part the result of microbial activities. Nitrogen fixation (red arrow) is the enzymatic reduction of N 2 gas in the atmosphere to ammonium. A small but significant amount of ammonia is also formed from N 2 by lightening. Ammonification (black arrow) also produces ammonia via the decomposition by fungi and bacteria of excreted waste or of dead organisms. Nitrification (blue arrow), the conversion of ammonium to nitrate is the result of the activity of two groups of bacteria, known as the ammonia-oxidizers and the nitrite-oxidizing bacteria, which generate energy from these oxidations. Nitrification is carried out under aerobic conditions. Ammonia oxidation occurring under anoxic conditions is carried out by a completely different group of bacteria and is called Anammox (green arrows). Denitrification (purple arrow) is the result of the reduction of nitrate and other oxidized forms of nitrogen by bacteria that use these N-compounds as electron acceptors in an alternative anaerobic respiration. The denitrification process results in the production of N 2 and its release in the atmosphere, thereby closing the nitrogen cycle. Figure taken from Nature Education.

(http://www.nature.com/scitable/knowledge/library/the-nitrogen-cycle-processes-players-and-human-15644632).

Only a select group of prokaryotic microorganisms is able to perform the energetically extremely costly enzymatic nitrogen fixation. Certain Eubacteriae and Archaea possess the nitrogenase enzyme complex capable to reduce N 2 into the biologically useful form NH 4 + [START_REF] Young | Phylogenetic classification of nitrogen-fixing organisms[END_REF]. Nitrogenase enzymes are related in all nitrogen fixers [START_REF] Zehr | Nitrogenase gene diversity and microbial community structure: a cross-system comparison[END_REF].

Many bacteria and archaea are free-living nitrogen fixing organisms reducing nitrogen solely for their own growth but some bacteria are symbiotic nitrogen fixers, reducing nitrogen not only for their own subsistence but also contributing to the nitrogen supply of a eukaryotic symbiotic partner. Most notorious symbiotic nitrogen fixers are rhizobia, which fix nitrogen within legume plants and Frankia inside actinorhizal plants. But also animals establish associations with nitrogen fixing bacteria. Interesting examples are nitrogen fixing bacteria living in the gut of termites [START_REF] Ceja-Navarro | Compartmentalized microbial composition, oxygen gradients and nitrogen fixation in the gut of Odontotaenius disjunctus[END_REF] or in the fungus gardens of leaf-cutter ants (Pinto-Tomas et al., 2009).

Nitrogen fixing microbes are nitrogen autotrophs while all other organisms, including plants and animals, require uptake of oxidized or reduced nitrogen from nutrition.

Leguminous plants (Fabaceae) and actinorhizal plants are an exception since they are able to fix atmospheric nitrogen (N 2 ) and thus to grow in the absence of available nitrogen in the soil or of nitrogen-containing chemical fertilizers in agricultural settings. Plants of the legume family have solved their nitrogen nutrition needs through the symbiotic association with nitrogen fixing rhizobium bacteria. Rhizobia are a polyphyletic group of soil bacteria belonging to the "-or #-proteobacteria (Fig. 2; [START_REF] Sawada | Changing concepts in the systematics of bacterial nitrogen-fixing legume symbionts[END_REF][START_REF] Masson-Boivin | Establishing nitrogen-fixing symbiosis with legumes: how many rhizobium recipes?[END_REF]. Actinorhizal plants on the other hand associate with a completely different group of bacteria, the Frankia, which are gram-positive actinobacteria and from a monophyletic clade [START_REF] Nouioui | Phylogeny of members of the Frankia genus based on gyrB, nifH and glnII sequences[END_REF].

Figure 2. Phylogeny of rhizobia

Unrooted phylogenetic tree of 16S rDNA sequences from selected "-, ß-and $-proteobacteria. The numbers correspond to clades 1-9 are defined by [START_REF] Sawada | Changing concepts in the systematics of bacterial nitrogen-fixing legume symbionts[END_REF]. Genera in bold font contain rhizobia. Rhizobial species or strains whose genomes have been fully sequenced are indicated in parentheses. [START_REF] Sawada | Changing concepts in the systematics of bacterial nitrogen-fixing legume symbionts[END_REF][START_REF] Masson-Boivin | Establishing nitrogen-fixing symbiosis with legumes: how many rhizobium recipes?[END_REF].

For the purpose of the symbiosis, legumes and actinorhizal plant hosts form new specific organs on its roots, called nodules inside which the symbiotic rhizobia or Frankia are housed (Fig. 3D-G), fix nitrogen and transfer the ammonium to the plant. This highly improves the plant's nitrogen nutrition and makes them independent from external nitrogen supplies. Currently, the process of biological nitrogen fixation is of great importance to the environment and to global agriculture. The Rhizobium-legume and Frankia-actinorhizal symbioses on a global scale are responsible every year for 50 million tons of nitrogen injected into agriculture, without greenhouse effects, as compared to 90 million tons on N-fertilizers [START_REF] Fields | Global nitrogen : Cycling out of control[END_REF]. Legumes are thus plants of extreme ecological and agricultural importance. In addition legumes are a major feed (soybean) and food resource in South and Central America.

Legume seeds are very rich in proteins and thus should become in future decades, a valuable alternative to animal proteins. Furthermore, largely thanks to their N 2 -fixing capacity, legumes are often pioneering plant for (re) colonization of poor and degraded soils (Pérez-Montano et al., 2013). Actinorhizal plants comprise mostly trees and shrubs that are longlived.

They play important roles in wild-land, polar and mountainous ecosystems including forests, bogs, swamps, coastal dunes, landslides, glacial deposits, riparian zones, shrub lands, prairies and deserts. As such they play key pioneer roles in ecological transitions such as following glacier retreat, forest fires or volcanic eruptions. They are used in agro-forestry and horticulture and have been used by humans to re-vegetate nitrogen-poor substrates such as hydrodam dykes or mine spoils.

Because of those agronomic and ecological stakes, the nitrogen fixing symbioses of plants are subject to extensive scientific research worldwide. This thesis work is part of these research efforts. It is focused on the Rhizobium-legume symbiosis. And more specifically, my work has contributed to the understanding how legume plants and rhizobium bacteria are adapted to form a very intricate relationship that is taking place within the nodules. The following paragraphs describe the current knowledge of these plant and bacterial functions, which forms the basis for my thesis work.

Selection of the symbiont in the Rhizobium-legume symbiosis

Rhizobia are horizontally transmitted symbionts, taken up from the soil every time the formation of a new nodule is initiated. Soils are among the richest known environments in terms of bacterial diversity and density. Nevertheless, nodules are generally infected with a clonal population of rhizobia only. Moreover, the legumes only accept specific rhizobial strains. Therefore, the recognition between the symbionts is extremely specific and is mainly based on a two-step recognition of both partners. Major determinants of this specificity are: 1) the plant-produced flavonoids found in root exudates when the plant grows in nitrogen limiting conditions and 2) the secreted bacterial Nod factor molecules, which are produced after rhizobia have sensed the cognate flavonoid and which have a lipo-chitooligosaccharide backbone in common but which are differentiated by Rhizobium strain-specific substitutions on this backbone [START_REF] Mergaert | Molecular mechanisms of Nod factor diversity[END_REF]. Binding of Nod factors to their cognate plant lysin motif (LysM) type receptor-like kinases activates a signaling process, which culminates in the initiation of nodule formation and in the infection of the incipient nodules by the rhizobia [START_REF] Oldroyd | The rules of engagement in the legume-rhizobial symbiosis[END_REF]. Along the infection process, from its initiation till the settlement of the rhizobia inside symbiotic nodule cells, other bacterial patterns like lipopolysaccharides and exopolysaccharides are recognized by the host tissues [START_REF] Gibson | Molecular determinants of a symbiotic chronic infection[END_REF]. These sensing mechanisms function as checkpoints and failure of recognition will result in abortion of the interaction. Together, these mechanisms assure that the proper symbiotic bacteria are selected from the environment.

Symbiotic nodule cell formation

The details of the developmental processes leading to a mature nodule differ considerably among legumes. In the most common mechanism [START_REF] Gibson | Molecular determinants of a symbiotic chronic infection[END_REF][START_REF] Oldroyd | The rules of engagement in the legume-rhizobial symbiosis[END_REF], Nod factors produced by rhizobia located at the root surface, induce cell divisions in root cortical cells forming the nodule primordium. Concomitantly, the bacteria penetrate the host tissues through tubular infection structures called infection threads, which guide the bacteria towards the cells of the growing primordium. They release rhizobia by an endocytosis-like mechanism in the primordium cells, enclosed in organelle-like, membranebound structures called symbiosomes. Infected cells stop dividing but grow further by repeated endoreduplication cycles to become giant polyploid symbiotic cells [START_REF] Vinardell | Endoreduplication mediated by the APC activator CCS52A is required for symbiotic cell differentiation in Medicago truncatula nodules[END_REF]. The differentiating symbiotic cells are gradually infected and filled with Rhizobiumcontaining symbiosomes. Continuous bacterial release from infection threads in combination with symbiosome divisions leads to a complete packing of the cytosolic space of the symbiotic cells. Mature symbiotic cells have about 80-fold larger cell size and endoploidy levels up to 64C compared to the diploid progenitor cells (Fig. 4).

Nodule organogenesis is accompanied with major changes in the gene expression program [START_REF] El Yahyaoui | Expression profiling in Medicago truncatula identifies more than 750 genes differentially expressed during nodulation, including many potential regulators of the symbiotic program[END_REF][START_REF] Manthey | Transcriptome profiling in root nodules and arbuscular mycorrhiza identifies a collection of novel genes induced during Medicago truncatula root endosymbioses[END_REF][START_REF] Lohar | Transcript analysis of early nodulation events in Medicago truncatula[END_REF][START_REF] Starker | Nitrogen fixation mutants of Medicago truncatula fail to support plant and bacterial symbiotic gene expression[END_REF][START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF][START_REF] Moreau | Transcrioptional reprogramming during root nodule development in Medicago truncatula[END_REF]. Two waves of transcriptional reprogramming occur during Medicago truncatula nodule formation, which involves the repression and the massive induction of hundreds of genes (Fig. 4) [START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF].

A first wave occurs concomitantly with the establishment of an incipient nodule containing differentiating symbiotic cells while a second wave is observed at the establishment of a functional nodule. In plant and bacterial symbiotic mutants forming nonfunctional nodules, the transcriptional activation of these genes is only observed when endoreduplication takes also place suggesting that most of them are expressed in the endoreduplicated symbiotic cells and that these cells undergo a tremendous transcriptional reprogramming [START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF]. Several examples of these genes were confirmed to be indeed specifically and only expressed in the symbiotic nodule cells by in situ transcript localization techniques [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF][START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF][START_REF] Van De Velde | Plant peptides govern terminal differentiation of bacteria in symbiosis[END_REF]. Moreover, the in situ transcript localization revealed that genes of the first wave are activated in young differentiating symbiotic cells with intermediate ploidy levels and the second wave genes are activated in mature symbiotic cells with high ploidy levels (Fig. 4). A first wave occurs concomitantly with the establishment of an incipient nodule containing differentiating symbiotic cells while a second wave is observed at the establishment of a functional nodule. In plant and bacterial symbiotic mutants forming nonfunctional nodules, the transcriptional activation of these genes is only observed when endoreduplication takes also place suggesting that most of them are expressed in the endoreduplicated symbiotic cells and that these cells undergo a tremendous transcriptional reprogramming [START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF]. Several examples of these genes were confirmed to be indeed specifically and only expressed in the symbiotic nodule cells by in situ transcript localization techniques [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF][START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF][START_REF] Van De Velde | Plant peptides govern terminal differentiation of bacteria in symbiosis[END_REF]. Moreover, the in situ transcript localization revealed that genes of the first wave are activated in young differentiating symbiotic cells with intermediate ploidy levels and the second wave genes are activated in mature symbiotic cells with high ploidy levels (Fig. 4).

These transcriptome switches put in place the specific metabolic conditions in the symbiotic nodule cell which are required for their symbiotic role, including the maintenance of a low free-oxygen concentration, feeding the microsymbionts and assimilating and transporting the fixed nitrogen [START_REF] Oldroyd | The rules of engagement in the legume-rhizobial symbiosis[END_REF]. An additional striking feature of the symbiotic nodule cell-specific transcriptome in M. truncatula, representing more than 50% of the induced genes, is the activation of the secretory pathway, including genes encoding regulators of the pathway, a large number of secretory proteins and transmembrane proteins [START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF]. The most likely role of this pathway is to feed host proteins to the membrane-bound symbiosomes and their bacteroids. In agreement with this dominant role of these cells, they have a remarkably well developed and abundant endoplasmic reticulum, which is the gateway to the secretory pathway [START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF]. The most notable secretory proteins produced by the symbiotic nodule cells are several classes of peptides including the Glycine Rich Peptides (GRPs) [START_REF] Kevei | Glycine-rich proteins encoded by a nodule-specific gene family are implicated in different stages of symbiotic nodule development in Medicago spp[END_REF][START_REF] Alunni | Genomic organization and evolutionary insights on GRP and NCR genes, two large nodule-specific gene families in Medicago truncatula[END_REF], the Small Nodulin Acidic RNA-binding Proteins (SNARPs) [START_REF] Laporte | A novel RNA-binding peptide regulates the establishment of the Medicago truncatula-Sinorhizobium meliloti nitrogen-fixing symbiosis[END_REF] and the Nodule-specific Cysteine-Rich (NCR) peptides [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF][START_REF] Alunni | Genomic organization and evolutionary insights on GRP and NCR genes, two large nodule-specific gene families in Medicago truncatula[END_REF].

Determinate and indeterminate nodule development

The histology, form and size of nodules can be quite different in the various legume hosts. Nevertheless, they can be classified in two major types, either as the determinate or the indeterminate type (Fig. 5) [START_REF] Terpolilli | What determines the efficiency of N(2)-fixing Rhizobium-legume symbioses?[END_REF][START_REF] Kondorosi | A paradigm for endosymbiotic life: Cell differentiation of Rhizobium bacteria provoked by host plant factors[END_REF][START_REF] Maróti | Nitrogen-fixing Rhizobium-legume symbiosis: are polyploidy and host peptide-governed symbiont differentiation general principles of endosymbiosis?[END_REF].

The major difference between the determinate and indeterminate nodules is the transient or persistent nature of the nodule meristem, respectively. In addition, determinate nodules originate from divisions of outer cortical root cells while the indeterminate ones from inner cortical root cells. Determinate nodules are spherical since cell division is only maintained at the primordium stage and later, while the cell number does not increase, nodule growth is achieved by cell growth and determinate nodules contain homogenous populations of symbiotic cells (Fig. 5). Nevertheless, this process of cell enlargement also involves endoreduplication and symbiotic plant cells can reach the 32C stage (Gonzales-Sama et al., 2006). In a way, determinate nodule organogenesis can be considered as a sequential process, whereas indeterminate nodule formation may represent a spatial differentiation gradient.

In contrast, in the indeterminate nodules, the active cell division is maintained. A nodule meristem is present in the apical region (zone I), which, by constant generation of new cells, provokes the continuous growth of the organ resulting in an elongated nodule shape.

The cells leaving the meristem no longer divide and enter a differentiation phase. The infection thread releases the bacteria into the submeristematic cells, which differentiate gradually along the 12-15 cell layers of the infection zone (zone II), leading to the development of nitrogen-fixing symbiotic cells in nodule zone III (Fig. 5) (Pop and Ott, 2011).

Bacteroid formation in the course of nodulation

The nitrogen fixing rhizobia within the symbiosomes are called bacteroids. Bacteroids are differentiated bacteria entirely dissimilar to the free-living rhizobia in soil. This differentiation results from a massive transcriptome switch, which is in large part regulated by the FixLJ oxygen-sensing two-component regulator. Upon detection of the low free-oxygen in the symbiotic nodule cells, the regulator activates the genes for nitrogenase subunits and cofactor synthesis and for a specific respiratory chain which functions under microaerobic conditions. Moreover, most of the bacterial household genes for cell growth and division, synthesis of ribosomal proteins, nucleic acid synthesis and repair, outer membrane proteins and peptidoglycan are shut down in bacterioids [START_REF] Barnett | A dual-genome Symbiosis Chip for coordinate study of signal exchange and development in a prokaryote-host interaction[END_REF][START_REF] Becker | Global changes in gene expression in Sinorhizobium meliloti 1021 under microoxic and symbiotic conditions[END_REF][START_REF] Uchiumi | Expression islands clustered on the symbiosis island of the Mesorhizobium loti genome[END_REF][START_REF] Capela | Sinorhizobium meliloti differentiation during symbiosis with alfalfa: a transcriptomic dissection[END_REF][START_REF] Karunakaran | Transcriptomic analysis of Rhizobium leguminosarum biovar viciae in symbiosis with host plants Pisum sativum and Vicia cracca[END_REF]. Ammonium has to be produced by the nitrogenase in the bacteroids in sufficient amounts to meet the needs of the plant. Therefore, the nitrogen stress response which regulates the nitrogen economy in the bacterial cell has to uncouple nitrogen fixation from the bacterial nitrogen needs in bacteroids contrary to their strict coordination in free-living nitrogen fixing bacteria which produce only that amount of ammonium they need for their own growth [START_REF] Yurgel | A mutant GlnD nitrogen sensor protein leads to a nitrogen-fixing but ineffective Sinorhizobium meliloti symbiosis with alfalfa[END_REF]. Even ammonium assimilation into amino acids and the synthesis of branched chain amino acids is switched off in bacteroids which become dependent on amino acid supply by the host plant [START_REF] Prell | Legumes regulate Rhizobium bacteroid development and persistence by the supply of branched-chain amino acids[END_REF][START_REF] Mulley | Mutation of GOGAT prevents pea bacteroid formation and N 2 fixation by globally downregulating transport of organic nitrogen sources[END_REF]. This phenomenon is called symbiotic auxotrophy.

The dependency of bacteroids on the host plant is further illustrated by the inability of most rhizobia to produce autonomously the FeMo-cofactor of nitrogenase. They depend on supply of homocitrate by the host cell for the production of this very specific nitrogenase cofactor [START_REF] Hakoyama | Host plant genome overcomes the lack of a bacterial gene for symbiotic nitrogen fixation[END_REF]. This far-stretched metabolic integration of the endosymbiont in its host cell advocates the hypothesis that symbiosomes are organelle-like compartments.

In many but not all legume plants, the differentiation of bacteroids is accompanied by a morphological metamorphosis whereby the bacteroid cell becomes enlarged. Three different bacteroid morphotypes have been described (Fig. 6) [START_REF] Vasse | Correlation between ultrastructural differentiation of bacteroids and nitrogen fixation in alfalfa nodules[END_REF][START_REF] Mergaert | Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis[END_REF][START_REF] Bonaldi | Nodulation of Aeschynomene afraspera and A. indica by photosynthetic Bradyrhizobium Sp. strain ORS285: the nod-dependent versus the nod-independent symbiotic interaction[END_REF][START_REF] Kondorosi | A paradigm for endosymbiotic life: Cell differentiation of Rhizobium bacteria provoked by host plant factors[END_REF] and U-morphotype bacteroids in the latter legume hosts [START_REF] Mergaert | Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis[END_REF]. Some Rhizobium isolates can nodulate both Arachis and Vigna unguiculata legumes in which they transform into S-morphotype and U-morphotype bacteroids respectively [START_REF] Sen | A basis for different rates of N 2 -fixation by the same strains of Rhizobium in peanut and cowpea root nodules[END_REF]. In the Aeschynomene genus, bacteroids can be of the E-or S-morphotype.

The Bradyrhizobium strain ORS285 which can nodulate Aeschynomene afraspera and Aeschynomene indica or Aeschynomene evenia transforms into E-morphotype bacteroids in nodules of the former legume and in S-morphotype bacteroids in the latter two legumes [START_REF] Bonaldi | Nodulation of Aeschynomene afraspera and A. indica by photosynthetic Bradyrhizobium Sp. strain ORS285: the nod-dependent versus the nod-independent symbiotic interaction[END_REF]. Moreover, Bradyrhizobium japonicum strain USDA110 which is the Bradyrhizobium type-strain, symbiont of Glycine max (soybean), is able to efficiently nodulate A. afraspera [START_REF] Renier | Photosynthetic Bradyrhizobium Sp. strain ORS285 synthesizes 2-O-methylfucosylated lipochitooligosaccharides for nod gene-dependent interaction with Aeschynomene plants[END_REF]. This strain makes U-morphotype bacteroids on soybean but E-morphotype bacteroids on A. afraspera.

These observations indicate that the bacteroid morphotype is controlled by the host plant and results from host factors that induce the bacteroid metamorphosis [START_REF] Mergaert | Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis[END_REF].

The Sinorhizobium meliloti E-morphotype bacteroids in M. truncatula and Medicago sativa nodules reach up to 10 µm in length, compared to the 1-2 µm of free-living rhizobia.

The genome of these bacteroids is highly amplified and condensed in multiple nucleoids of variable size [START_REF] Vasse | Correlation between ultrastructural differentiation of bacteroids and nitrogen fixation in alfalfa nodules[END_REF][START_REF] Mergaert | Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis[END_REF]. This implies that the differentiation process is the result of a bacterial cell cycle switch which blocks division but permits further genome replication. In agreement with this, genetic or pharmacological interference with the S. meliloti cell cycle machinery leads to morphological transformations, which are Emorphotype bacteroid-like [START_REF] Latch | Generation of buds, swellings, and branches instead of filaments after blocking the cell cycle of Rhizobium meliloti[END_REF][START_REF] Wright | The CcrM DNA methyltransferase is widespread in the alpha subdivision of proteobacteria, and its essential functions are conserved in Rhizobium meliloti and Caulobacter crescentus[END_REF][START_REF] Sibley | The Sinorhizobium meliloti chromosomal origin of replication[END_REF][START_REF] Cheng | A Sinorhizobium meliloti minE mutant has an altered morphology and exhibits defects in legume symbiosis[END_REF][START_REF] Pini | The DivJ, CbrA and PleC system controls DivK phosphorylation and symbiosis in Sinorhizobium meliloti[END_REF]. Moreover, also interference with peptidoglycan biosynthesis results in bacterial morphologies resembling E-or S-morphotype differentiated bacteroids [START_REF] Popham | Role of penicillin-binding proteins in bacterial cell morphogenesis[END_REF][START_REF] Young | Bacterial shape[END_REF][START_REF] Lam | Damino acids govern stationary phase cell wall remodeling in bacteria[END_REF] suggesting that the rhizobial peptidoglycan might be affected during bacteroid differentiation. The envelopes of the bacteroids in Medicago and other, related legumes of the IRLC clade like P. sativum (pea) or V. sativa (vetch) are considerably modified. The lipopolysaccharides, which are the primary component of the outer leaflet of the outer membrane in gram-negative bacteria, become more hydrophobic during bacteroid differentiation [START_REF] Kannenberg | Lipid A and O-chain modifications cause Rhizobium lipopolysaccharides to become hydrophobic during bacteroid development[END_REF][START_REF] Ferguson | Importance of unusually modified lipid A in Sinorhizobium stress resistance and legume symbiosis[END_REF]. Moreover, a propidium iodide uptake assay demonstrated that bacteroid membranes in these legumes are permeable to propidium iodide, contrary to the membranes of the free-living bacteria or to the U-morphotype bacteroids from L. japonicus or P. vulgaris nodules [START_REF] Mergaert | Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis[END_REF]. Finally, the E-morphotype differentiation in the IRLC legumes is an irreversible process because these bacteroids cannot reproduce anymore.

Therefore, this has been called terminal bacteroid differentiation. This is in contrast to the Umorphotype bacteroids, which can reproduce and are therefore reversibly differentiated [START_REF] Mergaert | Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis[END_REF].

The NCR peptide family

The symbiotic nodule cells in M. truncatula express factors inducing E-morphotype bacteroid formation. Candidate factors were identified by interrogating nodule transcriptome datasets for M. truncatula as well as for L. japonicus, V. unguiculata, G. max and P. vulgaris, which form U-morphotype bacteroids. The criteria that were applied were nodule-specific gene expression and expression in the symbiotic nodule cells, presence of orthologous genes in other species of the IRLC clade but they are absence in the tested legumes with Umorphotype bacteroids and finally, the candidate genes should encode secretory proteins compatible with transport to the symbiosomes. On the basis of this analysis, the NCR peptides (Nodule-specific Cysteine-rich Peptides) were proposed as likely candidates for the bacteroid differentiation factors in IRLC legumes that controls the bacteroid elongation and polyploidization [START_REF] Mergaert | Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis[END_REF][START_REF] Alunni | Genomic organization and evolutionary insights on GRP and NCR genes, two large nodule-specific gene families in Medicago truncatula[END_REF][START_REF] Van De Velde | Plant peptides govern terminal differentiation of bacteria in symbiosis[END_REF].

Highly remarkable features of the M. truncatula NCR gene family are its composition of about 600 homologous genes [START_REF] Branca | Whole-genome nucleotide diversity, recombination, and linkage disequilibrium in the model legume Medicago truncatula[END_REF][START_REF] Zhou | Detecting small plant peptides using SPADA (Small Peptide Alignment Discovery Application)[END_REF] and the exclusive nodule-specific expression of nearly all the tested genes [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF][START_REF] Alunni | Genomic organization and evolutionary insights on GRP and NCR genes, two large nodule-specific gene families in Medicago truncatula[END_REF]; see below, Chapter I). The almost 600 NCR genes are spread all over the M.

truncatula genome but the majority of them appears in clusters of up to 11 members [START_REF] Alunni | Genomic organization and evolutionary insights on GRP and NCR genes, two large nodule-specific gene families in Medicago truncatula[END_REF][START_REF] Branca | Whole-genome nucleotide diversity, recombination, and linkage disequilibrium in the model legume Medicago truncatula[END_REF]. This suggests that the family has expanded through multiple gene duplications mediated by mobile elements such as transposons or viruses. The microarray dataset in the M. truncatula gene expression atlas [START_REF] Benedito | A gene expression atlas of the model legume Medicago truncatula[END_REF], containing 254 genome-wide expression profiles, and similarly complex EST datasets (http://compbio.dfci.harvard.edu/tgi/) revealed expression exclusively in nodules for about 300 different NCR genes except for 5, which were also expressed in roots and some aerial tissues [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF][START_REF] Guefrachi | Extreme specificity of NCR gene expression in Medicago truncatula[END_REF], see below, Chapter I). Expression was not found in any other plant organ or in other biotic interactions with mycorrhizal fungi, rhizosphere bacteria, pathogens, or insects. Also in nematode infected roots, NCR expression could not be detected [START_REF] Favery | The endosymbiosis-induced genes ENOD40 and CCS52a are involved in endoparasitic-nematode interactions in Medicago truncatula[END_REF][START_REF] Guefrachi | Extreme specificity of NCR gene expression in Medicago truncatula[END_REF]. Transcriptome analysis of the nodules of a collection of M. truncatula and S. meliloti symbiotic mutants revealed a correlation between NCR gene expression and the formation of symbiotic nodule cells [START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF]. Moreover, when tested with promoter-GUS fusion or in situ hybridization, the expression of NCR genes was always found to be restricted to the Rhizobium-infected plant cells (Fig. 4). However, subsets of NCR genes were expressed in young symbiotic cells while others in older or mature symbiotic cells [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF].

Some NCR genes are expressed at very high levels [START_REF] Guefrachi | Extreme specificity of NCR gene expression in Medicago truncatula[END_REF], see below, Chapter I) comparable to leghemoglobin, which is one of the most strongly expressed nodule genes. On the basis of EST counts, it was estimated that the combined transcripts of the NCR family constitute close to 5% of the total mRNA pool in nodules [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF].

NCR homologs are present in other IRLC legumes including M. sativa [START_REF] Jimenez-Zurdo | Expression profiles of 22 novel molecular markers for organogenetic pathways acting in alfalfa nodule development[END_REF], P. sativum [START_REF] Scheres | Sequential induction of nodulin gene expression in the developing pea nodule[END_REF][START_REF] Kato | Expression of genes encoding late nodulins characterized by a putative signal peptide and conserved cysteine residues is reduced in ineffective pea nodules[END_REF][START_REF] Chou | Thirteen nodule-specific or noduleenhanced genes encoding products homologous to cysteine cluster proteins or plant lipid transfer proteins are identified in Astragalus sinicus L. by suppressive subtractive hybridization[END_REF], A.

sinicus [START_REF] Chou | Thirteen nodule-specific or noduleenhanced genes encoding products homologous to cysteine cluster proteins or plant lipid transfer proteins are identified in Astragalus sinicus L. by suppressive subtractive hybridization[END_REF], T. repens [START_REF] Crockard | A white clover nodulin gene, dd23b, encoding a cysteine cluster protein, is expressed in roots during the very early stages of interaction with Rhizobium leguminosarum biovar trifolii and after treatment with chitolipooligosaccharide Nod factors[END_REF], G. orientalis [START_REF] Kaijalainen | Cloning of nodule-specific cDNAs of Galega orientalis[END_REF] and V. faba [START_REF] Frühling | Genomic organization and expression properties of the VfENOD5 gene from broad bean (Vicia faba L.)[END_REF] where they belong to multi-gene families although it is presently unknown if the NCR gene families in these plants are as large as in M. truncatula.

However, we know that each of the known genes in those plants is expressed specifically in the nodules as in M. truncatula. In contrast, by interrogating EST databases (http://compbio.dfci.harvard.edu/tgi/) no orthologous genes were found in other legume species, including species of closely related clades forming nodules with U-morphotype bacteroids. Moreover, the comparison of genomic loci carrying NCR genes in M. truncatula with microsyntenic loci in L. japonicus confirmed the absence of the NCR genes in L.

japonicus [START_REF] Alunni | Genomic organization and evolutionary insights on GRP and NCR genes, two large nodule-specific gene families in Medicago truncatula[END_REF]. Also other plant families or members of the animal kingdom lack NCR genes [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF][START_REF] Graham | Computational identification and characterization of novel genes from legumes[END_REF][START_REF] Alunni | Genomic organization and evolutionary insights on GRP and NCR genes, two large nodule-specific gene families in Medicago truncatula[END_REF]. For the sake of completeness, 3 sequences in the Arabidopsis genome were reported that potentially encode peptides with the typical NCR cysteine motif [START_REF] Zhou | Detecting small plant peptides using SPADA (Small Peptide Alignment Discovery Application)[END_REF] although no experimental evidence for the transcription of those genes and production of the peptides is available at present, leaving open the question whether those sequences are truly genes. Thus

NCRs are so-called lineage-specific or taxonomically restricted genes (TRGs). TRGs generally constitute an important part of eukaryotic genomes with 10-20% of genes lacking recognizable homologs in other species. TRGs are associated with evolutionary novelties and phylum-specific processes [START_REF] Khalturin | More than just orphans: are taxonomically-restricted genes important in evolution?[END_REF].

Despite their uniqueness, the protein structure, gene organization and family structure of the NCRs strikingly resemble those of antimicrobial peptides (AMPs) even if the NCRs have only limited amino acid sequence homology with known AMPs. AMPs are a diverse class of secreted and ribosomally synthesized peptides that are crucial effectors of innate immunity in both animals and plants [START_REF] Zasloff | Antimicrobial peptides of multicellular organisms[END_REF][START_REF] Brogden | Antimicrobial peptides: pore formers or metabolic inhibitors in bacteria?[END_REF][START_REF] Maróti | Natural roles of antimicrobial peptides in microbes, plants and animals[END_REF].

AMPs are constitutively produced or induced in response to microbial infections and have the capacity to attack and kill the invading microbes. AMPs in both plants and animals are often part of multigene families with dozens of members [START_REF] Schutte | Discovery of five conserved b-defensin gene clusters using a computational search strategy[END_REF][START_REF] Silverstein | Small cysteine-rich peptides resembling antimicrobial peptides have been under-predicted in plants[END_REF][START_REF] Amid | Manual annotation and analysis of the defensin gene cluster in the C57BL/6J mouse reference genome[END_REF] similarly as the NCRs, which are an extreme example of a multigene family. The exon-intron structure of the NCR genes is conserved with plant AMP genes [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF]. NCR peptide sequences contain an N-terminal signal peptide, encoded by the first exon of the genes, which is a cleavable tag for targeting of the peptide in the secretory pathway (Fig. 7). By expressing fusions of NCRs or their signal peptides with fluorescent proteins in a heterologous system, it was shown that these signal peptides are indeed functional and mediate entry of the peptides in the secretory pathway [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF]. The mature NCR peptides, obtained after processing of the signal peptide, are encoded in the second exon, are around 40 amino acids long and contain a highly conserved pattern of 4 or 6 cysteine residues (Fig. 7).

!

Figure 7. NCR-mediated differentiation of E-morphotype bacteroids in IRLC legumes

In a Medicago truncatula nodule cell, the Sinorhizobium meliloti wild-type (WT) and BacA mutant bacteria are released from the infection threads into membrane-bound symbiosomes. The bacteria are then challenged with host NCRs (purple), which are synthesized on the endoplasmic reticulum (ER) and transported to the symbiosomes via vesicle transport and a symbiotic nodule-cell-specific secretory pathway. This pathway requires a nodule-specific signal peptidase complex (SPC). The SPC is located in the ER and removes the signal peptide of the NCR pre-proteins (inset). The NCRs mediate the differentiation of the bacteria into a nitrogenfixing bacteroid, resulting in the chronic infection. The conserved cysteines in mature NCR peptides are shown in the inset ("x" are variable amino acids in the NCR peptide sequence).

This structure is typical for certain classes of AMPs and notably the defensins which have a signal peptide and a mature domain with conserved cysteines although the number of cysteine residues and their configuration is different from the NCRs [START_REF] Ganz | Defensins: antimicrobial peptides of innate immunity[END_REF][START_REF] Silverstein | Small cysteine-rich peptides resembling antimicrobial peptides have been under-predicted in plants[END_REF][START_REF] Aerts | The mode of antifungal action of plant, insect and human defensins[END_REF]. The NCRs are clearly related to each other because their cysteine signature is strongly conserved as well as a few amino acids flanking the cysteine residues and also their signal peptides are typical and highly similar. However, the sequences of mature domains as a whole are highly variable amongst the NCR family members [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF]. This reflects an evolutionary pattern with the signal peptides subjected to purifying selection in contrast to the mature peptides, which were subjected to diversifying selection [START_REF] Alunni | Genomic organization and evolutionary insights on GRP and NCR genes, two large nodule-specific gene families in Medicago truncatula[END_REF][START_REF] Nallu | Patterns of divergence of a large family of nodule cysteine-rich peptides in accessions of Medicago truncatula[END_REF]. Similar observations are frequently made for AMP gene families, including conservation of signal peptides and cysteine residues but strong divergence in the functional mature peptide [START_REF] Patil | Rapid evolution and diversification of mammalian adefensins as revealed by comparative analysis of rodent and primate genes[END_REF][START_REF] Semple | The changing of the guard: Molecular diversity and rapid evolution of b-defensins[END_REF]. Finally, AMPs are often TRGs similarly to the NCRs [START_REF] Khalturin | More than just orphans: are taxonomically-restricted genes important in evolution?[END_REF]. AMPs encoded by TRGs may be effective defense molecules against taxon-specific microbial interactors.

NCR transport to the symbiosomes

The possible involvement AMP-like peptides in the interaction with symbiotic bacteria, which are deliberately maintained by the host, may seem surprising at first sight because AMPs are known in the first place to counteract and eliminate invading bacteria.

Nevertheless symbiosomes and bacteroids and this localization of the peptides indicates that the bacteroids are their targets (Fig. 7).

The symbiosome localization of the NCR peptides is also in agreement with the presence of the characteristic signal peptide and a transport mechanism of the peptides through the secretory pathway. The characterization of the M. truncatula DNF1 gene has indeed highlighted the importance of the secretory pathway for NCR transport. One of the earliest steps in this pathway is the proteolytic cleavage of the signal peptide (Fig. 7), which is critical for the correct targeting of secretory proteins. It takes place during translocation of the secretory proteins from their site of synthesis by the ribosomes at the cytosolic face of the endoplasmic reticulum into the lumen of the endoplasmic reticulum and it is performed by the signal peptidase complex (SPC) which is located in the membrane of the endoplasmic reticulum [START_REF] Paetzel | Signal peptidases[END_REF]. The SPC is composed of four subunits conserved in all eukaryotes. The SPC18 subunit contains the proteolytic catalytic site [START_REF] Paetzel | Signal peptidases[END_REF].

SPC18 together with SPC22 are the critical subunits for SPC activity while the other two subunits, SPC12 and SPC25 are apparently dispensable for SPC activity, at least in yeast [START_REF] Fang | In addition to SEC11, a newly identified gene, SPC3, is essential for signal peptidase activity in the yeast endoplasmic reticulum[END_REF][START_REF] Meyer | The yeast SPC22/23 homolog Spc3p is essential for signal peptidase activity[END_REF]. The M. truncatula genome contains two gene copies for SPC12, SPC18 and SPC22, which are 77% to 82% identical to each other and one encoding SPC25. Interestingly, four of these genes, one copy for each of the four subunits, named DAS12, DAS18, DNF1 and DAS25 have a highly correlated gene expression profile with a major expression in nodules, a lower expression in seeds and a basal expression in other plant tissues [START_REF] Wang | A nodule-specific protein secretory pathway required for nitrogen-fixing symbiosis[END_REF]. The other three genes have a more uniform expression in all plant tissues. The M. truncatula dnf1 (does not fix 1) mutant was identified on the basis of its symbiotic phenotype, forming non-functional nodules [START_REF] Starker | Nitrogen fixation mutants of Medicago truncatula fail to support plant and bacterial symbiotic gene expression[END_REF]. The mutant was found to be deficient in the DNF1 nodule-enhanced SPC22 subunit of the SPC [START_REF] Wang | A nodule-specific protein secretory pathway required for nitrogen-fixing symbiosis[END_REF]. The dnf1 nodules contain infected nodule cells; however, the symbiosome bacteria do not differentiate into elongated bacteroids (Van de Velde et al., 2010;[START_REF] Wang | A nodule-specific protein secretory pathway required for nitrogen-fixing symbiosis[END_REF].

NCRs are produced in the nodules of the mutant but their signal peptide is not cleaved in agreement with the known essential role of the SPC22 subunit in signal peptide processing [START_REF] Fang | In addition to SEC11, a newly identified gene, SPC3, is essential for signal peptidase activity in the yeast endoplasmic reticulum[END_REF][START_REF] Meyer | The yeast SPC22/23 homolog Spc3p is essential for signal peptidase activity[END_REF]. The inactivation of NCR processing resulted in the accumulation of NCR peptides in the endoplasmic reticulum and the blockage of their further transport to the symbiosomes. Thus, obstructing NCR transport is correlated with the absence of bacteroid differentiation in the symbiosomes, in agreement with a role of the NCRs in this bacterial differentiation process (Van de Velde et al., 2010).

NCR-mediated bacteroid differentiation

Inhibiting NCR transport inactivated the function of the whole NCR family but this strategy could not exclude the possibility that the resulting phenotype was the consequence of other proteins or peptides produced by the symbiotic nodule cells and transported to the symbiosomes through the secretory pathway. An opposite, gain-of-function approach, consisted in ectopic expression of individual NCR genes in the infected nodule cells of L.

japonicus, a legume with reversible bacteroid differentiation and lacking NCR genes. In control nodules of L. japonicus, symbiosomes contained multiple U-morphotype bacteroids while in the NCR-expressing nodules a significant number of symbiosomes contained single, E-morphotype bacteroids thereby mimicking the symbiosomes and bacteroids in M.

truncatula [START_REF] Van De Velde | Plant peptides govern terminal differentiation of bacteria in symbiosis[END_REF]. Nevertheless, the effect was only partial, possibly because the sequential and cooperative action of multiple peptides in the right proportions is required to obtain an optimal response such as is observed in the M. truncatula nodules.

Similarly, in vitro application of single NCR peptides to S. meliloti free-living bacteria resulted in E-morphotype bacteroid-like features. Notably, peptides induced high permeability of the membrane, inhibition of bacterial proliferation, DNA accumulation and cell elongation (Van de Velde et al., 2010;[START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF] but again, the effects were partial probably because an optimal cocktail of peptides is required.

The polyploidy of the bacteroids is the testimony of a switch in their cell cycle that must have taken place during their formation after their release from the infection threads in the host cell. In a regular bacterial cell cycle, DNA replication and division are alternating resulting in cells, which have a haploid (1C) or diploid (2C) genome content while the polyploid bacteroids must result from an inhibition of the division accompanied with ongoing DNA replication (Fig. 8A). Thus one of the major effects of the NCR peptides may be an interference with the regulatory circuit of the bacteria that controls the cell cycle. The perturbation of the bacterial cell cycle during bacteroid formation is nicely illustrated when the expression of bacterial cell cycle regulatory genes is followed in situ within a mature nodule of M. truncatula (Fig. 8B). The expression pattern of these genes undergoes a complete switch, which coincides with the formation of the bacteroids and with the expression of the NCR genes. For example, the master regulator CtrA, which promotes the cell division cells and inhibits the DNA replication stage, is strongly expressed in the apical parts of the nodule where bacteria are not yet differentiated and divide within the infection threads. However, CtrA expression drops strongly where the bacteria start the differentiation in the proximal infection zone (pI) and interzone II-III (IZ), in agreement with the absence of division. DnaA on the other hand which is the master regulator controlling DNA replication has an opposite pattern and its highest expression level is in pI and IZ thus coinciding with the genome amplification of the differentiating bacteroids. How the NCR peptides affect the cell cycle remains to be elucidated. However, a recent study shed some light on this question. It was shown that a specific NCR peptide, NCR247, affected very strongly the expression of cell cycle regulated genes in S. meliloti. Particularly the expression of the CtrA regulated genes was inhibited by the NCR247 peptide [START_REF] Penterman | Host plant peptides elicit a transcriptional response to control the Sinorhizobium meliloti cell cycle during symbiosis[END_REF] (Fig. 8C). Interestingly, when S. meliloti is depleted genetically from CtrA, the bacteria stop further growth but elongate very strongly and become polyploid [START_REF] Pini | The DivJ, CbrA and PleC system controls DivK phosphorylation and symbiosis in Sinorhizobium meliloti[END_REF] (Fig. 8D). Together these findings suggest that NCR peptides drive the endoreduplication and enlargement of the intracellular S. meliloti in the symbiotic nodule cells by altering directly or more likely indirectly the regulation of the cell-cycle, leading to the loss of CtrA activity and the consequent arrest in cell division. A second recent study described that NCR247 is taken up by S. meliloti cells and that the peptide interacts with multiple targets including FtsZ, a cell division protein, ribosomes which are inhibited by the interaction with NCR247 and the chaperone GroEL which is required for normal bacteroid formation [START_REF] Farkas | The Medicago truncatula symbiotic peptide NCR247 contributes to bacteroid differentiation through multiple mechanisms[END_REF]. Therefore, the mechanism of action of NCR247 seems to be complex and multiple and more work is required to fully elucidate the precise mechanism of the NCR-induced cell cycle switch. In addition, another NCR peptide, NCR035, was found to be localized in vitro at the division site of S. meliloti cells but not NCR247 (Van de Velde et al., 2010), suggesting that the NCR035 peptide interferes with the bacterial cell division machinery but probably by a different mechanism than NCR247.

Why morphological bacteroid differentiation?

Since morphological differentiation of bacteroids does not take place in all legumes, this process is not required per se for the maintenance of the endosymbiotic bacterial population, prompting the question why certain legumes impose this differentiation process on the nodule rhizobia. While this question remains entirely open at present, some studies suggest a selective advantage to the plant [START_REF] Sen | A comparison of nitrogen-fixing ability of peanut, cowpea and siratro plants nodulated by different strains of Rhizobium[END_REF]Oono and Denison, 2010;Oono et al., 2010). Phylogenetic analysis and ancestral state reconstruction suggested that the ancestral morphotype of bacteroids was the undifferentiated U-morphotype while the differentiated E-and S-morphotypes have evolved several times independently in the legume family (Oono et al., 2010). This conclusion, together with the facts that the plant imposes the morphological differentiation and that the bacteria are affected in their reproductive capacity by the differentiation, is a strong argument for a selective advantage to the plant associated with the morphological bacteroid differentiation. Other studies demonstrated a higher symbiotic performance for E-or S-morphotype bacteroids compared to U-morphotype bacteroids [START_REF] Sen | A comparison of nitrogen-fixing ability of peanut, cowpea and siratro plants nodulated by different strains of Rhizobium[END_REF]Oono and Denison, 2010). However, even if these studies provide attractive arguments, the molecular and physiological basis for the selective advantage to the plant remain entirely unknown. Could the NCRs affect the symbiotic efficiency of the bacteroids?

As discussed above, some NCR peptides are implicated in the interference with the bacterial cell division machinery. However, the NCRs display a high sequence variety including a large number of anionic (acidic) or neutral peptides, which have no antimicrobial activity and are unable to induce bacterial differentiation. Moreover, some of these anionic peptides (e.g. NCR001) are expressed at very high levels [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF] and accumulate inside the bacteroids (Van de Velde et al., 2010). This suggests a multiplicity of functions, modes of action and bacterial targets for the NCR peptides. Together, the net effect of the peptides might be an optimization of the efficiency of the nitrogen fixation process in the E-morphotype bacteroids. Perhaps some peptides interfere with bacteroid metabolism in order to channel bacterial metabolism towards the nitrogen fixation process. For example, the accumulation of the storage compound polyhydroxybutyrate (PHB), which takes the hostsupplied carbon away from nitrogen fixation, is very frequently observed in U-morphotype bacteroids but not in the E-morphotype bacteroids of IRLC legumes. Inhibition of PHB accumulation in those bacteroids could be a direct or indirect consequence of the NCR peptides.

Additional, non-exclusive hypotheses can be put forward as to explain a better performance of E-or S-morphotype bacteroids. Polyploidy of the bacteroids may support higher metabolic activity of the cells in a general way as it is the case for eukaryotic cells [START_REF] Kondorosi | Endoreduplication and activation of the anaphase-promoting complex during symbiotic cell development[END_REF]). E-and S-morphotype bacteroids are present as a single bacterium per symbiosome, which may permit a very efficient nutrient exchange with the host cell. On the contrary, multiple U-morphotype bacteroids are present in a single symbiosome and these bacteroids have therefore a more limited and less efficient contact with the symbiosome membrane.

Besides the better performance of differentiated bacteroids, other benefits to the host plant could result from the bacterial differentiation. Polyploid bacteroids could be more resistant to DNA damaging oxidative stress because of the multiple gene copies. Since bacteroids are metabolically very active, oxidative stress could affect the longevity of bacteroids, which might be enhanced by the polyploidy. Moroever, bacteroids in M.

truncatula, which are fragilized by the NCR challenge, are efficiently digested by the host during the senescence process, at the end of the nodule life [START_REF] Vasse | Correlation between ultrastructural differentiation of bacteroids and nitrogen fixation in alfalfa nodules[END_REF][START_REF] Van De Velde | Aging in legume symbiosis. A molecular view on nodule senescence in Medicago truncatula[END_REF]. This is in contrast to reversibly differentiated bacteroids in L. japonicus, which can efficiently survive nodule senescence [START_REF] Müller | Redifferentiation of bacteria isolated from Lotus japonicus root nodules colonized by Rhizobium sp. NGR234[END_REF]. During nodule senescence in M. truncatula, catabolic genes for macromolecule degradation and a variety of transporters for the mobilization of small molecules are activated, indicating that the degradation products generated during senescence are recycled as nutrients for the plant (Van de [START_REF] Van De Velde | Aging in legume symbiosis. A molecular view on nodule senescence in Medicago truncatula[END_REF]. Thus degradation of weakened bacteroids may provide supplementary nutrients and be an advantage to the host plant.

Even if a variety of effects is supposed for the NCR peptides, their extreme diversity remains astonishing. Another raison d'être for the high diversity of NCR peptides could be an adaptation to the high diversity of rhizobia in soils. The diversifying selection that has shaped the NCR family [START_REF] Alunni | Genomic organization and evolutionary insights on GRP and NCR genes, two large nodule-specific gene families in Medicago truncatula[END_REF][START_REF] Nallu | Patterns of divergence of a large family of nodule cysteine-rich peptides in accessions of Medicago truncatula[END_REF] is compatible with such a hypothesis.

Additional factors that may come into play are balancing effects of certain NCRs to counteract cationic, antimicrobial NCR peptides, expression of NCR subclasses at different stages of host cell differentiation [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF] and a large number of genes are expressed at low or very low levels [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF] suggesting that they have less important roles.

Other nodule peptides interacting with bacteroids

The GRP and SNARP peptides are two other families of secretory peptides produced by symbiotic nodule cells [START_REF] Kevei | Glycine-rich proteins encoded by a nodule-specific gene family are implicated in different stages of symbiotic nodule development in Medicago spp[END_REF][START_REF] Alunni | Genomic organization and evolutionary insights on GRP and NCR genes, two large nodule-specific gene families in Medicago truncatula[END_REF][START_REF] Laporte | A novel RNA-binding peptide regulates the establishment of the Medicago truncatula-Sinorhizobium meliloti nitrogen-fixing symbiosis[END_REF][START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF] and are therefore most likely transported to the symbiosomes.

Moreover, they are encoded by TRGs. GRPs have a similar distribution as the NCRs and are restricted to the IRLC legumes [START_REF] Küster | The nodule-specific VfENOD-GRP3 gene encoding a glycine-rich early nodulin is located on chromosome I of Vicia faba L. and is predominantly expressed in the interzone II-III of root nodules[END_REF][START_REF] Schröder | The temporal and spatial transcription pattern in root nodules of Vicia faba nodulin genes encoding glycine-rich proteins[END_REF][START_REF] Kevei | Glycine-rich proteins encoded by a nodule-specific gene family are implicated in different stages of symbiotic nodule development in Medicago spp[END_REF][START_REF] Alunni | Genomic organization and evolutionary insights on GRP and NCR genes, two large nodule-specific gene families in Medicago truncatula[END_REF]. At present the SNARPs were only detected in M. truncatula. They are not present in legumes outside of the IRLC clade, e.g. in L. japonicus, soybean or P. vulgaris for which rich genomic and transcriptomic data exist, but insufficient data are available to conclude on their presence in other IRLC legumes. The taxonomic distribution of these genes and their likely localization in the symbiosomes suggest that they have, like the NCRs, a role in a bacteroid-related process which is specific for the IRLC legumes. The SNARP1 and SNARP2 peptides were identified as RNA-binding peptides in a yeast three-hybrid screen and the SNARP2 peptide has in vitro RNA-binding capacity. Knocking-down the SNARP2 gene by RNA interference (RNAi) resulted in non-functional nodules in which bacteroids were formed but could not be maintained. Further work will be required to determine how this in vivo function is related to the RNA-binding capacity of these peptides. In contrast, the symbiotic function of the GRP peptides remains to be analyzed.

Antimicrobial peptides in other symbiotic systems

The symbiotic nodule cells show a striking similarity with symbiotic cells, called bacteriocytes, formed by aphids and other insect groups. These insects exploit restricted nutritional resources such as plant sap and their bacterial symbionts produce the nutrients, which are lacking in the host's diet. The insect bacteriocytes are organized in specific symbiotic organs called bacteriomes. Bacteriocytes are large polyploid cells, virtually filled with intracellular bacteria (Fig. 3). Even more striking are the morphological modifications these endosymbionts are undergoing inside the host cell such as formation of spheres (Fig. 3)

or elongated cells similar to the S-morphotype or E-morphotype bacteroids [START_REF] Shigenobu | Genomic revelations of a mutualism: the pea aphid and its obligate bacterial symbiont[END_REF]The International Aphid Genomics Consortium, 2010;[START_REF] Tsuchida | Symbiotic bacterium modifies aphid body color[END_REF][START_REF] Login | Antimicrobial peptides keep insect endosymbionts under control[END_REF].

The pea aphid Acyrthosiphon pisum is one of the best studied insects in respect to bacterial symbiosis. A. pisum is in fact a species complex consisted of multiple biotypes each adapted to a specific legume species (e.g. pea, clover or Medicago). The primary (obligate) symbiont of A. pisum is Buchnera aphidicola, which produces essential amino acids lacking in the plant sap diet of the aphid. RNA-seq transcriptome analysis of bacteriocytes and in situ hybridization experiments in A. pisum has led to the discovery of a class of genes encoding secreted cysteine-rich peptides, which were called BCR peptides for "bacteriocytes-specific cysteine rich peptides", in analogy to the NCR nodule peptides [START_REF] Shigenobu | Aphids evolved novel secreted proteins for symbiosis with bacterial endosymbiont[END_REF].

The genes are specifically expressed in the bacteriocytes and not in other parts of the insect.

Following their expression by in situ hybridization during bacteriome formation in embryos, it was found that they are induced when the early-stage bacteriome becomes first infected with Buchnera. Thus, drawing the analogy with the NCR peptides in nodules it is an inevitable and exciting hypothesis that the BCRs are targeted to and affect the bacterial symbionts in a similar fashion as the NCRs do in symbiotic nodule cells.

In a different insect group, in the stinkbugs or Heteroptera, symbiotic bacteria are not located in bacteriomes as intracellular endosymbionts but in a specialized, posterior region of the midgut where numerous crypts harbor in their lumen a single and specific extracellular bacterial symbiont [START_REF] Kikuchi | Endosymbiotic bacteria in insects: their diversity and culturability[END_REF]. The stinkbug Riptortus pedestris which is another legume pest, particularly of soybean, carries in its crypts a symbiont belonging to the genus Burkholderia, a soil bacterium which is acquired every generation from the environment during a specific developmental window coinciding with the development of the crypts in the midgut [START_REF] Kikuchi | Specific developmental window for establishment of an insect-microbe gut symbiosis[END_REF]. A transcriptome analysis of symbiotic (crypt-carrying) and non-symbiotic midgut regions from Burkholderia-infected and uninfected (aposymbiotic) R.

pedestris identified among the most abundantly expressed genes in the midgut 97 different cysteine-rich secretory peptides many of which were specifically expressed in the Burkholderia-containing crypts of the midgut [START_REF] Futahashi | Gene expression in gut symbiotic organ of stinkbug affected by extracellular bacterial symbiont[END_REF]. The authors speculated that these crypt-specific cysteine-rich peptides (CCRs) are secreted into the lumen of the crypts by the epithelial cells and act on the proliferation and physiology of the symbiotic Burkholderia, similarly to the NCRs in legume nodules and the BCRs in aphid bacteriomes [START_REF] Futahashi | Gene expression in gut symbiotic organ of stinkbug affected by extracellular bacterial symbiont[END_REF].

Another recent study in the beetle Sitophilus identified the specific transcription of an AMP gene in bacteriocytes [START_REF] Anselme | Identification of the weevil immune genes and their expression in the bacteriome tissue[END_REF]. The encoded peptide, called coleoptericin-A (ColA), is unrelated to the legume NCRs, aphid BCRs or stinkbug CCRs and has no cysteine residues. ColA is targeted to the endosymbionts and regulates the formation of the elongated, filamentous morphology of these endosymbionts [START_REF] Login | Antimicrobial peptides keep insect endosymbionts under control[END_REF].

Moreover, AMPs are not only employed in the maintenance of endosymbiotic bacteria but also of the gut microbiota. Studies on gut microbiota in Hydra, Drosophila and mouse (Fig. 3) demonstrated that in these very divergent animals the profile and the level of expression of AMPs produced by the gut epithelia are crucial to create and maintain a specific and favorable composition of the microbiota [START_REF] Wehkamp | Reduced Paneth cell alpha-defensins in ileal Crohn's disease[END_REF][START_REF] Ryu | Innate immune homeostasis by the homeobox gene Caudal and commensal-gut mutualism in Drosophila[END_REF][START_REF] Fraune | In an early branching metazoan, bacterial colonization of the embryo is controlled by maternal antimicrobial peptides[END_REF][START_REF] Salzman | Enteric defensins are essential regulators of intestinal microbial ecology[END_REF][START_REF] Vaishnava | The antibacterial lectin RegIIIgamma promotes the spatial segregation of microbiota and host in the intestine[END_REF].

Thus the legume symbiosis and the animal symbiotic systems constitute together a striking case of convergent evolution between plants and animals. This observation suggests that the employment of AMPs is an optimal modus operandi for a eukaryotic host cell to tolerate large numbers of intracellular bacteria.

All current data point towards a restriction of the NCR gene family to the IRLC legumes. However morphologically differentiated bacteroids are also observed in other legume clades (Oono et al., 2010) such as the E-morphotype and S-morphotype bacteroids in Aeschynomene species (Fig. 6), which are also induced by plant factors [START_REF] Bonaldi | Nodulation of Aeschynomene afraspera and A. indica by photosynthetic Bradyrhizobium Sp. strain ORS285: the nod-dependent versus the nod-independent symbiotic interaction[END_REF].

Since these species have most likely no NCR genes of the type found in the IRLC legumes, these host factors are different but in light of the recurrent use of AMPs to control symbiotic bacteria, they are possibly also AMP-like. In my here presented thesis work, I have participated in the characterization of a gene family in Aeschynomene legumes encoding such peptides. Also actinorhizal plants such as Alnus glutinosa, Casuarina glauca and Datisca glomerata produce nodule-specific cysteine-rich peptides [START_REF] Hocher | Tanscriptomics of Actinorhizal Symbiosis Reveals Homologs of the Whole Common Symbiotic Signaling Cascade[END_REF][START_REF] Demina | Comparison of the nodule vs. root transcriptome of the actinorhizal plant Datisca glomerata: actinorhizal nodules contain a specific class of defensins[END_REF] and ongoing research aims at verifying the hypothesis that these peptides are implicated in the control of the Frankia endosymbionts in the nodules of those plants.

Together, these recent studies demonstrate that although AMPs are commonly linked with innate immunity and the clearance of infecting microbes, symbiotic bacteria are equally challenged with AMPs. The recruitment of these immune effectors has in all likelihood been a crucial evolutionary step in efficiently maintaining large symbiotic bacterial populations.

BacA is required in legumes forming E-or S-morphotype bacteroids but not in other legumes

As described above, NCRs are similar to the defensin-type of AMPs and several NCR peptides have been found to have antimicrobial activity, killing many microorganisms including gram-positive and gram-negative bacteria as well as fungi at similar concentrations as other known AMPs. [START_REF] Tiricz | Antimicrobial nodule-specific cysteine-rich peptides induce membrane depolarizationassociated changes in the transcriptome of Sinorhizobium meliloti[END_REF]Ördögh et al., 2014;[START_REF] Balogh | Anti-chlamydial effect of plant peptides[END_REF]. Also

Rhizobium bacteria including S. meliloti are killed by NCR peptides when applied at high concentration. AMPs are mostly cationic (basic) peptides although some anionic (acidic) peptides are also known [START_REF] Zasloff | Antimicrobial peptides of multicellular organisms[END_REF][START_REF] Brogden | Antimicrobial peptides: pore formers or metabolic inhibitors in bacteria?[END_REF][START_REF] Maróti | Natural roles of antimicrobial peptides in microbes, plants and animals[END_REF]. The peptides in the NCR family range from acidic to basic. Strikingly, only the basic peptides were found to have antimicrobial activity.

Thus NCRs are a two-edged sword with dramatically different effects on S. meliloti in vivo and in vitro: on the one hand NCR-challenged bacteroids in nodule symbiosomes differentiate and maintain, despite their growth inhibition, an active metabolism for nitrogen fixation but on the other hand, in vitro NCRs are detrimental and kill the rhizobia. The in vivo conditions are significantly different from an in vitro treatment of rhizobia with a pure peptide. Several tens to hundreds of different peptides are accumulating in symbiosomes, each probably at low concentration and with possibly antagonistic effects. Moreover, the in vivo effect of the NCR peptides on the bacteroids may be further influenced by particular physiological conditions in the symbiosomes such as low free oxygen and low pH in such a way that the bacteroids remain alive, although with a complete loss of their reproductive capacity.

In addition, the rhizobia may have defense mechanisms to counteract the antimicrobial activity of the NCR peptides and one important mechanism requires the BacA protein. The first rhizobial bacA gene was identified more than 20 years ago, long before NCR peptides were known, in a screen of S. meliloti mutants inducing ineffective (Fix -) nodules on M.

sativa [START_REF] Glazebrook | A Rhizobium meliloti homolog of the Escherichia coli peptide-antibiotic transport protein SbmA is essential for bacteroid development[END_REF]. In Medicago nodules formed by the S. meliloti bacA mutant, the bacteria infect the plant cells but do not elongate into the typical E-morphotype shape and the rod-shaped bacteria in the symbiosomes quickly degenerate and lyse (Fig. 9 and 10) [START_REF] Glazebrook | A Rhizobium meliloti homolog of the Escherichia coli peptide-antibiotic transport protein SbmA is essential for bacteroid development[END_REF][START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF][START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF]. The gene's name reflects this role in bacteroid differentiation.

More recently, bacA genes in other Rhizobium species were also investigated. It was demonstrated that the bacA gene of Mesorhizobium huakuii and 2 different Rhizobium leguminosarum bv. viciae strains was required for the formation of Fix + nodules on the legumes Astragalus sinicus and Pisum sativum, respectively [START_REF] Tan | BacA is indispensable for successful Mesorhizobium-Astragalus symbiosis[END_REF][START_REF] Karunakaran | BacA is essential for bacteroid development in nodules of galegoid, but not phaseoloid, legumes[END_REF]. Both legumes are closely related to Medicago, belonging to the IRLC legumes [START_REF] Wojciechowski | A phylogeny of legumes (Leguminosae) based on analysis of the plastid matK gene resolves many well-supported subclades within the family[END_REF] and forming E-morphotype bacteroids. Histological analysis of Pisum nodules revealed that the bacA-deficient R. leguminosarum bv. viciae did not differentiate into bacteroids and could not survive in the symbiotic nodule cells but lysed after their release from the infection threads [START_REF] Karunakaran | BacA is essential for bacteroid development in nodules of galegoid, but not phaseoloid, legumes[END_REF], a phenotype similarly to the S. meliloti mutant in Medicago. In contrast, the bacA genes of Mesorhzobium loti, Rhizobium leguminosarum bv.

phaseoli, Rhizobium etli or Sinorhizobium fredii strain NGR234 are dispensable for the formation of a functional symbiosis on various plants like L. japonicus, P. vulgaris, Lablab purpureus, Leucaena leucocephala, Tephrosia vogelii, and Vigna unguiculata [START_REF] Maruya | The bacA Gene Homolog, mlr7400, in Mesorhizobium loti MAFF303099 is dispensable for symbiosis with Lotus japonicus but partially capable of supporting the symbiotic function of bacA in Sinorhizobium meliloti[END_REF][START_REF] Karunakaran | BacA is essential for bacteroid development in nodules of galegoid, but not phaseoloid, legumes[END_REF][START_REF] Ardissone | Role of BacA in lipopolysaccharide synthesis, peptide transport, and nodulation by Rhizobium sp. strain NGR234[END_REF]. Contrary to the IRLC legumes, all these legumes form U-morphotype bacteroids and lack NCR peptides.

An eloquent example for the bacA requirement in symbiosis is provided by the R. leguminosarum bv. viciae strain A34 and the R. leguminosarum bv. phaseoli strain 4292 which have the same genetic background except for their symbiotic plasmids. These strains have opposing requirements for the bacA gene in symbiosis with P. sativum which requires bacA and forms E-morphotype bacteroids and with P. vulgaris which does not require bacA and forms U-morphotype bacteroids [START_REF] Karunakaran | BacA is essential for bacteroid development in nodules of galegoid, but not phaseoloid, legumes[END_REF].

Bradyrhizobium species have a gene that is only distantly related to bacA, classified as bacA-like or bclA. We have found that bclA is required in Bradyrhizobium sp. for effective symbiosis on A. indica or A. afraspera whose bacteroids have the S-or E-morphotype, respectively. Mutants in the bclA gene do not differentiate and remain rod-shaped in the symbiosomes (see Chapter III; Guefrachi et al., 2015a). Strikingly, in Bradyrhizobium japonicum, the orthologous gene is not required for symbiosis with G. max, which forms Umorphotype bacteroids (Guefrachi et al., 2015a). Taking together, these examples give ample evidence for a correlation between the need of the bacA/bclA genes for symbiosis and the formation of E-or S-morphotype bacteroids while the gene is dispensable when Umorphotype bacteroids are formed.

The bacterial BacA protein provides protection against the antimicrobial activity of NCRs

NCR peptides induce E-morphotype bacteroid formation in IRLC legumes. It is also known that NCR-like peptides are involved in bacteroid differentiation in Aeschynomene legumes (see Chapter II; [START_REF] Czernic | Convergent evolution of endosymbiont differentiation in Dalbergoid and IRLC legumes mediated by nodule-specific cysteine-rich peptides[END_REF], the requirement of BacA or BclA proteins only for E-or S-morphotype bacteroid formation suggests that this protein plays a central role in the rhizobial response towards the NCR peptides [START_REF] Kereszt | Bacteroid development in legume nodules: evolution of mutual benefit or of sacrificial victims?[END_REF]. Two hypotheses were put forward that could take into account the requirement of BacA in S. meliloti [START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF]. The protein could be directly required for the bacterial metamorphosis. However, synthetic NCR peptides induced in vitro bacterial elongation and DNA amplification to the same extent in the S. meliloti bacA mutant as in the wild-type strain suggesting that BacA is not required for the differentiation process per se [START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF]. The second tested possibility is related to the above described antimicrobial activity of NCR peptides. The killing activity of the NCRs is related to the damage they provoke on the bacterial membrane, in a similar way as the defensins (Van de Velde et al., 2010;[START_REF] Tiricz | Antimicrobial nodule-specific cysteine-rich peptides induce membrane depolarizationassociated changes in the transcriptome of Sinorhizobium meliloti[END_REF]. Importantly, the S. meliloti bacA mutant was between 10-to 100-fold more sensitive towards the antimicrobial action of NCR peptides [START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF][START_REF] Haag | Role of cysteine residues and disulfide bonds on the activity of a legume root nodule-specific, cysteine-rich peptide[END_REF]. The hypersensitivity of the bacA mutant was not specific for NCRs but applied also to the human defensin, β-defensin 2 [START_REF] Arnold | Partial complementation of Sinorhizobium meliloti bacA mutant phenotypes by the Mycobacterium tuberculosis BacA protein[END_REF]. This hypersensitivity provided a likely explanation for the mutant's symbiotic phenotype and suggested that BacA is required to survive the challenge with NCRs in the symbiotic nodule cells. Testing this hypothesis in vivo, in M. truncatula nodules, it was first shown that the S. meliloti bacA mutant bacteria, like wild-type, are challenged with NCR peptides once they are released from infection threads in the symbiotic nodule cells [START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF]. With a live/dead staining procedure which marks bacteria with a different dye according to whether they are alive or dead, it was found that bacA mutant bacteria are healthy in the infection threads but are rapidly killed after their uptake in symbiotic cells, while a wild-type strain remained, as expected, viable and differentiated. Moreover, the bacA mutant was able to survive in the symbiotic nodule cells of the M. truncatula dnf1 mutant [START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF] which is affected in a nodule-specific signal peptidase complex [START_REF] Wang | A nodule-specific protein secretory pathway required for nitrogen-fixing symbiosis[END_REF] and which cannot transport NCR peptides to the symbiosomes (Van de Velde et al., 2010). These observations are consistent with a role of BacA in preventing bacterial death by the potentially toxic NCR peptides and permitting the bacterial differentiation (Fig. 10). Perhaps, cell elongation and genome amplification is a secondary effect of a bacterial defense response towards the antimicrobial activity of the peptides. By synthesizing alternative versions of the NCR247 peptide with different disulfide bridge configurations between its 4 cysteine residues, it was demonstrated that the peptide with disulfide bridges between cysteines 1-2 and 3-4 was significantly more active than the peptide with disulfide bridges between cysteines 1-3 and 2-4 for both induction of cell elongation and DNA amplification at low peptide concentrations and for antimicrobial activity at higher peptide concentrations [START_REF] Haag | Role of cysteine residues and disulfide bonds on the activity of a legume root nodule-specific, cysteine-rich peptide[END_REF]. Furthermore, substitution of the cysteine residues with serines diminished the antimicrobial potency of NCR247 while chemically reducing the peptide by dithiothreitol treatment increased its antimicrobial potency significantly [START_REF] Haag | Role of cysteine residues and disulfide bonds on the activity of a legume root nodule-specific, cysteine-rich peptide[END_REF]. This is similar to the behavior observed for the human βdefensin 1, which is produced in the colon and thought to be stored in an inactive oxidized form and to be activated by reduction through the action of thioredoxin proteins [START_REF] Schroeder | Reduction of disulphide bonds unmasks potent antimicrobial activity of human β-defensin 1[END_REF]. Thioredoxins are ubiquitous redox proteins, which regulate the oxidation state of cysteine residues and disulfide bridges in proteins. Intriguingly, Medicago nodules express a pair of unusual thioredoxins, Trx s1 and Trx s2 [START_REF] Alkhalfioui | A novel type of thioredoxin dedicated to symbiosis in legumes[END_REF]. They are nodule specific and are thus co-regulated with NCR genes. These thioredoxins differ from classical thioredoxins by the presence of an N-terminal signal peptide for the secretory pathway, like the NCR peptides, which indicates that they are potentially co-transported with the NCRs to the symbiosomes. Thus an interesting hypothesis is that these thioredoxins regulate the oxidative state of the peptides and thereby control their activity.

Importantly, no difference in sensitivity against either the chemically reduced NCR247 peptide or the linear peptide in which the cysteine residues were replaced with serine was observed between wild-type S. meliloti and the bacA mutant [START_REF] Haag | Role of cysteine residues and disulfide bonds on the activity of a legume root nodule-specific, cysteine-rich peptide[END_REF]. This suggests that the natural form of the NCR peptides is the oxidized one and raises the possibility that BacA can directly or indirectly influence the oxidation state of the NCR peptides in the periplasm of the bacteroids and promote the formation of the less detrimental oxidized state, thereby limiting damage of the inner membrane.

BacA homologues are required in pathogens for chronic infection

Besides its role in intracellular accommodation of rhizobia during symbiosis with legume plants, BacA or BclA homologues have been identified and characterized in several pathogenic bacteria, in which they play critical roles during the interaction with their eukaryotic host. Notably, functional studies have been performed on BacA homologues from avian pathogenic E. coli (APEC) and in the mammalian pathogens Mycobacterium tuberculosis and Brucella abortus.

APEC can colonize the respiratory tract and cause pneumonia in poultry, and in addition, they are also often associated with internal organ infections. In its most acute form, colibacillosis caused by APEC triggers septicemia, often resulting in sudden death of infected birds [START_REF] Dziva | Colibacillosis in poultry: unravelling the molecular basis of virulence of avian pathogenic Escherichia coli in their natural hosts[END_REF]. As other pathogenic E. coli, APEC remains exclusively extracellular and the pathogen resides in the host tissues without penetrating the cells. In a signature-tagged mutagenesis screen for altered pathogenicity on chicken, a mutant in the BacA homologue (in E. coli named SbmA) was identified. The mutant was shown to be attenuated in the colonization of different internal organs as compared to the wild-type strain, indicating that BacA is required for survival in the host and establishment of chronic infection even in the context of a non-intracellular pathogen [START_REF] Li | Identification of genes required for avian Escherichia coli septicemia by signature-tagged mutagenesis[END_REF].

Brucella abortus is a zoonotic pathogen that causes brucellosis (Malta fever) in humans and spontaneous abortions in animal hosts like cattle. It infects its host intracellularly through an endocytotic process and survives in phagocytic cells such as macrophages [START_REF] Roop | Survival of the fittest: how Brucella strains adapt to their intracellular niche in the host[END_REF]. A B. abortus BacA homologue has been identified and the corresponding mutant displayed reduced intracellular survival in macrophages compared to the wild-type strain.

Moreover, bacterial clearance from infected BALB/c mice was increased in the bacA mutant by five orders of magnitude at 8 weeks post inoculation. Taken together, these results indicate that, similarly to the Rhizobium-legume symbiosis, BacA is required for intracellular accommodation of Brucella and the establishment of a chronic infection [START_REF] Levier | Similar requirements of a plant symbiont and a mammalian pathogen for prolonged intracellular survival[END_REF].

Mycobacterium tuberculosis, the causal agent of tuberculosis in humans, is a respiratory pathogen that colonizes the lung through intracellular infection. The bacterium can enter alveolar epithelial cells and macrophages and survive despite the high levels of defensins and other AMPs produced in these cells [START_REF] Tan | Macrophages acquire neutrophil granules for antimicrobial activity against intracellular pathogens[END_REF][START_REF] Rivas-Santiago | Human β-defensin 2 is expressed and associated with Mycobacterium tuberculosis during infection of human alveolar epithelial cells[END_REF][START_REF] Rivas-Santiago | β-Defensin gene expression during the course of experimental tuberculosis infection[END_REF]. In a murine infection model, M. tuberculosis can reside in the lung and spleen for months and lead subsequently to host death. Although the M. tuberculosis bacA mutant was not compromised in infection of both organs over more than two hundred days, it induced significantly retarded host mortality compared to the wild-type strain in a long term time-to-death experiment [START_REF] Domenech | BacA, an ABC transporter involved in maintenance of chronic murine infections with Mycobacterium tuberculosis[END_REF].

Conservation of BacA function across bacterial species has been evaluated in different combinations of trans-complementation assays. Expression of the BacA homologues of either E. coli or B. abortus in the S. meliloti bacA mutant was able to complement its symbiotic defects and restored the formation of functional, nitrogen-fixing nodules on M. sativa [START_REF] Ichige | Genetic analysis of the Rhizobium meliloti bacA gene: functional interchangeability with the Escherichia coli sbmA gene and phenotypes of mutants[END_REF][START_REF] Wehmeier | Internalization of a thiazole-modified peptide in Sinorhizobium meliloti occurs by BacA-dependent and independent mechanisms[END_REF].

Even the M. tuberculosis BacA homologue rescued the S. meliloti bacA mutant phenotype on M. sativa, although only partially. In this case, nodules were not functional and did not support growth of the plants but the nodules were much more infected than in the case of the mutant suggesting that the bacteria persisted in the nodules contrary to the bacA mutant, which is quickly degraded in the plant cells. Moreover, bacA mutant bacteria carrying the M. tuberculosis gene were enlarged although they displayed aberrant morphologies and did not become elongated as wild-type bacteroids [START_REF] Arnold | Partial complementation of Sinorhizobium meliloti bacA mutant phenotypes by the Mycobacterium tuberculosis BacA protein[END_REF].

These examples from human and animal pathogens highlight the requirement of a functional BacA protein for inter-and intracellular accommodation of bacteria within eukaryotic cells and tissues and for maintenance of chronic infections. Similarly to S. meliloti dealing with NCRs in Medicago nodules, these pathogens colonizing host tissues are facing the host innate immune system comprising AMPs like defensins [START_REF] Roop | Survival of the fittest: how Brucella strains adapt to their intracellular niche in the host[END_REF][START_REF] Tan | Macrophages acquire neutrophil granules for antimicrobial activity against intracellular pathogens[END_REF][START_REF] Rivas-Santiago | Human β-defensin 2 is expressed and associated with Mycobacterium tuberculosis during infection of human alveolar epithelial cells[END_REF][START_REF] Rivas-Santiago | β-Defensin gene expression during the course of experimental tuberculosis infection[END_REF]. It is tempting to speculate that the BacA proteins of the pathogens are important to resist to these AMPs and are therefore key in the establishment of chronic infections. In agreement with this possibility, it was shown that the M tuberculosis bacA mutant has impaired tolerance to human β defensin 2 [START_REF] Arnold | Partial complementation of Sinorhizobium meliloti bacA mutant phenotypes by the Mycobacterium tuberculosis BacA protein[END_REF]. Moreover, the Mycobacterium and Brucella bacA homologues can restore the sensitivity towards NCRs or human β defensin 2 of the S. meliloti bacA mutant to wild-type levels [START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF][START_REF] Arnold | Partial complementation of Sinorhizobium meliloti bacA mutant phenotypes by the Mycobacterium tuberculosis BacA protein[END_REF]. [START_REF] Arnold | Partial complementation of Sinorhizobium meliloti bacA mutant phenotypes by the Mycobacterium tuberculosis BacA protein[END_REF]. This observation suggests that BacA-like proteins indeed function as ABC proteins. Thus potentially also the BacA/SbmA proteins are ABC transporters. Indeed, the E. coli SbmA protein forms homodimers with a topology closely resembling the membrane-spanning region of the ABC transporter family [START_REF] Corbalan | Functional and structural study of the dimeric inner membrane protein SbmA[END_REF][START_REF] Runti | Functional characterization of SbmA, a bacterial inner membrane transporter required for importing the antimicrobial peptide Bac7(1-35)[END_REF]. A putative ATPase domain of SbmA or BacA could be encoded by a separate gene but remains to be identified. No candidate gene has been identified in the vicinity of the bacA/sbmA genes. Alternatively, it was proposed that SbmA-mediated transport is driven by the electrochemical gradient across the inner-membrane of the bacterial cell in which the transporter is located rather than by ATP hydrolysis [START_REF] Runti | Functional characterization of SbmA, a bacterial inner membrane transporter required for importing the antimicrobial peptide Bac7(1-35)[END_REF].

Certain AMPs like microcin B17, microcin J25 and proline-rich cathelicidin type of AMPs are internalized and have intracellular targets leading to bacterial death. These AMPs kill bacteria without affecting the bacterial membrane integrity, in contrast to NCRs or defensins [START_REF] Scocchi | Proline-rich antimicrobial peptides: converging to a non-lytic mechanism of action[END_REF]. The uptake of these peptides is mediated at least in part by the BacA/SbmA/BclA proteins and mutations strongly reduce peptide uptake and thus confer resistance to the peptides. This has led to the suggestion that BacA/SbmA/BclA proteins are peptide transporters [START_REF] Laviña | Identification, mapping, cloning and characterization of a gene (sbmA) required for microcin B17 action on Escherichia coli K12[END_REF][START_REF] Yorgey | Posttranslational modifications in microcin B17 define an additional class of DNA gyrase inhibitor[END_REF][START_REF] Salomon | The peptide antibiotic microcin 25 is imported through the TonB pathway and the SbmA protein[END_REF][START_REF] Mattiuzzo | Role of the Escherichia coli SbmA in the antimicrobial activity of proline-rich peptides[END_REF][START_REF] Pränting | Mechanism and fitness costs of PR-39 resistance in Salmonella entericas erovar Typhimurium LT2[END_REF][START_REF] Marlow | Essential role for the BacA protein in the uptake of a truncated eukaryotic peptide in Sinorhizobium meliloti[END_REF][START_REF] Karunakaran | BacA is essential for bacteroid development in nodules of galegoid, but not phaseoloid, legumes[END_REF][START_REF] Ardissone | Role of BacA in lipopolysaccharide synthesis, peptide transport, and nodulation by Rhizobium sp. strain NGR234[END_REF][START_REF] Arnold | Partial complementation of Sinorhizobium meliloti bacA mutant phenotypes by the Mycobacterium tuberculosis BacA protein[END_REF]Guefrachi et al., 2015a; see Chapter III). Thus the BacA/SbmA transporters have opposing effects on the bacterial sensitivity towards peptides with intracellular targets like the proline-rich cathelicidins (BacA/SbmA promotes sensitivity) and towards membrane-disrupting peptides like NCRs or defensins (BacA promotes resistance). A possible explanation is that the internalization of AMPs by the transporter facilitates their contact with their target for AMPs with intracellular action such as the proline-rich cathelicidins and on the contrary, takes them away from their membrane targets for membrane-disrupting AMPs such as the NCRs or defensins. It is interesting to note that other ABC transporter systems are described which provide resistance to membranedisrupting AMPs. The SapABCDF transporters in S. typhimurium [START_REF] Para-Lopez | Molecular genetic analysis of a locus required for resistance to antimicrobial peptides in Salmonella typhimurium[END_REF][START_REF] Para-Lopez | A Salmonella protein that is required for resistance to antimicrobial peptides and transport of potassium[END_REF] or the plant pathogen Dickeya dadantii (previously named Erwinia chrysanthemi) [START_REF] Lopez-Solanilla | Inactivation of the sapA to sapF locus of Erwinia chrysanthemi reveals common features in plant and animal bacterial pathogenesis[END_REF] and the Salmonella YejABEF transporter [START_REF] Eswarappa | The yejABEF operon of Salmonella confers resistance to antimicrobial peptides and contributes to its virulence[END_REF] were proposed to import host-produced membrane-damaging AMPs taking them away from their membrane target site.

The BacA/SbmA/BclA proteins also mediate the uptake and sensitivity to the glycopeptide antibiotic bleomycin [START_REF] Yorgey | Posttranslational modifications in microcin B17 define an additional class of DNA gyrase inhibitor[END_REF][START_REF] Ichige | Genetic analysis of the Rhizobium meliloti bacA gene: functional interchangeability with the Escherichia coli sbmA gene and phenotypes of mutants[END_REF]Marlow et al., 2009;[START_REF] Wehmeier | Internalization of a thiazole-modified peptide in Sinorhizobium meliloti occurs by BacA-dependent and independent mechanisms[END_REF][START_REF] Domenech | BacA, an ABC transporter involved in maintenance of chronic murine infections with Mycobacterium tuberculosis[END_REF][START_REF] Tan | BacA is indispensable for successful Mesorhizobium-Astragalus symbiosis[END_REF][START_REF] Karunakaran | BacA is essential for bacteroid development in nodules of galegoid, but not phaseoloid, legumes[END_REF][START_REF] Maruya | The bacA Gene Homolog, mlr7400, in Mesorhizobium loti MAFF303099 is dispensable for symbiosis with Lotus japonicus but partially capable of supporting the symbiotic function of bacA in Sinorhizobium meliloti[END_REF]Guefrachi et al., 2015a; see Chapter III) and sensitivity towards several aminoglycoside antibiotics [START_REF] Ichige | Genetic analysis of the Rhizobium meliloti bacA gene: functional interchangeability with the Escherichia coli sbmA gene and phenotypes of mutants[END_REF]. Moreover, the M.

tuberculosis BacA-like protein was recently reported to facilitate vitamin B 12 uptake [START_REF] Gopinath | A vitamin B 12 transporter in Mycobacterium tuberculosis[END_REF]. Although vitamin B 12 is required for bacteroid differentiation [START_REF] Campbell | Sinorhizobium meliloti bluB is necessary for production of 5,6-dimethylbenzimidazole, the lower ligand of B12[END_REF][START_REF] Taga | Sinorhizobium meliloti requires a cobalamin-dependent ribonucleotide reductase for symbiosis with its plant host[END_REF], it is unlikely that the symbiotic role of BacA or BacA-like is the uptake of plant-provided vitamin B 12 because rhizobia can produce this cofactor and more importantly, plants do not use or produce it [START_REF] Roth | Cobalamin (coenzyme B 12 ): Synthesis and biological significance[END_REF].

Since aminoglycoside uptake is driven by membrane potential [START_REF] Taber | Bacterial uptake of aminoglycoside antibiotics[END_REF] and in light of the structural diversity of all the compounds whose uptake is affected by the BacA/SbmA/BclA proteins, an alternative to the peptide uptake hypothesis could be that the proteins affect in a direct or indirect way the membrane potential or membrane composition.

An altered membrane could have changed interaction properties with membrane-damaging peptides like NCRs or defensins.

Support for a role of BacA in the determination of cell envelope features derives from the observations that the S. meliloti mutant has an increased sensitivity to detergents and ethanol [START_REF] Ferguson | Deficiency of a Sinorhizobium meliloti BacA mutant in alfalfa symbiosis correlates with alteration of the cell envelope[END_REF]. Similar behavior was described for bacA mutants in M.

huakuii [START_REF] Tan | BacA is indispensable for successful Mesorhizobium-Astragalus symbiosis[END_REF], M. loti [START_REF] Maruya | The bacA Gene Homolog, mlr7400, in Mesorhizobium loti MAFF303099 is dispensable for symbiosis with Lotus japonicus but partially capable of supporting the symbiotic function of bacA in Sinorhizobium meliloti[END_REF], R. leguminosarum or R. etli [START_REF] Karunakaran | BacA is essential for bacteroid development in nodules of galegoid, but not phaseoloid, legumes[END_REF]. The bacA mutation in S. meliloti and B. abortus also reduces by about half the modification of the outer-membrane lipopolysaccharide (LPS) with typical very long chain fatty acids (VLCFA) [START_REF] Ferguson | Similarity to peroxisomal-membrane protein family reveals that Sinorhizobium and Brucella BacA affect lipid-A fatty acids[END_REF]. Therefore, it was proposed that BacA could be involved in the export of fatty acids out of the cytoplasm where they are synthesized [START_REF] Ferguson | Similarity to peroxisomal-membrane protein family reveals that Sinorhizobium and Brucella BacA affect lipid-A fatty acids[END_REF]. Also in M. huakuii, the bacA mutation affected the VLCFA content in LPS [START_REF] Tan | BacA is indispensable for successful Mesorhizobium-Astragalus symbiosis[END_REF]. However, no major changes associated with the cell envelope, including sensitivity to ethanol and detergents and lipid content of cell membranes, were detected in bacA deficient Sinorhizobium fredii strain NGR234 and M. tuberculosis [START_REF] Domenech | BacA, an ABC transporter involved in maintenance of chronic murine infections with Mycobacterium tuberculosis[END_REF][START_REF] Ardissone | Role of BacA in lipopolysaccharide synthesis, peptide transport, and nodulation by Rhizobium sp. strain NGR234[END_REF]. Moreover, the symbiotic defect of the S. meliloti bacA mutant does not seems to be directly linked to the VLCFA LPS defect because additional mutants in the acpXL and lpxXL genes, which completely lack VLCFA in their LPS, are much less severely affected than the bacA mutant and can form nitrogen-fixing nodules [START_REF] Ferguson | Importance of unusually modified lipid A in Sinorhizobium stress resistance and legume symbiosis[END_REF] although bacteroid abnormalities were observed with for example the formation of hypertrophied bacteroids [START_REF] Haag | The Sinorhizobium meliloti LpxXL and AcpXL proteins play important roles in bacteroid development within alfalfa[END_REF].

In addition, the BacA mediated uptake of cathelicidin type of AMPs such as the peptide Bac7 by S. meliloti can also be uncoupled from its symbiotic role because two point mutants in the bacA gene were identified that were symbiotically defective but maintained the capacity to take up the Bac7 peptide [START_REF] Marlow | Essential role for the BacA protein in the uptake of a truncated eukaryotic peptide in Sinorhizobium meliloti[END_REF]. Similarly, the M. tuberculosis bacA-like gene can restore the resistance defect of the S. meliloti bacA mutant towards the NCR247 peptide as well as human β-defensin 2. Nevertheless, the M. tuberculosis bacA-like gene is only partially able to support symbiosis and bacteroid development [START_REF] Arnold | Partial complementation of Sinorhizobium meliloti bacA mutant phenotypes by the Mycobacterium tuberculosis BacA protein[END_REF]. Probably, the multifaceted phenotypes of the bacA mutant together with the complex mixture of NCR peptides produced by the symbiotic nodule cells has to be taken into account to fully explain the essential role of BacA in symbiosis.

General Envelope Stress Response function of BacA/SbmA?

Bacteria that interact with eukaryotic cells are often confronted with AMPs produced by the host cells. Many of these bacteria have evolved resistance mechanisms permitting them to survive these peptides. Examples are the SapABCDF and YejABEF transporters described above. Another frequently observed mechanism is the modification of the electrical charge of the bacterial envelope, rendering it less negative and thereby reducing electrostatic interactions with the cationic AMPs [START_REF] Maróti | Natural roles of antimicrobial peptides in microbes, plants and animals[END_REF]. Related to this, it is interesting to note that the LPS of bacteroids in Medicago and pea nodules becomes more hydrophobic and displays different epitopes when compared to cultured bacteria [START_REF] Kannenberg | Lipopolysaccharide epitope expression of Rhizobium bacteroids as revealed by in situ immunolabelling of pea root nodule sections[END_REF][START_REF] Kannenberg | Lipid A and O-chain modifications cause Rhizobium lipopolysaccharides to become hydrophobic during bacteroid development[END_REF][START_REF] Ferguson | Importance of unusually modified lipid A in Sinorhizobium stress resistance and legume symbiosis[END_REF]. This LPS modification could reflect an adaptive response of the bacteroids to the exposure with the NCR peptides.

The bacterial resistance mechanisms are usually induced after perception of the AMPs. The presence of AMPs can be directly sensed with AMP-binding two-component regulators [START_REF] Bader | Recognition of antimicrobial peptides by a bacterial sensor kinase[END_REF][START_REF] Gryllos | Induction of group A Streptococcus virulence by a human antimicrobial peptide[END_REF] or with ABC transporter-derived sensors, which in that case have lost their transporter function but have a role as receptor and signal transduction protein [START_REF] Hiron | Bacitracin and nisin resistance in Staphylococcus aureus: a novel pathway involving the BraS/BraR two-component system (SA2417/SA2418) and both the BraD/BraE and VraD/VraE ABC transporters[END_REF][START_REF] Falord | GraXSR proteins interact with the VraFG ABC transporter to form a five-component system required for cationic antimicrobial peptide sensing and resistance in Staphylococcus aureus[END_REF]. Alternatively, bacteria can sense the membrane damage provoked by the membrane-disrupting peptides through different envelope (or extracytoplasmic) stress response (ESR) systems. ESR pathways detect and induce appropriate responses to stresses that affect the bacterial membranes and periplasmic and membrane proteins, including membrane stresses provoked by AMPs [START_REF] Majdalani | The Rcs phosphorelay: A complex signal transduction system[END_REF][START_REF] Rowley | Pushing the envelope: extracytoplasmic stress responses in bacterial pathogens[END_REF][START_REF] Bury-Moné | Global analysis of extracytoplasmic stress signalling in Escherichia coli[END_REF]. Generally, bacteria have different co-existing ESR systems. AMP-responsive ESRs include, in E. coli and Salmonella typhimurium, the Rcs phosphorelay system [START_REF] Farris | Antimicrobial peptides activate the Rcs regulon through the outer membrane lipoprotein RcsF[END_REF], the CpxAR two-component system [START_REF] Weatherspoon-Griffin | The CpxR/CpxA two-component system up-regulates two Tatdependent peptidoglycan amidases to confer bacterial resistance to antimicrobial peptide[END_REF] and the extracytoplasmic function sigma factor σ E -regulon (Hymphreys et al., 1999;[START_REF] Crouch | The alternative sigma factor σ E is required for resistance of Salmonella enterica serovar Typhimurium to anti-microbial peptides[END_REF].

Interestingly, SbmA is part of the ESR σ E -regulon in E. coli [START_REF] Rowley | Pushing the envelope: extracytoplasmic stress responses in bacterial pathogens[END_REF][START_REF] Bury-Moné | Global analysis of extracytoplasmic stress signalling in Escherichia coli[END_REF] suggesting that the SbmA protein has a role in ESR, which needs to be further defined. σ E and its homologues control to various degrees pathogenesis. For example, σ E mutants of S. typhimurium are defective for intracellular survival in macrophage and epithelial host cell lines and have a strongly attenuated virulence in mice [START_REF] Humphreys | The alternative sigma factor, σ E , is critically important for the virulence of Salmonella typhimurium[END_REF][START_REF] Crouch | The alternative sigma factor σ E is required for resistance of Salmonella enterica serovar Typhimurium to anti-microbial peptides[END_REF]. The attenuated virulence of the σ E -mutant of S. typhimurium has been attributed to its inability to protect itself against host-produced AMPs [START_REF] Crouch | The alternative sigma factor σ E is required for resistance of Salmonella enterica serovar Typhimurium to anti-microbial peptides[END_REF]. The mutant phenotypes are in part depending on the σ E -regulated genes htrA (degP)

and surA, encoding a periplasmic serine protease and a periplasmic peptidyl-prolyl-isomerase, respectively. But other σ E -regulated genes are possibly important since σ E mutants are more affected than individual htrA or surA mutants. A good candidate among them could be sbmA in light of the above described critical function of SbmA/BacA proteins in symbiotic and pathogenic interactions.

Interestingly, a genetic link between bacA and degP was also demonstrated in S.

meliloti [START_REF] Glazebrook | Genetic analysis of Rhizobium meliloti bacA-phoA fusion results in identification of degP: Two loci required for symbiosis are closely linked to degP[END_REF] suggesting that also in S. meliloti, bacA is implicated in ESR. However, the biochemical link between bacA and degP remains to be further unraveled.

The association of SbmA in E. coli and potentially also BacA in S. meliloti with ESR suggests that these proteins are part of a membrane stress sensing system or that they function in the bacterial response to alleviate this stress. The above-mentioned hypersensitivity of bacA mutants towards the membrane-damaging stresses induced by ethanol or detergents are in agreement with this.

The σ E -regulon in E. coli includes also the dsbC gene [START_REF] Rowley | Pushing the envelope: extracytoplasmic stress responses in bacterial pathogens[END_REF][START_REF] Bury-Moné | Global analysis of extracytoplasmic stress signalling in Escherichia coli[END_REF], which is involved in oxidative protein folding in the periplasm [START_REF] Depuydt | How proteins form disulfide bonds[END_REF]. It is unknown whether SbmA is also involved in oxidative folding but a speculative link could be that BacA keeps NCR peptides in an oxidized and less damaging state in the periplasm as is suggested above [START_REF] Haag | Role of cysteine residues and disulfide bonds on the activity of a legume root nodule-specific, cysteine-rich peptide[END_REF].

ESR regulators have been described in several Rhizobium species [START_REF] Bastiat | Dual control of Sinorhizobium meliloti RpoE2 sigma factor activity by two PhyR-type two-component response regulators[END_REF][START_REF] Sauviac | An extracytoplasmic function sigma factor acts as a general stress response regulator in Sinorhizobium meliloti[END_REF][START_REF] Gourion | The PhyR-σ EcfG signaling cascade is involved in stress response and symbiotic efficiency in Bradyrhizobium japonicum[END_REF][START_REF] Martínez-Salazar | Role of the extracytoplasmic function sigma factor RpoE4 in oxidative and osmotic stress responses in Rhizobium etli[END_REF]. But none of them were reported to regulate the bacA gene and thus the regulon to which rhizobial bacA belongs remains to be discovered but could be very informative for understanding better how BacA functions, both as a household function and in symbiosis.

Hypotheses for the mechanism of BacA/BclA functioning

Recent studies on BacA and BclA in Rhizobium species indicate that these transporters are critical for chronic intracellular infections of legume hosts, which produce AMPs in the symbiotic cells. BacA homologues are also contributing to the pathogenesis and chronic infection in pathogens and they do this most likely by protecting bacteria against host AMPs.

However, how BacA/SbmA proteins function mechanistically remains unknown. We propose the following hypotheses: i) the proteins could be part of a peptide uptake system, leading to internalization of AMPs which would move them away from membrane targets but bring them in proximity to intracellular targets; ii) on the contrary, it cannot be excluded that they are exporters, affecting the bacterial envelop and thereby indirectly promoting the uptake of certain peptides with intracellular mode of killing and reducing the affinity of membranedamaging peptides for the bacterial envelope or iii) alternatively, they could export compounds that directly modify the peptides (e.g. oxidizing peptides so that they are less harmful); iv) they could function as sensors rather than transporters, activating other resistance determinants. Testing these possibilities is a challenge for the future.

Understanding the household function of bacA in Rhizobium and its regulation in stress response or other cellular functions could provide important clues for understanding its interference with the action of NCR peptides and other AMPs.

Objectives

Objectives and outline of the PhD work

The host laboratory of my PhD work, the group "Cell Differentiation in the Legume-Rhizobium Symbiosis", has a long-standing interest in legume-rhizobia interaction from both the bacterial and plant perspective. The research activity in the last decade was focused on cell differentiation in nodules and notably on the differentiation of symbiotic cells as well as the bacterial endosymbionts. Among the meaningful results were the identification of Medicago nodulation genes and their functional analysis to unravel the molecular mechanism of symbiotic cell formation and intracelullar accomodation of rhizobia [START_REF] Cebolla | The mitotic inhibitor ccs52 is required for endoreduplication and ploidy-dependent cell enlargement in plants[END_REF][START_REF] Kevei | Glycine-rich proteins encoded by a nodule-specific gene family are implicated in different stages of symbiotic nodule development in Medicago spp[END_REF][START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF][START_REF] Vinardell | Endoreduplication mediated by the APC activator CCS52A is required for symbiotic cell differentiation in Medicago truncatula nodules[END_REF][START_REF] Van De Velde | Aging in legume symbiosis. A molecular view on nodule senescence in Medicago truncatula[END_REF][START_REF] Alunni | Genomic organization and evolutionary insights on GRP and NCR genes, two large nodule-specific gene families in Medicago truncatula[END_REF][START_REF] Kevei | A 3-Hydroxy-3-Methylglutaryl Coenzyme A reductase interacts with NORK in the nodulation signaling pathway[END_REF][START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF].

One of the primordial results from previous research of the group was the finding that bacteroids in the IRLC legume clade which form E-type bacteroids differentiate under the influence of plant derived antimicrobial peptides called the Nodule-specific Cysteine-Rich (NCR) which are targeted to the symbiosomes via the secretory pathway [START_REF] Mergaert | Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis[END_REF][START_REF] Van De Velde | Plant peptides govern terminal differentiation of bacteria in symbiosis[END_REF][START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF]. NCRs are similar to the defensin-type of antimicrobial peptides, and some NCR peptides have been found to have antimicrobial activity, killing rhizobium when applied at high concentration. To counteract the antimicrobial activity of the NCR peptides, S. meliloti, the symbiont of Medicago, requires the BacA protein, which is an ABC transporter protein that thus potentially acts as a peptide transporter.

This previous work of the group that formed the context of my PhD work at its onset and is described in detail in the introduction of this PhD manuscript and part of has been published as a review in the book entitled "Biological Nitrogen Fixation Book", edited by

Results

& Discussion

Chapter. I

Extreme specificity of NCR gene expression in

Medicago truncatula

Extreme specificity of NCR gene expression in Medicago truncatula *

Abstract

Legumes form root nodules to house nitrogen fixing bacteria of the rhizobium family.

The rhizobia are located intracellularly in the symbiotic nodule cells. In the legume Medicago truncatula these cells produce high amounts of Nodule-Specific Cysteine-Rich (NCR) peptides, which induce differentiation of the rhizobia into enlarged, polyploid and noncultivable bacterial cells. NCRs are similar to innate immunity antimicrobial peptides. The NCR gene family is extremely large in Medicago with about 600 genes. Here we used the Medicago truncatula Gene Expression Atlas (MtGEA) and other published microarray data to analyze the expression of 334 NCR genes in 267 different experimental conditions. We find that all but five of these genes are expressed in nodules but in no other plant organ or in response to any other biotic interaction or abiotic stress tested. During symbiosis, none of the genes are induced by Nod factors. The NCR genes are activated in successive waves during nodule organogenesis, correlated with a specific spatial localization of their transcripts from the apical to the proximal nodule zones, and are not associated with nodule senescence.

According to their Shannon entropy, a measure expressing tissue specificity of gene expression, the NCR genes are among the most specifically expressed genes in M. truncatula.

Moreover, when activated in nodules, their expression level is among the highest of all genes.

Together, these data show that the NCR gene expression is subject to an extreme tight regulation and is only activated during nodule organogenesis in the polyploid symbiotic cells.

* Modified from: Guefrachi, I., Nagymihaly, M., Pislariu, C.I., Van de Velde, W., Ratet, P., Mars, M., Udvardi, M.K., [START_REF] Guefrachi | Extreme specificity of NCR gene expression in Medicago truncatula[END_REF]. BMC Genomics.

15: 712

Introduction

Legume plants establish a symbiotic relationship with nitrogen fixing soil bacteria, known as rhizobia. For the purpose of this symbiosis, the plant host forms new, specific organs on its roots called nodules, inside which the symbiotic rhizobia are housed, fix nitrogen (i.e. the enzymatic reduction of nitrogen gas to ammonium) and transfer the ammonium to the plant. Nodules contain several thousand endoreduplicated giant symbiotic cells, which are each infected with thousands of intracellular rhizobia. These symbiotic cells are adapted to the symbiosis, to the metabolic exchange with the nitrogen fixing rhizobia and to the intracellular accommodation of this large bacterial population. The symbiotic cells originate from dividing progenitor cells in the nodule meristem. A key step in the differentiation of the symbiotic cells is the exit of the cell division cycle of nodule meristematic cells and a switch to an endoreduplication cycle in these post-meristematic cells.

An endoreduplication cycle is a modified cell cycle with repeated replication of the genome without mitosis and cytokinesis, resulting in polyploid cells with increased DNA content and cell volume. The cell cycle switch is under the control of the Anaphase Promoting Complex (APC) and its activator Ccs52A [START_REF] Cebolla | The mitotic inhibitor ccs52 is required for endoreduplication and ploidy-dependent cell enlargement in plants[END_REF][START_REF] Vinardell | Endoreduplication mediated by the APC activator CCS52A is required for symbiotic cell differentiation in Medicago truncatula nodules[END_REF]. The differentiating symbiotic cells are gradually infected and filled with rhizobia. Bacteria are released in the host cells through an endocytosis-like process releasing the intracellular bacteria in organelle-like structures called symbiosomes. Mature symbiotic cells have about 80-fold larger cell volume and endoploidy levels up to 64C compared to the diploid (2C) progenitor cells. Their cytosolic space is entirely packed with symbiosomes and their physiology is adapted for symbiosis, feeding the microsymbionts and assimilating and transporting the fixed nitrogen.

Remarkably, Wildermuth, (2010) noticed, by comparing different biotrophic interactions of plants, that host cell polyploidy is a common feature of symbiotic interactions with rhizobium bacteria and arbuscular mycorrhizal fungi, as well as parasitic interactions with fungi and nematodes. Even in symbiotic interactions of insects with endosymbiotic bacteria, host cells that house the endosymbionts are endoreduplicated cells [START_REF] Koga | Cellular mechanism for selective vertical transmission of an obligate insect symbiont at the bacteriocyte-embryo interface[END_REF]. Thus polyploid host cells may be a well suited adaptation as an interaction site for nutrient exchange with symbiotic microorganisms and some parasites may have evolved to exploit this.

Synchronously with the differentiation of their host cells, the symbiosome bacteria differentiate into nitrogen-fixing bacteria called bacteroids. These bacteroids have a specific physiology and metabolism adapted to the symbiotic life and nitrogen fixation, which are dramatically different from those of a free-living bacterium [START_REF] Kereszt | Bacteroid development in legume nodules: evolution of mutual benefit or of sacrificial victims?[END_REF].

Interestingly, the differentiation of bacteroids is often accompanied by a morphological and cytological metamorphosis whereby the bacteroid cell becomes enlarged, its envelope fragilized and its genome amplified (polyploid) and condensed [START_REF] Mergaert | Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis[END_REF][START_REF] Kondorosi | A paradigm for endosymbiotic life: Cell differentiation of Rhizobium bacteria provoked by host plant factors[END_REF]. In Medicago truncatula, a class of peptides named NCRs (Nodulespecific Cysteine-rich Peptides) controls the bacteroid elongation and polyploidization (Van de Velde et al., 2010). The NCR peptides are produced by the infected symbiotic cells and are transported to the bacteroid-containing symbiosomes. The NCR peptides can induce bacterial elongation and polyploidization in vitro on cultured rhizobium or in planta when expressed in transgenic Lotus japonicas plants, which lack NCRs and, form non-elongated bacteroids without genome amplification (Van de Velde et al., 2010). Some NCRs accumulate to a significant extent in the cytosol of mature bacteroids (Van de Velde et al., 2010) suggesting that these peptides may have additional functions, other than inducing the morphological transformation and notably, it has been suggested that these intracellular NCRs may affect the bacteroid metabolism [START_REF] Kereszt | Bacteroid development in legume nodules: evolution of mutual benefit or of sacrificial victims?[END_REF]. Indeed, it has been demonstrated for the peptide NCR247 that it has multiple bacterial targets leading to inhibition of cell division and affecting the bacterial transcriptome and translation that collectively contribute to the altered physiology of the endosymbionts [START_REF] Tiricz | Antimicrobial nodule-specific cysteine-rich peptides induce membrane depolarizationassociated changes in the transcriptome of Sinorhizobium meliloti[END_REF][START_REF] Farkas | The Medicago truncatula symbiotic peptide NCR247 contributes to bacteroid differentiation through multiple mechanisms[END_REF][START_REF] Penterman | Host plant peptides elicit a transcriptional response to control the Sinorhizobium meliloti cell cycle during symbiosis[END_REF].

NCRs are similar to the defensin-type of antimicrobial peptides, and some NCR peptides have antimicrobial activity, killing rhizobium and other bacteria when applied at high concentration (Van de Velde et al., 2010;[START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF][START_REF] Haag | Role of cysteine residues and disulfide bonds on the activity of a legume root nodule-specific, cysteine-rich peptide[END_REF][START_REF] Tiricz | Antimicrobial nodule-specific cysteine-rich peptides induce membrane depolarizationassociated changes in the transcriptome of Sinorhizobium meliloti[END_REF]. Defensins and other types of antimicrobial peptides are found in all eukaryotes where they are part of the first line of defense against invading microbes. Thus the NCR peptides likely evolved from the ancestral immune repertoire. NCR genes were originally thought to be unique to the IRLC legume clade [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF]. The bacteroids in the nodules of the tested species of this clade all share the elongation and polyploidization feature [START_REF] Mergaert | Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis[END_REF]. However, refined bioinformatics tools for the prediction of small peptides in genome sequences led to the discovery of three putative Arabidopsis genes that encode peptides with the typical pattern of cysteine residues of the NCRs [START_REF] Zhou | Detecting small plant peptides using SPADA (Small Peptide Alignment Discovery Application)[END_REF].

The existence of multiple NCR genes in several species of the IRLC clade suggests that the ancestral genes may have gained a new function in symbiosis in the common ancestor of IRLC and that increasing its copy number through gene duplications may have conferred a selective advantage. To counteract the antimicrobial activity of the NCR peptides, Sinorhizobium meliloti, the symbiont of Medicago, requires the BacA protein. In the absence of this protein, the bacteroids are immediately killed by the NCR peptides as soon as they are released in the symbiosomes in nodule cells [START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF].

A striking and unusual feature of the NCR gene family in M. truncatula is that it is composed of about 600 genes, which are seemingly all expressed in the nodules (Mergaert et al., 2003;[START_REF] Alunni | Genomic organization and evolutionary insights on GRP and NCR genes, two large nodule-specific gene families in Medicago truncatula[END_REF][START_REF] Branca | Whole-genome nucleotide diversity, recombination, and linkage disequilibrium in the model legume Medicago truncatula[END_REF]Nallu et al., 2013;[START_REF] Zhou | Detecting small plant peptides using SPADA (Small Peptide Alignment Discovery Application)[END_REF]. In situ expression analysis of individual NCR genes or microarray analysis of a large subset of the family has demonstrated that they are all expressed in the symbiotic nodule cells [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF][START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF][START_REF] Van De Velde | Plant peptides govern terminal differentiation of bacteria in symbiosis[END_REF]Nallu et al., 2013).

Moreover, EST analysis and microarray experiments, testing a number of different plant organs as well as different growth conditions, revealed NCR gene expression only in nodules [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF][START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF]Nallu et al., 2013).

The Medicago truncatula Gene Expression Atlas (MtGEA) [START_REF] Benedito | A gene expression atlas of the model legume Medicago truncatula[END_REF]He et al., 2009) was generated with the whole genome Affymetrix Medicago Gene Chip and compiles microarray data for the majority of M. truncatula genes (50,900 probe-sets) over a large set of experiments (254 different experiments in MtGEA version 3). The compendium is a unique and currently the richest resource for analysing gene expression in M. truncatula. In this study, we used the MtGEA compendium and additional unpublished and published microarray experiments [START_REF] Limpens | Cell-and tissue-specific transcriptome analyses of Medicago truncatula root nodules[END_REF][START_REF] Jayaraman | Response of Medicago truncatula seedlings to colonization by Salmonella enterica and Escherichia coli O157:H7[END_REF] to describe in great detail the expression profiles of the majority of the M. truncatula NCR genes. We show that this gene family has an extreme tissue specific expression profile with undetectable expression in all tissues and conditions except in nodules where they become transcriptionally active to very high levels. In addition, promoter-GUS plants were produced for three NCR genes and used to confirm expression pattern specificity in various conditions, most particularly during biotic interactions.

Results

Global analysis of NCR gene expression

The probe-sets and expression data corresponding to the NCRs described before [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF] and [START_REF] Alunni | Genomic organization and evolutionary insights on GRP and NCR genes, two large nodule-specific gene families in Medicago truncatula[END_REF] were searched in MtGEA (version 3) using the BLAST search option of the database. Expression profiles of 334 probe-sets were obtained (Table 1; derived from the M. truncatula genotype 'Jemalong' but also contains data sets obtained from the genotypes 'R108' and 'F83005.5' although all specific experiments discussed below were obtained with the 'Jemalong' genotype. As specified in more detail below, the compendium covers the plant' s major organs, various kinds of abiotic and biotic stresses and data from specific cell and tissue types. In the heat map of Fig. 11, the experiments are organized in three major groups, namely the profiles of nodule samples, root samples and samples of all other plant organs regardless of the treatment they were submitted to. An obvious global pattern instantly revealed by the heat map is that the nearly complete 334 NCR gene set is only expressed in nodules except for one experiment marked with the red arrowhead, which is annotated in the MtGEA compendium as mycorrhizal roots, but is contaminated with nodules (Fig. 12). The expression of the NCR gene set in mycorrhizal roots was inconsistent with our previous results [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF] and also with other mycorrhizal samples present in the MtGEA compendium in which the NCR genes are not expressed. The Affymetrix gene Chip contains also probe-sets for monitoring the genes of S. meliloti. The S. meliloti probe-set can thus reveal whether samples of the MtGEA contain this bacterium. Analysing 28 genes of the S. meliloti probe-sets (Table 1), corresponding to the nitrogenase locus, we found that these genes were expressed in nodule samples, as expected, as well as in the particular mycorrhizal sample in which the NCRs are also active, while in all other samples including other mycorrhizal samples, roots and other organs, the S. meliloti probe-sets gave background signals (Fig. 12). Considering that the nitrogenase locus is known to be exclusively expressed in nodule bacteroids [START_REF] Becker | Global changes in gene expression in Sinorhizobium meliloti 1021 under microoxic and symbiotic conditions[END_REF], this mycorrhizal sample is most likely impure and contaminated with nodules. The heat map shows the expression of 334 NCR genes (rows) in 267 experimental conditions (columns). Experiments are ordered as indicated above the columns. The color scale bar indicates the expression level from background level (blue) to maximum level (yellow). The red arrowhead indicates the mycorrhizal sample that is contaminated with nodules. The green arrowheads locate the NCR genes with relaxed specificity; from top to down: NCR247, NCR235, NCR122, NCR218 and NCR077. The dataset used for the generation of the heat map is provided in Table 1.

Figure 12. Heat map of S. meliloti gene expression in the MtGEA compendium

The heat map shows the expression of 28 S. meliloti genes (rows) in 267 experimental conditions (columns). The probe-set included 28 genes of the locus carrying the nifHDK operon encoding the nitrogenase enzyme and the dataset used for the generation of the heat map is provided in Table 1. Experiments are ordered as in Fig. 11 The expression profiles of individual NCR genes show expression to very high levels in the nodule conditions and only background levels in the other experiments (Fig. 13A,B).

Such profiles are typical for nearly all NCR genes (Table 1) but by surveying all NCR probesets, 5 exceptions were discovered with more or less relaxed nodule specific expression patterns (Fig. 11, red arrowheads; Fig. 13C-G). NCR247 and NCR077 are still mainly expressed in nodules but are also weakly active in other conditions. The NCR247 gene seems to be expressed in different root samples and in some samples from areal tissues although at lower levels than in nodules. It is not evident from the available information of the different experiments to determine what may activate this expression. NCR077 has a higher than usual background level, possibly because of a less specific probe set, but the gene seems to be also expressed in some mycorrhizal samples (Fig. 13D) including a laser capture microdissection sample of arbuscule-containing cells [START_REF] Gaude | Arbuscule-containing and non-colonized cortical cells of mycorrhizal roots undergo extensive and specific reprogramming during arbuscular mycorrhizal development[END_REF]. NCR218 and NCR122 on the other hand have a completely relaxed specificity and are expressed to similar levels in nodules and in other conditions, mostly roots (Fig. 13E,F). NCR235 expression is similarly specific to most other NCR genes except for a weak expression in stems and shoots, which is about 10to 100-fold lower than in nodules (Fig. 13G). Also NCR247 is expressed in some of the stem samples. Thus, except for these 5 genes, the complete tested NCR gene set is only expressed in nodules and in none of the other conditions that are present in MtGEA.

Spatio-temporal expression of NCR genes in nodules

The MtGEA compendium contains 42 different nodule samples including samples of wild-type nodules harvested at different days post inoculation with S. meliloti and thus at different stages of nodule development. In uninoculated root samples and nodule primordia of 3dpi, none of the NCR genes are activated (Fig. 14A,B). The NCR transcriptomes at 0 dpi and 3 dpi have a correlation coefficient close to 1 (Fig. 14D). This suggests that NCRs have no function in the early stages of the interaction. In agreement with this, Nod factor or Myc factor treatments [START_REF] Czaja | Transcriptional responses toward diffusible signals from symbiotic microbes reveal MtNFP-and MtDMI3-dependent reprogramming of host gene expression by arbuscular mycorrhizal fungal lipochitooligosaccharides[END_REF] do not induce NCR gene expression (Fig. 15).

Nod factors and Myc factors are similar lipochitooligosaccharide signals, produced by rhizobium and mycorrhizal fungi symbionts inducing in the legume host the early stages of nodule and mycorrhizal formation respectively [START_REF] Maillet | Fungal lipochitooligosaccharide symbiotic signals in arbuscular mycorrhiza[END_REF][START_REF] Downie | Legume nodulation[END_REF]. A subset of about 70 NCR genes is activated at 4 dpi with S. meliloti (Fig. 14). Some of these genes are already activated at their maximal level while others are only induced to a fraction of their maximal level and reach full expression at a more advanced stage of nodule development (Fig. 14). At 6 dpi, most genes are activated but many of them not yet at their maximal level (Fig. 14A-C). At 10 dpi the NCR transcriptome seems to be fully activated, to a similar extent as a later time point at 14 dpi. The NCR transcriptomes at 10 dpi and 14 dpi have a correlation coefficient close to 1 (Fig. 14D). This pattern hints at a link between the activation of NCR gene expression and the progression of bacterial infection in the incipient nodules.

!

Figure 15. NCR gene expression in response to Nod factors and Myc factors

The expression pattern of 334 NCR probe-sets (x-axis) is shown for nodules 10 dpi, untreated control roots, 24h 10 -8 M Nod factor treatment, 24h 10 -7 M non-sulfated Myc factor treatment (nsMyc) and 24h 10 -8 M sulfated Myc factor treatment (sMyc) [START_REF] Czaja | Transcriptional responses toward diffusible signals from symbiotic microbes reveal MtNFP-and MtDMI3-dependent reprogramming of host gene expression by arbuscular mycorrhizal fungal lipochitooligosaccharides[END_REF]. The y-axis represents the strength of the hybridization signal and the graphs for all treatments are at the same scale (maximum 25,000).

Because of the presence of an apical meristem, nodules in M. truncatula are of the socalled indeterminate type and all stages of symbiotic cell differentiation, from undifferentiated meristem cells till fully differentiated and functional symbiotic cells, are present in any mature nodule, independent of its age. Therefore, mature nodules are organized in welldefined histological zones: zone I, meristem; zone II, zone of infection and differentiation (polyploidization) of the symbiotic cells; interzone II-III, characterized by amyloplast accumulation; zone III, nitrogen fixation zone with mature functional symbiotic cells; zone IV, senescent zone [START_REF] Kondorosi | A paradigm for endosymbiotic life: Cell differentiation of Rhizobium bacteria provoked by host plant factors[END_REF]. This nodule structure suggests that the temporal NCR expression profiles defined above could correlate with a spatial pattern in the nodule tissues. To test this possibility, we analyzed transcriptome data from 4-week-old nodules that were hand-sectioned in five different parts (see Methods section). These samples correspond to the nodule tissues from the most apical part of the nodule to the most proximal part and are approximately overlapping with the nodule zones I, II, II-III, III and IV. Sample A is the meristem and the underlying few cell layers of post-meristematic cells which start the infection and differentiation process. Sample B corresponds mainly to the infection and differentiation zone II. Sample C corresponds essentially to the interzone II-III. Cluster analysis of the NCR abundance profiles in these five samples distinguishes groups of NCR genes that have preferential expression in defined zones of the nodule and that are sequentially activated from nodule apex to proximal tissues (Fig. 16A and Fig. 17 The transcriptome in specific tissues and cells of nodules was also obtained by lasercapture microdissection (LCM) [START_REF] Limpens | Cell-and tissue-specific transcriptome analyses of Medicago truncatula root nodules[END_REF]. In this experiment, the transcriptome was analyzed in nodule meristems, the distal and the proximal infection zone, the infected cells of the fixation zone and the uninfected cells of the fixation zone (Fig. 16B). Although the type of samples are not entirely overlapping, a good correspondence can be observed between the LCM dataset and the hand-dissected dataset (Fig. 16A,B). The LCM dataset shows that NCR genes are not expressed in the nodule meristem which is free of rhizobia (Fig. 16B). This observation is in agreement with their activation by the infecting rhizobia. It also indicates that the expression of the early NCR genes in the hand-dissected sample I results from the significant presence of cells from the infection zone in that sample. NCR genes were reported to be expressed in infected cells [START_REF] Kondorosi | A paradigm for endosymbiotic life: Cell differentiation of Rhizobium bacteria provoked by host plant factors[END_REF]. Unexpectedly, the LCM dataset revealed a relatively high expression of several NCR genes in the uninfected cells (Fig. 16B, sample UC).

However, among them were the genes NCR001, NCR084, NCR035 and NCR247 for which in situ hybridization, promoter-GUS or-mCherry fusions or immunolocalization of the peptide demonstrated a specific expression in the infected symbiotic nodule cells and no expression in uninfected cells [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF][START_REF] Van De Velde | Plant peptides govern terminal differentiation of bacteria in symbiosis[END_REF][START_REF] Farkas | The Medicago truncatula symbiotic peptide NCR247 contributes to bacteroid differentiation through multiple mechanisms[END_REF]. Possibly, the signal in the sample of the uninfected cells is the result of some contamination with infected cells or due to background hybridization coming from the RNA amplification procedure used with the very low amount of RNA obtained from the LCM samples [START_REF] Limpens | Cell-and tissue-specific transcriptome analyses of Medicago truncatula root nodules[END_REF]. Nevertheless, when comparing the expression level of all NCRs in the uninfected and infected cells, only two genes had a significant higher expression in the uninfected cells. Those two genes are NCR218 (9,6 fold higher; t-test P=0,002) and NCR122 (3,2 fold higher; t-test P=0,018), which are the only 2 NCRs that are consistently expressed to high levels in roots (Fig. 13E,F).

When matching the spatial patterns with the corresponding temporal patterns, a fairly good correspondence can be observed: genes expressed in the apex are mostly also fully activated early during the nodule development at 4 or 6 dpi, while genes expressed in the more proximal tissues are activated late in nodule development (Fig. 16). The correspondence between the spatial and temporal pattern of NCR gene expression is also obvious whenconsidering the clusters of genes representing the major expression profiles (Fig. 17).

For example, genes of clusters 1 and 2 (Fig. 17 Together these spatio-temporal patterns reveal that the NCR genes are activated in different waves, in agreement with our previous results that identified two key points in nodule development associated with major transcriptional activation, one at the formation of symbiotic cells and another one when bacteroids differentiate [START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF].

Nevertheless, the present analysis is refining this description and shows that NCR genes are activated in at least 3 waves and moreover they can be distinguished by the maintenance or the decline of their expression in the older nodule cell.

NCR genes are not directly involved in nodule senescence

Because of their resemblance to the defensin-type of antimicrobial peptides [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF] and because they have in vitro and in vivo antimicrobial activity (Van de Velde et al., 2010;[START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF][START_REF] Haag | Role of cysteine residues and disulfide bonds on the activity of a legume root nodule-specific, cysteine-rich peptide[END_REF][START_REF] Tiricz | Antimicrobial nodule-specific cysteine-rich peptides induce membrane depolarizationassociated changes in the transcriptome of Sinorhizobium meliloti[END_REF], part of the NCR family could be involved in the killing of the rhizobia during the senescence process of nodules as has been suggested (Nallu et al., 2013). Nodule senescence leads to a complete digestion of the bacteroids and the symbiotic host cells (Van de [START_REF] Van De Velde | Aging in legume symbiosis. A molecular view on nodule senescence in Medicago truncatula[END_REF] and is controlled by the transcriptional activation of a battery of genes which are involved in the regulation of the process, the digestion of macromolecules and the remobilization of the liberated nutrients (Van de [START_REF] Van De Velde | Aging in legume symbiosis. A molecular view on nodule senescence in Medicago truncatula[END_REF]. Senescence of nodules can also be induced artificially by supplying a high concentration of nitrate to roots which prefer this new nitrogen source over the energy costly nitrogen fixation [START_REF] Cabeza | A RNA-Seq transcriptome analysis reveals novel insights into molecular aspects of the nitrate Impact on nodule activity of Medicago truncatula[END_REF] or alternatively by cutting off the nutrient supply to nodules by putting plants in the dark [START_REF] Pérez-Guerra | Comparison of developmental and stress-induced nodule senescence in Medicago truncatula[END_REF] or by applying a herbicide [START_REF] Seabra | Inhibition of glutamine synthetase by phosphinothricin leads to transcriptome reprograming in root nodules of Medicago truncatula[END_REF]. To test whether NCR genes are induced by senescence, we compared nodules at 14 dpi with nodules of 14 dpi followed by 2 days of nitrate supply (Fig. 18A). Another opportunity in the MtGEA database to test the effect of senescence on NCR expression is provided by samples corresponding to a treatment of M. truncatula with the herbicide phosphinothricin, which also induces nodule senescence [START_REF] Seabra | Inhibition of glutamine synthetase by phosphinothricin leads to transcriptome reprograming in root nodules of Medicago truncatula[END_REF]) (Fig. 18B; Fig. 19).

Besides the NCR genes, 8 senescence marker genes were included in the analysis (Table 1) [START_REF] Van De Velde | Aging in legume symbiosis. A molecular view on nodule senescence in Medicago truncatula[END_REF]. Both treatments induced senescence as indicated by the strong activation of the senescence marker genes. However, all the NCR genes without exception are reduced in expression by the senescence-inducing treatments, in agreement with a recent report [START_REF] Cabeza | A RNA-Seq transcriptome analysis reveals novel insights into molecular aspects of the nitrate Impact on nodule activity of Medicago truncatula[END_REF]. The down regulation of NCR gene expression is induced very rapidly, within 4 hours of nitrate application [START_REF] Cabeza | A RNA-Seq transcriptome analysis reveals novel insights into molecular aspects of the nitrate Impact on nodule activity of Medicago truncatula[END_REF] or 8 hours of phosphinothricin treatment (Fig. 19) and occurs before the degeneration of the symbiotic cells and their bacterial symbionts [START_REF] Seabra | Inhibition of glutamine synthetase by phosphinothricin leads to transcriptome reprograming in root nodules of Medicago truncatula[END_REF]. The down-regulation of the NCRs most likely reflects the shutdown of the symbiotic process. This expression pattern suggests that none of the NCR genes has a direct role in senescence.

Figure 18. NCR expression during nodule senescence

(A) Scatter plot of the expression of 334 NCRs (blue) and 8 senescence marker genes (red) in 14 dpi nodules (xaxis) and 16 dpi nodules, which have been treated before harvest with nodule senescence-inducing nitrate for 2 days (y-axis) [START_REF] Benedito | A gene expression atlas of the model legume Medicago truncatula[END_REF]. (B) Scatter plot of the expression of 334 NCRs (blue) and 8 senescence marker genes (red) in 20 dpi nodules (x-axis) and 20 dpi nodules, which were treated for 24h with phosphinothricin before harvest (y-axis) [START_REF] Seabra | Inhibition of glutamine synthetase by phosphinothricin leads to transcriptome reprograming in root nodules of Medicago truncatula[END_REF]. (C) Scatter plot of the expression of 334 NCRs (blue) and 8 senescence marker genes (red) in hand-sectioned nodule zone D (x-axis) and in hand-sectioned nodule zone E (y-axis). The scale of all axes is the same and is 25,000 at maximum. The red line in the three graphs indicates a ratio of 1 between the two conditions that are compared. The probe-sets for the senescence marker genes, encoding cysteine proteinases, a chitinase, a nuclease a nucleoside transporter and a metalnicotinamide transporter, are provided in Table 1.

This conclusion is also confirmed by comparing the hand-dissected nodule samples D and E, which are enriched for the nitrogen fixation zone III, and the senescence zone IV respectively. None of the NCRs is significantly (t-test P<0,05) higher expressed in sample IV (Fig. 18C).

Promoter-GUS analysis and immunolocalization of selected NCRs in nodules

In order to confirm the expression data from MtGEA, stable transgenic M. truncatula R108 lines were generated carrying promoter-GUS fusion constructs for 3 different NCR genes, representing different temporal classes of NCRs and inoculated with Sinorhizobium meliloti strain 1021 or Sinorhizobium arboris strain B554. NCR001 is not activated before the late stages of the nodule formation, NCR084 is slightly induced in early time points (4 dpi) and fully activated at the mature stage of the nodule and finally NCR121 is an early gene, which is already fully activated at 4 dpi. GUS expression in the 3 transgenic lines was not detected in root tips or other root parts (Fig. 20). In agreement with its temporal regulation during nodulation, NCR121 expression was induced in young nodule primordia as early as 5dpi and remained expressed throughout the experiment in the entire infection zone and the fixation zone of mature nodules (Fig. 20). NCR084 expression was detected from 11 dpi on and was mainly confined to the proximal infection zone, the interzone II-III and to the distal part of the fixation zone (Fig. 20). NCR001 expression was detectable from 11 dpi in the developing fixation zone and its expression extends in the following days as the fixation zone is growing (Fig. 20). All 3 genes are only expressed in the symbiotic nodule cells. In older nodules, at 30 dpi, displaying a senescence zone, NCR expression was never detected in the senescing tissues, nor was their expression enhanced in the proximal fixation zone adjacent to the senescent tissue (Fig. 20), confirming that NCR genes are not involved in the senescence process. Overall, the temporal and spatial promoter-GUS expression patterns are in very good agreement with the expression profiles deduced from MtGEA.

The particular expression pattern of NCR122 with its relaxed tissue specificity (Fig. 13F) and its apparent expression in the uninfected nodule cells, together with the availability of an anti-NCR122 antibody prompted us to specifically analyze the localization of the NCR122 peptide in nodules. Immunolocalization of the peptide revealed indeed a specific presence of NCR122 in the uninfected cells of the central zone of a mature nodule as well as in the uninfected cortical cells of the nodule (Fig. 21). Together with the transcriptome data, this indicates that NCR122 and most likely also NCR218 are the only NCR peptides that are specific to uninfected root and nodule cells.

Expression of NCR genes in plant organs

Previously, NCR expression in conditions other than the symbiosis with rhizobium was tested by EST analysis [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF] and with dedicated microarrays [START_REF] Tesfaye | Spatio-temporal expression patterns of Arabidopsis thaliana and Medicago truncatula defensin-like genes[END_REF], indicating the absence of expression. The MtGEA database offers the possibility to extend this analysis to more plant organs and biotic and abiotic stress conditions.

Besides nodules, 8 other plant organs [START_REF] Benedito | A gene expression atlas of the model legume Medicago truncatula[END_REF] were interrogated for NCR expression (Fig. 22) as well as plant treatments with the phytohormones auxin, cytokinin and auxin transport inhibitors [START_REF] Imin | Genome-wide transcriptional analysis of super-embryogenic Medicago truncatula explant cultures[END_REF][START_REF] Rightmyer | Pseudonodule formation by wild-type and symbiotic mutant Medicago truncatula in response to auxin transport inhibitors[END_REF]) (Fig.

23). Figure 22. NCR expression in plant tissues

The expression pattern of 334 NCR probe-sets is shown for nodules, roots, hypocotyls, stems, petioles, leaves, flowers, seed pods and seeds. The graphs for all organs are at the same scale (maximum hybridization signal value is 25,000). The small peaks that can be seen in some of the organ samples correspond to the NCR genes with relaxed expression (Fig. 13).

Interestingly, treatment of roots with the auxin transport inhibitors TIBA or NPA leads to the formation of nodule-like structures [START_REF] Rightmyer | Pseudonodule formation by wild-type and symbiotic mutant Medicago truncatula in response to auxin transport inhibitors[END_REF]. However, in none of these conditions were NCR genes expressed except for the NCR genes with relaxed expression described above.

NCRs resemble the defensin-type of antimicrobial peptides and plant defensins are often expressed to high levels in "infection-sensitive" organs like flowers or seeds. Because of the complete lack of detectable expression of NCRs in these organs (Fig. 22), they most probably do not have a defensive function in these organs. Nevertheless, many non-NCR defensin-like genes were found to be expressed in seeds, potentially involved in their protection [START_REF] Tesfaye | Spatio-temporal expression patterns of Arabidopsis thaliana and Medicago truncatula defensin-like genes[END_REF].

Expression of NCR genes after biotic and abiotic stress

Defensins are also induced during infection with pathogens or during salt and drought stresses [START_REF] De Coninck | Arabidopsis thaliana plant defensin AtPDF1.1 is involved in the plant response to biotic stress[END_REF]Maroti et al., 2011). Therefore, we specifically analyzed how the NCR gene family is expressed during such conditions (Fig. 24) in roots after infection with the pathogenic fungi Phymatotrichopsis omnivora [START_REF] Uppalapati | Global gene expression profiling during Medicago truncatula-Phymatotrichopsis omnivora interaction reveals a role for jasmonic acid, ethylene, and the flavonoid pathway in disease development[END_REF] and Macrophomina phaseolina [START_REF] Mah | Gene expression profiling of Macrophomina phaseolina infected Medicago truncatula roots reveals a role for auxin in plant tolerance against the charcoal rot pathogen[END_REF] and the symbiotic mycorrhizal fungus Glomus intraradices [START_REF] Hogekamp | Laser microdissection unravels cell-type-specific transcription in arbuscular mycorrhizal roots, including CAAT-box transcription factor gene expression correlating with fungal contact and spread[END_REF], the oomycete Aphanomyces euteiches [START_REF] Rey | Nfp, a lysM protein controlling nod factor perception, also intervenes in Medicago truncatula resistance to pathogens[END_REF], the bacterial pathogen Ralstonia solanacearum, the nematode Meloidogyne incognita [START_REF] Damiani | Plant genes involved in harbouring symbiotic rhizobia or pathogenic nematodes[END_REF] and the human enteric bacterial pathogens Escherichia coli O157:H7 and Salmonella enterica which are frequent sources of legume food contamination and are capable of surface and internal colonization of M. truncatula roots [START_REF] Jayaraman | Response of Medicago truncatula seedlings to colonization by Salmonella enterica and Escherichia coli O157:H7[END_REF] (Fig. 25A,B). In addition transcriptomes in M. truncatula cell suspensions treated with the defense response-inducing yeast elicitor or methyl jasmonate signals [START_REF] Naoumkina | Different mechanisms for phytoalexin induction by pathogen and wound signals in Medicago truncatula[END_REF] were also included (Fig. 26C,D). Finally, salt and drought stresses are also reported to induce defensin genes in plants [START_REF] De Coninck | Mining the genome of Arabidopsis thaliana as a basis for the identification of novel bioactive peptides involved in oxidative stress tolerance[END_REF]. Thus transcriptomes in NaCltreated roots [START_REF] Li | An expression database for roots of the model legume Medicago truncatula under salt stress[END_REF] as well as drought stressed roots [START_REF] Zhang | Global reprogramming of transcription and metabolism in Medicago truncatula during progressive drought and after re-watering[END_REF] were analyzed (Fig. 26). Not considering the 5 NCR genes with relaxed specificity, we could not detect expression of any of the NCR genes in all these data sets together (with the possible exception of giant cells formed by the nematode Meloidogyne incognita (Fig. 24B).

Figure 24. Expression of NCR genes during microbial infections and elicitor treatment (A,B) NCR expression in M. truncatula control roots and roots infected with the pathogenic fungi Phymatotrichopsis omnivora [START_REF] Uppalapati | Global gene expression profiling during Medicago truncatula-Phymatotrichopsis omnivora interaction reveals a role for jasmonic acid, ethylene, and the flavonoid pathway in disease development[END_REF] and Macrophomina phaseolina (Mah et al., 2013) and the symbiotic mycorrhizal fungus Glomus intraradices [START_REF] Hogekamp | Laser microdissection unravels cell-type-specific transcription in arbuscular mycorrhizal roots, including CAAT-box transcription factor gene expression correlating with fungal contact and spread[END_REF], the oomycete Aphanomyces euteiches [START_REF] Rey | Nfp, a lysM protein controlling nod factor perception, also intervenes in Medicago truncatula resistance to pathogens[END_REF], the bacterial pathogen Ralstonia solanacearum, the nematode Meloidogyne incognita [START_REF] Damiani | Plant genes involved in harbouring symbiotic rhizobia or pathogenic nematodes[END_REF] and the human enteric bacterial pathogens Escherichia coli O157:H7 and Salmonella enterica which are frequent sources of legume food contamination and are capable of surface and internal colonization of M. truncatula roots [START_REF] Jayaraman | Response of Medicago truncatula seedlings to colonization by Salmonella enterica and Escherichia coli O157:H7[END_REF]. (C,D) NCR expression in a M. truncatula cell suspension derived from root cells (CS) after treatments for 2h or 24h withthe defense response-inducing signals methyl jasmonate (500 !M) (MJ) and yeast elicitor (50 !g glucose equivalents ml -1 ) (YE) [START_REF] Naoumkina | Different mechanisms for phytoalexin induction by pathogen and wound signals in Medicago truncatula[END_REF]. Several NCRs showed a hybridization signal in giant cells although the level was about 1 to 2 orders of magnitude lower than the signal in nodules for the same NCR gene.

However, it should be noted that the giant cells were isolated by LCM and that the array hybridization was performed with an amplified cDNA sample [START_REF] Damiani | Plant genes involved in harbouring symbiotic rhizobia or pathogenic nematodes[END_REF], which could be a source of background hybridization. In any case, besides the possible exception of the giant cells, the data indicate that the NCR genes seem not to be used by the plant to control infections other than the rhizobium bacteria in nodules.

Promoter-GUS analysis of NCR expression during pathogenic interactions

We used the 3 NCR promoter-GUS reporter lines to confirm the absence of NCR expression during pathogenic responses and to complement these observations. The MtGEA dataset includes M. truncatula responses to root pathogens and therefore we analyzed leaf or stem pathogens that encompass also other trophic interactions and infection strategies.

Inoculation of M. truncatula leaflets with the necrotrophic soft rotting bacterium Dickeya dadantii3937 induced maceration symptoms from 1dpi on, but failed to induce NCR expression (Fig. 26). Similarly infiltration of the virulent strain Pseudomonas syringae pv. tomato DC3000 (Pst) induced necrosis in the infiltrated zone within 2 dpi, whereas the hrcC mutant strain that is unable to form a functional type three secretion system (TTSS) did not induce any visible reaction (Fig. 26).

Figure 26. Promoter-GUS analysis of NCR genes in infected and wounded leaves

Wild-type M. truncatula R108 and transgenic R108 carrying NCR promoter-GUS fusions or a constitutive GUS under the control of the 35S promoter were mock infected, infected with Dickeya dadantii 3937 (Dd 3937), Pseudomonas syringae pv. tomato DC3000 (Pst DC3000), the Pseudomonas syringaepv. tomato DC3000 hrcC TTSS mutant (Pst DC3000 hrcC) or Botrytis cinerea (B. cinerea), or wounded and then stained for GUS activity, which is observed as blue color. Note that in certain leaves, the leaf base and the veins are faintly stained blue. Also the wounding print is slightly stained (arrows). This staining is also observed in untransformed R108 and thus represents background signal. The left panels show the leaf phenotype after the treatment and before the staining procedure. Scale bars are 2 mm. NCR expression was not detected in the infiltrated leaflets in either condition (Fig. 26). Although Pst DC3000 is not described as a natural pathogen of M. truncatula, the necrosis induced by the wild-type strain and its absence in the presence of the TTSS mutant suggest that at least some bacterial effector proteins can be specifically recognized by the plant resulting in a necrotic response similar to the hypersensitive response it provokes on non-host Nicotiana benthamiana plants [START_REF] Hann | Early events in the pathogenicity of Pseudomonas syringae on Nicotiana benthamiana[END_REF]. Similarly, inoculation of the same M. truncatula lines with the necrotrophic polyphagous grey mold-causing fungus Botrytis cinerea yielded typical symptoms at 7 dpi without inducing any detectable NCR expression (Fig. 26). The results of our pathoassays are also in line with a recent study showing that NCR expression was not detected during the compatible interaction of M. truncatula with the hemibiotrophic leaf pathogen Colletotrichum trifolii or with the biotrophic soil pathogen Phytophthora medicaginis [START_REF] Tesfaye | Spatio-temporal expression patterns of Arabidopsis thaliana and Medicago truncatula defensin-like genes[END_REF]. Altogether, our data and the study from [START_REF] Tesfaye | Spatio-temporal expression patterns of Arabidopsis thaliana and Medicago truncatula defensin-like genes[END_REF] are in agreement with the MtGEA dataset and broaden the conclusion that NCRs are not involved in pathogen responses, whatever the trophic (bio-, hemibio-or necrotrophic) interaction, the host or non-host status and the outcome of the interaction (disease or resistance). Finally, as herbivory and more generally wounding may induce plant defenses around the wounded zone, we also tested the effect of mechanical wounding on NCR expression but again no NCR expression could be detected in the wounded leaflet (Fig. 26).

NCR genes have very high tissue specificity as measured by Shannon entropy

The above analyses reveal an extreme specificity in expression for the NCR gene family: the genes are only expressed in nodules and not in any other organ or physiological condition. To express this specificity quantitatively and to compare it to other types of specifically expressed genes, the complete MtGEA probe-set was analysed and their Shannon entropy was calculated. Shannon entropy is a metric for characterizing the uniformity of the expression pattern of a gene over the tested conditions [START_REF] Schug | Promoter features related to tissue specificity as measured by Shannon entropy[END_REF]. Low entropy values indicate high tissue specificity while high entropy levels characterize ubiquitous expression.

Ten different tissues were taken into consideration: leaf, petiole, stem, bud, flower, seed, pod, root, nodule and mycorrhiza. For nodule, seed and root the mean value of different developmental stages or experiments was used (for "nodule": 4, 10, 14 and 28 dpi stages; for "seed": 10, 12, 16, 20, 24 and 36 dap; for "root" the 0 dpi control for nodulation and an independent experiment); thus in total 19 experiments were used. The Shannon entropy was calculated as described (see materials and methods) (Shug et al., 2005;[START_REF] Zhang | Genome-wide high-resolution mapping and functional analysis of DNA methylation in Arabidopsis[END_REF] for each of the 50,900 probe-sets using these 10 tissue datasets. The 9000 probe-sets with the lowest entropy (and therefore the highest tissue specificity) were selected for further analysis (Table 2; http://www.biomedcentral.com/content/supplementary/1471-2164-15-712-S3.xlsx). A hierarchical cluster analysis of these 9000 genes was calculated (Fig. 27A).

Clusters of tissue specific genes can be distinguished for root, seed, pod, flower, aerial tissues (l/p/s/b), root tissues (r/n/m), nodule and mycorrhiza. The seed, flower, mycorrhiza and especially the nodule clusters are enriched in genes with low entropy value E g (Fig. 27A). The strong enrichment of nodule-specific genes among the low entropy genes becomes more obvious when re-ordering the dataset according the increasing entropy levels (Fig. 27B). This analysis shows that in M. truncatula the genes with the lowest entropy and thus the highest tissue specific expression are mostly nodule-specific genes and to a lesser extent, seed-and flower-specific genes. Strikingly, among the nodule-specific genes, the NCRs are the most represented ones (Fig. 27B). The expression heat map is in green-black-red colour scheme. The entropy heat map is in blue-black-yellow scheme. (B) The data set of panel (A) was ordered according to increasing entropy of the genes. The left panel shows the relative expression level of the 9,000 genes in the 10 tissues (green-black-red heat map) and the entropy values E g (blue-black-yellow heat map). The location of the NCR genes is indicated with the black-red heat map (NCR): red means NCR, black means other gene type. The right panel is an enlargement for the first 1,800 genes (entropy values E g from 0.07 to 1.64). The scale bar for the expression level heat map is green: 0, black: 0.5 and red: 1 and for the entropy heat map blue: 0, black 1.44 and yellow: 2.88 (maximum entropy in the complete data set is 3.32 (log 2 (N)).

Although the NCR family is by far the most represented among the nodule-specific low entropy genes, many other known nodule specific genes have very low entropy (Table 2; http://www.biomedcentral.com/content/supplementary/1471-2164-15-712-S3.xlsx). These include for example leghemoglobin genes, the glycine-rich peptide (GRPs) genes [START_REF] Kevei | Glycine-rich proteins encoded by a nodule-specific gene family are implicated in different stages of symbiotic nodule development in Medicago spp[END_REF][START_REF] Alunni | Genomic organization and evolutionary insights on GRP and NCR genes, two large nodule-specific gene families in Medicago truncatula[END_REF], the small nodulin acidic RNA-binding protein (SNARP) gene family [START_REF] Laporte | A novel RNA-binding peptide regulates the establishment of the Medicago truncatula-Sinorhizobium meliloti nitrogen-fixing symbiosis[END_REF], genes encoding a small family of secretory calmodulin-like proteins [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF][START_REF] Liu | Recruitment of novel calcium-binding proteins for root nodule symbiosis in Medicago truncatula[END_REF], the DNF2 gene involved in suppression of defence responses in the symbiotic cells [START_REF] Bourcy | Medicago truncatula DNF2 is a PI-PLC-XDcontaining protein required for bacteroid persistence and prevention of nodule early senescence and defense-like reactions[END_REF] and others. Most interestingly, also putative retrotransposons (probe-sets Mtr.9294.1.S1_at and Mtr.636.1.S1_at) and a Dicer 1-like ribonuclease III gene (probe-set Mtr.41531.1.S1_at) are among the nodule specific low entropy genes (Table 2; http://www.biomedcentral.com/content/supplementary/1471-

2164-15-712-S3.xlsx).

Besides the high tissue-specificity, another aspect of expression in which the NCRs stand out from the average M. truncatula genes is the strength of expression. We used the hybridization signal on the microarrays as a proxy to strength of gene expression. For each of the 50,900 probe-sets in MtGEA, we searched for its maximal signal in the 267 experiments.

These hybridization signals vary from 33,500 for the strongest expressed gene to 9 (background) for the weakest. One percent of the probe-sets have an expression level higher than 15,000, 3% higher than 10,000 and 10% higher than 5,000. The mean signal is 1,687 and the median 459 (Fig. 28A). The same analysis on the subset of probes corresponding to the NCR genes gives a completely different picture: 5% of NCR genes have signals above 15,000, 30% above 10,000 and 75% above 5,000 with a mean signal of 7,982 and a median of 7,758 (Fig. 28B). Thus, the NCR genes are among both the most specific and the most strongly expressed genes in the genome of M. truncatula.

Discussion

The expression of NCR genes has been studied in M. truncatula mostly but also in some other IRLC legumes at the level of individual genes by RT-PCR, in situ hybridization, immuno-localization and promoter-marker gene fusions or at the family level by ESTanalysis, macroarrays, dedicated microarrays or whole-genome microarrays [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF][START_REF] Alunni | Genomic organization and evolutionary insights on GRP and NCR genes, two large nodule-specific gene families in Medicago truncatula[END_REF][START_REF] Van De Velde | Plant peptides govern terminal differentiation of bacteria in symbiosis[END_REF][START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF][START_REF] Moreau | Transcrioptional reprogramming during root nodule development in Medicago truncatula[END_REF][START_REF] Tesfaye | Spatio-temporal expression patterns of Arabidopsis thaliana and Medicago truncatula defensin-like genes[END_REF]Nallu et al., 2013;[START_REF] Farkas | The Medicago truncatula symbiotic peptide NCR247 contributes to bacteroid differentiation through multiple mechanisms[END_REF].These studies detected NCR expression only in nodules and in no other tested tissues. By mining publicly available whole-genome transcriptome data, we have extended this analysis of NCR gene expression to a very large number of conditions, together covering most plant organs as well as different growth conditions including biotic and abiotic stresses. As a whole, our study suggests that, apart from 5 genes, all NCRs are only expressed in nodules. Moreover, quantifying the specificity of expression with the Shannon entropy factor reveals that the NCR genes, and more generally, nodule specific genes are among the most specifically expressed genes in M. truncatula. This suggests thus that nodulation in Medicago is in large part depending on genes solely dedicated to this symbiotic process. These genes may be resulting from gene duplications followed by neo-functionalization (for example the DNF2 protein which has non-symbiotic homologues) or they may be unique for the symbiosis (possibly the NCRs, GRPs, SNARPs and others). In addition to that, the expression of the NCR genes in nodules reaches very high levels. Even if certain NCR genes are expressed at a low level, the majority of them are among the highest expressed genes in the whole genome of Medicago. This is in agreement with our previous estimation, based on EST counts, that all NCR mRNAs together constitute almost 5% of the total mRNA population in nodules [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF].

In accordance with their resemblance to antimicrobial peptides of the innate immunity such as defensins, many NCR peptides, in particular the most cationic ones, have a strong antimicrobial activity against a diversity of bacteria as well as fungi (Van de Velde et al., 2010;[START_REF] Tiricz | Antimicrobial nodule-specific cysteine-rich peptides induce membrane depolarizationassociated changes in the transcriptome of Sinorhizobium meliloti[END_REF][START_REF] Farkas | The Medicago truncatula symbiotic peptide NCR247 contributes to bacteroid differentiation through multiple mechanisms[END_REF]. Despite this, the NCR genes are not expressed in any of the pathogenic interactions of Medicago tested here or by [START_REF] Tesfaye | Spatio-temporal expression patterns of Arabidopsis thaliana and Medicago truncatula defensin-like genes[END_REF]. This included interactions with bacteria, fungi, oomycetes and nematodes. They are also not expressed in organs like leaves, seeds and flowers which often express high levels of innate immunity antimicrobial peptides [START_REF] Sels | Plant pathogenesisrelated (PR) proteins: a focus on PR peptides[END_REF]. Therefore, it seems that the NCR peptides have no function in innate immunity. In situ detection of NCR expression has demonstrated for all the tested genes that they are specifically expressed in the symbiotic nodule cells but different subsets of NCR genes are activated at different stages of differentiation of these host cells [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF][START_REF] Van De Velde | Plant peptides govern terminal differentiation of bacteria in symbiosis[END_REF][START_REF] Farkas | The Medicago truncatula symbiotic peptide NCR247 contributes to bacteroid differentiation through multiple mechanisms[END_REF]This work).

Transcriptome analysis extended this pattern to the whole family. The NCR genes are not activated by Nod factors or during the very early stages of the nodule organogenesis when infected cells are not yet formed [START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF]Nallu et al., 2013;This work).

During the development of wild-type nodules, they are activated in consecutive waves and their first appearance coincides with the formation of infected symbiotic cells [START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF]. We show here that NCR genes are activated during nodule development in at least 3 temporal waves corresponding to specific spatial expression patterns. Genes activated early in nodule development are expressed in the more distal nodule parts (close to the apex) while genes activated late during development are expressed in the proximal nodule tissues. In addition, certain clusters of genes, once activated, maintain their activity when the tissues grow older while other clusters are characterized by a decline of their expression in the older nodule cells. Our spatial analysis of NCR expression is in strong agreement with a recently published study [START_REF] Roux | An integrated analysis of plant and bacterial gene expression in symbiotic root nodules using laser-capture microdissection coupled to RNA sequencing[END_REF] that used LCM of nodule zones coupled to RNA-Sequencing (Fig. 29)

Figure 29. Comparison of the spatial and temporal NCR expression profiles (A)

The expression profiles for the NCR gene set described in this study were extracted from the data obtained by laser-capture microdissection coupled to RNA-Seq from the website https://iant.toulouse.inra.fr/symbimics [START_REF] Ovchinnikova | IPD3 controls the intracellular accommodation of rhizobia in pea and Medicago[END_REF]. FI corresponds to meristem, FIID to the distal zone II or infection zone, FIIP to the proximal zone II, IZ to the interzone between the infection zone and the fixation zone and ZIII to the fixation zone. Clustering was performed using Pearson correlation. (B) The corresponding profiles obtained by handdissection of nodules in this study. Samples I, II, II-III, III and IV correspond to the nodule tissues from the most apical part of the nodule with the youngest symbiotic cells to the most proximal part containing the oldest symbiotic cells. (C) The corresponding profiles obtained by laser-capture microdissection coupled to affymetrix microarray analysis described by [START_REF] Limpens | Cell-and tissue-specific transcriptome analyses of Medicago truncatula root nodules[END_REF]. M corresponds to meristem, dIZ to the distal zone II or infection zone, pIZ to the proximal zone II, IC to infected cell and UC to uninfected cell. (D) The corresponding profiles in function of nodule age (dpi). For all data sets, the expression patterns in the different dissected nodule zones are expressed in percentage from their total. The NCR genes are ordered identically in all 4 panels according to a hierarchical clustering of the dataset from panel (A).

Transcriptome analysis of non-functional nodules that are formed by bacterial or plant symbiotic mutants and that are arrested at different stages of nodule development, is also in agreement with specific expression of all NCR genes in the symbiotic nodule cells: their transcriptional activation is only observed when polyploid symbiotic cells are formed in the mutant nodules [START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF]. For example, in nodules of the M. truncatula TE7 mutant which is affected in the IPD3 gene [START_REF] Horvath | Medicago truncatula IPD3 is a member of the common symbiotic signaling pathway required for rhizobial and mycorrhizal symbioses[END_REF][START_REF] Ovchinnikova | IPD3 controls the intracellular accommodation of rhizobia in pea and Medicago[END_REF] and in nodules infected by the S. meliloti exoY mutant, no infected and polyploid symbiotic cells are formed and these nodules do not express any of the NCR genes [START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF]. Conversely, in nodules infected by the S. meliloti bacA mutant which contain symbiotic cells with undifferentiated bacteroids, only a subset of NCR genes is activated while in other mutants, forming normal symbiotic cells with differentiated bacteroids, NCR genes are activated to a similar extent as in the wild-type [START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF]Nallu et al., 2013). Together, the expression pattern of all the tested NCR genes suggests that the endosymbiotic rhizobia in the symbiotic nodule cells are the only targets of the peptides.

However, the distinct spatio-temporal profiles clearly suggest that NCR peptides have many different roles. Subsets of NCR genes that are expressed during the early stages of symbiotic cell formation might be involved in the elongation and polyploidization of the bacteroids while other subsets that are active in later stages of symbiotic cell formation or even after the completion of the symbiotic cell differentiation might have other functions in the bacteroids.

Moreover, we find that the expression of the NCRs is shut down when nodule senescence is activated, meaning that the antimicrobial NCR peptides have no direct role in the lysis and digestion of the bacteroids that is taking place during senescence of nodules.

Very little is known about how the very specific regulation of NCRs is achieved. Since their expression is correlated with bacterial infection of the symbiotic cells, the perception of bacterial signals such as components of the bacterial envelope could be involved. The transcription factor EFD, belonging to the ethylene response factor family, may control, directly or indirectly, the expression of a subset of NCR genes since a mutant forms nodules in which some of the NCR genes are down regulated and in which bacteroid differentiation is partially impaired [START_REF] Vernié | EFD Is an ERF transcription factor involved in the control of nodule number and differentiation in Medicago truncatula[END_REF]. The IPD3 protein is another transcription factor that might be involved, directly or more likely indirectly, in the regulation of the NCR genes and the other symbiotic cell specific genes [START_REF] Singh | CYCLOPS, a DNA-binding transcriptional activator, orchestrates symbiotic root nodule development[END_REF][START_REF] Limpens | CYCLOPS: a new vision on Rhizobium-induced nodule organogenesis[END_REF]. Indeed, in the M. truncatula ipd3 mutant nodules, the symbiotic cells do not form and the symbiotic cell specific genes, including the NCRs, are not activated [START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF].

In agreement with this, the IPD3 gene is expressed in the whole nodule and its expression domain overlaps with the NCR expression zone [START_REF] Messinese | A novel nuclear protein interacts with the symbiotic DMI3 calcium-and calmodulin-dependent protein kinase of Medicago truncatula[END_REF].

Searching for potential cis-elements in the promoters of NCR genes with different algorithms yielded 5 different conserved motifs of 41 to 50 bp, which are specifically enriched in the 1000 bp promoter regions (Nallu et al., 2013). Some of these motifs show resemblance to previously described motifs conferring nodule-specific gene expression.

However, the role, if any, of these motifs in the remarkable expression pattern of the NCR genes needs further investigation. Interestingly, some of these motifs include Auxin Response Factor binding sites that may suggest a role for auxin in NCR regulation (Nallu et al., 2013).

However, MtGEA dataset from auxin treated seedlings do not show any NCR induction, pointing out a more complex regulatory mechanism controlling NCR expression.

The very tight regulation of the NCR genes that was revealed here might indicate that besides cis-and trans-acting factors, regulation at the level of chromatin might also be involved in the activation of the NCR genes. Moreover, endoreduplication seems to be a prerequisite for their activation [START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF] and might thus, by a still unknown mechanism, be implicated in the activation of this gene family. In that respect, it is interesting to note that the giant feeding cells induced by the nematode M. incognita are highly polyploid cells and appear to express faintly a few NCR genes. Nevertheless, this observation could be an experimental artefact and will require further experimental confirmation.

Genes with high tissue-specific expression are often actively silenced during most of the plant growth by epigenetic mechanisms. Since in M. truncatula the nodule-specific genes display the highest level of expression specificity, it might be worthwhile to investigate if epigenetic control is important in the regulation of the symbiotic cell-specific genes. The nodule-specific expression of putative retrotransposons, which are usually epigenetically silenced, and the Dicer 1-like ribonuclease III gene, which may have a role in epigenetic regulation, as well as the identification of small RNAs potentially targeting NCR genes [START_REF] Lelandais-Brière | Genome-wide Medicago truncatula small RNA analysis revealed novel microRNAs and isoforms differentially regulated in roots and nodules[END_REF] are all in agreement with such an epigenetic control of the symbiotic cell-specific genes.

Plant genomes contain large numbers, several hundreds to thousands, of resistance genes (R) of the NB-LRR family that recognize specific pathogen effectors and trigger resistance. Silencing by microRNAs has been proposed as a mechanism to avoid unregulated expression of Rgenes, which may be a threat to the plant and represent a fitness cost (Li et al., 2012). Why M. truncatula maintains such a large repertoire of NCR genes is not known. It is

Introduction

Legumes, thanks to their ability to develop a symbiotic interaction with nitrogen fixing bacteria, collectively called rhizobia, are among the agronomically and ecologically most important plants. This symbiosis results in the formation of new organs, the nodules, inside which the bacteria fix nitrogen and transfer the ammonium to the plant. Two distinct phases can be observed during the establishment of a functional nodule, the early phases that lead to nodule organogenesis and the release of the bacteria in the host cells and, the late phases, in which the rhizobia differentiate into an endosymbiotic form, the bacteroids, able to fix atmospheric nitrogen. During this second phase, profound modifications of the metabolism of the rhizobia are observed and this can be accompanied by a marked change in the bacterial cell shape and size [START_REF] Arnold | Partial complementation of Sinorhizobium meliloti bacA mutant phenotypes by the Mycobacterium tuberculosis BacA protein[END_REF]. Three different bacteroid morphotypes have been observed in different legume species (Oono et al., 2010;[START_REF] Kondorosi | A paradigm for endosymbiotic life: Cell differentiation of Rhizobium bacteria provoked by host plant factors[END_REF]: i) elongated or E-morphotype bacteroids described in legumes of the Inverted Repeat-Lacking Clade (IRLC) (Medicago, Pisum, Vicia, …) and some Aeschynomene species such as A. afraspera, ii) enlarged, spherical bacteroids (S-morphotype) encountered in some species of the Dalbergoid clade (such as Aeschynomene indica, Aeschynomene evenia and Arachis hypogaea) and iii) unmodified bacteroids (U-morphotype) which display a rod-shape morphology similar to free-living bacteria found in e.g. Phaseoloid or Robinoid legumes (i.e.

Phaseolus, Vignia, Lotus, Glycine, Sesbania). The fact that the same rhizobium strain nodulating legumes of different clades can display different morphotypes is strong evidence for the host plant governing bacteroid morphotype [START_REF] Sen | A basis for different rates of N 2 -fixation by the same strains of Rhizobium in peanut and cowpea root nodules[END_REF][START_REF] Mergaert | Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis[END_REF]Oono et al., 2010). The change of shape is probably the tip of the iceberg of the control exerted by the plant of the bacterial physiology during symbiosis. Indeed, besides their Emorphotype, the Sinorhizobium meliloti bacteroids in Medicago truncatula differ from their free-living state in several aspects, they become polyploid, their membrane permeability dramatically increases and they lose their reproductive capacity. Bacteroids that display such drastic changes are considered terminally differentiated because they are unable to revert to their bacterial form [START_REF] Mergaert | Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis[END_REF].

In Medicago truncatula, a class of peptides, named NCRs (Nodule-specific Cysteine-Rich peptides) plays a key role in this terminal bacteroid differentiation (Van de Velde et al., 2010). The M. truncatula NCR family is composed of about 600 highly divergent genes, which are nearly all exclusively expressed in nodules [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF][START_REF] Alunni | Genomic organization and evolutionary insights on GRP and NCR genes, two large nodule-specific gene families in Medicago truncatula[END_REF][START_REF] Branca | Whole-genome nucleotide diversity, recombination, and linkage disequilibrium in the model legume Medicago truncatula[END_REF]. Within nodules, the NCR peptides are only produced by the infected symbiotic cells and are transported to the bacteroid-containing symbiosomes. NCRs are similar to the defensin-type of antimicrobial peptides. The peptides contain an N-terminal secretion signal, the mature peptides are usually no longer than 60 amino acids and have 4 to 6 conserved cysteine residues [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF]. The cleavage of the signal peptide by a nodule-specific signal peptidase complex (SPC) located in the endoplasmic reticulum is required to permit the trafficking of NCR peptides to the symbiosome compartment.

Bacteroid and symbiosome development is blocked in a M. truncatula dnf1 mutant that is affected in the SPC22 subunit of this nodule-specific SPC [START_REF] Wang | A nodule-specific protein secretory pathway required for nitrogen-fixing symbiosis[END_REF][START_REF] Van De Velde | Plant peptides govern terminal differentiation of bacteria in symbiosis[END_REF]. Some NCR peptides have been found to have antimicrobial activity, killing rhizobium when applied at high concentration (Van de Velde et al., 2010). However, these antimicrobial peptide-like NCRs can at lower concentration induce typical features of Emorphotype bacteroids in vitro on cultured rhizobium or in planta when expressed in transgenic Lotus japonicus plants which form normally U-morphotype bacteroids (Van de Velde et al., 2010). The NCR gene family has been identified in all investigated legume species of the IRLC clade but in no other plant species outside of this clade. This raises the question about the nature of the factors used by the other legumes to control bacteroid metamorphosis.

In the Dalbergoid legume clade, bacteroids can be of the E-or S-morphotype. For example, within the Aeschynomene genus, A. afraspera has E-type bacteroids but A. indica or A. evenia have S-type bacteroids [START_REF] Bonaldi | Nodulation of Aeschynomene afraspera and A. indica by photosynthetic Bradyrhizobium Sp. strain ORS285: the nod-dependent versus the nod-independent symbiotic interaction[END_REF][START_REF] Arrighi | Aeschynomene evenia, a model plant for studying the molecular genetics of the nod-independent Rhizobium-legume symbiosis[END_REF]. The bacteroid morphotype in the Aeschynomene genus is also under plant control since the Bradyrhizobium strain ORS285 which can nodulate A. afraspera and A. indica or A. evenia transforms into Emorphotype bacteroids in nodules of the former legume and in S-morphotype bacteroids in the latter two legumes [START_REF] Bonaldi | Nodulation of Aeschynomene afraspera and A. indica by photosynthetic Bradyrhizobium Sp. strain ORS285: the nod-dependent versus the nod-independent symbiotic interaction[END_REF][START_REF] Arrighi | Aeschynomene evenia, a model plant for studying the molecular genetics of the nod-independent Rhizobium-legume symbiosis[END_REF]. The Aeschynomene symbiosis with Bradyrhizobium sp. has several other particular features, including the formation of nodules on both roots and stems, photosynthesis by the Bradyrhizobium symbionts which is required for optimal stem nodulation [START_REF] Giraud | Effect of Bradyrhizobium photosynthesis on stem nodulation of Aeschynomene sensitiva[END_REF] and the use of a Nod factor independent nodulation pathway in some Aeschynomene species including A. evenia and A. indica [START_REF] Giraud | Legumes symbioses: Absence of Nod genes in photosynthetic Bradyrhizobia[END_REF][START_REF] Arrighi | Aeschynomene evenia, a model plant for studying the molecular genetics of the nod-independent Rhizobium-legume symbiosis[END_REF]. Furthermore, in contrast to classical determinate and indeterminate nodules, aeschynomenoid nodules originate from the successive divisions of only one or a few root cortical cells initially infected by an infection thread independent mechanism [START_REF] Bonaldi | Nodulation of Aeschynomene afraspera and A. indica by photosynthetic Bradyrhizobium Sp. strain ORS285: the nod-dependent versus the nod-independent symbiotic interaction[END_REF]. This infection mechanism has two consequences: i) all the nodule primordium cells are infected and ii) all the nodule primordium cells and bacteria are synchronized. Thus, during nodule development of A. indica nodules, the change of bacteroid shape occurs simultaneously for all nodule cells and takes place between the 4 th and 5 th day after inoculation [START_REF] Bonaldi | Nodulation of Aeschynomene afraspera and A. indica by photosynthetic Bradyrhizobium Sp. strain ORS285: the nod-dependent versus the nod-independent symbiotic interaction[END_REF]. The possibility to obtain sampling of homogenous materials at different differentiation stages and the existence of two distinct bacteroid morphotypes in closly related species make the Bradyrhizobium-Aeschynomene symbiotic couples an appealing model system for unraveling the mechanisms of bacteroid morphotype determination in legume species outside the IRLC clade.

In this study, we show that E-and S-morphotype bacteroids from A. afraspera and A.

indica nodules display typical features of terminal differentiation. Furthermore, in these two Aeschynomene species, we provide evidences of the presence of a NCR-like peptide family which is specifically produced in the nodules and targets the bacteroids. Alteration of the trafficking of these peptides by silencing the dnf1 homolog identified in A. evenia correlated with the absence of bacterial differentiation indicating that the NCR-like peptides identified in Aeschynomene species play a similar role to those described in Medicago. Altogether, these results suggest a convergent evolution of the host molecular mechanisms governing bacteroid differentiation in legumes.

Results

E and S-bacteroids in Aeschynomene species are terminally differentiated

Previous studies revealed that Bradyrhizobium strain ORS285 displayed two distinct bacteroid morphotypes depending on the Aeschynomene host: the E-morphotype in A. indica and the S-morphotype in A. afraspera (Fig. 30A) [START_REF] Bonaldi | Nodulation of Aeschynomene afraspera and A. indica by photosynthetic Bradyrhizobium Sp. strain ORS285: the nod-dependent versus the nod-independent symbiotic interaction[END_REF]. As this difference in shape could imply different properties, we analyzed in each bacteroid type, isolated from mature nodules at 14 days post inoculation (dpi), several characteristics that have been shown to change during bacteroid metamorphosis in Medicago, i.e. the bacterial DNA content, the membrane permeability and the viability. As revealed by flow cytometry analysis (Fig. 30B), the DNA content of E-and S-bacteroids peaked respectively at around 7C and 16C in comparison to the 1C/2C DNA content of free-living bacteria. These ploidy levels are lower than that described for S. meliloti bacteroids (24C) (Mergeart et al., 2006) although other studies have shown lower ploidy levels also in Medicago [START_REF] Sinharoy | The C2H2 transcription factor regulator of symbiosome differentiation represses transcription of the secretory pathway gene VAMP721a and promotes symbiosome development in Medicago truncatula[END_REF], Berrabah et al., 2014).

Bacteroid membrane integrity was estimated using PI, a DNA stain that is excluded from living cells but penetrates cells displaying impaired membrane integrity (Mergeart et al., 2006). Fluorescence images of the PI-stained bacteroids showed that the membrane permeability of E-and S-morphotype bacteroids was greater than that of the free-living bacteria (Fig. 30A). Indeed, 88% of S-bacteroids and 95% of E-bacteroids were found to be permeable to PI compared with 7% for free-living bacteria. Whereas, PI is known to stain about 50% of S.

meliloti bacteroids (Mergeart et al., 2006). The loss of viability measured for S-bacteroids from A. indica nodules (98%) was also comparable to the one previously estimated for S.

meliloti bacteroids (99%) (Mergeart et al., 2006). In contrast, intracellular bacteroids of A.

afraspera appeared more viable as 22 % formed colonies on agar plates. However, this higher value could result to the fact that A. afraspera nodules displayed the unusual presence of two infected tissues, a large central tissue in which the bacteria elongated and a superficial tissue in which the shape of the bacteria remained unmodified [START_REF] Bonaldi | Nodulation of Aeschynomene afraspera and A. indica by photosynthetic Bradyrhizobium Sp. strain ORS285: the nod-dependent versus the nod-independent symbiotic interaction[END_REF].

The intracellular cells extracted from nodules consisted therefore of a mixture of differentiated and non-differentiated bacteria suggesting that the loss of viability of Ebacteroids could be far greater. Taken together, we observed that the E-and S-bacteroids from Aeschynomene nodules share the same features as the S. meliloti bacteroids in Medicago and therefore, they could be considered as terminally differentiated.

Distribution of S-and E-morphotype bacteroids among Aeschynomene species

To obtain more insight into the distribution of the E-and S-bacteroid morphotypes among the Aeschynomene species, we analyzed by confocal microscopy the shape of intracellular bacteria from mature nodules of various Aeschynomene species elicited by the ORS285 strain tagged with GFP. Unlike the other photosynthetic Bradyrhizobium strains, such as ORS278 and BTAi1, this bacterium contains the canonical nodABC genes and displays a broader host range due to its ability to use a Nod-dependent and a Nod-independent symbiotic process depending on the host plant [START_REF] Bonaldi | Nodulation of Aeschynomene afraspera and A. indica by photosynthetic Bradyrhizobium Sp. strain ORS285: the nod-dependent versus the nod-independent symbiotic interaction[END_REF]. We observed in all tested species using a Nod-independent process (A. indica, Aeschynomene evenia ssp.

serrulata, A. evenia. ssp. evenia, Aeschynomene virginica, Aeschynomene scabra, and Aeschynomene sensitiva) that the bacteroids displayed an S-morphotype whereas in the three Nod-dependent species tested (A. afraspera, Aeschynomene nilotica, Aeschynomene aspera)

the bacteroids displayed an E-morphotype (Fig. 31). We also analyzed the bacteroid shape in another group of Aeschynomene species not nodulated by the ORS285 strain but by nonphotosynthetic Bradyrhizobium strains containing nod genes such as ORS301, ORS302 and ORS305 [START_REF] Molouba | Photosynthetic bradyrhizobia from Aeschynomene spp. are specific to stem-nodulated species and form a separate 16S ribosomal DNA restriction fragment length polymorphism group[END_REF]. We also observed in these species (Aeschynomene americana, Aeschynomene schimperi, Aeschynomene pfundii) that the bacteroids, stained using the live/dead staining procedure, displayed an E-morphotype. Altogether these data indicate that S-morphotype bacteroids are specific for the Aeschynomene species using a Nodindependent symbiotic process, while the E-morphotype is specific to the Aeschynomene using a Nod-dependent one.

Aeschynomene species contain a new class of nodule-specific cysteine-rich peptides-encoding genes

Our results indicate that in Aeschynomene nodules, the endosymbiotic bacteria undergo terminal differentiation similar to that described in nodules of legume species belonging to the IRLC clade. This raises the question if NCR peptides are also recruited in Aeschynomene to govern this differentiation step. To check this possibility, we analyzed 4 expressed sequence tag (EST) libraries previously constructed in our laboratory and corresponding to non-inoculated roots and mature nodules from A. afraspera and A. indica plants inoculated with ORS285 strain. Although the total number of cDNAs sequenced for each library was relatively small, around 9500 ESTs/library (Table 3; Annex 1), we postulated that if NCR peptides were also involved in bacteroid differentiation in Aeschynomene, they should be highly expressed specifically in the nodules and hence easily detectable. We first performed a BLAST search on the EST libraries using several M.

truncatula NCR genes as a query but no conclusive results were obtained. Considering that NCR genes are rapidly evolving and are highly diverse, even within M. truncatula [START_REF] Alunni | Genomic organization and evolutionary insights on GRP and NCR genes, two large nodule-specific gene families in Medicago truncatula[END_REF][START_REF] Branca | Whole-genome nucleotide diversity, recombination, and linkage disequilibrium in the model legume Medicago truncatula[END_REF], we re-analyzed the available EST databases using the following parameters determined from typical features of the NCR family: i) candidate genes should encode small proteins (fewer than 100 aa) containing a signal peptide, ii) the gene expression should be nodule specific with a significant read count in the EST database (we arbitrarily fixed a lower limit of 5 reads) and iii) the candidate genes should encode peptides rich in Cys residues (at least 4). This second in silico analysis has permitted identification of 15 genes encoding putative NCR-like peptides from the A. afraspera nodule library and 18 from the library of A. indica nodules (Table 4 grey shaded; Annex 2). These genes were named Aa-or AiNCRs and numbered according their rank in read counts. The deduced mature peptides (i.e. without their signal peptide) are between 37 and 85 residues long and contain 4

to 10 Cys (Table 4; Annex 2). The alignment of these NCR-like peptides highlighted, for 31 out of the 33 sequences, a conserved pattern of 6 Cys within the sequence of the mature peptides (Fig. 32A). This pattern is very close to the one previously described for a group of Medicago NCRs but distinct by the spacing between the first three Cys. This led us to propose a specific Aeschynomene NCR-like motif. Interestingly, 11 of these NCR-like peptides contained 2 extra-Cys, giving rise to a typical defensin signature that was named motif 2 (Fig. 32B).

By analogy with M. truncatula, for which the NCR family is composed of about 600 genes that exhibit distinct temporal and spatial expression patterns in nodules [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF][START_REF] Branca | Whole-genome nucleotide diversity, recombination, and linkage disequilibrium in the model legume Medicago truncatula[END_REF][START_REF] Guefrachi | Extreme specificity of NCR gene expression in Medicago truncatula[END_REF], the involvement of a larger number of NCR genes is also expected in Aeschynomene. We therefore considered the possibility that our criteria were too selective leading to a restricted number of candidate genes identified. By performing BLAST searches against the same EST libraries but using this time the first candidate genes that emerged, we identified a total of 38 and 44 NCR-like genes in A.

afraspera and A. indica, respectively (Table 4; Annex 2). All these additional genes were found to be specifically expressed in the nodules and a global alignment show that all of them except 3 contained the Aeschynomene NCR-motif 1 or 2.

Furthermore, a BLAST search against transcriptome data obtained from A. evenia ssp.

serrulata mature nodules made by the 454 technology which allows the detection of a larger number of transcripts, revealed an even higher number of NCR-like genes of more than 80 (Table 4; Annex 2). Altogether, these data suggest that the NCR-like genes constitute also in Aeschynomene a large family, which could count several tens or even hundreds of candidates.

NCR-like genes of Aeschynomene sp. are expressed before or concomitantly with bacteroid differentiation

In order to analyze the temporal expression of the Aa-and AiNCR genes, we performed a RT-qPCR analysis of 4 of them for each Aeschynomene species. Root tissues of A. afraspera and A. indica were sampled at T0 and at different time points after inoculation with Bradyrhizobium strain ORS285 (6h, 1, 2, 3, 4, 5, 6, 7 and 14 days). As a symbiotic marker, we also monitored Leghemoglobin (LegHb) mRNA accumulation. As shown in Fig. 33A and 33B, mRNA corresponding to LegHb started to accumulate at 5 dpi for both species, i. e. when the bacteroid differentiation process is achieved and a nitrogenase activity is detectable [START_REF] Bonaldi | Nodulation of Aeschynomene afraspera and A. indica by photosynthetic Bradyrhizobium Sp. strain ORS285: the nod-dependent versus the nod-independent symbiotic interaction[END_REF]. Interestingly, while the NCR-like gene transcripts were not detected in control roots or during the early time points of nodule formation, they were all strongly expressed at 5dpi and some of them even started to accumulate 1 or 2 days before, which coincided with the beginning of bacteroid morphogenesis [START_REF] Bonaldi | Nodulation of Aeschynomene afraspera and A. indica by photosynthetic Bradyrhizobium Sp. strain ORS285: the nod-dependent versus the nod-independent symbiotic interaction[END_REF]. To complete this expression analysis, we also checked the expression by RT-qPCR, in both roots and nodules, of 3 candidates NCR genes identified in A. evenia by BLAST. As expected, the 3 genes appeared specifically and highly expressed in A. evenia nodules (Fig.

34).

Figure 34. Nodule specific expression of NCR genes in A. evenia RNA was extracted from A. evenia spp. serrulata roots or matures nodules 14 days after inoculation with the Bradyrhizobium strain ORS285. Expression levels were calculated relative to EF1#. Error bars indicate standard errors of the means of 3 biological replicates.

NCR-like genes are expressed specifically in infected cells of Aeschynomene sp. nodules

In Medicago, the NCRs are specifically synthetized in the infected plant cells and are targeted to the bacteria (Van de Velde et al., 2010). To check if the expression of the identified Aa-and AiNCR genes was also restricted to the infected cells, we performed an in situ hybridization on 14-days old nodules of A. afraspera and A. indica, infected with the ORS285 strain. We used as probes AaNCR01 and AiNCR01, which were found to be the most highly expressed NCR genes in each species. In addition, the LegHb gene, known to be expressed in infected nodule cells, was included as a positive control for the hybridization experiment. As shown in Fig. 35, a strong signal was obtained using the LegHb antisense DNA as a probe. The signal was restricted to the infected cells (Fig. 35B andE), consistent with the known role of this protein in symbiosis. Both A. afraspera and A. indica NCR antisense probes gave also a signal circumscribed to the infected cells (Fig. 35C andF)

similarly as observed for LegHb. Control hybridizations without probe or with a sense LegHb probe or sense NCR-like probes (Fig. 35A andD) showed a complete absence of signal.

NCR-like peptides target bacteroids in Aeschynomene nodules

To test the possibility that Aa-and AiNCR mature peptides target the endosymbiotic bacteria in the nodule cells that express the corresponding genes, we set up a proteome approach. Total protein extracts were prepared from nodules and from purified bacteroids from both A. afraspera and A. indica. In addition, a control extract was prepared from freeliving ORS285 bacteria. Proteins were subjected to Tricine-PAGE analysis. Slight differences in the patterns of low molecular weight proteins (between 4 to 9 kDa), where we expected to identify Aa-or AiNCR peptides, could be observed between nodule or bacteroid extracts and extracts from free-living bacteria (Fig. 36). We identified the corresponding proteins by mass spectrometry. From the bacteroid extracts isolated from A. afraspera and A. indica nodules, 51 and 42 proteins were identified respectively. Among them, in both nodule contexts, 4 corresponded to plant proteins, of which three were identified as NCR peptides. The additional plant protein identified in A. indica bacteroids was a putative monosaccharideproton symporter corresponding probably to a symbiosome located transporter whereas there was an extensin-like protein of unknown function in A. afraspera bacteroids. Among the 40 proteins identified in free-living bacterial cells, all corresponded to bacterial proteins and none displayed the characteristics of cysteine-rich peptides.

The DNF1 signal peptidase complex is recruited for bacteroid differentiation in Aeschynomene

To obtain genetic evidence for a role of the Aeschynomene NCR peptides in bacterial differentiation, we applied a strategy aiming to interfere with their targeting to the symbiosomes, similarly as described before in M. truncatula [START_REF] Van De Velde | Plant peptides govern terminal differentiation of bacteria in symbiosis[END_REF][START_REF] Wang | A nodule-specific protein secretory pathway required for nitrogen-fixing symbiosis[END_REF]. NCR peptides are secretory proteins with an N-terminal signal peptide.

Therefore, their transport requires the removal of this signal peptide by the signal peptidase complex machinery, which is located in the membrane of the endoplasmic reticulum.

Reduced SPC activity is supposed to hamper the transport of the NCR peptides to the symbiosomes, similarly as in the dnf1 mutant of M. truncatula [START_REF] Van De Velde | Plant peptides govern terminal differentiation of bacteria in symbiosis[END_REF][START_REF] Wang | A nodule-specific protein secretory pathway required for nitrogen-fixing symbiosis[END_REF]. To develop such loss-of-function strategy in Aeschynomene, we chose to conduct this experiment on A. evenia ssp. serrulata which is diploid contrary to A. indica and A. afraspera which are allopolyploid [START_REF] Renard | Nombres chromosomiques de quelques Angiospermes du Rwanda, du Burundi et du Kenya[END_REF][START_REF] Arrighi | Radiation of the Nod-independent Aeschynomene relies on multiple allopolyploid speciation events[END_REF]. A BLAST search using MtDNF1 allowed the identification in the A. evenia ssp. serrulata transcriptome database of a single candidate gene, which we named AeDNF1, encoding a protein of 167 aa with 83% identity (Fig. 37A). We used an RNAi approach to knock down the AeDNF1. The morphology of the bacteroids was observed in mature nodules elicited by the GFP-tagged ORS285 strain. In parallel, we monitored the level of AeDNF1 extinction by RT-qPCR. As shown in Fig. 37B, in transgenic lines in which the extinction level of AeDNF1 was not affected, the bacteroid differentiation occurred normally giving rise to spherical bacteroids, while in the lines with the highest extinction level (about 65 % reduction of AeDNF1 expression) defects in bacteroid differentiation were observed with the presence of various bacteroid morphotypes within the same cell (Fig. 37C) or elongated bacteroids instead of spherical (Fig. 37D andE).

Discussion

The use of cysteine rich peptides called NCRs, to govern bacteroid differentiation was assumed to be specific to legume species belonging to the IRLC clade, no homologs of NCRs being found in legume species outside of this clade [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF]. In this study, we provide several independent pieces of evidence indicating that Aeschynomene species use a similar molecular mechanism to impose a differentiation process on their rhizobium endosymbionts.

First, we demonstrated that the distinguishing characters associated with bacteroid differentiation in nodules of IRLC legumes, i.e. marked cell enlargement, polyploidy, membrane permeability modification, and loss of viability, are conserved in Aeschynomene nodules. Second, we identified in several Aeschynomene species a class of cysteine rich peptides which have similar characteristics as the Medicago NCR-peptides and which are synthetized specifically in nodule symbiotic cells and targeted to the bacteroids. Finally, silencing of the dnf1 homolog in A. evenia spp. serrulata, impaired bacteroid differentiation.

Moreover, we show in the accompanying paper that Bradyrhizobium strains ORS278 and ORS285 require the BclA peptide transporter for both E-and S-morphotype bacteroid differentiation in Aeschynomene (Guefrachi et al., 2015a). BclA is similar to S. meliloti BacA which is needed for bacteroid differentiation in Medicago and which imports NCR peptides into the bacterial cells. BclA can functionally replace BacA for peptide transport. Together, these observations strongly suggest that in Aeschynomene, E-or S-morphotype bacteroid differentiation is under the control of the NCR peptide family, which we identified here.

These findings might be surprising, considering that according the Aeschynomene species the bacteroids display two distinct morphotypes (S-and E-morphotypes). This difference of morphological shape suggests distinct mechanisms governing bacterial morphogenesis.

The change from a rod to a spherical shape as observed in A. indica and A. evenia during bacteroid differentiation resemble bacterial morphologies resulting from mutations, which affect peptidoglycan synthesis and which were described in e.g. E. coli [START_REF] Young | Bacterial shape[END_REF].

Similarly, it has been shown in Vibrio that some non-canonical D-amino acids could led to a rod to sphere shape transition both via their incorporation into the peptidoglycan polymer and by regulating enzymes that synthesize and modify it [START_REF] Lam | Damino acids govern stationary phase cell wall remodeling in bacteria[END_REF]. Therefore, the bacteroid morphology in A. indica or A. evenia is possibly associated with changes in the peptidoglycan cell wall architecture. This raises the question whether the morphogenesis to the S-morphotype is governed by a specific class of NCR peptides targeting enzymes or genes involved in peptidoglycan synthesis or if additional plant effectors of a different nature such as particular D-amino acids are involved in the control of bacteroid morphogenesis.

Even if the underlying mechanisms for E-and S-morphotype bacteroid formation are obviously different, some observations strongly suggest that the two processes are closely related. In A. indica and A. evenia, the formation of the spherical bacteroids is taking place synchronously in all nodule cells between 4 and 5 dpi [START_REF] Bonaldi | Nodulation of Aeschynomene afraspera and A. indica by photosynthetic Bradyrhizobium Sp. strain ORS285: the nod-dependent versus the nod-independent symbiotic interaction[END_REF]. In this one-day time frame, all intracellular rod-shaped bacteria transform into spheres. However, when making observations between 4 and 5 dpi, one can discern intermediate differentiation stages, which strongly resemble E-morphotype bacteroids (Fig. 38). In addition, we observed in the A. evenia spp. serrulata knock-down lines in which the level of expression of AeDNF1 was reduced, bacteroids with an E-morphotype instead of an S-morphotype or a mixture of both shapes within the same cell. Finally, the bacterial gene bclA is similarly required for E-morphotype bacteroid formation in A. afraspera or Smorphotype bacteroid formation in A. indica or A. evenia (Guefrachi et al., 2015a).

Interestingly, analysis of the distribution of the E-and S-morphotype bacteroids among Aeschynomene revealed that all the tested species using a Nod-independent symbiotic process have S-morphotype bacteroids whereas those using a Nod-dependent process have Emorphotype bacteroids, similar to the IRLC species. Furthermore, a recent phylogenetic analysis revealed that all Aeschynomene species using a Nod-independent symbiotic process form a monophyletic clade that does not include other species using a Nod-dependent process [START_REF] Chaintreuil | Evolution of symbiosis in the legume genus Aeschynomene[END_REF]. It could therefore be tempting to speculate that the S-morphotype and Nod-independent symbiosis are both derived characters, which are correlated. However, in the Dalbergoid clade that contains the Aeschynomene genus, other species are present, such as Arachis hypogaea or Stylosanthes hanata, that also host spherical bacteroids [START_REF] Chandler | Infection and root-nodule development in Stylosanthes species by Rhizobium[END_REF][START_REF] Sen | Structure and organization of effective peanut and cowpea root nodules induced by rhizobial strain 32H1[END_REF]. It has been established that at least A. hypogaea uses a Noddependent symbiotic process [START_REF] Ibáñez | Rhizobial Nod factors are required for cortical cell division in the nodule morphogenetic programme of the Aeschynomeneae legume Arachis[END_REF]. This suggests that the S-character has possibly emerged several times during the diversification of the Dalbergoid clade independent of the Nod-independent character.

The mode of action of the NCRs and their bacterial targets remains unclear in

Medicago. However, it has been shown that some cationic NCRs, particularly those with a high isoelectric point, are able, in vitro, to provoke symptoms of terminal differentiation (loss of viability, bacteria elongation, membrane permeabilization and genome endoreplication)

( [START_REF] Van De Velde | Plant peptides govern terminal differentiation of bacteria in symbiosis[END_REF]. In Medicago, the proportion of anionic and cationic NCRs is similar but no in vitro activity has been reported to date for the anionic or neutral NCRs. This does not imply that this last class of NCRs is not functional in vivo considering their high level of expression, which is similar to the cationic NCRs [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF]. It was proposed that the cationic NCRs interact preferentially with the negatively charged outer membrane of rhizobia and by compromising its permeability facilitate the entry of anionic NCRs to the bacterial cytosol [START_REF] Kondorosi | A paradigm for endosymbiotic life: Cell differentiation of Rhizobium bacteria provoked by host plant factors[END_REF]. In Aeschynomene, except for very few exceptions, no cationic NCR-like peptides could be identified among the several tens of candidate genes identified in each species (Table 4; Annex 2). We have tested in vitro the effect of four synthetic Aeschynomene NCRs, corresponding to the most abundant ones identified in the A. afraspera and A. indica nodule EST libraries but no activity could be detected on Bradyrhizobium strain ORS285 or S. meliloti. We also tested on the strain ORS285 the Medicago cationic NCRs (NCR035, NCR247 and NCR335), known to have an antimicrobial effect on a large panel of bacteria [START_REF] Tiricz | Antimicrobial nodule-specific cysteine-rich peptides induce membrane depolarizationassociated changes in the transcriptome of Sinorhizobium meliloti[END_REF] but surprisingly no activity was detected also for those peptides whereas on the control Sinorhizobium strain a strong antimicrobial activity and strong membrane damage could be observed in our conditions. The membrane of Bradyrhizobium strains differs from rhizobium strains by the presence of hopanoids, which may represent up to 40% of the total lipid extract [START_REF] Kannenberg | The occurrence of hopanoid lipids in Bradyrhizobium bacteria[END_REF]. It has recently been shown that this class of compounds, which display some structural similarity with eukaryotic sterols, reinforce the stability and rigidity of the outer membrane of photosynthetic Bradyrhizobium (Silipo et al., 2014). It is therefore possible that hopanoids reduce the antimicrobial activity of cationic Medicago NCRs by reinforcing the outer membrane barrier function. In agreement, hopanoid mutants of Burkholderia strains displayed increase sensitivity to the antimicrobial peptide polymyxin B [START_REF] Schmerk | Hopanoid production is required for low-pH tolerance, antimicrobial resistance, and motility in Burkholderia cenocepacia[END_REF][START_REF] Malott | Identification of hopanoid biosynthesis genes involved in polymyxin resistance in Burkholderia multivorans[END_REF]. How the Aeschynomene NCRs can enter in Bradyrhizobium cells and modulate their physiology and morphology remains an open question. It is possible that these NCRs only act synergistically or are active only on a particular physiological state of the bacteria or that bacteria use specific transporters such as BclA (Guefrachi et al., 2015a), to import them or use specific membrane receptors to detect them and activate appropriate signaling pathways.

In silico analyses of the available transcriptome databases suggest the NCR-like genes constitute a large family in Aeschynomene species counting several tens or even hundreds of candidates. As observed for Medicago, the amino acids sequence of these NCR peptides is highly divergent except the distribution of 6 Cys residues, which is conserved in 93% of the identified peptides. A phylogenetic analysis based on the sequence of the mature peptides shows that the NCR peptides identified in Aeschynomene are distinct from the NCRs identified in IRLC (Fig. 39). A recent study mapping the bacteroid morphology in 40 legume species of the Papilionoideae suggests that legumes inducing bacteroid morphogenesis evolved independently at least five times from an ancestral papilionoid legume hosting Umorphotype bacteroids (Oono et al., 2010). It has been proposed that the NCR family identified in IRLC legumes evolved from defensin ancestor(s) [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF]. This large family of antimicrobial peptides is found in all eukaryotes where they are part of the first line of defense against invading microbes. The fact that the NCR peptides identified in Aeschynomene are phylogenetically distant from the NCRs identified in the IRLC legumes but also display a defensin signature suggest that they have similarly evolved from defensin(s) but likely from a distinct ancestral repertoire. The amino acid sequences of NCR genes were aligned using Muscle3.6. The evolutionary distances were computed using the Poisson correction method, and a tree was inferred from the distance matrix by Neighbor Joining using MEGA5. All ambiguous positions were removed for each sequence pair. There were a total of 210 positions in the final dataset. Percent of bootstrap replicates (1000 replicates) are indicated at each tree node (only if >50%). NCRs from A. afraspera and A. indica are in green and red respectively and those from other IRLC legumes are in blue (The accession numbers are indicated). A defensin from Lotus, a non-IRCL legume was used for tree rooting.

It will be interesting to analyze the transcriptome of nodules from species belonging to the two other lineages, the Genistoids and the Mirbelioids, in which bacteroid morphogenesis was reported, using the selective criteria defined in this study, to determine if the use of cysteine rich peptides is a general strategy in legumes to keep their endosymbionts under control. Remarkably, it has been shown recently that also insects produce antimicrobial peptides to control the development of their endosymbiotic bacteria which display in certain cases a spherical or elongated shape similar to the S-or E-morphotype bacteroids [START_REF] Login | Antimicrobial peptides keep insect endosymbionts under control[END_REF][START_REF] Shigenobu | Aphids evolved novel secreted proteins for symbiosis with bacterial endosymbiont[END_REF]. This suggests that the enrolment of antimicrobial peptides as symbiosis effectors is an optimal modus operandi for a eukaryotic host cell to control large population of intracellular bacteria.

Chapter. III

Bradyrhizobium

Introduction

An evolutionary strategy, frequently used by certain groups of organisms such as insects or plants to solve the lack of specific nutrients in their food, is the establishment of symbiotic interactions with micro-organisms that are able to produce those nutrients. A wellknown example, studied intensively for its agronomic and ecological impact, is the interaction of legumes with nitrogen-fixing bacteria. Bio-available nitrogen limits plant growth, yet legumes have the capacity to thrive on nitrogen-poor soils thanks to the reduction of air-borne nitrogen gas to ammonia by their symbiotic bacteria. These legume symbiotic bacteria, commonly called rhizobia, are phylogenetically disparate α-and β-proteobacteria belonging respectively to the Rhizobiales and the Burkholderiales orders [START_REF] Masson-Boivin | Establishing nitrogen-fixing symbiosis with legumes: how many rhizobium recipes?[END_REF].

Like in other nutritional symbioses, fulfilling all the nutritional needs of the host plant by the endosymbiotic rhizobia requires the hosting of millions to billions of them. To achieve this challenging task, legumes form specific organs called nodules on their roots, and certain species also on their stems. Nodules are in large part composed of specialized symbiotic cells, which are giant polyploid cells, each carrying several hundreds to thousands of intracellular rhizobia [START_REF] Kondorosi | A paradigm for endosymbiotic life: Cell differentiation of Rhizobium bacteria provoked by host plant factors[END_REF]. This high bacterial load requires the establishment of homeostasis between the two symbionts whereby the host immune response does not eliminate the micro-symbionts and inversely, the massive colonization of the host cells does not result in collapse of those cells. A recently unveiled pathway in the study-model

Medicago truncatula that prevents the activation of the host-defense reactions by the presence of the rhizobia in the symbiotic cells implies the receptor-like kinase SymCRK (Berrabah et al., 2014) and the phosphatidylinositol phospholipase C-like protein DNF2 [START_REF] Bourcy | Medicago truncatula DNF2 is a PI-PLC-XDcontaining protein required for bacteroid persistence and prevention of nodule early senescence and defense-like reactions[END_REF]. On the other hand, to maintain the endosymbiotic rhizobial population under control, some legumes, enforce the intracellular bacteria into a terminal differentiation process, which is characterized by cell enlargement, polyploidy and loss of reproductive capacity. Bacteroids in this type of legumes are elongated, called E-morphotype bacteroids or spherical, called Smorphotype bacteroids. Such a mechanism has been described in detail in IRLC and Dalbergoid legumes [START_REF] Mergaert | Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis[END_REF][START_REF] Czernic | Convergent evolution of endosymbiont differentiation in Dalbergoid and IRLC legumes mediated by nodule-specific cysteine-rich peptides[END_REF] and exists also in other legume clades (Oono et al., 2010). However, in still other legumes this terminal differentiation does not exist and such legumes carry in their nodules unmodified or so-called U-morphotype bacteroids [START_REF] Mergaert | Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis[END_REF]Oono et al., 2010;[START_REF] Kondorosi | A paradigm for endosymbiotic life: Cell differentiation of Rhizobium bacteria provoked by host plant factors[END_REF].

The ancestral state of the bacteroids is considered to be the undifferentiated state and the other morphotypes have appeared later in evolution (Oono et al., 2010). Since IRLC and Dalbergoids are distantly related legume clades [START_REF] Wojciechowski | A phylogeny of legumes (Leguminosae) based on analysis of the plastid matK gene resolves many well-supported subclades within the family[END_REF], it is inferred that the molecular mechanisms that enforce bacteroid differentiation have evolved independently in these clades (Oono et al., 2010).

In the Sinorhizobium meliloti -Medicago symbiosis, the formation of the Emorphotype bacteroids is under the control of a particular class of effectors called the Nodulespecific Cysteine-Rich (NCR) peptides which are massively produced by the symbiotic cells and targeted by the secretory pathway to the endosymbionts, inducing them into their polyploid differentiated state (Van de Velde et al., 2010). The polyploidy of the bacteroids implies that the peptides interfere with the bacterial cell cycle, inhibiting division while DNA replication continues. This is achieved, at least in part, by interfering with the bacterial cell cycle regulator CtrA [START_REF] Pini | The DivJ, CbrA and PleC system controls DivK phosphorylation and symbiosis in Sinorhizobium meliloti[END_REF][START_REF] Penterman | Host plant peptides elicit a transcriptional response to control the Sinorhizobium meliloti cell cycle during symbiosis[END_REF]. NCRs are conserved in all tested E-morphotype-forming IRLC legumes but other legumes forming U-morphotype bacteroids, such as for example Lotus japonicus, Phaseolus vulgaris (bean) or Glycine max (soybean), lack NCR genes explaining why bacteroids in those legumes remain unmodified [START_REF] Kondorosi | A paradigm for endosymbiotic life: Cell differentiation of Rhizobium bacteria provoked by host plant factors[END_REF].

NCR peptides are similar to antimicrobial peptides and have in vitro antimicrobial activity [START_REF] Van De Velde | Plant peptides govern terminal differentiation of bacteria in symbiosis[END_REF][START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF][START_REF] Haag | Role of cysteine residues and disulfide bonds on the activity of a legume root nodule-specific, cysteine-rich peptide[END_REF][START_REF] Arnold | Partial complementation of Sinorhizobium meliloti bacA mutant phenotypes by the Mycobacterium tuberculosis BacA protein[END_REF][START_REF] Tiricz | Antimicrobial nodule-specific cysteine-rich peptides induce membrane depolarizationassociated changes in the transcriptome of Sinorhizobium meliloti[END_REF]. The BacA transporter protein of S. meliloti provides protection against this antimicrobial activity and is required for the long term maintenance of S. meliloti in the NCR-producing symbiotic cells of Medicago nodules. A bacA mutant does not differentiate in these symbiotic cells but dies off very rapidly after its release and exposure to the NCR peptides inside the symbiotic cells due to its hypersensitivity to these peptides [START_REF] Glazebrook | A Rhizobium meliloti homolog of the Escherichia coli peptide-antibiotic transport protein SbmA is essential for bacteroid development[END_REF][START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF]. The bacA gene is not required for all Rhizobium-legume interactions and based on a limited number of case studies, a correlation was found between the requirement of the bacA gene for symbiosis, the production of NCR peptides by the host cells and bacterial differentiation [START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF].

Legumes of the Aeschynomene genus belong to the Dalbergoid clade and form either E-or S-morphotype bacteroids depending on the species [START_REF] Bonaldi | Nodulation of Aeschynomene afraspera and A. indica by photosynthetic Bradyrhizobium Sp. strain ORS285: the nod-dependent versus the nod-independent symbiotic interaction[END_REF][START_REF] Czernic | Convergent evolution of endosymbiont differentiation in Dalbergoid and IRLC legumes mediated by nodule-specific cysteine-rich peptides[END_REF]. We have shown recently that Aeschynomene afraspera nodules housing E-morphotype bacteroids or Aeschynomene indica and Aeschynomene evenia nodules housing S-morphotype bacteroids produce NCR peptides. However, these Aeschynomene NCR peptides have no sequence homology to the NCR peptides of IRLC legumes and are therefore of an independent evolutionary origin [START_REF] Czernic | Convergent evolution of endosymbiont differentiation in Dalbergoid and IRLC legumes mediated by nodule-specific cysteine-rich peptides[END_REF]. These peptides, which are specifically produced by the symbiotic cells and targeted to the bacteroids are most likely involved in the formation of the differentiated bacteroids [START_REF] Czernic | Convergent evolution of endosymbiont differentiation in Dalbergoid and IRLC legumes mediated by nodule-specific cysteine-rich peptides[END_REF]. Thus, despite the independent evolution of bacterial differentiation, similar mechanisms seem to operate in both legume clades. To further corroborate this notion, we identified in the present study a bacA-like gene in the Bradyrhizobium species which nodulate Aeschynomene plants and demonstrate its role in bacteroid differentiation in those plants while the same gene is not required for nodulation of U-morphotype forming soybean.

BacA-related proteins are transporters composed of solely a SbmA_BacA transmembrane domain [START_REF] Levier | Genetic analysis of the Sinorhizobium meliloti BacA protein: differential effects of mutations on phenotypes[END_REF]. BacA of S. meliloti and Mycobacterium tuberculosis and SbmA, a homolog in Escherichia coli, have the notable feature of internalizing a variety of structurally diverse peptides and other molecules, including the antibiotic bleomycin and the mammalian peptide Bac7 [START_REF] Laviña | Identification, mapping, cloning and characterization of a gene (sbmA) required for microcin B17 action on Escherichia coli K12[END_REF][START_REF] Yorgey | Posttranslational modifications in microcin B17 define an additional class of DNA gyrase inhibitor[END_REF][START_REF] Salomon | The peptide antibiotic microcin 25 is imported through the TonB pathway and the SbmA protein[END_REF][START_REF] Mattiuzzo | Role of the Escherichia coli SbmA in the antimicrobial activity of proline-rich peptides[END_REF][START_REF] Pränting | Mechanism and fitness costs of PR-39 resistance in Salmonella entericas erovar Typhimurium LT2[END_REF][START_REF] Marlow | Essential role for the BacA protein in the uptake of a truncated eukaryotic peptide in Sinorhizobium meliloti[END_REF][START_REF] Wehmeier | Internalization of a thiazole-modified peptide in Sinorhizobium meliloti occurs by BacA-dependent and independent mechanisms[END_REF][START_REF] Ghosal | Role of SbmA in the uptake of peptide nucleic acid (PNA)-peptide conjugates in E. coli[END_REF][START_REF] Puckett | Bacterial resistance to antisense peptide phosphorodiamidate morpholino oligomers[END_REF][START_REF] Arnold | Partial complementation of Sinorhizobium meliloti bacA mutant phenotypes by the Mycobacterium tuberculosis BacA protein[END_REF][START_REF] Gopinath | A vitamin B 12 transporter in Mycobacterium tuberculosis[END_REF][START_REF] Narayanan | Mechanism of Escherichia coli Resistance to Pyrrhocoricin[END_REF]. The uptake of these antimicrobial compounds by the SbmA/BacA 'multidrug' transporter alters the sensitivity of the bacteria towards these molecules. Here we find that the Bradyrhizobium BacA-related protein increases sensitivity towards Bleomycin and Bac7 suggesting that they are transporters of these molecules.

Moreover we show that S. meliloti BacA and its homolog in Bradyrhizobium are capable of importing NCR peptides.

Results

Identification of BacA homologues in Bradyrhizobium species

The BacA protein of S. meliloti belongs to a subfamily of ABC transporters characterized by the transmembrane domain pfam06472 (ABC_membrane_2) or pfam05992 (SbmA_BacA) [START_REF] Punta | The Pfam protein families database[END_REF][START_REF] Marchler-Bauer | CDD: conserved domains and protein three-dimensional structure[END_REF]. We call these ABC transporters the SbmA_BacA domain transporters. Besides BacA, S. meliloti has a second SbmA_BacA domain ABC transporter called ExsE [START_REF] Levier | Genetic analysis of the Sinorhizobium meliloti BacA protein: differential effects of mutations on phenotypes[END_REF]. The exsE gene is located next to a gene cluster important for symbiosis and involved in queuosine and in exopolysaccharide biosynthesis (the queCDE and exo-exs clusters) [START_REF] Becker | Extension of the Rhizobium meliloti succinoglycan biosynthesis gene cluster: identification of the exsA gene encoding an ABC transporter protein, and the exsB gene which probably codes for a regulator of succinoglycan biosynthesis[END_REF][START_REF] Marchetti | Queuosine biosynthesis is required for Sinorhizobium meliloti-induced cytoskeletal modifications on HeLa Cells and symbiosis with Medicago truncatula[END_REF]. Therefore, we analyzed the symbiotic phenotype of two independent S. meliloti exsE mutants. Both formed a normal symbiosis, inducing nodules on M. truncatula that were normally infected, that contained symbiotic cells with elongated bacteroids and that fixed nitrogen to the same level as wild-type nodules (Fig. 40). Thus exsE does not seem to have a crucial role in symbiosis. The bacA gene of S. meliloti is conserved in the genomes of Sinorhizobium, Rhizobium or Mesorhizobium strains in which these genes are located in synthenic regions and display similarities of 57 to 100%. Also exsE genes are conserved in different rhizobium strains (45 to 100%) although they are not located in synthenic regions. Genes with similar high homology levels to bacA or exsE or located in a similar synthenic region are missing from the genomes of the Bradyrhizobium strains ORS278, ORS285 and USDA110. The former two strains are natural symbionts of Aeschynomene species [START_REF] Giraud | Legumes symbioses: Absence of Nod genes in photosynthetic Bradyrhizobia[END_REF] while the latter is a soybean symbiont that can also nodulate A. afraspera [START_REF] Renier | Photosynthetic Bradyrhizobium Sp. strain ORS285 synthesizes 2-O-methylfucosylated lipochitooligosaccharides for nod gene-dependent interaction with Aeschynomene plants[END_REF].

Nevertheless, we identified by BLASTP searches in each of these 3 Bradyrhizobium genomes 250005 and blr0517 genes have a fused ATPase domain while the proteins encoded by the BRADO1574, 950010 and blr4660 genes are lacking such a domain (Fig. 41).

A phylogenetic tree of SbmA_BacA domain transporters (Fig. 42; Table 6;Annex The tree was generated with ClustalW2. The protein sequences used for the alignment and tree generation are provided in Table 5 (Annex 3). The proteins discussed in this study are highlighted in orange.

Symbiotic role of the Bradyrhizobium SbmA_BacA domain transporters

To determine whether the identified BacA homologs of Bradyrhizobium have a symbiotic role and more particularly in the differentiation of E-or S-morphotype bacteroids, we constructed mutants in the 3 genes of Bradyrhizobium strain ORS285 (BRAO285v1_1320006, BRAO285v1_250005 and BRAO285v1_950010). Strain ORS285 was chosen for mutagenesis rather than ORS278 because the latter strain can nodulate only Smorphotype producing Aeschynomene species while the former strain has a broad host range including species forming S-or E-morphotype bacteroids [START_REF] Czernic | Convergent evolution of endosymbiont differentiation in Dalbergoid and IRLC legumes mediated by nodule-specific cysteine-rich peptides[END_REF]. The 3 mutants were tested for their symbiotic phenotype on the host plants A. indica and A.

afraspera. Only the mutant in the gene 1320006 had an abnormal symbiotic phenotype, forming on both host plants nodules which looked aberrant in size, form and color, that had a strongly reduced nitrogen fixation activity (about 5% and 20% of wild-type levels in A. indica and A afraspera respectively) and that could not fully support plant growth in nitrogen poor growth conditions, obvious from the reduced plant shoot mass (Fig. 43). The mutants in the other two genes (250005 and 950010) on the other hand, formed normal nitrogen-fixing nodules indicating that these genes are not required for symbiosis (Fig. 43). Interestingly, in a screen of a collection of Tn5 transposon mutants of Bradyrhizobium strain ORS278, we previously identified a mutant in the gene BRADO6119, orthologous to the ORS285 gene 1320006, which formed non-functional nodules [START_REF] Bonaldi | Large-scale transposon mutagenesis of photosynthetic Bradyrhizobium sp. strain ORS278 reveals new genetic loci putatively important for nod-independent symbiosis with Aeschynomene indica[END_REF]. These observations together with the further characterization described below of the gene 1320006 and its orthologues in strains ORS278 and USDA110, indicate that they encode BacA-like proteins. Therefore, we named them bclA for Bradyrhizobium bacA-like.

BclA is not required for nitrogen fixation per se or for bacterial growth

Contrary to most other known rhizobium strains, the Bradyrhizobium symbionts of Aeschynomene species such as strains ORS285 and ORS278 fix nitrogen during free-living growth. This capacity makes it possible to test whether the BclA transporter is involved in the nitrogen fixation process itself. Such a test was not possible for the S. meliloti bacA gene because that strain cannot fix nitrogen during free growth. The ORS285 and ORS278 bclA mutants were as active as the wild-type strain in the acetylene reduction assay (Fig. 44A).

This indicates that BclA is not required for the nitrogen fixation metabolism.

It was recently shown that the bacA homologous gene of Mycobacterium tuberculosis is involved in vitamin B12 uptake [START_REF] Gopinath | A vitamin B 12 transporter in Mycobacterium tuberculosis[END_REF]. We found that both the wild-type strain and the bclA mutant were grew and fixed nitrogen normally in minimal medium BNM-B to which vitamin B12 was added or not (Fig. 44A-D). Thus Bradyrhizobium ORS285 does not need external vitamin B12 for growth, most likely because it has the genetic repertoire for vitamin B12 synthesis [START_REF] Rodionov | Comparative genomics of the vitamin B 12 metabolism and regulation in prokaryotes[END_REF]. Therefore, the symbiotic phenotype of the bclA mutant is unlikely to result from a defect in vitamin B12 uptake. These results also suggest that the symbiotic defect of the bclA mutant does not result from a general growth defect caused by the mutation.

The bclA gene of Bradyrhizobium strains ORS285 and ORS278 and of Bradyrhizobium japonicum is flanked by genes of the glycolate metabolism pathway (Fig. 41A). In E. coli the glcCglcDEFGBA locus confers the ability to utilize glycolate as carbon source [START_REF] Pellicer | glc locus of Escherichia coli: characterization of genes encoding the subunits of glycolateoxidase and the glc regulator protein[END_REF]Nunez et al., 2001). However, Bradyrhizobium ORS285 is unable to grow on glycolate as carbon source (Fig. 44E). In agreement with this, the genes glcB and glcG are absent in the Bradyrhizobium locus and are replaced by bclA and two other genes, one of them encoding a secreted protein (gene 1320003 in strain ORS285) (Fig. 41A). To investigate whether the glc metabolic genes and the secreted protein are required for symbiosis along with the bclA gene, we constructed mutants in these genes and inoculated them on A. afraspera and A. indica. We found that both mutants formed normal nodules, which fixed nitrogen and supported plant growth in a nitrogen-free medium on both host plants (Fig. 45). Thus the bclA-flanking genes are not involved in the symbiotic interaction with Aeschynomene and the function of bclA is probably not related to glycolate metabolism.

Bradyrhizobium BclA is required for polyploid bacteroid differentiation

The macroscopic phenotype of the A. afraspera and A. indica nodules infected with the ORS285!bclA mutant is compatible with a possible defect in bacteroid formation. To verify this hypothesis, we analyzed the histology of these nodules by light and confocal microscopy and compared it to wild-type infected nodules. Thin sections of resin-embedded nodules were stained with toluidine blue and observed by light microscopy (Fig. 46). The diameter of the nodules infected with the mutant strain were significantly smaller than the nodules infected with the wild-type strain, particularly for the A. indica nodules, but their central tissue was fully occupied with symbiotic cells and the surrounding cortical cell layers were normally formed (Fig. 46A-D). The cytosolic space of the symbiotic cells was completely filled with rhizobia similarly to symbiotic cells in wild-type nodules (Fig. 46E-H). However, in both A. afraspera and A. indica the morphology of the bacteroids was aberrant. In A. afraspera nodules infected with wild-type ORS285, the bacteroids are strongly elongated (E-morphotype) while in nodules infected with the mutant, the bacteroids remain small very similar to undifferentiated bacteria (Fig. 46E,F). Similarly, in A. indica nodules infected with wild-type ORS285, the bacteroids are spherical (S-morphotype) while in nodules infected with the mutant, the bacteroids are similar to undifferentiated bacteria (Fig.

46G,H).

The non-differentiated state of the bacteroids formed by the mutant strain was further verified and quantified precisely. Bacteroids were isolated from nodules and purified on a density gradient and then analyzed by flow cytometry. The forward scatter (FS) of the bacteria is relative to their size. The bacteroids isolated from A. afraspera and A. indica nodules have a higher FS than the cultured ORS285 bacteria (Fig. 46M,N) in agreement with their enlarged size. The bclA mutant bacteroids on the other hand have a FS that is intermediate between the FS of cultured bacteria and wild-type bacteroids, supporting their differentiation defect observed by microscopy. The bacteroids in A. afraspera and A. indica nodules have a high DNA content [START_REF] Czernic | Convergent evolution of endosymbiont differentiation in Dalbergoid and IRLC legumes mediated by nodule-specific cysteine-rich peptides[END_REF]. Therefore, we also measured the DNA content of the bclA mutant bacteroids, isolated from A. afraspera and A. indica nodules and stained with the DNA fluorescent stain 4',6-diamidino-2-phenylindole (DAPI). The mean DNA content of the wild-type bacteroids is 7 genome complements (7C) and 16C in A.

afraspera and A. indica respectively while the mutant bacteroids reach only a 1 to 3C ploidy levels which is close to the 1C-2C ploidy of free growing Bradyrhizobium strain ORS285 (Fig. 46O,P), confirming the nearly undifferentiated state of the bacteroids formed by the bclA mutant observed by microscopy.

Next, we used live/dead staining [START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF] of thick nodule sections to analyze the survival ability of the bclA mutant bacteria within the symbiotic cells. Imaging with confocal microscopy again clearly reveals the undifferentiated state of the mutant bacteria in the nodules of both plant species (Fig. 46I-L). Despite this undifferentiated state, the bacteria remained viable in A. afraspera nodules as indicated by the green syto9 staining at least until 14 days after inoculation (Fig. 46I,J). In contrast, in A. indica nodules, the survival capacity of the mutant is more strongly affected and at 14 days post inoculation, many bacteria were dead as indicated by their red staining due to Propidium Iodide (PI) uptake (Fig. 46K,L). The degradation of the bacteroids in A. indica nodules can also be seen on the resin-embedded nodule sections (Fig. 46D,H inset).

To further explain the strongly reduced nitrogen fixation ability of the bacteroids in A.

afraspera nodules, we used the acidophilic DND-99 LysoTracker probe which marks actively fixing bacteroids by red fluorescence due to acidification of the peribacteroid space (Pierre et al., 2013). We find that in the A. afraspera nodules, the bclA mutant bacteroids are not stained by DND-99 in many symbiotic cells (Fig. 47). Thus although the mutant bacteroids are not PI positive, their activity is affected by the absence of BclA function. To confirm that the described phenotypes are linked to the deletion of the bclA gene in strain ORS285, we also analyzed the phenotype of the bclA Tn5 insertion mutant in strain ORS278 [START_REF] Bonaldi | Large-scale transposon mutagenesis of photosynthetic Bradyrhizobium sp. strain ORS278 reveals new genetic loci putatively important for nod-independent symbiosis with Aeschynomene indica[END_REF]. Nodulation of A. evenia and A. indica with this mutant cannot support the growth of these plants in the absence of external nitrogen, the nodules are abnormally formed, indicative that they are non-functional, bacteroids do not become spherical but remain similar in morphology to free-living rhizobia and these bacteroids are PI positive (Fig. 48). We also confirmed by confocal microscopy and live/dead staining that the above mentioned mutants in the two additional bacA-homologous genes and in the genes encoding the secreted protein and glcA have a wild-type bacteroid phenotype (Fig. 49).

Bradyrhizobium BclA is functionally equivalent to S. meliloti BacA in symbiosis

Since the phenotype of the Bradyrhizobium bclA mutants is reminiscent of the S.

meliloti bacA mutant phenotype, we wanted to determine whether bacA and bclA are interchangeable for their symbiotic function. No vectors are available for Bradyrhizobium that can be maintained stably in the absence of antibiotic selection. Therefore, we chose to investigate the capacity of the bclA gene of Bradyrhizobium strain ORS285 to complement the symbiotic defect of a S. meliloti bacA deletion mutant [START_REF] Ferguson | Deficiency of a Sinorhizobium meliloti BacA mutant in alfalfa symbiosis correlates with alteration of the cell envelope[END_REF]. This mutant forms nodules in which symbiotic cells are filled with non-differentiated bacteria which lose viability very rapidly due to their hyper-sensitivity to the antimicrobial action of certain NCR peptides produced by these symbiotic cells [START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF]. The phenotype of the mutant can be easily scored by the aspect of the nodules which are small and white compared to the large elongated and pink wild-type nodules (Fig. 50A-C). This pink color results from the abundant production of leghemoglobin in the cytosol of symbiotic cells during the late stages of their formation, just before onset of nitrogen fixation. Also live/dead staining of nodule sections and observation by confocal microscopy easily reveals the mutant phenotype: in nodules infected with the bacA mutant, the symbiotic cells contain dead bacteria, stained red by PI, and only the infection threads carry life bacteria, stained green by Syto9, while in nodules infected with the wild-type strain, symbiotic cells are entirely occupied with live bacteria which are stained green by Syto9 (Fig. 50E-G,I-K).

The absence of differentiation of the bacteroids in the mutant can also be measured by flow cytometry analysis of bacteria extracted from nodules since the mutant bacteria do not amplify their genome contrary to the wild-type strain (Fig. 50M-O). The S. meliloti bacA mutant strain carrying the bclA gene of ORS285 induces the formation of nodules, which are elongated although not to the same level as the same strain carrying the bacA gene and those nodules are slightly pinkish indicating the leghemoglobin production is restored (Fig. 50C,D).

The confocal microscopy of live/dead stained nodule sections shows that the bacteroids of the bclA expressing strain are elongated and mostly stained green although in older symbiotic cells many dead bacteria were also detected which were nevertheless elongated (Fig. 50H,L).

Flow cytometry analysis of the DNA content in nodule bacteria also shows that the presence of the bclA gene restores the polyploidization of the bacteroids to similar levels as in the strain carrying the bacA gene or the wild-type strain (Fig. 50M-P). Thus altogether, these results indicate that the bclA gene can complement the endogenous bacA gene although this complementation is only partial and insufficient to restore the nitrogen fixation capacity of the strain (data not shown).

Bradyrhizobium japonicum USDA110 BclA is not required for symbiosis

A. afraspera can be nodulated by the B. japonicum strain USDA110 which is a soybean symbiont although that strain is a less efficient nitrogen fixer than strain ORS285 [START_REF] Renier | Photosynthetic Bradyrhizobium Sp. strain ORS285 synthesizes 2-O-methylfucosylated lipochitooligosaccharides for nod gene-dependent interaction with Aeschynomene plants[END_REF]. Soybean belongs to the Millettioid legume clade, which includes also Phaseolus or Vigna [START_REF] Wojciechowski | A phylogeny of legumes (Leguminosae) based on analysis of the plastid matK gene resolves many well-supported subclades within the family[END_REF]. All these legumes form nodules housing Umorphotype bacteroids because they lack NCR peptides [START_REF] Kondorosi | A paradigm for endosymbiotic life: Cell differentiation of Rhizobium bacteria provoked by host plant factors[END_REF]. For a few tested examples of legumes forming U-morphotype bacteroids, it has been found that the bacA gene is dispensable for symbiosis [START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF][START_REF] Kondorosi | A paradigm for endosymbiotic life: Cell differentiation of Rhizobium bacteria provoked by host plant factors[END_REF]. Since the bclA gene is highly conserved in strain USDA110 (Fig. 42; Table 6; Annex 4), we were wondering whether this gene is required in strain USDA110 for symbiosis with A. afraspera or soybean. A bclA deletion mutant was created and its symbiotic phenotype was compared to the parent strain on both host plants (Fig. 51). As expected, USDA110 formed undifferentiated, U-morphotype bacteroids on soybean as revealed by microscopy of nodule sections (Fig. 51E) and flow cytometry analysis of purified bacteroids (Fig. 51H). But more surprisingly, we found that USDA110 was within A. afraspera nodules only slightly elongated (Fig. 51C), contrary to strain ORS285 on the same host plant. The absence of a pronounced differentiation was confirmed in flow cytometry analysis by the minor increase of the FS and the slight increase in DNA content to a mean value of 3C (Fig. 51C). Thus the USDA110 wild-type bacteroids have very similar features as the ORS285!bclA mutant in terms of size and DNA content.

The bclA mutant of USDA110 was undistinguishable from the wild-type strain for nodulation of both host plants for all parameters analyzed, including nodule tissue structure and bacterial occupation, bacterial viability, morphology, size and DNA content as well as nitrogen fixation (Fig. 51). Thus the Bradyrhizobium bclA gene, similarly to bacA, is not required for symbiosis when bacteroids are not constrained by the host plant to differentiate in an elongated or spherical morphotype. We also confirmed the inessentiality of bacA for soybean symbiosis in another soybean symbiont, Sinorhizobium fredii strain HH103. The inactivation of the bacA gene in this strain, closely related to S. meliloti bacA (Fig. 42), has no consequence on the symbiosis with soybean (Fig. 52). To exclude the possibility that bclA of strain USDA110 is an inactive gene, we confirmed its capacity to complement the S. meliloti bacA mutation in a similar way as the bclA gene of strain ORS285 does (Fig. 53).

Bradyrhizobium BclA has multidrug transport activity

Different peptide-derived antibiotic compounds have been shown to be taken up by bacterial cells in a SbmA/BacA dependent manner. Among them is bleomycin, which is a DNA damaging antibiotic. In both E. coli and S. meliloti, the resistance to bleomycin is increased by mutations in sbmA and bacA, respectively [START_REF] Ichige | Genetic analysis of the Rhizobium meliloti bacA gene: functional interchangeability with the Escherichia coli sbmA gene and phenotypes of mutants[END_REF][START_REF] Wehmeier | Internalization of a thiazole-modified peptide in Sinorhizobium meliloti occurs by BacA-dependent and independent mechanisms[END_REF]. We observed that the bclA mutants of Bradyrhizobium strains ORS278, ORS285 and USDA110 were more resistant to bleomycin (Fig. 55A-C) and the ORS285 and USDA110

bclA genes conferred sensitivity to bleomycin in an E. coli sbmA mutant or in an S. meliloti bacA mutant (Fig. 54) indicating that BclA can mediate the import of bleomycin.

Bac7 is a proline-rich cathelicidin-type of antimicrobial peptide with an intracellular mode of action, binding ribosomes and thereby inhibiting the translation process [START_REF] Mardirossian | The host antimicrobial peptide Bac7 1-35 binds to bacterial ribosomal proteins and inhibits protein synthesis[END_REF]. The uptake of the Bac7 peptide is depending on the SbmA or BacA transporter in E. coli or S. meliloti respectively [START_REF] Mattiuzzo | Role of the Escherichia coli SbmA in the antimicrobial activity of proline-rich peptides[END_REF][START_REF] Marlow | Essential role for the BacA protein in the uptake of a truncated eukaryotic peptide in Sinorhizobium meliloti[END_REF]. Therefore we wondered whether the BclA proteins can also import the Bac7 peptide into bacterial cells. Unfortunately, the Bac7 peptide has no antimicrobial activity in the tested concentration range, up to 4 $M, on the 3 Bradyrhizobium strains used in this study (data not shown). Therefore, we tested the effect of the BclA proteins in E. coli or S. meliloti. The expression of the bclA genes of strains ORS285 and USDA110 in the E. coli sbmA or S.

meliloti bacA mutants rendered them more sensitive to the Bac7 peptide in agreement with the BclA proteins able to internalize also the Bac7 peptide (Fig. 54). S. meliloti BacA enhances the resistance of the bacterium towards NCR peptides produced by the symbiotic cells in Medicago nodules [START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF]. Therefore, we analyzed how the BclA transporter affects the bacterial sensitivity to NCR peptides.

Unfortunately, none of the currently tested Aeschynomene NCR peptides nor any of the Medicago NCR peptides had an antimicrobial activity on Bradyrhizobium or their bclA mutants [START_REF] Czernic | Convergent evolution of endosymbiont differentiation in Dalbergoid and IRLC legumes mediated by nodule-specific cysteine-rich peptides[END_REF]. Therefore, we assayed the BclA proteins in the S. meliloti background as above for the Bac7 peptide. The S. meliloti"bacA mutant is highly sensitive to 3 tested NCR peptides, NCR247, NCR335 and NCR035 and this sensitivity is strongly reduced when the bacA gene is reintroduced on a plasmid, as we reported before [START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF], or when the sbmA gene of E. coli is replacing bacA (Fig. 55D-F). Importantly, also the bclA genes of Bradyrhizobium strains ORS285 and USDA110 are able to provide protection against the activity of the tested NCR peptides. It is interesting to note that the BacA, SbmA and ORS285 or USDA110 BclA transporters have a differential effect on the different tested peptides. For example, BacA is more active against NCR335 than the two BclA proteins (Fig. 55E) while USDA110 BclA is the most active against NCR035 (Fig. 55F) but the least against Bac7 (Fig. 54).

The SbmA and BacA transporters modulate the bacterial susceptibility to bleomycin and Bac7 by promoting the uptake of these peptides [START_REF] Mattiuzzo | Role of the Escherichia coli SbmA in the antimicrobial activity of proline-rich peptides[END_REF][START_REF] Marlow | Essential role for the BacA protein in the uptake of a truncated eukaryotic peptide in Sinorhizobium meliloti[END_REF][START_REF] Wehmeier | Internalization of a thiazole-modified peptide in Sinorhizobium meliloti occurs by BacA-dependent and independent mechanisms[END_REF][START_REF] Mardirossian | The host antimicrobial peptide Bac7 1-35 binds to bacterial ribosomal proteins and inhibits protein synthesis[END_REF]. Whether the NCR peptides are also taken up by the BacA, SbmA or BclA transporters is unknown. We used a fluorescent derivative of the NCR247 peptide, carrying at its N-terminus a fluorescein isothiocyanate (FITC) modification. FITC-NCR247 had been shown to be taken up by free bacteria and bacteroids [START_REF] Farkas | The Medicago truncatula symbiotic peptide NCR247 contributes to bacteroid differentiation through multiple mechanisms[END_REF]. We used flow cytometry to measure the uptake of FITC-NCR247 by the S. meliloti strain Sm1021 or by Sm1021ΔbacA derivatives expressing from a plasmid-born copy the bacA, ORS285 bclA or USDA110 bclA gene or no gene (Fig. 56).

The mutant without any transporter was unable to take up the peptide under the applied conditions while the wild type strain or the mutant strains expressing one of the tested genes were able to transport the fluorescent peptide as revealed by the appearance of a fluorescent bacterial population. Fluorescence microscopy of sorted FITC-positive bacteria confirmed that these bacteria had taken up the NCR peptide, which appeared in the cells as discrete foci (Fig. 56). In this assay, only a fraction of the bacteria were FITC positive. The reason for this is presently not understood and requires further analysis. Possibly the NCR peptide or its FITC moiety was not stable in the cells or FITC fluorescence is quenched and only a portion of the cells have maintained sufficient FITC to be detected in the assay. In a control experiment, we used FITC alone in the assay and this compound was unable to label the cells (Fig. 57) demonstrating that the uptake of fluorescence is NCR247dependent. Together our experiments show that BclA proteins mediate resistance to NCR peptides similarly as BacA and possibly this resistance mechanism involves the uptake of the peptides by the BclA or BacA transporter proteins.

Discussion

Since its initial description in M. truncatula [START_REF] Mergaert | A novel family in Medicago truncatula consisting of more than 300 nodule-specific genes coding for small, secreted polypeptides with conserved cysteine motifs[END_REF], the use of cysteine-rich peptides or other types of antimicrobial peptides in the management of large endosymbiotic bacterial populations has now been found in several other symbiotic interactions, including in other nitrogen fixing interactions of actinorhizal plants with grampositive Frankia bacteria [START_REF] Carro | Alnus peptides modify the porosity and induce the release of nitrogen rich metabolites from nitrogen-fixing Frankia[END_REF][START_REF] Hocher | Tanscriptomics of Actinorhizal Symbiosis Reveals Homologs of the Whole Common Symbiotic Signaling Cascade[END_REF][START_REF] Demina | Comparison of the nodule vs. root transcriptome of the actinorhizal plant Datisca glomerata: actinorhizal nodules contain a specific class of defensins[END_REF] but also in interactions of several groups of insects with endosymbiotic bacteria [START_REF] Login | Antimicrobial peptides keep insect endosymbionts under control[END_REF][START_REF] Shigenobu | Aphids evolved novel secreted proteins for symbiosis with bacterial endosymbiont[END_REF][START_REF] Futahashi | Gene expression in gut symbiotic organ of stinkbug affected by extracellular bacterial symbiont[END_REF]. Even the microbiota in the gut from the most basal animals to the mammals are kept in homeostasis by antimicrobial peptides produced by host epithelial cells [START_REF] Wehkamp | Reduced Paneth cell alpha-defensins in ileal Crohn's disease[END_REF][START_REF] Ryu | Innate immune homeostasis by the homeobox gene Caudal and commensal-gut mutualism in Drosophila[END_REF][START_REF] Fraune | In an early branching metazoan, bacterial colonization of the embryo is controlled by maternal antimicrobial peptides[END_REF][START_REF] Salzman | Enteric defensins are essential regulators of intestinal microbial ecology[END_REF][START_REF] Vaishnava | The antibacterial lectin RegIIIgamma promotes the spatial segregation of microbiota and host in the intestine[END_REF]. We recently reported the production of NCR peptides in the symbiotic nodule cells of legume species belonging to the Aeschynomene genus and that their bacteroids have similar features, strong cell enlargement, polyploidy, membrane permeabilization and loss of cell viability, to those found in Medicago and other IRLC legumes [START_REF] Czernic | Convergent evolution of endosymbiont differentiation in Dalbergoid and IRLC legumes mediated by nodule-specific cysteine-rich peptides[END_REF]. These observations suggest thus the distantly related IRLC and Dalbergoid legumes have converged during evolution independently to the same strategy: controlling their endosymbionts by forcing them in an irreversible, differentiated state and using cysteine-rich peptides for this. In the present study, we show that this convergent evolution extends also to the endosymbionts' use of the BacA-related multidrug transporters to be able to respond to the NCR peptides produced by the host cells and to differentiate into functional bacteroids. We showed before that BacA is required for S.

meliloti to withstand the NCR peptides within the symbiotic nodule cells of Medicago nodules and to differentiate into elongated and polyploid bacteroids [START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF]. Here, we provide multiple lines of evidence that the Bradyrhizobium symbionts of Aeschynomene use the homologous protein BclA for the same purpose. We show that the bclA mutant of Bradyrhizobium strains ORS285 and ORS278 does not differentiate (no or little cell enlargement and DNA amplification) within the symbiotic cells of Aeschynomene nodules and that on the contrary bclA is not needed for the formation of U-morphotype bacteroids in soybean which does not produce NCR peptides in the symbiotic cells. Furthermore, we demonstrate that the bclA gene complements partially the S. meliloti bacA function in symbiosis with Medicago as well as in free living growth, conferring resistance to antimicrobial peptides that have intracellular targets and whose uptake by E. coli or S. meliloti is dependent on a SbmA-BacA-type of transporter. Finally, we show that BclA, as BacA of S.

meliloti, promotes the uptake of NCR peptides and confers protection against them.

Although BacA and BclA have a key role in symbiosis, BacA-related proteins do not have an exclusive symbiotic function since they are conserved in non-symbiotic bacteria (e.g. Mesorhizobium loti or Rhizobium leguminosarum bv. phaseoli [START_REF] Ardissone | Role of BacA in lipopolysaccharide synthesis, peptide transport, and nodulation by Rhizobium sp. strain NGR234[END_REF][START_REF] Karunakaran | BacA is essential for bacteroid development in nodules of galegoid, but not phaseoloid, legumes[END_REF][START_REF] Maruya | The bacA Gene Homolog, mlr7400, in Mesorhizobium loti MAFF303099 is dispensable for symbiosis with Lotus japonicus but partially capable of supporting the symbiotic function of bacA in Sinorhizobium meliloti[END_REF]This work). The 'housekeeping' role of these transporters is hitherto unknown but apparently their capacity to import peptides has been co-opted at least twice among the rhizobia to respond to or provide protection against peptides produced by the host cell. It will be interesting in the future to determine whether BacA-related proteins have a role in other symbiotic interactions as well.

E. coli or

The S. meliloti bacA mutant dies rapidly after its release from the infection threads in Medicago nodules as a consequence of its hypersensitivity to the NCR peptides and is then lysed [START_REF] Glazebrook | A Rhizobium meliloti homolog of the Escherichia coli peptide-antibiotic transport protein SbmA is essential for bacteroid development[END_REF][START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF]. This results in nodules in which the symbiotic cells filled with bacterial cells rapidly disappear. Here we find that the Bradyrhizobium bclA mutant bacteria have a much higher capacity to persist within the symbiotic cells of Aeschynomene nodules than the S. meliloti bacA mutant in Medicago symbiotic cells. This suggests that the Aeschynomene symbiotic cells produce a much less aggressive arsenal of peptides than the Medicago symbiotic cells. In agreement with this, none of the tested Aeschynomene peptides displayed antimicrobial activity [START_REF] Czernic | Convergent evolution of endosymbiont differentiation in Dalbergoid and IRLC legumes mediated by nodule-specific cysteine-rich peptides[END_REF]. Alternatively, Bradyrhizobium could have a better capacity than S. meliloti to maintain its cell integrity after exposure to membrane disrupting molecules such as the NCRs. Such a capacity could be related to the unusual membrane composition of these bacteria which is extremely rich in hopanoids (Silipo et al., 2014). Hopanoids are pentacyclic triterpenoids, structural similarity with eukaryotic sterols. Hopanoids stabilize bacterial membranes, controlling its fluidity, permeability and integrity under stress conditions.

Even if the survival of the bclA mutant is not immediately affected after their release in the symbiotic cells, it is clearly so at later time points in the A. indica and A. evenia symbiotic cells but not in the A. afraspera symbiotic cells, suggesting that the former may produce a spectrum of peptides which are more virulent to the bacteria than those of the latter.

This difference translates to a more severely reduced nitrogen fixation capability of the bclA mutant bacteroids in A. indica or A. evenia nodules compared to A. afraspera. The different NCR spectrum could also be at the basis of the S-morphotype versus E-morphotype bacteroids formed by A. indica and A. evenia compared to A. afraspera [START_REF] Czernic | Convergent evolution of endosymbiont differentiation in Dalbergoid and IRLC legumes mediated by nodule-specific cysteine-rich peptides[END_REF].

The bclA genes of both Bradyrhizobium strains ORS285 and USDA110 partially complement the bacA mutation of S. meliloti in symbiosis with Medicago. The complementation is obvious by the production of the late developmental marker leghemoglobin which is never produced in the mutant nodules and by the elongated bacteroids which are not stained with the PI marker for bacterial death. Nevertheless, these nodules remain smaller than wild-type nodules, they do not accumulate leghemoglobin to the same extent, they senesce earlier than normal as indicated by the PI staining of the bacteroids when symbiotic cells grow older, and the nodules do not fix nitrogen or support plant growth. This partial complementation resembles the complementation level obtained by the Mycobacterium tuberculosis bacA gene [START_REF] Arnold | Partial complementation of Sinorhizobium meliloti bacA mutant phenotypes by the Mycobacterium tuberculosis BacA protein[END_REF]. We show that BacA of S. meliloti and the BclA proteins from Bradyrhizobium strains ORS285 and USDA110 have a differential activity towards different NCR peptides. Also the Mycobacterium and S. meliloti BacA proteins have a differential response towards derivatives of the Bac7 peptide [START_REF] Arnold | Partial complementation of Sinorhizobium meliloti bacA mutant phenotypes by the Mycobacterium tuberculosis BacA protein[END_REF]. Therefore, the partial complementation in symbiosis could result from a different spectrum of peptides that can be transported or different transport kinetics by the endogenous BacA of S. meliloti and the Bradyrhizobium BclA proteins or M. tuberulosis BacA.

In this respect, it is interesting to note that although BacA, SbmA and BclA share a SbmA_BacA transmembrane domain, they have an otherwise different architecture and transport mechanism: SbmA and BacA lack an ATPase domain and transport is driven by the membrane electrochemical gradient [START_REF] Runti | Functional characterization of SbmA, a bacterial inner membrane transporter required for importing the antimicrobial peptide Bac7(1-35)[END_REF] while BclA and Mycobacterium

BacA have a C-terminal cytosolic ATPase domain that provides the energy for transport [START_REF] Arnold | Partial complementation of Sinorhizobium meliloti bacA mutant phenotypes by the Mycobacterium tuberculosis BacA protein[END_REF].

Medicago produces NCR peptides in different waves and the large majority of peptides are produced at later stages [START_REF] Maunoury | Differentiation of symbiotic cells and endosymbionts in Medicago truncatula nodulation are coupled to two transcriptome-switches[END_REF][START_REF] Guefrachi | Extreme specificity of NCR gene expression in Medicago truncatula[END_REF]. Possibly the BclA proteins or M. tuberculosis BacA are sufficient to tolerate the early waves of NCR production but become limited when the later waves are activated, ultimately resulting in the death of the bacteroids, even after their differentiation.

The very weak differentiation of B. japonicum strain USDA110 in A. afraspera nodules is unlikely to be the result of its bclA gene because the USDA110 bclA mutant has a similar response to bleomycin as the corresponding mutants in strains ORS285 and ORS278.

Moreover, the USDA110 bclA gene complements S. meliloti bacA and E. coli sbmA mutants similarly to the corresponding genes of ORS285 and ORS278. The Bradyrhizobium strains have a high proportion of shared genes but have nevertheless also a substantial number of unique genes [START_REF] Giraud | Legumes symbioses: Absence of Nod genes in photosynthetic Bradyrhizobia[END_REF]. Probably the different genetic repertoires between the strains are at the basis of the differential response of the strains to the bacteroid differentiation factors produced by the symbiotic host cells. This difference could be exploited to identify additional bacterial factors involved in the differentiation process and the response to the NCR peptides. Alternatively, it is possible that the NCR genes are not or only weakly induced in A. afraspera nodules infected with B. japonicum strain USDA110.

The M. tuberculosis bacA gene can complement the S. meliloti bacA mutation [START_REF] Arnold | Partial complementation of Sinorhizobium meliloti bacA mutant phenotypes by the Mycobacterium tuberculosis BacA protein[END_REF]. It was also shown recently that the M. tuberculosis transporter can promote the uptake of vitamin B12 [START_REF] Gopinath | A vitamin B 12 transporter in Mycobacterium tuberculosis[END_REF]. However, we believe that a defect in vitamin B12 uptake is unlikely at the origin of the symbiotic phenotype of the bacA or bclA mutants.

Bradyrhizobium and Sinorhizobium have a complete biosynthesis pathway [START_REF] Rodionov | Comparative genomics of the vitamin B 12 metabolism and regulation in prokaryotes[END_REF] and in S. meliloti the pathway genes are needed for symbiosis suggesting that the pathway is active in the bacteroids and that the bacteroids produce their own vitamin B12 [START_REF] Campbell | Sinorhizobium meliloti bluB is necessary for production of 5,6-dimethylbenzimidazole, the lower ligand of B12[END_REF][START_REF] Taga | BluB cannibalizes flavin to form the lower ligand of vitamin B 12[END_REF][START_REF] Taga | Sinorhizobium meliloti requires a cobalamin-dependent ribonucleotide reductase for symbiosis with its plant host[END_REF]. In addition, Bradyrhizobium and

Sinorhizobium have an identified putative vitamin B12 transport system [START_REF] Rodionov | Comparative genomics of the vitamin B 12 metabolism and regulation in prokaryotes[END_REF]. Moreover, plants do not produce and use vitamin B12 as a cofactor [START_REF] Roth | Cobalamin (coenzyme B 12 ): Synthesis and biological significance[END_REF] and thus a vitamin B12 transporter would be superfluous within the symbiotic cells. Finally, our experimental data suggest that bclA has no impact on cell growth nor nitrogen fixation in the presence or absence of external vitamin B12. Of course, this does not exclude that the BacA or BclA proteins can transport vitamin B12, depending on the environmental context of the bacteria.

BacA-related transporters are not only critical in rhizobia for chronic intracellular infections of legume hosts, which produce NCRs in the symbiotic cells. They also contribute to the pathogenesis and chronic infection in pathogens and they do this most likely by protecting the bacteria against host antimicrobial peptides ( [START_REF] Levier | Similar requirements of a plant symbiont and a mammalian pathogen for prolonged intracellular survival[END_REF][START_REF] Li | Identification of genes required for avian Escherichia coli septicemia by signature-tagged mutagenesis[END_REF][START_REF] Tan | Macrophages acquire neutrophil granules for antimicrobial activity against intracellular pathogens[END_REF][START_REF] Rivas-Santiago | Human β-defensin 2 is expressed and associated with Mycobacterium tuberculosis during infection of human alveolar epithelial cells[END_REF][START_REF] Rivas-Santiago | β-Defensin gene expression during the course of experimental tuberculosis infection[END_REF][START_REF] Domenech | BacA, an ABC transporter involved in maintenance of chronic murine infections with Mycobacterium tuberculosis[END_REF][START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF][START_REF] Arnold | Partial complementation of Sinorhizobium meliloti bacA mutant phenotypes by the Mycobacterium tuberculosis BacA protein[END_REF]. However, how the transporters function mechanistically and contribute to resistance against antimicrobial peptides requires further investigation. An attractive hypothesis is that the internalization of peptides would move them away from the bacterial membrane which is the main target of many antimicrobial peptides, including the toxic NCRs, and at the same time bring them into proximity of intracellular targets or set them up for degradation. In the case of peptides such as Bac7 or bleomycin, the transporter would be a Trojan horse for the bacterial cell, bringing the enemy inside its walls, within reach of its intracellular targets.

Material & Methods

Material & Methods

Analysis of MtGEA data

The MtGEA transcriptome compendium was downloaded from the website of the Samuel Roberts Noble foundation (http://mtgea.noble.org/v3/). The data from [START_REF] Limpens | Cell-and tissue-specific transcriptome analyses of Medicago truncatula root nodules[END_REF] and [START_REF] Jayaraman | Response of Medicago truncatula seedlings to colonization by Salmonella enterica and Escherichia coli O157:H7[END_REF] were obtained from the NCBI Gene Expression Omnibus (accession n° GSE53406). All the data were imported in Excel ( 1). Each individual NCR nucleotide sequence resulted in the identification of multiple probe-sets due to the homology between NCR gene sequences. In total 334 different probesets were retrieved. This collection represent likely nearly all NCR probe-sets present on the Affymetrix Medicago GeneChip and the remaining genes identified in [START_REF] Branca | Whole-genome nucleotide diversity, recombination, and linkage disequilibrium in the model legume Medicago truncatula[END_REF] and [START_REF] Zhou | Detecting small plant peptides using SPADA (Small Peptide Alignment Discovery Application)[END_REF] are missing from these arrays because they were not yet annotated at the time of array design.

Cluster analysis of the complete MtGEA dataset was performed using the MeV software package (http://sourceforge.net/projects/mev-tm4/). Briefly, the Excel datasheet extracted from MtGEA was analysed using the Euclidean distance application with average linkage settings. Heatmaps were generated with MeV and histograms and graphs with Excel.

Entropy calculations

Calculations were performed on the MtGEA dataset in Excel. For the normalization of expression levels in N tissues, the relative expression P t/g of a gene g in a tissue t was calculated as P t/g = W t/g /S 1≤t≤N W t/g where W t/g is the expression level of the gene g in the tissue t.

The Shannon entropy E g of gene g is calculated as E g = S 1≤t≤N -P t/g log 2 (P t/g ). E g ranges from zero for genes expressed in a single tissue to log 2 (N) for genes expressed uniformly in all tissues considered. Heatmaps of entropy values were generated by the MeV software package.

Transcriptome analysis of hand-dissected nodule zones

Using the leghemoglobin colour gradient along the nodule as guideline, five regions from 28 dpi nodules were hand-dissected, as previously described [START_REF] Zhou | From model to crop: Functional analysis of a STAY-GREEN gene in the model legume Medicago truncatula and effective use of the gene for alfalfa improvement[END_REF]. These samples correspond to the nodule tissues from the most apical part of the nodule with the youngest symbiotic cells to the most proximal part containing the oldest symbiotic cells.

Sample I is the meristem and the underlying few cell layers of post-meristematic cells which start the infection and differentiation process. Sample II corresponds mainly to the infection and differentiation zone II. Sample II-III corresponds essentially to the interzone II-III.

Sample III is the nitrogen fixation zone III, easily characterized by its pink color due to the accumulation of high amounts of leghemoglobin and finally sample IV is the senescence zone IV that is recognized by its green color resulting from the accumulation of biliverdin, a product of the catabolism of leghemoglobin-derived heme. It should be noted that each of these hand-dissected samples is enriched for the indicated zone but can contain cell layers form the adjacent zones as well.

Total RNA extraction and purification were conducted as described [START_REF] Benedito | A gene expression atlas of the model legume Medicago truncatula[END_REF]. For hybridization onto the Affymetrix M. truncatula Genechip Array probes were synthesized and labelled from 500 ng RNA using the Gene Chip 3'IVT express kit following manufacturer's guidelines (Affymetrix). Global normalization of expression was carried out using the Robust Multiarray Average Express software [START_REF] Irizarry | Exploration, normalization, and summaries of high density oligonucleotide array probe level data[END_REF].

Transgenic plants and GUS analysis

The promoters of NCR001, NCR084 and NCR121 (respectively 2.5kb, 1.5kb and 1kb fragments upstream of the ATG) were obtained by an Amplified Fragment-Length Polymorphism (AFLP) based PCR protocol as described [START_REF] Ratet | The Handbook of Plant Mutation Screening (Mining of Natural and Induced Alleles)[END_REF] and recombined in the Gateway vector pDONRP4-P1R according to the manufacturer's instructions (Invitrogen). Primers used for the amplification and cloning of the promoters are listed in Entry clones for the GUS ORF and the 35S terminator were obtained in the Gateway vectors pDONR221 and pDONRP2R-P3, respectively (Van de Velde et al., 2010). Entry clones were recombined in the binary vector pKm43GW [START_REF] Karimi | Modular cloning in plant cells[END_REF]. Leaf explants from the M. truncatula line R108 were transformed using Agrobacterium tumefaciens according to the method described in [START_REF] Cosson | Medicago truncatula transformation using leaf explants[END_REF].

For GUS analysis, three independent T2 transgenic lines were each time analysed to avoid positional effects of the transgene insertion. No variations were observed between independent lines. Untransformed plants and the constitutive GUS line pG3.3 (35S promoter fused to GUS) [START_REF] Mondy | GOLLUM [FeFe]-hydrogenase-like proteins are essential for plant development in normoxic conditions and modulate energy metabolism[END_REF] were used as negative and positive controls respectively.

For nodulation kinetics, R108 plants were cultivated on BNM agar plates and inoculated with OD 600nm = 0.1 suspensions of S. meliloti strain 1021 or Sinorhizobium arboris strain B554

(synonymous strain names LMG 14919 and HAMBI 1552) [START_REF] Nick | Sinorhizobium arboris sp. nov. and Sinorhizobium kostiense sp. nov., isolated from leguminous trees in Sudan and Kenya[END_REF] which is an excellent symbiont of M. truncatula R108 forming numerous large, nitrogen fixing nodules.

Samples were collected at indicated time points and embedded in 6% agarose. Tissue sections of 70 µm were prepared with a Leica VT1200S vibratome. GUS staining was done as described [START_REF] Vanstraelen | APC/C-CCS52A complexes control meristem maintenance in the Arabidopsis root[END_REF] and was allowed to proceed for 1h (Fig. 20). Overnight staining did not alter the expression patterns (data not shown). The pattern of expression of the NCR genes in nodules induced by both Sinorhizobium strains were very similar.

For all pathogen assays, plants were cultivated on perlite/sand (3/1 vol/vol) substrate and watered with a commercial nutrient solution. Six weeks old plants were transferred to a growth chamber with saturating humidity the day before the inoculations and stayed in these conditions all along the assay. Dickeya dadantii 3937, Pseudomonas syringaepv. tomato DC3000 and its hrcC derivative strain were cultivated at 30°C in LB medium. Inocula of OD 600nm = 0.1 were resuspended in 10 mM MgCl 2 and were syringe infiltrated in the terminal leaflet of 5-8 leaves per plant. Sterile 10 mM MgCl 2 solution was infiltrated as mock control.

Botrytis cinerea strain B05.10 was cultivated on PDA medium [START_REF] Amselem | Genomic analysis of the necrotrophic fungal pathogens Sclerotinia sclerotiorum and Botrytis cinerea[END_REF] at 20°C. Spores were collected in ½ potato dextrose broth with 0.01% Tween 20 and inocula were normalized to 10 6 spores/mL using a Malassez cell. Five microliter drops of mock/inoculum were put on 5 to 8 terminal leaflets per plant. Symptoms were scored at 1, 2 or 7 dpi and leaflets were collected for GUS staining. For wounding experiments, the terminal leaflet of 5-8 leaves per plant were pinched with forceps and collected 24 hours post wounding. Staining for all infections or treatments was allowed for 24 hours in the GUS staining solution at 37°C. The leaflets were transferred to bleach to remove chlorophyll before photographing.

Antibodies and immunolocalization

The mature region of the NCR122 peptide was amplified from cDNA and cloned into the expression vector pBADgIII/A (Invitrogen). Recombinant proteins were purified according to the manufacturer's instructions and used for immunization of rabbits by a commercial service (Agro-bio). Immunolocalisations were done exactly as described before [START_REF] Van De Velde | Plant peptides govern terminal differentiation of bacteria in symbiosis[END_REF]. For the SYTO13 nucleic acid staining, nodules sections were incubated for 5 minutes with 1 μM SYTO13 in H 2 O. Immuno-or SYTO13-stained sections were mounted in deionised water for confocal imaging. Fluorescence images were acquired at Leica, using 10X water-immersion and 63X oil-immersion objectives and Leica software.

Images were processed with Adobe Photoshop for adjustment of contrast and brightness.

Bacterial strains and growth media

Bradyrhizobium strains were grown in Yeast Mannitol (YM) [START_REF] Giraud | Effect of Bradyrhizobium photosynthesis on stem nodulation of Aeschynomene sensitiva[END_REF] or BNM-B medium [START_REF] Renier | Photosynthetic Bradyrhizobium Sp. strain ORS285 synthesizes 2-O-methylfucosylated lipochitooligosaccharides for nod gene-dependent interaction with Aeschynomene plants[END_REF], E. coli in Luria Bertani (LB) or Mueller-Hinton (MH) medium and S. meliloti strains in LB medium. Bacteria were grown on agar plates (1.5%) or in liquid cultures under agitation at 30°C for Bradyrhizobiumor S. meliloti and at 37°C for E.

coli. When required, the media were supplemented with the appropriate antibiotics.

Antibiotics were used at the following concentrations: streptomycin (Sm), 500 μg/ml; spectinomycin (Sp), 100 μg/ml; chloramphenicol (Cm), 12.5 μg/ml; gentamycin (Gm), 15 μg/ml or 50 μg/ml; carbenicilin (Cb), 50 μg/ml; tetracycline (Tc), 10 μg/ml, kanamycin (Km), 50 μg/ml or 200 μg/ml, neomycin (Nm), 120 μg/ml and nalidixic acid (Nal) 25 μg/ml. All bacterial strains and plasmids used in this study are listed in Table 7 (Annex 5).

Construction of bacterial mutants and complemented strains

Standard molecular biology techniques were used for all cloning work. All primers used for cloning of DNA fragments of interest are listed in Bradyrhizobium strain ORS285 by triparental mating using the helper strain HB101.pRK600 [START_REF] Finan | Second symbiotic megaplasmid in Rhizobium meliloti carrying exopolysaccharide and thiamine synthesis genes[END_REF]. ORS285 clones with the plasmid integrated in the genome were selected on YM medium with gm (15 µg/ml) for counter-selection of the E. coli and Km for selection of the plasmid. The integration of the plasmids in the target genes by homologous recombination was verified by PCR. The bacA mutant in S. fredii strain HH103 was constructed with a similar strategy but using plasmid pK18mob [START_REF] Schafer | Small mobilizable multi-purpose cloning vectors derived from the Escherichia coli plasmids pK18 and pK19: Selection of defined deletions in the chromosome of Corynebacterium glutamicum[END_REF].

Deletion mutants of the bclA genes in strains ORS285 and USDA110 (genes BRAO285v1_1320006 and blr7537, respectively) were obtained as follows. The 600 bp upstream and downstream regions of the bclA open reading frames were obtained by PCR (see Table 8, Annex 6 for primer sequences). The two regions were merged by sewing PCR and cloned in the pGEM-Teasy vector (Promega). The Gm resistance cassette from plasmid p34S-Gm (Dennis and Zylstra, 1998) was cloned in between the upstream and downstream regions. The fragments of the upstream region-Gm cassette-downstream region were subsequently transferred into the vector pK18mob-sacB [START_REF] Schafer | Small mobilizable multi-purpose cloning vectors derived from the Escherichia coli plasmids pK18 and pK19: Selection of defined deletions in the chromosome of Corynebacterium glutamicum[END_REF]. This plasmid

Plant growth and nodulation

A. indica, A. evenia or A. afraspera seeds were surface sterilized with concentrated sulfuric acid for 45 min followed by 30 min with bleach. The seeds were germinated overnight at 34°C in the dark on tap-water agar plates. One day-old seedlings were transferred to test tubes containing BNM medium [START_REF] Ehrhardt | Depolarization of alfalfa root hair membrane potential by Rhizobium meliloti nod factors[END_REF]. Seedlings were grown at 28°C with a 16 h light regime and 70% humidity. Seven days after germination, each seedling was inoculated with 2 mL of a bacterial suspension adjusted to and OD 600nm of 0.2.

Glycine max (Williams 82) seeds were cleaned with 100% ethanol for 30 sec and sterilized with bleach for 15 min. Seeds were germinated on tap-water agar plates at room temperature for two days. Seedlings were then transferred to bottles filled with BNM medium, inoculated and grown as described above for Aeschynomene plants.

Medicago sativa (alfalfa) seeds were sterilized with bleach for 15 min and germinated on tap-water agar plates at room temperature overnight. Seedlings were planted in pots containing a mixture of perlite and sand (3:1, v/v) and grown at 24°C with a 16 h light regime and 40% humidity. Three-days old plantlets were inoculated with a bacterial suspension as described above. Plants were watered with BNM medium.

Acetylene reduction assay on plants

Acetylene reduction assays were performed on plants 2 weeks after inoculation with a protocol modified from [START_REF] Koch | Reduction of acetylene to ethylene by soybean root nodules[END_REF]. Single whole plants were placed into 20 ml glass vials sealed with rubber septa prefilled with 200 μL of sterile water to avoid drying of the plant during the experiment. Two hundred μL of acetylene was injected into each vial.

Gas samples (200 μL) werewithdrawn after 3 hours of incubation at 28°C and the ethylene produced was measured by gas chromatography on a 7820A GC system (Agilent Technologies). The assay was done with a minimum of 5 replicates and included always measurements on uninoculated plants.

Bacterial growth and nitrogen fixation assays

Bradyrhizobium ORS285 and ORS285ΔbclA were grown in YM medium, washed and the pellet was resuspended in different media: in rich YM medium; in minimal BNM-B medium; in BNM-B medium in which the vitamin B12 component was omitted and in BNM-B medium in which the only carbon source was glycolate (BNM-B glycolate). The bacterial suspensions were diluted to OD 600nm = 0.01 and then distributed in microtiter plates, 100 μl per well. The plates were incubated with agitation (200 rpm) at 30°C and the bacterial growth was monitored by optical density measurements (OD 600nm ) in a plate reader at different times.

For free-living nitrogen fixation activity measurements with the acetylene reduction assay, pre-cultures in YM medium were pelleted by centrifugation and resuspended in BNM-B medium without nitrogen source. When indicated, the vitamin B12 component was omitted from the medium. Two hundred μl of cell suspensions at OD 600nm = 1.5 were added to 10 ml glass vials containing 7 mL melted 1% agar medium, precooled at 45°C. The tubes were sealed with rubber septa, 0.85 ml air from the container was removed with a syringe, and 0.8 ml of 100% acetylene was injected. The cultures were incubated at 28°C in the dark during 7 days. Gas samples were analyzed for the amount of ethylene formed by gas chromatography as above.

Microscopy

Histological analysis of nodules

Nodules were fixed in 1% glutaraldehyde, 4% formaldehyde in 0,1 M Phosphate buffer pH 7.2 then washed, dehydrated and embedded in Technovit 7100 resin (KulzerHistoTechnik). 5 μm sections were cut with a Leica RM2155 microtome, stained with 0.005% Toluidine Blue and then mounted with DPX (VWR International Ltd). Bright filed images were acquired with an Eclipse 80i microscope (Nikon).

In vivo Live-Dead staining and confocal microscopy

Nodules were harvested, embedded with 6% agarose and then freshly sectioned with a Leica VT1200S vibratome (Leica Microsystems GmbH, Germany) into 70 μm tissue slices.

Slices were incubated in Live/Dead BacLight (Molecular Probes) staining solution for 20 min [START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF]. Section were washed from excess of dye and observed using a Leica TCS SP8X confocal microscope. 488 and 543 nm laser lines from a white light laser (Leica) were sequentially used and SYTO9 and PI fluorescence intensity were recorded into separated channels.

In vivo DND-99 staining and confocal microscopy

Nodules were freshly sectioned as described above and incubated 20 min with 5 μM DND-99 LysoTracker Red (Molecular Probes) in a 50 mM Tris buffer pH 7.0 [START_REF] Pierre | Peribacteroid space acidification: a marker of mature bacteroid functioning in Medicago truncatula nodules[END_REF]. Excess of dye was removed with buffer and sample were imaged with a Leica TCS SP8X confocal microscope after a 543 nm laser line illumination.

Fluorescent in situ Hybridization

A. indica and A. afraspera nodules were harvested at 14 dpi and fixed immediately with 4% paraformaldehyde containing 0.1% Tween 20 and 0.1% Triton X100 at 4°C overnight. Nodule tissues were dehydrated by passing in progressively higher concentrations of ice-cold ethanol series. They were then embedded in paraffin blocks as described by [START_REF] Javelle | In situ hybridization for the precise localization of transcripts in plants[END_REF]. Sections (7 μm thick) were cut with a microtome RM2155 (Leica, Wetzlar, Allemagne). Sections were adhered on poly-L-lysine-coated slides and paraffin was subsequently removed with histoclear and histoclear/ethanol series. To produce the hybridization probes, PCR products of approximately 200 bp of the genes of interest were cloned into the pGEM-T Easy plasmid. The inserts were then amplified by PCR using the vector-specific T7 and SP6 primers flanking the insert. These primers correspond to the bacteriophage SP6 and T7 RNA polymerase promoters. RNA probes, labeled with digoxigenin (DIG), were obtained by in vitro transcription using the PCR product as a template and the T7 RNA polymerase of the DIG RNA Labeling Kit (SP6/T7) (Roche, Boulogne-Billancourt, France). Antisense probes were used to detect the location of transcripts of interest and sense probes were used as negative controls. A control without probe was also performed.

Pre-hybridization, hybridization, post-hybridization treatments and mounting were done by a method derived from the procedure described by [START_REF] Javelle | In situ hybridization for the precise localization of transcripts in plants[END_REF]. To reveal the hybridization signal, an anti-DIG antibody coupled to alkaline phosphatase was used with its substrate NBT/BCIP (4-nitro blue tetrazolium chloride/5-bromo-4-chloro-3-indolylphosphate) which forms a brownish precipitate. The slides were finally observed under a Reichert Polyvarepifluorescence microscope (Buffalo, USA) combined with a Retiga 2000 CCD camera (QImaging, Surrey, Canada).

Bacteroid purification and flow cytometry analysis

Bacteroids or cultered bacteria were purified as described [START_REF] Mergaert | Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis[END_REF].

Bacteria were fixed by heat treatment (70°C, 10min) and stained by 20 μg/ml of DAPI before flow cytometry analysis.

The bacteroid preparations and similarly prepared suspensions of bacteria grown in culture were analyzed by flow cytometry using a MoFlo ADTRIOS flow cytometer (Beckman Coulter). Bacterial size was estimated by the Forward Scatter (FS) and the bacterial DNA content was assessed by the DAPI fluorescence with a 355 nm laser line. Each single event was recorded and analyzed with the Summit 6.2 software (Beckman Coulter).

Experiments have been performed in at least 3 biological replicates.

Bleomycin, Bac7 and NCR sensitivity assays

E. coli, S. meliloti and Bradyrhizobium stains were grown in liquid MH, LB and YM medium at 37°C, 30°C or 30°C, respectively in the presence of appropriate antibiotics for plasmid selection. Within an experiment, cultures were diluted until all strains grew at the same optical density. Sensitivity tests were performed on cultures that had not grown over OD 600nm = 0.5. All experiments have been performed at least in triplicate with identical results.

A representative example is shown in the figures.

For bleomycin sensitivity tests, bacterial suspensions were diluted with fresh medium until OD 600nm = 0.005 for E. coli and S. meliloti or OD 600nm = 0.01 for Bradyrhizobium strains.

The bacterial suspensions were distributed in microtiter plates, 100 μl per well, and bleomycin stock solution was added to reach the desired final concentration. The plates were incubated with agitation (200 rpm) at 30°C for Bradyrhizobium and S. meliloti or at 37°C for E. coli. Growth was determined after 24h for E. coli, 48h for S. meliloti or 72h for Bradyrhizobium by measurement of the OD 600nm with a plate reader.

For Bac7 sensitivity tests, S. meliloti or E. coli suspensions were diluted to OD 600nm = 0.005 in LB medium and 100 μl suspensions were incubated in the presence of 2 μM or 4 μM Bac7 peptide at 30°C or 37°C for 4h or 3 and 6h respectively. Ten-fold dilution series were then spotted on agar plates and the colony forming units were determined after 72h or 24h growth at 30°C or 37°C.

NCR sensitivity assays were performed as before [START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF]. S. meliloti strains were re-suspended in 10 mM sodium phosphate buffer, pH 7.0. Hundred microliter of the suspensions were incubated with the indicated NCR peptides at a final concentration of 50 μg/ml for 4h at 30°C. Ten-fold dilution series were then spotted on agar plates and the colony forming units were determined after 72h at 30°C.

Single cell NCR peptide uptake assay and flow cytometry

Strains were growth in LB medium to mid-log phase, below OD 600nm = 0.5, washed and resuspend at OD 600nm = 0.05 in 50 mM phosphate buffer pH 7.0. Then, 1 μM FITC-NCR247 [START_REF] Farkas | The Medicago truncatula symbiotic peptide NCR247 contributes to bacteroid differentiation through multiple mechanisms[END_REF] or free FITC were added and cells were incubated for 1h at 30°C. Cells were washed and resuspended in 50 mM phosphate buffer pH 7.0, supplemented with 1 mg/ml Trypan Blue used as extracellular fluorescence quencher [START_REF] Mattiuzzo | Role of the Escherichia coli SbmA in the antimicrobial activity of proline-rich peptides[END_REF]. FITC uptake was assayed using a MoFlo ADTRIOS flow cytometer (BeckmanCoulter) with a 488 nm laser and fluorescence was recorded through a 526/552 nm band pass filter. More than two hundred thousand events were acquired and analyzed with Summit 6.2 software (BeckmanCoulter). To further confirm the internalization of FITC-NCR247, FITC-positive cells were sorted and imaged with an Eclipse 80i microscope (Nikon) equipped with a FITC-appropriate filter set.

General Discussion & Perspectives

General Discussion and Perspectives

Symbiosis

« No organism is an island. » This variation of the saying « No man is an island. » means that no organism can survive independently of the others and its environment.

Interactions between organisms can take many forms. Symbiosis is a particular form of a sustainable interaction between different organisms that appeared several times during evolution. These associations play a central role in contemporary ecosystems. They are, to name just a few, essential for atmospheric nitrogen fixation by plants, for development of coral reefs or for food digestion and many other complex tasks in the human gut. Intracellular symbioses between eukaryotes and bacteria are wide-spread in most ecosystems. Hostassociated bacteria have broad phylogenetic distribution and are found in most bacterial phyla. Nevertheless, some bacterial clades such as the α-, β-, and γ-proteobacteria are particularly rich in intracellular symbionts (Toft and Andersson, 2010). Reciprocally, the eukaryotic hosts for these bacteria also span the tree of live ranging from fungi, protists, animals to plants.

Symbiont transmission maintains symbioses through host generations and has a pivotal role in their evolution [START_REF] Ewald | Transmission modes and evolution of the parasitism-mutualism continuum[END_REF]Kato el al., 1999). Two fundamentally different modes of transmission can be distinguished: horizontal (that is, from an environmental, freeliving symbiont source) and vertical (that is, inheritance of the symbiont from the mother or, more rarely, from both parents). Traditionally, bacterial symbionts are divided into two groups. The first consists of obligate mutualists required to support normal host development.

These symbionts, also called primary symbionts are typically restricted to a specialized symbiotic organ [START_REF] Baumann | Biology of bacteriocyte-associated endosymbionts of plant sap-sucking insects[END_REF]. They occur in many terrestrial arthropods as well as some marine invertebrates. In contrast, facultative or secondary symbionts resemble invasive pathogens in that they may invade various cell types, including reproductive organs, and may reside extracellularly in the body cavity (hemolymph) [START_REF] Dobson | Wolbachia infections are distributed throughout insect somatic and germ line tissues[END_REF][START_REF] Fukatsu | The secondary endosymbiotic bacterium of the pea aphid Acyrthosiphon pisum (Insecta : Homoptera)[END_REF][START_REF] Moran | Genomics and evolution of heritable bacterial symbionts[END_REF].

Bacterial intracellular symbiosis (endosymbiosis) is widespread in invertebrates and exhibits a large variety of phenotypes, ranging from mutualism to pathogenesis. A single bacterium can be pathogenic on some hosts and beneficial for some others, like Xenorhabdus nematophila that colonize the entomopathogenic nematode Steinernema carpocapsae in a mutualistic manner and is pathogenic to insects notably by induction of macrophage apoptosis (Herbert and Goodrich-Blair, 2007). Another example is Wolbachia commonly found in arthropods and nematodes, being partly pathogenic to the firsts and mutualistic to the latter (Iturbe- [START_REF] Ormaetxe | Wolbachia-host interactions: connecting phenotype to genotype[END_REF][START_REF] Werren | Wolbachia: Master manipulators of invertebrate biology[END_REF]Olivier et al., 2010).

Other well documented symbioses involve organ development and often require an active participation of the microsymbiont in the organogenesis. The marine squid Euprymna scolopes for example has a light-emitting organ which is colonized by luminescent Vibrio fischeri bacteria [START_REF] Nyholm | The winnowing: establishing the squid-vibrio symbiosis[END_REF]. Aphids and other insect groups harbor intracellular symbionts, in specialized cells called bacteriocytes which are organized in the bacteriome organ located on the abdomen of the insect [START_REF] Baumann | Biology of bacteriocyte-associated endosymbionts of plant sap-sucking insects[END_REF][START_REF] Moran | Genomics and evolution of heritable bacterial symbionts[END_REF]. These insects exploit restricted nutritional resources such as plant sap and their bacterial symbionts produce nutrients that are lacking in the host's diet. These examples constitute fascinating models of endosymbioses that can be used for comparisons with the Rhizobium-legume model [START_REF] Baumann | Genetics, physiology, and evolutionary relationships of the genus Buchnera: intracellular symbionts of aphids[END_REF][START_REF] Visick | Vibrio fischeri and its host: it takes two to tango[END_REF]. Indeed, some aspects of bacteroid physiology are reminiscent of stages of obligatory endosymbionts in insects. For example genome amplification coupled to cell enlargement has been observed, as we reported for the microsymbionts of IRLC legumes or the Dalbergoid legumes [START_REF] Komaki | Intracellular bacterial symbionts of aphids possess many genomic copies per bacterium[END_REF][START_REF] Komaki | Genomic copy number of intracellular bacterial symbionts of aphids varies in response to developmental stage and morph of their host[END_REF][START_REF] Mergaert | Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium-legume symbiosis[END_REF] Chapter II). The morphological modifications these endosymbionts are undergoing inside the host cell such as formation of spheres or elongated cells is similar to the formation of Smorphotype or E-morphotype bacteroids [START_REF] Shigenobu | Genomic revelations of a mutualism: the pea aphid and its obligate bacterial symbiont[END_REF]The International Aphid Genomics Consortium, 2010;[START_REF] Tsuchida | Symbiotic bacterium modifies aphid body color[END_REF][START_REF] Login | Antimicrobial peptides keep insect endosymbionts under control[END_REF]. Strikingly, this symbiont differentiation occurs in parallel to the differentiation of the bacteriocytes by enlargement and endoredupliaction, similarly to the nodule symbiotic cells [START_REF] Buchner | Endosymbiosis of animals with plant microorganisms[END_REF][START_REF] Douglas | The mycetocyte symbiosis of aphids: Variation with age and morph in virginoparae of Megoura viciae and Acyrthosiphon pisum[END_REF][START_REF] Cebolla | The mitotic inhibitor ccs52 is required for endoreduplication and ploidy-dependent cell enlargement in plants[END_REF].

Symbiotic antimicrobial peptides are wide-spread in plants and animals

The involvement of host antimicrobial peptides in controlling the endosymbionts' multiplication and cell fate was described first in the legume Medicago but is not unique for the Rhizobium-legume symbiosis. In insects of the Sitophilus genus (weevils), the bacteriocytes contain giant filamentous endosymbionts and produce the antimicrobial peptide coleoptericin-A (ColA) [START_REF] Login | Antimicrobial peptides keep insect endosymbionts under control[END_REF]. Silencing the colA gene resulted in reduced elongation of the endosymbionts and their spreading from the bacteriocytes into insect tissues, indicating that ColA is essential for symbiont differentiation, regulating their growth through inhibition of cell division [START_REF] Login | Antimicrobial peptides keep insect endosymbionts under control[END_REF]. Interestingly, this scenario also operates in the bacteroids of symbiotic nodule cells.

In the bacteriocytes of aphids, the expression of a unique class of genes was discovered encoding small proteins with signal peptides that were often cysteine-rich, reminiscent of NCRs, and named bacteriocyte-specific cysteine-rich peptides (BCR) [START_REF] Shigenobu | Aphids evolved novel secreted proteins for symbiosis with bacterial endosymbiont[END_REF]. The genes are specifically expressed in the bacteriocytes and not in other parts of the insect. Following their expression by in situ hybridization during bacteriome formation in embryos, it was found that they are induced when the early-stage bacteriome becomes first infected with Buchnera. Thus, drawing the analogy with the NCR peptides in nodules, it is an inevitable and exciting hypothesis that the BCRs are targeted to and affect the bacterial symbionts in a similar fashion as the NCRs do in symbiotic nodule cells.

Similarly, in another insect, the stinkbug Riptortus pedestris, cysteine-rich peptides are specifically produced by a gut-derived symbiotic organ carrying extracellular bacterial symbionts in crypts [START_REF] Futahashi | Gene expression in gut symbiotic organ of stinkbug affected by extracellular bacterial symbiont[END_REF]. The authors speculated that these crypt-specific cysteine-rich peptides (CCRs) are secreted into the lumen of the crypts by the epithelial cells and act on the proliferation and physiology of the symbiotic Burkholderia, similarly to the NCRs in legume nodules and the BCRs in aphid bacteriomes [START_REF] Futahashi | Gene expression in gut symbiotic organ of stinkbug affected by extracellular bacterial symbiont[END_REF].

Although the functions of these aphid and stinkbug peptides are entirely unknown, it is interesting that legume and insect symbiotic cells evolved similar, but not homologous types of peptides, that are specifically produced in symbiotic organs.

Moreover, AMPs are not only employed in the maintenance of endosymbiotic bacteria but also of the gut microbiota. Studies on gut microbiota in Hydra, Drosophila and mouse demonstrated that in these very divergent animals the profile and the level of expression of AMPs produced by the gut epithelia are crucial for creating and maintaining a specific and favorable composition of the microbiota [START_REF] Wehkamp | Reduced Paneth cell alpha-defensins in ileal Crohn's disease[END_REF][START_REF] Ryu | Innate immune homeostasis by the homeobox gene Caudal and commensal-gut mutualism in Drosophila[END_REF][START_REF] Fraune | In an early branching metazoan, bacterial colonization of the embryo is controlled by maternal antimicrobial peptides[END_REF][START_REF] Salzman | Enteric defensins are essential regulators of intestinal microbial ecology[END_REF][START_REF] Vaishnava | The antibacterial lectin RegIIIgamma promotes the spatial segregation of microbiota and host in the intestine[END_REF].

Before the start of my PhD thesis work, all available data pointed towards a restriction of the NCR gene family to the IRLC legumes. However morphologically differentiated bacteroids were also observed in other legume clades (Oono et al., 2010) such as the Emorphotype and S-morphotype bacteroids in Aeschynomene species, which are also induced by plant factors [START_REF] Bonaldi | Nodulation of Aeschynomene afraspera and A. indica by photosynthetic Bradyrhizobium Sp. strain ORS285: the nod-dependent versus the nod-independent symbiotic interaction[END_REF]. In my here presented thesis work, I have participated in the characterization of these bacteroids in Aeschynomene species, showing that similarly to Medicago bacteroids, they are polyploid. Moreover, a gene family encoding NCR peptides was identified and we demonstrated that they are involved in the bacteroid differentiation.

The Aeschynomene NCRs are unrelated to the NCR sequences of the IRLC legumes. Thus the mechanism of bacteroid differentiation in Aeschynomene is similar in its principles to the mechanism employed by IRLC legumes but relies on a distinct class of NCR peptides. Thus two distinct legume clades, the Dalbergoids and the IRLCs, use a very similar molecular mechanism to control their nitrogen-fixing intracellular endosymbionts despite an inferred independent acquisition of that mechanism [START_REF] Czernic | Convergent evolution of endosymbiont differentiation in Dalbergoid and IRLC legumes mediated by nodule-specific cysteine-rich peptides[END_REF].

Moreover, actinorhizal plants such as Alnus glutinosa, Casuarina glauca and Datisca glomerata also produce nodule-specific cysteine-rich peptides [START_REF] Hocher | Tanscriptomics of Actinorhizal Symbiosis Reveals Homologs of the Whole Common Symbiotic Signaling Cascade[END_REF][START_REF] Demina | Comparison of the nodule vs. root transcriptome of the actinorhizal plant Datisca glomerata: actinorhizal nodules contain a specific class of defensins[END_REF] and recent work showed that these peptides are indeed implicated in the interaction with the Frankia endosymbionts in the nodules of those plants [START_REF] Carro | Alnus peptides modify the porosity and induce the release of nitrogen rich metabolites from nitrogen-fixing Frankia[END_REF].

NCRs are similar to AMPs such as defensins and the analysis of in vitro NCR activity demonstrated that some NCR peptides indeed possess genuine antimicrobial properties and effectively kill not only S. meliloti (Van de Velde et al., 2010) but also other gram-negative and gram-positive bacteria, as well as fungi [START_REF] Tiricz | Antimicrobial nodule-specific cysteine-rich peptides induce membrane depolarizationassociated changes in the transcriptome of Sinorhizobium meliloti[END_REF](Tiricz et al., , Ördögh et al., 2014)). Thus despite the key role of these peptides for the establishment of the symbiosis, some of them have an antimicrobial activity. This raises the question how the symbionts protect themselves against the arsenal of these effectors?

Previous work in the laboratory in S. meliloti and my work in Bradyrhizobium sp.

have shown that the BacA and BclA proteins are key factor in the circumvention of the possible harmful consequences of NCR exposure within the symbiosomes [START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF], Guefrachi et al., 2015). Interestingly, BacA function is also crucial for the pathogenicity of bacteria such as Brucella and Mycobacterium [START_REF] Levier | Similar requirements of a plant symbiont and a mammalian pathogen for prolonged intracellular survival[END_REF][START_REF] Domenech | BacA, an ABC transporter involved in maintenance of chronic murine infections with Mycobacterium tuberculosis[END_REF].

These pathogens establish chronic infections in animal hosts where they need to withstand cocktails of AMPs to survive.

How BacA, BclA or SbmA function at the molecular level remains a matter of speculation at present. The S. meliloti BacA, the E. coli SbmA and the Bradyrhizobium BclA proteins facilitate the uptake of proline-rich peptides as well as the antibiotic bleomycin, which is a peptide derivative, suggesting that those proteins can function as peptide transporters [START_REF] Mattiuzzo | Role of the Escherichia coli SbmA in the antimicrobial activity of proline-rich peptides[END_REF][START_REF] Marlow | Essential role for the BacA protein in the uptake of a truncated eukaryotic peptide in Sinorhizobium meliloti[END_REF][START_REF] Wehmeier | Internalization of a thiazole-modified peptide in Sinorhizobium meliloti occurs by BacA-dependent and independent mechanisms[END_REF]Guefrachi et al., 2015). These transporters also facilitate the uptake of NCR peptides providing a possible mechanism for protection against NCR antimicrobial activity: by taking them away from the bacterial envelop through import, the NCRs are removed from the site where they are toxic [START_REF] Guefrachi | Bradyrhizobium BclA is a NCR peptide transporter required for bacterial differentiation in symbiosis with Aeschynomene Haag[END_REF]. The BacA/SbmA/BclA transporters seem thus to have a broad spectrum of substrates. The structural basis for this low specificity is an interesting question worth to be followed up in the future. X-ray crystallography could provide proof of peptide transport by these proteins.

The BacA/SbmA/BclA are clearly not symbiosis specific proteins since they are conserved in a broad range of bacteria including non-symbiotic ones. Thus they likely have a housekeeping function. Understanding this housekeeping function might provide more information on their symbiotic role as well. Studying the regulation of the bclA/bacA genes might be one road to take to learn more about the molecular function and mode of action of these genes. An interesting hypothesis is that the expression of the bclA gene is induced by the NCR-like peptides produced by the host cells. This would fit to the role of sbmA in E. coli, which is part of the membrane stress response [START_REF] Rowley | Pushing the envelope: extracytoplasmic stress responses in bacterial pathogens[END_REF][START_REF] Bury-Moné | Global analysis of extracytoplasmic stress signalling in Escherichia coli[END_REF].

Indeed, NCRs challenge the symbionts by targeting the rhizobial cell wall [START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF] and therefore could induce an envelope stress response. Thus it is possible that BclA and BacA are part of the membrane stress response and that their expression is induced by the NCR peptides produced by the host cells.

Thus the legume symbiosis and the animal symbiotic systems constitute together a striking case of convergent evolution between plants and animals. The similarities suggest that large polyploid host cells, morphologically adapted endosymbionts and the employment of AMP-like peptides constitute an optimal modus operandi for symbiotic exchange between a eukaryotic host cell and its endosymbionts and for a host cell to tolerate large numbers of bacteria. Moreover, our studies have uncovered a remarkable conservation in innate immune defense mechanisms among plants, insects, and mammals, which suggests also a common ancestry of the system [START_REF] Kang | A peptidoglycan recognition protein in innate immunity conserved from insects to humans[END_REF][START_REF] Cao | Common mechanisms for pathogens of plants and animals[END_REF]. It is predicted that further comparative studies between legume and insect symbioses will reveal further common principles, for example, in the mechanisms used for bacterial recognition and selection.

The physiological meaning of bacteroid differentiation

Focusing back on Rhizobium-legume symbiosis, morphological differentiation of bacteroids is not taking place in all legumes and it is thus not required per se for the maintenance of the endosymbiotic bacterial population or for nitrogen fixation. This prompts the question why certain legumes impose this differentiation process on their nodule rhizobia. While this question remains entirely open at present, two published studies suggest a selective advantage to the plant (Oono et al., 2010;Oono and Denison, 2010). Phylogenetic analysis and ancestral state reconstruction suggested that the ancestral morphotype of bacteroids was the undifferentiated U-morphotype and that the differentiated E-and Smorphotypes appeared several times independently in the legume family (Oono et al., 2010).

This conclusion, together with the facts that the plant imposes the morphological differentiation and that the bacteria are affected in their reproductive capacity by the differentiation, is a strong argument for a selective advantage to the plant associated with the morphological bacteroid differentiation. Another study by the same researchers suggested a higher symbiotic performance for E-or S-morphotype bacteroids compared to U-morphotype bacteroids (Oono and Denison, 2010). However, even if these studies are attractive arguments for a host benefit, la « raison d'être » of morphological bacteroid differentiation (E-and S-morphotype bacteroids) remain entirely unknown.

An original strategy to answer this question would be the use of rhizobium strains with large host specificity, capable of nodulating legume species with NCR peptides and E-or Smorphotype bacteroids as well as species with U-morphotype bacteroids. As a matter of facts, we are aware of very few examples of such strains. A well-known example of a broad host range strain is Sinorhizobium fredii strain NGR234 which nodulates more than 100 genera (Pueppke and Broughton). However this strain does not nodulate legumes of the IRLC or the Dalbergoids, the two clades that we have characterized for their bacteroid morphotype and presence of NCR peptides. Another strain of interest is S. fredii strain HH103 which is reported to nodulate soybean as well as alfalfa. However in our hands we could never obtain alfalfa nodulation with this strain despite testing many different plant growth conditions as well as different alfalfa varieties. Probably alfalfa nodulation occurs with very low efficiency.

In the "Dalbergoid" legume clade, bacteroids can be of the E-or S-morphotype. For example, Bradyrhizobium strain ORS285 which can nodulate A. afraspera and A. indica or A.

evenia transforms into E-morphotype bacteroids in nodules of the former legume and in Smorphotype bacteroids in the latter two legumes [START_REF] Bonaldi | Nodulation of Aeschynomene afraspera and A. indica by photosynthetic Bradyrhizobium Sp. strain ORS285: the nod-dependent versus the nod-independent symbiotic interaction[END_REF]. Along the same line, themselves better than the U-morphotype. These encouraging results suggest thus that the higher the ploidy levels of the bacteroids the higher symbiotic efficiency is. If these results are confirmed by additional experiments, then the next challenge will be to understand the molecular and physiological basis of this difference.

A possible approach for this question is a comparison by transcriptome, proteome and metabolome analyses of the different bacteroid types. Such approaches will identify genes and pathways that are specific to one or more bacteroids morphotypes. The results coming from these three approaches will also allow better understanding of the molecular mechanisms involved in the bacteroid differentiation and generally speaking in the functioning of the Rhizobium-legume symbiosis.

Concluding remarks

The bacA gene came into the light of research, when it was identified as essential for bacteroid development in Sinorhizobium meliloti in the early 1990's [START_REF] Glazebrook | A Rhizobium meliloti homolog of the Escherichia coli peptide-antibiotic transport protein SbmA is essential for bacteroid development[END_REF]. This aroused curiosity about bacA or homologs in other bacteria. Indeed, bacA is not a symbiotic gene and in fact it is widespread throughout the bacterial kingdom and can also be found in non-symbiotic bacteria like E. coli, Mycobacterium and Brucella. The B. abortus bacA null mutant was found to induce protective immunity in mice against subsequent challenge with wild-type B. abortus, suggesting it could be developed as a potential vaccine strain [START_REF] Roop | Seeking a niche: putative contributions of the hfq and bacA gene products to the successful adaptation of the brucellae to their intracellular home[END_REF]. A BacA protein was also recently identified to be involved in the maintenance of chronic intracellular infections of Mycobacterium tuberculosis within mice [START_REF] Domenech | BacA, an ABC transporter involved in maintenance of chronic murine infections with Mycobacterium tuberculosis[END_REF]. Accordingly, our effort in understanding the function(s) of BacA and BclA in legume symbiosis will thus also be relevant for identifying novel antibiotic targets and/or vaccine candidates against medically important chronic human infections.

Moreover, the study of NCR genes can be of major interest also outside of symbiosis.

The ever-increasing prevalence of antibiotic-resistant strains has made it critical that new antibiotics with novel modes of action and more effective means of killing be developed.

Our studies on the NCR will reveal whether these peptides have the potential to be part of a solution to this urgent need.

This thesis provides insights into both the bacterial and the host functions important for the housing of intracellular bacteria. These results will pave the way towards mediumterm strategies for improving symbiotic performance in legumes with non-optimal bacteroid morphotype by for example introducing factors for bacteroid differentiation into legumes lacking these factors (e.g soybean). On a long-term perspective, one possible aim could be to transfer efficient symbiotic nitrogen fixation to major agricultural crops like wheat or maize to enhance yield and reducing the need of fertilizers at the same time. Insights from our studies on the management of bacterial populations within host cells might be also relevant in the pursuit of this extremely ambitious goal.

Annex 3

Bacteroid differentiation in Aeschynomene legumes

Abstract

The ability of legumes to acquire sufficient nitrogen from the symbiosis with Rhizobium relies on the intimate contact between the endosymbiotic, intracellular rhizobia, called bacteroids, and their host cells, the symbiotic nodule cells. A well-studied example is the symbiotic nitrogen fixing bacterium Sinorhizobium meliloti, which nodulates the legume Medicago truncatula. Nodules of M. truncatula produce an enormous diversity of peptides called NCRs, which are similar to antimicrobial peptides (AMPs) of innate immune systems. These NCRs are involved in maintaining the homeostasis between the host cells in the nodules and the large bacterial population they contain. Although many NCRs are genuine AMPs, which kill microbes in vitro, in nodule cells they do not kill the bacteria but induce them into the terminally differentiated bacteroid state involving cell elongation, genome amplification, membrane fragilization and loss of cell division capacity. Protection against the antimicrobial action of NCRs by the bacterial BacA protein is critical for bacteroid survival in the symbiotic cells and thus for symbiosis. As a part of my PhD thesis, I have shown that the differentiation of the symbiotic cells in M. truncatula is associated with a tremendous transcriptional reprogramming involving hundreds of genes, mainly NCR genes, which are only expressed in these cells.

Although the extensive work on the model M. truncatula/S. meliloti, little is known how the plant controls its intracellular population and imposes its differentiation into a functional form, the bacteroids in other symbiotic systems.

In my PhD work, I provide several independent pieces of evidence to show that tropical legumes of the Aeschynomene genus, which belong to the Dalbergoid legume clade use a different class of cysteine rich peptides (NCR-like) to govern bacteroid differentiation. This mechanism is similar to the one previously described in Medicago which was up to now assumed to be restricted to the advanced IRLC legume clade, to which it belongs.

I have also shown that the Bradyrhizobium symbionts of Aeschynomene legumes possess a multidrug transporter, named BclA, which mediates the import of a diversity of peptides including NCR peptides. In the absence of this transporter, the rhizobia do not differentiate and do not fix nitrogen. BclA has a transmembrane domain of the same family as the transmembrane domain of the BacA transporter of Rhizobium and Sinorhizobium species, which is known to be required in these rhizobia to respond to the NCR peptides of IRLC legumes. Again this is a mechanism which is analogous to the one described in S. meliloti the symbiont of Medicago.

This study broaden our knowledge on the evolution of symbiosis by showing that the modus operandi involving peptides derived from innate immunity used by some legumes to keep their intracellular bacterial population under control is more widespread and ancient than previously thought and has been invented by evolution several times. Key Words: Symbiosis, Bacteroid, Medicago truncatula, Sinorhizobium meliloti, IRLC, NCR, BacA, peptide transporter, Aeschynomene, Bradyrhizobium, Dalbergoid.

Différenciation des bactéroïdes chez Aeschynomene

Résumé

Les Légumineuses ont développé une interaction symbiotique avec des bactéries du sol, les rhizobia, qui fixent l'azote atmosphérique et le transfèrent à la plante sous forme assimilable. Cette interaction a lieu, au sein des nodosités, des organes racinaires où les bactéries intracellulaires se différencient en bactéroïdes. Chez Medicago truncatula, ces bactéroïdes correspondent à un stade de différentiation terminale corrélée à une endoréplication de leur génome, une augmentation de la taille des cellules, une modification des membranes et une faible capacité à se propager. Cette différentiation est induite par des facteurs de la plante appelés NCR (Nodule-specific Cysteine Rich). Les peptides NCRs ressemblent à des défensines, des peptides antimicrobiens ayant une activité antimicrobienne in vitro, tuant des bactéries. Ainsi, un élément clef dans la différenciation des bactéroïdes est la protéine bactérienne BacA, un transporteur membranaire qui confère une résistance contre l'activité antimicrobienne des peptides. Dans le cadre de ce travail de thèse, j'ai montré que l'expression des NCR est soumise à une régulation stricte et qu'ils sont activés dans trois vagues dans les cellules symbiotiques polyploïdes.

Les mécanismes de contrôle par la plante sur les rhizobia intracellulaires demeurent à ce jour peu connus et le seul modèle étudié, au début de ce travail de thèse, restait l'interaction entre M. truncatula et S. meliloti. Je me suis donc intéressée à la symbiose de certaines Légumineuses tropicales du genre Aeschynomene appartenant au clade des Dalbergoïdes où je montre qu'ils utilisent une classe différente de peptides riches en cystéine (NCR-like) pour induire la différenciation des bactéroïdes. Ce mécanisme est analogue à celui décrit précédemment chez Medicago qui était jusqu'à présent supposé être limitée aux légumineuses appartenant au clade des IRLC. J'ai également montré que Bradyrhizobium, symbionte d'Aeschynomene possèdent un transporteur de type ABC homologues à BacA de Sinorhizobium nommé BclA. Ce gène permet l'importation d'une variété de peptides comprenant des peptides NCR. En l'absence de ce transporteur, les rhizobiums sont incapables de se differencier et de fixer l'azote.

Cette étude a permis d'élargir nos connaissances sur l'évolution de la symbiose en montrant qu'au cours de l'évolution, deux clades de Légumineuses relativement éloignés (IRLC et Dalbergoïdes) aient convergé vers l'utilisation de peptides de l'immunité innée afin de contrôler leur symbionte bactérien et d'en tirer un bénéfice maximal au cours de l'interaction symbiotique. Mots clés: Symbiose, Bactéroïdes, Medicago truncatula, Sinorhizobium meliloti, IRLC, NCR, BacA, Transporteur de peptide, Aeschynomene, Bradyrhizobium, Dalbergoïdes

  gènes NCR dans 267 différentes conditions expérimentales. Nous avons également généré des plantes transgéniques portant des fusions transcriptionnelles pNCR-GUS pour trois NCRs représentant différentes classes temporelles d'expression. Ces lignées transgéniques ont été utilisées pour analyser l'activité des promoteurs (révélant ainsi le patron d'expression génique des NCR considérés) au cours d'une cinétique d'infection par S. meliloti ou en réponse à l'inoculation par différents agents pathogènes. Nous avons trouvé que l'ensemble des environ 300 gènes NCR testés (sauf cinq) n'est exprimé que dans les nodosités, ils ne sont pas exprimés dans d'autres organes de la plante, ni lors d'une infection par des agents pathogènes. De plus l'expression des NCR n'est induite en réponse à aucune interaction biotique ou abiotique testée. Dans les nodosités, les NCR ne sont pas encore exprimés à des stades précoces du développement, avant que les cellules symbiotiques ne soient formées et que les rhizobia ne soient libérés dans les symbiosomes des cellules hôtes. Ils ne sont pas impliqués dans la dégradation des bactéroïdes pendant la sénescence des nodosités et leur expression s'arrête lorsque la sénescence est lancée. Cependant, le profil d'expression des NCR par des vagues successives au cours de la formation de nodosités suggère que les bactéroïdes sont les seules cibles des peptides et qu'un ensemble de peptides pourrait être impliqué dans la différenciation des bactéroïdes et d'autres dans leur fonctionnement. Ces analyses expérimentales, ensemble avec des calculs d'entropie de Shannon, montrent que les gènes NCRs ont une spécificité d'expression exceptionnelle indiquant qu'ils subissent une régulation extrêmement stricte. L'ensemble de ces résultats a mené à une publication dans la revue BMC Genomics (voir chapitre I des résultats) dont je suis co-premier auteur. Cette étude montre que l'expression des NCR est soumise à une régulation stricte et qu'ils sont activés pendant l'organogenèse et au cours du développement nodulaire dans les cellules symbiotiques polyploïdes.

:

  Quels sont les effecteurs de la plante forçant les bactéroïdes à se différencier en bloquant les divisions et en favorisant l'endoréplication ? ! Identification de gènes NCRs chez Aeschynomene Les mécanismes de différenciation pourraient être conservés chez les trois plantes (A. indica, A. afraspera et Medicago). Le contrôle de cette étape, chez les Aeschynomene, pourrait également impliquer des NCR. En accord avec cette hypothèse, une étude a été menée par l'équipe d'Eric Giraud pour identifier des facteurs de la plante qui imposent une différenciation morphologique des bactéroïdes par des analyses transcriptomiques (banque EST) dans les nodosités d'A. afraspera et A. indica. Cette approche a permis de mettre en évidence l'existence d'une famille de gènes NCR-like spécifiquement et fortement exprimés dans les nodosités des deux plantes au moment de la différenciation des bactéroïdes (qui a lieu de façon synchrone entre le 4 ème et le 5 ème jour après inoculation). Les NCRs d'Aeschynomene sont semblables structuralement aux peptides NCR de Medicago avec un peptide signal en N-terminal et une séquence mature en C-terminal comprenant des cystéines en positions conservées. Toutefois les NCRs de Medicago et d'Aeschynomene présentent des différences quant à la position des cystéines et vis-à-vis du peptide signal, suggérant qu'ils ont une origine évolutive différente. On peut donc émettre l'hypothèse que ces NCR pourraient agir sur la différenciation des bactéroïdes chez Aeschynomene. Afin d'évaluer cette hypothèse, une approche par ARN interférant a été réalisée visant à empêcher l'adressage des NCR au bactéroïdes. En effet, l'interrogation du transcriptome d'A. evenia a révélé l'existence d'un orthologue de DNF1 et une construction permettant l'atténuation de ce gène par ARNi a été réalisée et introduite in planta via Agrobacterium rhizogenes (une bactérie tellurique phytopathogène, induisant l'émergence et la prolifération de chevelu racinaire (hairy root) à partir du point d'infection de la bactérie). Les résultats obtenus ont montré un défaut de différenciation des bactéroïdes. Elles présentent soit de différents morphotypes au sein de la même cellule soit des bactéroïdes allongées au lieu des sphériques. Parallèlement, une analyse de la localisation spatiale de leurs expressions chez A. indica et A. afraspera a été effectuée par une approche d'hybridation in situ (HIS). Une technique qui permet la détection et la localisation d'ARN messagers sur Résumé coupes histologiques de tissus grâce à l'utilisation de sondes ARN complémentaires marquées. Les résultats de l'HIS ont permis d'affirmer que les gènes NCR d'Aeschynomene sont bien exprimés au niveau des cellules symbiotiques contenant les bactéroïdes. Finalement, une approche de protéomique par spectrométrie de masse a montré que les NCRs sont adressés aux bactéroïdes. L'ensemble des résultats de cette étude confortent l'hypothèse selon laquelle les mécanismes de différenciation des bactéroïdes en morphotypes E et S chez les Aeschynomene sont semblables à ceux identifiés chez M. truncatula. Il semble donc qu'au cours de l'évolution, deux clades de Légumineuses relativement éloignés (IRLC et Dalbergoïdes) aient convergé vers l'utilisation de peptides de l'immunité innée afin de contrôler leur symbionte bactérien et d'en tirer un bénéfice maximal au cours de l'interaction symbiotique. Ces données ont été valorisées dans un article dont je suis co-auteure : Convergent evolution of endosymbiont differentiation in Dalbergoid and IRLC legumes mediated by nodule-specific cysteine-rich peptides, soumis à Plant Physiology (voir chapitre II des résultats). Dans le mesure où les NCR-like chez Aeschynomene ressemblent aux peptides antimicrobiens NCR de Medicago, l'étape suivante a consisté en la recherche de facteurs bactériens qui permettraient aux rhizobactéries de survire dans les symbiosomes en présence des NCR. Quels sont alors les facteurs bactériens qui déterminent la différenciation bactérienne chez les Aeschynomene ? ! Le transporteur BclA de Bradyrhizobium Le gène bacA de S. meliloti, auparavant caractérisé dans le laboratoire, est nécessaire pour la réponse de la bactérie aux peptides NCR de Medicago. Un gène ressemblant fortement au gène bacA a été identifié par analyse in silico chez les bradyrhizobia et il a été nommé bclA pour bacA-like.

  Nous avons donc testé la nécessité du gène bclA de Bradyrhizobium en cas de formation de bactéroïdes du morphotype U. B. japonicum USDA110 induit des nodosités fonctionnelle chez le soja et aussi chez A. afraspera. Chez le soja les bactéroïdes sont du morphotype U tandis que chez A. afraspera les bactéroïdes sont très légèrement allongées et ne sont quasiment pas polyploïdes (3C). Cette dernière observation était inattendue, vu la production des NCRs par A. afraspera. Effectivement, nous avons montré par qPCR qu'aussi dans les nodosités d'A. afraspera infectées par USDA110, les gènes NCRs sont normalement exprimés. Possiblement des facteurs génétiques de la souche USDA110 rendent la bactérie insensible au peptide NCRs. L'analyse des phénotypes symbiotiques du mutant B. japonicum USDA110∆bclA chez A. afraspera et le soja a montré que le gène bclA de B. japonicum n'est pas nécessaire pour la formation de bactéroïdes non différenciées. Le contenu en ADN des bactéroïdes est alors très similaire à celui des bactéries en culture. ! BclA est un transporteur de NCRs et autres peptides antimicrobiens Bien que BacA et BclA ont un rôle clé au cours du processus symbiotique, ces deux protéines n'ont pas une fonction exclusivement symbiotique, car des gènes homologues sont conservées chez d'autres bactéries, inclus des agents pathogènes de Mammifères comme Brucella abortus, Mycobacterium tuberculosis et Escherichia coli (le gène homologue de bacA est alors nommé sbmA) et s'avèrent cruciaux pour la persistance de la bactérie dans son hôte et donc pour le succès de l'infection

  . Il serait par exemple possible de tester la capacité des NCRs d'induire la différenciation des bactéroïdes chez le soja ou des NCRs d'A. indica à induire un morphotype S chez A. afraspera. En outre, nous avons montré que Bradyrhizobium ORS278 et ORS285 nécessitent le transporteur de peptides BclA à la fois pour la différenciation des bactéroïdes de morphotypes E et S. BclA possède une fonction similaire à la protéine BacA de S. meliloti qui est nécessaire pour la différenciation des bactéroïdes chez Medicago et qui joue aussi un rôle dans le transport des peptides antimicrobiens, les NCR, dans les cellules bactériennes. BclA peut aussi remplacer fonctionnellement BacA dans le transport des peptides. Ces observations confortent fortement la conclusion que chez les Aeschynomene, la différenciation des bactéroïdes en morphotype E ou S est sous le contrôle de la famille des peptides NCRs.

  , et iii) cette même fragilisation permettrait de favoriser les échanges nutritionnels entre les deux partenaires. Nos observations, conduites chez deux espèces d'Aeschynomene (A. indica et A. afraspera) en utilisant la même bactérie (ORS285), suggèrent qu'il peut y avoir aussi des différences de performance entre des bactéroïdes différenciés morphologiquement. La comparaison des résultats de fixation de l'azote et de cytométrie en flux indique que le niveau d'endoréplication des bactéroïdes est corrélé à leur efficacité symbiotique. En effet les bactéroïdes de morphotype U (B. japonicum USDA110/soja), qui n'ont pas effectué d'endoréplication, fixent moins bien l'azote (fixation normalisée par masse de nodosités) que des bactéroïdes de morphotype E (B. japonicum USDA110-Bradyrhizobium sp. ORS285/A. afraspera). Ces bactéroïdes endorépliqués (7C) fixent eux-mêmes moins bien l'azote que les bactéroïdes de morphotype S (Bradyrhizobium sp. ORS285/A. indica) qui ont effectués davantage d'endoréplication (16C).

  le cas d'A. indica. Il est par ailleurs possible, comme le suggère l'analyse en cytométrie en flux que le niveau d'endoréplication soit plus important sous cette conformation, et que le métabolisme bactérien soit plus actif dans cet état. Il est donc tentant de suggérer que plus l'endoréplication subie par les bactéroïdes est importante, plus ils sont efficaces pour la fixation de l'azote. Résumé Ces résultats préliminaires sont très intéressants et méritent d'être approfondis en plus grand détail. En outre, il sera très intéressant de faire des analyses transcriptomiques, protéomiques et métabolomiques sur les trois morphotypes (U, E et S) afin de comprendre les bases moléculaires et métaboliques de cette différence d'efficacité entre les différentes formes de bactéroïdes. L'intégration des analyses transcriptomiques, protéomiques et métaboliques permettra d'identifier des gènes et des voies métaboliques qui sont spécifiques à un ou plusieurs morphotypes de bactéroïdes. Les résultats qui découleraient d'une telle étude devraient permettre d'améliorer plus largement la compréhension des mécanismes contrôlant et régulant la différenciation des bactéroïdes lors de la mise en place de la nodosité ainsi que l'impact sur la fixation symbiotique d'azote. Enfin, dans le cas où des déterminants majeurs de la différenciation des bactéroïdes seraient identifiés, l'obtention de souches bactériennes, dans lesquelles ces déterminants moléculaires sont modifiés, pourrait permettre, à plus long terme, de créer des systèmes symbiotiques plus efficaces chez des espèces d'intérêt agronomique majeur, tel que le soja. De la même manière, on pourrait introduire des gènes NCRs chez des plantes qui n'en possèdent pas afin d'induire la différenciation des bactéroïdes et ainsi d'améliorer l'efficacité de la fixation d'azote.

Figure 1 .

 1 Figure 1. The global nitrogen cycle ...........................................................................................

Figure 2 .

 2 Figure 2. Phylogeny of rhizobia ................................................................................................

Figure 3 .

 3 Figure 3. The house of symbiosis: symbiotic niches in animals and plants ..............................

Figure 4 .

 4 Figure 4. Differentiation of symbiotic nodule cells ...................................................................

Figure 5 .

 5 Figure 5. Development of determinate and indeterminate root nodules ...................................

Figure 6 .

 6 Figure 6. Bacteroid morphotypes ............................................................................................

Figure 7 .

 7 Figure 7. NCR-mediated differentiation of E-morphotype bacteroids in IRLC legumes .......

Figure 8 .Figure 9 .

 89 Figure 8. Cell cycle deregulation induced by NCR peptides leading to polyploid bacteroids Figure 9. BacA is required for bacteroid differentiation in Medicago ....................................

Figure 10 .

 10 Figure 10. BacA is required for survival in the symbiotic nodule cells ..................................

Figure 11 .

 11 Figure 11. Heat map of NCR gene expression in the MtGEA compendium ...........................

Figure 12 .

 12 Figure 12. Heat map of S. meliloti gene expression in the MtGEA compendium ..................

Figure 13 .

 13 Figure 13. Expression profile examples of 7 NCR genes ........................................................

Figure 14 .

 14 Figure 14. Successive activation of NCR genes during nodule development .........................

Figure 15 .

 15 Figure 15. NCR gene expression in response to Nod factors and Myc factors .......................

Figure 16 .

 16 Figure 16. Spatio-temporal expression of the NCR gene family in nodules ...........................

Figure 17 .

 17 Figure 17. Representative clusters of spatial and temporal NCR expression profiles ............

Figure 18 .

 18 Figure 18. NCR expression during nodule senescence ...........................................................

Figure 19 .

 19 Figure 19. Effect of Phosphinothricin treatment on NCR gene expression .............................

Figure 20 .

 20 Figure 20. Promoter-GUS analysis of NCR genes in nodules .................................................

Figure 21 .

 21 Figure 21. Immunolocalization of NCR122 in nodule sections ..............................................

Figure 22 .

 22 Figure 22. NCR expression in plant tissues .............................................................................

Figure 23 .

 23 Figure 23. NCR expression in response to phytohormones ....................................................

Figure 24 .

 24 Figure 24. Expression of NCR genes during microbial infections and elicitor treatment .......

Figure 25 .

 25 Figure 25. Expression of NCR genes during drought and salt stress .......................................

Figure 26 .

 26 Figure 26. Promoter-GUS analysis of NCR genes in infected and wounded leaves ...............

Figure 27 .

 27 Figure 27. Shannon entropy .....................................................................................................

Figure 28 .

 28 Figure 28. NCR genes are among the most actively expressed genes in M. truncatula ..........

Figure 29 .

 29 Figure 29. Comparison of the spatial and temporal NCR expression profiles ........................

Figure 30 .

 30 Figure 30. Properties of free-living cultured Bradyrhizobium sp. ORS285 bacteria and ORS285 bacteroids isolated from A. afraspera or A. indica nodules ......................................

Figure 31 .Figure 32 .Figure 33 .Figure 34 .Figure 35 .Figure 36 .Figure 37 .Figure 38 .Figure 39 .Figure 40 .Figure 41 .Figure 42 .Figure 43 .Figure 44 .Figure 45 .Figure 46 .Figure 47 .Figure 48 .Figure 49 .Figure 50 .Figure 51 .Figure 52 .Figure 53 .Figure 54 .

 313233343536373839404142434445464748495051525354 Figure 31. Distribution of S and E-Bacteroid morphotypes among Aeschynomene species .. 76 Figure 32. Alignment and cysteine-signature of Aeschynomene NCRs .................................. 77 Figure 33. NCR expression during the symbiotic process ...................................................... 79 Figure 34. Nodule specific expression of NCR genes in A. evenia ......................................... 79 Figure 35. NCR genes are specifically expressed in the infected symbiotic cells of Aeschynomene nodules ............................................................................................................. 80 Figure 36. NCR peptides colocalize with bacteroids in A. afraspera and A. indica nodules .. 81 Figure 37. DNF1 silencing affects bacteroids differentiation in Aeschynomene .................... 82 Figure 38. Change in the bacterial cell shape during the symbiotic interaction between Bradyrhizobium strain ORS285 and Aeschynomene indica ..................................................... 84 Figure 39. Neighbor joining phylogeny of NCR protein sequences ....................................... 87 Figure 40. The exsE gene of S. meliloti is not required for symbiosis .................................... 93 Figure 41. Three loci in Bradyrhizobium genomes carrying SbmA_BacA domain ABC transporters ............................................................................................................................... 94 Figure 42. Phylogenetic tree of SbmA-BacA domain proteins ............................................... 95 Figure 43. Symbiotic phenotype of Bradyrhizobium strain ORS285 mutants ........................ 96 Figure 44. The mutation in the bclA gene of Bradyrhizobium strain ORS285 is not affecting free-living growth and nitrogen fixation .................................................................................. 98 Figure 45. Glycolate metabolism and the secretory protein of the bclA locus are not required for symbiosis ............................................................................................................................ 99 Figure 46. Small nodule size and abnormal bacteroids of ORS285ΔbclA in A. afraspera and A. indica .................................................................................................................................. 100 Figure 47. ORS285ΔbclA bacteroid functioning is impaired in A. afraspera nodules ......... 102 Figure 48. Symbiotic phenotype of the bclA mutant in Bradyrhizobium strain ORS278 ..... 103 Figure 49. Bacteroids in A. afraspera and A. indica 14 dpi nodules infected with ORS285 mutants ................................................................................................................................... 104 Figure 50. Complementation of the S. meliloti bacA mutation with Bradyrhizobium bclA .. 105 Figure 51. The B. japonicum bclA mutant is not affected in symbiosis ................................ 107 Figure 52. The bacA gene of S. fredii strain HH103 is not required for symbiosis with soybean ................................................................................................................................... 108 Figure 53. The bclA gene of B. japonicum USDA110 complements the bacA mutation in S. meliloti Sm1021ΔbacA ........................................................................................................... 109

Figure 55 .

 55 Figure 55. The bclA gene confers sensitivity to the antibiotic bleomycin and resistance to antimicrobial NCR peptides ...................................................................................................

Figure 56 .

 56 Figure 56. Uptake of FITC-NCR247 mediated by the BacA and BclA transporters ............

Figure 57 .

 57 Figure 57. FITC is not taken up by S. meliloti derivatives ...................................................

Figure 3 .

 3 Figure 3. The house of symbiosis: symbiotic niches in animals and plants

Figure 4 .

 4 Figure 4. Differentiation of symbiotic nodule cells

Figure 5 .

 5 Figure 5. Development of determinate and indeterminate root nodulesIn legumes that develop interterminate nodules (top), rhizobial infection of the plant root induces periclinical cell divisions in the pericycle folloxed by inner cortical cell proliferation. Development of a nodule primordium is accompanied by the presence of a persistent mersitem leading to a zonation of an inderterminate nodule with the meristem (zone I), the infection zone (II), an interzone (zone II-III), the fixation zone (III) and the senescence zone (IV). By contrast, determinate nodules (bottom) derive from cell divisions in the outer root cortex where the meristimatic activity is lost in mature nodules. Red are rhizobia attached to root hairs are present in infection threads. Yellow cells are cortical and epidermal cells. Grey cells are dividing cells in nodule primordia or nodule meristem. Large pinkish cells are the differentiation or differentiated symbiotic cells, carrying bacteroids. Adapted from[START_REF] Popp | Regulation of signal transduction and bacterial infection during root nodule symbiosis[END_REF].

!Figure 6 .

 6 Figure 6. Bacteroid morphotypes Bacteroids display different morphotypes according the host plant. (A) E-morphotype: Bradyrhizobium ORS285 bacteroids on A. afraspera. (B) S-morphotype: Bradyrhizobium ORS285 bacteroids on A. evenia. (C) Umorphotype: Bradyrhizobium japonicum USDA110 bacteroids on soybean. The bacteroids are green due to GFP expression (A and B) or by Syto9 DNA staining (C) and the contours of the symbiotic host cell are blue due to calcofluor staining. Scale bars, 10 !m. (D) Schematic presentation of the paths leading to the different bacteroid morphotypes. In all cases, bacteroids are released in the host cells by an endocytotic-like process and are enclosed in a symbiosome vesicle. The bacteria are released either from an infection thread penetrating the cell or directly from the extracellular space. The cell is gradually filled with bacteroids by repeated endocytosis, by division and multiplication of the symbiosomes and by division of the bacteroids within de symbiosomes in the case of U-morphotype bacteroids leading to multiple bacteroids per symbiosome or by host cell factors-induced enlargement in the case of E-and S-morphotype bacteroids, which remain single in the symbiosomes.

Figure 8 .

 8 Figure 8. Cell cycle deregulation induced by NCR peptides leading to polyploid bacteroids (A) Analysis by flow cytometry of the DNA content of cultured S. meliloti stained with the DNA-labelling fluorescent dye DAPI (left). The x-axis (logarithmic scale) represents the DNA content (DAPI fluorescence) and the y-axis is the number of bacterial cells. The histogram shows bacteria with 1C and 2C genome content reflecting the cycling of the bacteria between genome duplication and cell division. Under the influence of the NCR peptides during bacteroid differentiation the endosymbiont cells become polyploid because division is inhibited while replication is ongoing (right). (B) In situ analysis of gene expression in a nodule of M. truncatula by laser micro-dissection coupled to RNA-seq analysis. Left panel: nodule section showing the different nodule zones in which gene expression was analyzed. M: nodule meristem; dI: distal infection zone; pI: proximal infection zone; IZ: interzone; Fix: fixation zone. Central panel: expression of the major cell cycle regulators of S. meliloti in the different nodule tissues. Yellow is high expression and blue is low expression. The genes encoding DnaA and CtrA are highlighted by arrows. Right panel: expression profile of 334 NCR genes in nodule tissues. Data was extracted from the study by Roux et al. (2014). (C) Expression of CtrA regulated cell cycle genes in synchronized S. meliloti cultures that were treated or not with the NCR247 peptide. The time points above the columns indicate the time of synchronous growth. Image adapted from Penterman et al. (2014). (D) Effect of the genetic depletion of CtrA on the morphology of S. meliloti compared to a wild-type cell and to a bacteroid cell. Data adapted from Pini et al. (2013).

Figure 9 .

 9 Figure 9. BacA is required for bacteroid differentiation in Medicago Semi-thin sections stained with toluidine bleue are shown of M. truncatula nodules infected with wild-type S. meliloti (a-c) or its bacA mutant (d-f). (a,d) Whole nodules. (b,e) Enlargement of a region containing symbiotic nodule cells. (c,f) Close-up of a single symbiotic cell, showing the elongated (E-morphotype) bacteroids in the wild-type infected cell (c) and the non-differentiated bacteria in the cell infected with the bacA mutant (f). Scale bars are 200 !m (a,d) or 50 !m (b,c,e,f).

Figure 10 .

 10 Figure 10. BacA is required for survival in the symbiotic nodule cells (a) In a M. truncatula wild-type nodule cell, the S. meliloti wild-type (WT) and bacA mutant bacteria are released from the infection threads into membrane-bound symbiosomes. The bacteria are then challenged with host NCRs (purple), which are synthesized on the endoplasmic reticulum (ER) and transported to the symbiosomes via vesicle transport and a symbiotic nodule-cell-specific secretory pathway. This pathway requires an ER-located nodule-specific signal peptidase complex (SPC) containing the DNF1 subunit. The NCRs mediate the differentiation of the wild-type strain into a nitrogen-fixing bacteroid (green bacteroids), resulting in the chronic infection. In contrast, the bacA-deficient mutant is hypersensitive towards the host NCRs and is killed and then lysed (red symbiosome bacteria) rather than forming the chronic infection. (b) Wild-type S. meliloti in wild-type host cells with live/dead staining marking live bacteria in green and dead bacteria in red. The wild-type S. meliloti are healthy and differentiated. (c) Staining as in (b) of bacA mutant S. meliloti in wildtype host cells. The bacteria are non-differentiated and dead except for the infection thread-located ones (arrow). (d) In a M. truncatula dnf1 mutant nodule cell, the S. meliloti wild-type (WT) and bacA mutant bacteria are released from the infection threads into membrane-bound symbiosomes. The NCRs are not transported to the symbiosomes but remain blocked in the ER because of the absence of the DNF1 SPC subunit. Therefore both the wild-type and bacA mutant bacteria remain alive (green) within the symbiosomes but do not differentiate. (e) Staining as in (b) of wild-type S. meliloti in dnf1 mutant host cells. The bacteria are alive but non-differentiated. (f) Staining as in (b) of bacA mutant S. meliloti in dnf1 mutant host cells. The bacteria are alive but nondifferentiated. Scale bars are 10 !m.

  Frans J. de Bruijn. The article is entitled "Bradyrhizobium BclA is a NCR peptide transporter required for bacterial differentiation in symbiosis with Aeschynomene". The first objective of my PhD was to explore further the expression behavior of this amazing family of NCR genes in Medicago truncatula. The differentiation of bacteroids in the IRLC legumes to which Medicago belongs, is now well described. In other legumes like the Dalbergoids, bacteroid differentiation evolved most likely independently form the IRLC(Oono et al., 2010). The major part of my thesis work was devoted to the analysis of the mechanisms of differentiation in Aeschynomene species belonging to the Dalbergoids. The results I describe are divided in three main chapters. In the first part of my thesis work (see Chapter I), I have exploited the transcriptome database provided by the Medicago truncatula Gene Expression Atlas (MtGEA) and other published microarray data to characterize in detail the expression behavior of a large set of NCR genes. This studied consisted mainly in a data mining and a bioinformatics analysis, complemented with a number of experimental confirmations. This work was recently published in BMC Genomics with the title "Extreme specificity of NCR gene expression in Medicago truncatula". The NCR gene family has been identified in all investigated legume species of the IRLC clade to control bacteroid differentiation but in no other plant species outside of this clade and the molecular mechanisms underlying bacterial differentiation in legumes with other bacteroid morphotypes remain unknown at present. I have participated during the second part of my PhD in research aiming at understanding the mechanisms implicated in differentiation of bacteroids in the Dalbergoid legume clade to which belong the Aeschynomene species. In the second Chapter of my results, I present the article "Convergent evolution of endosymbiont differentiation in Dalbergoid and IRLC legumes mediated by nodulespecific cysteine-rich peptides", submitted in Plant Physiology. This paper was devoted to the study of the factors used by the Aeschynomene legumes to control bacteroid metamorphosis. My contribution in this work was the in situ hybridization experiments which demonstrated that the expression of the identified NCR-like genes was restricted to the infected nodule cells in A. afraspera and A. indica and the measurement of the DNA content by flow cytometry in both type of nodule harboring spherical and elongated bacteroids in A. indica and A. afraspera respectively. Finally, in the third Chapter, I present "Bradyrhizobium BclA is a NCR peptide transporter required for bacterial differentiation in symbiosis with Aeschynomene". In this chapter, I used an attractive model system composed of Bradyrhizobium strains interacting with Aeschynomene (A. indica, A. evenia and A. afraspera) and soybean legumes in which bacteroids have three different host-dependent morphotypes. I describe a bacterial determinant, which is required for bacteroid differentiation. Specifically, I studied the role of a gene of Bradyrhizobium species, named bclA, which is homologous to bacA of S. melilotiand is required for the formation of polyploid bacteroids in nodules of the Aeschynomene legumes. This work shows that not only these Dalbergoid host plants use similar effectors as the legumes of the IRLC but that also the bacteria has co-opted the same type of genes to be able to respond to those effectors, indicating that IRLC and Dalbergoid legumes together with their cognate rhizobia underwent convergent coevolution towards similar bacteroid differentiation strategies. The results of this chapter will be submitted in two separate papers, one dealing with the bclA gene of Bradyrhibium strain ORS285 and the second one with the corresponding gene of B. japonicum USDA110. The first paper was accepted in MPMI and the second one is nearly ready for submission.

  http://www.biomedcentral.com/content/supplementary/1471-2164-15-712-S1.xlsx) and their expression patterns in 267 different experimental conditions (254 from MtGEAv3, five unpublished conditions, three from Jayaraman et al. (2014) and five from Limpens et al. (2013) are summarised in Fig. 11. The transcriptome compendium is mostly

Figure 11 .

 11 Figure 11. Heat map of NCR gene expression in the MtGEA compendium

  and as indicated above the columns. The color scale bar indicates the expression level from background level (blue) to maximum level (yellow). The red arrowhead indicates the mycorrhizal sample that is contaminated with nodules.

Figure 13 .

 13 Figure 13. Expression profile examples of 7 NCR genes (A-G) The expression pattern of 7 individual NCR genes in 267 experiments (x-axis) is provided. The experiments are ordered as in Fig. 11. N is nodules; R is root conditions; O is all other conditions. The y-axis is the expression level according to the fluorescence hybridization signal and is scaled according to the maximum hybridization signal as indicated on the top of the axis. The red arrowheads in panels A and B indicate the mycorrhizal sample that is contaminated with nodules.

Figure 14 .

 14 Figure 14. Successive activation of NCR genes during nodule development(A) Expression level of the 334 NCR probe-sets (x-axis) at 0, 3, 4, 6 and 10 dpi. The y-axis scale is the same for all graphs and is 25,000 at maximum. (B) Heat map of the same expression data as in (A). (C) The ratios of NCR expression levels at 4 dpi, 6 dpi or 10 dpi compared to 14 dpi. For each time point, the genes were ordered from high ratio to low ratio. At 4 dpi, only few NCR genes are activated; at 6 dpi most are activated but not yet at maximum scale; at 10 dpi, nearly all NCR genes are activated at similar level to 14 dpi (ratio close to 1). (D) Heat map of the Pearson correlation of the NCR expression profiles at 0,3, 4, 6, 10 and 14 dpi.

  ).

Figure 16 .

 16 Figure 16. Spatio-temporal expression of the NCR gene family in nodules (A) Heat map and hierarchical clustering of NCRs in function of their spatial expression in nodules (samples I to IV, from the youngest, most apical part of the nodules to the oldest, most proximal part). Clustering was performed using Pearson correlation. (B) Heat map of NCR expression profiles obtained by laser-capture microdissection coupled to affymetrix microarray analysis described by Limpens et al. (2013) M corresponds to meristem, dIZ to the distal zone II or infection zone, pIZ to the proximal zone II, IC to infected cell and UC to uninfected cell. (C) Heat map of NCR expression in function of the nodule age (dpi). The genes in the rows in (B) and (C) are in the same order as in panel (A). For each probe-set in the 3 panels, the relative expression was converted to the log2 value of the ratio representing the zone-specific expression over the average expression in all five zones.

  ) are expressed in the most apical part of the nodule and this correlates with an early activation at 4 or 6 dpi in the temporal pattern. On the Results & Discussion Chapter. I 51 other hand, genes in cluster 5 have maximum expression only in sample II-III and this corresponds with an activation late in nodule development at 10 dpi (Fig. 17).

Figure 17 .

 17 Figure 17. Representative clusters of spatial and temporal NCR expression profilesFive representative expression profiles are shown (cluster 1 to 5). The spatial pattern (left), the temporal pattern (centre) and the genes constituting the cluster (right) are shown. Genes in cluster 1 display the highest expression in the nodule apex sample I, are still high in sample II and then decline rapidly (Fig.29). Cluster 2 genes are already activated in sample I but their expression increases in sample II and remains high in the whole nodule. Cluster 3 genes have a very sharp pattern and are highly expressed in sample II, low in sample I and II-III and close to zero in the samples III and IV. The genes of cluster 4 have a similar profile but are expressed in a broader zone with high levels of expression in samples II and II-III. Finally, cluster 5 genes are absent in sample I, partially activated in sample II and then fully activated in sample II-III, III and IV. The genes are identified by their probe-set annotation number as provided in Table1.

Figure 19 .

 19 Figure 19. Effect of Phosphinothricin treatment on NCR gene expression

Figure 20 .

 20 Figure 20. Promoter-GUS analysis of NCR genes in nodules Sections (70 !m) of root tips and nodules of different ages of transgenic plants carrying promoter-GUS fusions for the NCR121, NCR084 and NCR001 genes were stained for GUS activity observed as blue color. Scale bars are 50 !m. The location of the nodule zones in mature nodules is indicated: I is the meristem; II is the infection zone; II-III is the interzone; III is the fixation zone and IV is the senescence zone. The nodules shown in the figure were induced by S. arboris strain B554 but indistinguishable patterns were obtained in nodules formed by S. meliloti strain 1021 (data not shown).

Figure 21 .

 21 Figure 21. Immunolocalization of NCR122 in nodule sections Sections of nodules were stained with the DNA label SYTO13 (green) to reveal the bacteria (A,D) and were immunolabelled with the anti-NCR122 antibody (red) (B,E). (C,F) Overlays of (A,B) and (D,E) respectively. (D-F) enlargement of the top part of the nodule shown in (A-C). Arrowheads or double asterisks mark NCR122labeled cells in the nodule cortex and the double arrowhead or asterisks mark uninfected cells of the nitrogen fixation zone III labeled by NCR122. II is the infection zone; II-III is the interzone. Scale bars are 200 !m in panels A-C and 40 !m in panels D-F.

Figure 23 .

 23 Figure 23. NCR expression in response to phytohormones (A,B) NCR expression after treatments for 1 (1d) or 7 days (7d) of roots with the auxin transport inhibitors naphthylphthalamic acid (NPA; 200 !M) and 2.3.5-triidobenzoid acid (TIBA; 200 !M) (Rightmyer and Long, 2011). (C,D) NCR expression after treatments of leaves for 1 (1wk) or 2 (2wk) weeks with the auxin1naphthaleneacetic acid (NAA; 10 !M) or the cytokinin 6-benzylaminopurine (BAP; 4 !M) (Imin et al., 2008). (A) and (B) on the one hand and (C) and (D) on the other hand represent the same data but the scale of the graphs are at relative expression level 25,000 maximum in (A) and (C) and at maximum level 500 in (B) and (D).

  (A) and (B) on the one hand and (C) and (D) on the other hand represent the same data but the scale of the graphs are at relative expression level 25,000 maximum in (A) and (C) and at maximum level 500 in (B) and (D).

Figure 25 .

 25 Figure 25. Expression of NCR genes during drought and salt stress (A-G) NCR expression in roots exposed to NaCl salt stress (Hann and Rathjen, 2007) or drought stress (Schug et al., 2005). (A) and (B), (C) and (D), (E) and (F) and (G) and (H) represent pairwise the same data but the scale of the graphs is at relative expression level 25,000 maximum in (A), (C), (E) and (G) and at level 500 in (B), (D), (F) and (H).

Figure

  Figure 27. Shannon entropy (A) Hierarchical clustering of the 9,000 probe-sets with the highest tissue specificity (lowest entropy values E g ).The expression heat map is in green-black-red colour scheme. The entropy heat map is in blue-black-yellow scheme. (B) The data set of panel (A) was ordered according to increasing entropy of the genes. The left panel shows the relative expression level of the 9,000 genes in the 10 tissues (green-black-red heat map) and the entropy values E g (blue-black-yellow heat map). The location of the NCR genes is indicated with the black-red heat map (NCR): red means NCR, black means other gene type. The right panel is an enlargement for the first 1,800 genes (entropy values E g from 0.07 to 1.64). The scale bar for the expression level heat map is green: 0, black: 0.5 and red: 1 and for the entropy heat map blue: 0, black 1.44 and yellow: 2.88 (maximum entropy in the complete data set is 3.32 (log 2 (N)).

Figure 28 .

 28 Figure 28. NCR genes are among the most actively expressed genes in M. truncatula (A) Maximum expression level of the 50,900 M. truncatula probe-sets. (B) Maximum expression level of 334 NCR genes.

  also not known whether closely related legumes of the IRLC have an arsenal of NCRs of Convergent evolution of endosymbiont differentiation in Dalbergoid and IRLC legumes mediated by nodule-specific cysteine-rich peptides * 1. Abstract Nutritional symbiotic interactions require the housing of large numbers of microbial symbionts, which produce essential compounds for the growth of the host. In the legumerhizobium nitrogen fixing symbiosis, thousands of rhizobium microsymbionts, called bacteroids, are confined intracellularly within highly specialized symbiotic host cells. In IRLC legumes such as Medicago, the bacteroids are kept under control by an arsenal of nodule-specific cysteine-rich (NCR) peptides, which induce the bacteria in an irreversible, strongly elongated and polyploid state. Here we show that in Aeschynomene legumes belonging to the more ancient Dalbergoid lineage, bacteroids are elongated or spherical depending on the Aeschynomene species and that these bacteroids are terminally differentiated and polyploid, similarly to bacteroids in IRLC legumes. Transcriptome, in situ hybridization and proteome analysis demonstrated that the symbiotic cells in the Aeschynomene nodules produce a large diversity of NCR-like peptides, which are transported to the bacteroids. Blocking NCR transport by RNAi-mediated inactivation of the secretory pathway inhibits bacteroid differentiation. Together, our results suggest that bacteroid differentiation in the Dalbergoid clade, which likely evolved independently from the bacteroid differentiation in the IRLC clade, is based on very similar mechanisms used by IRLC legumes.

Figure 30 .

 30 Figure 30. Properties of free-living cultured Bradyrhizobium sp. ORS285 bacteria and ORS285 bacteroids isolated from A. afraspera or A. indica nodules (A) Normaski bright field (BF, upper panels) and fluorescence microcopy of bacteria and bacteroids stained with DAPI (middle panels) and PI (lower panels). Scale bars, 1 "m. (B) DNA content of DAPI-stained bacteria and bacteroids measured by flow cytometry. In blue, free-living bacteria, in green, bacteroids isolated from A. afraspera and in red, bacteroids isolated from A. indica.

Figure 31 .

 31 Figure 31. Distribution of S and E-Bacteroid morphotypes among Aeschynomene speciesThe species A. indica, A. evenia spp. serrulata, A. evenia spp. evenia, A. virginica, A. scabra, A. sensitiva, A. afraspera, A. nilotica, A. aspera were nodulated by the ORS285 GFP-tagged strain; the species A. americana, A. schimperi, A. pfundii were nodulated respectively by Bradyrhizobium strains ORS301, ORS302 and ORS305 and bacteroids were stained by the SYTO9 fluorescent probe of the Live/dead stain.

Figure 32 .

 32 Figure 32. Alignment and cysteine-signature of Aeschynomene NCRs (A) The deduced protein sequences from the up-regulated NCR genes from A. indica (AiNCRs) or A. afraspera (AaNCRs) were aligned using Multalin (http://multalin.toulouse.inra.fr/multalin/). Highly conserved amino acids are in red and conservative substitutions in blue. The putative location of the signal peptide is indicated below the alignment. (B) Comparison of the cysteine-rich motifs of Aeschynomene NCRs to those of Medicago and to defensin-like peptides. Red and blue Cysteines correspond to the NCR-and defensin-signatures respectively.

Figure 33 .

 33 Figure 33. NCR expression during the symbiotic process (A) NCR expression during A. afraspera and (B) A. indica nodule development. Plants were inoculated with the Bradyrhizobium ORS285 strain and the roots were sampled at T0 and at different time points as indicated. The relative expression level of the NCRs was determined by RT-qPCR and normalized by the expression of EF1#. As a control of nodule development, the expression of Leghemoglobin (LEG) was also measured.

Figure 35 .

 35 Figure 35. NCR genes are specifically expressed in the infected symbiotic cells of Aeschynomene nodules Sections of 14 dpi A. indica and A. afraspera nodules, infected with the Bradyrhizobium ORS285 strain, were analysed by in situ hybridization with antisense probes of leghemoglobin (B, E) or NCR (C, F). Control hybridizations were done with the sense NCR probe (A, D). Bars represent 200 "m.

Figure 36 .

 36 Figure 36. NCR peptides colocalize with bacteroids in A. afraspera and A. indica nodules SDS-PAGE analysis and Coomassie blue staining of total protein extracts of nodules (nod), bacteroids (Boid), and cultured Bradyrhizobium strain ORS285 cells (B) reveal the specific presence of low molecular weight peptides in nodules and bacteroids. The rectangle in the dashed line corresponds to the region of the gel analysed by LC-MS/MS.

Figure 37 .

 37 Figure 37. DNF1 silencing affects bacteroids differentiation in Aeschynomene (A) Alignment of the deduced protein sequence of the Aeschynomene evenia ssp. serrulata DNF1 ortholog with the corresponding Medicago protein (accession number: TC121074). The 2 protein sequences were aligned using Multalin (http://multalin.toulouse.inra.fr/multalin/). (B-E) Roots of different lines of A. evenia ssp. serrulata transformed with a RNAi construct directed against AeDNF1 encoding the SPC22 subunit of the SPC were inoculated with a GFP-tagged Bradyrhizobium ORS285 strain. At 21 dpi, the nodules were harvested, and the shape of the bacteroids was observed by confocal microscopy on nodule sections. As a control, a line with no RNAi effect (1% reduction) was used (B). The line 3, 7 and 53 with extinction levels of 62, 65 and 66 % respectively are presented in panel (C-E). Bars represent 10 "m.

Figure 38 .

 38 Figure 38. Change in the bacterial cell shape during the symbiotic interaction between Bradyrhizobium strain ORS285 and Aeschynomene indica To easy bacteroids visualization, A. indica nodules were elicited by a Bradyrhizobium ORS285 strain expressing GFP. Nodule were harvested at 4, 4.5 and 5 dpi then sectioned and analysed by confocal microscopy. Red and white arrows show respectively early (U-morphotype) and final (S-morphotype) shape of bacteroids while yellow arrows point the transitory elongated shape of bacteroids. Bars represent 10 $m.

Figure 39 .

 39 Figure 39. Neighbor joining phylogeny of NCR protein sequences

  on their roots, which harbor nitrogen fixing endosymbiotic rhizobium bacteria called bacteroids. Several legume species produce in the symbiotic nodule cells which house the bacteroids, effector molecules, called NCRs, that are related to antimicrobial peptides of innate immunity and that induce the endosymbiotic rhizobia in a differentiated, enlarged and polyploid state. Here we identified the bclA gene of Bradyrhizobium species, which is required for the establishment of a functional symbiosis with the legumes Aeschynomene indica and Aeschynomene afraspera. Bacteroids in Aeschynomene nodules induced by the bclA mutant are less enlarged and have reduced ploidy compared to the wild-type strain. This bacterial differentiation defect is associated with a strongly reduced nitrogen fixation capability of the bacteroids. The BclA ABC transporter promotes the import of the NCR peptides and provides protection against the antimicrobial activity of these peptides. Moreover, BclA can complement the role of the related BacA transporter of Sinorhizobium meliloti, which has a similar symbiotic function in the interaction with Medicago legumes.* Modified from: Guefrachi, I.,Pierre, O., Bourge, M., Timchenko, T., Alunni, B., Barrière, Q., Czernic, P., Villaécija-Aguilar, J.A., Verly, C., Fardoux, J., Mars, M., Kondorosi, E., Giraud, E., and Mergaert, P. (2015a) and from Guefrachi, I.,Pierre, O., Crespo, J.C., Timchenko, T., Alunni, B., Mars, M., Giraud, E., Vinardell, J.M., and Mergaert, P. (2015a/b).

Figure 40 .

 40 Figure 40. The exsE gene of S. meliloti is not required for symbiosis (A) M. truncatula plants were inoculated with S. meliloti wild-type strains Sm2011 and Sm1021 and their exsE mutant derivatives or not inoculated (NI). The nitrogen fixation capacity per plant at 14 dpi was determined by the acetylene reduction assay and expressed as the relative activity to the Sm2011 wild-type infected plants. Error bars are standard deviations (n=10). (B,C) Symbiotic cells of a nodule infected with the Sm1021 exsE mutant observed by confocal microscopy after Live/Dead staining of a nodule section. IT indicates an infection thread. (B) Symbiotic cells in the infection zone where bacteroids are not yet fully differentiated. (C) Symbiotic cells in the fixation zone of the nodule where the bacteroids are normally differentiated into elongated bacterial cells. The green staining of the bacteroids indicates their viability. Scale bars are 10 $m.

  4) clusters the Bradyrhizobium proteins in 3 distinct subgroups but none of these subgroups is more closely related to the S. meliloti BacA clade than the others. The clade containing the BRADO6119, 1320006 and blr7537-encoded proteins is closer to the Mycobacterium BacA clade carrying a functional ortholog of BacA (Arnold et al., 2013) while the other Bradyrhizobium proteins encoded by BRADO0237, 250005, blr0517, BRADO1574, 950010 and blr4660 are closer to the non-symbiotic ExsE clade (Fig. 42). Thus based on the topology of the tree, the prediction of a symbiotic SbmA_BacA domain transporter is not easy.

Figure 42 .

 42 Figure 42. Phylogenetic tree of SbmA-BacA domain proteins

Figure 43 .

 43 Figure 43. Symbiotic phenotype of Bradyrhizobium strain ORS285 mutants (A) Plant growth and nodule phenotype at 14 dpi of A. afraspera (top panel) and A. indica (bottom panel) plants inoculated with ORS285 wild-type and its mutant derivatives in the 1320006 (bclA), 250005, 950010, 1320003 (Sec Prot) and 1320007 (glcA) genes. Scale bars are 2 cm and 2.5 mm respectively. (B) The nitrogen fixation activity at 14 dpi of A. afraspera (top panel) and A. indica (bottom panel) plants inoculated with the indicated strains, measured by the acetylene reduction assay per mg of fresh nodule weight (nmol ethylene produced per hour incubation and per mg nodule weight). Box-plots represent in the rectangle the first quartile to the third quartile, divided by the median value, whiskers above and below the box show the minimum and maximum measured values. Statistical analysis was made with a Mann-Whitney U-test (n=5; **: p<0.001). (C) Fresh weight shoot mass (left panel) or ratio of the shoot to root fresh weight (right panel) of A. afraspera and A. indica plants at 14 dpi inoculated with ORS285, ORS285!bclA or uninoculated. The experiment was carried out in duplicate with 10 plants per conditions. Error bars represent standard deviation and significance was determined by the Krsukall and Wallis test (**: p<0.001; ***: p<0.0001).

Figure 44 .

 44 Figure 44. The mutation in the bclA gene of Bradyrhizobium strain ORS285 is not affecting free-living growth and nitrogen fixation (A) Nitrogen fixation in nitrogen-free BNM medium (BNM-B) or nitrogen-free BNM to which no vitamin B12 was added (BNM-B, vitB12-) by the indicated strains. The nifK gene is an essential gene for the activation of the nitrogenase genes. A nifK mutant strain (ORS278nifK::Tn5) was included as a negative control for nitrogen fixation. The "control" sample is a non-inoculated flask. (B-E) Growth curve of ORS285 and ORS285!bclA in rich YM medium (B), minimal BNM-B medium (C), BNM-B to which vitamin B12 was omitted (BNM-B vitB12-) (D) or BNM medium in which the only carbon source was glycolate (BNM-B glycolate) (E). Growth was monitored by optical density measurements (OD 600nm ) in a plate reader at the indicated time points.

Figure 45 .

 45 Figure 45. Glycolate metabolism and the secretory protein of the bclA locus are not required for symbiosis (A) Plant growth and nodule phenotype at 14 dpi of A. afraspera (top panel) and A. indica (bottom panel) plants inoculated with ORS285 wild-type and its mutant derivatives in the 1320003 (Sec Prot) and 1320007 (glcA) genes. Scale bars are 2 cm and 2,5 mm respectively. (B) The nitrogen fixation activity of A. afraspera (top panel) and A. indica (bottom panel) plants inoculated with the indicated strains at 14 dpi, measured by the acetylene reduction assay per mg of fresh nodule weight (nmol ethylene produced per hour incubation and per mg nodule weight). Box-plots represent in the rectangle the first quartile to the third quartile, divided by the median value, whiskers above and below the box show the minimum and maximum measured values.

Figure 46 .

 46 Figure 46. Small nodule size and abnormal bacteroids of ORS285!bclA in A. afraspera and A. indica (A-H) Toluidine blue stained thin sections of 14 dpi A. afraspera and A. indica nodules induced by ORS285 or ORS285!bclA. The location of the symbiotic cells shown in (H) is indicated with an asterisk in (D) while the inset of (H) shows degenerating cells located in the central zone of the nodule (D, double asterisk). (I-L) Bacteroid viability determined by live/dead staining of nodule sections and confocal microscopy in A. afraspera and A. indica nodules induced by ORS285 or ORS285!bclA. Scale bars are 100 $m (A-D) or 10 $m (E-L). (M-P) Flow cytometry analysis of bacteroid size and DNA content. (M,N) Forward scatter and (O,P) DAPI fluorescence in free living Bradyrhizobium ORS285 (blue) or bacteroids isolated from A. afraspera (M,O) or A. indica (N,P) nodules infected with ORS285 wild-type (green) or ORS285!bclA (red).

Figure 47 .

 47 Figure 47. ORS285!bclA bacteroid functioning is impaired in A. afraspera nodules Sections of 14 dpi nodules were stained with the acidophilic DND-99 red fluorescent probe which marks active bacteroids by red fluorescence due to acidification of the peribacteroid space (Pierre et al., 2013). DIC is the differential interference contrast image and merge is the overlay of the fluorescence and DIC images. Pictures were acquired by confocal microscopy. The scale bars are 25 $m (top panels) or 10 $m (lower panels).

Figure 48 .

 48 Figure 48. Symbiotic phenotype of the bclA mutant in Bradyrhizobium strain ORS278 (A) Plant growth of A. evenia at 14 dpi inoculated with ORS278, ORS278 bclA::Tn5 or uninoculated. (B) Plant growth of A. indica inoculated with ORS278, ORS278 bclA::Tn5 or uninoculated. (C) Nodule phenotype of A. evenia inoculated with ORS278 or ORS278 bclA::Tn5. (D) Nodule phenotype of A. indica inoculated with ORS278 or ORS278 bclA::Tn5. (E) The nitrogen fixation activity of A. indica plants inoculated with ORS278 or ORS278 bclA::Tn5, measured by the acetylene reduction assay per plant (nmol ethylene produced per min and per plant). Box-plots represent in the rectangle the first quartile to the third quartile, divided by the median value, whiskers above and below the box show the minimum and maximum measured values. (F-I) Confocal microscopy of bacteroid viability determination by live/dead staining of A. evenia (F,H) and A. indica (G,I) nodules induced by ORS278 (F,G) or ORS278 bclA::Tn5 (H,I).

Figure 49 .

 49 Figure 49. Bacteroids in A. afraspera and A. indica 14 dpi nodules infected with ORS285 mutants Confocal microscopy of bacteroid viability determination by live/dead staining of A. afraspera (A) and A. indica (B) nodules induced by ORS285 wild-type and mutants in genes 1320006 (bclA), 250005 (SbmA-BacA domain transporter), 950010 (SbmA-BacA domain transporter), 1320003 (Sec Prot) and 1320007 (glcA). The scale bars are 10 !m.

Figure 50 .

 50 Figure 50. Complementation of the S. meliloti bacA mutation with Bradyrhizobium bclA (A-D) Phenotype of M. sativa nodules at 28 dpi, infected with the indicated strains. The scale bar is 1 mm. (E-L) Bacteroid viability determined by live/dead staining of nodule sections and confocal microscopy in M. sativa nodules induced by the indicated strains. Scale bars are 100 $m (E-H) or 10 $m (I-L). (M-P) Flow cytometry analysis of the DNA content by DAPI fluorescence in free living S. meliloti (blue in all panels) or bacteroids isolated from M. sativa nodules infected with the S. meliloti wild-type (M, green), the S. meliloti bacA mutant (N, orange), the S. meliloti bacA mutant complemented with the S. meliloti bacA gene (O, pale blue) or the S. meliloti bacA mutant complemented with the ORS285 bclA gene (P, red).

Figure 51 .

 51 Figure 51. The B. japonicum bclA mutant is not affected in symbiosis (A) Plant growth and nodule phenotype of A. afraspera (left panels) and G. max (right panels) plants inoculated with B. japonicum USDA110 wild-type and the bclA mutant at 14 dpi. Scale bars are 2 cm (top left panel), 3,5 cm (top right panel) and 1 mm (bottom panels). (B) The nitrogen fixation activity of A. afraspera (left panel) and G. max (right panel) plants inoculated with B. japonicum USDA110 wild-type and the bclA mutant, measured by the acetylene reduction assay per mg of fresh nodule weight (nmol ethylene produced per hour incubation and per mg nodule weight). Box-plots represent in the rectangle the first quartile to the third quartile, divided by the median value, whiskers above and below the box show the minimum and maximum measured values. (C-F) Toluidine blue stained thin sections (two top rows) and confocal microscopy of bacteroid viability determination by live/dead staining (bottom row) of A. afraspera (C,D) and G. max (E,F) nodules induced by USDA110 or USDA110!bclA. Scale bars are 100 $m for the top panels and 10 $m for the other panels. (G,H) Flow cytometry analysis of bacteroid size by forward scatter (top) and DNA content by DAPI fluorescence (bottom) in free living B. japonicum USDA110 (pink) or bacteroids isolated from A. afraspera (G) or G. max (H) nodules infected with USDA110 wild-type (blue) or USDA110!bclA (orange).

Figure 52 .

 52 Figure 52. The bacA gene of S. fredii strain HH103 is not required for symbiosis with soybean (A) Soybean nodule infected with HH103. (B) Soybean nodule infected with the bacA mutant HH103bacA::pK18mob. (C,D) Confocal microscopy of bacteroid viability determination by live/dead staining of a soybean nodule infected with strain HH103 (C) or strain HH103bacA::pK18mob (D). (E) The nitrogen fixation activity of soybean plants inoculated with HH103 or HH103bacA::pK18mob, measured by the acetylene reduction assay per mg of fresh weight nodules (nmol ethylene produced per min and per plant). Box-plots represent in the rectangle the first quartile to the third quartile, divided by the median value, whiskers above and below the box show the minimum and maximum measured values. Plants and nodules were harvested at 14 dpi. Scale bars are 4 mm (A-B) or 10 $m (C-D).

Figure 53 .

 53 Figure 53. The bclA gene of B. japonicum USDA110 complements the bacA mutation in S. meliloti Sm1021!bacA (A,C,E) Phenotype of M. sativa nodules at 28 dpi, infected with the indicated strains. (B,D,F) Bacteroid viability determined by live/dead staining of nodule sections and confocal microscopy in M. sativa nodules induced by the indicated strains. Scale bars are 1 mm (A-C-E) or 10 $m (D-E-F).

Figure 54 .

 54 Figure 54. The bclA gene confers sensitivity to the antibiotic bleomycin and the antimicrobial peptide Bac7 (A) Growth in the presence of bleomycin in BW25113!sbmA and derivatives expressing the indicated SbmA_BacA domain transporters located on plasmid pRF771. (B) Growth in the presence of bleomycin in Sm1021!bacA and derivatives expressing the indicated SbmA_BacA domain transporters located on plasmid pRF771. Bleomycin concentrations were applied as indicated and growth was determined after 24h incubation for E. coli and 48h for S. meliloti by optical density measurement at 600 nm with a plate reader. (C-G) Survival of E. coli strain BW25113!sbmA (C-E) and S. meliloti strain Sm1021!bacA (F,G) derivatives expressing the indicated SbmA_BacA domain transporters located on plasmid pRF771 after treatment with the peptide Bac7 at the indicated concentration and for the indicated times. The surviving bacteria were counted and expressed as % from the water control treatment.

Figure 55 .

 55 Figure 55. The bclA gene confers sensitivity to the antibiotic bleomycin and resistance to antimicrobial NCR peptides (A-C) Bleomycin sensitivity in Bradyrhizobium strains ORS278, ORS285 and USDA110 (blue bars) and their bclA mutant derivatives (red bars). Bleomycin concentrations were applied as indicated and growth was determined after 72h incubation by optical density measurement at 600nm with a plate reader. (D-F) S. meliloti strain Sm1021!bacA derivatives expressing no bacA-related gene (empty vector), the Sm1021 bacA gene, the E. coli BW25113 sbmA gene, the ORS285 bclA gene or the USDA110 bclA gene were incubated with NCR247 (D), NCR335 (E) or NCR035 (F) or with water (control) and the surviving bacteria were counted and expressed as % from the control treatment. Error bars in all panels are standard deviation.

Figure 56 .

 56 Figure 56. Uptake of FITC-NCR247 mediated by the BacA and BclA transporters (A) Epifluorescence microscopy of FITC-NCR247 treated Sm1021!bacApRF771bacA cells and sorted as FITC positive by the flow cytometer-cell sorter. (B-G) FITC-NCR247 uptake by S. meliloti strain Sm1021 (B) and Sm1021!bacA derivatives expressing no bacA-related gene (F), the Sm1021 bacA gene (D), the ORS285 bclA gene (E) or the USDA110 bclA gene (G) was measured by flow cytometry in the presence of trypan blue to quench extracellular fluorescence. FITC-positive cells are marked with a green box.

Figure 57 .

 57 Figure 57. FITC is not taken up by S. meliloti derivatives (A-E) FITC treated bacteria were analyzed by flow cytometry in the presence of trypan blue to quench extracellular fluorescence. The vertical bars separate FITC negative and positive signals. No FITC positive cells were detected in any of the strains.

  Mycobacterium) or in symbiotic bacteria in which they do not have a role in symbiosis such as the bclA gene in B. japonicum or the bacA gene in Sinorhizobium fredii,

  cannot replicate in Bradyrhizobium and carries the sacB gene that induce bacterial death in the presence of sucrose. The pK18mob-sacB derivatives were introduced in the respective Bradyrhizobium strains by triparental mating as above. Single recombinant clones were obtained by antibiotic selection and verified by PCR. Double recombinant clones were then obtained by growth on sucrose and Gm (50 µg/ml). Candidate clones were verified for the loss of Km resistance from the pK18mob-sacB plasmid and the replacement of the bclA gene by the Gm cassette was verified by PCR.For complementation experiments of bacA or sbmA mutations in S. meliloti Sm1021 or E. coli BW25113, the genes of interest were introduced into the broad host range vector pRF771 under the control of the trp promoter[START_REF] Wells | The Sinorhizobium meliloti stringent response affects multiple aspects of symbiosis[END_REF]. The plasmid pRF771bacA Sm1021[START_REF] Haag | Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis[END_REF] carries the S. meliloti bacA gene. The sbmA gene from E. coli strain BW25113 and the bclA genes from Bradyrhizobium strains ORS285 and USDA110 were obtained by PCR and cloned in the multicloning site of pRF771. The plasmids were introduced in the S. melilot mutant Sm1021ΔbacA by triparental conjugation and in the E. coli mutant BW25113ΔsbmA by electroporation (Table7; Annex 5).

  the closely related B. japonicum strain USDA110 makes U-morphotype bacteroids on soybean but E-type bacteroids on A. afraspera. In terms of ploidy levels of the bacteroids, these symbiotic systems cover bacteroids from very high ploidy (16C in the S-morphotype bacteroids of A. indica) over intermediate levels(7C or 3C in A. afraspera) to no polyploidy (in soybean). Thus, The Aeschynomene-Bradyrhizobium / soybean symbiotic couples constitute an appealing model system for studying the impact of bacteroid morphotype and polyploidy on the functioning of the symbiosis and for unraveling the mechanisms of bacteroid morphotype determination. Thus to better understand the mechanisms involved in bacteroid differentiation and the evolutionary advantage that it can provide to one or the other symbiotic partner, a comparative study of the different morphotypes can be undertaken using this model system. A preliminary study of the symbiotic efficiency of different morphotypes indicates that S-morphotypes are more efficient than E-morphotypes,

Table 1 .

 1 NCR probe-sets and expression data.. ....................................................................... 69

Table 2 .

 2 Entropy of gene expression.. ..................................................................................... 69

Table 3 .

 3 Aeschynomene afraspera and Aeschynomene indica root and nodule expressed sequenced ............................................................................................................................... 156

Table 4 .

 4 Nodule-specific Cys-rich putative peptides from Aeschynomene indica, A. afraspera and A. evenia. ......................................................................................................................... 157

Table 5 .

 5 Sequence of BacA-related proteins ......................................................................... 164

Table 6 .

 6 Homology table between BacA-related proteins ..................................................... 173

Table 7 .

 7 List of bacterial strains and plasmids used in this study .......................................... 174

Table 8 .

 8 List of primers used in this study .............................................................................

177 Introduction 1. The Rhizobium-legume symbiosis

  Rhizobium strains nodulating the IRLC-legumes pea or Vicia and the non-IRLC species Phaseolus vulgaris or Lotus japonicus have E-morphotype bacteroids in the former

	see below, Chapter II): elongated
	bacteroids which are sometimes also branched (designated as the E-morphotype; e.g. in
	legumes of the IRLC which includes the model legume M. truncatula or in some species of
	the distantly related Aeschynomene genus), enlarged spherical bacteroids (S-morphotype; in
	Aeschynomene or Arachis species of the Dalbergoid legume clade) and finally bacteroids
	which remain unmodified and have a rod-shaped morphology similar to free-growing bacteria
	(U-morphotype; e.g. in the clades comprising Lotus, Phaseolus or Glycine). Rhizobium strains
	that can nodulate legumes of different clades adapt a bacteroid morphotype, which is
	determined by the host rather than the bacterial genetic repertoire. For example, nearly
	isogenic

11. BacA is an ABC transporter. But what does it transport?

  [START_REF] Levier | Genetic analysis of the Sinorhizobium meliloti BacA protein: differential effects of mutations on phenotypes[END_REF]. The function of the M. tuberculosis BacA protein is abolished by deletion of its ATPase domain or by introduction of a point mutation in a conserved residue in this domain, which is known to be important for transport activity in other ABC transporters

	Walker, 2001), these ATPase containing proteins show large regions of similarity with BacA
	and SbmA. In particular, transmembrane predictions revealed that all members have 6
	transmembrane domains at conserved positions in the protein, shared with BacA and SbmA,

S. meliloti BacA and E. coli SbmA proteins have 8 transmembrane-domains but they lack any other identified functional domain. However, a class of BacA/BclA proteins have an additional ATPase domain. The above mentioned Bradyrhizobium and Mycobacterium genes belong to this class but also S. meliloti and E. coli have besides bacA or sbmA an additional bacA-like gene, named exsE and yddA respectively. Albeit they are divergent (LeVier and suggesting that these proteins have similar topologies. Importantly, the C-terminal ATPase domain of these BacA proteins is typical for ATP-binding cassette (ABC) transporters (

Table 1 ; http://www.biomedcentral.com/content/supplementary/1471-2164-15-712-S1.xlsx) for

 1 extracting the expression profiles of the 334 NCR probe-sets and for further treatments. The NCR probe-sets on the Affymetrix Medicago GeneChip, which was used for the MtGEA transcriptome compendium, were obtained by BLASTn searches on the MtGEA website (Table

Table 8 (Annex 6).

 8

Table 8 (Annex 6).

 8

	For the construction of Bradyrhizobium strain ORS285 mutants in the genes 250005
	(SbmA_BacA domain ABC transporter), 950010 (SbmA_BacA domain ABC transporter),
	1320007 (glcA) and 1320003 (secreted protein); (full accession numbers are
	BRAO285v1_250005,	BRAO285v1_950010,	BRAO285v1_1320007	and
	BRAO285v1_1320003), 400 to 600 base pairs (bp) fragments were amplified by PCR and
	cloned into the plasmid pVO155nptIIgfp (plasmid pVO155 (Oke and Long, 1999) carrying a
	constitutively expressed gfp gene) which does not replicate in S. meliloti. The resulting
	pVO155 derivatives were verified by PCR and sequencing and subsequently introduced in

Table 5 .

 5 Sequence of BacA-related proteins

	Bradyrhizobium BclA proteins

Remerciements Résumé

similar size. However, it seems likely to us that expressing such a large gene family might be a fitness cost for the plant that is not to be neglected. Therefore, keeping the whole gene family under a very tight regulatory control might be essential for the plant.

Supporting Information

Additional supporting information may be found in the online version of the article on the web site of the journal BMC Genomics.

Additional file 1: Table S1 (named in this thesis as Table 1). NCR probe-sets and expression data. The NCR probe-set annotations are from the Affymetrix Medicago Gene Chip. The expression data were extracted from MtGEA, NCBI GEO, and from the spatially-resolved nodule dataset. http://www.biomedcentral.com/content/supplementary/1471-2164-15-712-S1.xlsx Additional file 3: Table S2 (named in this thesis as Table 2). Entropy of gene expression. The 9,000 probe-sets with the lowest entropy. Bradyrhizobium genes is strongly conserved among the 3 tested strains (Table 5;Annex 3) and is located in synthenic regions in the Bradyrhizobium genomes (Fig. 41). These proteins display a relatively weak homology ranging from 21 to 31% identity and 38 to 51% similarity to BacA and ExsE (Table 5 and6 Within this family of ABC transporters, the BacA protein of S. meliloti and its homologues in Sinorhizobium, Mesorhizobium and Rhizobium species and the E. coli SbmA protein are 8-transmembrane domain proteins but they lack the typical ATPase domain of ABC transporters. In contrast, ExsE (LeVier and Walker, 2001) and the Mycobacterium BacA protein encoded by gene Rv1819c [START_REF] Domenech | BacA, an ABC transporter involved in maintenance of chronic murine infections with Mycobacterium tuberculosis[END_REF][START_REF] Arnold | Partial complementation of Sinorhizobium meliloti bacA mutant phenotypes by the Mycobacterium tuberculosis BacA protein[END_REF] have in addition to the transmembrane domains a fused ATPase domain. The here identified Bradyrhizobium proteins encoded by the BRADO6119, 13200006, blr7537, BRADO0237, Annex 1 The NCR were named with the initials of the plant species and numbered according to the read count level in the EST nodule libraries (for A. indica and A. afraspera) or arbritarily for A. evenia. The NCRs with a read count above 5 in the nodules are grey shaded. Signal peptides indicated in bold were determined using SignalP 4.0 (Petersen el al., 2011) and ajusted manually. Theoretical pI and MW were calculated for mature peptides from Expasy web site [START_REF] Gasteiger | Protein Identification and Analysis Tools on the ExPASy Server[END_REF]. For the A. evenia NCRs, the pI and MW were calculated for the mature peptide even if the signal peptide was truncated or contained stop codon (indicated in the table ). The presence of a signature corresponding to the NCR motif 1 (1) or motif 2 (2), as defined in Fig. 32, is indicated in the last column.