Keywords: Arabidopsis, defense signaling, root development, DAMP, phytocytokines, secreted peptide, oxidative stress DAMP: Damage/Danger-Associated Molecular Pattern, PA: Phosphatidic Acid, PTMP: Post-Translationally Modified Peptide, CRP: Cysteine-Rich Peptide, ROS: Reactive Oxygen Species

A secreted peptide, member of a Brassicaceae specific gene family, acts on pathogen defense response and root development through phospholipid pathway and ROS regulation.

2.

Summary

Due to the sessile nature, plants cannot simply escape a stressful situation. However, plants developed a multilayered immune system to counteract certain stresses. The immune system can for instance be triggered by herbivore feeding and pathogen or virus infections. Highly specialized components of the immune system enable the plant to detect a dangerous situation. These danger signals can have exogenous or endogenous origins. Exogenous danger signals derived from microbes are called microbe associated molecular patterns (MAMPs).

The best studied MAMP is flg22, a conserved motif of the bacterial flagella. On the other hand, endogenous danger signals are referred to as damage/danger associated molecular patterns (DAMPs). Actively processed and/or secreted peptides upon an infection that modulate immune response are referred as phytocytokines. Next to their activity as defense amplifiers, phytocytokines have been shown to regulate developmental processes. In addition to these classes of exogenous and endogenous elicitors there are also synthetic elicitor molecules.

These molecules are known to induce a systemic defense activation at distant non-challenged sites. This effect is called systemic acquired resistance (SAR). Along with SAR comes an extensive transcriptional reprogramming of defense-related genes and gene priming. Priming describes a mechanism in which a subset of genes is kept at a "ready-state" to facilitate a subsequent transcriptional regulation. Priming is often connected to epigenetic regulation of gene expression.

In this work, I describe the discovery and characterization of a novel phytocytokine. I will begin with the bioinformatics-based discovery of a peptide family that we termed SCOOP (for Serine riCh endOgenOus Peptide). I then show that a peptide, covering a conserved motif present in all members of the SCOOP family, induces various defense responses in Arabidopsis. Moreover, I show that the SCOOP12 peptide promotes resistance against Pseudomonas syringae and I demonstrate that perception of the SCOOP12 relies on the BAK1 coreceptor. I show the SCOOP peptide family contains several members apart from SCOOP12 that induce defense responses. Finally, I present a role of the SCOOP peptide family in plant developmental processes. The knock-out mutant of the putative SCOOP12 precursor ix proscoop12 shows an increased root length while treatments with three different SCOOP peptides induce severe phenotypical changes in the root tissue.

In the second part I investigate the effect of two other classes of elicitors (exogenous and synthetic elicitors). I show that treatments with a synthetic elicitor (BTH, a salicylic acid analogue) can lead to long-term transcriptional memory at certain genes. I found that subsequent challenging of such plants with the exogenous elicitor flg22 reverted this transcriptional memory bringing their expression back to the original pre-treatment level.

This memory behaviour we describe as "de-priming" memory response. I made very similar observations in apple (Malus x domestica), suggesting that this response is highly conserved in plants. Finally, I describe a potential role for DNA methylation in the observed transcriptional memory behaviour. I show that plants defective in DNA methylation pathways showed a different memory behaviour.

In conclusion, my thesis investigates effects on plant transcription, development and defense by endogenous (the SCOOP peptide family), exogenous (flg22) and synthetic (BTH) plant elicitors. My thesis shows (1) how diverse the function of these elicitors can be and (2) how the plant defense system can affect plant development and trigger memory.

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Résumé en français

De par leur nature d'organismes sessiles, les plantes ne peuvent fuir une condition stressante.

Toutefois, les plantes ont développé un système immunitaire multicouche pour contrer certains stress. Le système immunitaire peut, par exemple, être déclenché par le broutage d'herbivores ainsi que par des infections bactériennes ou virales. Des composés hautement spécialisés du système immunitaire permettent à la plante de détecter les situations dangereuses pour elle. Ces signaux de danger peuvent avoir des origines endogènes ou exogènes. Les signaux de danger exogènes sont dérivés des micro-organismes se nomment microbe associated molecular patterns (MAMPs). Le MAMP le plus étudié est flg22, un motif conservé présent dans les flagelles bactériens. D'un autre côté, les signaux de danger endogènes sont identifiés en tant que damage/danger associated molecular patterns (DAMPs). Les phytocytokynes sont, quant à elles, des peptides activement produits et sécrétés durant une infection et modulant la réponse. De manière conjointe à leur activité d'amplificateur des mécanismes de défense de la plante, les phytocytokines jouent un rôle dans le processus développemental. En plus de ces catégories d'éliciteurs exogènes et endogènes, il existe également des éliciteurs synthétiques. Ces molécules peuvent induire une activation des défenses systémiques sur des sites distants et non-infectés. Cet effet est dénommé résistance acquise systémique (SAR). Le SAR génère une reprogrammation transcriptionnelle extensive des gènes associés à la réponse défensive et au priming. Le priming décrit un mécanisme par lequel un petit nombre de gènes est maintenu dans un « état de réponse » pour faciliter une régulation transcriptionnelle subséquente. Le priming est souvent connecté à la régulation épigénétique de l'expression génique.

Dans ce travail, je décris la découverte et caractérisation d'une nouvelle phytocytokine. Je commencerai par présenter l'identification bioinformatique d'une famille de peptides que nous avons baptisé SCOOP (Serine riCh EndOgenOus Peptide). Je montrerai ensuite qu'un peptide, présentant un motif conservé par tous les membres de la famille SCOOP, induit des réponses défensives variées chez Arabidopsis. De plus, je mets en évidence que le peptide SCOOP12 induit une résistance contre le pathogène Pseudomonas syringae et démontre que la perception de SCOOP12 nécessite le corécepteur BAK1. Je présente également que la famille de peptides SCOOP contient plusieurs membres, autres que SCOOP12, qui induisent xi une réponse défensive. Finalement, j'expose un rôle de la famille SCOOP dans le processus développemental de la plante. Le mutant knock-out du précurseur putatif de SCOOP12, proscoop12, présente une augmentation de la longueur des racines alors que des traitements avec d'autres peptides SCOOP induisent des changements phénotypiques sévères au niveau des tissus racinaires.

Dans la seconde partie, j'investigue les effets de deux autres classes d'éliciteurs (exogènes et synthétiques). Je montre que les traitements avec un éliciteur synthétique (BTH, un analogue de l'acide salicylique) peuvent conduire à une mémoire transcriptionnelle de longue durée de certains gènes. J'ai observé que le challenge expérimental de ces plantes avec l'éliciteur exogène flg22 désactive cette mémoire transcriptionnelle en ramenant le niveau d'expression de ces gènes à leur niveau initial pré-traitement. Nous avons identifié ce comportement comme un « désamorçage » (de-priming) de la réponse-mémoire. J'ai effectué des observations similaires sur le pommier (Malus x domestica), suggérant que cette réponse puisse être hautement conservée chez les plantes. Finalement, je décris le rôle potentiel de la méthylation de l'ADN dans le phénomène observé de mémoire transcriptionnelle. Je démontre que les plantes avec des voies métaboliques de la méthylation de l'ADN défectueuses présentent un comportement mémoire différent.

En conclusion, ma thèse investigue les effets éliciteurs endogènes (la famille de peptides SCOOP), exogènes (flg22) et synthétiques (BTH) sur la transcription, le développement et les défenses des plantes. Ma thèse met en lumière (1) la diversité des fonctions de ces éliciteurs et (2) de quelle manière les mécanismes de défense des plantes peuvent affecter le développement et activer une mémoire.

3.

General Introduction

Why working on plants? What makes it so important to work on plants and why is this research so relevant but still often underrated in a cross-border context of science?

These are questions I often asked myself during the last years. However, during my studies I discovered how multifaceted, fascinating and unexpected this part of science can be. Why working on plants? -not only the work is fascinating but also important. Plants are the main source of energy to terrestrial ecosystems. Plants are capable to convert sunlight, CO2 and water into sugar. Carbohydrates are therefore available for other organisms. Especially at the IRHS in Angers we profit a lot from this in the form of delicious apple fruits.

Because of their sessile nature, plants cannot avoid danger by simply moving away. Therefore, plants need to be protected in a different manner compared to animals. In fact, plants are home to millions of potentially disease-causing pathogenic microbes that can have a negative influence on plant growth. Whereas beneficial microbes can have positive effects [START_REF] Delmotte | Community proteogenomics reveals insights into the physiology of phyllosphere bacteria[END_REF]. Despite this apparent challenging condition plant were able to entrench in most environments. As a key to success, plants have developed a very reliable defense system. This plant immune system consists of several layers of constitutive and inducible responses to fight back the millions of microbes [START_REF] Jones | The plant immune system[END_REF].

Next to the constitutive defense system, which consists of properties such as the waxy cuticular that covers the leaf surface, tichomes, thorns, secondary metabolites which harm invading pathogens, as well as lignified cell walls, defense responses can be induced [START_REF] Thordal-Christensen | Fresh insights into processes of nonhost resistance[END_REF]. One key to activating the plant immunity is the sensing of danger.

These danger signals can originate from the infectious agent or from the plant itself.

According to the latest classification of these immunogenic agents, plant host factors can be divided into two categories. The damage-associated molecular patterns (DAMPs) and peptides which are actively processed and/or secreted upon infection in order to modulate the immune response (phytocytokines) (Gust et al., 2017).

Whereas peptides derive from infectious agents are referred to as Microbe-Associated Molecular Patterns (MAMPs) or as Pathogens-Associated Molecular Patterns (PAMPs) one of the best characterized molecule among them is flagellin (Boller and Felix, 2009).

The activation of defense responses at a MAMP recognition site is followed by systemic defense activation even at distant non-challenged sites. This mechanism is called systemic acquired resistance (SAR). Along with SAR comes an extensive transcriptional reprogramming of defense-related genes and gene priming. Priming describes a mechanism in which a subset of genes is kept at a "ready-state" to facilitate a subsequent transcriptional regulation.

Epigenetic mechanisms, such as DNA methylation and histone modification and their influence on the chromatin reconfiguration, are shown to have influence on plant adaptation to different biotic stresses [START_REF] Espinas | Epigenetic Control of Defense Signaling and Priming in Plants[END_REF].

MAMPs and DAMPs -defense inducing molecular signatures

Plants have developed mechanisms to detect various forms of danger, including the attack by pathogens as well as tissue and cellular damage. Since plants lack an adaptive immune system they have strong need for rapid detection of all kinds of pathogens. Thus, the perception of defense-inducing molecular signatures like MAMPs and DAMPs is viable for the fast initiation of defense responses. Exogenous as well as endogenous elicitors will be described in the following chapter.

Microbe-associated Molecular Patterns (MAMPs)

MAMPs are highly conserved and crucial molecules, often they are found in a whole clade of microbes. Formally MAMPs were referred as PAMPs. However, these structures are not exclusively restricted to pathogens and the term MAMPs is more accurate (Boller and Felix, 2009). MAMPs have the ability to elicit defense responses upon their perception. Prominent examples for MAMPs are peptidoglycans. These are the building blocks of the bacterial cell wall. Another example are bacterial elongation factors and finally flagellin monomers, which are required for the movement of motile bacteria [START_REF] Newman | MAMP (microbe-associated molecular pattern) triggered immunity in plants[END_REF][START_REF] Choi | DAMPs, MAMPs, and NAMPs in plant innate immunity[END_REF]. The class of MAMPs can be separated according to their origin. Fungal MAMPs can also be perceived by plants. One of best studied receptors is the chitin elicitor receptor kinase 1 (CERK1). This receptor plays an important role in chitin triggered immunity [START_REF] Miya | CERK1, a LysM receptor kinase, is essential for chitin elicitor signaling in Arabidopsis[END_REF][START_REF] Wan | A LysM receptor-like kinase plays a critical role in chitin signaling and fungal resistance in Arabidopsis[END_REF]. Next to chitin, ergosterol, which is an important building block of the fungus itself, can serve as defense elicitors [START_REF] Klemptner | Ergosterol, an orphan fungal microbe-associated molecular pattern (MAMP)[END_REF]. Moreover, recently it was shown that the linear 1,3-β-D-glucans, which is also present in the walls of fungi and oomycetes, are recognized by the plant [START_REF] Mélida | Non-branched β-1,3-glucan oligosaccharides trigger immune responses in Arabidopsis[END_REF]. Additional MAMPs can derive from viruses.

Viruses are often transmitted through vector organisms like aphids or nematodes. These organisms wound the plant by sucking or feeding on it and thereby transmit the virus into the plant cells [START_REF] Bragard | Status and prospects of plant virus control through interference with vector transmission[END_REF]. Typical viral molecules that can be observed by the plant are virus-specific coat proteins, movement proteins or replicases [START_REF] Conti | Modulation of host plant immunity by Tobamovirus proteins[END_REF]. However, in contrast to other MAMPs, virus derived MAMPs mostly induce virus-specific defense responses. This includes the activation of the RNA silencing machinery and located cell-death in order to restrict viral spread [START_REF] Padmanabhan | The conformational and subcellular compartmental dance of plant NLRs during viral recognition and defense signaling[END_REF]. Finally, bacterial MAMPs include the bacterial flagellin. Flagellin is the hallmark of a peptide MAMP (Felix et al., 1999). As mentioned before, flagellin monomers are the building blocks of the bacterial flagellum [START_REF] Taguchi | Effects of glycosylation on swimming ability and flagellar polymorphic transformation in Pseudomonas syringae pv. tabaci 6605[END_REF]. The N-and C-terminus of this protein are highly conserved.

It was shown that only nanomolar concentrations of a conserved part of 22 amino acids (aa) from the N-terminus of Pseudomonas syringae pv. tomato (Pst DC3000) flagellin elicits defense responses [START_REF] Felix | Plants have a sensitive perception system for the most conserved domain of bacterial flagellin[END_REF][START_REF] Smith | Toll-like receptor 5 recognizes a conserved site on flagellin required for protofilament formation and bacterial motility[END_REF]. This small conserved peptide is referred to as flg22 and is perceived by most plant species via the leucin-rich repeat receptor like kinase (LRR-RLK) FLAGELLIN SENSING 2 (FLS2) [START_REF] Gomez-Gomez | A single locus determines sensitivity to bacterial flagellin in Arabidopsis thaliana[END_REF][START_REF] Bauer | Sensitivity of different ecotypes and mutants of Arabidopsis thaliana toward the bacterial elicitor flagellin correlates with the presence of receptor-binding sites[END_REF][START_REF] Chinchilla | The Arabidopsis receptor kinase FLS2 binds flg22 and determines the specificity of flagellin perception[END_REF]. Plants defective in FLS2 are completely "blind" to flagellin and were found to be more susceptible to Pst. DC3000 infections. Interestingly, the exposure of Arabidopsis pants to flg22 results in a protective effect. Pre-treatments with exogenously applied flg22 leads to enhanced resistance to Pst. DC3000 [START_REF] Zipfel | Bacterial disease resistance in Arabidopsis through flagellin perception[END_REF]. However, not all epitopes of flagellin are as universal as flg22, for instance the flgII-28 has only been found to be perceived by several solanaceous species. A C-terminal epitope of flagellin, called CD2-1 has only been found to be perceived by rice [START_REF] Cai | The plant pathogen Pseudomonas syringae pv. tomato is genetically monomorphic and under strong selection to evade tomato immunity[END_REF][START_REF] Veluchamy | Natural variation for responsiveness to flg22, flgII-28, and csp22 and Pseudomonas syringae pv. tomato in heirloom tomatoes[END_REF][START_REF] Katsuragi | CD2-1, the C-Terminal Region of Flagellin, Modulates the Induction of Immune Responses in Rice[END_REF]. Although flg22 recognition by FLS2 is very efficient some microbial species or strains have diverged their MAMPs to render them unrecognizable by the plant. It was shown that the flg22 peptide covering flagellin of Erwinia amylovora does not elicit a defense response in Arabidopsis or only in very high concentration. However, only two mutations of FLS2 can increase the recognition of E. amylovora flagellin peptide [START_REF] Helft | Directed Evolution of FLS2 towards Novel Flagellin Peptide Recognition[END_REF]. This indicates that plants can readily evolve in order to adapt to novel or modified microbes. Interestingly, flg22 can be transported by the plant into distal tissues. This process is established by endocytosis of flg22 together with the FLS2 and results in long-distance transport of flg22 [START_REF] Jelenska | Flagellin peptide flg22 gains access to long-distance trafficking in Arabidopsis via its receptor, FLS2[END_REF].

Another well-known MAMP is the ELONGATION FACTOR THERMO UNSTABLE (EF-Tu). This MAMP is essential for elongation during protein synthesis [START_REF] Jeppesen | Crystal structure of the bovine mitochondrial elongation factor Tu.Ts complex[END_REF]. EF-Tu is a highly conserved 18 or 26 amino acids (aa) (elf18/26) and shows a 90% homology between hundreds of bacteria species. EF-Tu is perceived via the PRR EF-Tu RECEPTOR (EFR) [START_REF] Kunze | The N terminus of bacterial elongation factor Tu elicits innate immunity in Arabidopsis plants[END_REF][START_REF] Zipfel | Perception of the bacterial PAMP EF-Tu by the receptor EFR restricts Agrobacteriummediated transformation[END_REF].

Next to these examples also non-proteinaceous MAMPs like peptidoglycans (PGN), β-glucans and lipopolysaccharides (LPS) have been identified and shown to elicit defense responses [START_REF] Proietti | Xanthomonas campestris lipooligosaccharides trigger innate immunity and oxidative burst in Arabidopsis[END_REF][START_REF] Zipfel | Plant pattern-recognition receptors[END_REF]Gust et al., 2017).

Damage/Danger-associated Molecular Patterns (DAMPs)

Peptides that are produced by the plant itself and trigger a defense response by the same plant are generally referred as DAMPs. However, with the increasing knowledge of the action of DAMPs the classification is more and more diverse. The endogenous danger signals can be divided into primary, "classical" DAMPs, which are passively released upon plant tissue damage and secondary endogenous danger signals. This second class can again be divided into (1) secondary endogenous peptides which are processed and released upon herbivore or microbial infections and induced plant defense responses and (2) secondary endogenous danger signals which are linked to the regulation of plant growth and development. These endogenous peptides are also referred to as phytocytokines. However, a large proportion of phytocytokines play roles in plant development and growth as well as in plant defense (Gust et al., 2017) Primary endogenous danger signals are passively released upon wounding. Wounding and herbivore attacks results in the release of intracellular and cell wall-associated molecules into the apoplastic space [START_REF] Mithöfer | Plant defense against herbivores: chemical aspects[END_REF][START_REF] Ferrari | Oligogalacturonides: plant damage-associated molecular patterns and regulators of growth and development[END_REF][START_REF] Choi | DAMPs, MAMPs, and NAMPs in plant innate immunity[END_REF][START_REF] Duran-Flores | Sources of specificity in plant damaged-self recognition[END_REF]. Well-known primary DAMPs include the oligomeric fragments of plant cell-wall pectin, termed oligogalacturonides (OGs). OGs are produced by the polygalacturonase enzyme. This enzyme is secreted by fungi during plant infection [START_REF] D'ovidio | Polygalacturonases, polygalacturonase-inhibiting proteins and pectic oligomers in plant-pathogen interactions[END_REF]. OGs can be sensed by the plant with the wall-associated (receptor) kinase 1 (WAK1) [START_REF] Brutus | A domain swap approach reveals a role of the plant wall-associated kinase 1 (WAK1) as a receptor of oligogalacturonides[END_REF]. Other molecules that are considered as DAMPs are extracellular (e) nucleotides like eATP. In Arabidopsis eATP is perceived by the lectin receptor kinase DOES NOT RESPOND TO NUCLEOTIDES 1 (DORN1) [START_REF] Choi | Identification of a plant receptor for extracellular ATP[END_REF]. Interestingly, eATP has strong influence on jasmonate (JA) signaling [START_REF] Tripathi | Extracellular ATP Acts on Jasmonate Signaling to Reinforce Plant Defense[END_REF]. JA signaling is typically induced upon infection by necrotrophic pathogens and chewing herbivores [START_REF] Reymond | Jasmonate and salicylate as global signals for defense gene expression[END_REF]. Upon herbivory, the green-leaf volatiles (GLV) are released from wounded leaves [START_REF] Scala | Green leaf volatiles: a plant's multifunctional weapon against herbivores and pathogens[END_REF]. GLV have direct antimicrobial effects and can thus serve as primers of systemic immunity upon local damage. It was found that also fructans in fructansaccumulating plants have actions as primary DAMPs [START_REF] Versluys | Fructans As DAMPs or MAMPs: Evolutionary Prospects, Cross-Tolerance, and Multistress Resistance Potential[END_REF].

Additionally, to passively released danger signals plants produce secondary endogenous danger peptides. These molecules are shown to modulate immune responses to herbivory and microbial infections (Albert, 2013;[START_REF] Mott | Peptides and small molecules of the plant-pathogen apoplastic arena[END_REF]. Typically, the molecules are produced as larger precursor proteins and are processed by proteolytic cleavage and then secreted upon wounding, microbial infection or MAMP treatment [START_REF] Yamaguchi | Endogenous peptide elicitors in higher plants[END_REF]. One of the first danger peptides was initially discovered in tomato. There, systemin was shown to be an 18aa long polypeptide that is processed from a 200aa prosystemin precursor upon wounding and herbivory [START_REF] Pearce | A polypeptide from tomato leaves induces wound-inducible proteinase inhibitor proteins[END_REF][START_REF] Ryan | Systemins: A functionally defined family of peptide signal that regulate defensive genes in Solanaceae species[END_REF]. For many years the processing mechanism was unknown, but just recently, phytaspases and aspartate-specific proteases, have been found to be implicated in systemin processing [START_REF] Beloshistov | Phytaspase-mediated precursor processing and maturation of the wound hormone systemin[END_REF].

Systemin was shown to induce various defense reactions including the induction of the expression of protease inhibitors which are crucial for defense reactions against herbivores [START_REF] Zavala | Manipulation of endogenous trypsin proteinase inhibitor production in Nicotiana attenuata demonstrates their function as antiherbivore defenses[END_REF][START_REF] Zhu-Salzman | Arthropod-inducible proteins: Broad spectrum defenses against multiple herbivores[END_REF][START_REF] Hartl | The multiple functions of plant serine protease inhibitors: defense against herbivores and beyond[END_REF]. In several solanaceous plants a second class of systemins are produced. They are referred to as hydroxyproline-rich systemins (HypSys) [START_REF] Pearce | Production of multiple plant hormones from a single polyprotein precursor[END_REF][START_REF] Pearce | Isolation and Characterization of Hydroxyproline-Rich Glycopeptide Signals in Black Nightshade Leaves[END_REF]. In Nicotiana tabacum two HypSys (NtHySysI/II) have been identified and shown to induce defense responses in a similar way that Systemin does while not sharing any sequence homology to the prosystemin identified in tomato [START_REF] Pearce | Production of multiple plant hormones from a single polyprotein precursor[END_REF][START_REF] Pearce | Systemin, hydroxyproline-rich systemin and the induction of protease inhibitors[END_REF].

Another family of endogenous danger peptides are the Arabidopsis PAMP-induced peptides (PIP). The AtPIP family harbors 11 members in Arabidopsis of which two members (AtPIP1 and AtPIP2) have been shown to trigger immune responses similar to that of flg22 (Hou et al., 2014). AtPIP1 and AtPIP2 are produced as preproteins (PrePIP1 and PrePIP2) upon microbial infection as well as MAMP treatment. The preproteins harbor a signal domain that enables the secretion in a signal peptide-dependent manner. AtPIPs are perceived by the receptor LRR-RK RLK7 (Hou et al., 2014).

Another peptide family that has been shown to be widespread throughout the plant kingdom are the rapid alkalization factors (RALFs). They have been identified in flowering plants, lycophytes and mosses [START_REF] Murphy | Understanding the RALF family: a tale of many species[END_REF]. All 39 RALF peptides, encoded by the Arabidopsis genome carry a N-terminal signal peptide [START_REF] Sharma | Comprehensive analysis of plant rapid alkalization factor (RALF) genes[END_REF]. RALFs were first identified by their ability to cause alkalization of tobacco cell suspensions (Pearce et al., 2001a). Not all RALF peptides are the result of the cleavage of propeptides, those that require processing are processed during secretion or in the apoplast by the Arabidopsis subtilase SITE-1-PROTEASE (AtS1P) (Pearce et al., 2001a;Stegmann et al., 2017). Next to wide abundance of different plant species in which RALFs can be found, the high number of biological processes in which RALFs are involved is remarkable. Individual RALFs play roles in combining biotic and abiotic stresses, induce the plant defense, play roles in fungal infection resistance and act with elf18 to induce plant immunity [START_REF] Gupta | Primary metabolism of chickpea is the initial target of wound inducing early sensed Fusarium oxysporum f. sp. ciceri race I[END_REF][START_REF] Atkinson | Identification of genes involved in the response of Arabidopsis to simultaneous biotic and abiotic stresses[END_REF]Stegmann et al., 2017). RALF members have also been shown to be involved in controlling the pollen tube and therefore to be important in the regulation of sexual reproduction (Stegmann and Zipfel, 2017). The perception of AtRALF23 is established by the malectin-like receptor kinase FERONIA (FER) (Stegmann et al., 2017).

Another family that has been well-studied and originally identified in Arabidopsis is the plant elicitor peptide (Pep) family. This family harbors 8 members named AtPep1-8 (Huffaker et al., 2006;[START_REF] Yamaguchi | Endogenous peptide elicitors in higher plants[END_REF]Bartels and Boller, 2015) and has been identified in various plant species [START_REF] Lori | Evolutionary divergence of the plant elicitor peptides (Peps) and their receptors: interfamily incompatibility of perception but compatibility of downstream signalling[END_REF]. The precursors of the eight AtPep peptides (PROPEP1-8) have a tissue-specific expression pattern and the expression of individual members is induced by wounding, MAMP treatment, microbial infection and by treatments with their own AtPep peptides (Huffaker et al., 2006;Albert, 2013;[START_REF] Mott | Peptides and small molecules of the plant-pathogen apoplastic arena[END_REF]Bartels and Boller, 2015). The Atpep peptides are perceived by two leucin-rich repeat receptor kinases, named AtPEPR1 and AtPEPR2 [START_REF] Yamaguchi | The cell surface leucine-rich repeat receptor for AtPep1, an endogenous peptide elicitor in Arabidopsis, is functional in transgenic tobacco cells[END_REF]Krol et al., 2010;[START_REF] Yamaguchi | PEPR2 is a second receptor for the Pep1 and Pep2 peptides and contributes to defense responses in Arabidopsis[END_REF].

AtPeps are involved in various defense responses against bacteria, fungus, oomycetes as well as in responses to insect attacks (Huffaker et al., 2006;[START_REF] Yamaguchi | PEPR2 is a second receptor for the Pep1 and Pep2 peptides and contributes to defense responses in Arabidopsis[END_REF][START_REF] Huffaker | Plant elicitor peptides are conserved signals regulating direct and indirect antiherbivore defense[END_REF][START_REF] Yasuda | A look at plant immunity through the window of the multitasking coreceptor BAK1[END_REF]. However, it is still not known how PROPEP proteins are processed and secreted. It has been proposed that the peptides are processed (probably by metacaspase 4 (personal communication with Tim Hander)) and released only upon wounding (Bartels and Boller, 2015). An indication on how wide-spread this peptide family is, was provided by recent work on Rosaceae species. In 95 Rosaceae varieties up to 180 PROPEP 1-4 sequences could be identified. Homologues of PROPEPs could be found in different varieties of apple, pear, peach and strawberry [START_REF] Ruiz | Diversity of plant defense elicitor peptides within the Rosaceae[END_REF]Ruiz et al., 2018a).

Phytocytokines and small endogenous signaling peptides

Secondary endogenous danger signals which are involved in plant growth and development, but are not primarily involved in plant defense responses, could be referred to as phytocytokines (Gust et al., 2017). According to this classification prominent small peptides including CLAVATA3/EMBRYO-SURROUNDING REGION (CLE), C-TERMINAL ENCODED PEPTIDE 1 (CEP1), ROOT MERISTEM GROWTH FACTOR (RGF), phytosulfokines (PSK), AtPeps, AtPIPs and RALF and many more belong to this group. The full overview of all endogenous signaling peptides is shown in Figure 1. AtPeps, AtPIPs and RALF belong to phytocytokines because, next to their involvement in defense responses, they have also been shown to influence plant growth and development (Hou et al., 2014;[START_REF] Murphy | Understanding the RALF family: a tale of many species[END_REF][START_REF] Gully | Perception of Arabidopsis AtPep peptides, but not bacterial elicitors, accelerates starvation-induced senescence[END_REF].

Several of these endogenous signaling peptides have also been shown to be involved in development and functioning of nodulation [START_REF] Kereszt | Impact of Plant Peptides on Symbiotic Nodule Development and Functioning[END_REF]. Phytocytokines belong to small endogenous signaling peptides, they are also referred to as plant peptide hormones in order to separate them from classical plant hormones such as auxin, cytokinin and ethylene, which have been assumed to be the main players in cell to cell communication and intercellular signaling processes [START_REF] Davies | The plant hormones: Their nature, occurrence, and functions[END_REF]. responses. This illustration of small endogenous peptides involved in several plant physiological processes was taken from (Tavormina et al., 2015). Not all small peptides presented in the figure are discussed in this introduction. For more information the original review as well as [START_REF] Czyzewicz | Message in a bottle: small signalling peptide outputs during growth and development[END_REF] provide an exhaustive overview.

In Arabidopsis, the CLE peptide (CLAVATA3/EMBRYO SURROUNDING REGION-RELATED) family is a large family with 32 members. CLE genes play various roles in stem cell homeostasis and in different types of meristems as well as many biological roles in growth and development [START_REF] Wang | CLE Peptide Signaling and Crosstalk with Phytohormones and Environmental Stimuli[END_REF][START_REF] Dao | CLE peptide-mediated signaling in shoot and vascular meristem development[END_REF]. A common feature of all CLE members is that they all derive from a roughly 150aa long precursor protein containing an Nterminal secretion signal and the C-terminal 14aa long CLE motif [START_REF] Cock | A large family of genes that share homology with CLAVATA3[END_REF][START_REF] Wang | CLE peptide signaling during plant development[END_REF]. One of the best characterized CLE family members is CLV3 (Clavata 3).

CLV3 is involved in the maintenance of the shoot apical meristem (SAM), while other CLE family members are involved in cell fate regulation in the root apical meristem (RAM) and (pro) cambium [START_REF] Kondo | A plant peptide encoded by CLV3 identified by in situ MALDI-TOF MS analysis[END_REF]Betsuyaku et al., 2011;Matsubayashi, 2014;[START_REF] Czyzewicz | Modulation of Arabidopsis and monocot root architecture by CLAVATA3/EMBRYO SURROUNDING REGION 26 peptide[END_REF]. The perception of CLV3 is not yet been completely elucidated. Three major receptor kinase complexes have been proposed for signaling in the SAM, namely CLV1, CLV2, CORYNE as well as RPK2 [START_REF] Clark | CLAVATA3 is a specific regulator of shoot and floral meristem development affecting the same processes as CLAVATA1[END_REF][START_REF] Kinoshita | RPK2 is an essential receptor-like kinase that transmits the CLV3 signal in Arabidopsis[END_REF]Matsubayashi, 2014). However, in other tissues and for other CLE peptides the perception has been proposed to be established by other receptors (Betsuyaku et al., 2011). Interestingly, signaling of various CLE peptide family members have been shown to integrate with signaling of classic plant hormones [START_REF] Wang | CLE Peptide Signaling and Crosstalk with Phytohormones and Environmental Stimuli[END_REF]. These interactions include auxin, brassinosteroids and cytokinin signaling [START_REF] Whitford | Plant CLE peptides from two distinct functional classes synergistically induce division of vascular cells[END_REF][START_REF] Kondo | CLE peptides can negatively regulate protoxylem vessel formation via cytokinin signaling[END_REF][START_REF] Kondo | Plant GSK3 proteins regulate xylem cell differentiation downstream of TDIF-TDR signalling[END_REF]. The same observations have been made in other plant species apart from Arabidopsis. 47 CLE genes have been identified from rice [START_REF] Kinoshita | Gain-offunction phenotypes of chemically synthetic CLAVATA3/ESR-related (CLE) peptides in Arabidopsis thaliana and Oryza sativa[END_REF] one of which (OsCLE48) has been shown to be induced by auxin application [START_REF] Guo | An auxin responsive CLE gene regulates shoot apical meristem development in Arabidopsis[END_REF].

Many small signaling peptides are involved in root growth and root development. The number of identified peptides is steadily increasing for different root tissues [START_REF] Oh | Signaling Peptides and Receptors Coordinating Plant Root Development[END_REF]. The C-TERMINAL ENCODED PEPTIDE (CEP) family was identified by a in silico approach and contains 15 members [START_REF] Ohyama | Identification of a biologically active, small, secreted peptide in Arabidopsis by in silico gene screening, followed by LC-MSbased structure analysis[END_REF]. One member, CEP1 is a 15aa long small peptide and has been shown to be involved in the regulation of lateral root growth by regulating root meristem activity [START_REF] Ohyama | Identification of a biologically active, small, secreted peptide in Arabidopsis by in silico gene screening, followed by LC-MSbased structure analysis[END_REF]. Moreover, CEP family members have been shown to lead to increased expression in response to Nitrogen (N) starvation. During N starvation CEPs are transported from the roots to the shoots and there perceived by two LRR-RLK receptors (CEPR1 and CEPR2). This leads to a systemic signal which results in the upregulation of N transporter genes in the roots [START_REF] Tabata | Perception of root-derived peptides by shoot LRR-RKs mediates systemic N-demand signaling[END_REF]. The nature of this signal was recently uncovered, the two polypeptides CEPD1 and CEPD2 are transported from the shoot to the root [START_REF] Ohkubo | Shootto-root mobile polypeptides involved in systemic regulation of nitrogen acquisition[END_REF]. Moreover, CEP members have been shown to be involved in the development of lateral roots (LR). CEP1 overexpression leads to the inhibition of root growth and is therefore, in line with CEP5, a negative regulator of root growth [START_REF] Ohyama | Identification of a biologically active, small, secreted peptide in Arabidopsis by in silico gene screening, followed by LC-MSbased structure analysis[END_REF][START_REF] Roberts | CEP5 and XIP1/CEPR1 regulate lateral root initiation in Arabidopsis[END_REF] A peptide family mainly involved in the development of the primary root is ROOT MERISTEM GROWTH FACTOR (RGF). Members of this family play roles in the root meristem maintenance [START_REF] Oh | Signaling Peptides and Receptors Coordinating Plant Root Development[END_REF]. RGF1 requires sulfation of a tyrosine residue to achieve its activity in the maintenance of root stem cells (Matsuzaki et al., 2010). The perception of RGF is not yet been investigated in great detail. The rgf insensitive (rgi) is a quintuple mutant consisting of the genes PLETHORA1, PLETHORA2, RGFR1,2,3 [START_REF] Ou | RGF1 INSENSITIVE 1 to 5, a group of LRR receptor-like kinases, are essential for the perception of root meristem growth factor 1 in Arabidopsis thaliana[END_REF][START_REF] Shinohara | Identification of three LRR-RKs involved in perception of root meristem growth factor in Arabidopsis[END_REF]. It is up to now not clarified whether all five receptors are equally involved in the perception of RGF peptides.

A well-known small peptide family are the phytosulfokines (PSK). PSKs were originally found to act as growth factors in low-density suspension cultures [START_REF] Matsubayashi | Phytosulfokine -Sulfated peptides that induce the proliferation of single mesophyll cells of Asparagus officinalis L[END_REF][START_REF] Matsubayashi | The endogenous sulfated pentapeptide phytosulfokine-alpha stimulates tracheary element differentiation of isolated mesophyll cells of zinnia[END_REF][START_REF] Yang | Oryza sativa PSK gene encodes a precursor of phytosulfokine-alpha, a sulfated peptide growth factor found in plants[END_REF]. In Arabidopsis five PSK precursor genes have been identified, coding for 80aa long precursor proteins [START_REF] Yang | Diversity of Arabidopsis genes encoding precursors for phytosulfokine, a peptide growth factor[END_REF]. The PSK peptide is only 5aa long and was shown to regulate cell expansion in the root elongation and differentiation zone [START_REF] Kutschmar | PSK-alpha promotes root growth in Arabidopsis[END_REF]. PSK has a close homologue, PLANT PEPTIDE CONTAINING SULFATED TYROSINE (PSY1) which is an 18aa long peptide and that has been shown to possess similar functions as PSK (Amano et al., 2007). PSK is recognized by two LRR-RK, PSKR1 and PSKR2 while PSY1 is perceived by the close homologue PSYR1 (Amano et al., 2007). PSKR1 interacts with the well-known co-receptor BAK1 in PSK signaling [START_REF] Ladwig | Phytosulfokine Regulates Growth in Arabidopsis through a Response Module at the Plasma Membrane That Includes CYCLIC NUCLEOTIDE-GATED CHANNEL17, H+-ATPase, and BAK1[END_REF]. Next to the control of root growth the interaction of PSK with its receptors is involved in hypocotyl cell expansion control [START_REF] Stührwohldt | Phytosulfokine-α controls hypocotyl length and cell expansion in Arabidopsis thaliana through phytosulfokine receptor 1[END_REF]. Moreover, a role for PSKRs as negative regulators of innate immunity has been proposed (Mosher et al., 2013;[START_REF] Tang | Receptor Kinases in Plant-Pathogen Interactions: More Than Pattern Recognition[END_REF]. The pskr1 mutant showed enhanced resistance against the hemibiotrophic pathogen Pseudomonas syringae as well as increased defense responses following elf18 application [START_REF] Igarashi | The peptide growth factor, phytosulfokine, attenuates pattern-triggered immunity[END_REF].

Plant innate immunity

In contrast to higher animals, plants do not possess mobile cells as part of their immune system. The mobile immune system is integrated by specialized cells, which detect and destroy enemies. The plant immune system however, is supposed to be present in every single cell and needs to allow each one of those to detect danger. In order to induce an effective defense response and to initiate signal cascades to alert parts of the plant about an imminent attack [START_REF] Schilmiller | Systemic signaling in the wound response[END_REF][START_REF] Jones | The plant immune system[END_REF]. The plant immune system was shaped by millions of years of coevolution between plants and the corresponding pathogens. This convergent evolution results in a tremendous complexity at the molecular level [START_REF] Asai | Plant cells under siege: plant immune system versus pathogen effectors[END_REF]. While the host plant is defending like a fortified castle, the invading pathogens developed special weapons to conquer the established walls of the plant immune system. The first layer of defense and therefore the "watch towers" of the plant defense can be found in the cell surface in form of pattern-recognition receptors (PRR). These receptors detect broadly conserved pathogen molecules (pathogen/ microbe-associated molecular patterns, PAMP/MAMPs). This system is known as PAMP-(or pattern)-triggered immunity (PTI) or sometimes also referred as MTI (MAMP-triggered immunity). A weapon of pathogens to overcome the first layer of defense are effectors. Effectors are targeted to the pant apoplast or cytoplasm. However, also against these weapons plants developed specific defense mechanisms. Effectors are recognized by receptors called Resistance (R) proteins, which recognize the presence of pathogen effectors directly or indirectly and induce a strong counter attack. This mechanism is known as effector-triggered immunity (ETI) [START_REF] Jones | The plant immune system[END_REF].

They reveal themselves -the first layer of defense

The first layer of defense must be a broad system since plants are under continuous attack from various pathogens. Plants need to be able to detect a large diversity of different pathogens. The plant PRRs can recognize highly conserved structures common amongst numerous classes of microbes. Most of these structures are vital for the microbial life style and therefore underlie a negative selection pressure which is altered by the microbes to overcome the recognition (Boller and Felix, 2009;[START_REF] Monaghan | Plant pattern recognition receptor complexes at the plasma membrane[END_REF]. PTI is therefore discussed as an evolutionary old defense system, which is strengthened by the fact that many PRRs are conserved amongst higher plants (Boller and Felix, 2009). The first layer of defense is established by the recognition of MAMPs by PRRs.

Recognize the enemy -pattern recognition receptors (PRRs)

The innate immunity system of multicellular organisms requires the acquisition of cell surface receptors that can differ between "self" and "non-self" molecules. This is achieved by the large family of pattern recognition receptors (PRRs). PRRs are capable of activating one or more signaling pathways. Microbes are often found in the plant apoplast and therefore separated from the plant cell interior. To sense environmental information across the plasma membrane in a selective manner, plants use families of plasma membrane-localized PRRs.

Therefore, PRRs transduce the signal of danger into intracellular signals [START_REF] Macho | Plant PRRs and the activation of innate immune signaling[END_REF][START_REF] Zipfel | Plant pattern-recognition receptors[END_REF][START_REF] Tang | Receptor Kinases in Plant-Pathogen Interactions: More Than Pattern Recognition[END_REF]. PRRs can be broadly categorized into receptor-like kinases (RLKs) and receptor-like proteins (RLPs). The structure of RLKs consists of an extracellular receptor domain, a membrane-spanning domain and an intracellular kinase domain. This last domain is missing in RLPs [START_REF] Morillo | Functional analysis of receptor-like kinases in monocots and dicots[END_REF][START_REF] Toer | Receptor-mediated signalling in plants: molecular patterns and programmes[END_REF][START_REF] Ben Khaled | A moving view: subcellular trafficking processes in pattern recognition receptor-triggered plant immunity[END_REF][START_REF] Boutrot | Function, Discovery, and Exploitation of Plant Pattern Recognition Receptors for Broad-Spectrum Disease Resistance[END_REF]. In the Arabidopsis genome, more than 600 RLKs have been identified and were associated with numerous different signaling pathways and responses [START_REF] Shiu | Receptor-like kinases from Arabidopsis form a monophyletic gene family related to animal receptor kinases[END_REF][START_REF] Shiu | Comparative analysis of the receptor-like kinase family in Arabidopsis and rice[END_REF]. This large family can be separated into two classes [START_REF] Shiu | Expansion of the receptor-like kinase/Pelle gene family and receptor-like proteins in Arabidopsis[END_REF][START_REF] Tör | Receptor-mediated signalling in plants: molecular patterns and programmes[END_REF]. The first one includes RD kinases containing a conserved arginine (R) residue in front of an aspartate (D), which provides the catalytic activity and is important for the function as a kinase [START_REF] Schwessinger | Phosphorylation-dependent differential regulation of plant growth, cell death, and innate immunity by the regulatory receptor-like kinase BAK1[END_REF]. The second class are non-RD kinases, these kinases lack these two specific amino acids. To achieve the initiation and amplification of phosphorylation signals non-RD kinases may require a coreceptor [START_REF] Dardick | Non-arginine-aspartate (non-RD) kinases are associated with innate immune receptors that recognize conserved microbial signatures[END_REF]. A key characteristic for RLKs is their extracellular domain which determines their classification into further classes like leucin-rich repeat (LRR), epidermal growth factor (EGF)-like, lysine-motif (LysM) or leucin motif. Whereas, RLK-LRR are the receptors for peptide ligands such as MAMPs and DAMPs [START_REF] Chinchilla | The Arabidopsis receptor kinase FLS2 binds flg22 and determines the specificity of flagellin perception[END_REF][START_REF] Yamaguchi | The cell surface leucine-rich repeat receptor for AtPep1, an endogenous peptide elicitor in Arabidopsis, is functional in transgenic tobacco cells[END_REF][START_REF] Zipfel | Perception of the bacterial PAMP EF-Tu by the receptor EFR restricts Agrobacteriummediated transformation[END_REF]. The second class of PRRs are the RLPs. They bind their ligands at the ectodomain but lack the intracellular kinase domain. Therefore, they require to bind the assembly with a co-receptor upon ligand binding in order to transduce the signal [START_REF] Zipfel | Plant pattern-recognition receptors[END_REF]. Various defense-associated, physiological and developmental functions rely on PRRs [START_REF] Shiu | Receptor-like kinases from Arabidopsis form a monophyletic gene family related to animal receptor kinases[END_REF][START_REF] Ten Hove | Probing the roles of LRR RLK genes in Arabidopsis thaliana roots using a custom T-DNA insertion set[END_REF][START_REF] Araya | CLE-CLAVATA1 peptide-receptor signaling module regulates the expansion of plant root systems in a nitrogendependent manner[END_REF].

The most well-studied PRR in plants is the receptor for the bacterial flagellin FLS2 [START_REF] Chinchilla | The Arabidopsis receptor kinase FLS2 binds flg22 and determines the specificity of flagellin perception[END_REF]. FLS2 is the analogue of the mammalian TLR5 and perception and defense responses caused by the perception of flg22 by these receptors have been well studied in plants and mammals [START_REF] Fliegmann | Immunity: Flagellin seen from all sides[END_REF][START_REF] Hajam | Bacterial flagellin-a potent immunomodulatory agent[END_REF]. The extensive search for FLS2 orthologues lead to the identification of FLS2 orthologues in tomato, rice, grapevine and tobacco [START_REF] Hann | Early events in the pathogenicity of Pseudomonas syringae on Nicotiana benthamiana[END_REF][START_REF] Robatzek | Molecular identification and characterization of the tomato flagellin receptor LeFLS2, an orthologue of Arabidopsis FLS2 exhibiting characteristically different perception specificities[END_REF][START_REF] Takai | Analysis of flagellin perception mediated by flg22 receptor OsFLS2 in rice[END_REF][START_REF] Trdá | The grapevine flagellin receptor VvFLS2 differentially recognizes flagellin-derived epitopes from the endophytic growth-promoting bacterium Burkholderia phytofirmans and plant pathogenic bacteria[END_REF].

PRRs with a non-RD kinase domain have been shown to interact with a RD receptor kinase.

The RD receptor kinase functions as a co-receptor upon ligand binding. One of the best characterized co-receptors is the LRR-RLK BRI1-ASSOCIATED KINASE 1 (BAK1). BAK1 is a member of the SERK family and interacts with BRI1 to regulate brassinosteroid signaling.

AtBAK1 form ligand-dependent heteromeric complexes with several defense inducing signals [START_REF] Chinchilla | A flagellin-induced complex of the receptor FLS2 and BAK1 initiates plant defence[END_REF][START_REF] Roux | The Arabidopsis leucine-rich repeat receptor-like kinases BAK1/SERK3 and BKK1/SERK4 are required for innate immunity to hemibiotrophic and biotrophic pathogens[END_REF][START_REF] Schwessinger | Plant Innate Immunity: Perception of Conserved Microbial Signatures[END_REF][START_REF] Jordá | ERECTA and BAK1 Receptor Like Kinases Interact to Regulate Immune Responses in Arabidopsis[END_REF].

The BAK1 mutants are drastically impaired in the perception of defense inducing signals.

A recent study in the field of PRR receptors could be a breakthrough in the research on receptor interactions. In the last years the number of identified and characterized receptor families increased continually. It is assumed that these receptors interact with each other. By investing of 40.000 potential extracellular domains of receptors and their interactions it was found that BAK1 interacts with a high number of receptors [START_REF] Smakowska-Luzan | An extracellular network of Arabidopsis leucine-rich repeat receptor kinases[END_REF]. This research could help to uncover further receptor co-receptor complexes and their interactions.

It is time the defend -Effector-triggered immunity (ETI), the second layer of defense

The perception of a of a MAMP or DAMP and the induction of a PTI response is in general a very efficient defense system. However, certain pathogens or pathogen strains found a way to inhibit PTI responses. By introducing virulence effectors into the host pathogens developed a possibility to inhibit specific steps of pathogen detection or subsequent downstream signaling processes. This process is referred to as effector-triggered susceptibility (ETS) [START_REF] Jones | The plant immune system[END_REF][START_REF] Boller | Innate Immunity in Plants: An Arms Race Between Pattern Recognition Receptors in Plants and Effectors in Microbial Pathogens[END_REF][START_REF] Dodds | Plant immunity: towards an integrated view of plantpathogen interactions[END_REF]. This mechanism is a crucial element of the host-pathogen coevolution cycle. Also, the host side developed a mechanism to defend against pathogen effectors by introducing a robust resistance response called effector-triggered immunity (ETI) [START_REF] Jones | The plant immune system[END_REF][START_REF] Jacob | Evolution and Conservation of Plant NLR Functions[END_REF][START_REF] Cui | Effector-triggered immunity: from pathogen perception to robust defense[END_REF].

ETI is famously also known as gene-for-gene resistance [START_REF] Flor | Current Status of the Gene-For-Gene Concept[END_REF]. This second layer of defense is based on a surveillance system detecting either directly secreted effectors or indirectly detecting a modified internal signal originating from the effector attack [START_REF] Boller | Innate Immunity in Plants: An Arms Race Between Pattern Recognition Receptors in Plants and Effectors in Microbial Pathogens[END_REF]. This detection system is based on resistance (R) genes. These genes code mostly for intracellular NB-LRR proteins (Nucleotide Binding Proteins with Leucin-Rich Repeat domains) [START_REF] Jones | The plant immune system[END_REF]. The induced defense response by the perception of effectors by NB-LRR is regarded to be more rapidly induced, longer lasting and more severe than PTI. The ETI response culminates in the hypersensitive response (HR). HR is the apoptosis of the infected and the surrounding cells [START_REF] Greenberg | The role and regulation of programmed cell death in plant-pathogen interactions[END_REF][START_REF] Jones | The plant immune system[END_REF][START_REF] Truman | Type III effectors orchestrate a complex interplay between transcriptional networks to modify basal defence responses during pathogenesis and resistance[END_REF][START_REF] Tsuda | Comparing signaling mechanisms engaged in patterntriggered and effector-triggered immunity[END_REF]. The induction of HR response is under tight regulation. The HR signaling is strongly inhibited by PTI signaling, indicating a strict separation of the two layer of plant defense [START_REF] Hatsugai | A plant effector-triggered immunity signaling sector is inhibited by pattern-triggered immunity[END_REF].

Regarding the co-evolution of plants and their host, ETI is seen as the more dynamic process in comparison to PTI. ETI is often highly specific between a particular plant cultivar and a pathogen race [START_REF] Dangl | Plant pathogens and integrated defence responses to infection[END_REF]. Plants constantly adopt their R genes, while the pathogens change their effectors. This race of effector evolution and adaptation is known as the "arms race" and is nicely outlined in the zigzag model proposed by [START_REF] Jones | The plant immune system[END_REF] (Figure 2).

Defense responses induced during PTI

Plants developed a physical barrier against pathogens that make them generally resistant against the majority of invaders. This barrier is termed non-host resistance and consist of the plant cuticle, the cell wall and constitutively produced antimicrobial compounds. All land plants protect their external surface of the aerial epidermis with a waxy cuticle [START_REF] Osbourn | Preformed Antimicrobial Compounds and Plant Defense against Fungal Attack[END_REF][START_REF] Somerville | Toward a systems approach to understanding plant cell walls[END_REF][START_REF] Yeats | The formation and function of plant cuticles[END_REF]. The detection of an elicitor induces a complex set of responses intended for resisting against a pathogen attack [START_REF] Bigeard | Signaling mechanisms in pattern-triggered immunity (PTI)[END_REF]. Moreover, the perception of MAMPs and DAMPs triggers similar PTI responses [START_REF] Yamaguchi | Endogenous peptide elicitors in higher plants[END_REF]. However, the kinetics and kind of induced defense response depends on the perceived elicitor. For example, the induction of protease inhibitors is specifically induced after perception of HAMPs (Herbivore associated molecular patterns) and some DAMPs but not MAMPs [START_REF] Zebelo | Role of early signalling events in plant-insect interactions[END_REF][START_REF] Zhu-Salzman | Insect response to plant defensive protease inhibitors[END_REF]. The most common PTI responses will be discussed in the following paragraph.

Electrical signaling

The fastest defense response appears to take place at the only cell compartment that is in direct contact to the environment. At the plasma membrane a modulation of ion fluxes is among the earliest cellular responses to biotic and abiotic stresses [START_REF] Ebel | Early events in the elicitation of plant defence[END_REF][START_REF] Shabala | Non-invasive microelectrode ion flux measurements to study adaptive responses of microorganisms to the environment[END_REF][START_REF] Fürstenberg-Hägg | Plant Defense against Insect Herbivores[END_REF]. In form of an altered membrane potential a signal can travel though the plant. The generated action potential-like signal serve as a systemic defense signal [START_REF] Maffei | Electrophysiology and Plant Responses to Biotic Stress[END_REF]. This signal on the cell surface can propagate with a speed of up to 40 m sec -1 [START_REF] Volkov | Electrochemistry of Plant Life[END_REF]. A strong membrane depolarization was observed induced by the MAMPs elf18 and flg22. The depolarization was induced within 1-5 minutes after elicitor treatment and lasted for around 1 to 1.5 hours [START_REF] Jeworutzki | Early signaling through the Arabidopsis pattern recognition receptors FLS2 and EFR involves Ca-associated opening of plasma membrane anion channels[END_REF]

Ion fluxes

Upon elicitor perception plasma membrane channels are opened within 1-2 min. This causes a strong increase of intracellular Ca 2+ and H + [START_REF] Lecourieux | Analysis and effects of cytosolic free calcium increases in response to elicitors in Nicotiana plumbaginifolia cells[END_REF]. On the other hand the elicitor perception also causes a anion efflux [START_REF] Boller | Chemoperception of Microbial Signals in Plant Cells[END_REF]. The strongly increased Ca 2+ concentration is of particular interest since Ca 2+ is known to function as second messenger in various cellular processes [START_REF] Lecourieux | Calcium in plant defence-signalling pathways[END_REF]. Moreover, innate immunity is regulated by calcium-dependent protein kinases (CDPKs) and act as sensor for Ca 2+ (Boudsocq et al., 2010). Also, other defense responses are regulated by CDPKs. The production of reactive oxygen species (ROS) is regulated by phosphorylation of the ROS producing enzyme NADPH oxidase [START_REF] Kobayashi | Calcium-dependent protein kinases regulate the production of reactive oxygen species by potato NADPH oxidase[END_REF].

Oxidative burst

One of the fastest defense response is the burst of reactive oxygen species (Torres et al., 2006). The plasma membrane-localized NADPH oxidase (RbohD) is mainly responsible for the ROS burst. This enzyme produces membrane-impermeable superoxide (O•-2), which is converted into hydrogen peroxide (H2O2) in the apoplast [START_REF] Liu | Regulation of plant reactive oxygen species (ROS) in stress responses: learning from AtRBOHD[END_REF]. It was shown that the aquaporin AtPIP1;4 is crucial for the transport of H2O2 across the plasma membrane into the cytoplasm and therefore important for defense response induction [START_REF] Tian | Plant Aquaporin AtPIP1;4 Links Apoplastic H2O2 Induction to Disease Immunity Pathways[END_REF]. The plant is capable to produce tremendous amounts of ROS, which directly inhibit the pathogen or herbivore growth [START_REF] Apel | Reactive oxygen species: Metabolism, oxidative stress, and signal transduction[END_REF]. The plasma membrane-associated cytoplasmic kinase BIK1 interacts and phosphorylates RbohD upon elicitor perception [START_REF] Li | The FLS2-associated kinase BIK1 directly phosphorylates the NADPH oxidase RbohD to control plant immunity[END_REF]. Furthermore, by triggering the synthesis of SA and MAPK activation, ROS serves as a second messenger [START_REF] O'brien | Reactive oxygen species and their role in plant defence and cell wall metabolism[END_REF].

Changes in protein phosphorylation and MAPK activation

External stimuli are transduced into intracellular responses by the activation of MITOGEN-ACTIVATED PROTEIN KINASES (MAPK). This cascade starts with the phosphorylation of a MAP Kinase Kinase Kinase (MAPKKK), which phosphorylates a MAPKK, which in turn phosphorylates MAPK. These phosphorylated MAPK have various protein targets in the nucleus and the cytoplasm [START_REF] Meng | MAPK cascades in plant disease resistance signaling[END_REF]. Regarding defense responses, MAPK cascades are activated after MAMPs and DAMPs perception. The Phosphorylation and activation of MPK3 and MPK6 is used as an assay for defense signaling [START_REF] Asai | MAPK Signaling Regulates Nitric Oxide and NADPH Oxidase-Dependent Oxidative Bursts in Nicotiana benthamiana[END_REF][START_REF] Rodriguez | Mitogen-Activated Protein Kinase Signaling in Plants[END_REF][START_REF] Galletti | Arabidopsis MPK3 and MPK6 Play Different Roles in Basal and Oligogalacturonide-or Flagellin-Induced Resistance against Botrytis cinerea[END_REF].

Callose deposition

One of the later defense responses in the deposition of callose. In Arabidopsis, callose deposition can be detected from 16h onwards after MAMP treatment by fixing and staining the tissue with aniline blue [START_REF] Gomez-Gomez | A single locus determines sensitivity to bacterial flagellin in Arabidopsis thaliana[END_REF].In the leaf tissue callose is located in the papillae, a tissue that stretches from the plasma membrane to the cell wall. However, the exact role of callose deposition in plant defense remains unclear. Callose deposition goes hand in hand with that of ROS, phenolic compounds and several cell wall proteins. These events have confirmed antimicrobial or cell wall reinforcing functions [START_REF] Voigt | Callose-mediated resistance to pathogenic intruders in plant defenserelated papillae[END_REF].

Transcriptional changes

Upon pathogen invasion as well as perception of MAMPs and DAMPs plants launch a profound and dynamic reprogramming of gene expression. Nearly all previously described defense responses are regulated by a set of different genes [START_REF] Tsuda | Transcriptional networks in plant immunity[END_REF]Li et al., 2016). Studies of Arabidopsis revealed that already after 30min after flg22 treatment, expression of roughly 1000 genes were up regulated and only 200 genes down regulated [START_REF] Zipfel | Bacterial disease resistance in Arabidopsis through flagellin perception[END_REF]. Among the induced genes are also the PRR FLS2 and EFR. These results reveal a positive feedback loop of PTI activation [START_REF] Shiu | Comparative analysis of the receptor-like kinase family in Arabidopsis and rice[END_REF][START_REF] Zipfel | Bacterial disease resistance in Arabidopsis through flagellin perception[END_REF].

Comparative expression analysis of gene expression after infection with virulent

pseudomonas bacteria and a nonpathogenic strain showed that transcriptional response to the conserved bacterial patterns starts before the bacteria itself starts to multiply. The earliest induced genes are related to defense responses and salicylic acid (SA) biosynthesis.

Genes involved in photosynthesis are down regulated. This indicates that sources are shifted towards the limitation of pathogen growth [START_REF] Lewis | Transcriptional Dynamics Driving MAMP-Triggered Immunity and Pathogen Effector-Mediated Immunosuppression in Arabidopsis Leaves Following Infection with Pseudomonas syringae pv tomato DC3000[END_REF].

Inhibition of seedling growth

The addition of MAMPs and DAMPs to the seedling growth medium leads to a strong inhibition of growth in a concentration dependent manner. This effect depends on the interaction of the elicitor and its receptor (Krol et al., 2010). The exact molecular mechanism behind the arrested seedling growth remains unclear. One possible explanation would be the shift of resources from growth towards defense mechanism (Walters and Heil, 2007;Boller and Felix, 2009).

Hormonal integration of immune responses

Several classical plant hormones are shown to be involved in defense responses. Among them jasmonic acid (JA), and salicylic acid (SA) are the major defense-related phytohormones.

Moreover, other hormones are involved namely, ethylene, abscisic acid, auxin, gibberellins, cytokinins and brassinosteroids [START_REF] Shigenaga | No hormone to rule them all: Interactions of plant hormones during the responses of plants to pathogens[END_REF]. In general, JA and SA are positive regulator of plant defense. JA regulates immunity against necrotrophic pathogens and SA immunity against biotrophic pathogens [START_REF] Pieterse | Networking by small-molecule hormones in plant immunity[END_REF][START_REF] Berens | Evolution of Hormone Signaling Networks in Plant Defense[END_REF].

The fatty acid derivate JA has been reported to be an important downstream signaling upon necrotrophic pathogen attacks. JA levels increase locally after tissue damage [START_REF] Wasternack | Jasmonates: an update on biosynthesis, signal transduction and action in plant stress response, growth and development[END_REF][START_REF] Bari | Role of plant hormones in plant defence responses[END_REF]. The active form of JA, JA-Ile is perceived by coronatine-insensitive 1 (COI1) This perception regulates a group of MYC transcription factors in initialize transcriptional reprogramming [START_REF] Wasternack | Jasmonates: biosynthesis, perception, signal transduction and action in plant stress response, growth and development. An update to the 2007 review in[END_REF]. Differently regulated genes are defensins, proteins with antimicrobial and enzyme inhibitory functions and marker genes for JA-dependent defense signaling [START_REF] Manners | The promoter of the plant defensin gene PDF1.2 from Arabidopsis is systemically activated by fungal pathogens and responds to methyl jasmonate but not to salicylic acid[END_REF]. JA application triggers immunity against necrotrophic pathogens in Arabidopsis but also in rice and Medicago truncatula as well as a systemic priming of defense responses which is referred as induced systemic resistance (ISR) [START_REF] Bostock | Signal crosstalk and induced resistance: Straddling the line between cost and benefit[END_REF][START_REF] Schilmiller | Systemic signaling in the wound response[END_REF][START_REF] Taheri | Riboflavin induces resistance in rice against Rhizoctonia solani via jasmonate-mediated priming of phenylpropanoid pathway[END_REF][START_REF] Berens | Evolution of Hormone Signaling Networks in Plant Defense[END_REF].

A pathogen infection causes increased accumulation of SA in various eudicots while exogenous application of SA or its analogues triggers immune responses and resistance against pathogens [START_REF] Berens | Evolution of Hormone Signaling Networks in Plant Defense[END_REF]. SA is perceived by a receptor complex containing several NPR (non expressor of PR) proteins [START_REF] Yan | Perception of the plant immune signal salicylic acid[END_REF]. This signaling acts as activator of a large set of defense-related genes. These genes are referred to as pathogenesisrelated (PR) genes [START_REF] Dong | NPR1, all things considered[END_REF]. PR genes are diverse however several of these genes are shown the code for proteins with direct anti-microbial activity (van Loon et al., 2006).

Moreover, the activation of PR genes at the site of infection often leads to similar responses in distal tissue. This tissue activates defense responses in order to be in a "ready state" to an imminent pathogen threat. This effect is called systemic acquired resistance (SAR) [START_REF] Conrath | Systemic acquired resistance[END_REF][START_REF] Vlot | Systemic acquired resistance: the elusive signal(s)[END_REF]. However, SA seems not to serve as a mobile signal per se inducing immunity in uninfected tissue while several other molecules have been proposed to fulfill such a role [START_REF] Shah | Long-distance communication and signal amplification in systemic acquired resistance[END_REF]). An overview of the chronological induction of PTI responses is given in the Figure 3. Finally, metabolic changes can lead to profound defense responses and long-lasting adaptations. (adopted from [START_REF] Maffei | Before gene expression: early events in plant-insect interaction[END_REF])

Systemic acquired resistance (SAR)

Already in the year 1933 the concept of a acquired immunity in order to increase resistance upon a reinfection in plants was postulated [START_REF] Chester | The Problem of Acquired Physiological Immunity in Plants[END_REF]. The term SAR was first proposed 30 years later upon work on tobacco. When the three lower leaves of a tobacco plant were infected with Tobacco mosaic virus (TMV) the upper leaves developed much weaker infection symptoms after a second infection 7 day subsequent to the first infection [START_REF] Ross | SYSTEMIC ACQUIRED RESISTANCE INDUCED BY LOCALIZED VIRUS INFECTIONS IN PLANTS[END_REF][START_REF] Klessig | Systemic Acquired Resistance and Salicylic Acid: Past, Present, and Future[END_REF]. SAR is a complex mechanism of induced defense that can lead to a long-lasting resistance against a broad spectrum of unrelated pathogens [START_REF] Durrant | Systemic acquired resistance[END_REF]. Upon a local infection a transport of defense signals throughout the plant is initiated. These signals are generated and transported through the phloem via the apoplast to the uninfected distal tissue [START_REF] Tuzun | Movement of a factor in tobacco infected with Peronospora tabacina Adam which systemically protects against blue mold[END_REF][START_REF] Gao | Signal regulators of systemic acquired resistance[END_REF][START_REF] Singh | Transport of chemical signals in systemic acquired resistance[END_REF]. The nature of the systemic signal has not yet been uncovered however, in the last years at least 13 different possible signals have been proposed [START_REF] Gao | Signal regulators of systemic acquired resistance[END_REF]. One of the first proposed signals was SA. Indeed, SA, methyl SA (MeSA) as well as the accumulation of PR transcripts is required for SAR [START_REF] Mishina | Pathogen-associated molecular pattern recognition rather than development of tissue necrosis contributes to bacterial induction of systemic acquired resistance in Arabidopsis[END_REF]. Arabidopsis mutants unable to accumulate SA do not acquire systemic resistance upon infection with necrotizing pathogens while Arabidopsis plants overproducing SA show enhanced defense to pathogens [START_REF] Conrath | Systemic acquired resistance[END_REF]. PR gene accumulation is often seen as the molecular basis of SAR. Some of the typical SAR-induced PR genes code for enzymes possible able to hydrolyze microbial cell wall components [START_REF] Van Loon | The families of pathogenesis-related proteins, their activities, and comparative analysis of PR-1 type proteins[END_REF]. However, the accumulation of PR proteins does not per se explain the SAR phenomenon [START_REF] Conrath | Systemic acquired resistance[END_REF]. A key component for SAR is NPR1 (NON-EXPRESSOR OF PR1). NPR1 is an important downstream signaling element of SA. Overexpression of NPR1 leads to stronger PR gene expression after pathogen infection and a strongly enhanced disease resistance [START_REF] Cao | Generation of broad-spectrum disease resistance by overexpression of an essential regulatory gene in systemic acquired resistance[END_REF][START_REF] Friedrich | NIM1 overexpression in Arabidopsis potentiates plant disease resistance and results in enhanced effectiveness of fungicides[END_REF]. SA can directly bind to NPR1 which leads to its monomerization [START_REF] Wu | The Arabidopsis NPR1 protein is a receptor for the plant defense hormone salicylic acid[END_REF]. NPR1 monomers are transported to the nucleus and directly interact with basic leucin zipper transcription factors (bZIP) in order to activate defense gene expression [START_REF] Gao | Signal regulators of systemic acquired resistance[END_REF][START_REF] Birkenbihl | Transcriptional events defining plant immune responses[END_REF]. Another hallmark of SAR is the priming of defense responses whereas SAR-dependent priming is associated with faster and stronger defense responses to a secondary infection [START_REF] Martinez-Medina | Recognizing Plant Defense Priming[END_REF]

Synthetic SAR activators

Since it was discovered that SA is an endogenous signal for SAR activation synthetic chemicals able to mimic SA and SAR induction have been identified. These synthetic elicitors are often termed plant activators and they trigger defense reactions and an induction of defense responses by mimicking interactions of natural elicitors or defense signaling molecules [START_REF] Bektas | Synthetic plant defense elicitors[END_REF]. One of the first identified synthetic elicitors is polyacrylic acid and was shown to induce resistance of tobacco against TMV and to activate PR1 gene expression [START_REF] Gianinazzi | Virus Resistance Induced in Plants by Polyacrylic Acid[END_REF]. The first indication that exogenous application of SA contributes to the establishment of SAR is given by treatments with acetylsalicylic acid (Aspirin). Aspirin treatment causes induced resistance against TMV of tobacco by PR protein accumulation [START_REF] White | ACETYLSALICYLIC-ACID (ASPIRIN) INDUCES RESISTANCE TO TOBACCO MOSAIC-VIRUS IN TOBACCO[END_REF]. This discovery paved the way to the discovery of more potent plant defense activators that are more suitable in crop protection. Two have the first discovered ones that are still widely used compounds are 2,6-dichloro-isonicotinic acid (INA), Beta-aminobutyric acid (BABA) and (1,2,3) thiadiazole-7-carbothioic acid S-methyl ester (BTH) (Metraux et al., 1990;[START_REF] Ward | Coordinate Gene Activity in Response to Agents That Induce Systemic Acquired Resistance[END_REF][START_REF] Görlach | Benzothiadiazole, a novel class of inducers of systemic acquired resistance, activates gene expression and disease resistance in wheat[END_REF][START_REF] Baccelli | Beta-aminobutyric acid priming of plant defense: the role of ABA and other hormones[END_REF].

INA was discovered by Ciba-Geigy and was shown to promote activation of defense in cucumber against fungal pathogens. Moreover, the resistance was also achieved in distal tissue indicating an activation of SAR [START_REF] Metraux | Induced Systemic Resistance in Cucumber in Response to 2,6-Dichloro-Isonicotinic Acid and Pathogens[END_REF]. INA provides also resistance against pathogens in pear, pepper and rice as well as in laboratory experiments tobacco and Arabidopsis [START_REF] Metraux | Induced Systemic Resistance in Cucumber in Response to 2,6-Dichloro-Isonicotinic Acid and Pathogens[END_REF][START_REF] Ward | Coordinate Gene Activity in Response to Agents That Induce Systemic Acquired Resistance[END_REF]. However, INA does not trigger any changes in plant SA levels [START_REF] Delaney | A CENTRAL ROLE OF SALICYLIC-ACID IN PLANT-DISEASE RESISTANCE[END_REF] but has been reported to mimic biochemical and physiological effects of SA such as inhibition of catalase and the induction of cellular H2O2 accumulation [START_REF] Conrath | Two inducers of plant defense responses, 2,6-dichloroisonicotinec acid and salicylic acid, inhibit catalase activity in tobacco[END_REF][START_REF] Bektas | Synthetic plant defense elicitors[END_REF]. INA-induced PR gene expression is blocked in the npr1 mutant this strengthen the role of INA in SAR [START_REF] Wang | A genomic approach to identify regulatory nodes in the transcriptional network of systemic acquired resistance in plants[END_REF]. However, INA has phytotoxic side effects in crops and is therefore it has never been commercialized as agrochemical but is still used as tool to study SAR [START_REF] Oostendorp | Induced Disease Resistance in Plants by Chemicals[END_REF].

BABA induces resistance against a very high number of stresses such as attacks by viruses, bacteria, fungi, oomycetes, nematodes and arthropods and also abiotic stresses like heat, cold and salt stresses [START_REF] Balmer | The 'prime-ome': towards a holistic approach to priming[END_REF]. After spraying of Arabidopsis plants with BABA, BABA get transported inside the plant whereas the young plant tissue act as sink [START_REF] Jakab | β-Aminobutyric Acid-induced Resistance in Plants[END_REF]. Also, BABA was shown to boost SA pathway by enhanced PR1 gene expression [START_REF] Zimmerli | Potentiation of pathogenspecific defense mechanisms in Arabidopsis by beta -aminobutyric acid[END_REF]. The enhanced defense response of BABA treated plants is explained by enhanced callose deposition and ROS burst [START_REF] Ton | Beta-amino-butyric acid-induced resistance against necrotrophic pathogens is based on ABA-dependent priming for callose[END_REF][START_REF] Flors | Interplay between JA, SA and ABA signalling during basal and induced resistance against Pseudomonas syringae and Alternaria brassicicola[END_REF][START_REF] Pastor | Fine tuning of reactive oxygen species homeostasis regulates primed immune responses in Arabidopsis[END_REF]. BABA induced drought and salt stress is regulated by interference with abscisic acid (ABA) [START_REF] Zimmerli | The xenobiotic β-aminobutyric acid enhances Arabidopsis thermotolerance[END_REF]. Moreover, drought stressed wheat plants have been shown to have a reduced water use and therefore increased desiccation tolerance upon BABA treatment [START_REF] Du | β-Aminobutyric acid increases abscisic acid accumulation and desiccation tolerance and decreases water use but fails to improve grain yield in two spring wheat cultivars under soil drying[END_REF]. Another indication that BABA is a multifaceted synthetic elicitor is given by the observations that BABA boosts hormonal signaling pathways and the plant responses to the hormones [START_REF] Flors | Interplay between JA, SA and ABA signalling during basal and induced resistance against Pseudomonas syringae and Alternaria brassicicola[END_REF] BTH (benzothiadiazole) also known as acibenzolar-S-methyl ester (ASM) is a potent inducer of plant immune responses [START_REF] Oostendorp | Induced Disease Resistance in Plants by Chemicals[END_REF]. Until now BTH has been tested in more than 120 pathosystems [START_REF] Faize | Functional Analogues of Salicylic Acid and Their Use in Crop Protection[END_REF]. In Europe BTH is commercialized as "Bion".

Application of BTH has been shown to control downey mildew infections in vegetables and to control a range of fungal, bacterial and viral diseases of important crops like tomato, cucumber, broccoli, tobacco, melon, pear and apple trees [START_REF] Scarponi | Persistence and translocation of a benzothiadiazole derivative in tomato plants in relation to systemic acquired resistance against Pseudomonas syringae pv tomato[END_REF][START_REF] Zavareh | Effects of Acibenzolar-S-methyl on the specific activities of peroxidase, chitinase and phenylalanine ammonia-lyase and phenolic content of host leaves in cucumber-powdery mildew interaction[END_REF][START_REF] Pajot | Evidence that DL-3-aminobutyric acid and acibenzolar-S-methyl induce resistance against bacterial head rot disease of broccoli[END_REF][START_REF] Jiang | Ultrastructural Study on Acibenzolar-S-Methyl-Induced Scab Resistance in Epidermal Pectin Layers of Japanese Pear Leaves[END_REF]. In apple it was shown that application controls fire blight, which is caused by the bacterium Erwinia amylovora [START_REF] Brisset | Acibenzolar-S-methyl Induces the Accumulation of Defense-related Enzymes in Apple and Protects from Fire Blight[END_REF][START_REF] Maxson-Stein | Effect of Treating Apple Trees with Acibenzolar-S-Methyl on Fire Blight and Expression of Pathogenesis-Related Protein Genes[END_REF]. BTH itself does not show any direct effect on plant pathogens and is therefore not antimicrobial [START_REF] Friedrich | A benzothiadiazole derivative induces systemic acquired resistance in tobacco[END_REF]. BTH is a functional analogue of SA since it induces the same SA-characteristic expression profile [START_REF] Friedrich | A benzothiadiazole derivative induces systemic acquired resistance in tobacco[END_REF]. It was suggested that BTH is metabolized by the plant into acibenzolar by the enzyme SABP2. SABP2 silenced tobacco plants do not induce SAR typical gene expression while acibenzolar fully induced SAR in the same plants [START_REF] Tripathi | SABP2, a methyl salicylate esterase is required for the systemic acquired resistance induced by acibenzolar-S-methyl in plants[END_REF]. One potent mechanism of BTH in order to prime plant defense is the potentiated activation of the defense associated MAPKs, MPK3 and MPK6. BTH induces the accumulation of the non-phosphorylated forms as well as an increased mRNA level of these two MAPK [START_REF] Beckers | Mitogen-activated protein kinases 3 and 6 are required for full priming of stress responses in Arabidopsis thaliana[END_REF]. Moreover, BTH has the capacity to induce the expression of the important PRRs; FLS2, BAK1 and CERK1 [START_REF] Tateda | Salicylic Acid Regulates <em>Arabidopsis</em> Microbial Pattern Receptor Kinase Levels and Signaling[END_REF]. This indicates that BTH primes defense responses at the receptor level and by mimicking SA signaling, also large sets of defense gene transcripts. This transcriptional response includes WRKY transcription factors. The genes AtWRKY29, AtWRKY6, AtWRKY53 are strongly transcribed after stress application following pre-treatment with BTH. Moreover, BTH pre-treatment triggers several histone modifications that are found at actively transcript genes. AtWRKY29 shows an elevated level of H3K4me3, H3K4me2, H3ac and H4ac and AtWRKY6 as well as AtWRKY53 enhanced levels of H3K4me3 and H3K4me2. These findings are strengthened by the observation that BTH-induced trimethylation of H3K4 is reduced in the npr1 mutant [START_REF] Jaskiewicz | Chromatin modification acts as a memory for systemic acquired resistance in the plant stress response[END_REF]. Since BTH was commercialized in the year 1989 it was demonstrated several times that BTH application has positive effects on plant health [START_REF] Faize | Functional Analogues of Salicylic Acid and Their Use in Crop Protection[END_REF]. However, it was shown that about 75% of applied BTH was photolyzed within 4 hours on apple leaf tissue and only a small fraction undergo metabolism by the plant [START_REF] Sleiman | Heterogeneous Photochemistry of Agrochemicals at the Leaf Surface: A Case Study of Plant Activator Acibenzolar-Smethyl[END_REF]. This study reveals that even after the long time BTH is available on the market still numerous plant responses to this molecule remain to be discovered.

DAMPs as plant vaccines

In the last years the idea was raised that synthetic peptides could be replaced by DAMPs since they are capable to trigger immunity in a similar fashion compared synthetic peptides [START_REF] Quintana-Rodriguez | Damage-associated molecular patterns (DAMPs) as future plant vaccines that protect crops from pests[END_REF]. Interestingly, it was assumed that plant and algal extracts, which contains many DAMPs, enhance the resistance against herbivore attacks in cabbage, tomato and maize. Moreover, algae extract lead to enhanced fungal and bacterial resistance in banana, apple, grapevine, melon, tomato, cucumber and carrot [START_REF] Quintana-Rodriguez | Damage-associated molecular patterns (DAMPs) as future plant vaccines that protect crops from pests[END_REF]. For example, leaf extract of devil`s trumpet elicits resistance against downy mildew in pearl millet [START_REF] Devaiah | Induction of systemic resistance in pearl millet (Pennisetum glaucum) against downy mildew (Sclerospora graminicola) by Datura metel extract[END_REF]. Seaweed extract has the capacity to enhance the resistance in cucumber plants to different fungal pathogens [START_REF] Jayaraman | Commercial extract from the brown seaweed Ascophyllum nodosum reduces fungal diseases in greenhouse cucumber[END_REF]. Resistance against herbivores and pathogens after an exposure with the plants own volatile organic compounds has been described in more than 30 plant species [START_REF] Heil | Explaining evolution of plant communication by airborne signals[END_REF]. Several volatile organic compounds can also have direct antimicrobial or insect-repellent effects (Quintana- [START_REF] Quintana-Rodriguez | Damage-associated molecular patterns (DAMPs) as future plant vaccines that protect crops from pests[END_REF]. In summary, it seems that DAMPs could be a good tool to increase plant resistance in agriculture. Especially, extracellular fragments of DNA (eDNA) induces resistance in various plant species by increased formation of H2O2 and the activation of MAPKs [START_REF] Duran-Flores | Extracellular self-DNA as a damage-associated molecular pattern (DAMP) that triggers self-specific immunity induction in plants[END_REF]. However, the increased "ready state" of the plant defense, which is referred as defense priming, was up to now not observed caused by other DAMPs apart of volatile organic compounds or crude plant extracts including eDNA. Indicating that, DAMPs as plant vaccines might not be the optimal choice for plant treatments.

Defense priming -The third layer of defense?

The ultimate achievement in plant immunity and maybe partly the compensation of a missing adaptative immune system, plants developed the capacity to memorize a previous stress by promoting a primed state of enhanced defense. Defense priming is established in the plants tissue exposed to a priming stimulus as well as systemically to unharmed, or untreated parts of the plant [START_REF] Reimer-Michalski | Innate immune memory in plants[END_REF]. When plants are in a primed state, plants are capable of responding to very low stimulation with a faster and stronger defense response than unprimed plants. This effect has also been defined as systemic immunity or systemic stress tolerance. The effect of defense priming is also referred as plant sensitization and trained immunity [START_REF] Kuć | Translocated Signals for Plant Immunizationa[END_REF][START_REF] Ding | Multiple exposures to drought 'train' transcriptional responses in Arabidopsis[END_REF][START_REF] Van Der Meer | Trained immunity: A smart way to enhance innate immune defence[END_REF]. In general, a primed plant can show a modified set of responses in comparison to unprimed (naïve) plants. The possible mechanisms are summarized in Figure 4. Plants that have been primed by a priming stress, including treatments with chemical SAR inducer, DAMPs or any abiotic stress, are capable of responding faster to a subsequent triggering stress. Other possible effects of priming include a stronger and more sensitive response to a triggering stress. Whereas plants react to a lower threshold of the triggering stress. It is also possible that primed plants induce different gene network(s) in response to subsequent stress, which might be better adapted to the specific stress than it is the case for unprimed plants [START_REF] Lämke | Epigenetic and chromatin-based mechanisms in environmental stress adaptation and stress memory in plants[END_REF]. After the priming stimulus, the plant undergoes a period of stress memory [START_REF] Stief | Epigenetic responses to heat stress at different time scales and the involvement of small RNAs[END_REF].

Priming involves the gathering of information. The duration of the priming memory may be in the range of several days to weeks and in some cases transgenerational [START_REF] Mauch-Mani | Defense Priming: An Adaptive Part of Induced Resistance[END_REF]. Two mechanisms involved in defense priming have been proposed. The first is the accumulation of signaling or transcription factors and the second epigenetic changes that allow plants to memorize the "ready state" [START_REF] Bruce | Stressful "memories" of plants: Evidence and possible mechanisms[END_REF].

Transcriptional memory -one way to manifest stress priming

It has been proposed that one layer of defense priming memory can be found at the transcriptional level. [START_REF] Bruce | Stressful "memories" of plants: Evidence and possible mechanisms[END_REF]. The priming stimulus can cause a continuous change in gene expression, although the plant is not facing the stimulus anymore. Whereas, the transcriptional modification includes an activation or repression as well as a modified transcriptional response, like a hyperinduction upon a second stimulus. Also, other typical transcriptional responses might contribute to priming memory. An induction of transcriptional feedback loops leading to the autoactivation of a transcription factor could take place after the initiative stress stimulus. Moreover, posttranscriptional modifications which influence the protein stability and a modification could contribute to an enhanced memory [START_REF] Lämke | Epigenetic and chromatin-based mechanisms in environmental stress adaptation and stress memory in plants[END_REF].

Synthetic elicitors in defense priming

An enhanced capacity to react to a subsequent stress might be also established by an enhanced perception of a certain stress. Evidence for this post challenge prime state at the receptor level is given by treatments with the previously mentioned BTH. BTH treated plants show enhanced responsiveness to flagellin and chitin by increased expression levels of the receptors FLS2, CERK1 and the co receptor BAK1 [START_REF] Tateda | Salicylic Acid Regulates <em>Arabidopsis</em> Microbial Pattern Receptor Kinase Levels and Signaling[END_REF]. Also, BABA has similar effects on plant receptors. The plasma membrane-localized and with FLS2 associated protein lectin receptor kinase VI.2 is required for BABA-induced resistance and is important for priming of MAMP-triggered immunity [START_REF] Huang | The Arabidopsis LecRK-VI.2 associates with the pattern-recognition receptor FLS2 and primes Nicotiana benthamiana pattern-triggered immunity[END_REF] At the downstream level primed plants show the capacity of an increased ROS response upon a challenge. Priming treatments with SA, BTH and BABA lead to an enhanced ROS burst in response to a challenge with pathogens, MAMPs and DAMPs [START_REF] Pastor | Fine tuning of reactive oxygen species homeostasis regulates primed immune responses in Arabidopsis[END_REF][START_REF] Tateda | Salicylic Acid Regulates <em>Arabidopsis</em> Microbial Pattern Receptor Kinase Levels and Signaling[END_REF][START_REF] Mauch-Mani | Defense Priming: An Adaptive Part of Induced Resistance[END_REF]. An interesting example of the enhanced primed resistance is the result that primed plants can block pathogen-mediated reopening of stomata during Pseudomonas infection. This led to the finding that BABA pre-treated plants might show drought stress tolerance mediated by the stomata closure [START_REF] Camañes | A deletion in NRT2.1 attenuates Pseudomonas syringae-induced hormonal perturbation, resulting in primed plant defenses[END_REF][START_REF] Baccelli | Beta-aminobutyric acid priming of plant defense: the role of ABA and other hormones[END_REF]. Furthermore, the previous described MAPKs also show enhanced activation in primed plants in response to pathogen challenge or biotic stresses [START_REF] Beckers | Mitogen-activated protein kinases 3 and 6 are required for full priming of stress responses in Arabidopsis thaliana[END_REF][START_REF] Yi | NPR1 is Instrumental in Priming for the Enhanced flg22-induced MPK3 and MPK6 Activation[END_REF]. Many other defense signals and transcription factors are activated by the initial triggering of the MAPK cascade [START_REF] Conrath | Priming for enhanced defense[END_REF]. In summary, priming at a transcriptional level is very multifaceted and affects all parts of plants metabolism and enables the plant to remember stressful situations for a limited time. However, epigenetic modifications enable plants to acquire memory and can cause long-term changes in gene responsiveness.

Epigenetic memory -the better way to manifest stress priming?

Epigenetic phenomena are widely spread among organisms and include genetic imprinting, paramutation, transposon activation, gene silencing and changes in the chromatin structure.

Epigenetics was first defined in the year 1942 by Conrad Waddington as "The branch of biology which studies the causal interactions between genes and their products which bring the phenotype into being" (Waddington, 2012)(reprint). It is hypothesized that priming and the way a plant reacts to biotic and abiotic stresses relies strongly on epigenetic regulations [START_REF] Bruce | Stressful "memories" of plants: Evidence and possible mechanisms[END_REF][START_REF] Van Den Burg | Does chromatin remodeling mark systemic acquired resistance?[END_REF]. It is assumed that the initial priming stimulus alters the chromatin structure or methylation level in such a way that gene promotors are more accessible and therefore facilitated to activate [START_REF] Mauch-Mani | Defense Priming: An Adaptive Part of Induced Resistance[END_REF][START_REF] Ramirez-Prado | Plant Immunity: From Signaling to Epigenetic Control of Defense[END_REF].

DNA methylation is a widely studied epigenetic mechanism and is shown to be a dynamic regulatory mechanism of defense genes and stress priming. In plants, methylation of cytosine DNA base residues can have been classified in three sequence contexts, methylation can be separated in symmetric (CG, CHG) and asymmetric (CHH) DNA methylation patterns (where H is very base excepted of G). DNA methylation in all sequence contexts can be triggered by small interfering RNAs (siRNA) via a de novo methylation pathway termed RNA-directed DNA methylation (RdDM). The onset of RdDM begins with the production of RNAs by the Polymerase IV (Pol IV). The produced RNAs undergo several processing steps leading to the production of siRNAs that are loaded into ARGONAUTE 4 (AGO4). The formed complex has been proposed to base-pair with a nascent RNA scaffold which is produced by POL V. This complex is established the physical interaction of AGO4 with the large subunit of POL V, named NUCLEAR RNA POLYMERASE E1 (NRPE1). The methylation of DNA is finally established by the subsequent interaction with DOMAINS REARRANGED METHYLTRANSFERASE2 (DRM2).

The second possible mechanism leading to DNA methylation is the POL II-RDR6-dependent RdDM pathway. Here, single stranded RNA (ssRNA) are transcribed by POL II and converted into double stranded RNA (dsRNA) by RNA-DEPENDENT RNA POLYMERASE 6 (RDR6) and then processed into 21-22 nucleotides (nt) long siRNA. These siRNA are loaded into AGO6 that can also interact with the scaffold RNA transcribed by POL V, which therefore establishes DNA methylation. The established DNA methylation marks in CG and CHG context are maintained through mitosis and meiosis via a pathway which is catalyzed by METHYLTRANSFERASE1 (MET1) and CHROMOMETHYLASE3 (CMT3), respectively [START_REF] Espinas | Epigenetic Control of Defense Signaling and Priming in Plants[END_REF][START_REF] Ramirez-Prado | Plant Immunity: From Signaling to Epigenetic Control of Defense[END_REF][START_REF] Zhang | Dynamics and function of DNA methylation in plants[END_REF]. Factors involved in the establishment and maintenance of methylation have been found to control plant immunity and therefore provide evidence of epigenetic regulation of plant immunity. For instance, plants that are globally defective in maintaining CG and non-CG methylation in met1-3 show enhanced defense responses when exposed to Pseudomonas syringae pv. tomato DC3000 [START_REF] Dowen | Widespread dynamic DNA methylation in response to biotic stress[END_REF]. Moreover, it was shown that the global level of DNA methylation during defense response upon bacterial infection is reduced and that expression of defense-related genes is promoted by DNA hypomethylation during pathogen infection [START_REF] Yu | Dynamics and biological relevance of DNA demethylation in Arabidopsis antibacterial defense[END_REF]. Evidence that DNA methylation is part of defense priming was given by a study that showed a different capacity of hypermethylated and hypomethylated mutants to prime the activity of defense-related genes and callose deposition [START_REF] López Sánchez | The role of DNA (de)methylation in immune responsiveness of Arabidopsis[END_REF]. Moreover, another important factor for DNA methylation namely AGO4 is involved in immunity. The ago4 mutant is more susceptible to Pseudomonas infection [START_REF] References Agorio | ARGONAUTE4 is required for resistance to Pseudomonas syringae in Arabidopsis[END_REF].

The claim that "priming smells of epigenetics" is mainly based on findings involving histone modifications at defense-related genes [START_REF] Waterborg | Plant histone acetylation: in the beginning[END_REF]. Histone modifications influence chromatin compaction and therefore the accessibility of genes for transcription as well as replication and recombination [START_REF] Mauch-Mani | Defense Priming: An Adaptive Part of Induced Resistance[END_REF]. Modifications of histones are posttranslational and can, among others, consist of the addition of methyl or acetyl residues.

Well characterized examples of these modification are acetylation of histone H3 at the lysine (K) 9 (H3K9ac). This histone mark is associated with the positive gene transcription activity.

On the other hand, the histone mark H3K27me3, which is a trimethylation of histone H3 at the lysine 27, is associated with repressed gene transcription activity [START_REF] Pasini | Coordinated regulation of transcriptional repression by the RBP2 H3K4 demethylase and Polycomb-Repressive Complex 2[END_REF][START_REF] Zhou | Genome-wide profiling of histone H3 lysine 9 acetylation and dimethylation in Arabidopsis reveals correlation between multiple histone marks and gene expression[END_REF]. Especially, the histone mark H3K4me3 is considered as a primary marker of stress memory [START_REF] Conrath | Priming for enhanced defense[END_REF]. It was shown that the maintenance of heat stress memory is mediated by H3K4 methylation at the side of a heat-inducible transcription factor [START_REF] Lämke | A hit-and-run heat shock factor governs sustained histone methylation and transcriptional stress memory[END_REF]. Participation in defense priming was also shown in the case of histone acetyltransferases and deacetylases. The mutant hac1-1 (histone acetyltransferase 1) is involved in bacterial resistance and defense priming following PTI reactions [START_REF] Singh | Environmental History Modulates Arabidopsis Pattern-Triggered Immunity in a HISTONE ACETYLTRANSFERASE1-Dependent Manner[END_REF]. It seems that HAC1 links repetitive stress responses activation to defense priming. In line with these results is the finding that knock out of RPD3/HDA1-class histone deacetylase (HDA19) results in a de-repression of SA-based defense [START_REF] Choi | HDA19 is required for the repression of salicylic acid biosynthesis and salicylic acid-mediated defense responses in Arabidopsis[END_REF]. Moreover, the great majority of genes involved in SAR seems to be primed resulting by an interplay with different histone modifications [START_REF] Conrath | Priming for enhanced defense[END_REF][START_REF] Mauch-Mani | Defense Priming: An Adaptive Part of Induced Resistance[END_REF]. One report that shows concrete involvement of histone methylation of defense priming reported that treatment with BTH increases the appearance of H3K4me2 and H3K4me3 on three important defense-related transcription factors (WRKY29, WRKY6, WRKY53). BTH induced priming at these genes showed a transcriptional memory after a lag phase of several days [START_REF] Jaskiewicz | Chromatin modification acts as a memory for systemic acquired resistance in the plant stress response[END_REF]. However, since also abiotic stress can induce defense priming [START_REF] Singh | Environmental History Modulates Arabidopsis Pattern-Triggered Immunity in a HISTONE ACETYLTRANSFERASE1-Dependent Manner[END_REF] and defense inducing agents reduce between 20 and 85% of plant infection, it might be that plants in the field are already primed due to the various stresses they have to face continuously [START_REF] Walters | Are plants in the field already induced? Implications for practical disease control[END_REF].

Transgenerational stress priming

The finding that certain DNA methylation patterns are inheritable paved the way to the hypothesis that some traits that are regulated by DNA methylation could be passed on to subsequent generations. Several studies showed an effect in the progeny of plants infected with tobacco mosaic virus or exposure to UV light or flg22 treatment. Progeny of plants infected with tobacco mosaic virus showed greater resistance while plants of which parents were exposed to UV or flg22 resulted in a greater homologous recombination frequency [START_REF] Roberts | Acquired resistance to tobacco mosaic virus transmitted to the progeny of hypersensitive tobacco[END_REF][START_REF] Molinier | Transgeneration memory of stress in plants[END_REF]. Interestingly, also the chemical SAR activator BABA was shown to induce resistance in the progeny [START_REF] Slaughter | Descendants of primed Arabidopsis plants exhibit resistance to biotic stress[END_REF]. After comparison of transgenerational resistance in RdDM mutants with wild type plants, it was suggested that transgenerational SAR is achieved through induced hypomethylation at non-CG DNA sites. (Luna et al., 2012;Luna and Ton, 2012). Next to the possibility of inherited DNA methylation marks, an alternative is that histone modifications are inherited through nucleosome recycling or the copying of modifications onto newly incorporated histones. This hypothesis is based on findings on the widely studied gene FLOWERING LOCUS C (FLC). This transcription factor acts as repressor of floral transition and is regulated by the histone mark H3K27me3.

During embryogenesis the vernalized state of FLC is reset by the activity of an H3K27 demethylase [START_REF] Crevillén | Epigenetic reprogramming that prevents transgenerational inheritance of the vernalized state[END_REF]. Mutants lacking the demethylase inherit the vernalization state to their offspring. Intergenerational stress memory was confirmed in a study on hyperosmotic stress priming. Plants which were stressed during their vegetative development passed on the stress memory for at least two generations. However, this stress memory was reset after one stress-free generation [START_REF] Wibowo | Hyperosmotic stress memory in Arabidopsis is mediated by distinct epigenetically labile sites in the genome and is restricted in the male germline by DNA glycosylase activity[END_REF]. Transgenerational epigenetic stress memory is meiotically stable and extends for at least one stress-free generation. One study showed that stress-dependent mobilization of retrotransposons and their directed integration in the genome can be stably inherited. This stable integration could possibly lead to a more stress-resistant progeny [START_REF] Thieme | Inhibition of RNA polymerase II allows controlled mobilisation of retrotransposons for plant breeding[END_REF]. This finding could open a door for the introduction of beneficial plant traits.

Memory is not for free -costs and omission of stress priming

Induced transgenerational resistance could possibly results in a costs for the plant. On the level of hormonal regulation of plant defense, it was shown that the progeny of plants primed with a SA pathway inducing pathogen downregulate JA-dependent defense. This results in an increased susceptibility in these plants against JA-pathway dependent bacterial infections (Luna et al., 2012). Overall, defense priming is assumed to be beneficial for the plants with a generally positive cost-benefit balance in times of stresses. However, the advantage of a primed "ready state" becomes only obvious upon a subsequent exposure to a second stress, whereas a primed plant can outperform an unprimed plant. If this second stress is not accruing only the costs of priming influence the plants fitness. The activation and maintenance of the prime state of enhanced defense in form of the deposition of dormant signaling enzymes as well as the storage in form of epigenetic marks on defense gene promotors could result in fitness consequences [START_REF] Conrath | Priming for enhanced defense[END_REF][START_REF] Martinez-Medina | Recognizing Plant Defense Priming[END_REF]. However, defense priming has lower costs than the direct activation of defense. In summary, research on the impact of defense priming on possibly negative effects is currently underrepresented. A possible model of the impact of defense priming on defense responses and plant fitness is summarized in Figure 5. In the last years it became clear, that plants are also able to forget certain stresses. It might be an advantage for the plant to forget a previous stress in case the plant is sensing a false alarm signal [START_REF] Crisp | Reconsidering plant memory: Intersections between stress recovery, RNA turnover, and epigenetics[END_REF]. In the case of a maladaptive effect of stress memory it is reasonable to consider possible mechanisms for resetting. A screen for factors involved in the erasure of epigenetic stress memory resulted in the identification of two genes. DECREASE IN DNA METHYLATION 1 (DDM1) and MORPHEUS´MOLECULE 1 (MOM1) are key factors to prevent transgenerational memory [START_REF] Iwasaki | Identification of genes preventing transgenerational transmission of stress-induced epigenetic states[END_REF]. Moreover, the role of RNA metabolism has great potential as a regulatory mechanism in memory resetting. Three mechanisms have been proposed to facilitate resetting of the transcriptome. At the transcriptional level are fast activation exonucleases (1) as well as miRNA silencing (2) and siRNA silencing (3) that could play a role in a directed resetting of gene expression [START_REF] Crisp | Reconsidering plant memory: Intersections between stress recovery, RNA turnover, and epigenetics[END_REF]. However, not much is known how about how changes in mRNA stability influence transcriptional memory.

One research group showed a rather unusual memory response pattern by a subset of hydration stress response genes. Several exposures to hydration stresses has been assumed to prime the response to a subsequent hydration stress. However, it was found that a subset of genes responds to a first stress but then returns to a basic pre-stressed expression level during watered recovery and do not respond to subsequent stresses. The transcription factor MYC2 was identified to be a critical component for the memory behaviour of this subset of genes [START_REF] Liu | Different gene-specific mechanisms determine the 'revised-response' memory transcription patterns of a subset of A. thaliana dehydration stress responding genes[END_REF]Liu et al., 2016).

Antisense transcription

Next to the epigenetic regulation of gene expression another emerging regulation of transcription involves the expression of antisense transcripts. It has been previously reported that next to transcription of genes in sense they can also be transcribed in antisense orientation. Antisense transcripts include partial or complete sequences complementary to other transcripts and are endogenous RNA molecules [START_REF] Wang | Genome-wide prediction and identification of cis-natural antisense transcripts in Arabidopsis thaliana[END_REF]. They play an important role in various processes, including the response to biotic and abiotic stresses [START_REF] Terryn | The sense of naturally transcribed antisense RNAs in plants[END_REF]. Antisense transcripts are widespread in both prokaryotes [START_REF] Wagner | Antisense RNA control in bacteria, phages, and plasmids[END_REF] and eukaryotes [START_REF] Vanhée-Brossollet | Do natural antisense transcripts make sense in eukaryotes?[END_REF]. Moreover, evidence suggest that transcription of antisense RNAs have the potential to alter RNA processing, transport, stability and translation [START_REF] Vanhée-Brossollet | Do natural antisense transcripts make sense in eukaryotes?[END_REF].

Interestingly, antisense transcripts can modulate transcript levels via an RNA silencing mechanism. Double stranded RNAs emerging from sense and corresponding antisense transcripts could be processed into siRNA which then trigger silencing [START_REF] Borsani | Endogenous siRNAs derived from a pair of natural cis-antisense transcripts regulate salt tolerance in Arabidopsis[END_REF][START_REF] Wang | Genome-wide prediction and identification of cis-natural antisense transcripts in Arabidopsis thaliana[END_REF]. Moreover, it was shown that antisense transcripts are widely presents in apple. Notably, the percentage of antisense in apple is higher than that identified in annual plants like Arabidopsis [START_REF] Celton | Widespread anti-sense transcription in apple is correlated with siRNA production and indicates a large potential for transcriptional and/or post-transcriptional control[END_REF].

The aims of this thesis.

The plant immune system is under complex genetic and epigenetic regulation and can be triggered by external as well as endogenous signaling peptides. At the beginning of my thesis the number of discovered and characterized small endogenous signaling peptides was constantly increasing. However, the number of potential small endogenous peptides in the Arabidopsis genome is tremendously high. In fact, the Arabidopsis genome harbors more than thousand genes that potentially code for secreted peptides (Lease and Walker, 2010). Apart from the induction and regulation of defense responses by small endogenous peptides, the plant is capable of remembering previous stresses by a mechanism referred as priming. At the beginning of my thesis the research focused mainly on the molecular mechanism of priming memory maintenance. Less published research focused on the plants capability to forget previous stresses. Thus, the overall focus of my thesis is to uncover to molecular mechanisms of induction, maintenance and omission of plant defense responses and memory by endogenous, exogenous and synthetic plant elicitors.

To achieve the first aim of my thesis I was working in close collaboration with the bioinformatic group at IRHS Angers. This group developed a bioinformatic pipeline to predict previously unidentified small endogenous plant peptide families. As a proof of concept, my goal was to characterize the properties and effects of the newly predicted small peptide family termed SCOOP (Serine riCh endOgenOus Peptide). The first very promising results opened the door for the discovery of various interesting effects of several small peptides of the SCOOP family. I show that the SCOOP family peptides are involved in defense responses and root development in Arabidopsis. My work on the SCOOP peptide family is described in the chapters 4 and 5.

The second aim of my thesis was to study the ability of plants to memorize treatments with defense priming elicitors as well as the influence of a subsequent exposure to exogenous elicitors. The goals here were (1) to uncover the long-term transcriptional memory a salicylic acid-related defense priming compound induce in a plant (2) to test if this memory is stable following a subsequent exogenous elicitor treatment. To demonstrate the generality of my findings I carried out experiment on Arabidopsis and apple. My work on the plant stress memory behavior and the loss of the primed state of transcription by a subsequent stress is described in chapter 6.

4.

The SCOOP12 peptide regulates defense responses and root development in Arabidopsis thaliana.

The work described in this chapter was resulting from a collaboration between the bioinformatic group and me. A modified and extended version of this chapter was published in Gully et al., (2018) (Appendix in this thesis). The SCOOP peptide family regulates defense response and root development in Arabidopsis thaliana. My contribution was all experiments involving the application of peptides and certain pathogen infections (protection assay and pseudomonas infection of proscoop12) as well as creating a CRISPR-Cas9 mutant in the Col-0 background and data compilation. The phospholipid pathway activating capacity of SCOOP12 was investigated in a collaboration with the iEES-Paris.

Abstract

Small secreted peptides are important actors in plant development and stress response. Even though numerous Arabidopsis thaliana genes have the potential to produce these peptides, the vast majority of them have not yet been characterized for their biological functions. In this study, using a targeted in silico approach, we identified a small family of 14 Arabidopsis genes encoding precursors of Serine rich endogenous peptides (PROSCOOP). Plants defective in one of the family members (PROSCOOP12) show enhanced root growth. Searching through all Brassicaceae homologs of PROSCOOP12 we identified a conserved motif indicating that it encodes for a putative secreted peptide. The exogenous application of the synthetic peptide SCOOP12 induced various defense responses and pathogen tolerance in Arabidopsis. Our findings show that SCOOP12 has numerous properties of damage/danger-associated molecular pattern (DAMP) and is perceived in a BAK1 co-receptor dependent manner. In conclusion, we demonstrate that SCOOP12 fulfills all structural features of a posttranslationally modified peptide that modulates defense responses and root elongation.

Introduction

Plants are facing continuous attacks by pathogens. To counter constant pathogen aggressions, plants have developed sophisticated perception and defense systems. These plant responses are regulated by complex networks involving regulatory proteins and hormones and are associated with massive changes in gene expression (Buscaill and Rivas, 2014). Among the involved actors, it has been shown that small secreted peptides, also named peptide phytohormones, play an important role through their direct interaction with pathogens or through their function in development and cell-to-cell communication involving ligand-receptor interactions (Murphy et al., 2012;Marmiroli and Maestri, 2014).

The secreted peptides derive from protein precursors having a shared N-terminal signal peptide which target the protein to the secretory pathway. They can be categorized into two major classes: (i) the small post-translationally modified peptides (PTMP) which are the targets of posttranslational maturations and are produced through a proteolytic processing and (ii) the cysteine-rich peptides (CRP) characterized by an even number of cysteine residues involved in intramolecular disulfide bonds (Tavormina et al., 2015). Although they are mainly involved in plant growth and development processes, it has been shown that numerous genes encoding secreted peptides are also involved in plant defense mechanisms (Albert, 2013). For instance, the CRP class includes the antimicrobial peptides (such as knottins and defensins) which interact and disrupt the pathogen cell membrane (Goyal and Mattoo, 2014). Regarding PTMPs, families such as the phytosulfokines, CLE/CLV3, IDA/IDL or PSY are actors in processes regulating a large panel of plant-pathogen interactions (Lee et al., 2011;Shen and Diener, 2013;Vie et al., 2015;[START_REF] Rodiuc | Evolutionarily distant pathogens require the Arabidopsis phytosulfokine signalling pathway to establish disease[END_REF].

Among secreted peptides, those showing immunity-inducing activity have been classified as damage/danger associated molecular pattern, i.e. DAMPs (Heil et al., 2012), by analogy with the exogenous microbe-associated molecular patterns, i.e. MAMPs (Boller and Felix, 2009).

By using lytic enzymes, a pathogen can penetrate the plant cell wall and the cell wall fragments released into the apoplastic space can be perceived by neighboring cells, resulting in defense reactions. Oligalacturonides and cutin monomers are examples of DAMPs which get released upon fungal infection (Fauth et al., 1998). Also, other molecules, which are not located in the extracellular space under normal conditions such as DNA, ATP and some sugar molecules can serve as DAMPs. Their perception by neighboring cells elicits innate immunity as well (De Lorenzo et al., 2011). The small peptide AtPep1 is a well-documented DAMP (Bartels and Boller, 2015). A first induction of AtPep1 and other peptides of this gene family by wounding or pathogen attack has a positive feedback on the expression of its own precursors as well as defense marker genes that is thought to amplify defense signaling pathways (Huffaker and Ryan, 2007).

However, only a small fraction of the gene space likely to encode signaling peptides has been described and their diversity appears to be largely underestimated (Matsubayashi, 2014).

Indeed, the Arabidopsis genome contains more than a thousand genes harboring secreted peptide features for which the biological function is currently unknown (Lease andWalker, 2006, 2010). This lack of data can be explained by the fact that this type of genes has only recently been detected due to their small size and their low sequence conservation (Silverstein et al., 2007). Furthermore, the frequent functional redundancy inside these gene families (Matsubayashi, 2014) renders mutant knock-out approaches less successful. The mining of previously published transcriptomes is an efficient way to explore this unknown gene-space and decipher functions of new genes for which, without reference, the inference of function by similarity cannot be applied. Based on transcriptome meta-analysis and bioinformatics predictions in a 'guilt by association' approach, we identified a peptide family, whose one member at least is involved in plant immunity and root development.

This work describes the identification of a gene family specific to the Brassicaceae genus encoding putative secreted peptides. The functional characterization of PROSCOOP12, one of its members in Arabidopsis, shows that this small gene could act as moderator in the response to different pathogen aggressions and in root development presumably via controlling ROS detoxification. Based on the prediction of conserved motifs present in this family, we then illustrate that the small endogenous SCOOP12 peptide displays most properties of a DAMP.

Results

Identification of the PROSCOOP gene family

As previously mentioned, the Arabidopsis genome harbors more than a thousand genes with properties of a potentially secreted peptide. Among them, one gene that we termed PROSCOOP12 (AT5G44585 in TAIR10) caught the attention of the bioinformatic group at INRA Angers. This gene has an uncommon and highly informative transcription profile. By metaanalysis of the CATMA micro-array dataset (Gagnot et al., 2008) PROSCOOP12 was found to be constrictively expressed in roots but strongly induced in leaves in response to a large panel of different biotic stresses. In roots PROSCOOP12 shows a constitutive expression in normal growth condition but is down-regulated in numerous conditions affecting root elongation such as nitrogen starvation (Krapp et al., 2011). These observations led us to investigate the role of PROSCOOP12 in root development in greater detail.

The screening of the Arabidopsis genome revealed that PROSCOOP12 belongs to a small family of 14 unknown genes with a similar intron-exon structure (2 or 3 exons), encoding proteins ranging from 72 to 117 amino-acids (aa) with a N-terminal signal peptide, targeting proteins to the endoplasmic reticulum present in all members of the family. The genes of the PROSCOOP family are organized in two tandemly arrayed clusters on chromosomes 1 and 5 (Figure 6A). The largest 37 kb long gene cluster on chromosome 5 contains numerous vestiges of transposable elements (Helitron type) which could have impacted evolution of this family through local duplication events. Manual annotation revealed two additional yet nonannotated genes (PROSCOOP2 and PROSCOOP3) located between AT5G44565 and AT5G44568. Both share significant similarities with the other clustered PROSCOOP genes. Our manual annotation also led to the correction of the structure of AT5G44570 (PROSCOOP5) in which an over-predicted 3' coding exon has been removed. The size of the proteins, the number and the organization of paralogs, the aa composition (with the absence of cysteine) and the presence of a signal peptide are common features shared by the PTMP families previously published (Matsubayashi, 2014). This newly identified family has been named SCOOP, for Serine riCh endOgenOus Peptide. Its members termed PROSCOOP1 to 14 encode putative precursors of the mature SCOOP1 to 14 peptides.

In order to identify divergent yet still conserved smaller regions (ranging from 6 to 12 aa), Sebastien Aubourg and his colleagues of the bioinformatic group used the MEME algorithm (Bailey et al., 2015), excluding full length alignments, on the 74 identified homologs. This sensitive approach allowed the identification of two significantly conserved 11 aa-long motifs (Figure 6B). These motifs were good candidates for functional mature peptides following the putative proteolytic processing of the corresponding precursor. Indeed, both motifs are proline-, serine-, arginine-and glycine-rich, as in previously described PTMP families such as CLV3/CLE (Betsuyaku et al., 2011), IDA (Vie et al., 2015), PIP (Hou et al., 2014) and CEP (Roberts et al., 2013). Motif 1 is more ubiquitous than Motif 2 since it was detected in 72 sites (e-value of 9.8e-213) compared to 39 sites (e-value of 3.4e-179) out of the 74 PROSCOOP homologs (present in all Brassicaeae species). Therefore, we have focused our downstream analysis on motif 1 (Figure 6B), named SCOOP thereafter. In order to assess the evolutionary conservation of the PROSCOOP family, an extensive search for homologs in GenBank was carried out. We identified this family in several Brassicaceae genomes reaching from Eutrema salsugineum to Camelina sativa and the number of identified homologs in these genomes ranged from 1 to 13. Outside Brassicaceae genus, no similar protein has been detected despite low stringency searches. The phylogenetic tree built from the multiple alignment of the 74 identified PROSCOOP homologs shows that tandem duplications occurred before speciation of the 8 different Brassicaceae species (Figure 7)

Use of the CRISPR-Cas9 system to generate proscoop12 in Col-0 background

While a T-DNA mutation in PROSCOOP12 was available in the Wassilewskija (Ws) Arabidopsis accession, no mutation in that gene was available for Columbia (Col-0). In order to obtain a PROSCOOP12 mutant in Col-0 accession, we used the CRISPR-Cas9 approach to knock-out PROSCOOP12 in this accession. A guide RNA (gRNA) was designed to target the first exon of PROSCOOP12. Following transformation PROSCOOP12 was genotyped by sequencing for the presence of mutations. In one line we detected a single alanine (A) insertion within the gRNA binding sequence (Figure 8A). The resulting frame shift leaded to an early stop codon only 10aa downstream the editing event. To confirm the sequencing result a 450 base pairs (bp) long PCR product was digested with the restriction enzyme HaeII, which enzyme recognition sequence (RGCGCY) is disrupted by the CRISPR-Cas9-targeted mutation in PROSCOOP12. The PCR product could not be digested in proscoop12 confirming the sequencing results (Figure 8B). In the previously mentioned T-DNA mutant proscoop12 in the Ws background PROSCOOP12 was not transcribed (Figure 8C).

PROSCOOP12 is involved in root development

Previous results on the AtPep/PROPEP peptide family, showed that this small peptide could play an important role in developmental processes. It was shown that AtPep1 perception might inhibit root growth via regulation of GLUTAMINE DUMPER (GDUs) genes encoding amino acid exporters [START_REF] Ma | AtPEPTIDE RECEPTOR2 mediates the AtPEPTIDE1 induced cytosolic Ca rise which is required for the suppression of Glutamate Dumper gene expression in Arabidopsis roots[END_REF], and second a publication uncovered an acceleration of starvation-induced senescence upon Pep perception [START_REF] Gully | Perception of Arabidopsis AtPep peptides, but not bacterial elicitors, accelerates starvation-induced senescence[END_REF]. Based on these findings and the transcriptomic analysis that suggested PROSCOOP12 may play a role in root development, we compared the root length of wild type and proscoop12 plants. Indeed, proscoop12 plants developed significant longer roots than control plants in both accessions (Figure 9).

The SCOOP12 peptide has the main features of DAMPs

The structural features of the PROSCOOP12 protein suggested that it should be classified as a secreted PTMP. However, at the functional level, its transcriptional behavior suggested that it may play a role as a DAMP. Indeed, the induction of PROSCOOP12 expression by a large panel of biotic stresses and the root phenotypes identified in proscoop12 revealed some analogies with the AtPROPEP1 and AtPROPEP2 genes which encode for the precursors of the AtPep1 peptide, a well-characterized DAMP (Bartels and Boller, 2015). Likewise, both genes are also induced by biotic stress (Huffaker et al., 2006) and the AtPep1 DAMP is involved in root development since the overexpression of PROPEP1 and PROPEP2 resulted in significantly longer roots (Huffaker et al., 2006). Therefore, we wanted to test if PROSCOOP12 encodes for a peptide that may act as a DAMP and compare it to AtPep1

The SCOOP12 peptide induces immune responses in Arabidopsis

Based on the identification of the conserved motif 1 (Figure 6A), a putative mature peptide SCOOP12 was defined (PVRSSQSSQAGGR) and synthetized in order to explore its biological function. Despite the non-predictable post-translational modifications of the mature peptide, we tested the exogenous application of the synthetic SCOOP12 peptide as previously described for the CLE and RGF PTMP families (Matsuzaki et al., 2010;Murphy et al., 2012;Whitford et al., 2012). Treatment of plants with SCOOP12 induced a wide range of long-and short-term immune responses (Figure 10). One of the fastest defense responses is the production of ROS (Torres et al., 2006). We show here that SCOOP12 induced a more intensive ROS burst compared to AtPep1 but weaker than flg22 (Figure 10A). Next, we wanted to study the effect of SCOOP12 on genes closely linked to early defense mechanisms. The FLG22-INDUCED RECEPTOR-LIKE KINASE1 (FRK1) has previously been shown to be induced by pathogens, pathogen-derived elicitors and salicylic acid through MAPK (mitogen-activated protein kinases)-mediated signaling (Asai et al., 2002;Boudsocq et al., 2010) and AtPep1 (Flury et al., 2013). Therefore, we wanted to test if exposure to SCOOP12 could influence FRK1 expression. Here, we measured FRK1 steady-state transcript levels in detached leaves floating for 2h in solutions supplemented with SCOOP12 or AtPep1. Compared to controls, AtPep1 and SCOOP12 treatments resulted in a 15-fold and 8.5-fold increase in FRK1 expression, respectively (Figure10B). The deposition of callose is known to be triggered by conserved PAMPs (Luna et al., 2011) as well as DAMPs such AtPep1. Callose staining after 24h of treatment with the elicitor peptides showed that SCOOP12 induced a callose deposition, yet at a weaker level compared to flg22 or AtPep1 (Figure 10C and10D). One of the long-lasting defense responses is an inhibition of growth caused by the elicitor. The addition of MAMPs or DAMPs to the medium can lead to a strong inhibition of seedling growth in a concentrationdependent manner, which is dependent on the receptor-MAMP interaction (Krol et al., 2010).

Our results indicate that perception of SCOOP12 also lead to an arrest of growth. The effect is comparable to the flg22 MAMP and the AtPep1 DAMP (Figure 10 Small endogenous peptides are known to be active at a very low centration. In order to confirm this property for the SCOOP12 peptide we performed dose effect experiment by using the seedling growth experiment. The results show that flg22 causes an inhibition of seedling growth at concentrations as low as 1nM. A concentration of 100nM SCOOP12 causes a highly significant reduction in seedling fresh weight while the root length is less severely affected. Moreover, SCOOP12 inhibits seedlings growth with a concentration of 50nM. (Figure 11).

Pre-treatment with the SCOOP12 peptide protects Arabidopsis against Pseudomonas infection

It has previously been shown that priming of plants with the flg22 elicitor as well as with oligogalacturonides could result in enhanced tolerance against subsequent bacterial infections. For instance, plants pre-treated with these elicitors showed significantly reduced lesion size following an infection with Botrytis cinerea (Raacke et al., 2006;Ferrari et al., 2007).

Using a similar assay, we found that plants pre-treated with flg22 as well as with SCOOP12 and AtPep1 were less susceptible to Pseudomonas syringae pv. tomato DC3000 infection (Figure 10H). The effect of the two endogenous peptides SCOOP12 and AtPep1 was weaker than flg22, which is consistent with the fact that flg22 induced stronger defense response compared to SCOOP12 (Figure 10 AC).

SCOOP12 induce the expression of PROSCOOP12 and PROPEP2

It has previously been shown that small endogenous peptides can induce the expression of their own precursors resulting in a positive feed-back loop. For instance, expression of several PROPEP genes can be induced by different AtPep peptides (Huffaker and Ryan, 2007). This led us to investigate the change in steady state transcript level of PROSCOOP12 after SCOOP12 exposure. Moreover, we decided to add AtPep1 in our assay for comparison since it is also known to induce transcription of another peptide precursor, prePIP1 (Hou et al., 2014). The results show, that PROSCOOP12 is upregulated by SCOOP12 in comparison to the control treatment (Figure 10 I). Therefore, there is a positive feedback loop linking SCOOP12 to its putative precursor PROSCOOP12. Next, we wanted to know if SCOOP12 is capable of inducing the expression of PROPEP family members. However, SCOOP12 does not induce expression of PROPEP1, while PROPEP2 is induced by a SCOOP12 application (Figure 12 AB). Indicating a cross regulation between the two peptide families.

proscoop12 does not show an altered response to Pseudomonas infection

It was shown that the overexpression of the AtPep1 precursor PROPEP1 results in an enhanced resistance to the root pathogen Pythium irregulare (Huffaker et al., 2006).

Moreover it was shown that the two precursors PROPEP2 and PROPEP3 are enhanced expressed by a Pseudomonas infection [START_REF] Ross | The Arabidopsis PEPR pathway couples local and systemic plant immunity[END_REF]. However, it was so far not shown that a mutant of a PROPEP member is more susceptible to Pseudomonas infection. We decided to infect the proscoop12 plants in the Ws background with Pseudomonas (Figure 13).

Even though we have found that the SCOOP12 peptide induces various defense responses, a knock-out in PROSCOOP12 does not result in an altered response to Pseudomonas (Figure 13 AB).

SCOOP12 activity depends on the correct amino-acid order

In order to demonstrate the specificity of SCOOP12 sequence, we synthesized a peptide based on a randomized version of the same 13 amino acids and tested plant responses to this scrambled SCOOP12 (scSCOOP12). Furthermore, we synthesized peptides with double alanine replacements (SCOOP12 S5/7A) and single replacements (SCOOP12 S5A; SCOOP12 S7A) to test the importance of the two highly conserved serine residues on positions 5 and 7 of SCOOP12 (Figure 6B) for its activity. Plants treated with scSCOOP12 as well as with the modified peptides did not show seedling growth inhibition. Total seedling fresh weight as well as root length were not different from that of control plants (Figure 14). Finally, treatments with scSCOOP12, SCOOP12 S5/7A and SCOOP12 S5A did not induce a ROS burst and only SCOOP12 S7A resulted in a low, still significant ROS burst (Figure 14). These results highlight the importance of the amino acid order and the presence of the highly conserved serine residues for the perception of SCOOP12 by the plant.

SCOOP12 is only perceived by Arabidopsis and Brassica napus

Next, we wanted to test the conservation of plant responses to SCOOP12. For that purpose, plants were selected in which we identified PROSCOOP homologues (Brassica napus, Figure 7) and plants for which this gene family has not been identified (Nicotiana benthamiana and Lycopersicon esculentum). We measured ROS production following application of SCOOP12 in these plants and included flg22 as a positive control. We detected a ROS burst caused by flg22 in all four plant species. On the other hand, SCOOP12 only resulted in a ROS burst in A. thaliana and at a lower, yet still significant level, in B. napus (Figure 15). Therefore, only the two plant species containing homologues of the PROSCOOP gene-family members, showed a response to SCOOP12 treatments.

The BAK1 co-receptor is involved in SCOOP12 perception

A well characterized co-receptor of several receptors of small peptides is BRI1-associated kinase1 (BAK1). It has been shown to dimerize with BRI1 (Brassinosteroid-Insensitive 1), the brassinosteroid receptor [START_REF] Li | BAK1, an Arabidopsis LRR receptor-like protein kinase, interacts with BRI1 and modulates brassinosteroid signaling[END_REF]. Interaction of BAK1 with receptor like kinases that act as elicitor receptors, was proposed to be due to conformational changes occurring after ligand binding which results in the formation of the receptor complex (Chinchilla et al., 2009;Liu et al., 2017). To test if BAK1 is involved in the perception of SCOOP12, a seedling growth inhibition assay was performed on bak1-4 plants. Compared to wild-type controls, bak1-4 plants did not display any significant growth inhibition upon SCOOP12 treatment (Figure 16A).

In order to identify the SCOOP12 receptor, the same approach was carried out on fls2 (the flg22 receptor) and pepr1/pepr2 plants. Contrary to BAK1, our results suggest that these receptors are not involved in the perception of SCOOP12 (Figures 16BC). In order to confirm the involvement of BAK1 in the perception of SCOOP12 we tested a double mutant bak1-5 bkk1-1. The mutant bak1-5 harbors a single amino acid substitution and does, in contrast to bak1-4, still accumulate wild type level of BAK1-5 protein [START_REF] Roux | The Arabidopsis leucine-rich repeat receptor-like kinases BAK1/SERK3 and BKK1/SERK4 are required for innate immunity to hemibiotrophic and biotrophic pathogens[END_REF].

However, the double mutant bak1-5 bkk1-1 is nearly completely impaired in defense responses caused by flg22, elf18 and AtPep1 [START_REF] Roux | The Arabidopsis leucine-rich repeat receptor-like kinases BAK1/SERK3 and BKK1/SERK4 are required for innate immunity to hemibiotrophic and biotrophic pathogens[END_REF]. The results show that bak1-5 bkk1-1 is nearly completely impaired in the perception of SCOOP12. However, the root length of bak1-5 bkk1-1 treated with SCOOP12 is significantly shorter compared to the untreated control (Figure 17).

SCOOP12 rapidly activates phospholipid signaling pathways in Arabidopsis cell suspensions

Lipid signaling pathways act as multifunctional regulatory mechanisms in plants. They incorporate several groups of inducible enzymes that convert membrane phospholipids into signaling molecules. Phosphatidic acid (PA) is a well-known biologically active lipid that is produced in response to numerous hormonal and stress signals including, notably, flg22 (van der Luit et al., 2000). Here, we show (experiments done by collaborators at the iEES in Paris) that application of SCOOP12 induces an accumulation of PA in Arabidopsis cell suspensions (Figure 18). This effect is observed as early as 5 min following SCOOP12 application in a low concentration of 100nM (Figure18 BC). The scSCOOP12 had no effect on PA accumulation. Two modes of PA accumulation are known: PLD-dependent via direct hydrolysis of membrane phospholipids and DGK-dependent via phosphorylation of diacylglycerol (DAG). In our experiment a labelling protocol that favors visualization of DGK-derived PA was used (Arisz and Munnik, 2013). PIP2 is a substrate to phosphatidylinositol-specific phospholipase C (PI-PLC) that produce DAG. We have also observed that the level of phosphatidylinositol 4,5bisphosphate (PIP2) is transiently reduced following SCOOP12 treatment (Figure18 B). These results suggest that SCOOP12 initiates a signaling cascade implicating PI-PLC (causing the depletion of PIP2) and subsequent production of PA via phosphorylation of DAG by DGK.

Discussion

The comparison of the PROSCOOP family with other previously published genes encoding such secreted peptides highlights numerous shared features but also interesting specificities.

At the structural level, the PROSCOOP proteins distinguish themselves by the absence of a highly conserved C-terminal region. Indeed, the detected motives are quite divergent compared to the other PTMP precursors (Matsubayashi, 2011). This divergence could explain the fact that no PROSCOOP homologs could be detected outside of the Brassicaceae genomes. This restricted phylogenetic profile is opposite to the other described secreted peptides which are conserved both in monocot and eudicot plants. Furthermore, contrary to the majority of the known PTMPs, the conserved motifs are not localized at the C-terminal extremity of their precursors, and their maturation could involve two steps of proteolytic processing or a trimming step (Matsubayashi, 2011).

Out of the 14 Arabidopsis PROSCOOP proteins, three include two duplicated SCOOP motifs (Figure 6), reminiscent of the previously described cases of the CEP and PIP families (Roberts et al., 2013;Vie et al., 2015) and also of the CLE18 protein in which each copy of the conserved CLE motifs has a specific function (Murphy et al., 2012). The motif composition classifies SCOOP in the superfamily of 'SGP-rich peptide' among PIP, CLE, IDA, PEP and CEP families (Hou et al., 2014). At the functional level, the triggering of ROS burst, FRK1 transcription and callose deposition move SCOOP12 close to the cytosolic AtPep and apoplastic PIP families (Huffaker et al., 2006). Our results suggest a functional link between AtPep1 and SCOOP12 since both peptides induce the transcription of PROSCOOP12 (Figure10 I). This collaboration between different peptide families has also been described with AtPEP1 and PIP1 which act cooperatively to amplify triggered immunity. Furthermore, the signaling induced by AtPep1 (Schulze et al., 2010), PIP1 (Hou et al., 2014) and SCOOP12 (Figure16,17) is dependent on the BAK1 co-receptor. Our results also show that proscoop12 in WS background does not show an altered resistance to Pseudomonas syringae infection. Similar results were obtained for the AtPep receptor mutants (pepr1 and pepr2). These mutants where not more resistant, while a pre-treatment of AtPep1-6 peptides increased the resistance to a subsequent Pseudomonas infection [START_REF] Yamaguchi | PEPR2 is a second receptor for the Pep1 and Pep2 peptides and contributes to defense responses in Arabidopsis[END_REF].

In addition to the role of peptides as amplifiers of the immune response, these peptides are involved in root development but via different mechanisms. A high number of PTMP are expressed in the root tip and play a role in primary root development. One of the most prominent one is the CLE family, which is involved in developmental processes in the root apical meristem [START_REF] Casamitjana-Martínez | Root-specific CLE19 overexpression and the sol1/2 suppressors implicate a CLV-like pathway in the control of Arabidopsis root meristem maintenance[END_REF]Stahl and Simon, 2009;Murphy et al., 2012). The cle40 mutant, one member of this family, shows an enlarged root phenotype. This is due to repression of WUSCHUL (WUS) expression [START_REF] Hobe | Loss of CLE40, a protein functionally equivalent to the stem cell restricting signal CLV3, enhances root waving in Arabidopsis[END_REF]Stahl et al., 2009). It is possible that also PROSCOOP12 influences the expression of important root growth factors.

In figure 12 we show that transcription of PROPEP2 is induced by SCOOP12 application.

Previously it was reported that an overexpression of PROPEP1 and PROPEP2 lead to an increased root and aerial growth (Huffaker et al., 2006). The overexpression of the PIP1 precursor or its exogenous application inhibits Arabidopsis root growth as described for CEP (Roberts et al., 2013), AtPep1 (Poncini et al., 2017) and SCOOP12 peptide (Figure 10F). Acting as growth factors and contrary to SCOOP12, the PTMP PSK and PSY1 are involved in root elongation (Amano et al., 2007;Matsuzaki et al., 2010). Our results could be a first indication of a complex root growth regulation system implemented by the interaction of multiple peptide families.

However, these comparisons show that despite common structural and functional characteristics, the SCOOP family is different from previously described secreted peptides.

The divergence observed in the C-terminal sequence of PROSCOOP proteins suggest a large set of biological functions through a diversity of receptors which will be the targets of future studies. The functions of such plant secreted peptides at the boundaries of development-and stress-signaling pathways open the way to future strategies that jointly consider product quality/quantity and new resistance traits.

In conclusion, SCOOP12 belongs to a new family of putatively secreted peptides (specific to the Brassicaceae species) displaying all the structural features of post-translationally modified peptides. SCOOP12 could play a role in the moderation of defense responses, as well as root growth.

Material and methods

Plant material

Plant material used was wild-type Arabidopsis thaliana L. Heynh cultivar 6 Columbia (Col-0) as well as the cultivar Wassilewskija (Ws) and the mutants proscoop12 (T-DNA line FLAG_394H10 in Ws background, primers used for genotyping are detailed in Table 1). The proscoop12 mutant in Col-0 background was created using the CRISPR-Cas9 approach. We searched proscoop12 gene-specific sgRNA and potential off-target sites in the Arabidopsis genome using the crispor tefor program (http://crispor.tefor.net). bak1-4 (T-DNA line SALK_116202), fls2 (Gomez-Gomez and Boller, 2000) and pepr1/pepr2 described by (Flury et al., 2013). All in vitro plants (on Murashige and Skoog) were grown under short day conditions (photoperiod of 8h light at 22°C/16h dark at 21°C, with 70% of relative humidity). Plants used for all other assays were grown under long day conditions (photoperiod of 16h light at 22°C/8h dark at 21°C, with 60% relative humidity). B. napus (Darmor-bzh) and L. esculentum (Sweet Baby) were grown under short day conditions.

Root length experiment

Proscoop12 and Ws as well as proscoop12 and Col-0 seedlings were grown on solid MS plates for 2 weeks. Plates were placed in a vertical position. After 11 days, the primary roots length was measured and compared. For each genotype, two repetitions have been done with at least 10 seeds per plate.

Plant inoculation with pseudomonas syringae pv Tomato DC3000 (Pst DC3000)

Wassilewskija and proscoop12 mutant plants were grown 5 weeks on soil. Eight leaves per plant were infiltrated with bacterial suspensions of at a concentration of 10 5 colony forming units (cfu/ml) (OD600 of 0.02) in sterile 10mM MgCl (also used for mock-inoculation) using a needleless syringe. Plants were maintained at high humidity. Samples were taken using a cork-borer (d=8mm) to cut one leaf discs per infected leaf. Leaf discs were ground in 10mM MgCl, diluted to the indicated concentration and plated as droplets of 10µl on YEB plates with the appropriate selection. Plates were incubated at 28°C and colonies counted two hours after the infection (0 dpi) as well as 1 and 2 dpi. Pictures were taken after 1 and 2 dpi.

Protection assay

Mature leaves of Arabidopsis thaliana plants were infiltrated by needless syringe infiltration with the indicated elicitor peptide or control solution and were kept under long day growth conditions for 24h. The P. syringae pv tomato DC3000 (Pto. DC3000) strain was grown in overnight culture on YEB medium plates supplemented with appropriate antibiotics. Cells were harvested from the plate and re-suspended in sterile 10mM MgCl and diluted to an OD600 of 0.02. Bacteria solution was needles syringe infiltrated into the pre-treated leaves.

Plants were maintained at high humidity. Samples were taken using a cork-borer (d=8mm) to cut one leaf discs per infected leaf. Leaf discs were ground in 10mM MgCl, diluted to the indicated concentration and plated as droplets of 10µl on YEB plates with the appropriate selection. Plates were incubated at 28°C and colonies counted two hours after the infection (0 dpi) as well as 1 and 2 days post infection. Eight plants were infected for each pre-treatment and sampling time point. The experiment was performed two times with similar results.

Seedling growth inhibition assay

Seedlings were germinated on MS agar and grown for 5 days before transferring one seedling per well to 24 well plates containing 500 µl MS media or MS media supplied with the indicated elicitor peptide to a final concentration of 1 µM (six replicates per elicitor peptide treatment).

Photos were taken, fresh weight and root length were measures after 8 additional days.

Elicitor peptides

Peptides of flg22 (QRLSTGSRINSAKDDAAGLQIA), Arabidopsis thaliana Plant Elicitor Peptide 1 (AtPep1) (ATKVKAKQRGKEKVSSGRPGQHN), SCOOP12 (PVRSSQSSQAGGR), scSCOOP12 (GRPRSASSGSVQQ), SCOOP12 S5/7A (PVRSAQASQAGGR), SCOOP12 S5A (PVRSAQSQAGGR) and SCOOP12 S7A (PVRSSQASQAGGR) were obtained from Eurogentec SA (Angers, France) and diluted in water to the final concentration used for the assays.

Measurement of reactive oxygen species

For ROS assays leaf discs of three weeks old soil grown plants, were placed into each well of a white 96-well plate (Thermo Scientific, Waltham, USA) in 0,1 ml of water and kept in the dark overnight. For elicitation and ROS detection, horseradish peroxidase and luminol were added to a final concentration of 10 µg ml -1 and 100 µM, respectively. Luminescence was measured directly after addition of elicitor peptides in a FLUOstar OPTIMA plate reader (BMG LABTECH, Offenburg, Germany).

Callose deposition

Leaf discs were vacuum infiltrated for 10 min with the indicated elicitor solution and kept floated in elicitor or control solution for 24h. After leaf discs were fixed and destained in 1:3 acetic acid/ ethanol until leaf tissue was completely transparent. After washing the leaf discs in 150 mM K2HPO4 for 30 min, the plant material was stained for 2 h in 150 mM K2HPO4 and 0,01% aniline blue. Callose depositions were quantified with a Leica DM1000 microscope equipped with a Qimaging Micropublisher 3.3 RTV camera using a DAPI filter.

Determination of gene expression by qPCR

Detached leaves of three weeks old plants were collected and floated for two hours in water supplied with the indicated elicitor or control solution. After the treatment, material was frozen and ground in liquid nitrogen. RNA from 100 mg of tissue was extracted using the NucleoSpin RNA plant extraction kit (Macherey-Nagel Hoerdt, France). The DNase treatment was performed according to the manufacturer's recommendations. Per PCR reaction, complementary DNA was synthesized from 10 ng of total RNA extract with oligo(dT) primers using Moloney Murine Leukemia Virus Reverse Transcriptase according to the manufacturer's instructions (Promega). For quantitative real-time reverse transcription PCR (qPCR) in a 96well format, the Chromo4™ System (Bio Rad) was used. Expression was normalized to that of the gene ACR12 (AT5G04740) using the qGene protocol (Muller et al., 2002). All the genespecific primers used are detailed in Table 1. This includes the meristem maintenance, lateral root development, gravitropic response and vascular formation. Here we extend the knowledge of the recently discovered small peptide family SCOOP. Here, we show that the SCOOP family contains several defense responses inducing peptide in Arabidopsis. Additionally, we show an interesting effect of SCOOP peptides on root morphology and development. In conclusion we show that the SCOOP family is very complex and these peptides play an important role in root metabolisms or development.

Introduction

Plants are complex organisms and regulation of plant growth, development, reproduction and response the various environmental stresses need to be tightly regulated by a complex network of signaling pathways. For a long time, it was considered that plant hormones like salicylic acid, ethylene, jasmonic acid, auxins, gibberellins, cytokinins, abscisic acid and brassinosteroids are the main signaling molecules during these processes. However, in recent years it was found that the plant regulation system is more complex than initially proposed.

An increasing number of small peptides have been discovered and lead to an increased intricacy in nearly all facets of plant development and defense. The large number of small peptides can be classified into two different categories. The first contains primary endogenous danger signals which are passively released upon host damage and secondary endogenous peptides. The secondary endogenous peptides are actively processed and released upon tissue damage. Peptides of the second category can be divided into peptides which are processed and released upon an herbivore attack or a microbial infection and peptides which have been linked to the regulation of plant growth and development. These peptides are referred to as phytocytokines (Gust et al., 2017). This category contains peptides like CLAVATA3/EMBYRO-SURROUNDING REGION (CLE) peptides [START_REF] Kondo | A plant peptide encoded by CLV3 identified by in situ MALDI-TOF MS analysis[END_REF], AtPeps [START_REF] Yamaguchi | Endogenous peptide elicitors in higher plants[END_REF], AtPIPs (Hou et al., 2014), RALFs [START_REF] Murphy | Understanding the RALF family: a tale of many species[END_REF], phytosulfokines (PSK) [START_REF] Matsubayashi | Disruption and overexpression of Arabidopsis phytosulfokine receptor gene affects cellular longevity and potential for growth[END_REF] and the SCOOP peptide family described in this thesis. Out of these examples PSK, AtPeps, AtPIPs, RALF and SCOOP have been shown to be involved in regulation of plant immune responses to herbivore or microbial attack (Mosher et al., 2013;Hou et al., 2014;[START_REF] Murphy | Understanding the RALF family: a tale of many species[END_REF][START_REF] Yasuda | A look at plant immunity through the window of the multitasking coreceptor BAK1[END_REF].

The perception of members of these peptide families is dependent on the SERK (SOMATIC EMBRYOGENESIS RECEPTOR KINASE) family member BAK1 (Hou et al., 2014;[START_REF] Yasuda | A look at plant immunity through the window of the multitasking coreceptor BAK1[END_REF]. BAK1 serves as coreceptor to various of these peptides. At a functional level phytocytokines have been shown to play key roles in root development. After proteolytic post-translational modification, whereas the peptide is processed from a precursor protein, it is secreted out of the cell and regulates many aspects of root growth and development including gravitropism, meristem maintenance, lateral root development and protoxylem differentiation [START_REF] Delay | Regulation of Arabidopsis root development by small signaling peptides[END_REF]. The root growth is also strongly influenced by stresses.

Plants can redirect root growth and morphology to diminish stress exposure. This plant response is referred to as stress-induced morphogenic response. These responses can be summarized by three components. The elongation of cells can be inhibited, cell division can be locally stimulated, and cell differentiation can be altered [START_REF] Potters | Stress-induced morphogenic responses: growing out of trouble?[END_REF]. Small peptide families contain a different number of members. The CLE is a large family that contains 32 peptides which are expressed in various tissues [START_REF] Jun | Comprehensive Analysis of CLE Polypeptide Signaling Gene Expression and Overexpression Activity in Arabidopsis[END_REF]. The AtPIP family contains at least 11 members (Hou et al., 2014) and the AtPep family at least 8 members (Bartels and Boller, 2015). This shows that peptide families can contain various numbers of small peptides, whereas members may play different roles in plant development and defense.

For certain families (AtPep and PIP) it was shown that the small processed peptide can induce the expression of its own precursor protein, effectively resulting in a positive feedback loop.

Expression of different PROPEP proteins have been shown to be induced by various processed peptides (Huffaker and Ryan, 2007). Moreover, expression of prePIP1 is induced by treatments with the AtPIP1 small peptide (Hou et al., 2014). In the previous chapters, we

show that the recently discovered SCOOP peptide family, includes various defense response activating peptides. In this chapter we report on the roles of three SCOOP peptides on root development and root stress responses.

Results

SCOOP12 and AtPep1 induce the expression of various PROSCOOP family members

It has previously been shown, that small endogenous peptides can induce the expression of their own precursors resulting in a positive feed-back loop. For instance, expression of several PROPEP genes can be induced by different AtPep peptides (Huffaker and Ryan, 2007). This led us to investigate the change in steady state transcript level of all 14 PROSCOOP family members after SCOOP12 exposure. Moreover, we decided to add AtPep1 in our assay for comparison since it is also known to induce the transcription of another peptide precursor, prePIP1 (Hou et al., 2014). The results show that PROSCOOP 2, 7, 8, 12 and 13 are upregulated by the AtPep1 treatment (Figure 19). Most importantly, the direct precursor PROSCOOP12 is upregulated by SCOOP12 in comparison to the control treatment (Figure 10,19). Therefore, there is a positive feedback loop linking SCOOP12 to its precursor PROSCOOP12 but also of other members of the PROSCOOP family such as PROSCOOP7. These results suggest that there is a feedback loop of SCOOP12 to its precursor and to PROSCOOP7 and that AtPep1 is capable of inducing five members of the PROSCOOP family.

Several SCOOP peptides induce ROS burst in Arabidopsis

In line with our previous results and the induction of PROSCOOP family member gene expression by SCOOP12 we decided to focus on the highly conserved motif found in all PROSCOOP family members (Figure 6B). We synthesized peptides covering the motif of all 14 family members (Table 2). The peptides were named according to their putative precursor protein. The motif contains two highly conserved serine (S) residues on position 5 and 7 of the peptides. Excepted of SCOOP3 all synthesized peptides harbor these two amino acids.

Table 2: List of peptides, covering the identified motif on the corresponding PROSCOOP family member. In total 14 PROSCOOP family members were identified. All 14 harbor the highly conserved motif. The peptides covering two highly conserved serine residues on position 5 and 7 of the peptides (marked in red). SCOOP3 is the only exception, here the serine on position 7 is replaced by a glutamate (marked in green) residue. In this extensive screen we then wanted to test if apart of SCOOP12 also other peptides of this family have the potential to induce defense responses in Arabidopsis. Like previously described in chapter 4 we first tested short term defense and tested all SCOOP peptides for their ability to induce a ROS burst in Arabidopsis. The results show that indeed 9 peptides are capable to inducing a significant accumulation of ROS. The five peptides SCOOP3, 8, 10, 11 and 14 did not trigger a response. Notably, the response to flg22 was stronger in comparison to all of the active SCOOP peptides (Figure 20). The ROS induction by SCOOP12 is comparable to that of SCOOP6 and 7. Interestingly, SCOOP7 showed a response comparable to SCOOP12 in intensity. This is well in line with our finding that expression of PROSCOOP7, the putative precursor of SCOOP7 is induced by SCOOP12 (figure 19).

Peptide

The PROSCOOP family contains several growth inhibiting peptides

Next, we wanted to know if also long-term defense responses are induced by the SCOOP peptides. Therefore, we grew seedlings in media supplied with the 14 SCOOP peptides. A continuous activation of defense responses is known to inhibit the plants growth (Walters and Heil, 2007). Our results show that most SCOOP peptides induce an inhibition of Arabidopsis seedling growth. The peptides SCOOP3, 8 and 11 did not significantly inhibit seedling growth (Figure 21AB). In comparison with the obtained results of ROS accumulation we found that the peptides SCOOP10 and 14 do induce a seedling growth inhibition but no ROS accumulation (Figure 20,21). SCOOP10 induces a significant inhibition of root growth (Figure 21B) but the total fresh weight is not affected in comparison to the control treatment (Figure 21A). The repression of root growth and total fresh weight is comparable to that of control treatments with AtPep1 and flg22. The most pronounced inhibition of root growth was induced by SCOOP12, 6 and 7.

Exogenous application of SCOOP12 induces a strong root pigmentation

In chapter 4 we showed that proscoop12 displays an enhanced root growth. That was a reason for us to focus our observations on root tissue. During the seedling growth inhibition experiment (Figure 21) we noticed a strong root pigmentation caused by SCOOP12 but not by flg22 or AtPep1 (Figure 22A). The elicitors were added in a final concentration of 1µM to the growth media. We assumed that SCOOP12 causes a strong hypersensitive response in the roots and therefore cell death of affected cells. To investigate this hypothesis, we performed a cell death staining with neutral red. The staining solutions stains exclusively intact and alive cells. The experiment showed that the roots are still intact, and no cell death is caused by the SCOOP12 treatment (Figure 22B).

Root pigmentation is caused by SCOOP6 and SCOOP7 in a tissue specific manner

In our previous results, we showed that the inductions of defense responses are most pronounced for the peptides SCOOP12, 6 and 7. These three peptides have the strongest effect on root growth. After the observation of a strong root pigmentation caused by SCOOP12, we decided to focus on SCOOP6 and 7 as other strong elicitor peptides. By the treatment of SCOOP12 we confirmed our previous observations. SCCOP12 causes a strong pigmentation of the entire root tissue. Moreover, the root architecture seemed to be altered.

We could observe an increased number or an expansion of cells in the root transition and/or root elongation zone caused by SCOOP12 (Figure 22A). The roots treated with SCOOP12 have increased secondary root growth, while the primary root apical meristem show reduced growth (Figure 23). This altered root shape was not observed by treatments with SCOOP6 and 7. However, application of SCOOP6 causes a root pigmentation in a tissue specific manner.

Exclusively the root differential and/or the elongation zone show root pigmentation with this peptide. The treatments with SCOOP7 lead to similar, but less pronounced, observations. These results are in line with our previous results on the effect of the SCOOP peptides on root growth, since these peptides show the strongest inhibition of root growth (Figure 21).

Interestingly, the peptide SCOOP9 also had a similar effect on root growth yet no root pigmentation was observed (data not shown).

Perception of SCOOP6 and SCOOP7 is dependent on SERK family members

We have shown in chapter 4 that the perception of SCOOP12 depends on the SERK family member BAK1. In order to test if also SCOOP6 and SCOOP7 are perceived by members of the SERK family we decided to test the response of bak1-5 bkk1-1 plants to this treatment. The bak1-5 harbors a single amino acid substitution and does, in contrast to bak1-4, still accumulate the wild type level of BAK1-5 protein [START_REF] Roux | The Arabidopsis leucine-rich repeat receptor-like kinases BAK1/SERK3 and BKK1/SERK4 are required for innate immunity to hemibiotrophic and biotrophic pathogens[END_REF]. However, the double mutant bak1-5 bkk1-1 is almost completely impaired in defense responses caused by flg22, elf18 and AtPep1 [START_REF] Roux | The Arabidopsis leucine-rich repeat receptor-like kinases BAK1/SERK3 and BKK1/SERK4 are required for innate immunity to hemibiotrophic and biotrophic pathogens[END_REF]. Therefore, we used this double mutant for treatments with SCOOP6 and SCOOP7. The results show that bak1-5 bkk1-1 is almost completely insensitive to SCOOP6 (Figure 24AB), indicating the role of SERK family members as coreceptors of SCOOP family peptides perception. Moreover, the response to SCOOP7 is less pronounced than in the wildtype situation. However, this might be due to the higher variance within the replicates (Figure 24CD).

Discussion

The SCOOP family harbors various peptides with interesting properties. In Table3 we summarized our current knowledge on the 14 members of the SCOOP family. It is important to note that all SCOOP peptides are based on a bioinformatic prediction approach, whereas all 14 PROSCOOP family members harbor a conserved motif. Out of the 14 synthesized peptides only 4 (SCOOP3, 8, 10, 11) do not show any activity in the applied assays. SCOOP14 did not induce a ROS burst but showed an inhibition of seedling growth. The strongest activity was observed for SCOOP12, as this peptide induced short-and long-term defense responses and a strong pigmentation of root tissue. The peptides SCOOP6 and SCOOP7 show similar results and an induction of a ROS burst, an inhibition of seedling growth as well as a pigmentation of roots. In chapter 3 we show that bak1-4 is involved in the perception of SCOOP12. By testing the bak1-5 bkk1-1 double mutant we confirmed that also SCOOP6 and SCOOP7 were perceived by members of the SERK family. However, by challenging a double mutant we cannot be sure which mutant is mainly responsible for the perception of SCOOP6 and 7. We intended to find a reason for the different activity of the SCOOP peptides. The activity of the SCOOP peptides seems to rely on the amino acid composition. By sorting the peptides by their activity and the charge of the single amino acids, we could not find a clear indication which amino acid is crucial for SCOOP peptide perception and activity (Figure 25). However, the two highly conserved Serine residues on positions 5 and 7 of the 13 aa long peptides, seems to be important for the perception by the plant. SCOOP3 is the only peptide for which the polar Serine (S) residue is replaced with a negatively charged Glutamic acid (E) residue. SCOOP3 does not show any activity in the applied assays. Overall, it is notable that none of the inactive peptides (SCOOP3, 8, 10, 11) and weakly active peptide (SCOOP14) harbor positively charged aa at position 6 (Figure 25). Most of the active peptides (excepted of SCOOP4 and SCOOP12) have a positively charged aa at position 6. However, it remains unclear which effects the aa composition has on the peptide 3D structure since a reliable structure prediction is difficult for short peptides [START_REF] Gupta | In silico protein modeling: possibilities and limitations[END_REF]. The SCOOP family contains at least 9 active peptides. The AtPIP family has at least 11 prePIP proteins whereas only 2 peptides are shown to have a defense inducing effect in Arabidopsis (Hou et al., 2014).

The AtPep family contains at least eight PROPEP proteins and their corresponding small peptides. Out of the eight AtPep peptides the small peptides AtPep 1 and AtPep2 show the strongest defense inducing activity. The alignment of the 8 PROPEP proteins shows that the conserved motif is less variable and longer than it is the case for the SCOOP peptide family [START_REF] Bartels | The family of Peps and their precursors in Arabidopsis: differential expression and localization but similar induction of pattern-triggered immune responses[END_REF]. Indicating that the variability within the peptides and the short length of the conserved region might prevent a better prediction of members of peptide families.

The synthesized peptides are based on the conserved motif found in all PROSCOOP genes. However, the in planta peptides are for the moment not known. The experimental determination and confirmation of all peptides could improve our knowledge about their properties and functional roles in plants. Various peptides have been shown to effect and regulate root growth. As described in chapter 3 also PROSCOOP12 is a negative regulator of root growth (Figure 9). Moreover, we showed that PROSCOOP12 expression is induced by SCOOP12 and PROPEP2 expression is enhanced by SCOOP12 application (Figure 19). These results suggest a functional link between the two peptide families. A collaboration between different peptides has also been described for AtPep1 and PIP1 which act cooperatively to amplify triggered immunity. However, the SCOOP12 peptide has a similar effect on root growth inhibition like AtPep1 but still seems to have additional effects on roots. The observed radial swollen root tips phenotype is commonly reported to be caused by salt, drought and osmotic stress [START_REF] Burssens | Expression of cell cycle regulatory genes and morphological alterations in response to salt stress in Arabidopsis thaliana[END_REF][START_REF] Ji | PEG-mediated osmotic stress induces premature differentiation of the root apical meristem and outgrowth of lateral roots in wheat[END_REF]. In case of these stresses, the root tip swelling is connected to a decrease of cell cycle activity in the root meristem. The decrease appears in the zone with CycB1 expression.

CycB1 is associated with actively dividing cells [START_REF] Ferreira | Developmental expression of the arabidopsis cyclin gene cyc1At[END_REF]. This indicates a shrinkage of the meristematic region. The short peptide RGF (ROOT GROWTH FACTOR) was shown to be an important factor for the maintenance of the root stem cell niche by restoring CycB1 expression in a mutant with reduced root meristem size and loss of coordination between cell elongation and expansion in the elongation-differentiation zone. It is possible that the SCOOP peptide family members negatively regulate the expression of RGF. It would be interesting to apply the peptides to the CycB1 reporter line to further investigate the lateral swollen root tip phenotype. Moreover, the strong secondary root growth could be explained by a crossregulation of other peptide families with SCOOP peptides. Most of the RGF/GLV/CLEL gene family members are expressed during lateral root development [START_REF] Fernandez | Transcriptional and functional classification of the GOLVEN/ROOT GROWTH FACTOR/CLE-like signaling peptides reveals their role in lateral root and hair formation[END_REF]. The overexpression of many of these gene family members results in a reduction of lateral root growth [START_REF] Delay | Regulation of Arabidopsis root development by small signaling peptides[END_REF]. It is possible that SCOOP peptides have expression promoting effect on certain members of these peptide families, like we could show for PROPEP2 (Figure 12). It would be interesting to investigate to transcription expression level of members of these peptide families after SCOOP peptide treatments. A strong pigmentation of plant tissue is usually observed during various stress conditions. Often the darkening of leaf tissue is connected to the accumulation of anthocyanin. In many plants the anthocyanin accumulation is induced by biotic and abiotic stresses [START_REF] Brosché | Molecular events following perception of ultraviolet-B radiation by plants[END_REF][START_REF] Li | Anthocyanin accumulation and changes in CHS and PR-5 gene expression in Arabidopsis thaliana after removal of the inflorescence stem (decapitation)[END_REF]. However, our observations were not made in shoot tissue and Arabidopsis roots usually do not show an accumulation of anthocyanin. However, a pigmentation of roots could be observed by lowsulfur conditions in Arabidopsis [START_REF] Jackson | Large Cellular Inclusions Accumulate in Arabidopsis Roots Exposed to Low-Sulfur Conditions[END_REF]. For the moment we cannot explain the root pigmentation. We cannot exclude a possible interference of SCOOP peptides with the uptake and metabolization of molecules like sulfur. Moreover, the exact localization of cells which show the pigmentation will require further investigations. In conclusion, we show that the PROSCOOP family contains several peptides with defense activating properties.

Moreover, these peptides can have remarkable effects on root development and phenotype.

The SCOOP peptides could therefore bridge the gap between small peptides involved in plant developmental processes and peptides mainly serving as DAMPs.

Material and methods

Plant material

Plant material used was wild-type Arabidopsis thaliana L. Heynh cultivar 6 Columbia (Col-0) as well as bak1-5 bkk1-1 as described by [START_REF] Roux | The Arabidopsis leucine-rich repeat receptor-like kinases BAK1/SERK3 and BKK1/SERK4 are required for innate immunity to hemibiotrophic and biotrophic pathogens[END_REF] were grown under short day conditions (photoperiod of 8h light at 22°C/16h dark at 21°C, with 70% of relative humidity).

Plants used for expression analyzation were grown under long day conditions (photoperiod of 16h light at 22°C/8h dark at 21°C, with 60% relative humidity).

Elicitor peptides

SCOOP elicitor peptides were used as described in table 2. As well as AtPep1 (ATKVKAKQRGKEKVSSGRPGQHN) and flg22(QRLSTGSRINSAKDDAAGLQIA)

Seedling growth inhibition assay

Seedlings were germinated on MS agar and grown for 5 days before transferring one seedling per well to 24 well plates containing 500 µl MS media or MS media supplied with the indicated elicitor peptide to a final concentration of 1 µM (six replicates per elicitor peptide treatment).

Photos were taken, fresh weight and root length were measures after 8 additional days.

ROS measurements

For ROS assays leaf discs of three weeks old soil grown plants, were placed into each well of a white 96-well plate (Thermo Scientific, Waltham, USA) in 0,1 ml of water and kept in the dark overnight. For elicitation and ROS detection, horseradish peroxidase and luminol were added to a final concentration of 10 µg ml -1 and 100 µM, respectively. Luminescence was measured directly after addition of elicitor peptides in a FLUOstar OPTIMA plate reader (BMG LABTECH, Offenburg, Germany).

Determination of expression

Detached leaves of three weeks old plants were collected and floated for two hours in water supplied with the indicated elicitor or control solution. After the treatment, material was frozen and ground in liquid nitrogen. RNA from 100 mg of tissue was extracted using the NucleoSpin RNA plant extraction kit (Macherey-Nagel Hoerdt, France). The DNase treatment was performed according to the manufacturer's recommendations. Per PCR reaction, complementary DNA was synthesized from 10 ng of total RNA extract with oligo(dT) primers using Moloney Murine Leukemia Virus Reverse Transcriptase according to the manufacturer's instructions (Promega). For quantitative real-time reverse transcription PCR (qPCR) in a 96well format, the Chromo4™ System (Bio Rad) was used. Expression was normalized to that of the gene ACR12 (AT5G04740) using the qGene protocol (Muller et al., 2002). All the genespecific primers used are detailed in and in transcriptional reprogramming. A lot of work was done on the influence of chromatin modifications and remodeling of defense-related genes [START_REF] Berr | Chromatin modification and remodelling: a regulatory landscape for the control of Arabidopsis defence responses upon pathogen attack[END_REF]. However, most studies focused on epigenetic modifications initiated by pathogen perception and hormone homeostasis changes upon defense signaling but less on epigenetic changes upon perception and signaling of small endogenous peptides. One of the widely discussed epigenetic change involved in plant resistance is the modulation of the chromatin structure. The histone tails can undergo diverse reversible post-transcriptional histone modifications. These modifications include acetylation, methylation, phosphorylation, ubiquitination, sumoylation, carbonylation and glycosylation [START_REF] Kouzarides | Chromatin modifications and their function[END_REF]. The modifications can directly modulate the chromatin structure or promote the recruitment of specific effectors which determines the function of the chromatin modification and their functional outcome [START_REF] Yun | Readers of histone modifications[END_REF]. Depending on their targets, histone methylation and/or ubiquitination can activate or repress transcription. One of the most intensely studied chromatin marks connected with gene repression is H3K27me3 [START_REF] Roudier | Integrative epigenomic mapping defines four main chromatin states in Arabidopsis[END_REF]. Next to the influence of chromatin marks also the involvement of ATP dependent chromatin-remodeling enzymes as contributors to fast and reversible and/or heritable gene expression control is currently in focus of research in the plant innate immunity [START_REF] Berr | Chromatin modification and remodelling: a regulatory landscape for the control of Arabidopsis defence responses upon pathogen attack[END_REF]. The research on chromatin modifications and plant immunity focus mostly on histone modifications of hormonal pathways like salicylic acid and the jasmonic/ethylen mediated plant defense signaling. These signaling pathways are part of the multilayer defense system of plants, consisting of PTI and ETI responses, leading to SAR. The well-defined markers for SA-mediated basal and R genemediated defense signaling PR1 and PR2 genes are among the most intensely investigated genes regarding chromatin modifications. Several studies have shown that the activation of PR1 is correlated with an increase in the level of acetylated histones at the PR1 locus in Arabidopsis and tobacco [START_REF] Butterbrodt | Chromatin immunoprecipitation analysis of the tobacco PR-1a-and the truncated CaMV 35S promoter reveals differences in salicylic acid-dependent TGA factor binding and histone acetylation[END_REF][START_REF] Mosher | A comprehensive structure-function analysis of Arabidopsis SNI1 defines essential regions and transcriptional repressor activity[END_REF][START_REF] Koornneef | Histone modifications do not play a major role in salicylate-mediated suppression of jasmonate-induced PDF1.2 gene expression[END_REF]. The resulting induction of SAR by increased PR gene expression is often linked to priming for stronger activation of various defense responses that are induced following the attack by pathogens [START_REF] Van Hulten | Costs and benefits of priming for defense in Arabidopsis[END_REF]. Moreover, priming of innate immunity is correlated with chromatin modification of the promotor region of WRKY transcription factors as well as SA and PTI correlated genes [START_REF] Jaskiewicz | Chromatin modification acts as a memory for systemic acquired resistance in the plant stress response[END_REF]Luna et al., 2011;[START_REF] Po-Wen | Priming of the Arabidopsis pattern-triggered immunity response upon infection by necrotrophic Pectobacterium carotovorum bacteria[END_REF]. Regarding peptide triggered immunity much less is known. Recently it was shown that the histone methyl transferase SET DOMAIN GROUP (SDG8) and SDG25 regulate AtPep1 and flg22 triggered immunity, ETI as well as SAR. These two genes affect global and locusspecific H3K4 and H3K36 methylation leading to the regulation of plant immunity genes [START_REF] Lee | Global Regulation of Plant Immunity by Histone Lysine Methyl Transferases[END_REF]. In order to increase the knowledge of the influence of chromatin modification on small endogenous peptides we wanted to investigate the distribution of the chromatin mark H3K27me3 at PROSCOOP12 and PROSCOOP7. We choose these two members of the SCOOP family because we had a first identification, by chromatin immunoprecipitation sequencing (ChIP-seq) on WT seedlings, that these two genes might have an increased level of H3K27me3. In summary we show the influence of treatments with the recently discovered peptide SCOOP12, on the chromatin mark H3K27me3. Here we focus on the H3K27me3 chromatin mark present on the SCOOP peptide family members PROSCOOP7 and PROSCOOP12.

Results and discussion

After we investigated the effects of the SCOOP peptides on plant immunity as well as the effect on root growth, we were interested in the expression regulation of PROSCOOP. We showed in chapter 5 that PROSCOOP7 and PROSCOOP12 expression was enhanced by treatments with SCOOP12 (Figure 19). Next, we investigated possible mechanisms involved in the feedback loop of the small peptide to its putative precursor. Our in-house ChIP-seq of H3K27me3 data on untreated WT seedlings revealed that this chromatin mark is widely distributed over SCOOP family members (Figure 26A). Especially in the promotor regions of PROSCOOP1, 6, 7 and 8 as well as in the gene body of PROSCOOP12 a clear peak of H3K27me3 is noticeable. The genes PROSCOOP2 and 3 are not annotated but present between PROSCOOP1 and PROSCOOP4 (Figure 26A). The H3K27me3 peak on the first exon of PROSCOOP12 and the peak in the promotor of PROSCOOP7 raised our interest. In order to investigate the changes of H3K27me3 level upon treatments we designed primers covering the DNA sequences of the H3K27me3 peaks on PROSCOOP7 and PROSCOOP12. On PROSCOOP12 we designed three primer sets. Primer set 1 amplifies in the promotor of PROSCOOP12, while set 2 amplifies a region directly covering the H3K27me3 peak in the first exon and the set3 a region in the second exon (Figure 26B). The primer sets 4 and 5 amplify regions at or before PROSCOOP7. Set 4 at the H3K27me3 peak in the promotor and set5 after the peak in the first exon of PROSCOOP7 (Figure 26C). Next, we decided to treat Arabidopsis plants with SCOOP12. The treatments were applied with the same experimental setup than used in Figure 19. Chromatin of SCOOP12 treated and untreated control plants was extracted.

After, an immunoprecipitation (IP) with a specific antibody for the H3K27me3 chromatin mark was performed. A real-time PCR of input and IP samples with the primer sets was performed and normalized to the expression of 18S. Our results show that H3K27me3 is increased by SCOOP12 treatment at PROSCOOP12 at the primer sets 2 and 3 as well as at set 4 on PROSCOOP7 (Figure 26DE). On PROSCOOP12 at primer set 2 and 3 H3K27me3 is weakly increased by a fold change of 1.5 in comparison to the control (Figure 26D). On PROSCOOP7 set 4 H3K27me3 was found to be doubled upon SCOOP12 treatment (Figure 26E). However, the data presented here are preliminary and miss a repetition and thus only provide a first indication that the expression of PROSCOOP genes might be influenced by specific chromatin marks. The results are rather unexpected since the presence of H3K27me3 in the promotor of genes is related to the repression of expression [START_REF] Roudier | Integrative epigenomic mapping defines four main chromatin states in Arabidopsis[END_REF]. We show that the expression of PROSCOOP7 and 12 is increased by SCOOP12 treatment. However, the H3K27me3 fold change is not strong in comparison to previous studies. It might be possible that the regions at the two genes are not representative and do not contribute to the expression regulation. Moreover, it is possible that other chromatin marks influence the expression of the PROSCOOP genes. In order to further investigate the role of chromatin modifications at PROSCOOP genes it would be necessary to add additional replicates and to test additional chromatin marks such as H3K4me3. Moreover, it would be interesting to investigate the influence of SCOOP12 on the whole genome chromatin marks of H3K27me3 distribution as well as the with gene expression promoting associated chromatin mark H3K4me3. These experiments would provide new insights into the epigenetic regulation of small peptide influenced gene expression.

Material and methods

5.6.3.1.

ChIP

For chromatin extraction we used an adopted protocol from [START_REF] Ikeuchi | PRC2 represses dedifferentiation of mature somatic cells in Arabidopsis[END_REF]. For sonication we used the M220 Focused-ultrasonicator (Covartis USA). Immunoprecipitation was performed with specific H3K27me3 antibodies (ab8580) (Abcam USA).

5.6.3.2.

Real time PCR and specific primer

For quantitative real-time reverse transcription PCR (qPCR) in a 96-well format, the Chromo4™ System (Bio Rad) was used. The relative IP to input values are normalized to that of 18S (AT2G03810). The primer sequences are shown in table 5.

Biotic stress-induced priming and de-priming of transcriptional memory in Arabidopsis and apple

"Without forgetting it is quite impossible to live at all."

Friedrich Nietzsche -German philosopher This chapter contains results of a project I started before we discovered the SCOOP peptides, as described in the previous two chapters. The intention of this project was to uncover the influence of synthetic plant defense inducing compounds on Arabidopsis and apple. Our findings are rather unusual and have the potential to uncover an unknown mechanism of plant memory. My contribution to this chapter is the complete experimental part, data compilation and writing of the first manuscript draft as well as the data analyzation with help of Jean-Marc Celton, Alexandre Degrave, Marie-Noelle Brisset and Etienne Bucher. This chapter will serve as manuscript for a publication.

Abstract

Under natural growth conditions plants experience various and repetitive biotic and abiotic stresses. Salicylic acid (SA) is a key phytohormone involved in the response to biotic challenges. Application of synthetic SA analogues can efficiently prime defense responses and leads to improved pathogen resistance. Because SA analogues can result in long-term priming and memory, aimed at identifying genes with the potential to memorize treatments with an SA analogue and explored the role of DNA methylation in this memorization process. Here, we show that treatments with SA analogues can lead to long-term transcriptional memory of particular genes in Arabidopsis. We found that subsequent challenging of such plants with a bacterial elicitor reverted this transcriptional memory bringing their expression back to the original pre-treatment level. We also made very similar observations in apple (Malus domestica), suggesting that this expression pattern is highly conserved in plants. Finally, we found a potential role for DNA methylation in the observed transcriptional memory behavior.

We show that plants defective in DNA methylation pathways showed a different memory behavior. Our work improves our understanding of the role of transcriptional memory in priming and has important implication concerning the application of SA analogues in agricultural settings.

Introduction

Plants are under continuous attack by pathogens because they are rich sources of nutrients.

However, they protect themselves by physical barriers, such as a waxy cuticular layer or by protective periderm. In addition to these barriers plants have evolved an immune system comprising constitutive and inducible defenses. The inducible immune system is based on the specific recognition of pathogen-derived molecules [START_REF] Chisholm | Host-microbe interactions: shaping the evolution of the plant immune response[END_REF]Jones and Dangl, 2006a). This so-called pattern-triggered immunity (PTI) is achieved by plasma membrane localized pattern-recognition receptors (PRRs) which directly interact with highly conserved pathogen/microbe -associated molecular patterns (PAMPs/MAMPs) (Gomez-Gomez and Boller, 2002;[START_REF] Zipfel | Bacterial disease resistance in Arabidopsis through flagellin perception[END_REF]. One of the best-studied examples of a MAMP, that is capable of activating plant immunity, is the bacterial flagellin which is the major component of the bacterial motility organ [START_REF] Macnab | How bacteria assemble flagella[END_REF]. The perception system of flagellin is widely conserved across the plant kingdom since most plants respond to flagellin [START_REF] Felix | Plants have a sensitive perception system for the most conserved domain of bacterial flagellin[END_REF].

The fastest MAMP-triggered defense responses are typically induced within 5 min and decrease within a 30min time frame [START_REF] Maffei | Before gene expression: early events in plant-insect interaction[END_REF]. These events include the following plant responses: apoplastic alkalinisation, burst of reactive oxygen species (ROS), activation of mitogen-activated protein kinases (MAPKs) and intracellular calcium burst [START_REF] Romeis | Calcium-dependent protein kinases play an essential role in a plant defence response[END_REF]Asai et al., 2002;Boller and Felix, 2009). However, late responses are induced within several hours to days. Late defense responses include the accumulation of callose, the inhibition of growth, the differential expression of defense genes and the production of salicylic acid (SA) (Boller and Felix, 2009). In line with short and long-term defense responses, plants are also capable of inducing long-lasting systemic immunity. By local compatible or incompatible interactions, such systemic immunity can be initiated. This results in systemic acquired resistance (SAR). SAR-like responses can be induced by exogenous application of SA or SA analogues [START_REF] White | ACETYLSALICYLIC-ACID (ASPIRIN) INDUCES RESISTANCE TO TOBACCO MOSAIC-VIRUS IN TOBACCO[END_REF]Metraux et al., 1990;[START_REF] Ward | Coordinate Gene Activity in Response to Agents That Induce Systemic Acquired Resistance[END_REF]. However, SA does not seems to serve as a mobile signal per se inducing immunity in uninfected distal tissues, while several other small molecules have been proposed to fulfil such a role [START_REF] Shah | Long-distance communication and signal amplification in systemic acquired resistance[END_REF].

Defense-related stimuli enhance the capacity of plants to activate defense responses [START_REF] Conrath | Priming: Getting ready for battle[END_REF][START_REF] Beckers | Priming for stress resistance: from the lab to the field[END_REF]. Exogenous application of SA and other benzoic acid derivates have been shown to induce resistance of tobacco against tobacco mosaic virus (TMV) and to cause the accumulation of pathogenesis-related (PR) transcripts [START_REF] White | ACETYLSALICYLIC-ACID (ASPIRIN) INDUCES RESISTANCE TO TOBACCO MOSAIC-VIRUS IN TOBACCO[END_REF]. This discovery paved the way for companies to identify more potent related compounds. These compounds are referred to as synthetic plant defense elicitors [START_REF] Bektas | Synthetic plant defense elicitors[END_REF]. One of the most frequently used elicitors is benzo(1,2,3)thiadiazole-7-carbothioic acid S-methyl ester (BTH), which is commonly named acibenzolar S-methyl (ASM) and which is commercialized by Syngenta [START_REF] Cole | The efficacy of acibenzolar-S-methyl, an inducer of systemic acquired resistance, against bacterial and fungal diseases of tobacco[END_REF][START_REF] Friedrich | A benzothiadiazole derivative induces systemic acquired resistance in tobacco[END_REF]. In Europe this compound is commercialized as "Bion". Application of BTH has been shown to restrain downey mildew infections in vegetables and to control a range of fungal, bacterial and viral diseases of important crops like tomato, cucumber, broccoli, tobacco, melon, pear and apple trees [START_REF] Scarponi | Persistence and translocation of a benzothiadiazole derivative in tomato plants in relation to systemic acquired resistance against Pseudomonas syringae pv tomato[END_REF][START_REF] Zavareh | Effects of Acibenzolar-S-methyl on the specific activities of peroxidase, chitinase and phenylalanine ammonia-lyase and phenolic content of host leaves in cucumber-powdery mildew interaction[END_REF][START_REF] Pajot | Evidence that DL-3-aminobutyric acid and acibenzolar-S-methyl induce resistance against bacterial head rot disease of broccoli[END_REF][START_REF] Jiang | Ultrastructural Study on Acibenzolar-S-Methyl-Induced Scab Resistance in Epidermal Pectin Layers of Japanese Pear Leaves[END_REF]. In apple, it was shown that application can limit the extent of fire blight disease, which is caused by the bacterium Erwinia amylovora [START_REF] Brisset | Acibenzolar-S-methyl Induces the Accumulation of Defense-related Enzymes in Apple and Protects from Fire Blight[END_REF][START_REF] Maxson-Stein | Effect of Treating Apple Trees with Acibenzolar-S-Methyl on Fire Blight and Expression of Pathogenesis-Related Protein Genes[END_REF]. The SARinducing ability of compounds such as BTH is frequently associated with a primed state in which plants are able to `recall´ a previous infection or exposure to stress. Primed plants are therefore capable of responding more rapidly and/or effectively to a subsequent biotic or abiotic stress [START_REF] Conrath | Priming: Getting ready for battle[END_REF]. The molecular mechanisms behind priming are largely unclear. Priming has been associated with the accumulation of post-translational modification of cellular compounds. These compounds have important roles in signal transduction and/or amplification. In general, an accumulation or modification of these compounds does not activate a broad panel of plant's defense responses [START_REF] Conrath | Priming: Getting ready for battle[END_REF]. Epigenetic regulation of gene expression is another widely discussed mechanism involved in defense priming. It has been shown that histone modifications at promotors of defense-related transcription factors such as WRKY6, 53 and 29 contribute to priming of gene expression by BTH [START_REF] Jaskiewicz | Chromatin modification acts as a memory for systemic acquired resistance in the plant stress response[END_REF]). An additional epigenetic regulation mechanism is DNA methylation. In plants, DNA methylation is present in all three possible sequence contexts (CG, CHG, CHH , whereas H is A,T or C) and has been shown to influence defense responses [START_REF] Dowen | Widespread dynamic DNA methylation in response to biotic stress[END_REF][START_REF] López Sánchez | The role of DNA (de)methylation in immune responsiveness of Arabidopsis[END_REF]. DNA methylation in the CG context can be maintained by DNA METHYLTRANSFERASE1 (MET1) and in all sequence contexts, it can be triggered by the RNA-directed DNA methylation (RdDM) pathway. In RdDM, one of the main players is NUCLEAR RNA POLYMERASE D1 (NRPD1), the largest subunit of RNA Polymerase IV (Pol IV), which plays a key role in the initiation of siRNAs production [START_REF] Herr | RNA polymerase IV directs silencing of endogenous DNA[END_REF]. Another emerging regulation of gene expression involves antisense transcripts. It has been previously reported that genes transcribed in sense orientation, can also be transcribed in antisense orientation. Antisense transcripts include partial or complete sequences complementary to other transcripts and are endogenous RNA molecules [START_REF] Wang | Genome-wide prediction and identification of cis-natural antisense transcripts in Arabidopsis thaliana[END_REF]. They play an important role in various processes, including the adaptation to biotic and abiotic stresses [START_REF] Terryn | The sense of naturally transcribed antisense RNAs in plants[END_REF]. Antisense transcripts are widespread in both prokaryotes [START_REF] Wagner | Antisense RNA control in bacteria, phages, and plasmids[END_REF] and eukaryotes [START_REF] Vanhée-Brossollet | Do natural antisense transcripts make sense in eukaryotes?[END_REF]). Here we show that stresses can lead to long lasting transcriptional memory (priming)

and that priming can be reversed (de-priming) by subsequent stresses. Furthermore, our results show that antisense transcripts contribute to transcriptional memory and that DNA methylation is required for proper stress response.

Results

BTH induces short-and long-term defense responses in Arabidopsis.

While it is known that bacterial elicitors such as flg22 induce various defense responses in numerous different plants, less is known about the action of synthetic plant defense elicitors such as BTH. Here, we tested the short and long-term responses of plants treated with BTH.

One of the fastest plant defense response is the accumulation of ROS [START_REF] Bolwell | The apoplastic oxidative burst in response to biotic stress in plants: a three-component system[END_REF].

Therefore, we first tested if a BTH treatment resulted in a detectable ROS burst in Arabidopsis.

We found that BTH applied at a final concentration of 1mM induced a weak albeit significant (p-value < 0.05) ROS burst, whereas a control treatment with flg22 lead to a strong accumulation of ROS (Figure 27). However, it was shown that a lower BTH concentration of 100µM does not induce a significant ROS burst [START_REF] Tateda | Salicylic Acid Regulates <em>Arabidopsis</em> Microbial Pattern Receptor Kinase Levels and Signaling[END_REF]. The increased BTH concentration could explain these different results. (Figure 27A). One of the long-term defense responses is the deposition of callose. As shown by [START_REF] Kohler | Benzothiadiazole-induced priming for potentiated responses to pathogen infection, wounding, and infiltration of water into leaves requires the NPR1/NIM1 gene in Arabidopsis[END_REF][START_REF] Tateda | Salicylic Acid Regulates <em>Arabidopsis</em> Microbial Pattern Receptor Kinase Levels and Signaling[END_REF] BTH induces a deposition of callose in leaf tissue. We also observed that application of BTH lead to a strong accumulation of callose in Arabidopsis leaves (Figure 27B,C). Another long-lasting defense response is growth inhibition. Addition of bacterial elicitors to the growth medium was shown to result in a strong inhibition of growth (Krol et al., 2010). Our results indicate that repetitive spraying with BTH led to similar effects. Seven days after germination (dag) young plants were sprayed with BTH and then treated two additional times at three days intervals (Figure 27D). Plants treated three times with BTH showed a reduction of fresh weight after a recovery phase of 8 days following the last BTH exposure in comparison to the water control (Figure 27E,F). To test the more general effect of BTH on other plants, we choose to investigate the growth repression effect, observed in Arabidopsis, on apple grown under two different growth conditions. Grafted apple plants exposed six times to BTH showed a lower number of internodes in comparison to a mock treatment (Suppl. Figure 28A). Moreover, in-vitro apple plantlets grown for 4 weeks on media supplied with BTH showed a strong inhibition of growth (suppl. Figure 28B).

Transcriptional response and memory resulting from BTH treatment in Arabidopsis

BTH is associated with inducing a primed state of gene expression. In order to identify primed (and memorized) genes resulting from BTH application, we performed a series of microarraybased transcription profiles. Additionally, we investigated the role of a subsequent biotic stress exposure on BTH-induced long term transcriptional memory. To investigate this, we applied a series of treatments by spraying young Arabidopsis plants 7 dag. With a first application we primed gene expression with BTH. After a 3 days recovery phase, 10 days old plants were treated with the bacterial elicitor flg22. 11 days following the last treatment, plants were harvested and RNA extracted for subsequent gene expression profiling using two independent replicates (Figure 29A). Four different treatment combinations were used: A control treatment where plants were sprayed two times with water (w) (sample name: ww).

Three combinations whereby either BTH (b) or water (w) was used in the first treatment and water or flg22 (f) in the second treatment (sample names: bw, wf and bf, respectively, Figure 29B). Gene expression changes induced by these treatment combinations were assessed using microarrays. The long-term effect of flg22 or BTH was investigated by comparing these samples against ww (ww Vs. wf and ww Vs. bw, respectively). To determine the effect of flg22

after previous BTH application we compared the samples bw against bf. The effect of BTH before a subsequent flg22 application was determined by comparing wf against bf. The impact of both compounds in succession was studied by comparing ww against bf (Figure 29C). In order to facilitate this complex analysis, we decided to sort differently expressed transcripts (DETs, which include antisense transcripts) by using a binary code. For that purpose, every DET was assigned to a four digits number code. These four digits represent the examined microarray comparisons. The first position in the number code represents the comparison ww Vs. wf, the second ww Vs. bw, the third bw Vs. bf and the forth the comparison wf Vs. bf. A 1 in the binary code indicates the presence of DETs in the corresponding comparison, while a 0 the absence (Figure 29D). This binary code allowed us to provide a global overview of the different comparisons. By applying this binary code, we sorted all DETs into 16 categories (Figure 29E). The category with the binary code 1000 contains transcripts that are exclusively differently expressed in the comparison ww Vs. wf and not in any other comparison. The category 1100 contains genes that are differently expressed in the comparisons ww Vs. wf and ww Vs. bw but not in the other two comparisons.

Transcripts that are differently expressed in all four comparisons can be found in the category 1111 (Figure 29D andE). This categorization of DETs revealed that a substantial number of transcripts are only differently expressed in one comparison and not in any of the others.

Indeed, the categories 1000, 0100, 0010, 0001 contained between 2107 DETs (for 0010) and 4428 DETs (for 0001), respectively. 110 transcripts were found to be differently expressed in all four comparisons (Figure 29E, category 1111). Next, we investigated the impact of subsequent stresses on BTH-induced transcriptional priming. The category 0110 contains genes that are differently expressed by applying BTH alone and by applying flg22 after a previous BTH exposure (ww Vs. bw and bw Vs. bf, respectively). 541 DETs were found in this category. The expression profile of these transcripts caught our attention because it showed that transcripts that were initially up-regulated by BTH were down-regulated by a subsequent flg22 exposure. Conversely, transcripts that were first down-regulated by BTH were up-regulated by subsequent flg22 treatment (Figure 31A).

Only 2.6% of all transcripts in this category show a transgressive expression profile (Figure 31B). This suggested that subsequent stresses could erase the primed state of certain DETs.

Therefore, we investigated how many of those 0110 DETs went back to a basal expression level. Because the application of BTH is closely related to the priming of genes [START_REF] Katz | A benzothiadiazole primes parsley cells for augmented elicitation of defense responses[END_REF][START_REF] Kohler | Benzothiadiazole-induced priming for potentiated responses to pathogen infection, wounding, and infiltration of water into leaves requires the NPR1/NIM1 gene in Arabidopsis[END_REF] (and this study) we tested if a subsequent flg22 treatment lead to a de-priming of genes. Here, we define de-priming as a primed transcript expression returning to a basal (pretreatment-like) expression level by a subsequent stress. To assess the number version of the apple genome [START_REF] Daccord | Highquality de novo assembly of the apple genome and methylome dynamics of early fruit development[END_REF]. Plants were treated 14 days after propagation (dap) and 17 dap. After 14 days of recovery plantlets were harvested, RNA extracted and subjected to transcription profiling (Figure 33A). In total 3 different treatment combinations, on two independent biological replicates were applied (Figure 33B). Similar to our experiments in Arabidopsis we explored the effect of BTH as a first treatment (ww Vs. bw)

as well the influence of flg22 application after BTH treatment (bw Vs. bf). In order to identify DETs that returned to a basic expression level, we investigated the effect of a combination of BTH and flg22 treatment on apple plants (ww Vs. bf) (Figure 33C). In the comparison ww Vs. The plot of the 342 common DETs confirmed our observations in Arabidopsis (Figure 34A). In total, 82.7% of these transcripts are de-primed and only 8,2% are still significantly differently expressed and therefore primed (Figure 34B). 9.1% of the transcripts are transcribed in a transgressive way (Figure 34B). Next, we determined how many transcripts lost their priming status by comparing the common DETs with the comparison ww Vs. bf (Figure 34C). However, for apple an overrepresentation of antisense transcripts such as we have seen for Arabidopsis was not observed. The subcategory of common DETs which are up regulated by BTH showed 47.3% antisense transcripts and DETs down regulated by BTH are expressed with a 57.1%

antisense contribution (Figure 34C). In total, common DETs have a higher content of antisense transcripts than found in our Arabidopsis comparisons. This observation is in line with a previous report that apple has globally a high percentage of antisense transcripts [START_REF] Celton | Widespread anti-sense transcription in apple is correlated with siRNA production and indicates a large potential for transcriptional and/or post-transcriptional control[END_REF].

DNA methylation and de-priming of gene expression

To investigate the mechanism behind the de-priming of genes by a second stress, we chose to follow the expression profile of AMY1 (At4G25000), a gene found in the Arabidopsis subcategory 0110 up by BTH. To enhance the contrast resulting from the treatments on Arabidopsis we added a third treatment at 13 dag and harvested at 21 dag (Figure 35A). AMY1

has previously been shown to be induced by biotic and abiotic stress and play an important role in starch metabolism [START_REF] Stanley | Characterisation of putative α-amylases from apple (Malus domestica) and Arabidopsis thaliana[END_REF][START_REF] Doyle | An alphaamylase (At4g25000) in Arabidopsis leaves is secreted and induced by biotic and abiotic stress[END_REF]. Again, to be able to measure the long-term memory on AMY1 transcription, sampling was performed eight days after the last treatment. In the sample bbf AMY1 was found to be significantly down-regulated in comparison to the bbw treatment showing that we can follow de-priming by qPCR. Three BTH treatments (bbb) further enhanced the expression of AMY1 in comparison to bbw and bbf (Figure 35B). However, we could not detect a significant differential expression of AMY1 in the sample bww and bfw in comparison to the water control treatment. The expression profile of AMY1 confirms the previous observations with respect to the enhanced expression of this gene by BTH and de-priming upon a subsequent flg22 exposure after BTH. Therefore, we used AMY1 as a marker gene to investigate the molecular mechanisms involved in depriming gene expression. To test if DNA methylation was involved in the memory or depriming process, we applied the same combinations of treatments to nrpd1-3 and met1-3

plants. For nrpd1-3, the expression profile is similar to that of wild type plants. However, the amplitude is four times lower for all samples (Figure 35C). The met1 mutant did not show a significant response to any of the applied treatments, suggesting that AMY1 activation and priming may directly or indirectly depend on DNA methylation.

Discussion

BTH could have negative effects on plant vitality in an energy trade-off balance

In their natural environment plants are continuously exposed to a multitude of variable stresses. In an agricultural setting, plants such as apple need to be protected from various diseases with pesticides since the lack thereof may be causing tremendous declines in yield.

In order to reduce the use of pesticides, compounds are now being developed that can enhance natural pathogen defense mechanisms in plants. Because BTH can contribute to defense priming [START_REF] Jaskiewicz | Chromatin modification acts as a memory for systemic acquired resistance in the plant stress response[END_REF][START_REF] Conrath | Priming for enhanced defense[END_REF] (and this work) it could therefore have the potential to reduce yield loses. Here, we show that the application of BTH induces callose depositioning and a ROS burst in Arabidopsis (Figure 27A,B). While other reports did not see this [START_REF] Tateda | Salicylic Acid Regulates <em>Arabidopsis</em> Microbial Pattern Receptor Kinase Levels and Signaling[END_REF], this might be due to the tenfold higher BTH concentration of 1mM final concentration that was used in our assay. The induction of defense responses is widely connected to loss of energy in a trade-off balance (Walters and Heil, 2007). In line with the results we show here, the application of BTH induced an inhibition of growth in both, Arabidopsis and apple plants ((supp.) Figure 28). These results indicate that BTH application at a higher concentration could have negative effects on plant growth.

De-priming of transcription is tightly regulated

With the series of microarray experiments performed here, we found that flg22 and BTH treatments globally result in similar transcriptional changes in Arabidopsis. The category 11XX (Figure 30) contains transcripts that are differently expressed by flg22 and BTH treatments, these transcripts show the same trend in expression as well as sense and antisense transcript distribution. BTH is an analogue of SA that is naturally produced by the plant [START_REF] Bektas | Synthetic plant defense elicitors[END_REF]. SA is an important plant hormone that plays a role in the signaling pathway following flg22 perception [START_REF] Yi | The activated SA and JA signaling pathways have an influence on flg22-triggered oxidative burst and callose deposition[END_REF]. Therefore, it is reasonable that common deregulated transcripts resulting from the two different treatments globally show a similar expression profile in Arabidopsis. We then tested how a subsequent stress affects BTHinduced transcriptional memory. We show that, a flg22 treatment after BTH reverses the transcriptional memory in Arabidopsis (Figure 31A) and in apple (Figure 34A) of certain DETs.

We termed such effects de-priming. It has previously been shown that BTH pre-treatments resulted in increased transcription of a subset of genes upon a second stress [START_REF] Conrath | Molecular aspects of defence priming[END_REF][START_REF] Jaskiewicz | Chromatin modification acts as a memory for systemic acquired resistance in the plant stress response[END_REF]. Indeed, a similar de-priming phenomenon was observed for genes responding to repetitive drought stress. [START_REF] Liu | Different gene-specific mechanisms determine the 'revised-response' memory transcription patterns of a subset of A. thaliana dehydration stress responding genes[END_REF] showed that a subset of dehydration stress-response genes reacted to a first stress but did not respond to a second stress and stayed at a basic non-stressed expression level. These genes are referred as `revisedresponse` memory genes. Together with a follow-up work it was shown that the transcription factor MYC2 plays a critical role for gene activation upon a second drought stress [START_REF] Liu | Different gene-specific mechanisms determine the 'revised-response' memory transcription patterns of a subset of A. thaliana dehydration stress responding genes[END_REF]Liu et al., 2016). These results indicate that the regulation of de-priming in plants could depend on precise regulation of such a single gene. With our work we show that the number of de-primed transcripts by a subsequent exposure to a second biotic stress reflects the majority of commonly regulated transcripts (Figure 31B, Figure 34B). It is remarkable that we only find 2.6% of DETs in the Arabidopsis category 0110 and 9.6% of DETs for apple which show a transgressive expression profile and do not follow the global trend of de-priming (Figure 31B,34B). Therefore, the number of transgressive DETs is low. Notable is also that the total number of DETs of the comparison bw Vs. bf in Arabidopsis and apple respectively: this comparison represents the lowest number of DETs in comparison to all other examined microarrays of both plant species (Figure 29C,33C). This indicates that the global level of transcription is reduced by the subsequent flg22 treatment after BTH exposure.

DNA methylation could contribute to the priming properties of BTH

Two plant memory mechanisms have been proposed: The accumulation of proteins or transcription factors is one of them, epigenetic changes is a second potential mechanism [START_REF] Bruce | Stressful "memories" of plants: Evidence and possible mechanisms[END_REF]. Our results show a strong effect on priming and de-priming by global methylation decrease. We identified AMY1 in the Arabidopsis category 0110 as a good marker for transcriptional memory because: (1) It has been shown that AMY1 is a secreted protein that is expressed following biotic and abiotic stresses [START_REF] Doyle | An alphaamylase (At4g25000) in Arabidopsis leaves is secreted and induced by biotic and abiotic stress[END_REF], ( 2) it shows a memory accumulating property following multiple treatments (Figure 35B) and (3) of Its depriming property upon a succession of BTH and flg22 treatments (Figure 35B). Notably, we found that AMY1 activation is strongly reduced in met1-3 only being activated after three subsequent BTH applications. This suggests that DNA methylation may be required directly or indirectly to activate and/or prime AMY1. However, due to the weak activation of AMY1 in met1-3, the de-priming effect of flg22 after BTH treatment could not be observed with this method (Figure 35D). We thus cannot conclude on whether MET1 is required for the activation or the maintenance of the primed state of AMY1. In nrpd1-3 we found a low expression value but still detected the de-priming expression profile, indicating that DNA methylation via the RdDM pathway may be required to achieve the full potential of BTH treatments. Therefore, DNA methylation may be contributing to the priming and de-priming events which will have to be more closely investigated in the near future. to a basic expression level (dashed green line).

De-priming could limit the impact of sequential stresses

It was shown that treatment with BTH increases the expression of the flg22 receptor FLS2 as well as the closely related co-receptor BAK1 [START_REF] Tateda | Salicylic Acid Regulates <em>Arabidopsis</em> Microbial Pattern Receptor Kinase Levels and Signaling[END_REF]. An increased expression of BAK1 was shown to have strongly reduced growth and extensive cell death [START_REF] Domínguez-Ferreras | An Overdose of the Arabidopsis Coreceptor BRASSINOSTEROID INSENSITIVE1-ASSOCIATED RECEPTOR KINASE1 or Its Ectodomain Causes Autoimmunity in a SUPPRESSOR OF BIR1-1-Dependent Manner[END_REF]. It might be possible that the subsequent flg22 exposure after BTH could fine tune the plant expression profile to prevent negative effects of BTH and to enhance the resistance of the plant. The gene ontology analysis of the category 0110 in our Arabidopsis comparisons revealed that most DETs belong to cellular and metabolic processes ((Suppl.)

Figure 28). Indicating, that changes in the expression profile not only effects transcripts related with defense responses. In Figure 36 we propose a model of the effect of expressional de-priming. We hypothesize that the de-priming of a subset of transcripts could enhance plant fitness. The second treatment and the resulting de-priming of transcripts could prevent the plants from negative effects of the priming treatment and/or fine tune the plant response to the more recent stress. It was shown with several examples that plants deal with stressful situations by inducing silencing mechanisms via antisense transcription and that endogenous siRNAs derived from a pair of sense and antisense transcript can enhance the tolerance to various stresses [START_REF] Borsani | Endogenous siRNAs derived from a pair of natural cis-antisense transcripts regulate salt tolerance in Arabidopsis[END_REF][START_REF] Chinnusamy | Cold stress regulation of gene expression in plants[END_REF][START_REF] Jin | Small RNAs and the regulation of cis-natural antisense transcripts in Arabidopsis[END_REF][START_REF] Sunkar | MicroRNAs with macro-effects on plant stress responses[END_REF][START_REF] Khraiwesh | Role of miRNAs and siRNAs in biotic and abiotic stress responses of plants[END_REF] The strong overrepresentation of antisense transcripts in the Arabidopsis sub-category 0110 up by BTH is in line with our hypothesis, that plants induce depriming in order to prevent negative or unnecessary effects of the priming stimulus. These antisense transcripts are up-regulated by BTH and down-regulated by the subsequent flg22

treatments. Therefore, it may be that enhanced antisense expression suppresses negative effects that the BTH treatment may have on the plant. With our work we describe a rather unusual expression profile caused by subsequent stresses in Arabidopsis and apple in which plants have an efficient way to memorize stresses by transcriptional priming, and that such priming can readily be erased (de-primed) by subsequent stresses. Since our observations were made in both apple and Arabidopsis suggests that priming and de-priming may be conserved in the plant kingdom. These results have implications on the application of priming compounds in the field, as plants are constantly subjected to stresses, which may affect the efficiency of such compounds.

Material and methods

Plant material

Plant material used was wild-type Arabidopsis thaliana L. Heynh cultivar 6 Columbia (Col-0)

as well as met1-3 [START_REF] Saze | Maintenance of CpG methylation is essential for epigenetic inheritance during plant gametogenesis[END_REF] and nrpd1-3 [START_REF] Herr | RNA polymerase IV directs silencing of endogenous DNA[END_REF]. All Arabidopsis plants were grown under long day condition (photoperiod of 16h light at 22°C/ 8h light at 22°C, with 60% relative humidity). Apple cv. Golden Delicious double haploid (GDDH13 described by [START_REF] Lespinasse | Haploidy in apple and pear[END_REF]) in-vitro grown plantlets were propagated on MS based medium with BA 0.25mg/L and IBA 0.1mg/l under short day conditions (photoperiod of 8h light at 22°C/ 16h dark at 21°C). Grafted apple plants (GDDH13) were grown in the greenhouse on a MM106 rootstock. Dormant buds were grafted in winter (February). After 10 days in a cold chamber they were potted in 1L pots and grown in normal greenhouse conditions until they developed to about 30 nodes.

Quantification of the growth inhibiting effect of BTH

The growth inhibiting effect of BTH (Bion, Syngenta) was examined in grafted apple plants.

Two months after grafting the plants were treated six times every three days with BTH (1mM) by applying the solution with a paintbrush on all leaves and the meristem. The total number of internodes was counted 3 weeks after the first treatment. The growth inhibiting effect of BTH on apple plantlets was observed by growing plantlets after propagation on MS based medium with 0.25mg/L BA, 0.1mg/L IBA and 1µM BTH or the control media without BTH for four weeks before the pictures were taken.

Transcriptomic analysis

Four Arabidopsis plants represent one biological replicate. Plants were treated with flg22

(QRLSTGSRINSAKDDAAGLQIA -obtained from Eurogentec SA (Angers, France)) or BTH (Bion)

-obtained from Syngenta (Basel, Switzerland) and were applied by spraying with final concentrations of 1µM and 1mM, respectively. Leaves of 20 apple plantlets represent one biological replicate. Plantlets were treated by dipping the whole plantlet into filter sterilized flg22 (1µM) and BTH(1mM) solution and placed on fresh growth medium after every treatment. Microarray analysis was performed for Arabidopsis with the CATMA array ((V5) GPL21364)) and for apple with the ARIANE array (VXXX) Leaves of Arabidopsis and apple were collected from two independent biological replicates. The RNA was extracted using the NucleoSpin RNA plant extraction kit (Machery-Nagel Hoerdt, France) according the manufacturer`s recommendations. For Arabidopsis samples the Message AmpII aRNA amplification kit (Ambion) (Thermo fisher scientific USA) and for apple samples the Low Input Quick Amp Labeling Kit, two-color (Agilent USA) was used for cDNA synthesis and hybridization. The hybridizations were performed on a NimbleGen Hybridization System 4 (mix mode B) at 42° overnight. Afterwards, the slides were washed, dried and scanned at 2 µm resolution. NimbleGen MS 200 v1.2 software was used for microarray scans and the Agilent Feature Extraction 11.5 software was used to extract pair-data files from the scanned images. Statistical analysis was based on dye switch approach as described in [START_REF] Depuydt | An integrated genomics approach to define niche establishment by Rhodococcus fascians[END_REF]. All statistical analyses were performed using the R language (R Development Core Team, 2009); data were normalized with the lowess method, and differential expression analyses were performed using the lmFit function and the Bayes moderated t test using the package LIMMA [START_REF] Smyth | Use of within-array replicate spots for assessing differential expression in microarray experiments[END_REF] from the Bioconductor project. Differently expressed transcripts sorted by applying the binary code were selected for a p-value < 0.05.

Determination of gene expression by qPCR

4 plants per biological replicates were harvested, frozen and ground in liquid nitrogen. RNA from 100 mg of tissue was extracted using the NucleoSpin RNA plant extraction kit (Macherey-Nagel Hoerdt, France). The DNase treatment was performed according to the manufacturer's recommendations. Per PCR reaction, complementary DNA was synthesized from 10 ng of total RNA extract with oligo(dT) primers using Moloney Murine Leukemia Virus Reverse

Transcriptase according to the manufacturer's instructions (Promega). For quantitative realtime reverse transcription PCR (qPCR) in a 96-well format, the Chromo4™ System (Bio Rad) was used. Expression was normalized to ACR12 (AT5G04740) using the qGene protocol (Muller et al., 2002). The Primers used are as followed: ACR12 (AT5G04740) acr12FW:

TTGTTCGATGATCGCCGGAA, acr12REV: TGGAACAACGTCGTCATCGT; AMY1 (At4G25000) amy1FW: AATACGGTTCAGAGGCGGAA, amy1REV: CGGAAGTCCCACCTTCGAAA.

Measurement of reactive oxygen species

For ROS assays, leaf discs of three weeks old soil grown plants, were placed into each well of a white 96-well plate (Thermo Scientific, Waltham, USA) in 0.1 mL of water and kept in the dark overnight. For elicitation and ROS detection, horseradish peroxidase and luminol were added to a final concentration of 10 µg/mL and 100 µM, respectively. Luminescence was measured directly after addition of concentrated BTH solution (final concentration of 1mM) in a FLUOstar OPTIMA plate reader (BMG LABTECH, Offenburg, Germany).

Callose deposition

Leaf discs were vacuum infiltrated for 10 min with BTH (1mM) solution or water and kept floated in the solution for 24h. After leaf discs were fixed and destained in 1:3 acetic acid/ ethanol until leaf tissue was completely transparent. After washing the leaf discs in 150 mM

Appendix: Does BTH/flg22 application influence defense responses and

Pseudomonas resistance in a long-time memory fashion? 6.6.1. Introduction

Plants are under continuous stress but higher plants have evolved mechanisms that enables them to cope with environmental changes. This mechanism was established over a long period of evolution as sessile organisms [START_REF] Tardif | Interaction network of proteins associated with abiotic stress response and development in wheat[END_REF]. The mechanism of priming enables plants to respond and adopt to recurring biotic and abiotic stresses. Primed plants display faster and/or stronger activation of various defense responses that are induced by a subsequent attack by pathogens, insects or in response to abiotic stress [START_REF] Conrath | Priming: Getting ready for battle[END_REF].

The activation of systemic acquired resistance (SAR) is key for the plant to overcome a pathogen infection. Pre-treatment with various stress-signaling molecules like JA, SA, or ABA as well as, pre-exposure to pathogens or herbivores resulted in an increased SAR [START_REF] Goh | Stress memory in plants: a negative regulation of stomatal response and transient induction of rd22 gene to light in abscisic acidentrained Arabidopsis plants[END_REF][START_REF] Jakab | Enhancing Arabidopsis salt and drought stress tolerance by chemical priming for its abscisic acid responses[END_REF][START_REF] Conrath | Priming: Getting ready for battle[END_REF][START_REF] Bruce | Stressful "memories" of plants: Evidence and possible mechanisms[END_REF][START_REF] Conrath | Molecular aspects of defence priming[END_REF][START_REF] Rasmann | Herbivory in the previous generation primes plants for enhanced insect resistance[END_REF][START_REF] Slaughter | Descendants of primed Arabidopsis plants exhibit resistance to biotic stress[END_REF][START_REF] Bruce | Variation in plant responsiveness to defense elicitors caused by genotype and environment[END_REF]. SAR involves several steps and multiple pathways that lead to a strong defense response [START_REF] Conrath | Systemic acquired resistance[END_REF]. SAR induction and an increased resistance to a subsequent infection can be achieved by treatments with chemical stimuli. These compounds induce low-cost changes in the plant metabolism that include the accumulation of various metabolites [START_REF] Mauch-Mani | Defense Priming: An Adaptive Part of Induced Resistance[END_REF]. One of these compounds is the SA analogue BTH. BTH is the main active compound of the commercialized plant defense inducing product Bion [START_REF] Görlach | Benzothiadiazole, a novel class of inducers of systemic acquired resistance, activates gene expression and disease resistance in wheat[END_REF]. SA itself was shown to trigger several responses in plants. At a low dose it was reported to enhance the activation of the flg22-induced MITOGEN-ACTIVATED PROTEIN KINASE 3 (MPK3) and MPK6 [START_REF] Yi | NPR1 is Instrumental in Priming for the Enhanced flg22-induced MPK3 and MPK6 Activation[END_REF]. Also BTH induces the accumulation of inactive unphosphorylated MPK3 and MPK6 [START_REF] Beckers | Mitogen-activated protein kinases 3 and 6 are required for full priming of stress responses in Arabidopsis thaliana[END_REF]. It was also shown that a activation of the SA pathway influence flg22 triggered short-term defense responses like the oxidative burst and the deposition of callose [START_REF] Yi | The activated SA and JA signaling pathways have an influence on flg22-triggered oxidative burst and callose deposition[END_REF]. Several studies also showed that defense inducing compounds can also change the plant protein levels during a priming phase [START_REF] Balmer | The 'prime-ome': towards a holistic approach to priming[END_REF]. Importantly, the protein levels of pattern recognition receptors and coreceptors increase after BTH treatments [START_REF] Tateda | Salicylic Acid Regulates <em>Arabidopsis</em> Microbial Pattern Receptor Kinase Levels and Signaling[END_REF]. These findings suggest that the plants prepare their defensive system to enhance their sensitivity against potential attackers [START_REF] Conrath | Priming for enhanced defense[END_REF]. However, recently it was suggested that plants might be also able to forget a previous stress [START_REF] Crisp | Reconsidering plant memory: Intersections between stress recovery, RNA turnover, and epigenetics[END_REF] (and this thesis). Priming could result in a sensitization in a way they response to false alarm signals [START_REF] Mauch-Mani | Defense Priming: An Adaptive Part of Induced Resistance[END_REF]. The current hypothesis at the moment is that plants might forget previous stresses to avoid reduced development, yield and ultimate survival [START_REF] Crisp | Reconsidering plant memory: Intersections between stress recovery, RNA turnover, and epigenetics[END_REF]. This hypothesis is supported by the results that transgenerational defense priming may be linked to the severity of the originally faced stress and that the initial stress in only memorized when applied repeatedly (Luna et al., 2012). Epigenetics widely contributes to the stress memory in plants [START_REF] Bruce | Stressful "memories" of plants: Evidence and possible mechanisms[END_REF][START_REF] Tsai | Priming for enhanced defence responses by specific inhibition of the Arabidopsis response to coronatine[END_REF][START_REF] Ramírez-Carrasco | Transgenerational Defense Priming for Crop Protection against Plant Pathogens: A Hypothesis[END_REF]. Epigenetics involves fast and reversible modifications. Therefore, it is likely that epigenetic mechanisms are involved in the removal of immune priming after certain stress-free generations or after longer time in the same generation in order to alleviate the potential costs.

Results and discussion

After our investigations on the inversion and de-priming of transcriptional memory by a subsequent flg22 treatment after BTH exposure, we decided to explore the effect of BTH and flg22 treatment combinations on the induction of short-term defense responses. The activation of MAP Kinases (MAPK) is one of the fastest defense responses [START_REF] Pitzschke | MAPK cascade signalling networks in plant defence[END_REF]. Since it was shown that BTH treatment enhances the protein level of the receptor FLS2 and the coreceptors BAK1 [START_REF] Tateda | Salicylic Acid Regulates <em>Arabidopsis</em> Microbial Pattern Receptor Kinase Levels and Signaling[END_REF] as well as the level of unphosphorylated MAPK [START_REF] Beckers | Mitogen-activated protein kinases 3 and 6 are required for full priming of stress responses in Arabidopsis thaliana[END_REF] we studied the level of phosphorylated MAPK, induced by application of flg22, after a pre-treatment of BTH and/or flg22 (Figure 37). Arabidopsis plants were treated after 7dag with water (W) or BTH (B) and a second time after 10dag with water or flg22 (F). After a recovery period of 11 days plants were treated with flg22 in order to induce the phosphorylation of MAPK. Even though these preliminary results miss a loading control with coomassie blue staining it is clearly visible that no difference between the two times water treated plants the BTH/flg22 treatments can be observed (Figure 37 A). This might be due to the long recovery phase of 11 days after the last treatment. In line with this result is also our next observation. Plants pre-treated after 7dag and 10dag with water, BTH or flg22 (WW, WF, BW, BF) were treated after 21dag with flg22 to induce a ROS burst. All pre-treated plants induced a ROS burst with a comparable intensity (Figure 37 B). Finally, we decided to test the resistance of pre-treated plants to a Pseudomonas infection. It was previously reported that plants pre-treated repetitively with BTH show an increased resistance to Pseudomonas infections [START_REF] Kohler | Benzothiadiazole-induced priming for potentiated responses to pathogen infection, wounding, and infiltration of water into leaves requires the NPR1/NIM1 gene in Arabidopsis[END_REF]. Also, our results show that plants pre-treated three times with BTH after 7, 10 and 13dag show reduced bacteria growth 1dpi. However, all other pre-treatment combinations do not show any difference in bacterial growth (Figure 37 C). These results miss a control of bacteria extraction after the infection (0dpi) as well as a second extraction at 2dpi. However, these results indicate that the inversion and de-priming in transcription might not reflect in short-term defense responses as well as resistance to

Pseudomonas infection. The de-priming and return to a basic expression level pertain only a subset of transcripts. It is possible that these de-primed transcripts might be not critical for the induced resistance by BTH but might cause negative effects for the plant and loss of energy due to priming-related fitness costs. The subsequent stress after BTH could fine-tune the expression response of BTH pre-treated plants and could lead to a directed response to the current stress the plant is facing. However, the increased resistance against Pseudomonas after a three times BTH treatment is in line with the previously published results on BTH. This might due to the shorter recovery phase of 8 days after the last BTH treatment. The plant memory for BTH response might be limited and the induced resistance is increased after 8 days but not after 11 days. In conclusion we show, that Bion has an effect on resistance against Pseudomonas infection but that the inversion and de-priming of transcription shows no effect on the short-term defense responses MAPK activation and ROS bust as well as resistance. However, these results have a strong preliminary character and require additional experiments and replicates. Arabidopsis Plants were pre-treated by spaying after 7, 10 and 13 days with the indicated treatment.

Plants three times exposed to BTH (1mM) show a reduced bacteria growth 1dpi (days post infection).

None of the other applied pre-treatments caused an effect on plant resistance at 1dpi.

Material and methods

6.6.3.1.

MAPK phosphorylation

Plants were pre-treated with a first treatment after 7dag (water (W) or BTH (1mM)(B)) and a second treatment after 10dag (water (W) or flg22 (F)). After a 11days recovery phase, all leaves of one plant were treated by floating for 15min on solution supplied with 1µM flg22 or without (control). After 15min leaves were shock frozen and grounded to fine powder before adding 80µl extraction buffer (0.35M Tris-HCl pH6.8, 30% glycerol, 10% SDS, 0.6M DTT, 0.012% bromphenol blue). After boiling for 5min, 15µl of the protein extract was separated by electrophoresis in 12% SDS-polyacrylamide gel and electrophoretically transferred to a polyvinylidene fluoride membrane according to the manufactural instruction (Milipore). For exposure, monoclonal primary antibodies against phosphor-p44/42 MAP kinase (cell Signalling Technologies) and alkaline phosphatase-conjugated anti-rabbit secondary antibody (Sigma Aldrich). Signal was detected using CDPstar (Roche).

6.6.3.2.

Plant inoculation with Pseudomonas syringae pv. Tomato (pst. DC3000)

Plants were pre-treated with the indicated treatments after 7, 10 and 13dag with the indicated treatment combination and infected after 21dag. Eight leaves of one plant per pretreatment were infiltrated with bacterial suspensions of at a concentration of 10 5 colony forming units (cfu/ml) (OD600 of 0.02) in sterile 10mM MgCl (also used for mock-inoculation) using a needleless syringe. Plants were maintained at high humidity. Samples were taken using a cork-borer (d=8mm) to cut one leaf discs per infected leaf. Leaf discs were ground in 10mM MgCl, diluted to the indicated concentration and plated as droplets of 10µl on YEB plates with the appropriate selection. Plates were incubated at 28°C and colonies counted 1dap.

7.

General Discussion

The discovery of the SCOOP peptide family

Considering the complexity of the plant immune system and the tradeoff between defense responses and plant development it is reasonable for plants to merge these two fundamental mechanisms. The expression of phytocytokines is a plant achievement that facilitates the induction of defense responses upon an infection and wounding as well as the regulation of plant development. The latest trend in small endogenous peptide research is the use of bioinformatic tools. By using the continuous advances in genomics and transcriptomics technologies already several small peptides have been identified, among them well known and increasingly well-characterized peptides such as CLE, EPIDERMAL PATTERNING FACTOR (EPF1), C-terminally encoded peptide 1 (CEP1) and ROOT MERISTEM GROWTH FACTOR (RGF) (Matsubayashi, 2014). Finally, our research adds another small peptide family onto this list.

The bioinformatic prediction of the SCOOP family was only possible because of the advantage of using bioinformatics which can overcome the barrier posed by strong gene redundancy and the low abundance of peptides in the plant tissue (Matsubayashi, 2014). By finding analogues of the SCOOP peptide family in 74 Brassicaceae species and an alignment of the PROSCOOP sequences, we show that it is possible to precisely identify potentially active small peptide sequences. However, by using a bioinformatic approach we predict peptides that might not reflect real in planta peptides. The research on small peptides resulted in the identification of several peptides with unknown mature peptide structure. For instance, CLE40 was found in a comparable approach than the SCOOP family. By an in silico screen of CLV3 homologues CLE40 was found to control stem cell fate in the root meristem (Stahl et al., 2009). Other examples of small peptides without fully characterized mature peptide aa composition are CLE8, CLE45 and IDA. Even though, the full mature peptide has not yet been identified, the research on these peptides resulted in the identification of various interesting regulatory functions (Matsubayashi, 2014). The applied bioinformatic approach lead not only to the prediction of several active peptides but also predicted in which tissue the peptide precursors are expressed and helped us to predict functional partners. The use of the CATMA transcriptome database [START_REF] Crowe | CATMA: a complete Arabidopsis GST database[END_REF][START_REF] Sclep | CATMA, a comprehensive genome-scale resource for silencing and transcript profiling of Arabidopsis genes[END_REF]Gagnot et al., 2008) is an excellent tool to identify previously unknown proteins. The microarray produced in frame of the CATMA project contains 677 gene specific tags that map in intergenic regions according to an alternative Arabidopsis genome annotation. By investigating the regions of these tags, it is possible to reveal possible under-predicted genes in Arabidopsis. Especially small peptides could be found by this method (Aubourg et al., 2007).

Possible posttranscriptional modifications of the SCOOP family

Next to the identification of in planta peptides, the discovery of posttranscriptional modification and secretion processes of small peptides bear a great challenge for researchers.

Posttranscriptional modifications of small endogenous peptides are known to modulate the physiochemical properties, changes the net charge, alters the hydrophilicity and/or the conformation. These modulations therefore change the binding and specificity of the peptides and their receptors (Matsubayashi, 2014). Three such modifications have been identified for peptides up to now: tyrosine sulfation, proline hydroxylation and hydroxyproline arabinosylation (Matsubayashi, 2011). Upon them Proline hydroxylation is identified in almost all posttranscriptional modified small peptides. Only PSK does not carry this modification because it has no proline residues. Interestingly, out of the 4 inactive SCOOP peptides (SCOOP3, 8, 10, 11) the peptides SCOOP8 and SCOOP11 do not harbor any proline residue. Out of the active peptides only SCOOP5 does not contain a proline residue. However, the activity of SCOOP5 might be promoted by a tyrosine sulfation. Overall only SCOOP5 and SCOOP13 contain a tyrosine residue. Tyrosine sulfation is a modification that indicates a peptide which is synthesized through the secretory pathway [START_REF] Moore | The biology and enzymology of protein tyrosine O-sulfation[END_REF]. Three other tyrosine-sulfated peptides have been identified in plants namely, PSK, PSY and RGF [START_REF] Matsubayashi | Phytosulfokine -Sulfated peptides that induce the proliferation of single mesophyll cells of Asparagus officinalis L[END_REF]Amano et al., 2007;Matsuzaki et al., 2010). Another open question in regard to our SCOOP peptide discovery is the possibility of a posttranslational proteolytic processing of the precursor protein. The identification of the precursor cleavage site of plant peptides is challenging. There is no typical amino acid motif in plants which is directly adjacent to mature peptide domains. Moreover, the nature of the cleavage mechanism is not always clear in plants. In animals, small peptides are cleaved on the C-terminus of paired basic amino acids, such as lysine-lysine, lysine-arginine or argininearginine. The CLV3 peptide in plants is cleaved on the N-terminus at a single arginine residue of its precursor protein [START_REF] Ni | Evidence for Functional Conservation, Sufficiency, and Proteolytic Processing of the CLAVATA3 CLE Domain[END_REF]. Another example is CLE36 which is cleaved between a methionine and a serine residue is located two residues upstream of the mature peptide domain [START_REF] Djordjevic | Border sequences of Medicago truncatula CLE36 are specifically cleaved by endoproteases common to the extracellular fluids of Medicago and soybean[END_REF]. Therefore, the processing site does not define the boundary of the mature peptide domain because, next to the proteolytic cleavage, peptides undergo proteolytic trimming. In summary, the posttranslational modifications could protect the peptide from exoprotease digestion. The conserved active motif of the SCOOP peptides is not located at the extreme C-terminus of the precursor proteins which could mean that SCOOP peptides have to undergo two proteolytic cleavages. However, the majority of SCOOP peptides harbor a second conserved motif which is located closer to the N-terminus than the SCOOP motif, the second motif might reflect a possible cleavage site.

The identification of the SCOOP receptor(s) -the next big challenge

The next challenge which future research on SCOOP peptides will face is the identification of the SCOOP receptor. Several promising methods have been developed in order to identify pattern recognition receptors. The most efficient approaches are forward genetic mutant screens. The two tools which are often applied are the use of natural variations and the use of a mutagenized population. The most prominent example of a receptor that was identified by using an EMS mutagenized population of Arabidopsis is the flg22 receptor FLS2 (Gomez-Gomez and Boller, 2000). Treatments with the active SCOOP peptides causes a strong inhibition of seedling growth. This phenotype could be used to identify the receptor or other functional partner in the SCOOP perception by screening EMS mutagenized seeds for insensitivity to SCOOP peptides. The seedling growth inhibition assay was also the assay of choice for the identification of the FLS2 receptor. An alternative method to identify peptide receptors are biochemical approaches. One example is the discovery of the PEPR1 receptor which was identified using radiolabeled AtPep1 peptide, interacted with suspension-cultured Arabidopsis, followed by binding studies [START_REF] Yamaguchi | The cell surface leucine-rich repeat receptor for AtPep1, an endogenous peptide elicitor in Arabidopsis, is functional in transgenic tobacco cells[END_REF]. Also, possible and likely to be successful for the SCOOP peptides is the use of a co-receptor as a molecular bait. It was for example shown that it is also possible to identify FLS2 after flg22 treatment and immunoprecipitation of BAK1 (Schulze et al., 2010). We could show that perception of at least SCOOP6, 7 and 12 is dependent on BAK1. Moreover, we show that the bak1-4 mutant is almost completely impaired in SCOOP12 perception. A BAK1 immunoprecipitation could help to identify the SCOOP peptide receptor. However, BAK1 has been shown to serve as coreceptor to a high number of PRR receptors (Chinchilla et al., 2009). An immunoprecipitation might thus lead to many false positive receptor candidates. Another challenge for the discovery of the SCOOP peptides might be the possibility that not all SCOOP peptides are perceived by the same receptor. The SCOOP peptide motif is very diverse in comparison to other small peptides. It could be possible that different SCOOP peptides might be perceived by different receptors. This was also shown for the AtPeps. While AtPep 1-6 are mainly perceived by PEPR1, AtPep1 and 2 are perceived by PEPR2 [START_REF] Yamaguchi | PEPR2 is a second receptor for the Pep1 and Pep2 peptides and contributes to defense responses in Arabidopsis[END_REF]Bartels and Boller, 2015;[START_REF] Lori | Evolutionary divergence of the plant elicitor peptides (Peps) and their receptors: interfamily incompatibility of perception but compatibility of downstream signalling[END_REF]. It was recently shown that the complexity of receptor and coreceptor interactions can be very high. The investigation of 40,000 potential extracellular domains and their interactions revealed a regulatory network consisting of 567 interactions [START_REF] Smakowska-Luzan | An extracellular network of Arabidopsis leucine-rich repeat receptor kinases[END_REF]. Interestingly, the regulatory network confirms BAK1 as the most important and interconnected node while the authors identified a previously unknown LRR-RK (APEX). The double mutant of BAK1 and APEX is strongly impaired in plant development. The authors also confirmed APEX as an interaction partner of PEPR1 and PEPR2 in mediating AtPep2 induced ROS burst [START_REF] Smakowska-Luzan | An extracellular network of Arabidopsis leucine-rich repeat receptor kinases[END_REF]. This indicates that APEX might also be involved in the perception of the SCOOP peptides. However, our predictions of PROSCOOP12 coregulated genes could not confirm APEX to be coregulated.

Phytocytokines -peptides between defense and development

The strongly reduced growth phenotype of apex bak1-5 is another observation demonstrating the importance of small endogenous peptides and their receptors in plant development. With that respect, the SCOOP family is not an exception, since we show that the SCOOP peptides are involved in root development. We conclude that the SCOOP family is a new phytocytokine. The term phytocytokines was just recently defined as secondary endogenous peptides which require a processing step and are passively or actively released. The SCOOP family fulfills all properties of a phytocytokine and is comparable with other members of this classification. The AtPIP peptide family that was described in 2014 is probably the closest functional homologue of the SCOOP family (Hou et al., 2014). Similar to the PROSCOOP proteins the precursor of AtPIPs (prePIPs) harbor an N-terminal secretion signal and a conserved C-terminal peptide motive whereas the synthetic peptide PIP1 induces defense responses in Arabidopsis. Like SCOOP12, the PIP1 peptide induces the expression its own precursor. Both peptide families interact with AtPep signaling by inducing the expression of PROPEPs. PIP1 induces the expression of PROPEP1 and SCOOP12 the expression of PROPEP2.

PIP1 was also shown to amplify FLS2 signaling and therefore bridges the perception of MAMPs with the action of phytocytokines (Hou et al., 2014). The three peptide families AtPIP, AtPep and SCOOP seems to be tightly linked in their function as amplifier of defense. Even though, all three peptides have different receptors, BAK1 is involved in the perception of peptides of these families (Chinchilla et al., 2009;Hou et al., 2014). Moreover, the downstream signal after perception of members of these three peptides families seems to be identical. The expression of FRK1 is enhanced after treatments with peptide members of the three families (Flury et al., 2013;Hou et al., 2014). It was proposed that receptors operate in unified regulatory networks [START_REF] Smakowska-Luzan | An extracellular network of Arabidopsis leucine-rich repeat receptor kinases[END_REF]. This thought leads to the idea that the corresponding ligands might also act in larger networks. Different small peptides as well as their precursors have very similar effects on plant defense responses and plant development.

It might be, that the peptides cooperate in the action of defense stimulation while the precursors have antagonistic effect in root (or whole plant) development. In line of this idea the observation that overexpression of AtPROPEP1 results in a bushy root phenotype and the most AtPROPEPs and both AtPEPRs were found to be exclusively expressed in roots is an interesting novel observation (Huffaker et al., 2006;[START_REF] Ferrari | Oligogalacturonides: plant damage-associated molecular patterns and regulators of growth and development[END_REF]. Overexpression of prePIP1 prePIP2 reduces root growth and knocking out these genes leads to an increased root growth. This indicates that PROSCOOP12 and prePIP1 have comparable functions in root development which might be antagonistic regulated by PROPEP1. A high number of small endogenous peptide families are found to have a function in root development [START_REF] Delay | Regulation of Arabidopsis root development by small signaling peptides[END_REF]. An orchestration of peptides in order to control various biological functions was shown in the case of the CLE peptides. CLE peptide signaling integrated with phytohormone signaling controls developmental processes in various tissues. Especially auxin signaling conjugates with different CLE peptides in regulating vascular pattering [START_REF] Wang | CLE Peptide Signaling and Crosstalk with Phytohormones and Environmental Stimuli[END_REF]. The fact that AtPep, AtPIP and SCOOP are expressed in the same tissues and the growing evidence that the Pep-PROPEP-PEPR system is involved in developmental processes (Huffaker et al., 2006;[START_REF] Gully | Perception of Arabidopsis AtPep peptides, but not bacterial elicitors, accelerates starvation-induced senescence[END_REF] could indicate an orchestration of these families similar to the CLE peptides.

DAMPs and synthetic elicitors as plant defense stimulating compounds

The idea of using classical DAMPs as vaccines to promote the plant defense is also adopted by agricultural companies and the first products based on plant extracts are already available on the market (Quintana- [START_REF] Quintana-Rodriguez | Damage-associated molecular patterns (DAMPs) as future plant vaccines that protect crops from pests[END_REF]. This raises the question of possible advantages of these plant extract products compared to synthetic elicitors. One advantage of crude plant extracts is that is easily and rapidly prepared and has therefore a low economic cost. This could make them an option for low-income farmers worldwide. However, the exact concentration of the active compound in the plant extract remains unknown. Plant extracts contain a high diversity of DAMPs and therefore they are expected to trigger a more diverse set of resistance responses compared to an individual DAMP. Moreover, plant extracts could have direct pathogen repellent or antimicrobial effects [START_REF] Quintana-Rodriguez | Damage-associated molecular patterns (DAMPs) as future plant vaccines that protect crops from pests[END_REF].

However, the possible effects of plant extracts treatments are not completely clear. It might be possible that plant extracts contain pathogens or plant viruses that bear the risk of infection on the treated plant. Another disadvantage is the limited stability of the containing DAMPs that probably would limit the applicability on the field. Overall, the use of synthetic defense-promoting compounds is still the more adapted tool in crop protection. An intersection of DAMPs and synthetic elicitors as treatment is given by two studies that showed that treatment with BTH leads to a release of volatile organic compounds (VOCs), which can be classified as DAMPs, from lima beans. These VOCs elicited resistance against a pathogen in neighboring lime bean plants [START_REF] Yi | Airborne induction and priming of plant defenses against a bacterial pathogen[END_REF][START_REF] Heil | Short signalling distances make plant communication a soliloquy[END_REF].

The most important advantage of synthetic elicitors is the broad activation of defense responses including the accumulation of salicylic acid. SA is the most important hormone involved in systemic acquired resistance, which is induced in distal non-treated parts of the plant. SAR promotes resistance against a broad spectrum of pathogens [START_REF] Henry | Recognition of bacterial plant pathogens: local, systemic and transgenerational immunity[END_REF][START_REF] Faize | Functional Analogues of Salicylic Acid and Their Use in Crop Protection[END_REF]. The effect of SA in plant defense is therefore of great magnitude.

However, SA is not being directly used as defense promoting compound. Instead, the compounds of choice are SA analogues. SA itself gets rapidly glycosylated or methylated which reduces its activity [START_REF] Vlot | Salicylic Acid, a Multifaceted Hormone to Combat Disease[END_REF][START_REF] Yamasaki | Chloroplast envelope localization of EDS5, an essential factor for salicylic acid biosynthesis in Arabidopsis thaliana[END_REF]. Moreover, several examples showed a phytotoxic action of SA and, when applied in higher concentrations, an inhibition of growth and an induction of strong defense responses [START_REF] Heil | Fitness costs of induced resistance: emerging experimental support for a slippery concept[END_REF].

The methylation of SA generates MeSA, which is a possible mobile molecule active in SAR.

However, only a few commercialized functional analogous of SA carry a methyl modification.

Among them is BTH, which might enable the induction of SAR in various plants and against a wide range of pathogens. Even though, BTH is a functional analogue of SA, it just mimics a subset of SA functions and is interfering with the receptors and triggers similar transcriptional and physiological responses. However, SA analogous do not directly interfere with SA targets [START_REF] Bektas | Synthetic plant defense elicitors[END_REF]. Moreover, farmers face several problems by using SA analogues.

This includes a very incomplete disease reduction and the necessarily of frequent application.

Moreover, a recent study showed that the formulated BTH (Bion) photolyze quickly on detached apple leaves with a half-life of only 2,8h [START_REF] Sleiman | Heterogeneous Photochemistry of Agrochemicals at the Leaf Surface: A Case Study of Plant Activator Acibenzolar-Smethyl[END_REF]. This result might explain the requirement of repetitive applications. The problem of a reduced plant fitness, as it was previously described for SA, is still an issue for treatments with SA analogues [START_REF] Canet | Resistance and biomass in Arabidopsis: a new model for salicylic acid perception[END_REF]Faize and Faize, 2018) (and this work). This raises the question if SA analogues and especially BTH are really the optimal choice as defense activators. Also, our research shows that BTH application causes a growth inhibition effect in Arabidopsis and apple. The negative effects of SA analogues and the largely unknown molecular mechanism made it necessary to screen for new compounds in defense priming. One research group developed a highthroughput screening method to identify compounds which potentiate pathogen-activated cell death in Arabidopsis cell cultures. This group found and characterized in a series of publications five compounds that prime immune response [START_REF] Noutoshi | Sulfonamides identified as plant immune-priming compounds in high-throughput chemical screening increase disease resistance in Arabidopsis thaliana[END_REF]Noutoshi et al., 2012c;Noutoshi et al., 2012a). The group screened 10.000 molecules and only three induced a priming effect without causing cell death. The newly identified molecules are called imprimatins. Interestingly, two of these molecules archive their action by inhibiting the SA glucosyltransferase (SAGT) which then allows the accumulation of endogenous SA. (Noutoshi et al., 2012a). The idea of overcoming the tradeoff between defense and growth by the development of defense-priming compounds which do not directly interfere with SA receptors and pathways, but which cause changes in the SA metabolism, transport and perception might be the future of defense priming compounds. Another possible strategy to identify new priming compounds was described recently. Kuai and colleagues proposed that research should focus on the development of "Just in time" immunomodulating compounds.

These compounds should be chemicals that boost immunity on demand and only when needed [START_REF] Kuai | Combining Fungicides and Prospective NPR1-Based "Just-in-Time" Immunomodulating Chemistries for Crop Protection[END_REF]. They propose to develop an NPR1 agonist, since NPR1 is a key factor in SA signaling.

In fact, the action of NPR1 and the action on defense priming by SA analogues is mostly in focus of research while the problem of the defense and development tradeoff remains to be investigated in more detail. One of the key factors in the balance of defense and development are the target of rapamycin (TOR) kinase and raptor (regulatory-associated protein of mTOR).

A recent study on rice showed that plants with reduced TOR signaling display enhanced disease resistance against bacterial and fungal pathogens. TOR was found to antagonize the action of SA and JA. Moreover, the authors could show that silencing of raptor primes SA and JA dependent gene expression in response to BTH [START_REF] De | Target of rapamycin signaling orchestrates growth-defense trade-offs in plants[END_REF]. In human cancer research a lot of strong and weak inhibitors of mTOR are in focus of research.

Moreover, numerous anti-cancer drugs are plant natural compounds. Just one example out of many is resveratrol, a phytoalexin found in grapes (Zhang et al., 2012a). It would be interesting to investigate the effect of a combined treatment of a salicylic acid analogue such as BTH and a TOR inhibitor. A combined treatment of plant extracts containing natural TOR inhibitors and BTH could lead to stronger plant protection activity.

Is a plant able to forget?

One question that this thesis raised is whether plants are able to "forget" a previous stress or treatment. The quotation by Nietzsche "Without forgetting it is quite impossible to live at all" fits very well to the life of a plant. Plants are not able to escape a stressful situation and face many different stresses in short periods of time. The concept of stress memory is in the focus of research however the resetting of memory is at least of the same importance. A time when the concept of resetting versus consolidation of memory is essential is during the period of stress recovery. A stress recovery phase is defined to be a period of time following a stress until a new homeostasis is attained [START_REF] Crisp | Reconsidering plant memory: Intersections between stress recovery, RNA turnover, and epigenetics[END_REF]. A new higher homeostasis can be reached due to the priming and memory of a plant while in other cases the post-stress homeostasis resembles the prestress state. This is the case if the stress is brief and a memory of the stress would be a maladaptation for the plant. It might also be the case that a stress is prolonged but there is still a difference between pre-and post-stress state. This might be due to a progression of the plant to a new developmental stage [START_REF] Crisp | Reconsidering plant memory: Intersections between stress recovery, RNA turnover, and epigenetics[END_REF]. The possibility of different developmental stages we avoid in our research by using in-vitro apple plantlets.

In contrast to apple trees these plants keep their vegetative developmental stage and do not multiply sexually. The concept of stress recovery and therefore a reset of memory or transcription is especially important when the initial stress could have a long duration and the stressful situation could vanish rapidly which then requires a fast adaptation. This is the case for drought stress. Therefore, several investigations have analyzed rehydration responses during drought stress recovery. One study showed that drought stressed Medicago truncatula undergo a global reprogramming of transcription and metabolism during 14-days of drought and re-watering. Moreover, 90% of the drought-responsive genes react oppositely to the addition of water [START_REF] Zhang | Global reprogramming of transcription and metabolism in Medicago truncatula during progressive drought and after rewatering[END_REF]. Thus, a stress and a subsequent stress release leads to an inversion of transcription. Another interesting publication showed that not only a stress release but also a similar repeated stress can lead to a transcriptional inversion. Liu and colleagues showed that a subset of dehydration stress response genes respond to an initial drought stress and return to a basic expression level during watered recovery but do not respond to a subsequent stress exposure [START_REF] Liu | Different gene-specific mechanisms determine the 'revised-response' memory transcription patterns of a subset of A. thaliana dehydration stress responding genes[END_REF]. Interestingly, with our results we show that a subset of genes returns to a basic expression level by applying a second biotic stress in form of flg22. Even though BTH and flg22 have the capacity to induce or to prime defense responses, a subsequent treatment of flg22 after BTH inverts the expression of a subset of transcripts. An inversion of expression by a comparable treatment was, by our knowledge, not described before in the literature. A possible explanation could be that BTH application is more stressful for the plant than the subsequent flg22 treatment. The flg22 treatment condition is therefore the more favorable condition. Under natural growth conditions plants may sense flg22 continually and the "flg22-stress" condition could be considered as a baseline stress. On the other hand, BTH is not seen by the plant in an all-day fashion which therefore could be considered as a stress trigger. However, with our research we investigate the transcript expression profile several days after the last treatment. Thus, we can propose two mechanisms of flg22-induced de-priming of BTH induced transcripts. First it is possible that flg22 treatment leads to an accelerated returning to the basic expression level during the last recovery phase before sampling. The second possibility is that flg22 leads to an immediate de-priming of BTH-induced transcription expression. To further investigate these two possibilities, it would be important to follow the expression profile of the depriming marker gene AMY1 that was identified in this thesis directly after the treatments. It would also be possible to follow the expression of AMY1 by fusion of the promotor with a small and rapidly degrading reported gene such as the yellow florescence protein (YFP) of luciferase.

Molecular mechanism of de-priming

Investigations of de-priming velocity might give us an indication of the molecular mechanism behind our observations. As described in the introduction, stress priming can be established on the transcriptional level or by epigenetic changes. Also, the returning to a basic expression level could be regulated at the transcriptional or epigenetic level. It was shown that the mRNA half-live can have a range of 0.2 to 24 hours in Arabidopsis [START_REF] Narsai | Genomewide analysis of mRNA decay rates and their determinants in Arabidopsis thaliana[END_REF]. However, it was also shown that stress-responsive mRNAs can be stabilized by RNA binding proteins [START_REF] Frei Dit Frey | The RNA binding protein Tudor-SN is essential for stress tolerance and stabilizes levels of stress-responsive mRNAs encoding secreted proteins in Arabidopsis[END_REF]. It would be interesting to investigate the role of the RNA binding protein such as Tudor-SN in response to the subsequent flg22 treatment after BTH. Todor SN is one of the best studies RNA binding protein [START_REF] Frei Dit Frey | The RNA binding protein Tudor-SN is essential for stress tolerance and stabilizes levels of stress-responsive mRNAs encoding secreted proteins in Arabidopsis[END_REF]. With our research we show that among the differently expressed transcripts that show the de-priming pattern, an overrepresentation of antisense transcripts can be found. It was reported that naturally occurring antisense RNAs (natsiRNAs) have the potential to affect mRNA expression during stress in an inverse relationship by silencing sense transcripts, as serval reports have reported [START_REF] Zhang | Genome-wide analysis of plant nat-siRNAs reveals insights into their distribution, biogenesis and function[END_REF][START_REF] Wang | Non-coding RNAs and Their Roles in Stress Response in Plants[END_REF]. Although, natsiRNAs have the potential to be key players in the inversion of transcriptional memory their role in this process remains to be explored. It would be interesting to identify natsiRNAs accumulation after BTH and BTH with subsequent flg22 treatment by northern blot and to find the target genes. The "forgetting" of the transcriptional memory could be also be due to a change at the transcriptional level in from of removing template RNA molecules that could be used by the posttranscriptional gene silencing (PTGS) or RdDM pathways [START_REF] Crisp | Reconsidering plant memory: Intersections between stress recovery, RNA turnover, and epigenetics[END_REF]. Also, our results link de-priming with the RdDM pathway. In the nrpd1-3 mutant background the expression of AMY1 is strongly reduced. This could be an indication for the existence of a negative regulator for AMY1 which is epigenetically silenced in the wildtype situation and that shows an enhanced expression in hypomethylated mutant background. AMY1 is from an "epigenetic point of few" a gene which is at first sight not under epigenetic control. Our investigations showed no hypo or hypermethyltion at the AMY1 gene in the wildtype background (data not shown). However, since AMY1 is nearly not expressed in the met1-3 mutant background, we can assume a role of DNA methylation, and probably in CG context, in the regulation of AMY1. It would be necessary to repeat the microarrays in the met1-3 and nrpd1-3 mutant background to investigate the global gene expression profile after BTH and BTH with subsequent flg22

treatment. Moreover, it would be interesting to investigate the global methylation level by bisulfide sequencing of plants treated with BTH and BTH/flg22. With this experiment we would be able to identify differently methylated regions (DMRs) involved in de-priming of expression.

One sort of "forgetting" of stress-induced priming is often showed by a change in generations.

Not many publications showed a transgenerational transmission of acquired traits (over one stress-free generation). An interesting publication identified two chromatin regulators to be impaired in the resetting of stress induced loss of epigenetic silencing. DDM1 and MOM1 have been identified to be factors preventing transgenerational inheritance [START_REF] Iwasaki | Identification of genes preventing transgenerational transmission of stress-induced epigenetic states[END_REF]. The activity of MOM1 is linked to transcriptional gene silencing however, MOM1 action was also shown to contribute to the accumulation of small interfering RNAs in order to control gene silencing by the RdDM pathway [START_REF] Yokthongwattana | MOM1 and Pol-IV/V interactions regulate the intensity and specificity of transcriptional gene silencing[END_REF]. It would be interesting to repeat the microarray setup (or to follow the expression of AMY1) in Arabidopsis using the mom1 ddm1 double mutant in order to explore the role of these chromatin regulators in memory resetting in addition to their role in preventing transgenerational epigenetic memory. Interestingly the authors who identified MOM1 and DDM1 involvement in resetting transgenerational memory proposed that the prevention of transgenerational memory extends far beyond the activity of MOM1 and DDM1. They found around 3000 loci which are activated under stress conditions but still 340 transcripts remain transgenerationally active in the mom1 ddm1 double mutant [START_REF] Iwasaki | Identification of genes preventing transgenerational transmission of stress-induced epigenetic states[END_REF].

This indicates that also the mechanism of de-priming might be under complex epigenetic and/ or transcriptional regulation.

Conclusion

With this thesis I show a wide range of different aspects of plant immunity and development.

Due to the two projects I followed during my thesis I can provide results here describing the

Outlook

Despite, our investigations into the actions, processing and perception of the SCOOP family members, many open questions remain unanswered.

In the general discussion I propose several methods with the goal to identify the SCOOP receptor or the SCOOP receptors as well as the identification of the mature SCOOP peptides.

These experiments should be carried out to further characterize the SCOOP peptide family. It would also be important to expend our knowledge about other SCOOP precursor proteins apart from PROSCOOP12. The bioinformatic prediction of the SCOOP family expression profile revealed that PROSCOOP12 displays rather an exception in comparison to the other SCOOP peptides. This could indicate a different role in developmental processes, since also not all SCOOP family members induce defense responses. It would be interesting to confirm the bioinformatic prediction by fusion of the PROSCOOP genes with a GUS reporter gene. Besides the localization of the PROSCOOP genes it would be interesting to generate knock out and overexpression mutants of the PROSCOOP genes. These experiments would be especially important because mutants of the precursor proteins of small endogenous peptides did not receive a lot of attention so far [START_REF] Trivilin | Components of different signalling pathways regulated by a new orthologue of AtPROPEP1 in tomato following infection by pathogens[END_REF]. The PROPEPs were assumed to act redundantly and therefore the receptor mutant was used to indirectly study the lack of PROPEPs and PEPs (Bartels and Boller, 2015). However, we have a clear indication, by bioinformatic prediction, that members of the SCOOP family are involved in different cellular processes. Further, it could be possible to use the CRISPR-Cas9 approach to knock out several PROSCOOP members at the same time, due to the conserved motif. The research on SCOOP peptide family could be facilitated by specific antibodies targeting the PROSCOOP proteins.

By in situ hybridization we could generate a tissue specific map of PROSCOOP presence.

Further, due to the increasing evidence that phytocytokines are involved in developmental processes, it would be good to investigate possible interactions within different families like

AtPep, AtPIP and SCOOP.

The results presented here show that transcriptional regulation is a very plastic mechanism.

Since the effect of BTH and the de-priming of a subsequent flg22 exposure affects global expression, it would be important to investigate global effects on DNA-methylation of BTH and BTH/flg22. Since we have hints that the DNA methylation state is key to de-priming we should perform a bisulfide sequencing in order to find differentially methylated regions that might help us to identify factors involved in de-priming. Further, it would be important to not only focus on de-priming but also on mechanisms involved in the memory of BTH treatments.

Not many stresses have been shown to have a transgenerational effect. However, it was shown that next generational SAR requires the activity of NPR1, indicating that SA or SA signaling can lead to transgenerational resistance in Arabidopsis (Luna et al., 2012). This raises the question if this transgenerational inheritance can be also observed in apple. Apple plants are frequently treated with salicylic acid analogues like BTH. This question could be addressed by comparing methylomes of untreated plants with BTH treated trees. Priming is widely associated with histone modifications. Several histone modifications are connected with defense priming namely H3K4me3, H3K4me2, H3K9ac, H4K5ac, H4K8ac and H4K12ac.

Especially, H3K4me3 is considered as primary chromatin mark of stress memory [START_REF] Conrath | Priming for enhanced defense[END_REF]. Up to now, promoter regions of three defense-associated genes have been shown the be under epigenetic control upon BTH treatment [START_REF] Jaskiewicz | Chromatin modification acts as a memory for systemic acquired resistance in the plant stress response[END_REF].

8.

I want to thank the region pay de la Loire and Bayer CropScience for founding.

Then I want to thank the whole Epicenter team. Especially I want to thank the former members Tünde and Jean-Marc as well as Adrien, Marta and Estefania for after work activities and beer support. And David Roquis for kindly helping with the French translation.

Moreover, I am very thankful for all interesting and helpful discussions with other colleagues at IRHS Angers and more in particular Jean-Pierre, Marie-Charlotte, Sandra, Emilie, Philippe and Alexandre. For helping with technical issues and autoclaving as well as for being the friendly face in the building I want to thank Christophe. From the botanical institute in Basel I want to thank Michael and Tim.

Furthermore, the successful outcome of the SCOOP project was made possible by the extended discussions and teamwork with Sebastien. He showed me the power of bioinformatic and initialized the work on the SCOOP family.

And I want to thank the constructors of my Seat Arosa in the year 1997. The car with the conspicuous color brought me (more or less safe) to work and back during my PhD.

Finally, I would like to specially highlight and acknowledge the great support I received from my family and friends. Without them it would not have been possible to finish this dissertation.

Introduction

In order to counter constant pathogen aggressions, plants have developed sophisticated perception and defense systems. These plant responses are regulated by complex networks involving regulatory proteins and hormones and are associated with massive changes in gene expression (Buscaill and Rivas, 2014). Among the involved actors, it has been shown that small secreted peptides play an important role through their direct interaction with pathogens or through their function in development and cell-to-cell communication involving ligandreceptor interactions (Murphy et al., 2012;Marmiroli andMaestri, 2014, Gust et al., 2017).

The secreted peptides derive from protein precursors having a shared N-terminal signal peptide which target the protein to the secretory pathway. They can be categorized into two major classes: (i) the small post-translationally modified peptides (PTMP) which are the targets of posttranslational maturations and are produced through a proteolytic processing and (ii) the cysteine-rich peptides (CRP) characterized by an even number of cysteine residues involved in intramolecular disulfide bonds (Tavormina et al., 2015). Although they are mainly involved in plant growth and developmental processes, it has been shown that numerous genes encoding secreted peptides are also involved in plant defense mechanisms (Albert, 2013). For instance, the CRP class includes the antimicrobial peptides (such as knottins and defensins) which interact and disrupt the pathogen cell membrane (Goyal and Mattoo, 2014).

Regarding PTMPs, families such as the phytosulfokines (PSK), CLE/CLV3, IDA/IDL or PSY are actors in processes regulating a large panel of plant-pathogen interactions [START_REF] Rodiuc | Evolutionarily distant pathogens require the Arabidopsis phytosulfokine signalling pathway to establish disease[END_REF]Lee et al., 2011;Vie et al., 2015;Shen and Diener, 2013). Among secreted peptides, those showing immunity-inducing activity have been classified as damage/danger associated molecular pattern, i.e. DAMPs (Heil et al., 2012;Boller and Felix, 2009). Through the action of lytic enzymes, a pathogen can penetrate the plant cell wall; the cell wall fragments thus released into the apoplastic space can be perceived by neighboring cells, resulting in defense reactions. Oligalacturonides and cutin monomers are examples of non peptidic DAMPs which get released upon fungal infection (Fauth et al., 1998). Their perception by neighboring cells elicits immunity response as well (De Lorenzo et al., 2011). The small peptide AtPep1 is a welldocumented DAMP (Bartels and Boller, 2015). A first induction of AtPep1 and other peptides of this family by wounding or pathogen attack has a positive feedback on the expression of its own precursors as well as defense marker genes that is thought to amplify defense signaling pathways (Huffaker and Ryan, 2007).

It is considered that only a small fraction of the gene space likely to encode signaling peptides has been described and their diversity appears to be largely underestimated (Matsubayashi, 2014). Indeed, the Arabidopsis genome contains more than a thousand genes harboring secreted peptide features for which the biological function is currently unknown (Lease andWalker, 2006 and[START_REF] Sunkar | MicroRNAs with macro-effects on plant stress responses[END_REF]. This lack of data can be explained by the fact that this type of genes has only recently been detected due to their small size and their low sequence conservation (Silverstein et al., 2007). Furthermore, the frequent functional redundancy inside these gene families (Matsubayashi, 2014) rendering mutant knock-out approaches less successful. The mining of previously published transcriptomes is an efficient way to explore this unknown gene-space and decipher functions of new genes for which, without reference, the inference of function by similarity cannot be applied. Based on transcriptome meta-analysis and bioinformatics predictions in a 'guilt by association' approach, we identified a peptide family, of which at least one member is involved in plant immunity and root development. This work describes the identification of a gene family specific to the Brassicaceae genus encoding putative secreted peptides. The functional characterization of PROSCOOP12, one of its members in Arabidopsis, shows that this small gene could act as moderator in the response to different pathogen aggressions and in root development presumably via controlling ROS detoxification. We illustrate that the small endogenous SCOOP12 peptide displays most properties of phytocytokines, processed and actively transported actors of endogenous danger signal without cellular damage (Gust et al., 2017).

Materials and Methods

Plant material

Plant material used was wild-type Arabidopsis thaliana L. Heynh cultivar 6 Columbia (Col-0)

as well as the cultivar Wassilewskija (Ws) and the mutants proscoop12 (T-DNA line FLAG_394H10 in Ws background, primers used for genotyping are detailed in Table S1) bak1-4 (T-DNA line SALK_116202), fls2 (Gomez-Gomez and Boller, 2000) and pepr1/pepr2 described by Flury et al. (2013). The proscoop12 mutant in Col-0 background was created using the CRISPR-Cas9 approach. We searched proscoop12 gene-specific sgRNA and potential off-target sites in the Arabidopsis Col-0 genome using the Crispor Plant inoculation with E. amylovora Wassilewskija (Ws), Columbia (Col-0) and proscoop12 mutant in both genotypes were grown 5 weeks on soil. Four leaves of 20 plants were infiltrated with bacterial suspensions of wildtype strain of E. amylovora CFBP1430 at a concentration of 10 7 colony forming units (cfu/ml) in sterile water or with mock using a needleless syringe. Symptom severity was scaled as described in [START_REF] Degrave | Erwinia amylovora type three-secreted proteins trigger cell death and defense responses in Arabidopsis thaliana[END_REF]. For symptom rating (for Ws and proscoop12-Ws), at least 12 rosette leaves were used per condition in two biological replicates. Maximal symptoms appeared at 24 or 48hpi depending on biological replicates. Therefore, representative experiments are presented either at 24 or 48hpi. For bacterial counting (for Col-0 and proscoop12-Col-0), samples were taken 3 days post infection using a cork-borer (d=5mm) to cut one leaf disc per infected leaf. Leaf discs were ground in sterile water, diluted and plated as droplets of 10µl on LB plates. Plates were incubated, and colonies were counted the next day. Bacteria of 32 leaves of WT and proscoop12 were extracted and quantified.

Seed contamination and leave infection by A. brassicicola

Fifty surface sterilized seeds per petri dish of Ws and proscoop12 were immerged in a solution containing A. brassicicola (strain abra43) with 10 3 conidia/ml, for one hour and dried under sterile conditions. Leaves of Ws wild-type and proscoop12 mutant were inoculated with 5µl of an A. brassicicola solution, with a concentration of 10 3 conidia/ml. Symptoms were observed six days after infection. Necrosis areas were quantified using ImageJ. The experiments were repeated three times.

Protection assay

Mature leaves of A. thaliana plants were infiltrated by needless syringe infiltration with the indicated elicitor peptide or control solution and were kept under long day growth conditions for 24h. The Pseudomonas syringae pv tomato DC3000 strain was grown in overnight culture on YEB medium plates supplemented with appropriate antibiotics. Cells were harvested from the plate and re-suspended in sterile 10mM MgCl and diluted to an OD600 of 0.02. Bacteria solution was needles syringe infiltrated into the pre-treated leaves. Plants were maintained at high humidity. Samples were taken using a cork-borer (d=8mm) to cut one leaf disc per infected leaf. Leaf discs were ground in 10mM MgCl, diluted to the indicated concentration and plated as droplets of 10µl on YEB plates with the appropriate selection. Plates were incubated at 28°C and colonies counted two hours after the infection (0 dpi) as well as 1 and 2 days post infection. Eight plants were infected for each pre-treatment and sampling time point. The experiment was performed two times with similar results.

Transcriptomic analysis

Microarray analysis was performed with the CATMA array v5 [START_REF] Hilson | Versatile gene-specific sequence tags for Arabidopsis functional genomics: transcript profiling and reverse genetics applications[END_REF]. Leaves were collected 24h after inoculation from two independent biological replicates. Total RNA was extracted using the Qiagen RNeasy kit according to the supplier's instructions. RNA integrity, cDNA synthesis, hybridization and array scanning were performed as described in [START_REF] Lurin | Genome-wide analysis of Arabidopsis pentatricopeptide repeat proteins reveals their essential role in organelle biogenesis[END_REF]. cDNA from leaves inoculated with E. amylovora were hybridized against cDNA of leaves inoculated with water collected at the same time-point. Statistical analysis was based on two dye swaps as described in Gagnot et al. (2008). To determine differentially expressed genes, a paired t-test on the log ratios was performed. Spots displaying extreme variance were excluded. The raw p-values were adjusted by the Bonferroni method, which controls the Family Wise Error Rate. We considered differentially expressed genes with a Bonferroni p-value ≤ 0.05 Gagnot et al. (2008).

Determination of gene expression by qPCR

Detached leaves of three weeks old plants were collected and floated for two hours in elicitor or control solution. After the treatment, material was frozen and ground in liquid nitrogen.

RNA from 100 mg of tissue was extracted using the NucleoSpin RNA plant extraction kit (Macherey-Nagel Hoerdt, France). The DNase treatment was performed according to the manufacturer's recommendations. Per PCR reaction, complementary DNA was synthesized from 10 ng of total RNA extract with oligo(dT) primers using Moloney Murine Leukemia Virus

Reverse Transcriptase according to the manufacturer's instructions (Promega). For quantitative real-time reverse transcription PCR (qPCR) in a 96-well format, the Chromo4™ 0,01% aniline blue. Callose depositions were quantified with a Leica DM1000 microscope equipped with a Qimaging Micropublisher 3.3 RTV camera using a DAPI filter.

Cell culture conditions

A. thaliana cells were grown in a liquid MS based (Duchefa-Kalys, France) growth medium (pH 5.6) with the addition of 2,4-dichlorophenylacetic acid (0.2 mg/l), sucrose (30 g/l), and KH2PO4 (0,2 g/l). Cells were grown under continuous light (200 µE m -2 s -1 ) on a rotary shaker and weekly sub-cultured to a fresh medium. For radiolabeling experiments 7-day-old cell suspensions were used.

Radioisotope labelling of phospholipids

Arabidopsis cells were aliquoted (7 mL) in individual flasks and kept for 3h under mild rotation for equilibration. Radioisotope labelling was done by the addition of 53 MBq.L -1  33 Porthophosphate. Lipids were extracted according [START_REF] Krinke | Phospholipase D Activation Is an Early Component of the Salicylic Acid Signaling Pathway in Arabidopsis Cell Suspensions[END_REF]. Lipids were separated by thin layer chromatography (TLC) using an acidic solvent system composed of chloroform:acetone:acetic acid:methanol:water (10:4:2:2:1, v/v) [START_REF] Lepage | Identification and composition of turnip root lipids[END_REF] or in a solvent system composed of chloroform:methanol:ammonia:water (90:70:1:16, v/v) [START_REF] Munnik | Rapid turnover of polyphosphoinositides in carnation flower petals[END_REF]. Radiolabelled spots were quantified by autoradiography using a Storm phosphorimager (Amersham Biosciences, UK). Individual phospholipids were identified by comigration with non-labelled standards visualized by primuline staining or by phosphate staining.

Accession numbers

Transcriptome data are available at Gene Expression Omnibus with the accession GSE22683.

The samples used (including biological repetitions) are: GSM562282, GSM562283, GSM562284, GSM562285, GSM562286, GSM562287, GSM562288, GSM562289, GSM562294, GSM562295, GSM562296, GSM562297.

Results

Identification of the PROSCOOP gene family

Meta-analysis of CATMA micro-array data (Gagnot et al., 2008) has previously highlighted several hundreds of non-annotated small protein-coding genes of unknown functions in Arabidopsis (Aubourg et al., 2007). Further we investigated the whole CATMA resource available at this time in order to identify new genes induced by various stresses for further functional analyses. Among them, AT5G44585 caught our attention because of its highly informative profile: this gene was differentially expressed in 136 experiments (21%), being strongly induced in response to a large panel of biotic or oxidative stresses, Erwinia amylovora infection being in the top of them. Biological contexts were extracted from each CATdb experiment (http://tools.ips2.u-psud.fr/CATdb) and classified in 8 classes (Fig. 1 and Table S2). It is noteworthy that no less than 70% of the complete transcriptomic response of AT5G44585 could be summarized with three keywords: pathogen response, oxidative stress and root growth. Generally, we found this gene to be strongly up-regulated in most biotic and oxidative stress conditions, while it was down-regulated in conditions aiming at diminishing oxidative stress. Furthermore, AT5G44585 exhibited a constitutive expression in roots in growth conditions but is down-regulated in numerous conditions affecting root elongation such as nitrogen starvation (Krapp et al., 2011). This advocated for further exploration of this gene in oxidative stresses, root development, and in response to pathogen infections.

The screening of the Arabidopsis genome revealed that AT5G44585 belongs to a small family of 14 unknown homologous genes with similar intron-exon structure (2 or 3 exons), encoding proteins ranging from 72 to 117 amino-acids (aa). Analysis of the N-terminal regions using the SIGNALP v4.1 [START_REF] Nielsen | Predicting secretory proteins with SignalP[END_REF] and the PREDOTAR v1.04 [START_REF] Small | Predotar: A tool for rapidly screening proteomes for N-terminal targeting sequences[END_REF] software revealed a signal peptide targeting proteins to the endoplasmic reticulum to be present in all members of the family. DeepLoc v1.0 [START_REF] Armenteros | DeepLoc: prediction of protein subcellular localization using deep learning[END_REF] predict an extracellular localization for the 14 proteins with scores ranging from 0.88 to 1. The 14 genes are organized in two tandemly arrayed clusters on chromosomes 1 and 5 (Fig. 2A). The largest 37 kb long gene cluster on chromosome 5 contains numerous vestiges of transposable elements (Helitron type) which could have impacted evolution of this family through local duplication events. Manual annotation revealed two additional yet non-annotated genes located between AT5G44565 and AT5G44568. Both share significant similarities with the other tandemly arrayed homologs and cognate expressed sequence tags (ESTs) validate their transcription. Our manual annotation also led to the correction of the structure of AT5G44570 in which an overpredicted 3' coding exon has been removed. The size of the proteins, the number and the organization of paralogs, the aa composition (notably the absence of cysteine) and the presence of a signal peptide are common features shared by the PTMP families previously published (Matsubayashi, 2014). Furthermore, as described below, we identified a short conserved motif in the C-terminal region of these proteins, candidate to be mature functional peptides after proteolytic processing. For these reasons, this newly identified family has been named PROSCOOP as putative precursors of SCOOP peptide (Serine riCh endOgenOus Peptide). The genes are termed PROSCOOP1 to 14 (AT5G44585 being PROSCOOP12) and the corresponding mature peptides named SCOOP1 to 14 (Fig. 2A).

Previously reported RNA-seq approaches [START_REF] Hruz | Genevestigator v3: a reference expression database for the meta-analysis of transcriptomes[END_REF] allowed us to broaden our transcriptome analysis to the PROSCOOP family members that were missing on the microarrays (only 4 of them are present in the Affymetrix Ath1 chip). We could confirm the regulation of their transcription in several stress conditions and organs (Fig. 2B). These data show a large diversity of transcription profiles inside this family suggesting its involvement in different biological functions. Notably, PROSCOOP12 shows a distinct transcription profile as it is among the minority of paralogs to be highly induced by different pathogen aggressions and expressed in the whole root system.

In order to assess the evolutionary conservation of the PROSCOOP family, an extensive BLASTP search for homologs in GenBank was carried out. We identified this family in several Brassicaceae genomes reaching from Eutrema salsugineum to Camelina sativa and the number of identified homologs in these genomes ranged from 1 to 13. Outside Brassicaceae genus, no similar proteins could be detected despite low stringency searches. The phylogenetic tree built from the multiple alignment of the 74 identified PROSCOOP homologs shows that gene duplications occurred before speciation of the 8 different Brassicaceae species (Fig. S1).

In order to identify divergent yet still conserved smaller regions, the MEME algorithm (Bailey et al., 2015) was used, excluding full length alignments, on the 74 identified homologs. This sensitive approach allowed the identification of two significantly conserved 11 aa-long motifs (Fig. 3). These motifs are good candidates for functional mature peptides (or a part of them)

following the putative proteolytic processing of the corresponding precursor. Indeed, both motifs are proline-, serine-, arginine-and glycine-rich, as in previously described PTMP families such as CLV3/CLE (Betsuyaku et al., 2011), IDA (Vie et al., 2015), PIP (Hou et al., 2014) and CEP (Roberts et al., 2013). The motif 1 is more ubiquitous than the motif 2 since it was detected in 72 sites (e-value of 9.8e-213) compared to 39 sites (e-value of 3.4e-179) out of the 74 PROSCOOP homologs. Therefore, we have focused our downstream functional analysis on motif 1 (Fig. 3), named SCOOP thereafter.

PROSCOOP12 is co-expressed with genes involved in hormone signaling and defense

In order to make a first assessment of the potential biological relevance of PROSCOOP12 and to predict its putative functional partners, we further mined previously published Arabidopsis transcriptome data (Gagnot et al., 2008). Based on the assumption that genes with related biological functions are likely to be co-expressed [START_REF] Schöner | Network analysis of systems elements[END_REF], we used the results

of the Gaussian mixture model-based clustering method from the GEM2Net resource [START_REF] Maugis | Variable selection in model-based clustering: A general variable role modeling[END_REF][START_REF] Zaag | GEM2Net: from gene expression modeling to -omics networks, a new CATdb module to investigate Arabidopsis thaliana genes involved in stress response[END_REF]. The PROSCOOP12 gene was found to be co-expressed with 83 genes in a set of experimental samples gathering biotic stress triggered by necrotrophic bacteria and fungi. This cluster of 83 genes has been enriched by the integration of functional partners based on co-citations, protein-protein interactions and common biological pathways) in using TAIR, the Arabidopsis interactome (Arabidopsis Interactome Mapping Consortium, 2011) and the STRING database [START_REF] Szklarczyk | The STRING database in 2017: quality-controlled protein-protein association networks, made broadly accessible[END_REF]. This step resulted in a network of 117 genes (Tables S3A andS3B) mainly focused on hormone crosstalk (especially SA/JA signaling), pattern-triggered immunity (PTI), brassinosteroid and phenylpropanoid pathways and nitrogen metabolism (Fig. S2). Out of 117 genes, 53 are involved in response to stimulus (GO:0050896, fdr 1.31e-11), among them, 26 genes are classified in defense response (GO:0006952, fdr 5.72e-10), and 14 in transmembrane signaling receptor activity (GO:0004888, fdr 1.41e-09) . Numerous key defense actors were found to be clustered with PROSCOOP12 such as the NIMIN1, IOS1, NHL6, MLO12, FRK1, LECRKA4.1, CRK13, HA2 genes and the and -70 transcription factors. This relational network contains two other genes encoding PTMPs, namely PROVIR10 and PSK4, and two PTMP receptor kinases, PSKR1 and PSY1R that are involved in root development and modulation of SA/JA defense responses (Mosher et al., 2013). PROVIR10 has been found to correlate positively with disease triggered by necrotrophic pathogens [START_REF] Dobón | Novel disease susceptibility factors for fungal necrotrophic pathogens in Arabidopsis[END_REF] and PSK4

encodes a phytosulfokine, one of the peptide growth factors involved in disease establishment [START_REF] Rodiuc | Evolutionarily distant pathogens require the Arabidopsis phytosulfokine signalling pathway to establish disease[END_REF]. This approach led us to explore the role of PROSCOOP12 and its SCOOP12 peptide regarding fungal and bacterial infections.

PROSCOOP12 is involved in pathogen defense and root development

Screening Arabidopsis mutant collections [START_REF] Roudier | Integrative epigenomic mapping defines four main chromatin states in Arabidopsis[END_REF] we identified a T-DNA mutant proscoop12 in the Wassilewskija (Ws) background. Homozygous mutant plants did not transcribe PROSCOOP12 (Fig. S3). Based on PROSCOOP12 transcription induction in the presence of different pathogens, (Fig. 1, Fig. 2B and Table S2), the analysis of co-expressed putative partners, and its putative role as a secreted DAMP, we decided to challenge the mutant with the necrogenic bacterium Erwinia amylovora and the necrotrophic fungus Alternaria brassicicola.

Compared to wild-type plants, proscoop12 displayed a higher tolerance to E. amylovora induced cell death as observed by a reduction of necrotic symptoms in leaves (Fig. 4A). This phenotype has only been observed in wrky70 [START_REF] Moreau | EDS1 contributes to nonhost resistance of Arabidopsis thaliana against Erwinia amylovora[END_REF]. Like WRKY70, PROSCOOP12 acts as a negative regulator of defense against this bacterium. The transcription factor WRKY70 is known to positively regulate WRKY60 and it is involved in the JA/SA crosstalk [START_REF] Li | The WRKY70 transcription factor: a node of convergence for jasmonate-mediated and salicylate-mediated signals in plant defense[END_REF]. Notably, these two genes have been found clustered with PROSCOOP12 in our gene network analysis (Fig. S2). Then, we performed a micro-array transcriptomic comparison of proscoop12 versus wild-type following bacterial inoculation. Results show that 3,731 genes were differentially expressed in wild-type in response to E. amylovora, and 4,125 in proscoop12. Despite the difference in symptom intensity, the vast majority of the bacteriaresponsive genes did not display significant differences in both lines. Indeed, only 131 genes displayed a significantly different expression (Bonferroni p-value 5%) between wild-type and proscoop12 infected plants (Table S4): 126 up-regulated and 5 down-regulated genes, these latter corresponding only to hypothetical proteins or pseudogenes.

The 126 up-regulated genes that may contribute to the difference in symptoms between proscoop12 and wild-type were challenged by functional annotation adding literature references to GO terms to provide additional information (Table S4, summarized in Fig. 4B).

Indeed, 45% of them are connected to defense response (such as HR4, SQP1, AED1, MKK2, HD2B, NPR3) and/or protection against oxidative stress (such as ALDH24B, BiP2, APX1, ATOM1, APR1, PER50). Moreover, 18% were related to response to other stresses, mainly oxidative stress, and 10% could have indirect links with stress since involved in processes such as cell wall modifications or proteolysis. Only 13% could not be related to the phenotype, often because their function is currently unclear. Finally, the remaining 14% are unknown genes. The high percentage of genes directly related to protection to oxidative stress supports the hypothesis of a relationship between PROSCOOP12 and the control of ROS production.

The response of proscoop12 to a necrotrophic fungus infection was assessed using the Arabidopsis -A. brassicicola pathosystem [START_REF] Pochon | The Arabidopsis thaliana-Alternaria brassicicola pathosystem: a model interaction for investigating seed transmission of necrotrophic fungi[END_REF]. A. brassicicola inoculation of rosette leaves produced similar symptoms in wild-type and proscoop12 genotypes (Fig. S4).

Because seedling infection by A. brassicicola is mainly caused by seed transmission, we have also observed the fungus colonization during germination of infected seed lots under controlled conditions. Two days after sowing, proscoop12 showed a significantly lower rate of germinating seeds prone to A. brassicicola infection compared to the wild-type (Fig. 4C).

Because our transcriptome analysis suggested that PROSCOOP12 may play a role in root development (Fig. 1 and Fig. 2B), we compared the root lengths of wild-type and proscoop12

plants. Indeed, proscoop12 plants developed significantly longer roots than control plants (Fig. 5A and Fig. 5B). No significant difference was observed between wild-type and proscoop12 regarding the seedling fresh weight (Fig. 5C).

A second proscoop12 line was obtained in the Col-0 background using a CRISPR-Cas9 approach. The frameshift obtained in the first exon disrupts the coding frame 10 aa after the editing event, upstream the conserved motif. The phenotypes previously observed with the Ws proscoop12 mutant were confirmed in this Col-0 mutant line (Fig. S5).

The SCOOP12 peptide has main features of DAMPs

The structural features of the PROSCOOP12 protein suggested that it should be classified as a secreted PTMP. Besides, at the functional level, its transcriptional behavior suggested that it may play a role as a DAMP. Indeed, the induction of PROSCOOP12 expression by a large panel of biotic stresses and the root phenotypes identified in the proscoop12 mutant revealed some analogies with the AtPROPEP1 and AtPROPEP2 genes which are the precursors of the AtPep1 and AtPep2 peptides respectively, well-characterized DAMPs (Bartels and Boller, 2015). Likewise, both genes are also induced by biotic stress (Huffaker et al., 2006) and the AtPep1 DAMP is involved in root development since the overexpression of AtPROPEP1 and AtPROPEP2 causes significantly longer roots (Huffaker et al., 2006). Therefore, we wanted to test if PROSCOOP12 encodes for peptide that may act as a DAMP by comparing it to AtPep1.

The SCOOP12 peptide induces immune responses in Arabidopsis

Based on the identification of the conserved motif 1 (Fig. 3), a putative mature peptide SCOOP12 was defined (PVRSSQSSQAGGR) from PROSCOOP12 and synthetized in order to explore its biological function. Despite the non-predictable post-translational modifications, we tested the exogenous application of the synthetic SCOOP12 peptide as previously described for CLE and RGF PTMP families (Murphy et al., 2012;Matsuzaki et al., 2010;Whitford et al., 2012). Treatment of plants with SCOOP12 induced a wide range of long-and short-term immune responses (Fig. 6). One of the fastest defense responses is the production of ROS (Torres et al., 2006). We show here that SCOOP12 induced a more intensive ROS burst compared to AtPep1 but weaker than flg22 (Fig. 6A). Next, we wanted to study the effect of SCOOP12 on genes closely linked to early defense mechanisms. FRK1 has previously been shown to be induced by pathogens, elicitors, salicylic acid (Asai et al., 2002;Boudsocq et al., 2010) and AtPep1 (Flury et al., 2013). Furthermore, our co-expression network approach identified a co-expression of PROSCOOP12 with FRK1 (Fig. S2). Therefore, we measured the FRK1 expression level in detached leaves floating for 2h in solutions supplemented by SCOOP12 or AtPep1. Compared to controls, AtPep1 and SCOOP12 treatments resulted in a 15-fold and 8.5-fold increase in FRK1 expression, respectively (Fig. 6B). The deposition of callose is also known to be triggered by DAMPs (Luna et al., 2011). Callose staining after 24h of treatment with the elicitor peptides showed that SCOOP12 induces a callose deposition, yet at a weaker level compared to flg22 or AtPep1 (Fig. 6C and Fig. 6D). One of the long-lasting defense responses is an inhibition of growth caused by the elicitor (Krol et al., 2010). Our results indicate that perception of SCOOP12 also leads to an arrest of growth. The effect is comparable to the flg22 and the AtPep1 DAMP (Fig. 6E, Fig. 6F and Fig. 6G).

In order to demonstrate the specificity of SCOOP12 sequence, we synthesized a peptide based on a randomized version of the same amino acids and tested plant responses to this scrambled SCOOP12 (scSCOOP12). Furthermore, we synthesized peptides with double alanine replacements (SCOOP12 S5/7A) and single replacements (SCOOP12 S5A; SCOOP12 S7A) to test the importance of the two highly conserved serine residues on positions 5 and 7 of SCOOP12 (Fig. 3) for its activity. Plants treated with scSCOOP12 as well as with the modified peptides did not show seedling growth inhibition. Total seedling fresh weight as well as root length were not different from that of control plants (Fig. 7A). Finally, treatments with scSCOOP12, SCOOP12 S5/7A and SCOOP12 S5A did not induce a ROS burst and only SCOOP12

S7A resulted in a low, still significant ROS burst (Fig. 7B). These results highlight the importance of the amino acid order and the highly conserved serine residues for the perception of SCOOP12 by the plant.

Next, we wanted to test the conservation of plant responses to SCOOP12. For that purpose, plants were selected in which we identified PROSCOOP homologues (Brassica napus, Fig. S1) and plants that do not contain this gene family (Nicotiana benthamiana and Lycopersicon esculentum). We measured ROS production following application of SCOOP12 in these plants and included flg22 as a positive control. We detected a ROS burst caused by flg22 in all four plant species. On the other hand, SCOOP12 only resulted in a ROS burst in A. thaliana and at a lower, yet still significant level, in B. napus (Fig. S6). SCOOP12 seems to be similar enough to its closest B. napus homolog (BNCDY22858 with the motif FAGPSSSGHGGGR) to trigger a ROS burst. Therefore, only the two plant species containing homologues of the PROSCOOP gene-family members, showed a response to SCOOP12 treatments.

Pre-treatment with the SCOOP12 peptide protects Arabidopsis against Pseudomonas infection

It has previously been shown that priming of plants with the flg22 elicitor as well as with oligogalacturonides could result in enhanced tolerance against subsequent bacterial infections. For instance, plants pre-treated with these elicitors showed significantly reduced lesion size following an infection with Botrytis cinerea (Raacke et al., 2006;Ferrari et al., 2007). Using a similar assay, we found that plants pre-treated with flg22 as well as with SCOOP12 and AtPep1 were less susceptible to Pseudomonas syringae pv. tomato DC3000 infection (Fig. 8). The effect of the two endogenous peptides SCOOP12 and AtPep1 was weaker than flg22, which is consistent with the fact that flg22 induced stronger defense response compared to SCOOP12 (Fig. 6A and Fig. 6C).

SCOOP12 and AtPep1 induce the expression of several PROSCOOP genes

It has previously been shown that small endogenous peptides can induce the expression of their own precursors resulting in a positive feed-back loop. For instance, expression of several PROPEP genes can be induced by different AtPep peptides (Huffaker and Ryan, 2007). This led us to investigate the change in steady state transcript level of all 14 PROSCOOP family members after SCOOP12 exposure. Moreover, we decided to add AtPep1 in our assay for comparison since it is also known to induce the transcription of another peptide precursor, prePIP1 (Hou et al., 2014). The results show that PROSCOOP 2, 7, 8, 12 and 13 are upregulated by the AtPep1 treatment (Fig. S7). Most importantly, the direct precursor PROSCOOP12 is upregulated by SCOOP12 in comparison to the control treatment (Fig. S7L). Therefore, there is a positive feedback loop linking SCOOP12 to its precursor PROSCOOP12 but also of other members of the PROSCOOP family such as PROSCOOP7. However, SCOOP12 did not induce the expression of PROPEP1 (Fig. S7O). These results suggest that there is a feedback loop of SCOOP12 to its precursor and to PROSCOOP7 and that AtPep1 is capable of inducing five members of the PROSCOOP family.

The BAK1 co-receptor is involved in SCOOP12 perception

A well characterized co-receptor of several receptors of small peptides is BRI1-associated kinase1 (BAK1). Interaction of BAK1 with receptor-like kinases that act as elicitor receptors, was proposed to be due to conformational changes occurring after ligand binding which results in the formation of the receptor complex (Chinchilla et al., 2009;Liu et al., 2017). To test if BAK1 is involved in the perception of SCOOP12, a seedling growth inhibition assay was performed on bak1-4 plants. Compared to wild-type controls, bak1-4 plants did not display any significant growth inhibition upon SCOOP12 treatment (Fig. 9A). The same approach was carried out on fls2 (the flg22 receptor) and pepr1/pepr2 plants. Contrary to BAK1, our results

suggest that these receptors are not involved in the perception of SCOOP12 (Fig. 9B and Fig. 9C).

SCOOP12 rapidly activates phospholipid signaling pathways in Arabidopsis cell suspensions

Lipid signaling pathways act as multifunctional regulatory mechanisms in plants. They incorporate several groups of inducible enzymes that convert membrane phospholipids into signaling molecules.

Phosphatidic acid (PA) is a well-known biologically active lipid that is produced in response to numerous hormonal and stress signals including, notably, flg22 (van der Luit et al., 2000). We demonstrate that application of SCOOP12 induces an accumulation of PA in Arabidopsis cell suspensions (Fig. 10A). This effect is observed as early as 5 min following SCOOP12 application in a low concentration of 100nM (Fig. 10B and Fig. 10C). The scSCOOP12 had no effect on PA accumulation. Two modes of PA accumulation are known: Phospholipase D (PLD)-dependent via direct hydrolysis of membrane phospholipids and diacylglycerol kinase (DGK)-dependent via phosphorylation of diacylglycerol (DAG). In our experiment a labelling protocol that favors visualization of DGK-derived PA was used (Arisz and Munnik, 2013). Phosphatidylinositol 4,5bisphosphate (PIP2) is a substrate to phosphatidylinositol-specific phospholipase C (PI-PLC) that produce DAG. We have also observed that the level of PIP2 is transiently reduced following SCOOP12 treatment (Fig. 10B). These results suggest that SCOOP12 initiates a signaling cascade implicating PI-PLC (causing the depletion of PIP2) and subsequent production of PA via phosphorylation of DAG by DGK.

Discussion

Considered jointly, our transcriptome, mutant phenotyping and peptide assay results allow us to propose a model explaining the roles of the SCOOP12 peptide in Arabidopsis (Fig. 11).

The induction of numerous genes involved in the protection against oxidative stress such as peroxidases, glutathione transferase, phenylpropanoid synthases in proscoop12 in response to E. amylovora infection (Table S4) might indicate that its lack of expression could result in a decrease in H2O2 levels. This could impair E. amylovora progression in leaves, which is known to induce H2O2 production in plants in order to promote cell death and invade plant tissues [START_REF] Venisse | Evidence for the involvement of an oxidative stress in the initiation of infection of pear by Erwinia amylovora[END_REF][START_REF] Degrave | Erwinia amylovora type three-secreted proteins trigger cell death and defense responses in Arabidopsis thaliana[END_REF]. In parallel it is known that antioxidant responses in roots decrease the H2O2 level in the elongation zone, thereby contributing to root growth (Dunant et al., 2007;[START_REF] Tsukagoshi | Transcriptional regulation of ROS controls transition from proliferation to differentiation in the root[END_REF]. The constitutive expression of PROSCOOP12 in roots (Fig. 2) could therefore contribute to higher levels of H2O2 and act as a moderator of root elongation under normal conditions. This is consistent with the greater root length observed in proscoop12 (Fig. 5) and with the decrease of PROSCOOP12 expression in roots in conditions leading to root lengthening such as nitrogen starvation (Table S2).

In addition to its function in root elongation, we found PROSCOOP12 to be involved in response to biotic stress in aerial parts where its transcription is strongly induced in presence of pathogens (Fig. 1 and Fig. 2B). This induction triggers a ROS burst, putatively through the inhibition of the antioxidant responses and then participates to the increase of H2O2 level in the infected tissues. This mechanism occurs when we apply the synthetic SCOOP12 peptide on seedlings, as illustrated by its induction of ROS burst, transcription of the FRK1 defense gene, and callose deposition in leaf cells (Fig. 6). SCOOP12-induced PA production (Fig. 10) can be a part of a signaling cascade implicating several PA-binding proteins [START_REF] Pokotylo | The phosphatidic acid paradox: Too many actions for one molecule class? Lessons from plants[END_REF]. PA binds NADPH oxidase isoforms D and F and stimulates NADPH oxidase activity in guard cell protoplasts [START_REF] Zhang | ±SE of the mean of 4 replicates. (D) Localization of callose deposition by aniline blue staining. (E, F, G) Quantification of seedling growth inhibition. 5 days old seedlings were transferred from solid MS medium to liquid medium supplied with the indicated elicitors (all applied in a final concentration of 1µM) and are grown for additional 8 days before fresh weight and root length was quantified and pictures were taken. For all experiments, error bars show ±SE of the mean of 6 biological replicates[END_REF]. That is why PA production is likely to be upstream of ROS accumulation observed in response to SCOOP12. We have shown that the effects of SCOOP12 are BAK1-dependent (Fig. 9A). It is known that the activity of BAK1 in receptor complexes is dependent on its phosphorylation state and is controlled by protein phosphatase 2A (PP2A) [START_REF] Segonzac | Negative control of BAK1 by protein phosphatase 2A during plant innate immunity[END_REF]. PA interacts with the scaffolding A1 subunit of PP2A, tether it to membranes and induce its activity [START_REF] Gao | Phosphatidic Acid (PA) Binds PP2AA1 to Regulate PP2A Activity and PIN1 Polar Localization[END_REF]. This process was highlighted in connection to PIN1 dephosphorylation by PP2A in auxin signaling cascade.

However similar reactions are to be expected for BAK1 dephosphorylation in PAMP/DAMP receptor complexes and indicate that they may act as intrinsic part of SCOOP12 regulatory cascade in plants.

The negative action of SCOOP12 on antioxidant response is consistent with the reduction of symptoms observed in the proscoop12 defective mutant in presence of the necrogenic bacterium E. amylovora [START_REF] Degrave | Erwinia amylovora type three-secreted proteins trigger cell death and defense responses in Arabidopsis thaliana[END_REF]. In this case, the suppression of PROSCOOP12 seems to enhance the protection against oxidative stress, thus hampering bacterial development in infected Arabidopsis leaves.

The comparison of the PROSCOOP family with other previously published genes encoding such secreted peptides highlights numerous shared features but also interesting specificities.

At the structural level, the PROSCOOP proteins distinguish themselves by the absence of a highly conserved C-terminal region. Indeed, the motifs detected with the MEME tool are quite divergent compared to the other PTMP precursors (Matsubayashi, 2011). This divergence may explain the fact that no PROSCOOP homologs could be detected outside the Brassicaceae genomes. This restricted phylogenetic profile is opposite to the other described secreted peptides which are conserved both in monocots and eudicots. Furthermore, contrary to the majority of the known PTMPs, the conserved motifs are not localized at the C-terminal extremity of their precursors, and their maturation could involve two steps of proteolytic processing or a trimming step (Matsubayashi, 2011). Out of the 14 Arabidopsis PROSCOOP proteins, three include two duplicated SCOOP motifs (Fig. 3), reminiscent of the previously described cases of the CEP and PIP families (Roberts et al., 2013;Vie et al., 2015) and also of the CLE18 protein in which each copy of the conserved CLE motifs has a specific function (Murphy et al., 2012). The motif composition classifies SCOOP in the superfamily of 'SGP-rich peptide' among PIP, CLE, IDA, PEP and CEP families (Hou et al., 2014). At the functional level, the triggering of ROS burst, FRK1 transcription and callose deposition move SCOOP12 close to the cytosolic AtPEP and apoplastic PIP families (Huffaker et al., 2006). Our results suggest a functional link between AtPep1 and SCOOP12 since both peptides induce the transcription of PROSCOOP12 (Fig. S7L). This collaboration between different peptide families has also been described with AtPEP1 and PIP1 which act cooperatively to amplify triggered immunity.

Furthermore, the signaling induced by AtPep1 (Schulze et al., 2010), PIP1 (Hou et al., 2014) Figure 4B, keywords regarding functional pathways and references for justification. Pseudo color scale representing the differential expression is described in the first line of the table. S2. S4.

(C) Effect of A. brassicicola seed infection on proscoop12 during germination 2, 3 and 8 days post-imbibition. Significant differences according to Student's t-test results: *, P < 0.05.

Figure 1 :

 1 Figure 1: Small endogenous signaling peptides mediates a high number of plant physiological

Figure 2 :

 2 Figure 2: The "zigzag" model describing the coevolution of plants defense mechanisms and the pathogen countermeasures. The first level of plant defense is the induction of PTI. The detection of MAMPs induces these basal defense responses. Pathogens can evade from PTI orblock its activation by carrying specific effectors that attenuate defense response and render the plant susceptibility to the pathogen (ETS). The recognition of pathogen effectors induces a strong defense response, stronger than the PTI response. This response leads to the cell-death like HR and finally to ETI. (adopted from[START_REF] Jones | The plant immune system[END_REF]

Figure 3 :

 3 Figure 3: The chronology of PTI responses. After elicitor application various plant cell responses can be measured. Altered membrane potentials (Vm) and ion fluxes are the earliest events. The next responses are related to production of defense molecules and messenger (JA, SA and ROS/H2O2).

Figure 4 :

 4 Figure 4: Modified responses of primed plants in comparison to unprimed (naïve) plants. Plants that faced a priming stress show different responses to a triggering stress than naïve plants. Primed plant responses are indicated in purple and unprimed responses in black. The primed plants may respond to the triggering stress faster/earlier, stronger or more sensitized than the unprimed plants. Primed plants may induce different networks of genes than unprimed plants. (adopted from (Lämke and Bäurle, 2017)).

Figure 5 :

 5 Figure 5: Summary of the relation between defense responses (solid lines) and plant fitness (dashed lines) in primed (red) and unprimed plants (blue). A) Defense reaction during priming and in the primed state is only transient and weakly induced. B) The direct activation of defense and the resulting fitness costs without priming stimulus is higher than the fitness cost of the priming stimulus. C) Upon a triggering stress, primed plants are capable of mobilizing cellular defense in a faster, earlier, stronger and more sustained fashion than unprimed plants. D) Primed plants do defend better against the triggering stress than unprimed plants. Adapted from (Martinez-Medina et al., 2016).

Figure 6 :

 6 Figure 6: A) PROSCOOP family gene organization: coding exons and introns are represented by blue boxes and blue broken lines respectively. Remains of transposable elements (Helitron type) are represented by orange boxes and the green one indicates a putative non-coding RNA of unknown function. The TAIR gene names and corresponding PROSCOOP nomenclature are indicated. B) Conserved motifs identified in the PROSCOOP family proteins: The conserved motives were found within all 74 homologous PROSCOOP proteins in Brassicaceae genomes. P-values and motif locations are only shown for the 14 members from Arabidopsis.

Figure 7 :

 7 Figure7: Phylogenetic tree of PROSCOOP homologs. The tree was built with the neighbor-joining method from the multiple alignment of 74 homologous Brassicaceae proteins. Gaps were ignored for tree building and 1000 bootstrap replicates were used to determine the robustness of each node (values > 50% are highlighted in yellow). Except for Arabidopsis thaliana for which PROSCOOP nomenclature is used, each protein is labelled with two letters (species) and its GenBank ID or XP number.

Figure 8 :

 8 Figure 8: Confirmation of mutations in PROSCOOP12 in two accessions. (A) The mutant in Col-0 (Columbia) background was created using the CRISPR-Cas9 approach. The guide-RNA was designed in the first exon. DNA of 15 proscoop12 mutants was extracted and sequenced. The alignment to the reference sequence (TAIR10) shows that proscoop12 has a single base insertion. (B) A 450bp long fragment covering the guide-RNA sequence was amplified by PCR. The PCR product of Wt and proscoop12 DNA was digested using the restriction enzyme HaeII. (C) Confirmation of absence of transcription of PROSCOOP12 in the T-DNA knock-out line by RT-PCR in Ws (Wassilewskija). Actin2 expression (ACT2) is used as control.

Figure 9 :

 9 Figure 9: phenotypic comparison between wild-type and proscoop12 plants.

  EFG).

Figure 10 :

 10 Figure 10: Defense responses induced by SCOOP12. (A) Reactive oxygen species (ROS) measured in RLU (relative light units) production in wild-type Arabidopsis leaf-discs (Col-0), treated with 1µM for each peptide or without elicitor (control). Graphs display averages of 12 replicates. (B) Induction of (FRK1) gene transcription in soil-grown plants treated with 1µM of the indicated peptide

Figure 11 :

 11 Figure 11: Dose-dependent effect of SCOOP12. Quantification of seedling growth inhibition. 5 days old seedlings were transferred from solid MS medium to liquid medium supplied with flg22 or SCOOP12 with the indicated concentration and were grown for additional 8 days before fresh weight and root length was quantified and pictures were taken. The bars represent the mean of 6 replicates. Error bars show ±SE of the mean. Significant differences according to Student's t-test results: *, P < 0.05; ***, P < 0.001.

Figure 12 :

 12 Figure 12: Transcriptional response of PROPEP1 and PROPEP2 to SCOOP12 and AtPep1. A) transcription of PROPEP1 is induced by AtPep1 but not by SCOOP12. B) Transcription of PROPEP2 increases 3 folds by SCOOP12 application. Expression levels were determined by normalization to ACR12 transcripts, and bars indicate the fold change of transcription relative to the control treatment of two independent biological replicates. Error bars show the relative ±1 SE of the mean. Significant differences according to Student's t-test results: *, P < 0.05.

Figure 13 :

 13 Figure 13: P. syringae pv Tomato DC3000 (Pst DC3000) (P.s.t) infection Assay of Wild-Type Arabidopsis (WS) and proscoop12. A) Leaves were infected with 10 5 cfu ml -1 Pst. DC3000, and bacterial growth was assessed 1 and 2 days after infection. B) No phenotypic infection symptoms difference could be observed between wildtype and proscoop12 plants.

Figure 14 :

 14 Figure 14: SCOOP12 activity depends on the correct amino-acid order and two highlyconserved Serine. Assays were carried out with scrambled peptide (scSCOOP12) and alanine replacements of conserved serine residues at position 5 and 7 of SCOOP12 (PVRSSQSSQAGGR) (SCOOP12 S5/7A; SCOOP12 S5A; SCOOP12 S7A). (A) Quantification of seedling growth inhibition with the indicated elicitors. Bars of quantified fresh weight and root length represent mean of six replicates. (B) Reactive oxygen species (ROS) in RLU (relative light units) production in wild-type Arabidopsis leaf-discs (Col-0), treated with 1µM for each peptide or without elicitor (control). Graphs display averages of 12 replicates. Error bars show ±SE of the mean. Significant differences according to Student's t-test results: *, P < 0.05; ***, P < 0.001.

Figure 15 :

 15 Figure 15: ROS burst measurements on selected plant species treated with SCOOP12. ROS burst assay was performed on Arabidopsis thaliana (Col-0), Lycopersicon esculentum, Nicotiana benthamiana and Brassica napus. The flg22 and SCOOP12 peptides were added to a final concentration of 1µM. Bars display the average of the maximum ROS burst in RLU (relative light units) of 12 replicates.

Figure 16 :

 16 Figure 16: Seedling growth inhibition assay on selected receptor mutant backgrounds. (A) bak1 plants were insensitive to SCOOP12. Neither fresh weight (top) nor root length (center) were affected by SCOOP12 treatment. The FLS2 (B) and PEPR1/PEPR2 (C) receptor mutants were not affected in their perception of SCOOP12. Plants were grown for 8 days in presence of 1µM SCOOP12 or control solution. Bars of quantified fresh weight and root length represent mean of six replicates. Error bars show ±SE of the mean. Significant differences according to Student's t-test results ***, P < 0.001.

Figure 17 :

 17 Figure 17: Seedling growth inhibition assay using the bak1-5 bkk1-1 double mutant. bak1-5 bkk-1 plants are nearly completely impaired in SCOOP12 perception. Fresh weight is not affected by SCOOP12 treatment while root length shows a reduction upon SCOOP12 treatment. Plants were grown for 8 days in presence of 1µM SCOOP12 or control solution. Bars of quantified fresh weight and root length represent mean of six replicates. Error bars show ±SE of the mean. Significant differences according to Student's t-test results ***, P < 0.001; *, P< 0,05.

Figure 18 :

 18 Figure 18: Rapid activation of PA production in Arabidopsis cell suspensions following treatment with SCOOP12. (A) Separation of P 33 -labelled lipids using thin-layer-chromatography with contrasting effects of SCOOP12 (10µM) and scrambled scSCOOP12 (10µM) on the level of PA accumulation visible after 5 min of treatment. Significant differences according to Student's t-test results: ***, P < 0.001. (B) Time-scale of the SCOOP12 (1µM) influence on PA and PIP2 accumulation in Arabidopsis cell suspensions (C) Dose-scale of the SCOOP12 influence on PA and PIP2 accumulation in Arabidopsis cell suspensions after 5 min of treatment. All experiments were performed with at least three biological replicates. Error bars show ±SE of the mean. PA, phosphatidic acid; PIP2, phosphatidylinositol 4,5-bisphosphate; PI, phosphatidylinositol; PC, phosphatidylcholine; a.u., arbitrary units.

  Parts of this chapter are published in modified version inGully et al., (2018): "The SCOOP12 peptide regulates defense response and root development in Arabidopsis thaliana". In this chapter my contribution to this paper included the complete experimental part (excepted the in-silico prediction of the applied peptides) and data compilation. This chapter could be the onset of a second publication and additional future work introducing additional peptides of the SCOOP family apart of SCOOP12 as well as interesting root phenotypes caused by members of the SCOOP peptide family.5.1. AbstractSmall endogenous peptides are involved in controlling plant defense responses as well as most plant regulatory processes. Especially the regulation of root growth is under focus of research on small endogenous peptides. Small peptides have an important role in root development. Next to classical plant hormones Small peptides are a growing class of regulatory molecules. These molecules are involved in many aspects of root development.

Figure 19 :

 19 Figure 19: Transcriptional response of the PROSCOOP gene family to SCOOP12 and AtPep1. SCOOP12 and AtPep1 induce the expression of several PROSCOOP gene family members. Expression level of the PROSCOOP gene family members (A to N) were determined by qPCR with a normalization to ACR12 transcripts, bars indicate the fold change of transcription relative to the control treatment of at least five independent biological replicates. Error bars show the relative ±1 SE of the mean. Significant differences according to Student's t-test results: *, P < 0.05; **, P < 0.01; ***, P < 0.001.

  Sequence SCOOP1 ETP PSR SRR GGG G SCOOP2 PVR SSR SPR SPS F SCOOP3 ELR PSS EWR RKM I SCOOP4 ASF HSA SPK DKG P SCOOP5 IVR RSR SQR GRQ Y SCOOP6 EAR PSK SKK GGG R SCOOP7 RAG PSK SGQ GGG R SCOOP8 DFE GSI SGQ AGG G SCOOP9 GTG PSH SGH GGS S SCOOP10 FTG PSG SGH GGG R SCOOP11 DVG ASS SGQ GGG R SCOOP12 PVR SSQ SSQ AGG R SCOOP13 YLP PSK SRK GKG P SCOOP14 FVP PST SHK GQG P

Figure 20 :

 20 Figure 20: ROS induction by the SCOOP peptide family. Reactive oxygen species (ROS) in RLU (relative light units) production in wild-type Arabidopsis leaf-discs (Col-0), treated with 1µM for each peptide or without elicitor (control). Graphs display averages of 12 replicates. Error bars show ±SE of the mean. Significant differences according to Student's t-test results: ns (not significant), **, P < 0.01; ***, P < 0.001.

Figure 21 :

 21 Figure 21: Quantification of seedling growth inhibition. 5 days old seedlings were transferred from solid MS medium to liquid medium supplied with the indicated elicitors (all applied in a final concentration of 1µM) and were grown for additional 8 days before fresh weight and root length was quantified and pictures were taken. The bars represent the mean of 6 biological replicates. Error bars show ±SE of the mean. Significant differences according to Student's t-test results: ns (not significant) **, P < 0.01; ***, P < 0.001.

Figure 22 :

 22 Figure 22: SCOOP12 induces a strong root pigmentation. Seedlings were treated as before described for the seedling growth inhibition assay. A) Roots of seedlings grown for 8 days in media supplied with the indicated elicitor peptide applied in a final concentration of 1µM. B) neutral red staining of the same roots shown in A.

Figure 23 :

 23 Figure 23: Tissue specific root pigmentation caused by SCOOP6, 7, 12. Seedlings were grown for 8 days in media supplied with the indicated elicitor peptide with a final concentration of 1µM or with control solution.

Figure 24 :

 24 Figure 24: Seedling growth inhibition assay with plants defective in SERK family members. A) bak1-5 bkk1-1 plants are insensitive to SCOOP6 and SCOOP7. Neither fresh weight A, C nor root length B, D were significantly affected by SCOOP12 treatment. Plants were grown for 8 days in presence of 1µM SCOOP12 or control solution. Bars of quantified fresh weight and root length represent mean of six replicates. Error bars show ±SE of the mean. Significant differences according to Student's t-test results ***, P < 0.001.

Table 3 :

 3 Summary of our current knowledge on the SCOOP peptide family. Experimental results are summarized from chapter 3 and 4. The peptide sequence and results of ROS, seedling growth inhibition, callose deposition assay are presented. As well as the involvement of tested SERK family members and the observed root pigmentation. Abbreviations: Yes, shows a significant or visible induction; no: no significant or visible induction; nd: not determined

Figure 25 :

 25 Figure 25: Amino acid composition of the 14 SCOOP peptides. Peptides that showed an activity in the applied assays are indicated in white. Peptides without activity in red and peptides with partial activity in orange. The alignment of all 14 SCOOP peptides shows that most of the active peptides harbor a positively charged aa at position 6.

Figure 26 :

 26 Figure 26: H3K27me3 distribution at PROSCOOP genes. A) Distribution of H3K27me3 chromatin mark over all members of the SCOOP peptide family. B, C) H3K27me3 distribution at the two SCOOP family members PROSCOOP7 and PROSCOOP12. 3 Primer sets to investigate the level of H3K27me3 at PROSCOOP12 and 2 primer sets for PROSCOOP7. D,E) ChIP-real-time PCR result of H3K27me3 IP after SCOOP12 treatment for all 5 primer sets. The real-time results are represented relative IP to input expression normalized to 18S. The results show one biological replicate.

Figure 27 :

 27 Figure 27: BTH induces short-and long-term defense responses in Arabidopsis: A: Production of reactive oxygen species (ROS) measured in RLU (relative light units) in wild-type Arabidopsis leaf-discs (Col-0), treated with 1µM flg22, 1mM BTH or without elicitor (control). Graphs display averages of 12 replicates B: Quantification of callose deposition. The bars represent the mean of 4 replicates. C: Localization of callose deposition by aniline blue staining. D: Temporal order of applied Arabidopsis treatments. The first treatment was applied 7 dag (days after germination), the third treatment 13 dag. E: Quantification of fresh weight of 21 dag old Arabidopsis plants. Plants were previously sprayed three times with water or BTH (1mM) according the scheme shown in D. F: Pictures of 21 dag old plants previously sprayed with water or BTH (1mM). Error bars show ±SE of the mean. Significant differences according to Student's t-test results: *, P < 0.05; **, P < 0.01; ***, P < 0.001.

(

  Supplementary) Figure 28: Growth repression effect of BTH on grafted and in vitro apple plants. A: Total number of internodes of grafted apple plants treated six times with BTH (1mM) or water. Two months after grafting, plants were treated every three days. Number of internodes was counted three days after the last treatment. Bars represent the mean of at least two biological replicates. B: Pictures of in vitro grown apple plantlets. After propagation plantlets were grown for 4 weeks after propagation on media supplemented with or without 1mM BTH. Error bars show ±SE of the mean. Significant differences according to Student's t-test results: **, P < 0.01.

Figure 19 :

 19 Figure 19: Arabidopsis transcriptome analysis set-up and results. A: Experimental set-up of applied treatments. The first treatment was applied to young Arabidopsis plants 7 dag (days after germination), the second 10 dag and plants were harvested at 21 dag. B: List of applied treatments after 7 and 10 dag as well as the sample name. C: List of examined microarray comparisons and number of DETs. D: DETs of comparison 1 to 4 were sorted by applying a binary code whereas a 1represents the presents and a 0 the absence of a transcript. E: All DETs were sorted with help of applying the binary code into 15 categories (excluding category 0000). The table shows the number of DETs in all categories.

Figure 20 :

 20 Figure 20: Common differentially expressed transcripts resulting from flg22 and BTH treatments. A: Scatter plot of the log2 expression values of 11XX DETs showing ww Vs. wf on the X axes and ww Vs. bw on the Y axes. B: The category 11XX was divided into 11XX up by BTH and 11XX down by BTH subcategories. The graph represents the percentage of transcripts expressed in sense and antisense in both sub-categories as well as transcripts showing transgressive expression pattern and therefore do not follow the global expression trend.

  of de-primed genes, we compared the transcription level of DETs of category 0110 with that of DETs of the comparison ww Vs. bf (Figure29C). DETs of the category 0110, which are not significantly differently expressed in ww Vs. bf can be considered as de-primed. Out of the 541 DETs of category 0110 86.3% are not significantly differently expressed in the comparison ww Vs. bf anymore (p-value > 0.05) (Figure31B). Only 11.1% of these transcripts maintained their priming state after the flg22 challenge.Closer inspection of theDETs in category 0110 revealed an uneven distribution of sense and antisense DETs. We investigated the two sub-categories of this category. The sub-category 0110 up by BTH contains DETs that are up regulated by BTH and show globally the inverted expression by subsequent flg22 treatment. The sub-category 0110 down by BTH contains DETs down regulated by BTH and up regulated by subsequent flg22 treatment. While transcripts in the sub-category 0110 up by BTH consist of 52.4% antisense transcripts, transcripts in 0110 down by BTH only contain 18.7% antisense transcripts and 80.2% sense transcripts (Figure 31C).

Figure 31 :

 31 Figure 31: DETs of the categories 0110 show an inversion of gene expression profile by a second stress. A: Scatter plot of transcripts present in the category 0110. The category contains DETs of the comparison ww Vs. bw (Y axes) and bw Vs. bf (X axes). B: Classification of transcripts present in category 0110. The 541 transcripts were compared to DETs of ww Vs. bf. 86.3% of the transcripts are not differently expressed by the combination of both treatments and are considered as de-primed. 11.1% are differently expressed and therefore primed and 2.6% do not follow the global expression trend in 0110. C: Distribution of sense and antisense transcripts in the subcategories 0110 up by BTH and 0110 down by BTH. The sub-category 0110 up by BTH contains 52.4% antisense transcripts.

  bw 3920 DETs were detected. Notably, these DETs result from a treatment 17 days before harvesting and RNA extraction. In the comparison bw Vs. bf 847 DETs were found. 342 DETs are common in the comparison ww Vs. bw and bw Vs. bf. These common DETs are equivalent with the category 0110 of the Arabidopsis setup.

Figure 33 :

 33 Figure 33: Whole genome expression profiling and applied comparisons in apple. A: Experimental set up of apple plantlet treatments. Apple plantlets were treated the first time 14 dap (days after propagation) followed by one additional treatment 17 dap. Leave tissue was harvested 31 dap. B: Combinations of applied treatments and sample names. C: List of examined microarray comparisons and number of DETs.

Figure 34 :

 34 Figure 34: De-priming in apple. A: scatter Plots of common transcripts present in the comparisons ww Vs. bw (Y axes) and bw Vs. bf (X axes). B: The common transcripts are compared to ww Vs. bf. 82.7% are de-primed, 8.2% stay primed and 9.1% are transgressive and do not follow the global trend. C: The common DETs are divided into the sub-categories up by BTH and down by BTH

Figure 35 :

 35 Figure 35: Expression of AMY1 serves as marker for de-priming of transcription. A: set-up of treatments applied to Arabidopsis plants. The first treatment was applied 7 dag (days after germination) followed by two additional treatments. Plants were harvested 21 dag. B: Expression profile of AMY1 by different sequences of treatments of wild type plants. C, D: The same treatment orders are applied to nrpd1-3 and met1-3. All expression values are normalized to that of the gene ACR12 (AT5G04740). Bars represent the mean of at least four biological replicates. Error bars show ±SE of the mean. Significant differences according to Student's t-test results: *, P < 0.05; ***, P < 0.001; ns: not significant.

Figure 36 :

 36 Figure 36: Proposed model of de-priming effect on plant fitness. Here we suggest a model of a possible beneficial effect of transcriptional de-priming. If plants are not exposed to a first priming treatment the second treatment might cause stronger deficits on plant fitness (red line) in comparison to plants that are primed (blue line). However, the priming reflects a fitness costs for plants due to the induced defense response and the maintenance of the primed transcription and/or epigenetic memory. The second treatment could cause a less pronounced fitness cost in comparison to the unprimed plants. We propose that the de-priming of a DET subset could lead to an additional positive effect on plant fitness by fine tuning the plant defense response and returning non-beneficial DETs

Figure 37 :

 37 Figure 37: Short-term defense and resistance to Pseudomonas syringea may not influenced by de-priming. A) Western blot against phosphor-p44/42 MAP kinase. Pre-treated plants after 7 and 10, were exposed to flg22 to induce the phosphorylation of MAPK. Pre-treated plants do not show an altered level of phosphorylated MAPK in comparison to the water treated control. B) ROS burst of pre-treated plants. Arabidopsis plants were pre-treated after 7 and 10 days with the indicated treatment and exposed to 1µM flg22 in order to induce the ROS burst. Pre-treatments do not have an effect on the ROS burst intensity. C) Pseudomonas syringea infection of pre-treated plants.

  discovery and characterization of a new small endogenous peptide family called SCOOP and I describe an unusual memory response by a subsequent flg22 exposure after treatments with the defense priming compound BTH.In the first part of my thesis I show that one member of the SCOOP family, namely SCOOP12 can be classified as a new posttranslational modified phytocytokine with plays roles in plant defense responses and root development. Although, the SCOOP peptide family shares many features with other described small endogenous peptide families like AtPep and AtPIP, the precursor of the SCOOP peptides, PROSCOOPs have a very diverse C-terminus, which indicates a great variety of the processed peptides and therefore a broad spectrum of functions. Further, I conclude that the SCOOP peptide family contains various members with the capacity to induce defense responses and with strong effects on root development.However, the exact mode of action of the SCOOP peptides on the root defense and development remains for the moment unclear. Moreover, I show that the members of the SCOOP peptide family, similar to other peptide families, is perceived by members of the SERK receptor family. The perception of SCOOP12 is highly depended on BAK1. In conclusion, my research on the SCOOP peptide family could be the onset and initiation for the discovery of many additional functions of the SCOOP peptide family in Arabidopsis and other Brassicaceae species members.In the second part of my thesis I conclude that the priming state of BTH induced transcriptional memory can be forced to return to a basic expression by a subsequent flg22 treatment. I show that this effect is not only present in Arabidopsis but also in the economically important plant apple. The identification of a de-priming marker gene lead to General discussion 123 first evidence that the global methylation could be crucial for de-priming of BTH-induced differently expressed transcripts.In conclusion, my thesis investigates effects on plant transcription, development and defense by endogenous (the SCOOP peptide family), exogenous (flg22) and synthetic (BTH) plant elicitors. My thesis shows how diverse the function of these elicitors can be and how the plant defense system as well as the triggers of plant defense can cause tremendous effects on plant development and memory. A coarser overview over the main findings on Arabidopsis and apple are summarized in Figure38.

Figure 38 :

 38 Figure 38: Summary of the main findings. My thesis shows results on mainly on two plant species namely Arabidopsis and apple. The SCOOP peptide family was found to be restricted to Brassicaceae family while the de-priming expression profile was observed in Arabidopsis and apple. Results correlated with my work on the SCOOP peptide family are indicated in blue and results on the de-priming expression profile in orange. Dotted lines represent putative interactions.

  It would be interesting to apply ChIP experiments with subsequent sequencing with antibodies of the mentioned histone marks. These experiments would open a new door for the identification of memory genes under epigenetic control upon BTH and BTH/flg22 treatment. Once more, factors are identified showing different histone mark distribution it would be interesting totest other defense activating compounds apart from BTH, like BABA, in order to find reliable defense priming genes. Moreover, by using flg22 we induce a PTI response, it would be interesting to trigger an additional ETI response by using bacteria like Pseudomonas on Arabidopsis and Erwinia amylovora on apple plants. We assume that the flg22-induced depriming expression profile might counteract negative effects of BTH. This hypothesis needs to be experimental tested. We show that BTH causes an inhibition of growth that could be explained by a tradeoff between growth and defense. It might be that the subsequent flg22 treatment causes a shift of resources. Interesting would be to apply a metabolite profiling by liquid chromatography coupled with mass spectrometry of BTH and BTH/flg22 treated plants.Despite these open questions, the findings presented in this thesis could have an impact on the future research on small endogenous peptides and the agricultural use of plant defense inducing compounds. With the SCOOP peptides we add now a new peptide family to the class of phytocytokines. This discovery might pave the way to new findings regarding the involvement of small endogenous peptides in defense and development. The observed depriming memory response in Arabidopsis and apple could appeal the discussion of forgetting mechanisms in plants.

Figure 1 Fig. 1 .

 11 Figure 1

Figure 3 Fig. 3 .

 33 Figure 3

Figure 4 Fig. 4 .

 44 Figure 4

Figure 5 Fig. 5 .

 55 Figure 5

Figure 6 Fig. 6 .

 66 Figure 6

Figure 7 Fig. 7 .

 77 Figure 7

Figure 8 Fig. 8 .

 88 Figure 8

Figure 9 Fig. 9 .

 99 Figure 9

Figure 10 Fig. 10 .

 1010 Figure 10

Figure 11 Fig. 11 .

 1111 Figure 11

Table 1 : List of Primer used in this study

 1

	Name	Gene ID	Sequence fw	Sequence rev
	ACR12	AT5G04740 TTGTTCGATGATCGCCGGAA	TGGAACAACGTCGTCATCGT
	PROSCOOP1	AT5G44565 AGCATCCTCTTTCACCATACCG	ATTCTGACCACCACCACCTC
	PROPEP1	AT5G64900 TCTCCGACAACGTCCTCTCC	ACGGCCTGAGCTAACTTTCT
	PROPEP2	AT5G64890 CGGTAACTTTTAACCAGCCGG	TTAGTTTGGCCAGGACGACC
	FRK1	AT2G19190 TAGATGCAGCGCAAGGACTA	ACCGCTTCCTTCAACAGAGA
	primers used for genotyping of proscoop12
	ACT2	AT3G18780 CTAAGCTCTCAAGATCAAAGGC	AACATTGCAAAGAGTTTCAAGG
	PROSCOOP12 AT5G44585 ATGGGTCAAGTTCTAATTGTGC	TAATCTATGGCGATAGGATCAGC

Table 4 Table 4 : Primers used for experiments in this chapter

 44

	Name	Gene ID	Sequence forward	Sequence reverse
	PROSCOOP1	AT5G44565 AGCATCCTCTTTCACCATACCG	ATTCTGACCACCACCACCTC
	PROSCOOP2	NA	TAATTGTGCTGGTCTCATGCTC	GCGGTGGCGGCGGTTTTT
	PROSCOOP3	NA	GGTCCTTTGAATTTGAGACTTTTG	TAATACGAGCTCTTCGACCATAC
	PROSCOOP4	AT5G44568 ATCTCAAGTTGGAGTCGCCC	TTATCTTTAGGCGATGCAGAGTGA
	PROSCOOP5	AT5G44570 ATACAATCCACCGACGCTGC	GGATAGAGCATTTGTGGCTGC
	PROSCOOP6	AT5G44572 CTTGCAGCCTTAGCCAATCG	TCATCAATCTCCTCCCGTGG
	PROSCOOP7	AT5G44574 CACTTGCCTTAGCGTAACGG	TGGTGAGTTTTCTCCACGCT
	PROSCOOP8	AT5G44575 TCCCAACCCATACGGAGTCT	TTTGTTGACCACCACCGGC
	PROSCOOP9	AT5G44578 TCCGTATCCGTATGGTGGCA	ATGCTGCTACCACCATGTCC
	PROSCOOP10 AT5G44580 TGGGGAGGAAGCGGATGAAT	CACTGCCTGATGGTCCTGTA
	PROSCOOP11 AT5G44582 TTGTAATCACTGGAAGGAGG	GTTGCGACCACCACCTTGT

Table 5 :

 5 List of primers used for ChIP coupled with real time PCR

	Name	Sequence forward	Sequence reverse
	18S	TCCCTTCACGGCCGGCTTCT	TCGCGGGCGGCGAACCAC
	Set1	TTATGAAATGCTATGTACAACCGC	TAGAGGAATAACTGTAATAGAGCG
	Set2	GTTCCCTCCACCTGCATGTA	AAACCGGTTAGTAGGACTGAATG
	Set3	TCGATTTATATGTTTTGTTAATTTCTCA GATCTTACTGGTCCCGAAGC
	Set4	GTATGCATGAAATCGCCAGTTATA	CGATTCCAAGTTTAACTCCTTTTG
	Set5	ACTCAGTTTTTCTCTCTGCTTGG	GAGCCGTTACGCTAAGGCAA
	6.

  Soil grown plants used for Erwinia amylovora andAlternaria brassicicola inoculations as well as all in vitro plants (on Murashige and Skoog) were grown under short day conditions (photoperiod of 8h light at 22°C/16h dark at 21°C, with 70% of relative humidity). Plants used for all other assays were grown under long day conditions (photoperiod of 16h light at 22°C/8h dark at 21°C, with 60% relative humidity). B. napus (Darmor-bzh) and L. esculentum (Sweet Baby) were grown under short day conditions.

	The	20	bases	long-sgRNA	with	the	sequence
	AAGAACTTGACCCATTTTTG was used.

Tefor program (http://crispor.tefor.net).

Acknowledgements

First, I want to thank Dr. Etienne Bucher for giving me the opportunity to do my PhD in his lab, for the thought-provoking discussions and for the high degree of liberty I obtained to perform my work. Further, I want to thank Marie-Noëlle Brisset for being my co-supervisor, for discussions and for providing knowledge about apple treatments and BTH.

Acknowledgements

This work was supported by INRA and the 'Objectif Végétal' project funded by the Pays-de-la-Loire Region. K.G. and E.B. were funded by the EPICENTER ConnecTalent grant of the Paysde-la-Loire. The authors are grateful to Daniel Sochard (Phenotic, IRHS-UMR 1345) for growth chamber maintenance, Fabienne Simonneau for microscopy facilities (IMAC, SFR QuaSaV) as well as Sylvie Jolivet and Hervé Ferry (IJPB).

The SCOOP12 peptide regulates defense response and root elongation in Arabidopsis

are only overrepresented in the gene ontology response to stress and stimulus compared to the reference (Supplementary figure 32B).

(Supplementary) Figure 32 were available. DETs correlated with the response to stress and stimulus (GO:0006950, GO:0050896) are overrepresented in this category. Gene ontology overrepresentation test was probed using the PANTHER webtool using a fisher`s exact test [START_REF] Mi | PANTHER version 11: expanded annotation data from Gene Ontology and Reactome pathways, and data analysis tool enhancements[END_REF]

Transcriptional response and memory of stress treatments in apple

Next, we wanted to test if our observations in Arabidopsis were also relevant for other plant species. For that purpose, we performed a similar treatment regime as for Arabidopsis on in vitro grown apple plantlets followed by microarray-based expression profiling using the latest kay.gully@inra.fr, sandra.pelletier@inra.fr, marie-charlotte.guillou@inra.fr, sophie.aligon@agrocampus-ouest.fr, adrien.perrin@inra.fr, emilie.vergne@inra.fr, philippe.grappin@agrocampus-ouest.fr, etienne.bucher@inra.fr, jean-pierre.renou@inra.fr, sebastien.aubourg@inra.fr (AT5G04740, because of its constant transcription profile upon elicitor treatments) using the qGene protocol (Muller et al., 2002). All the gene-specific primers used are detailed in Table S1.

Seedling growth inhibition assay

Seedlings were germinated on MS agar and grown for 5 days before transferring one seedling per well to 24 well plates containing 500 µl MS media or MS media supplied with the indicated elicitor peptide to a final concentration of 1 µM (six replicates per elicitor peptide treatment).

Photos were taken, fresh weight and root length were measures after 8 additional days. Root length of proscoop12 and WT plants was determined on vertical MS plates.

Elicitor peptides

Peptides of flg22 (QRLSTGSRINSAKDDAAGLQIA), A. thaliana Plant Elicitor Peptide 1 (AtPep1) (ATKVKAKQRGKEKVSSGRPGQHN), SCOOP12 (PVRSSQSSQAGGR), scSCOOP12

(GRPRSASSGSVQQ), SCOOP12 S5/7A (PVRSAQASQAGGR), SCOOP12 S5A (PVRSAQSQAGGR) and SCOOP12 S7A (PVRSSQASQAGGR) were obtained from Eurogentec SA (Angers, France)

and diluted in water to the final concentration used for the assays.

Measurement of reactive oxygen species

For ROS assays leaf discs of three weeks old soil grown plants, were placed into each well of a white 96-well plate (Thermo Scientific, Waltham, USA) in 0,1 ml of water and kept in the dark overnight. For elicitation and ROS detection, horseradish peroxidase and luminol were added to a final concentration of 10 µg ml -1 and 100 µM, respectively. Luminescence was measured directly after addition of elicitor peptides in a FLUOstar OPTIMA plate reader (BMG LABTECH, Offenburg, Germany).

Callose deposition

Leaf discs were vacuum infiltrated for 10 min with the indicated elicitor solution and kept floated in elicitor or control solution for 24h. After leaf discs were fixed and destained in 1:3 acetic acid/ethanol until leaf tissue was completely transparent. After washing the leaf discs in 150 mM K2HPO4 for 30 min, the plant material was stained for 2 h in 150 mM K2HPO4 and and SCOOP12 (Fig. 9A) is dependent on the BAK1 co-receptor. In addition to their role as amplifiers of the immune response, these peptides are involved in root development but via different mechanisms. The overexpression of the PIP1 precursor or its exogenous application inhibits Arabidopsis root growth as described for CEP (Roberts at al., 2013) and SCOOP12 peptide (Fig. 6F). On the other hand, the constitutive overexpression of PROPEP1 increases the root development (Huffaker et al., 2006) whereas AtPep1 treatment inhibits root growth (Poncini et al., 2017). Acting as growth factors and contrary to SCOOP12, the PTMP PSK and PSY1 are involved in root elongation (Amano et al., 2007;Matsuzaki et al., 2010). These comparisons show that despite common structural and functional characteristics, the SCOOP family is different from previously described secreted peptides. The divergence observed in the C-terminal sequence of PROSCOOP proteins suggest a large broad of biological functions through a diversity of receptors which will be the targets of future studies.

In conclusion, SCOOP12 belongs to a new family of putatively secreted peptides specific to the Brassicaceae species. At the functional level, such secreted peptides are classified as phytocytokines (such RALFs, systemin and PIPs), i.e. secondary endogenous danger signal.

Indeed, this classification (Gust et al., 2017) distinguishes them from classical DAMPs (primary endogenous danger signals) which are passively released from injured tissue without biosynthesis and secretion processes. Through its negative action on antioxidant responses and its positive effect on PA/ROS production (PLC pathway), SCOOP12 could play a role in the moderation of defense responses, as well as root elongation, to prevent unnecessary energy loss in a 'trade-off' fashion (Walters and Heil, 2007). The functions of such plant secreted peptides at the boundaries of development-and stress-signaling pathways open the way to future strategies that jointly consider product quality/quantity and new resistance traits.

Supplementary data

Supplementary Figure S1. Phylogenetic tree of PROSCOOP homologs. The tree was built with the neighbor-joining method from the multiple alignment of 74 homologous Brassicaceae proteins. Supplementary Table S1. Gene-specific primer sequences used for mutant genotyping and qPCR analysis of all the PROSCOOP genes.

Supplementary

Supplementary Table S2. List of the 136 comparisons in which transcription of AT5G44585 was deregulated in CATdb (http://tools.ips2.u-psud.fr/CATdb). The AT5G44585 gene is tagged by the CATMA5A40400 probe. Column legends: CATdb project references (providing Gene Expression Omnibus accessions and complete description of samples), project titles, "yellow" and "blue" sample names, organs, Y/B ratios, phenotype keyword extracted from the project summary in CATdb pointing the expected phenotype, phenotype classes and color codes used in Fig. 1 (1.1: pathogen infections, 1.2: oxidative stress, 1.3: abiotic stresses, 1.4:

JA-SA related mutants, 2: root growth, 3: hypocotyl growth, 4: silencing mutants, 5: various experiments).

Supplementary Tables S3. List of 117 genes involved in the relational annotation of PROSCOOP12 (in addition to Fig. S2). Their protein name and complete functional annotation (from TAIR10 and/or literature) are indicated (Table S3A). Putative partner genes have been found co-expressed with PROSCOOP12 (according the GEM2Net resource) or have been added in the network by curated co-citations in literature, pathways, and protein-protein interactions (STRING, Arabidopsis Interactome, KEGG…). The type and the source of the each network edge are detailed in Table S3B.

Supplementary Table S4 Genevestigator platform [START_REF] Hruz | Genevestigator v3: a reference expression database for the meta-analysis of transcriptomes[END_REF]. The PROSCOOP12 gene is indicated by a red frame.

11.

Curriculum Vitae