Bruno Guillotin

Mohammad Etemadi

Corinne Audran

Mondher Bouzayen

Guillaume B Ecard

Jean-Philippe Combier
email: combier@lrsv.ups-tlse.fr

Sl-IAA27 regulates strigolactone biosynthesis and mycorrhization in tomato (var. MicroTom)

Keywords: Aux-IAA, auxin, mycorrhiza, strigolactones, tomato

Root colonization by arbuscular mycorrhizal (AM) fungi is a complex and finely tuned process. Previous studies have shown that, among other plant hormones, auxin plays a role in this process but the specific involvement of Aux/IAAs, the key regulators of auxin responses, is still unknown.

In this study, we addressed the role of the tomato Sl-IAA27 during AM symbiosis by using Sl-IAA27-RNAi and pSL-IAA27::GUS stable tomato lines.

The data show that Sl-IAA27 expression is up-regulated by the AM fungus and that silencing of Sl-IAA27 has a negative impact on AM colonization. Sl-IAA27-silencing resulted in down-regulation of three genes involved in strigolactone synthesis, NSP1, D27 and MAX1, and treatment of Sl-IAA27-silenced plants with the strigolactone analog GR24 complemented their mycorrhizal defect phenotype.

Overall, the study identified an Aux/IAA gene as a new component of the signaling pathway controlling AM fungal colonization in tomato. This gene is proposed to control strigolactone biosynthesis via the regulation of NSP1.

Introduction

The arbuscular mycorrhiza (AM), a symbiosis between soil fungi of the Glomeromycota phylum and nearly 80% of terrestrial plant species, is characterized by a two-way trade in which the fungus provides mineral nutrients to the plant in exchange for carbohydrates. The initiation of this symbiosis is known to require a molecular communication between the two partners. The plant secretes several signal molecules in its root exudates, including strigolactones (SLs), a class of plant hormones playing an important role in the rhizosphere for the establishment of AM symbiosis [START_REF] Gomez-Roldan | Strigolactone inhibition of shoot branching[END_REF]. SLs stimulate AM fungal metabolism and hyphal proliferation [START_REF] Akiyama | Plant sesquiterpenes induce hyphal branching in arbuscular mycorrhizal fungi[END_REF][START_REF] Besserer | Strigolactones stimulate arbuscular mycorrhizal fungi by activating mitochondria[END_REF][START_REF] Besserer | GR24, a synthetic analog of strigolactones, stimulates the mitosis and growth of the arbuscular mycorrhizal fungus Gigaspora rosea by boosting its energy metabolism[END_REF] and from its side, the AM fungus produces trace amount of chitinic signals [START_REF] Maillet | Fungal lipochitooligosaccharide symbiotic signals in arbuscular mycorrhiza[END_REF][START_REF] Genre | Multiple exocytotic markers accumulate at the sites of perifungal membrane biogenesis in arbuscular mycorrhizas[END_REF][START_REF] Genre | Short-chain chitin oligomers from arbuscular mycorrhizal fungi trigger nuclear Ca 2+ spiking in Medicago truncatula roots and their production is enhanced by strigolactone[END_REF]. Upon this successful mutual recognition, the fungus penetrates the roots through the epidermis, grows between root cells and forms highly branched structures called arbuscules inside cortical root cells, where most nutrient exchanges occur between the two partners.

The control of the mycorrhizal symbiosis is a finely tuned process at multiple levels. An increasing number of reports point to the important role of several plant hormones, besides that of SLs, in the regulation of early recognition/colonization steps up to the final arbuscular formation (reviewed in [START_REF] Hause | Jasmonates in arbuscular mycorrhizal interactions[END_REF][START_REF] De Los Santos | Altered pattern of arbuscular mycorrhizal formation in tomato ethylene mutants[END_REF][START_REF] Foo | Plant hormones in arbuscular mycorrhizal symbioses: an emerging role for gibberellins[END_REF][START_REF] Gutjahr | Phytohormone signaling in arbuscular mycorhiza development[END_REF]. For instance, auxin is involved in both the general development of the fungus in planta and the formation of arbuscules, whereas SLs are involved in presymbiotic growth of the fungus but not in arbuscule differentiation.

With regard to auxin, several studies have shown an increase in auxin content in AM roots and a stimulation of fungal growth and mycorrhization by exogenous auxin treatment (reviewed in [START_REF] Gutjahr | Phytohormone signaling in arbuscular mycorhiza development[END_REF]. This was recently confirmed in tomato by the observation that the synthetic auxin-responsive gene DR5-GUS promoter undergoes a net activation in mycorrhized roots and, more precisely, in arbuscule-containing cells [START_REF] Etemadi | Auxin perception is required for arbuscule development in arbuscular mycorrhizal symbiosis[END_REF]. Further supporting the role of auxin in the AM symbiosis, the mycorrhization rate was strongly decreased, although showing normal fungal structures and arbuscules, in the pea bushy mutant that produces three times less auxin in its roots, and also in the auxin-resistant tomato mutant diageotropica as well as in the auxin hypertransporting tomato mutant polycotyledon [START_REF] Hanlon | Genetic evidence for auxin involvement in arbuscular mycorrhiza initiation[END_REF][START_REF] Foo | Auxin influences strigolactones in pea mycorrhizal symbiosis[END_REF]. Interestingly, the low mycorrhization rate of bushy was attributed to a decreased SL biosynthesis, suggesting a possible crosstalk between auxin and SL in the regulation of AM [START_REF] Foo | Auxin influences strigolactones in pea mycorrhizal symbiosis[END_REF].

Auxin perception and/or signaling appear(s) to be critical for arbuscule development, as the inhibition of auxin receptors by the overexpression of the microRNA393 leads to a defect in arbuscule formation in addition to a reduced mycorrhization [START_REF] Etemadi | Auxin perception is required for arbuscule development in arbuscular mycorrhizal symbiosis[END_REF]. Upon auxin recognition, the auxin receptors TIR/AFBs (Transport Inhibitor Response/Auxin signaling F-Box) become associated with the SKP1-Cullin-F-box (SCF) complex, leading to a rapid proteasome-mediated degradation of Aux/IAAs [START_REF] Dharmasiri | Plant development is regulated by a family of auxin receptor F box proteins[END_REF][START_REF] Kepinski | The F-box protein TIR1 is an auxin receptor[END_REF][START_REF] Leyser | Dynamic integration of auxin transport and signalling[END_REF][START_REF] Tan | Mechanism of auxin perception by the TIR1 ubiquitin ligase[END_REF][START_REF] Chapman | Mechanism of auxin-regulated gene expression in plants[END_REF], a release of auxin response factors (ARFs) that can then activate the transcription of auxin-regulated genes through binding to auxinresponsive elements present in their promoter region [START_REF] Hagen | Auxin-induced expression of the soybean GH3 promoter in transgenic tobacco plants[END_REF][START_REF] Ulmasov | Aux/IAA proteins repress expression of reporter genes containing natural and highly active synthetic auxin response elements[END_REF][START_REF] Hagen | Auxin-responsive gene expression: genes, promoters and regulatory factors[END_REF]. Therefore, one can hypothesize that Aux/IAAs and/or ARFs play a role in the regulation of mycorrhization.

In tomato, 25 Aux/IAA genes were identified [START_REF] Imaizumi-Anraku | Plastid proteins crucial for symbiotic fungal and bacterial entry into plant roots[END_REF][START_REF] Herrera-Medina | Abscisic acid determines arbuscule development and functionality in the tomato arbuscular mycorrhiza[END_REF]Chaabouni et al., 2009a,b;[START_REF] Wang | Regulatory features underlying pollination-dependent and -independent tomato fruit set revealed by transcript and primary metabolite profiling[END_REF][START_REF] Audran-Delalande | Genome-wide identification, functional analysis and expression profiling of the Aux/IAA gene family in tomato[END_REF][START_REF] Bassa | Phenotypes associated with down-regulation of Sl-IAA27 support functional diversity among Aux/IAA family members in tomato[END_REF]Deng et al., 2012a,b;[START_REF] Su | The auxin Sl-IAA17 transcriptional repressor controls fruit size via the regulation of endoreduplication-related cell expansion[END_REF]. Among these, Sl-IAA27 was shown to display an intriguing expression pattern: a down-regulation upon exogenous auxin treatment and an up-regulation during mycorrhization [START_REF] Bassa | Phenotypes associated with down-regulation of Sl-IAA27 support functional diversity among Aux/IAA family members in tomato[END_REF][START_REF] Bassa | Sl-IAA27 gene expression is induced during arbuscular mycorrhizal symbiosis in tomato and in Medicago truncatula[END_REF].

To gain further insight into the role of auxin, and more specifically that of Aux/IAAs, in the mycorrhization process, we analyzed in MicroTom plants the expression pattern of Sl-IAA27 in mycorrhized and nonmycorrhized roots, the mycorrhizal phenotype of Sl-IAA27-silenced plants, and we compared the ability to produce SLs of wild-type and Sl-IAA27-silenced plants. The data suggest that Sl-IAA27 positively regulates mycorrhization via the induction of NSP1 transcription and SL biosynthesis.

Materials and Methods

Plant and fungal materials, growth and conditions

Seeds of tomato (Solanum lycopersicum cv MicroTom) wild-type, pSl-IAA27: GUS lines (three independent lines, 14/6/1, 10/10/1 and 37/4/1) and RNAi Sl-IAA27 (three independent lines, 5/3, 19/1 and 77/7, named Rline1, Rline2 and Rline3, respectively) were obtained as already described [START_REF] Bassa | Phenotypes associated with down-regulation of Sl-IAA27 support functional diversity among Aux/IAA family members in tomato[END_REF]. Seeds of the parasitic plant Phelipanche ramosa L. Pomel (genetic type 1; [START_REF] Voisin | Host specificity and genetic diversity of the parasitic plant Phelipanche ramosa on winter oilseed rape in France[END_REF] were provided by P. Simier (LBPV, University of Nantes, France).

Tomato seeds were surface-sterilized for 1 min in 2.3% sodium hypochlorite and washed eight times with sterile deionized water. They germinated on a solid water agar plate in the dark at 23°C for 6 d.

For mycorrhization assays and quantitative reverse transcription polymerase chain reaction (qRT-PCR) analyses, seedlings were grown in 250 ml pots (one seedling per pot) filled with Oil-Dri US special substrate (Damolin, Etrechy, France) for 12 wk, in a growth chamber (16 : 8 h, 24 : 22°C, day : night, 120-150 lmol m À2 s À1 ) and watered every 2 d with modified Long Ashton medium containing a low concentration (7.5 lM) of phosphate [START_REF] Balzergue | The regulation of arbuscular mycorrhizal symbiosis by phosphate in pea involves early and systemic signalling events[END_REF]. They were inoculated with Rhizophagus irregularis DAOM 197198 sterile spores (400 spores per liter of substrate) purchased from Agronutrition (Carbone, France). For b-glucuronidase (GUS) staining experiments, seedlings were inoculated with a higher inoculum (2000 spores l - 1 of substrate) and harvested 4 wk after inoculation.

For in vitro culture, germinated seedlings were grown on modified Long Ashton medium (7.5 lM of phosphate), gelled with 0.8% agar (Agar HP 696; Kalys Biotech, http://biotech. kalys.com) in 12-cm-square plates (five seedlings per plate) in a growth chamber (16 : 8 h, 24 : 22°C, day: night, 60 lmol m À2 s À1 ). After 6 d, a cellophane membrane (Couvre confiture, Hutchinson, Chalette/Loing, France) covered with 500 R. irregularis germinating spores was laid on seedling roots for another 3 days, so that the membrane prevented physical contact, but not the chemical communications, between the two partners. Before this step, the spores had been incubated on the cellophane membrane laid on the same solid modified Long Ashton medium for 6 d at 30°C and 2% CO 2 .

Homologous gene identification

BLASTN analyses were conducted on PHYTOZOME11 https://phyto zome.jgi.doe.gov/pz/portal.html, with the latest version of Solanum lycopersicum iTAG2.3 genome.

Strigolactone treatment

The SL analog GR24 was purchased from Chiralix BV (Nijmegen, the Netherlands). For P. ramosa seed germination tests (see later) 10 À8 -10 À13 M water solutions of SLs were prepared from a 10 À3 M stock solution in acetone. For treatment of tomato plants grown in pots, 10 À7 M GR24 was dissolved in the lowphosphate Long Ashton medium and watered (10 ml per pot) three times a week. Control plants were watered with 0.0001% (v/v) acetone. To minimize the amount of GR24 used, 12 plants for control and four plants per RNAi Sl-IAA27 line were used. For the RNAi Sl-IAA27 lines, the mean values of Fig. 4(b) (see later) represent the average obtained with the three lines.

Gene expression analyses

For qRT-PCR analyses, total RNA was extracted using a Plant RNeasy Mini kit (Qiagen) according to the manufacturer's instructions. Total RNA was treated by DNase I (Promega) to remove genomic DNA contamination. Reverse transcription was performed using M-MLV Reverse Transcriptase, RNase H Minus, Point Mutant (Promega) on 1 lg of total plant RNA. For each experiment, six to 12 independent plants were analyzed. Quantitative PCR amplifications were conducted on a Roche LightCycler 480 System (Roche Diagnostics, Meylan, France) under the following conditions: 95°C for 5 min, then 45 cycles of 95°C for 15 s and 60°C for 1 min. The various primer sets used are described in Supporting Information Table S1. The measured transcripts were normalized by using the Sl-Actin gene.

For histochemical GUS analysis, root tissues of pSL-IAA27:: GUS tomato lines were soaked in GUS staining solution (100 mM sodium phosphate buffer, pH 7.2, 10 mM EDTA, 0.1% Triton X-100, 0.3 mg ml À1 X-Gluc) under vacuum for 15 min. Root tissues were then incubated for 6-12 h in GUS staining solution at 37 °C. GUS pictures shown in Fig. 1(a-d,h, i) and Fig. S1 represent a staining pattern found in all pSl-IAA27: GUS lines. For fluorescent images shown Fig. 1(e,f), roots were GUS-stained as already described, then cleared with KOH and stained with fluorescein-conjugated wheat germ agglutinin (WGA-FITC) as described later for mycorrhizal phenotyping.

For transversal root sections, root tissues after GUS staining were included in low melting 5% agar and cut into 50 lm sections using a vibratome, before observation under an Axio Zoom V16 Zeiss stereomicroscope.

Mycorrhizal phenotyping and fungus staining

Roots were cleared in 10% w/v KOH for 8 min at 95°C and rinsed in sterile water. Then they were treated for 30 min with WGA-FITC (Invitrogen), which binds fungal chitin, washed three times for 10 min in PBS and observed using an Axio Zoom V16 Zeiss stereomicroscope. Alternatively, roots were stained with Schaeffer black ink as described by [START_REF] Vierheilig | Ink and vinegar, a simple staining technique for arbuscular-mycorrhizal fungi[END_REF]. The percentage of mycorrhization was established using the grid intersect method described by [START_REF] Giovannetti | An evaluation of techniques for measuring vesicular arbuscular mycorrhizal infection in roots[END_REF] and with two additional mycorrhization indices: F, mycorrhization frequency and a, arbuscule abundance in colonized root sections, according to [START_REF] Trouvelot | Mesure du taux de mycorhization VA d'un systeme radiculaire. Recherche de methodes d'estimation ayant une signification fonctionnelle[END_REF]. Arbuscule size and shape were analyzed using an Axio Zoom V16 Zeiss and an AxioPlan Zeiss.

Statistical analyses

Means were calculated with values of six to 15 replicates (n < 25) depending on the experiments (indicated in figure legends) and were therefore compared using the Kruskal-Wallis test. Each experiment was repeated two to three times.

P. ramosa germination assay

Root extracts One gram of powdered N 2 -frozen roots of tomato (12 wk old) grown in pots as described earlier was suspended in 2 ml of 100% ethyl acetate and sonicated for 10 min in 4°C water. After centrifugation at 450 g for 10 min at 4°C, the upper organic phase was transferred into a new tube and the extraction of the pellet was repeated with 2 ml of fresh ethyl acetate. The two organic phases were pooled before being washed with 0.2 M K 2 HPO 4 buffer and then dried under nitrogen flow. Root extracts used to stimulate germination of P. ramosa seeds were resuspended in 25% acetone and diluted 1000-100 000 times in sterile deionized water before use. Phelipanche ramosa seeds were surface-sterilized by vigorous agitation in a 2.3% sodium hypochlorite solution for 5 min. They were then washed with sterilized deionized water three times for 30 s and three times for 5 min, and then they were transferred for 10 d in the dark at 24°C in 12-well plates (c. 300 seeds per well) containing 0.5 ml of sterilized deionized water per well. After this preconditioning period, water was removed and replaced by 0.5 ml of diluted root extract. After 7 d, seeds were stained with 0.5% (w/v) neutral red and germination rate was assessed under a Leica MZ75 stereomicroscope.

Results

Sl-IAA27 expression is induced by the AM fungus R. irregularis

We have previously shown by qRT-PCR that Sl-IAA27 expression is globally up-regulated in mycorrhized roots of tomato [START_REF] Bassa | Sl-IAA27 gene expression is induced during arbuscular mycorrhizal symbiosis in tomato and in Medicago truncatula[END_REF]. To gain additional information on Sl-IAA27 expression in roots of mycorrhized plants, we used transgenic pSL-IAA27::GUS tomato lines [START_REF] Bassa | Phenotypes associated with down-regulation of Sl-IAA27 support functional diversity among Aux/IAA family members in tomato[END_REF]. We observed that in uncolonized roots of mycorrhized plants, Sl-IAA27 expression was higher than in roots of nonmycorrhized plants (Fig. 1a,b). In both types of roots, GUS expression was higher in young tissues and was mainly localized in the central cylinder and the inner cortex (Figs 1a,b, S1a-c). Interestingly, GUS expression was completely absent in the colonized root sections of mycorrhized plants (Fig. 1d,f). These observations suggest the occurrence of a subtle regulation of IAA27 expression in mycorrhized roots. While this expression seems to be positively regulated by the general presence of the fungus, it is negatively regulated in the immediate vicinity of intraradical fungal structures. To test if diffusible signal compounds released by the fungus are responsible for the positive regulation of IAA27 transcription, we cultivated pSl-IAA27::GUS tomato seedlings (7 d old) in vitro for 3 d in the presence of germinating fungal spores. Roots and spores were separated by a membrane allowing chemical exchanges but preventing physical contact. The presence of the fungus significantly increased Sl-IAA27 expression as shown by qRT-PCR (Fig. 1g) and GUS expression analyses (Fig. 1h,i), indicating that Sl-IAA27 up-regulation in mycorrhized roots could be caused by diffusible fungal compound(s).

Sl-IAA27 is a positive regulator of mycorrhization

We next investigated whether Sl-IAA27 plays a role during mycorrhization. We used three independent tomato lines silenced for the expression of Sl-IAA27 (named RNAi Sl-IAA27 lines, Rline1, Rline2 and Rline3; [START_REF] Bassa | Phenotypes associated with down-regulation of Sl-IAA27 support functional diversity among Aux/IAA family members in tomato[END_REF]. Silencing of Sl-IAA27 results in higher auxin sensitivity and reduced Chl content in leaves. Both ovule and pollen display a dramatic loss of fertility and the internal anatomy of the flower and the fruit are modified [START_REF] Bassa | Phenotypes associated with down-regulation of Sl-IAA27 support functional diversity among Aux/IAA family members in tomato[END_REF]. As Sl-IAA27-RNAi lines were also described to have longer primary roots and higher number of lateral roots when grown in vitro on rich medium (MS/2) [START_REF] Bassa | Phenotypes associated with down-regulation of Sl-IAA27 support functional diversity among Aux/IAA family members in tomato[END_REF], we assessed their root architecture in our growth conditions using low-phosphate Long Ashton medium. After 2 wk in vitro or 4 wk in pots, we observed no differences between the RNAi lines and the wild-type, for both the primary root length and the number of lateral roots (Fig. S2). We then inoculated the plants with R. irregularis spores and analyzed the root colonization rate at 12 wk after inoculation. Total root colonization was strongly reduced in the RNAi Sl-IAA27 lines (average of the three lines) compared with the wild-type (Fig. 2a). In agreement with this, the expression of the phosphate transporter gene Sl-PT4, the tomato homolog of Medicago truncatula PT4 [START_REF] Nagy | The characterization of novel mycorrhiza-specific phosphate transporters from Lycopersicon esculentum and Solanum tuberosum uncovers functional redundancy in symbiotic phosphate transport in solanaceous species[END_REF], which is specifically induced during mycorrhization [START_REF] Harrison | A phosphate transporter from Medicago truncatula involved in the acquisition of phosphate released by arbuscular mycorrhizal fungi[END_REF][START_REF] Nagy | The characterization of novel mycorrhiza-specific phosphate transporters from Lycopersicon esculentum and Solanum tuberosum uncovers functional redundancy in symbiotic phosphate transport in solanaceous species[END_REF], was also lower in the RNAi Sl-IAA27 lines (Fig. 3a). A closer look at the mycorrhization pattern showed that this lower colonization was a result of a strong decrease of the infection frequency and arbuscule abundance (Fig. 2b). On the other hand, the shape and size of arbuscules looked identical in control and Sl-IAA27-silenced roots (Fig. 2c-f). Altogether, these data suggest that Sl-IAA27 is not involved in the process of arbuscule differentiation but rather in the control of fungal root penetration and intraradical colonization.

Sl-IAA27 influences NSP1 expression

We have previously reported that, in M. truncatula, one important GRAS transcription factor of the nodulation process, NSP1, is involved in the control of mycorrhizal root colonization [START_REF] Delaux | NSP1 is a component of the Myc signaling pathway[END_REF]. To assess the potential link between Sl-IAA27 and NSP1, we compared the expression of its closer homologous gene in S. lycopersicum, in mycorrhized roots of control and RNAi Sl-IAA27 tomato plants. Only one homologous gene was found by direct BLAST on S. lycopersicum genome (Soly-c03g123400.1.1). Sl-NSP1 expression was down-regulated in Sl-IAA27-silenced roots compared with control roots (Fig. 3a). As [START_REF] Liu | Strigolactone biosynthesis in Medicago truncatula and rice requires the symbiotic GRAS-type transcription factors NSP1 and NSP2[END_REF] showed in M. truncatula and rice that NSP1 regulates the expression of D27 and MAX1, two genes involved in the SL biosynthetic pathway, we also measured the expression of the closest homolog of these two genes, Sl-D27 and Sl-MAX1, in mycorrhized tomato [START_REF] Challis | A role for MORE AXILLARY GROWTH1 (MAX1) in evolutionary diversity in strigolactone signaling upstream of MAX2[END_REF]. Our assumption that Sl-D27 and Sl-MAX1 were the best possible homologs of Mt-D27 and Mt-MAX1 was based on a previous study [START_REF] Challis | A role for MORE AXILLARY GROWTH1 (MAX1) in evolutionary diversity in strigolactone signaling upstream of MAX2[END_REF], a BLASTN analysis that we performed using the very last updated genome of S. lycopersicum (Solanum lycopersicum iTAG2.3 genome), confirming that only one homologous gene of Mt-D27 and Mt-MAX1 could be found with a good score and E-value, and on the fact that the S. lycopersicum D27 and MAX1 homologs had 3 and 5 exons like Mt-D27 and Mt-MAX1, respectively. Moreover, as expected, the four genes Sl-D27, Sl-MAX1, Sl-CCD7 and Sl-CCD8 were regulated by phosphate (Fig. S3a), as has been shown for Mt-MAX1, Mt-D27, Mt-CCD7 and Mt-CCD8 in M. truncatula [START_REF] Liu | Strigolactone biosynthesis in Medicago truncatula and rice requires the symbiotic GRAS-type transcription factors NSP1 and NSP2[END_REF][START_REF] Bonneau | Combined phosphate and nitrogen limitation generates a nutrient stress transcriptome favorable for arbuscular mycorrhizal symbiosis in Medicago truncatula[END_REF][START_REF] Van Zeijl | The strigolactone biosynthesis gene DWARF27 is co-opted in rhizobium symbiosis[END_REF]. We found that Sl-D27 and Sl-MAX1 expressions were also down-regulated in the three RNAi Sl-IAA27 tomato lines compared with control plants (Fig. 3a). The same results were obtained in nonmycorrhized plants (Fig. 3b), indicating that the observed down-regulation of NSP1, MAX1 and D27 in mycorrhizal Sl-IAA27-silenced roots was not a result of the lower mycorrhization rate.

We also examined if the silencing of Sl-IAA27 could affect Sl-CCD7 and Sl-CCD8, SL biosynthesis genes and presumably not linked to NSP1 [START_REF] Vogel | SlCCD7 controls strigolactone biosynthesis, shoot branching and mycorrhiza-induced apocarotenoid formation in tomato[END_REF][START_REF] Kohlen | The tomato carotenoid cleavage dioxygenase8 (SlCCD8) regulates rhizosphere signaling, plant architecture and affects reproductive development through strigolactone biosynthesis[END_REF]. In the three RNAi lines, we found no significant down-regulation of these three genes, suggesting that Sl-D27 and Sl-MAX1 expressions are specifically affected via Sl-NSP1 (Fig. S3b).

It has been reported that a close IAA27-related gene, AUX/ IAA9, was up-regulated in young fruits of the Sl-IAA27 RNAi lines [START_REF] Bassa | Sl-IAA27 gene expression is induced during arbuscular mycorrhizal symbiosis in tomato and in Medicago truncatula[END_REF]. Therefore we analyzed the expression of AUX/IAA9 in nonmycorrhized roots to check whether this upregulation in fruit was also effective in roots. We detected no significant changes of Sl-IAA9 expression in roots of silenced Sl-IAA27 lines. In addition, we assessed in the Sl-IAA27 RNAi lines the expression of the closest homologous genes of Sl-IAA27 and Sl-IAA9, Sl-IAA8 (Fig. S3c; Audran-Delalande et al., 2012), and we detected no significant changes.

These data suggest that the down-regulations of NSP1, MAX1 and D27 in Sl-IAA27 lines were not the result of some indirect IAA9 and/or IAA8 regulation. They support the hypothesis that Sl-IAA27, among other as yet unknown regulatory roles, could be an Aux/IAA specifically involved in the regulation of NSP1 expression and therefore indirectly involved in the regulation of SL biosynthesis.

Mycorrhizal defect of RNAi Sl-IAA27 lines can be complemented by GR24 addition

To investigate further the possible role of Sl-IAA27 in the regulation of SL biosynthesis, we performed mass spectrometry analyses of root extracts of wild-type and Sl-IAA27-silenced plants to compare their SL content. We could not detect the presence of SL in any of those extracts, probably because they are in trace amounts in S. lycopersicum cv MicroTom. Therefore we compared the ability of the extracts to stimulate seed germination of the parasitic plant Phelipanche ramosa. This in vivo assay has long been used to detect the presence of SL in plant extracts [START_REF] D€ Orr | Resistance of Helianthus to Orobanchehistological and cytological studies[END_REF][START_REF] Bouwmeester | Secondary metabolite signalling in host-parasitic plant interactions[END_REF][START_REF] Echevarr Ia-Zomeño | Prehaustorial resistance to broomrape (Orobanche cumana) in sunflower (Helianthus annuus): cytochemical studies[END_REF][START_REF] Yoneyama | Strigolactones as germination stimulants for root parasitic plants[END_REF][START_REF] Dor | Strigolactone deficiency confers resistance in tomato line SL-ORT1 to the parasitic weeds Phelipanche and Orobanche spp[END_REF]. It can detect SLs with a much higher sensitivity (down to 10 À13 M; Fig. S4) than that of a mass spectrometry analysis (10 À9 M; V. Puech-Pag es, pers. comm.), and it provides a better dynamic range for their quantification. As expected, when treated with the control solvent, the germination rate of P. ramosa seeds was null, while 73% germination was obtained in the presence of 10 À11 M GR24 (Fig. 4a). A similarly high rate of germination (55%) was obtained when seeds were treated with exudates of control roots, whereas zero or very few seeds germinated when treated with root extracts of RNAi Sl-IAA27 plants. Moreover, the addition of GR24 (10 À11 M) to the RNAi Sl-IAA27 root extract stimulated P. ramosa seed germination as efficiently as when added to the solvent or to the control root extract, showing the absence of germination inhibitors in the RNAi Sl-IAA27 root extracts (Fig. 4a). These results indicate that root extracts of RNAi Sl-IAA27 plants were at least 10 times less active than extracts of control roots (Figs 4, S4), therefore suggesting that SL synthesis of RNAi Sl-IAA27 roots could be strongly down-regulated.

To ask whether the mycorrhizal deficiency of the RNAi Sl-IAA27 plants could result from this SL down-regulation, we performed a mycorrhization assay with control and Sl-IAA27silenced plants in the presence or not of 10 À7 M GR24. The addition of the synthetic SLs complemented the mycorrhizal defect of RNAi Sl-IAA27 plants, especially by increasing the infection frequency as well as arbuscule abundance (Fig. 4b), strongly suggesting that the mycorrhizal defect of these plants was a result of a lower SL biosynthesis.

Discussion

Here we collected several pieces of experimental evidence suggesting that the auxin-related gene Sl-IAA27 positively regulates the mycorrhization process of tomato by controlling the SL synthesis via direct or indirect regulation of NSP1, a transcription factor that activates the SL biosynthesis genes D27 and MAX1 [START_REF] Liu | Strigolactone biosynthesis in Medicago truncatula and rice requires the symbiotic GRAS-type transcription factors NSP1 and NSP2[END_REF]. Indeed, we showed that the mycorrhizal defect of Sl-IAA27-silenced plants was correlated with a down-regulation of NSP1, D27 and MAX1 expression and arguably with a lower SL content in roots, which could be complemented by exogenous GR24 treatments.

We demonstrate for the first time the importance of an Aux/ IAA in the regulation of SLs biosynthesis, showing an additional crosstalk link between auxin and SL [START_REF] Foo | Auxin influences strigolactones in pea mycorrhizal symbiosis[END_REF][START_REF] Koltai | Cellular events of strigolactone signalling and their cross-talk with auxin in roots[END_REF]. Given that Aux/IAAs are known to interact with ARF partner proteins, preventing them from binding to target promoters, we can speculate that Sl-IAA27 represses an ARF that acts as a repressor of NSP1 expression. This repressor ARF remains to be identified, and it would be interesting to check the occurrence of this regulation in nonmycotrophic species, such as Arabidopsis thaliana.

Interestingly, we found that Sl-IAA27 expression is induced by the fungus very early in the mycorrhizal interaction, even before any root-fungus physical contact. We hypothesize that this induction is caused by some diffusible or volatile fungal signals and may result in the up-regulation of Sl-NSP1, thereby promoting Sl-D27 and Sl-MAX1 expression and SL synthesis in the roots. This would lead to an increase in SL content in root exudates and to the activation of the fungus metabolism in the rhizosphere [START_REF] Besserer | Strigolactones stimulate arbuscular mycorrhizal fungi by activating mitochondria[END_REF][START_REF] Besserer | GR24, a synthetic analog of strigolactones, stimulates the mitosis and growth of the arbuscular mycorrhizal fungus Gigaspora rosea by boosting its energy metabolism[END_REF].

We have previously shown in M. truncatula the importance of NSP1 in the mycorrhization process [START_REF] Delaux | NSP1 is a component of the Myc signaling pathway[END_REF]. The frequency of the fungal entries and the abundance of the arbuscules (with no change in their morphology) were reduced in the nsp1 mutant, similar to the mycorrhizal phenotype of Sl-IAA27-RNAi lines. Given that NSP1 positively regulates D27 and MAX1 expression [START_REF] Liu | Strigolactone biosynthesis in Medicago truncatula and rice requires the symbiotic GRAS-type transcription factors NSP1 and NSP2[END_REF], [START_REF] Delaux | NSP1 is a component of the Myc signaling pathway[END_REF] speculated that the nsp1 phenotype could result from a strong decrease in SL biosynthesis affecting fungal stimulation. Here, in tomato, we add some evidence to support this hypothesis: the perturbation of NSP1 expression, via the silencing of IAA27, has a negative impact on AM colonization that can be partially reversed by the addition of exogenous GR24. It would be interesting to determine whether a mutation of the ortholog of Sl-IAA27 in M. truncatula also down-regulates Mt-NSP1 expression and perturbs the mycorrhization and the nodulation processes.

Furthermore, the fact that the silencing of Sl-IAA27 only affects the expression of Sl-NSP1 and of the NSP1 target genes Sl-D27 and Sl-MAX1, but not of Sl-CCD7 and Sl-CCD8, points to the specificity of the Sl-IAA27 action. Intriguingly, the expression of Sl-CCD7 (but not of Sl-CCD8) was found to be upregulated during mycorrhization, suggesting the occurrence of another IAA27-independent signaling pathway for the activation of Sl-CCD7 expression in mycorrhizal tomato (L opez-R aez et al., 2014).

Our results indicate that in the root the presence of the fungus would switch off the transcription of Sl-IAA27 locally while upregulating this expression systemically in as yet uncolonized root tissue. Further investigation will be needed to determine if this IAA27 transcriptional activation is a result of intra-and/or extraradical diffusible fungal signals and what role this activation could have in the mycorrhization process. The local downregulation of Sl-IAA27 transcription in colonized root tissue is reminiscent of previous observation of a strong localized activation of DR5-GUS, an auxin-reporter construct, in arbusculecontaining cells [START_REF] Etemadi | Auxin perception is required for arbuscule development in arbuscular mycorrhizal symbiosis[END_REF]. Given that Sl-IAA27 expression of tomato can be down-regulated by treatment with exogenous auxin, at least in 12-d-old seedlings [START_REF] Bassa | Phenotypes associated with down-regulation of Sl-IAA27 support functional diversity among Aux/IAA family members in tomato[END_REF], the lack of Sl-IAA27 expression in the inner cortex and the vascular tissue of colonized root sections might be the result of an activation of auxin signaling in neighboring tissues.

We assume that a clear difference has to be made between the early colonization stages, when fungal growth has to be stimulated, and the later colonization stages, when mycorrhization and trophic exchanges have to be tightly controlled and balanced (notably to minimize the carbon cost for the host plant; [START_REF] Peng | Growth depression in mycorrhizal citrus at high-phosphorus supply (analysis of carbon costs)[END_REF]. During these late colonization stages, when the plant is well colonized, it is commonly known that SL content in roots decreases [START_REF] Lendzemo | Striga seed-germination activity of root exudates and compounds present in stems of Striga host and nonhost (trap crop) plants is reduced due to root colonization by arbuscular mycorrhizal fungi[END_REF][START_REF] Opez-R Aez | Arbuscular mycorrhizal symbiosis decreases strigolactone production in tomato[END_REF], 2014), while auxin content increases (reviewed in [START_REF] Fusconi | Regulation of root morphogenesis in arbuscular mycorrhizae: what role do fungal exudates, phosphate, sugars and hormones play in lateral root formation?[END_REF]. Here we speculate that the auxin-mediated down-regulation of Sl-IAA27 transcription in colonized root sections, by negatively regulating NSP1 expression and SL synthesis, participates in the complex process of autoregulation of mycorrhization and perhaps also in the process of arbuscule degeneration.

The present study illustrates the importance of careful spatiotemporal analyses in understanding the regulation mechanisms underlying the complex developmental process of mycorrhization. Further studies are necessary to fully understand why and how Sl-IAA27 expression is regulated at different stages of the mycorrhization process.

Fig. 1

 1 Fig. 1 Localization of Sl-IAA27 expression in roots of pSl-IAA27:: GUS plants inoculated or not with Rhizophagus irregularis. (a, b) b-glucuronidase (GUS) expression mainly found in the inner cortex and the central cylinder of young roots: (a) in a root of an uninoculated plant; (b) in a uncolonized root of an inoculated plant. (c, d) GUS expression in uncolonized (c) and colonized (d) root sections of an inoculated plant. (e, f) Corresponding fluorescent images confirming the absence (e) and presence (f) of the fungus stained with fluoresceinconjugated wheat germ agglutinin. (g) Quantification by quantitative reverse transcription polymerase chain reaction (qRT-PCR) of Sl-IAA27 gene expression in roots of plants cultivated in vitro in the presence or absence (control) of germinating spores separated with a cellophane membrane. Error bars represent AE SEM. Asterisks indicate a significant difference when compared with controls according to the Kruskal-Wallis test: n = 5, P < 0.05. (h, i) GUS expression in a control root (h) and in a root cultivated in the presence of germinating spores (i) separated by a cellophane membrane. The GUS pictures shown in this figure were obtained with transformed line 10/10/1. Bars: (a, b), 2 cm; (c-f, h, i), 200 lm.

Fig. 2 Fig. 3

 23 Fig.2Mycorrhizal phenotype of control and RNAi Sl-IAA27 lines 12 wk after inoculation with Rhizophagus irregularis. (a) Mycorrhizal rate in control and three RNAi Sl-IAA27 lines as measured by the grid-intersect method[START_REF] Giovannetti | An evaluation of techniques for measuring vesicular arbuscular mycorrhizal infection in roots[END_REF]. (b) Quantification of mycorrhization in control and in the three RNAi Sl-IAA27 lines separately, according to Trouvelot's method[START_REF] Trouvelot | Mesure du taux de mycorhization VA d'un systeme radiculaire. Recherche de methodes d'estimation ayant une signification fonctionnelle[END_REF]. 'F', frequency of colonization in the root system; 'a', arbuscule abundance (percentage) in the colonized root sections. (c-f) Root sections showing arbuscules of control (c, e) and Rline2 roots (d, f) stained with fluorescein-conjugated wheat germ agglutinin (c, d) and ink (e, f). Error bars represent AE SEM. Asterisks indicate a significant difference when compared with controls according to the Kruskal-Wallis test: n = 10, P < 0.05. Bars: (c, d), 50 lm; (e, f), 10 lm.

Fig. 4

 4 Fig.4Quantification of strigolactones in roots of control and RNAi Sl-IAA27 plants, and effect of GR24 on mycorrhization of control and RNAi Sl-IAA27 plants. (a) Germination rate of seeds of Phelipanche ramosa in response to solvent and to root exudates of control and RNAi Sl-IAA27 plants, with or without addition of 10 À11 M of synthetic strigolactone (GR24). (b) Percentage of mycorrhizal colonization in roots of control and RNAi Sl-IAA27 plants with or without 10 À7 M of GR24, 12 wk after inoculation, according to Trouvelot's method[START_REF] Trouvelot | Mesure du taux de mycorhization VA d'un systeme radiculaire. Recherche de methodes d'estimation ayant une signification fonctionnelle[END_REF]. 'F', frequency of colonization in the root system; 'a', arbuscule abundance (percentage) in the colonized root sections. Four replicates per RNAi Sl-IAA27 line were used here and mean values represent the average obtained with the three lines. Error bars represent AE SEM. Asterisks indicate a significant difference between control and RNAi Sl-IAA27 plants according to Kruskal-Wallis test: (a) n = 15, P < 0.01; (b) n = 12, P < 0.05.

Acknowledgements

This work was funded by the French ANR project miRcorrhiza (ANR-12-JSV7-0002-01). It was carried out in the LRSV, which belongs to the Laboratoire d'Excellence entitled TULIP (ANR-10-LABX-41). We thank P. Simier (LBPV, University of Nantes, France) for providing seeds of Phelipanche ramosa L. Pomel (genetic type 1) and V. Puech-Pag es (LRSV, Castanet-Tolosan, France) for the MS analyses. We also thank A. Le Ru (Genotoul, FRAIB 3450 CNR, Castanet-Tolosan, France) for her help with confocal microscopy.

Supporting Information

Additional Supporting Information may be found online in the Supporting Information tab for this article: Table S1 List of primers used, with the corresponding gene annotation