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Trees can be associated with dozens of fungi helping them to acquire resources from forest soils. The most widespread mutualistic association in boreal and temperate forests is the ectomycorrhizal symbiosis. This symbiosis involves mushroom-forming fungi of basidiomycota, ascomycota, and some zygomycota clades and the roots of woody plant species, including oaks, poplars or pines. Although the impact of this association on ecosystem production and tree nutrition is investigated for about a century, our understanding on the molecular mechanisms that control water and nutrient fluxes between plant and fungal partners is still limited. Here, we review the recent knowledge on the ectomycorrhizal contribution to tree nutrition. We specifically highlight the molecular mechanisms driving the acquisition, translocation and release of water and nutrients in ectomycorrhizal systems. We particularly focus on the transport of macronutrients, including nitrogen, phosphorus, potassium, sulphur and calcium, micronutrients, and water by the symbiotic partner. We also provide background on the evolution, diversity, and importance of this symbiosis, identify knowledge gaps, and propose future research directions.

International audience; Fungi are evolutionary shape shifters and adapt quickly to new environments. Ectomycorrhizal (EM) symbioses are mutualistic associations between fungi and plants and have evolved repeatedly and independently across the fungal tree of life, suggesting lineages frequently reconfigure genome content to take advantage of open ecological niches. To date analyses of genomic mechanisms facilitating EM symbioses have involved comparisons of distantly related species, but here, we use the genomes of three EM and two asymbiotic (AS) fungi from the genus Amanita as well as an AS outgroup to study genome evolution following a single origin of symbiosis. Our aim was to identify the defining features of EM genomes, but our analyses suggest no clear differentiation of genome size, gene repertoire size, or transposable element content between EM and AS species. Phylogenetic inference of gene gains and losses suggests the transition to symbiosis was dominated by the loss of plant cell wall decomposition genes, a confirmation of previous findings. However, the same dynamic defines the AS species A. inopinata, suggesting loss is not strictly associated with origin of symbiosis. Gene expansions in the common ancestor of EM Amanita were modest, but lineage specific and large gene family expansions are found in two of the three EM extant species. Even closely related EM genomes appear to share few common features. The genetic toolkit required for symbiosis appears already encoded in the genomes of saprotrophic species, and this dynamic may explain the pervasive, recurrent evolution of ectomycorrhizal associations.

Unravelling nutrient exchange in ectomycorrhizal symbiosis contributing to plant potassium nutrition. 12th Congress of the International Plant Molecular Biology

A major role of mutualistic interactions between plant roots and soil fungi is the improvement of plant nutrition by an equitable exchange of nutrients leading also to tolerance of environmental stress. Ectomycorrhizal symbiosis established between woody plants and soil fungi, widespread in forest ecosystems, is crucial for the plant partner to efficiently take up poorly available nutrients. Physiological studies as well as recent genome sequencing projects (Kohler et al. (2015) Nature Genet 47: 410-415) and transcriptome analyses (Doré et al. (2015) New Phytol 208: 1169-1187; Doré et al. (2017) Environ Microbiol 19:1338-1354) have allowed progress towards the identification and characterization of the symbiotic transportome (Garcia et al. (2016) Trends in Plant Science 21: 937-950). One of the major nutrients is potassium (K+ ), the most abundant cation in plant cells. We have shown improvement of potassium nutrition (Garcia et al. (2014) New Phytol 201: 951-960) under K+ shortage conditions by ectomycorrhizal symbiosis using two model species from European forests, Pinus pinaster and Hebeloma cylindrosporum. Questions are raised to identify the transport systems involved in the uptake of nutrients from the soil and in their transfer towards the plant at the symbiotic fungus-plant interface, called Hartig net. In the case of potassium (Garcia and Zimmermann (2014) Front Plant Sci 5: 337), we have identified two types of K+ transporters, Trk and HAK, as candidates to perform K+ uptake from the soil by the fungal extraradical hyphae, and two types of K+ channels, Shaker-like and TOK, that may release K+ by the hyphae of the Hartig net into the plant apoplasm. We have studied the three TOK (Two-pore Outward K+) channels identified in the genome of H. cylindrosporum, a channel family specific for fungi initially described in yeast (Ketchum et al. (1995) Nature 376: 690-695). These three TOK channels from the ectomycorrhizal fungus H. cylindrosporum belonging to two different subfamilies have been functionally characterized and localized (Carmen Guerrero-Galán et al. (2018) Env Microbiol , in press). Finally, we have analyzed whether these K+ channels might play specific roles within the fungus and within the symbiosis.

International audience; Symbiosis with ectomycorrhizal (ECM) fungi can be important for regeneration success. In a context of increasing regeneration failures in the coastal forest of maritime pine in Southwest France, we tried to identity whether differences in ECM communities could partly explain the variation of regeneration success and how they are influenced by forest practices and stand characteristics. In particular, we focused on the effects of harvesting methods (comparing mature forest with seed-tree regeneration and clear-cuts) and topography (bottom-, mid-, and top positions). Five field trials (two in regeneration failure areas and three in successful areas) were used to sample 450 one-year-old seedlings. Assessments of ECM of seedling nutrient concentrations and of seedling growth based on exploration types were made. ECM root colonisation was similar in all harvesting treatments, suggesting that enough inoculum remained alive after logging. Harvesting-induced effects modifying soil properties and light availability respectively impacted ECM composition and seedling growth. Topography-induced variations in water and nutrient availability led to changes in ECM composition, but had little impact on seedling growth. Contact, short-distance, and long-distance exploration types improved the nutritional status of seedlings (Ca, K, and N), showing that mycorrhization could play an important role in seedling vitality. However, neither ECM root colonisation nor exploration types could be related to regeneration failures.

The ascomycete Tuber borchii (Pezizomycetes) is a whitish edible truffle that establishes ectomycorrhizal symbiosis with trees and shrubs. This fungus is ubiquitous in Europe and is also cultivated outside Europe. Here, we present the draft genome sequence of T. borchii strain Tbo3840 (97.18 Mb in 969 scaffolds, with 12,346 predicted protein-coding genes).

New Caledonian serpentine (ultramafic) soils contain high levels of toxic heavy metals, in particular nickel, (up to 20 g kg−1) and are deficient in essential elements like carbon, nitrogen and phosphorus while having a high magnesium/calcium ratio. Although previous studies showed that ectomycorrhizal symbioses could play an important role in the adaptation of the endemic plants to ultramafic soils (FEMS Microbiol Ecol 72:238–49, 2010), none of them have compared the diversity of microbial communities from ultramafic vs non-ultramafic soils in New Caledonia. We explored the impact of edaphic characteristics on the diversity of ectomycorrhizal (ECM) fungi associated with different endemic species of Tristaniopsis (Myrtaceae) growing under contrasting soil conditions in the natural ecosystems of New Caledonia. ECM root tips were thus sampled from two different ultramafic sites (Koniambo massif and Desmazures forest) vs two volcano-sedimentary ones (Arama and Mont Ninndo). The molecular characterization of the ECM fungi through partial sequencing of the ITS rRNA gene revealed the presence of different dominant fungal genera including, both soil types combined, Cortinarius (36.1%), Pisolithus (18.5%), Russula (13.4%), Heliotales (8.2%) and Boletellus (7.2%). A high diversity of ECM taxa associated with Tristaniopsis species was found in both ultramafic and volcano-sedimentary soils but no significant differences in ECM genera distribution were observed between both soil types. No link could be established between the phylogenetic clustering of ECM taxa and their soil type origin, thus suggesting a possible functional—rather than taxonomical—adaptation of ECM fungal communities to ultramafic soils.

Ectomycorrhizal (ECM) symbiosis is defined as a mutual association between fungal symbionts and roots of woody plants. This symbiosis is strongly represented in temperate and boreal forests and is considered to be the most widespread means of increasing P acquisition by trees. Despite the importance of the ECM association, current knowledge about molecular mechanisms involved in membrane transport between the two partners is still limited. Therefore, the identification of the mechanisms essential for the symbiosis is a major issue. To better understand these mechanisms, we used a model association between the ECM basidiomycete Hebeloma cylindrosporum and the maritime pine Pinus pinaster. The fungal genome contains three transporters of Pi (HcPT1.1, HcPT1.2, and HcPT2), which have been already characterized as H+ Pi carriers. We hypothesized that one of these carriers could be responsible for both influx and efflux of Pi from the soil to the fungus and from the fungus to the plant. Our results suggest that HcPT2 would be the best candidate to fill both roles. An in vitro ectomycorrhizal system was used to investigate phosphate flux from H. cylindrosporum toward a liquid compartment containing pine roots. Interestingly, the expression pattern of HcPT2 was correlated to fungal phosphate efflux. Moreover, immuno-localization experiments showed that the protein is located (i) in the extraradical hyphae and the fungal sheath, compatible with Pi uptake, and (ii) within the Hartig net, which is the place of exchange between fungal and host cells. In order to study more specifically the role of HcPT2 at the fungus-plant interface, overexpressing and knock-down transgenic H. cylindrosporum strains were produced. Fungal strains with up regulated expression of HcPT2 increased phosphate efflux within the interaction medium, supporting a role of HcPT2 in phosphate transfer to the plant. In addition, the modification of HcPT2 expression strongly affects Pi nutrition of P. pinaster.

International audience; It is assumed that the most important role of (ecto)mycorrhizal symbiosis is to improve the P nutrition of the host-plant. Several hypothesis have been proposed to explain this positive role of the symbiosis that are (1) the increase of soil exploitation and P uptake by the fungus developing outside the roots, (2) the capacity to produce organic anions to mobilise adsorbed mineral P and (3) the capacity to produce enzymes, especially acid phosphatases able to mineralize organic P forms, to release free inorganic P (Pi) that could benefit to the host plant. The talk will present the results we obtained regarding the possible role of ectomycorrhizal symbiosis to mobilize soil mineral and organic P from forest soils. We worked mainly with Pinus pinaster as a model species. We used different approaches either in the field or in the laboratory. In the field, we explored the diversity of the ectomycorrhizal species associated with 14 year-old P. pinaster plantations as a function of fertilization (no fertilization, P or NPK fertilization) and irrigation. We also measured the enzyme activities produced by the ectomycorrhizal tips to assess their variability as a function of the fertilization treatment and/or the fungal species. In the laboratory, we cultivated young seedlings in rhizoboxes containing soil collected either near Montpellier (chromic cambisol) or from the same P. pinaster plantation, mentioned above. We cultivated young P. pinaster seedlings, whether or not associated with “laboratory” fungal species such as Rhizopogon roseolus (strong oxalate producer) or Hebeloma cylindrosporum (strong acid phosphatase producer), or with native fungal species from field soil samples. We studied also the interactions between ECM plants and other actors of the P cycle that are bacteria able to mineralize phytate, a recalcitrant organic P source, and their predators that are bacteria-feeding nematodes. In the field, we detected ≈ 20 species using DNA extracted from individual ECM tips and ITS sequencing. As expected, the fertilization regime modified the diversity of the ECM species associated with the roots with the maximum level of diversity found in irrigated, not fertilized plots (18 species) and the lowest one in non irrigated, fully fertilized plots (5 species). We found that N-acetlyl-glucosaminidase (NAG) and acid phosphatase (AcPAse) activities were the highest ones among the 8 activities assayed. Contrary to the NAG that did not vary much between treatments, the AcPAse could vary with fertilization but the variations seemed to depend first on the species. In the laboratory experiments, if our results demonstrated the importance of the fungal oxalate release to mobilize mineral P towards the host plant, our results are more in favour of an indirect role of the ectomycorrhizal fungi to mobilize organic P present in the medium. Therefore a new model of ectomycorrhizal functioning will be proposed.

Potassium (K+) is one of the most abundant element of soil composition but its very low availability limits plant growth and productivity of ecosystems. Because this cation participates in many biological processes, its constitutive uptake from soil solution is crucial for the plant cell machinery. Thus, the understanding of strategies responsible of K+ nutrition is a major issue in plant science. Mycorrhizal associations occurring between roots and hyphae of underground fungi improve hydro-mineral nutrition of the majority of terrestrial plants. The contribution of this mutualistic symbiosis to the enhancement of plant K+ nutrition is still underestimated and poorly studied so far. This mini-review examines the current knowledge about the impact of both arbuscular mycorrhizal and ectomycorrhizal symbioses on the transfer of K+ from the soil to the plants. A model summarizing plant and fungal transport systems identified and hypothetically involved in K+ transport is proposed. In addition, some data related to benefits for plants provided by the improvement of K+ nutrition thanks to mycorrhizal symbioses are presented.

Roots of most trees form symbiosis with mutualistic soil-borne fungi. The ectomycorrhizal basidiomycete Laccaria bicolor (Maire) P.D. Orton relies on mycorrhizal-induced small secreted proteins (MiSSP) to establish symbiotic tissues in the host-plant. The host proteins targeted by these fungal effectors are yet unknown. In the present study, we used the binary yeast two-hybrid (Y2H) system to determine direct interactions between MiSSP7 and the plant proteins in the L. bicolor-P. trichocarpa ectomycorrhizae. We showed that MiSSP7 interact with the jasmonic acid (JA) co-receptors JAZ5 and JAZ6 of P. trichocarpa, blocking JA signaling and promoting mutualism. L. bicolor transformants with severely reduced expression of MiSSP7 did not enter into symbiosis with poplar roots, a phenotype that could be complemented by transgenically varying the transcription of PtJAZ6 or through inhibiting JA signalling. Additional Y2H assays showed that PtJAZ6 protein form a regulatory complex involving 14-3-3 protein(s) and MYC transcriptional factors. Two others L. bicolor effectorlike proteins, MiSSP8 and MiSSP17, are secreted and are essential for the symbiosis development. MiSSP8 showed parietal and plasmalemma localization, while MiSSP17 was mainly cytoplasmic. Y2H assays suggested that these MiSSPs interact with plant proteins involved in plant defence signalling pathways. During symbiosis development, L. bicolor experiences important genetic reprogramming required for root colonization. Transcription factors (TFs) are key players of these genetic changes. Here, we developed high throughput analysis of TFs in L. bicolor to obtain a comprehensive inventory of significantly regulated transcription factors in ECM and showed that some of them display similarities with TFs involved in cell wall integrity, carbon, nitrogen, iron and sulfur metabolism. In addition, Yeast One Hybrid assays allowed the functional validation of 6% of the TFs predicted in silico and the identification of Secreted Transcriptional Activator Proteins (STAP) which may be a new class of effectors promoting symbiosis development or/and controlling rhizospheric microorganisms., Les racines de la plupart des arbres forment des symbioses ectomycorhiziennes avec les champignons mutualistes du sol. Le basidiomycète Laccaria bicolor (Maire) P.D. Orton secrète en contexte d’interaction des petites protéines effectrices (MiSSP) afin de faciliter la colonisation de la racine et établir les structures symbiotiques. Toutefois, les protéines de l’hôte ciblées par les MiSSPs de L. bicolor ne sont pas encore identifiées. Dans notre étude, nous démontrons, à l’aide du système double hybride chez la levure (Y2H), que la protéine MiSSP7 de L. bicolor interagit avec les co-récepteurs de l'acide jasmonique (AJ) JAZ5 et JAZ6 de P. trichocarpa. Cette interaction protéine-protéine entraine un blocage de la voie de signalisation de l’AJ et favorise le développement symbiotique. Des transformants de L. bicolor, dont l’expression de MiSSP7 est fortement réduite, ne sont plus capables de mycorhizer les racines du peuplier. Néanmoins, ce phénotype peut être complémenté en faisant varier de façon transgénique la transcription de PtJAZ6 ou par inhibition de la voie de signalisation de l'AJ. A l’aide du système Y2H, nous avons également mis en évidence que la protéine PtJAZ6 est capable d’interagir avec une protéine de type 14-3-3 et un facteur de transcription de type MYC, formant probablement un complexe de régulation. Deux autres protéines effectrices de L. bicolor, MiSSP8 et MiSSP17, sont sécrétées et essentielles au développement symbiotique. MiSSP8 présente alors une localisation pariétale et plasmalemmique, tandis que MiSSP17 est cytoplasmique. Les résultats des analyses Y2H suggèrent que MiSSP8 et MiSSP17 pourraient être impliquées dans le contournement des réactions de défense de la plantehôte. Par ailleurs, au cours du développement symbiotique, le champignon est le siège d’une reprogrammation génétique importante nécessaire à la colonisation des racines, au contournement des réactions de défense de la plante et à l'établissement d'échanges mutualistes. Les facteurs de transcription (TFs) sont les principaux acteurs de ces changements génétiques. Nous avons donc étudié les TFs de L. bicolor afin d’obtenir un inventaire complet des TFs régulés par la mycorhization. Certains d'entre eux présentent des similitudes avec des TFs impliqués dans le maintien de l'intégrité des parois cellulaires et la régulation du métabolisme du carbone, de l'azote, du fer et du soufre. De plus, le système simple hybride en levure a permis la validation fonctionnelle de 6% des TFs prédits in silico et l'identification de protéines sécrétées capables d’activer la transcription (STAP). Ces dernières pourraient constituer une nouvelle classe d'effecteurs favorisant le développement symbiotique ou le contrôle des microorganismes de la rhizosphère.

Les microorganismes agissent comme un puits et une source de N et P disponibles car ils sont responsables des cycles biogéochimiques de N et P. La boucle microbienne, basée sur la prédation des bactéries par les microprédateurs tels que les nématodes bactérivores, est considérée comme un facteur majeur de la minéralisation de Net de P dans les écosystèmes terrestres. Cependant, peu de données sont disponibles sur l'impact de la prédation par les nématodes sur la nutrition minérale des plantes ligneuses ectomycorhizées. Différentes expérimentations ont été conduites pour quantifier le rôle dela prédation des bactéries par les nématodes sur l'architecture et la croissance racinaire, la nutrition minérale (N et P) d'une espèce ligneuse, Pinus pinaster, associée ou non avec le basidiomycète ectomycorhizien Hebeloma cylindrosporum. Les plantes ont été cultivées dans un système expérimental simplifié et stérile, et inoculées ou non avec Bacillus subtilis et des nématodes bactérivores (de la famille des Rhabditidae ou des Cephalobidae) isolés à partir d'ectomycorhizes et de sol provenant d'une plantation de Pin maritime. L'effet de la prédation sur la croissance des plantes et le devenir du 15N bactérien vers les parties aériennes dépend très fortement de la disponibilité en P du milieu. De plus, la prédation des bactéries est indispensable pour permettre à la plante d'utiliser le P du phytate, une source de P organique très peu disponible pour la plante mais très facilement utilisable par B. subtilis car cette bactérie est capable de libérer de la phytase dans le milieu. Ces résultats ouvrent de nouvelles perspectives pour améliorer l'utilisation du phytate pour la nutrition phosphatée des plantes., Soil microorganisms act as a sink and a source of available N and P bymediating key processes in the biogeochemical N and P cycling. The microbial loop, based upon the grazing of bacteria by predators such as bacterial-feeding nematodes, is thought to play a major role in the mineralization of nutrients such as nitrogen (N) and phosphorus (P) in terrestrial ecosystems. However, little is known about the impact of grazing by nematodes on mineral nutrition of ectomycorrhizal woody plants. Different studies were undertaken to quantify the role of nematode grazing on bacteria on the root growth and architecture, mineral nutrition (N and P) of a woody species, Pinus pinaster, whether or not associated with the ectomycorrhizal basidiomycete Hebeloma cylindrosporum. Plants were grown in a sterile simplified experimental system, whether inoculated or not with Bacillus subtilis and bacterial-feeding nematodes (belonging to Rhabditidae or Cephalobidae families) that were isolated from ectomycorrhizae and from soil of a P. pinaster plantation. The effect of nematode grazing on plant growth and the fate of bacterial 15N towards plant shoots was strongly dependent upon medium P availability. In addition, nematode grazing was required to enable the plant to access P from phytate, a well-known poorly available P source to plants but that was used by bacterial populations of B. subtilis due to its ability to release phytase in the medium. These results open an alternative route to increase the use of phytate for plant P nutrition.

The EC-supported Network of Excellence Evoltree (http://www.evoltree.eu) formed a group of scientists involved in and actively contributing to Jointly Executed Research Activities on community genetics in forest ecosystems. In addition to the main authors, the group includes S. Augu-stin, M. Brandle, C. Burban, J. Burczyk, S. Cavers, I. Chybicki, C. Conord, E. Cremer, J. DeWoody, K. Donges, B. Fady,L. Karlinski, C. Kerdelhue, B. Kieliszewska-Rokicka, G. Kost, M. Kulczyk-Skrzeszewska, F. Lakatos, F. Lefevre, S.Liepelt, S. Oddou-Muratorio, K.-H. Rexer, M. Rudawska, M.Schadler, G. Taylor, K. Tuba, M. Viger, F. Villani, M. Villar.; Understanding the interactions of co-occurring species within and across trophic levels provides key information needed for understanding the ecological and evolutionary processes that underlie biological diversity. As genetics has only recently been integrated into the study of community-level interactions, the time is right for a critical evaluation of potential new, gene-based approaches to studying communities. Next-generation molecular techniques, used in parallel with field-based observations and manipulative experiments across spatio-temporal gradients, are key to expanding our understanding of community-level processes. Here, we introduce a variety of ‘-omics’ tools, with recent studies of plant–insect herbivores and of ectomycorrhizal systems providing detailed examples of how next-generation approaches can revolutionize our understanding of interspecific interactions. We suggest ways that novel technologies may convert community genetics from a field that relies on correlative inference to one that reveals causal mechanisms of genetic co-variation and adaptations within communities.

The development of ectomycorrhizal symbiosis leads to drastic changes in gene expression in both partners. However, little is known about the spatial regulation of symbiosis-regulated genes. Using cDNA array profiling, we compared the levels of expression of fungal genes corresponding to approximately 1,200 expressed sequenced tags in the ectomycorrhizal root tips (ECM) and the connected extraradical mycelium (EM) for the Paxillus involutus - Betula pendula ectomycorrhizal association grown on peat in a microcosm system. Sixty-five unique genes were found to be differentially expressed in these two fungal compartments. In ECM, a gene coding for a putative phosphatidylserine decarboxylase (Psd) was up-regulated by 24-fold, while genes coding for urea (Dur3) and spermine (Tpo3) transporters were up-regulated 4.1- and 6.2-fold in EM. Moreover, urea was the major nitrogen compound found in EM by gas chromatography-mass spectrometry analysis. These results suggest that (i) there is a spatial difference in the patterns of fungal gene expression between ECM and EM, (ii) urea and polyamine transporters could facilitate the translocation of nitrogen compounds within the EM network, and (iii) fungal Psd may contribute to membrane remodeling during ectomycorrhiza formation.

Ectomycorrhiza formation and function alter both fungal and plant gene expression. The identification of a large rumber of novel genes expressed exclusively or predominantly in the symbiosis will contribute greatly to the understanding of the function of the ectomycorrhizalassociation. We have constructed a cDNA library of 4-day-old Eucalyphls globulus-Pisolithus tinctorius ectomycorrhiza by random cloning and through suppression subtractive hybridization. We screened 715 arrayed cDNAs to identify symbiosis-regulated genes by using differential hybridization. Gene expression profiles obtained from free-living Pisolithus tinctorius, non-inoculated roots and ectomycorrhizas at various developmental stages, from theearliest contacts (4 days) to the functionning symbiotic organ (28 days) were analyzed. Comparisons of free-living partners and symbiotic tissues revealed significant changes in the expression levels (differential expression ratio> 2.0) for 11 to 23% of the genes analyzed at the different stages of mycorrhiza formation. No ectomycorrhiza-specific gene was detected. We derived groups of coordinately expressed genes (i.e. regulons) using hierarchical clustering and Self Organizing Maps. At least a dozen of distinct temporal patterns of induction/repressionwere observed. The main fungal regulons contained genes coding for cell-wall and membrane proteins, communication genes, and metallothionein-related poteins. In the host root, a major down-regulated regulon comprised genes involved in water transport and stress suggesting that mycorrhiza development improves water uptake.We have furthermore characterizedcDNA clones corresponding to Pisolithus signalling genes (ras, raf and calcineurine) and to a new hydrophobin gene (HydPt-3).; Les ectomycorhizes sont des associations symbiotiques établies entre les racines des arbres et certains champigpons du sol. Ces relations mutualistes, dont les bénéfices sont avant tout trophiques, entraînent des modifications morphologiques et métaboliques importantes chez lesdeux partenaires. La formation de l'ectomycorhize Eucalyprus globulus-Pisolithus tinctorius s'accompagne également d'un changement du protéome et de l'expression génique des racines et du champignon. Afin d'acquérir une vision globale des gènes exprimés et régulés au cours dela formation de l'ectomycorhize E. globulus-P. tinctorius, nous avons développé une analyse de l'expression sénique dans l'organe symbiotique via l'analyse du transcriptome. Une première étude, nous a permis d'obtenir les profils d'expression de 400 ESTs (Expressed Sequer..ce Tags)dans une ectomycorhize en formation. Nous avons ainsi pu identifier 17% des ESTs qui sont régulés par la symbiose. Nous avons poursuivi notre analyse sur plus de 700 ESTs au cours dudéveloppement de l'organe symbiotique, depuis les premiers contacts entre les symbiotes jusqu'à la mise en place de l'ectomycorhize fonctionnelle. L'analyse statistique des profils d'expression nous il permis de mettre en évidence (i) des ensembles de gènes co-régulés(régulons) et (ii) une chronoséquence dans l'expression génique. Parmi les gènes fortementrégulés nous avons identifié : ceux codant les protéines pariétales fongiques (hydrophobines et SRAP32), plusieurs familles de métallothionéines, des protéines de signalisation, des protéines de stress hydrique et des enzymes de la respiration mitocnondria.le. Parmi ces gènes régulés, nous avons caractérisé en détail, un gène codant une nouvelle hydrophobine, HydPt-3, et plusieurs gènes de communication fongiques (ras, raf, calcineurine).

Unravelling nutrient exchange in ectomycorrhizal symbiosis. 3rd international Molecular Mycorrhiza Meeting iMMM