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Septoria tritici blotch (STB), caused by the fungus Zymoseptoria tritici, is among the most threatening wheat diseases in Europe. Genetic resistance remains one of the main environmentally sustainable strategies to efficiently control STB. However, the molecular and physiological mechanisms underlying resistance are still unknown, limiting the implementation of knowledge-driven management strategies. Among the 22 known major resistance genes (Stb), the recently cloned Stb16q gene encodes a cysteine-rich receptor-like kinase conferring a full broad-spectrum resistance against Z. tritici. Here, we showed that an avirulent Z. tritici inoculated on Stb16q quasi near isogenic lines (NILs) either by infiltration into leaf tissues or by brush inoculation of wounded tissues partially bypasses Stb16q-mediated resistance. To understand this bypass, we monitored the infection of GFP-labeled avirulent and virulent isolates on Stb16q NILs, from germination to pycnidia formation. This quantitative cytological analysis revealed that 95% of the penetration attempts were unsuccessful in the Stb16q incompatible interaction, while almost all succeeded in compatible interactions. Infectious hyphae resulting from the few successful penetration events in the Stb16q incompatible interaction were arrested in the sub-stomatal cavity of the primary-infected stomata. These results indicate that Stb16q-mediated resistance mainly blocks the avirulent isolate during its stomatal penetration into wheat tissue. Analyses of stomatal aperture of the Stb16q NILs during infection revealed that Stb16q triggers a temporary stomatal closure in response to an avirulent isolate. Finally, we showed that infiltrating avirulent isolates into leaves of the Stb6 and Stb9 NILs also partially bypasses resistances, suggesting that arrest during stomatal penetration might be a common major mechanism for Stb-mediated resistances.

Fusarium graminearum, the primary cause of Fusarium head blight (FHB) in small-grain cereals, demonstrates remarkably variable levels of aggressiveness in its host, producing different infection dynamics and contrasted symptom severity. While the secreted proteins, including effectors, are thought to be one of the essential components of aggressiveness, our knowledge of the intra-species genomic diversity of F. graminearum is still limited. In this work, we sequenced eight European F. graminearum strains of contrasting aggressiveness to characterize their respective genome structure, their gene content and to delineate their specificities. By combining the available sequences of 12 other F. graminearum strains, we outlined a reference pangenome that expands the repertoire of the known genes in the reference PH-1 genome by 32%, including nearly 21,000 non-redundant sequences and gathering a common base of 9250 conserved core-genes. More than 1000 genes with high non-synonymous mutation rates may be under diverse selection, especially regarding the trichothecene biosynthesis gene cluster. About 900 secreted protein clusters (SPCs) have been described. Mostly localized in the fast sub-genome of F. graminearum supposed to evolve rapidly to promote adaptation and rapid responses to the host’s infection, these SPCs gather a range of putative proteinaceous effectors systematically found in the core secretome, with the chloroplast and the plant nucleus as the main predicted targets in the host cell. This work describes new knowledge on the intra-species diversity in F. graminearum and emphasizes putative determinants of aggressiveness, providing a wealth of new candidate genes potentially involved in the Fusarium head blight disease.

International audience; L'agriculture européenne est reconnue pour son haut niveau de rendement et de productivité. Parallèlement à un effort important de sélection et d'amélioration variétale dans la seconde moitié du 20 e siècle, cette productivité s'explique par un degré important de spécialisation et d'intensification des systèmes de cultures. La simplification écologique de ces systèmes conduit à de nombreux problèmes environnementaux et à une sensibilité accrue aux aléas climatiques et économiques. Récemment, le développement de la recherche sur les céréales pérennes, issues de graminées vivaces, propose une voie de diversification et d'amélioration des performances environnementales des rotations. La valorisation d'un couvert pérenne et de l'activité racinaire qui lui est associé offre des atouts indéniables pour la protection et la fertilité des sols. Bien qu'encore partiellement domestiquées et proposant des rendements en grain plus faibles que les céréales annuelles, l'utilisation de certaines populations de Thinopyrum intermedium ((Host) Barkworth & D.R. Dewey) est prometteuse en raison de leur capacité à combiner une production de grains et de biomasse fourragère. L'amélioration variétale et le développement d'idéotypes plus adaptés aux situations climatiques européennes et plus performants pour la production de grains représentent un enjeu majeur. Les premières années d'expérimentation en France et en Belgique ont permis d'identifier les principaux paramètres de croissance et de rendement de cette espèce. La production de grains varie entre 0,5 et 2 tonnes par hectare et est complétée par une production de matière sèche fourragère atteignant jusqu'à 11 tonnes MS par hectare. Dès la deuxième année, la biomasse racinaire produite est très supérieure aux standards annuels et influence positivement les caractéristiques structurales et microbiologiques du sol. La capacité de repousse post-récolte offre une valorisation additionnelle de fourrage. A ce jour, la phase d'implantation sensible à la concurrence des adventices (plante juvénile), l'induction des périodes phénologiques d'élongation et de floraison, l'influence des conditions d'alimentation hydrique et minérale et la gestion des valorisations fourrage-pâturage sont des objets de recherches développés dans le cadre d'un programme multipartenarial coordonné par l'ISARA.

Meeting abstract Molecular Plant-Microbe Interactions, 32(10): 203 ISSN: 0894-0282 eISSN: 1943-7706Meeting abstractMolecular Plant-Microbe Interactions, 32(10): 203ISSN: 0894-0282eISSN: 1943-7706; Fungal plant diseases are controlled by a complex molecular dialogue that involves pathogen effectors able to manipulate plant susceptibility factors at the earliest stages of the interaction. By probing the wheat-Fusarium graminearum pathosystem, we profiled the co-regulations of the fungal and plant proteins shaping the molecular responses of a 96 hour-long infection's dynamics. While no symptoms were yet detectable, fungal biomass swiftly increased along with an extremely diverse set of secreted proteins and candidate effectors supposed to target key plant organelles. Some showed to be early accumulated during the interaction or already present in spores, otherwise stored in germinating spores and detectable in an in vitro F. graminearum exudate. Wheat responses were swiftly set up and were evidenced before any visible symptom. Significant wheat protein abundance changes co-occurred along with the accumulation of putative secreted fungal proteins and predicted effectors. Regulated wheat proteins were closely connected to basal cellular processes occurring during spikelet ontogeny and particular co-regulation patterns were evidenced between chloroplast proteins and fungal proteins harboring a predicted chloroplast transit peptide. The plant and fungal coordinated responses provide a resourceful set of data and expand our understanding of the wheat-F. graminearum interaction.

International audience; Biological control is a great hope for reducing the overutilization of pesticides in agricultural soils. It often involves microorganisms or molecules produced by microorganisms that will be able to interact with either a plant or pathogens of this plant to reduce the growth of the pathogen and limit its negative impact on the host plant. When new biocontrol products are developed, strains were mostly selected based on their ability to inhibit a pathogen of interest under in vitro conditions via antagonistic effects. Strains with no in vitro effect are often discarded and not tested in planta. But is the in vitro selection of bacterial agents according to their antagonism activities towards a plant pathogen the best way to get effective biocontrol products? To answer this question, we used wheat and the fungal pathogen Fusarium graminearum as a study pathosystem model. A library of 205 soil bacteria was screened in 2 types of in vitro growth inhibition tests against F. graminearum, and in an in planta experiment. We find strains which do not have inhibition phenotypes in vitro but good efficacy in planta. Interestingly, some strains belong to species (Microbacterium, Arthrobacter, Variovorax) that are not known in the literature for their ability to protect plants against fungal pathogens. Thus, developing a biocontrol product against F. graminearum must be preferentially based on the direct screening of strains for their protective activity on wheat plants against fungal diseases, rather than on their in vitro antagonistic effects on fungal growth.

National audience

Molecular determinism of wheat susceptibility to FHB: How early events control the fate of infection?. 11. Effectome meeting