APHANO-Effect

Oomycetes form a phylogenetic lineage of fungus-like eukaryotic microorganisms related to photosynthetic organisms such as brown algae and diatoms. Oomycetes cause severe damages on important crops (potato, legumes) and in natural ecosystems. A variety of physiological differences mean that oomycetes are not inhibited by many of the chemicals used to control fungi. Regarding the impact of diseases caused by oomycetes, a better understanding of mechanisms that underlie pathogenicity is required. The recent completion of the genome sequence of several oomycete species (i.e Phytophthora infestans), has led to the discovery of two large and rapidly diverging superfamily of proteins translocated inside host cells. These proteins, also named effectors, possess a conserved N-terminal motif RXLR or LXLFLAK in the case of CRN (Crinkle and Necrosis) effectors and a highly divergent C-terminal part involved in effectors function. Once inside cells, effectors are presumed to interact with molecular targets to modify cellular functions, such as the suppression of host immune responses. CRN genes, and not RXLR genes, have been detected in all the plant pathogen oomycetes analyzed so far, suggesting that CRN are ancestral effectors in the oomycete lineage preceding the apparition of RXLR proteins. We are studying effectors and pathogenicity mechanisms in Aphanomyces euteiches, a root pathogen of legumes which is phylogenetically distant from Phytophthora and considered as an ancestral oomycete. We have recently identified A. euteiches CRN genes and showed that these proteins are translocated to host nuclei similarly to their Phytophthora orthologs. Within the phylum Oomycota, the genus Phytophthora is included in the Peronosporales lineage which includes most plant pathogens, whereas the Aphanomyces genus, which occurs within the Saprolegniales lineage, groups together both animal pathogens (crayfish, fish) and plant pathogens infecting various plant families (sugar beet, pea). Thus studying Aphanomyces pathogens offers the opportunity to identify key factors involved in host adaptation. The development in our research team of a powerful pathosystem involving A. euteiches and the legume model Medicago truncatula allows us to provide clues regarding the consequence of oomycete effectors production on interactions of plant roots with various type of microorganisms (pathogenic and symbiotic) occurring in the rhizosphere. By exploiting the advantages and originality of our biological system (host diversity of Aphanomyces sp., model plant able to interact with pathogenic and symbiotic microorganisms) the main objectives of the APHANO-Effect project are i) to evaluate the role of CRN effectors in the adaption of the pathogens by characterizing the repertoire and the evolution of CRN effectors in Aphanomyces species infecting animal and plant hosts, ii) to identify cellular targets of CRNs and iii) to study the impact of CRN expression on the perception of pathogenic and symbiotic signals and on their interaction with pathogenic and symbiotic microorganisms. To reach the objective 1, a comparative genomic approach based on transcriptomic analyses (RNAseq) will be conducted on Aphanomyces strains infecting various hosts (plants or animals) and by using the first genome draft of a Saprolegniale (A. euteiches, 69Mb) generated in the team in collaboration with the Genoscope.Molecular CRN targets (objectives 2) will be identified following a yeast 2-hybrid strategy as well as pull-down experiments upon in planta expression of tagged CRNs. Finally, M. truncatula roots expressing CRNs (composite plants) will be used to study the impact of CRN on pathogenic and symbiotic interactions. The project should impact appreciation about the diversity and conservation in functional evolutionary strategies used by pathogens, nevertheless genomic data obtained will be a valuable resource to identify keys actors implicated in host-specificity and pathogenicity of Aphanomyces sp.