Identification of major genes involved in rapeseed quantitative resistance to Leptosphaeria maculans by an innovative effectoromics approach.

International audience

English. Leptosphaeria maculans is a phytopathogenic fungus responsible for stem canker, a particularly damaging disease on rapeseed (Brassica napus). The infectious cycle of L. maculans is singularly long and complex by going through an ‘early’ phase of leaf infection and a ‘late’ phase of asymptomatic colonization of rapeseed stem, finally leading to stem canker. Genetic resistance is the principal way to control this disease, with numerous qualitative resistance genes of Brassica species recognizing L. maculans avirulence genes mainly expressed during the ‘early’ stage of infection. However, L. maculans has a high capability to bypass those resistances as the corresponding avirulence genes are located in genomic regions rich in transposable elements and rapidly evolving. Thus, there is an urge to search for more durable resistances to this pathogen. In this objective, we decided to focus on ‘late’ effectors expressed during stem colonization which are located in gene-rich regions of the genome and well conserved in the Leptosphaeria genus. We hypothesized that those effectors could trigger quantitative resistance in the stem, which would exert less selection pressure on the pathogen and be more durable. In a previous study, a set of ten ‘late’ effectors were selected and expressed at the cotyledon stage of infection using an innovative phenotyping method. Two of these ‘late’ effectors induced a resistance phenotype on several semi-winter rapeseed genotypes. To identify additional sources of quantitative resistance, we characterized the most complete set of potential L. maculans ‘late’ effectors for their expression kinetics during rapeseed infection, their conservation within the Leptosphaeria genus and among fungi, their location in the L. maculans genome, their enrichment in epigenetic marks and the 3D structure of the corresponding proteins. We then selected effector genes specifically expressed during petiole or stem infection, conserved in the Leptosphaeria genome, located in gene-rich regions, enriched in heterochromatin marks associated with genes, and belonging to new structural families of effectors. The selected ‘late’ effectors will be used to identify new resistances in B. napus genotypes that could be introduced into modern cultivars adapted to the agronomic context in France.