Identification of the determinants involved in the adaptation of Leptosphaeria maculans to different Brassica species

International audience

anglais. Plant pathogenic fungi have unique adaptive capacities that allow them to circumvent any new control method. Leptosphaeria maculans, a Dothideomycete fungus causing stem canker of several cruciferous species, has an extremely complex life cycle on oilseed rape (Brassica napus). The most effective control method against this disease relies on breeding resistant oilseed rape cultivars. The disease resistance roughly follows gene-for-gene relationships: a product of a host resistance gene (R gene) interacts directly or indirectly with the product of a matching avirulence gene (Avr) of the pathogen stimulating host immunity and resistance responses. These specific, single-gene, cultivardependent resistances are rapidly bypassed by the pathogen and a limited number of resistance genes are available and still efficient. Closely-related species of B. napus, Brassica carinata and Brassica nigra, are highly resistant to L. maculans, with B. carinata considered nonhost. A few field isolates were recently identified as partially adapted to these species. The availability of these isolates, while raising concerns about the ability of L. maculans to adapt to hosts considered resistant, represents an opportunity for the analysis of factors involved in adaptation to different hosts as well as the resistance mechanisms of species related to oilseed rape. The objective of this thesis is to identify the basis of adaptation of L. maculans to different Brassica species combining genetic,-omic and functional approaches. Here, we will focus on preliminary results of the genetic analyses based on a cross performed between one isolate adapted to B. carinata and the reference isolate adapted to B. napus. Phenotyping of the progeny isolates on oilseed rape and B. carinata showed a continuum of aggressiveness suggesting a complex, multigenic determinism of the adaptation. A QTL analysis has been set up, allowing identification of regions carrying potential candidate genes involved in the adaptation of L. maculans to B. napus and B. carinata. The study of this complex adaptation will shed light on new, possibly more sustainable, resistances to L. maculans in Brassica.