Résumé
Recognition of a pathogen avirulence (AVR) effector protein by a cognate plant resistance (R) protein triggers a set of immune responses that render the plant resistant. Pathogens can escape this resistance by different mechanisms including the acquisition of new effectors that suppress AVR recognition. Leptosphaeria maculans, causal agent of oilseed rape stem canker, develops a high diversity of mechanisms to efficiently escape plant resistance including suppression of R protein-mediated recognition by an AVR effector. Indeed, AvrLm4-7 suppresses the recognition of AvrLm3 and AvrLm5-9 by the R proteins Rlm3 and Rlm9, respectively [1,2]. To decipher the epistatic interaction between these L. maculans AVR effectors, we determined the crystal structure of AvrLm5-9 and obtained a 3D model of AvrLm3, based on the crystal structure of Ecp11-1, a homologous AVR effector from Fulvia fulva. Despite a lack of sequence similarity, AvrLm5-9 and AvrLm3 are structural analogues of AvrLm4-7 (structure previously characterized [3]). Structure-informed sequence database searches identified a larger number of putative structural analogues among L. maculans effector candidates, including AvrLmS-Lep2, as well as among effector candidates from other phytopathogenic fungi [4]. Remarkably, transformants of L. maculans expressing one of these structural analogues, Ecp11-1, triggered Rlm3-mediated immunity. Furthermore, this recognition was suppressed by AvrLm4-7. On these bases, we continued to decipher the interactions between Ecp11-1 / AvrLm3 (and their closely related homologues) and Rlm3 by: (i) performing L. maculans complementation experiments with different homologues; (ii) determining whether AvrLm4-7 could suppress that recognition; (iii) finely characterizing the amino-acids / regions of Ecp11-1 and AvrLm3 which induced recognition by Rlm3. This analysis is a first step towards the understanding of broad-spectrum resistances that may allow multi-pathogen disease management.