Genetic and molecular characterization of a locus involved in avirulence of Blumeria graminis f. sp. tritici on wheat Pm3 resistance alleles.

Wheat powdery mildew is caused by the obligate biotrophic fungus Blumeria graminis f. sp. tritici. The allelic series of the wheat Pm3 gene conferring race-specific resistance against powdery mildew has been well characterized functionally, and recently the corresponding avirulence gene AvrPm3a/f triggering the specific recognition by Pm3a and Pm3f alleles was cloned. Here, we describe the genetic and molecular analysis of two additional Blumeria loci involved in the resistance mediated by the Pm3c and Pm3f alleles. We genetically identified the two loci and mapped at high resolution one locus involved in the avirulence towards both Pm3c and Pm3f. The single candidate gene Bcg1 was identified in a physical target interval of 26kb defined by flanking genetic markers. Bcg1 encodes a small secreted protein sharing structural homology with ribonucleases and belongs to a family of clustered putative effector genes under diversifying selection. We found a very good, but not complete, correlation of Bcg1 haplotypes with the phenotypes of natural isolates. Two mutants were generated that were affected in their phenotypes towards Pm3a and Pm3f but did not show any sequence polymorphism in Bcg1. Our results suggest that avirulence to Pm3 in Blumeria is determined by a complex network of genes, in which Bcg1 might have a central role as a modifier of the Pm3/AvrPm3 interactions.