Loss of AvrSr50 by somatic exchange in stem rust leads to virulence for Sr50 resistance in wheat

Race-specific resistance genes protect the global wheat crop from stem rust disease caused by Puccinia graminis f. sp. tritici (Pgt), but often break down due to evolution of new virulent pathogen races. To understand virulence evolution in Pgt we identified the protein ligand (AvrSr50) recognized by the Sr50 resistance protein. A spontaneous mutant of Pgt virulent to Sr50 contained a 2.5Mbp loss-of-heterozygosity event. A haustorial secreted protein from this region triggers Sr50-dependent defense responses in planta and interacts with the Sr50 protein. Virulence alleles of AvrSr50 have arisen by DNA insertion and sequence divergence and our data provide molecular evidence that, in addition to sexual recombination, somatic exchange can play a role in the emergence of new virulence traits in Pgt.