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

Race-specific resistance genes protect the global wheat crop from stem rust disease caused by <i>Puccinia graminis</i> f. sp. <i>tritici</i> (<i>Pgt</i>) but are often overcome owing to evolution of new virulent races of the pathogen. To understand virulence evolution in <i>Pgt</i>, we identified the protein ligand (AvrSr50) recognized by the Sr50 resistance protein. A spontaneous mutant of <i>Pgt</i> virulent to <i>Sr50</i> contained a 2.5 mega-base pair loss-of-heterozygosity event. A haustorial secreted protein from this region triggers <i>Sr50</i>-dependent defense responses in planta and interacts directly with the Sr50 protein. Virulence alleles of <i>AvrSr50</i> have arisen through 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 <i>Pgt</i>.