Allelic variation of the AvrSr50 effector increased virulence of the Puccinia graminis f. sp. tritici on Sr50-carrying wheat lines

Wheat stem rust, caused by the biotrophic pathogen Puccinia graminis f. sp. tritici (Pgt), is an important disease worldwide. To overcome host immunity, Pgt secretes several effectors, including AvrSr35 and AvrSr50. These effectors are recognized by resistant cultivars carrying corresponding receptors, resulting in incompatible reactions. In this study, we assessed the resistance of Sr35- and Sr50-carrying wheat monogenic lines to 30 Pgt isolates through inoculation with urediospores of pure isolates. Virulence analyses were evaluated for resistant IF 0-2 and susceptible 3-4 reactions and revealed that Pgt20-6 and Pgt20-12 isolates showed increased virulence on the Sr50-carrying line. The presence of AvrSr35 and AvrSr50 genes was assessed in the genome of all isolates through PCR. Visualization of amplified fragments on agarose gel confirmed the presence of AvrSr35 and AvrSr50 genes in the genome of all 30 isolates. Despite the presence of Avr genes in all isolates, the lack of active recognition for two isolates on the Sr50-carrying line suggested point mutations in their sequence. Sequencing analysis of AvrSr50 confirmed an A178T mutation unique to these two virulent isolates. Homology modeling based on the solved crystal structure of AvrSr50 showed a nonsynonymous mutation at the protein surface that may be involved in the interaction with Sr50 to escape from recognition. This study showed the constant genetic change of fungus and recalled the necessity of continuously monitoring the fungal population to take appropriate strategies in wheat breeding programs.