Unraveling the Mechanisms of Stem Rust Resistance Conferred by the Sr35 Gene against Puccinia graminis f. sp. tritici (Pgt)

The identification of genetic factors in wheat and its pathogens that define the outcome of infection is critical for developing varieties with long lasting resistance. We have used the recently cloned Sr35 gene as entry point to unravel the molecular bases of resistance conferred by this gene to wheat stem rust caused by Pgt. We have performed the whole genome comparative analyses of multiple Sr35-virulent and –avirulent Pgt strains including natural field isolates and chemically mutagenized strains that acquired virulence to the Sr35 gene. Except for a Sr35-carrying wheat line, all mutagenized strains showed similar virulence response on the panel of lines carrying different stem rust resistance genes. This result suggested that the Sr35 avirulence phenotype in the RKQQC race is likely controlled by a single gene. We have used a forward genetic screen to identify an avirulence factor that is detected by Sr35. Whole genome sequencing of fifteen Sr35-virulent Pgt mutants detected strong effect mutations in a single candidate gene. This effector candidate gene is constitutively expressed in the infected wheat leaves and encodes a protein with a predicted N-terminal secretion signal. In planta experiments showed that the candidate gene can trigger a Sr35-dependent hypersensitive reaction. The candidate effector’s sequence diversity explained most of the virulence of Pgt isolates on the Sr35+ and Sr35- wheat lines. Based on our results we conclude that the identified fungal candidate effector is responsible for triggering the defense response by the Sr35 gene.