Wheat stem rust caused by the fungal pathogen Puccinia graminis f. sp. tritici (Pgt) is a devastating disease that impacts global wheat production. To combat Pgt, the cultivar ‘Hope’ was developed in the 1920s. Hope possesses an adult plant resistance (APR) gene Sr2, which provides durable and non-race specific resistance to Pgt, including the highly virulent strain, Ug99. Sr2 also confers resistance to wheat leaf rust and powdery mildew. Furthermore, Sr2-carrying wheat genotypes show a pseudo-black chaff phenotype involving cell necrosis in stems and glumes and anoxia-induced necrosis on leaf sections treated with Vaseline® petroleum jelly.
We found that Sr2 resistant lines contain a ~90 kb multi-copy tandem repeat at the Sr2 locus, while susceptible lines contain a single copy. Two genes within this repeat show high expression in resistant lines and induce necrosis when co-expressed in Nicotiana benthamiana but not when expressed individually, suggesting they act co-operatively. Homologues of these paired genes exist in other cereals, with the pair from Sorghum (BTx623) also inducing necrosis when co-expressed in N. benthamiana. Preliminary analysis of transgenic ‘Chinese Spring’ lines containing both genes indicates that they confer resistance to Pgt in an expression-dependent manner and also display anoxia-induced leaf necrosis. Intriguingly, these Sr2 genes represent a new APR gene class, encoding predicted multipass transmembrane proteins that are unrelated to other characterised APR transmembrane resistance proteins, Lr34 (ABC transporter) and Lr67 (hexose transporter). Confocal fluorescence microscopy of YFP-fused Sr2 proteins suggested localisation to the endomembrane system in N. benthamiana.