Resistance Mechanism of Yr36 to Wheat Stripe Rust

Puccinia striiformis f. sp. Tritici (PST) is a devastating disease of wheat. To understand the molecular basis of PST pathogenicity we re-sequenced four PST genomes from US and UK isolates and used experimental and bioinformatics approaches to prioritize a list of 2,999 secreted proteins. We used RNAseq analysis to identify secreted proteins enriched in haustoria compared to infected tissue and defined a list of 57 genes encoding secreted proteins that are enriched in haustoria. From the wheat side we characterized the resistance mechanisms of the partial resistance gene WKS1 (=Yr36). Using transgenic wheat plants we demonstrate that the alternative splice form WKS1.1 confers PST resistance whereas the WKS1.2 form does not. We show here that WKS1.1 from homodimers and is transported into chloroplast whereas WKS1.2 is not. Once inside the chloroplast the WKS1.1 protein interacts with the thylakoidal ascorbate peroxidase (tAPX1). WKS1.1 phosphorylates tAPX1 both in vitro and in planta and reduces its enzymatic activity up to 40%. We suggest that this results in a reduction of the ability of the cell to detoxify reactive oxygen species and the observed slow programmed cell death response associated with WKS1 resistance. This conclusion was further supported by early senescence in leaf tips in hexaploid wheat plants combining WKS1.1 over-expression with a deletion of one copy of APX1 (6B), which is not observed when the APX 6B deletion or the WKS1.1 is present separately.