Yr36 Confers Partial Resistance to Wheat Stripe Rust By a Novel Mechanism

Wheat provides roughly 20% of the energy and protein consumed by the human population and increased production is necessary to accommodate population growth. The reduction of yield losses due to diseases can contribute to these increases. We are currently focusing our efforts on wheat stripe rust, because new virulent races of Puccinia striiformis f. sp. tritici (Pst) that appear approximately in the year 2000 have been causing large yield losses worldwide. Five years ago we isolated a partial adult plant rust resistance gene, Yr36 (= WKS1), with a novel architecture including a functional kinase and a lipid binding START domain. Susceptible wheat transformed with the alternative splicing form WKS1.1 (full length protein) are resistant to Pst but those transformed with WKS1.2 (truncated START domain) are susceptible. WKS1.1 was shown to enter the chloroplast and phosphorylate the thylakoidal ascorbate peroxidase (APX1). Overexpression of WKS1 increased APX1 phosphorylation levels in planta. Phosphorylation of APX1 by WKS1.1 reduced its activity up to 40%. Overexpression of WKS1.1 in the presence of a deletion of the B-genome copy of APX1 results in early leaf senescence, which is not observed when the APX1 mutant or the WKS1.1 transgene are present separately separated. This data supports a model in which WKS1.1 protein reduces activity of APX1 resulting in the accumulation of reactive oxygen products that eventually trigger cell death. This process is slower than the rapid hypersensitive response associated with R genes, resulting in partial resistance.