Molecular and genetic characterization of barley mutants and genetic mapping of mutant rpr2 required for Rpg1-mediated resistance against stem rust

Upinder Gill, Robert Brueggeman, Jayaveeramuthu Nirmala, Yuan Chai, Brian Steffenson, Andris Kleinhofs

This study describes the generation, screening, genetic and molecular characterization, and high-resolution mapping of barley mutants susceptible to stem rust ( Puccinia graminis f. sp. tritici ) races MCCF and HKHJ. A single gene, Rpg1, has protected barley cultivars against many races of stem rust pathogen (Puccinia graminis f. sp. tritici) for the last 70 years in the United States and Canada. To identify signaling components of protein product RPG1, we employed a mutagenesis approach. Using this approach, six mutants exhibiting susceptibility to Puccinia graminis f. sp. tritici races MCCF and HKHJ were identified in the gamma irradiated M2 population of resistant cultivar Morex, which carries Rpg1 on chromosome 7H. The mutants retained a functional Rpg1 gene and an apparently functional protein, suggesting that the mutated genes were required for downstream or upstream signaling. Selected mutants were non-allelic, hence each mutant represents a unique gene. Low and high-resolution genetic mapping of the rpr2 mutant identified chromosome 6H (bin 6) as the location of the mutated gene. The target region was reduced to 0.6 cM and gene content analyzed. Based on the published barley genomic sequence, the target region contains approximately 157 genes, including a set that encodes putative leucine-rich receptor-like protein kinases, which may be strong candidates for the gene of interest. Overall, this study presents a strong platform for future map-based cloning of genes identified in this mutant screen.