Genetic Characterization of Virulence in a Pacific Northwest Stem Rust Population and Mapping of New Sources of Resistance in Barley

Arjun Upadhaya

doi:10.7273/000005216

Stem rust caused by the biotrophic fungal pathogen Puccinia graminis f. sp. tritici (Pgt) is an important disease of barley and wheat globally. In the last decade, the Pacific Northwest (PNW) region of the United States has experienced an increase in the incidence and severity of stem rust disease. This may be attributed to various factors, including increase in summer temperatures, lack of effective resistance, and early inoculum from sexual hosts. The PNW region’s conducive environment and the presence of the sexual hosts, Mahonia and barberry, present an opportunity for the stem rust pathogen to complete its sexual cycle, leading to the emergence of genotypes with novel virulence gene combinations. A total of 100 Pgt isolates collected from barley in eastern Washington during the 2019 growing season were tested for virulence on the two major barley stem rust resistance genes/loci, Rpg1 and the rpg4/5-mediated resistance locus (RMRL) at the seedling stage. Of these isolates, 99% were virulent on cultivar Morex containing the Rpg1 gene, 62% were virulent on Golden Promise transgenic line (H228.2c) carrying a single copy of the Rpg1 gene from Morex, 16% were virulent on the near isogenic line, HQ-1 containing RMRL, and 10% were virulent on barley line Q21861 carrying both Rpg1 and RMRL. This was the first report of Pgt isolates containing virulence on both Rpg1 and RMRL when stacked together, representing the most virulent Pgt population reported on barley worldwide. To identify and map effective resistances against the local virulent isolates, a genome wide association study (GWAS) was conducted on 440 accessions from the World Barley Core Collection (WBCC), genotyped with the 9K Illumina barley iSelect chip and phenotyped with two PNW Pgt isolates at the seedling stage. A total of 10 resistance loci were identified including four novel loci on chromosomes 1H, 2H, 5H, and 6H. On the pathogen side, GWAS was performed using 113 diverse Pgt isolates, collected from the PNW and the Midwest region of the US, to identify virulence and avirulence loci corresponding to the barley stem rust resistance gene Rpg1 and the six wheat stem rust R-genes, Sr5, Sr21, Sr8a, Sr17, Sr9a, and Sr9d. The genotype data for 96 isolates were generated by whole genome shotgun sequencing and for 17 isolates by RNA sequencing. The phenotype data for the Pgt isolates were collected by seedling disease assays on two barley lines, H228.2c and Morex (Rpg1+) and the six wheat stem rust differential lines, Sr5, Sr21, Sr8a, Sr17, Sr9a, and Sr9d. A total of five effector gene candidates were identified at two loci corresponding to the Rpg1 gene. Similarly, 24 putative effector genes were identified at 15 loci corresponding to the six wheat R-genes. Further characterization of these genes will be done to enhance our understanding of host pathogen interactions in both the barley-Pgt and wheat-Pgt pathosystems.

Genetic Characterization of Virulence in a Pacific Northwest Stem Rust Population and Mapping of New Sources of Resistance in Barley

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