Characterization of Stem Rust Resistance in US Wheat Germplasm

Abstract

In 1999 in Uganda a race of stem rust, Puccinia gramins f. sp. tritici was identified with virulence to Sr31. This race, designated as TTKS based on the North American nomenclature system, combined Sr31 virulence with virulence to the majority of Triticum aestivum L. derived stem rust resistance genes. The development of resistant cultivars is needed as TTKS may reach global dispersal due to its unique virulence to multiple known and unknown resistance genes and widespread cultivar susceptibility. The ability to detect the presence of specific stem rust resistance genes using molecular markers presents a viable method for identifying resistance to race TTKS in the absence of the pathogen itself. The frequency of DNA markers associated with resistance genes Sr24, Sr26, Sr36, and Sr1RSAmigo which confer resistance to TTKS was assessed in diverse wheat cultivars and breeding lines from breeding programs throughout the United States. The reliability of these markers in predicting the presence of the resistance genes in diverse germplasm was evaluated through comparison with phenotypic data. Introgression of undeployed seedling resistance genes is necessary to improve the availability of resistance to TTKS. The stem rust resistance gene Sr22 confers resistance to TTKS. Sr22 is present on a chromosomal translocation derived from Triticum boeoticum Boiss. which is homoeologous to the A genome of T. aesitivum Linkage analysis of SSR loci on 7AL was done to identify the loci most closely linked to Sr22. Individuals with reduced T. boeoticum segments due to recombination between wheat chromosome 7AL and the Sr22 introgression were identified with SSR markers in F2:3 populations of crosses between the germplasm stock Sr22Tb and the hard winter wheat lines 2174 and Lakin. From analysis of F3:4 populations derived from F2 recombinants, F3:4 individuals with further reduced translocation segments have been identified. Recombinant lines with reduced translocations will provide a more agronomically desirable source of Sr22 stem rust resistance in hard winter wheat germplasm that can be readily deployed utilizing molecular markers. The identification of molecular markers efficacious for the selection of genes for resistance to TTKS will hasten the development of resistant cultivars.