Molecular Mapping of Stem-Rust-Resistance Gene Sr40 in Wheat

Stem rust, caused by Puccinia graminis f. sp. tritici, was historically one of the most destructive diseases of wheat (Triticum, aestivum L.) worldwide. Deployment of resistant cultivars successfully prevented rust epidemics over the past several decades. Unfortunately, race TTKS (termed Ug99) has emerged in Africa to render several stem-rust-resistance genes ineffective. Sr40, a stem-rust-resistance gene from Triticum timopheevii ssp. armeniacum, was transferred to wheat on translocation chromosome T2BL/2G#2S and provides effective levels of seedling and adult plant resistance against Ug99. Two mapping populations were developed using Ug99-resistant line RL6088 and moderately susceptible to susceptible hard winter wheat cultivars Lakin and 2174. The parents were screened with 83 simple sequence repeats (SSR) from chromosome 2B and the polymorphic markers were analyzed on F(2) populations. F(2) and F(2:3) populations were inoculated with North American stem rust race RKQQ at the seedling stage. Marker locus Xwmc344 was most closely linked to Sr40 (0.7 cM) in the RL6088/Lakin linkage map, followed by Xwmc474 and Xgwm374. Marker locus Xwmc474 was mapped similar to 2.5 cM proximal to Sr40 in the RL6088/2174 population. Xwmc474 and Xwmc661 flanked Sr40 in both populations. Markers linked to Sr40 will be useful for marker-assisted integration and pyramiding of Sr40 into elite wheat breeding lines, and reduction in the size of the T timopheevii segment harboring this gene.