A "one-marker-for-two-genes" approach for efficient molecular discrimination of Pm12 and Pm21 conferring resistance to powdery mildew in wheat

Powdery mildew, caused by Blumeria graminis f. sp. tritici, is one of the most important wheat diseases worldwide. Pyramiding different resistance genes into single cultivar has been proposed as one remedy to provide durable resistance. Powdery mildew resistance genes Pm12 (T6BS-6SS.6SL), transferred from Aegilops speltoides to wheat cv. Wembley, and Pm21 (T6VS.6AL), introduced from Dasypyrum villosum to wheat cv. Yangmai5, conferred broad-spectrum resistance to B. graminis f. sp. tritici. Both Pm12 and Pm21 genes are located on the short arms of homologous group six involved translocated chromosomes 6SS.6BL and 6VS.6AL, respectively. Simple sequence repeat motifs of wheat simple sequence repeat (SSR) and expressed sequence tag (EST) sequences on the short arm of homologous group six chromosomes were analyzed to develop molecular markers for discriminating chromosome arms 6AS, 6BS, 6DS, 6VS, and 6SS. One EST-SSR marker, Xcau127, was polymorphic, and therefore can be used to distinguish the two resistance genes and the respective susceptible alleles. This marker allowed us to develop an efficient "one-marker-for-two-genes" procedure for identifying powdery mildew resistance genes Pm12 and Pm21 for marker-assisted selection and gene pyramiding in wheat breeding programs.