Positional cloning of PmCH1357 reveals the origin and allelic variation of the Pm2 gene for powdery mildew resistance in wheat

Powdery mildew, caused by the biotrophic fungus Blumeria graminis f. sp. tritici (Bgt), is a prevalent disease in common wheat (Triticum aestivum L.) and causes serious yield losses worldwide. We used a map-based approach to clone the major broad-spectrum powdery mildew resistance gene PmCH1357 from wheat breeding line CH1357. PmCH1357 was mapped to a 526 kb region containing only TraesCS5D01G044600. The TraesCS5D01G044600 sequence of the susceptibility allele in Taichung 29 (TC29) was identical to that in Chinese Spring, whereas the sequence of the resistance allele in CH1357 was identical to Pm2a previously cloned from the germplasm Ulka/*8Cc. The susceptibility allele in TC29 contained a 7 bp deletion in exon 1, resulting in loss of 856 of the 1277 amino acids in the predicted nucleotide-binding domain leucine-rich repeat containing Pm2a protein. PmCH1357/Pm2a sequence was also isolated from the Chinese wheat landraces and cultivars that were previously reported to possess the resistance gene Pm2b, Pm2c, PmLX66, or PmND399. The PmCH1357/Pm2a resistance allele was present in 10 of 495 accessions in core germplasm and contemporary cultivars from China and the USA. A newly developed diagnostic marker for the 7 bp InDel in the resistance gene can be used to eliminate the susceptibility allele in wheat breeding programs.