The Potential Role of Powdery Mildew-Resistance Gene Pm40 in Chinese Wheat-Breeding Programs in the Post-Pm21 Era

Powdery mildew, which is caused by Blumeria graminis f. sp. tritici (Bgt), is an important leaf disease that affects wheat yield. Powdery mildew-resistance (Pm) gene Pm21 was first transferred into wheat in the 1980s, by translocating the Heuchera villosa chromosome arm 6VS to the wheat chromosome arm 6AL (6VS·6AL). Recently, new Bgt isolates that are virulent to Pm21 have been identified in some wheat fields, indicating that wheat breeders should be aware of the risk of deploying Pm21, although pathological details regarding these virulent isolates still remain to be discovered. Pm40 was identified and mapped on the wheat chromosome arm 7BS from several wheat lines developed from the progenies of a wild cross between wheat and Thinopyrum intermedium. Pm40 offers a broad spectrum of resistance to Bgt, which suggests that it is likely to provide potentially durable resistance. Cytological methods did not detect any large alien chromosomal segment in the wheat lines carrying Pm40. Lines with Pm40 and promising agronomical traits have been released by several wheat-breeding programs in the past several years. Therefore, we believe that Pm40 will play a role in powdery mildew-resistance wheat breeding after Pm21 resistance is overcome by Bgt isolates. In addition, both Pm21 and Pm40 were derived from alien species, suggesting that the resistance genes derived from alien species are potentially more durable or effective than those identified from wheat.