Cellular basis of resistance to different formae speciales of Blumeria graminis in Hordeum chilense, wheat, and tritordeum and agroticum amphiploids

Tritordeum and agroticum amphiploids, derived from hybridization of Triticum species with Hordeum chilense or Agropyron cristatum, were assessed for resistance to Blumeria graminis. In glasshouse tests, all parental lines of H. chilense and A. cristatum showed complete resistance to wheat powdery mildew [Blumeria graminis f. sp. tritici] while all Triticum parents, except one, were highly susceptible. The amphiploids were all resistant to B. graminis f. sp. tritici, allowing the development of only a few sporulating colonies on leaves. Microscopic examination of host-cell responses to B. graminis revealed that resistance in H. chilense and amphiploids was attributable to two major factors: (i) a significant reduction in successful fungal penetration by rapid deposition of papillae in the host epidermal cell wall and (ii) a significant increase in frequency of death of penetrated epidermal cells. Papillae formation was associated with localized autofluorescence in host epidermal cells. Cell death prevented further fungal growth and differentiation for the establishment of biotrophy. Tritordeums showed both forms of resistance when tested with Blumeria graminis f. sp. hordei and Blumeria graminis f. sp. avenae. The tritordeum and agroticum amphiploids may offer novel sources of resistance to powdery mildew. They could be used either directly or as a bridge in genetic crosses to introgress resistance to powdery mildew into elite wheat breeding lines.