A Post-Haustorial Defense Mechanism is Mediated by the Powdery Mildew Resistance Gene,PmG3M, Derived from Wild Emmer Wheat

The destructive wheat powdery mildew disease is caused by the fungal pathogenBlumeria graminisf. sp.tritici(Bgt).PmG3M, derived from wild emmer wheatTriticum dicoccoidesaccession G305-3M, is a major gene providing a wide-spectrum resistance againstBgt.PmG3Mwas previously mapped to wheat chromosome 6B using an F(6)recombinant inbred line (RIL) mapping population generated by crossing G305-3M with the susceptibleT. durumwheat cultivar Langdon (LDN). In the current study, we aimed to explore the defense mechanisms conferred byPmG3MagainstBgt.Histopathology of fungal development was characterized in artificially inoculated leaves of G305-3M, LDN, and homozygous RILs using fluorescence and light microscopy. G305-3M exhibited H(2)O(2)accumulation typical of a hypersensitive response, which resulted in programmed cell death (PCD) inBgt-penetrated epidermal cells, while LDN showed well-developed colonies without PCD. In addition, we observed a post-haustorial resistance mechanism that arrested the development of fungal feeding structures and pathogen growth in both G305-3M and resistant RIL, while LDN and a susceptible RIL displayed fully developed digitated haustoria and massive accumulation of fungal biomass. In contrast, both G305-3M and LDN exhibited callose deposition in attempt to prevent fungal invasion, supporting this as a mechanism of a basal defense response not associated withPmG3Mresistance mechanism per se. The presented results shed light on the resistance mechanisms conferred byPmG3Magainst wheat powdery mildew.