Mechanistic and genetic overlap of barley host and non-host resistance to Blumeria graminis.

SUMMARY Non-host resistance of barley to Blumeria graminis f.sp. tritici (Bgt), an inappropriate forma specialis of the grass powdery mildew fungus, is associated with formation of cell wall appositions (papillae) at sites of attempted fungal penetration and a hypersensitive cell death reaction (HR) of single attacked cells. Penetration resistance and HR are also typical features of race-non-specific and race-specific resistance of barley to the appropriate Blumeria graminis f.sp. hordei (Bgh), raising the question of whether genotypic differences in the cellular response of barley to Bgt are detectable. First, we analysed fungal penetration frequencies and HR in different barley accessions known to show altered non-host resistance. In genotypes with limited resistance to inappropriate cereal rust fungi, we concomitantly detected low penetration resistance to Bgt and significant differences of HR rates during attack from Bgt. Second, we tested barley mutants known to show altered host responses to Bgh. The rar1-mutation that suppresses many types of race-cultivar-specific resistances did not influence the non-host response of the Bgt-isolate used in this study. However, mutants of Ror1 and Ror2, two genes required for full race non-specific penetration resistance of mlo-barley to barley powdery mildew fungus, exhibited altered defence response to Bgt, including higher frequencies of fungal penetration. On these mutants, growth of the inappropriate fungus was arrested subsequent to penetration by HR. Together, the data show that barley defence response to the wheat powdery mildew fungus is determined by similar factors as race-specific and race-non-specific resistance to appropriate Bgh.