Cell-autonomous expression of barley Mla1 confers race-specific resistanceto the powdery mildew fungus via a Rar1-independent signaling pathway

The barley Mla locus encodes 28 characterized resistance specificities to t he biotrophic fungal pathogen barley powdery mildew. We describe a single-c ell transient expression assay using entire cosmid DNAs to pinpoint Mla1 wi thin the complex 240-kb Mla locus. The MLA1 cDNA encodes a 108-kD protein c ontaining an N-terminal coiled-coil structure, a central nucleotide binding domain, and a C-terminal leucine-rich repeat region; it also contains a se cond short open reading frame at the 5' end that has a possible regulatory function. Although most Mla-encoded resistance specificities require Rar1 f or their function, we used the single-cell expression system to demonstrate that Mla1 triggers full resistance in the presence of the severely defecti ve rar1-2 mutant allele. Wheat contains an ortholog of barley Mla, designat ed TaMla, that is tightly linked to (0.7 centimorgan) but distinct from a t ested resistance specificity at the complex Pm3 locus to wheat powdery mild ew. Thus, the most polymorphic powdery mildew resistance loci in barley and wheat may have evolved in parallel at two closely linked homeoloci. Barley Mla1 expressed in wheat using the single-cell transformation system failed to trigger a response to any of the wheat powdery mildew Avr genes tested, indicating that AvrMla1 is not genetically fixed in wheat mildew strains.