Allelic barley MLA immune receptors recognize sequence-unrelated avirulence effectors of the powdery mildew pathogen

Significance Gene-for-gene immunity is frequently found in interactions between plants and host-adapted pathogens and reflects population-level diversification of immune receptors detecting matching pathogen effectors. We identified effector genes of a pathogenic powdery mildew fungus that are recognized by allelic variants of barley intracellular nucleotide-binding domain and leucine-rich repeat protein-type receptors. These pathogen effectors are phylogenetically unrelated, demonstrating that allelic immune receptors can evolve to recognize sequence-unrelated proteins. Conserved effector recognition in distantly related Arabidopsis indicates that the underlying mechanism is not restricted to monocotyledonous plants. Furthermore, our study reveals that the expression of a fungal avirulence effector alone is necessary and sufficient for allele-specific mildew resistance locus A receptor activation in planta .