A Unique Resistance Mechanism is Associated with RBgh2 Barley Powdery Mildew Adult Plant Resistance

Gene expression at the RBgh2 locus indicates involvement in cAMP/G-protein-coupled signalling and innate immunity in barley powdery mildew adult plant resistance. Barley powdery mildew is a globally significant disease, responsible for reduced grain yield and quality. A major effect adult plant resistance gene, RBgh2, was previously found in a landrace from Azerbaijan. The atypical phenotype suggested different underlying genetic factors compared to conventional resistance genes and to investigate this, genome-wide gene expression was compared between sets of heterogeneous doubled haploids. RBgh2 resistance is recessive and induces both temporary genome-wide gene expression changes during powdery mildew infection together with constitutive changes, principally at the RBgh2 locus. Defence-related genes significantly induced included homologues of genes associated with innate immunity and pathogen recognition. Intriguingly, RBgh2 resistance does not appear to be dependent on salicylic acid signalling, a key pathway in plant resistance to biotrophs. Constitutive co-expression of resistance gene homologues was evident at the 7HS RBgh2 locus, while no expression was evident for a 6-transmembrane gene, predicted in silico to contain both G-protein- and calmodulin-binding domains. The gene was disrupted at the 5' end, and G-protein-binding activity was suppressed. RBgh2 appears to operate through a unique mechanism that co-opts elements of innate immunity.