Key messageGene expression at theRBgh2locus indicates involvement in cAMP/G-protein-coupled signalling and innate immunity in barley powdery mildew adult plant resistance.AbstractBarley 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.RBgh2resistance is recessive and induces both temporary genome-wide gene expression changes during powdery mildew infection together with constitutive changes, principally at theRBgh2locus. Defence-related genes significantly induced included homologues of genes associated with innate immunity and pathogen recognition. Intriguingly,RBgh2resistance 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 7HSRBgh2locus, while no expression was evident for a 6-transmembrane gene, predictedin silicoto contain both G-protein- and calmodulin-binding domains. The gene was disrupted at the 5′ end, and G-protein-binding activity was suppressed.RBgh2appears to operate through a unique mechanism that co-opts elements of innate immunity.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00122-023-04392-0.