Involvement of a receptor-like kinase complex of FvFLS2 and FvBAK1 in brassinosteroids-induced immunity in postharvest strawberry fruit

Abstract

Conserved pathogen-derived features of microbial invaders are sensed by plants via pattern-recognition receptors (PRRs) and induce both local and systemic immune responses. The plant PRR flagellin sensitive 2 (FLS2), a leucine-rich repeat-receptor kinase, recognizes pathogen-derived molecules and initiates pattern-triggered immunity (PTI) responses by associating with other potential kinases to propagate downstream signalling cascades. Here, we identified a plasma membrane-localized receptor-like kinase BAK1 from strawberry (FvBAK1) that is rapidly induced upon exposure to the plant-specific hormone brassinosteroid (BR), which provides strawberry fruit with immunity to the necrotrophic fungus Botrytis cinerea. FvBAK1 reciprocally interacts with FvFLS2 and the receptor-like cytoplasmic kinase FvBIK1 in yeast cells in vivo. Subsequently, FvBIK1 can directly interact with respiratory burst oxidase homologue D (FvRBOHD), contributing to FvRBOHD-dependent ROS accumulation to amplify the SA signalling pathway in the BR-stimulated immunity of postharvest strawberry fruit to B. cinerea infection. Importantly, overexpression of bak1 promotes ROS accumulation and BR-induced defence responses, whereas bak1 loss-of-function mutants have compromised BR-induced SA signalling and immunity to necrotrophic fungal pathogens. In such a situation, FvBAK1 is an essential component of BR-mediated PTI signalling, which links the pathogen-derived receptor complex to downstream intracellular signalling pathways. Taken together, the findings of the current study highlight the importance of FvBAK1 as an upstream regulator of FvBIK1 and as a coreceptor of FvFLS2 in establishing BR-induced PTI signalling in postharvest strawberry fruit.