Modulation of multiple BAK1- dependent signaling pathways by two atypical receptor-like kinases

Abstract

Transmembrane leucine-rich repeat (LRR) receptors are commonly used innate immune receptors in plants and animals but can also sense endogenous signals to regulate development. BAK1, a plant LRR-receptor-like kinase (RLK) with a small extracellular domain, interacts with several ligand-binding LRR-RLKs to positively regulate their functions. BAK1 is involved in brassinosteroiddependent growth and development, innate immunity, and cell-death control by interacting with the brassinosteroid receptor BRI1, immune receptors, such as FLS2 and EFR, and the small receptor kinase BIR1, respectively. In vivo study of BAK1 complex partners by LC/ESI-MS/MS led to the identification of two novel BAK1-interacting RLKs, BIR2 and BIR3. Functional analyses of bir2 mutants show differential impact on BAK1- regulated processes, such as hyperresponsiveness to microbe-associated molecular patterns (MAMP), enhanced cell death, and resistance to bacterial pathogens, but have no effect on brassinosteroid-regulated growth. BIR2 interacts constitutively with BAK1, thereby preventing interaction with the ligand-binding LRRRLK FLS2. MAMP perception leads to BIR2 release from the BAK1 complex and enables the recruitment of BAK1 into the FLS2 complex. These results provide evidence for a new regulatory mechanism for innate immune receptors with BIR2 acting as a negative regulator of MAMP-triggered immunity by limiting BAK1- receptor complex formation in the absence of ligands. In a second part, functional analysis of BIR3 overexpressing lines has been performed. These lines present a strong decrease in BL signaling with a morphology resembling bri1 mutants. MAMP signaling and cell death control are also downregulated pointing to a negative regulatory role of BIR3 on BAK1.