Ligand-induced BIK1 monoubiquitination regulates plant immunity

Recognition of microbe-associated molecular patterns (MAMPs) via plasma membrane (PM)-resident pattern recognition receptors (PRRs) triggers the first line of inducible defense against invading pathogens(1–3). Receptor-like cytoplasmic kinases (RLCKs) are convergent regulators that associate with multiple PRRs in plants(4). The mechanisms underlying PM-tethered RLCK activation remain elusive. We report here that, upon MAMP perception, RLCK BOTRYTIS-INDUCED KINASE 1 (BIK1) is monoubiquitinated following phosphorylation, then released from the flagellin receptor FLAGELLIN SENSING 2 (FLS2)-BRASSINOSTEROID INSENSITIVE 1-ASSOCIATED KINASE 1 (BAK1) complex, and internalized dynamically into endocytic compartments. Arabidopsis E3 ubiquitin ligases RING-H2 FINGER A3A (RHA3A) and RHA3B mediate the monoubiquitination of BIK1, which is essential for the subsequent BIK1 release from the FLS2-BAK1 complex and immune signaling activation. Ligand-induced monoubiquitination and endosomal puncta of BIK1 exhibit spatial and temporal dynamics distinct from those of PRR FLS2. Our study reveals the intertwined regulation of PRR-RLCK complex activation by protein phosphorylation and ubiquitination, and elucidates that ligand-induced monoubiquitination contributes to the release of BIK1 family RLCKs from the PRR complex and activation of PRR signaling.