BAK1 protects the receptor-like kinase BIR2 from degradation to promote pattern-triggered immunity in Nicotiana benthamiana.

The detection of microbial infections by plants induces the rapid formation of immune receptor complexes at the plasma membrane. However, how this process is controlled to ensure proper immune signaling remains largely unknown. Here, we found that the Nicotiana benthamiana membrane-localized leucine-rich repeat receptor-like kinase BAK1-INTERACTING RLK 2 (NbBIR2) constitutively associates with BRI1-ASSOCIATED RECEPTOR KINASE 1 (NbBAK1) in vivo and in vitro and promotes complex formation with pattern recognition receptors. In addition, NbBIR2 is targeted by two RING-type ubiquitin E3 ligases, SNC1-INFLUENCING PLANT E3 LIGASE REVERSE 2a (NbSNIPER2a) and NbSNIPER2b, for ubiquitination and subsequent degradation in planta. NbSNIPER2a and NbSNIPER2b interact with NbBIR2 in vivo and in vitro and are released from NbBIR2 upon treatment with different microbial patterns. Furthermore, accumulation of NbBIR2 in response to microbial patterns is tightly associated with NbBAK1 abundance in N. benthamiana. NbBAK1 acts as a modular protein that stabilizes NbBIR2 by competing with NbSNIPER2a or NbSNIPER2b for association with NbBIR2. Similar to NbBAK1, NbBIR2 positively regulates pattern-triggered immunity and resistance to bacterial and oomycete pathogens in N. benthamiana, whereas NbSNIPER2a and NbSNIPER2b have the opposite effect. Together, these results reveal a feedback regulatory mechanism employed by plants to tailor pattern-triggered immune signaling.