The BIK1-mediated phosphorylation and SCFSKIP31-mediated ubiquitination coordinately control CNGC3 homeostasis and channel activity to fine tune plant immunity.

Cyclic nucleotide-gated ion channels (CNGCs) are a key component in the pattern-triggered immunity (PTI) signaling. Tight control of CNGC homeostasis is critical for plant growth-immunity balance. Nevertheless, the mechanisms for fine-tuning CNGC homeostasis remain largely unknown. Here, we report that Arabidopsis thaliana CNGC3 is a functional calcium channel to mediate pattern-induced Ca²⁺ influx, PTI and resistance to Sclerotinia sclerotiorum. We identified a CNGC interactor, Skp1-interacting protein 31 (SKIP31). In the absence of pathogen, SKIP31 ubiquitinates CNGC3 at Lys8 and Lys33 of the K-X-V-R motifs for degradation to repress plant immunity. When pathogen attacks, activated BOTRYTIS-INDUCED KINASE1 (BIK1) phosphorylates SKIP31 to inhibit its ubiquitin ligase activity and interaction with CNGC3 N-terminal region, thereby suppressing CNGC3 protein degradation to promote immunity. Phosphorylation within the F-box of SKIP31 at Ser88 and Ser93 and that at the C-terminal Ser261 prevent its interaction with Skp1 and CNGC3 respectively. These phosphorylation sites are conserved in SKIP31 of different plant species, and SKIP31 interacts with all examined CNGCs, suggesting the pivotal role of the SKIP31 phosphorylation for CNGC stability and plant immunity. Moreover, BIK1 directly phosphorylates CNGC3 cytoplasmic C-terminal region at four Ser residues to enhance its Ca²⁺ channel activity, demonstrating the dual roles of BIK1 in both promoting CNGC channel activity and stabilizing the channel protein. Our work unveils an SCF ubiquitin ligase-RLCK control system of CNGCs for plant immunity.