CAR modulates plasma membrane nano-organization and immune signaling downstream of RALF1-FERONIA ligand-receptor signaling system

The highly selective segregation of molecules at the plasma membrane (PM) to form domains is a widespread phenomenon. But how distinct signaling platforms are established and maintained by specific signals remains unclear. Here, we show that the receptor of Rapid Alkalinization Factor 1 (RALF1), the FERONIA (FER) receptor kinase, physically interacts with C2 domain ABA-related (CAR) proteins to control the nano-organization of the PM and to regulate extracellular signal transduction in Arabidopsis. During this process, the RALF1-FER pathway upregulates CAR protein synthesis, which then controls the membrane lipid order. This might act as a rapid feedforward loop to stabilize FER in PM nanodomains. FER then interacts with and phosphorylates CARs, reducing their lipid-binding ability, which might break the feedback regulation and downregulate CAR activity at latter time point. Similar to fer mutant, a pentuple car14569 mutant inhibits flg22-induced FLS2-BAK1 complex formation, which ultimately impacts plant immunity. Together, we proposed that the FER-CAR module controls the dynamics of the PM nano-organization during RALF signaling through a self-contained amplifying loop including both positive and negative feedbacks.