Abstract:In plants, perception of invading pathogens involves cell-surface immune receptor kinases. Here, we report that the Arabidopsis SITE-1 PROTEASE (S1P) cleaves endogenous RAPID ALKALINIZATION FACTOR (RALF) propeptides to inhibit plant immunity. This inhibition is mediated by the malectin-like receptor kinase FERONIA (FER), which otherwise facilitates the ligand-induced complex formation of the immune receptor kinases EF-TU RECEPTOR (EFR) and FLAGELLIN-SENSING 2 (FLS2) with their co-receptor BRASSINOSTEROID INSENSITIVE 1-ASSOCIATED KINASE 1 (BAK1) to initiate immune signaling. We show that FER acts as a RALF-regulated scaffold modulating receptor kinase complex assembly. A similar scaffolding mechanism may underlie RALF function in other signalling pathways.One Sentence Summary: A broadly accessible receptor perceives antagonistic endogenous peptides to regulate the formation of plant immune receptor complexes.Main Text: Plant immune pattern recognition receptors (PRRs) are often receptor kinases (1). The Arabidopsis thaliana (hereafter Arabidopsis) receptor kinases FLS2 and EFR bind bacterial flagellin (or the epitope flg22) and EF-Tu (or the epitopes elf18/elf24), respectively, and form ligand-induced complexes with their co-receptor BAK1 (1).

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Plant perception of pathogen-associated molecular patterns (PAMPs) triggers a phosphorylation relay leading to PAMP-triggered immunity (PTI). Despite increasing knowledge of PTI signaling, how immune homeostasis is maintained remains largely unknown. Here we describe a forward-genetic screen to identify loci involved in PTI and characterize the Arabidopsis calcium-dependent protein kinase CPK28 as a negative regulator of immune signaling. Genetic analyses demonstrate that CPK28 attenuates PAMP-triggered immune responses and antibacterial immunity. CPK28 interacts with and phosphorylates the plasma-membrane-associated cytoplasmic kinase BIK1, an important convergent substrate of multiple pattern recognition receptor (PRR) complexes. We find that BIK1 is rate limiting in PTI signaling and that it is continuously turned over to maintain cellular homeostasis. We further show that CPK28 contributes to BIK1 turnover. Our results suggest a negative regulatory mechanism that continually buffers immune signaling by controlling the turnover of this key signaling kinase.