Activation loop phosphorylation of a non-RD receptor kinase initiates plant innate immune signaling

Abstract

Receptor kinases (RKs) are fundamental for extracellular sensing and regulate development and stress responses across kingdoms. In plants, leucine-rich repeat receptor kinases (LRR-RKs) are primarily peptide receptors that regulate responses to myriad internal and external stimuli. Phosphorylation of LRR-RK cytoplasmic domains is among the earliest responses following ligand perception, and reciprocal transphosphorylation between a receptor and its coreceptor is thought to activate the receptor complex. Originally proposed based on characterization of the brassinosteroid receptor, the prevalence of complex activation via reciprocal transphosphorylation across the plant RK family has not been tested. Using the LRR-RK ELONGATION FACTOR TU RECEPTOR (EFR) as a model, we set out to understand the steps critical for activating RK complexes. While the EFR cytoplasmic domain is an active protein kinase in vitro and is phosphorylated in a ligand-dependent manner in vivo, catalytically deficient EFR variants are functional in antibacterial immunity. These results reveal a noncatalytic role for EFR in triggering immune signaling and indicate that reciprocal transphoshorylation is not a ubiquitous requirement for LRR-RK complex activation. Rather, our analysis of EFR along with a detailed survey of the literature suggests a distinction between LRR-RKs with RD- versus non-RD protein kinase domains. Based on newly identified phosphorylation sites that regulate the activation state of the EFR complex in vivo, we propose that LRR-RK complexes containing a non-RD protein kinase may be regulated by phosphorylation-dependent conformational changes of the ligand-binding receptor, which could initiate signaling either allosterically or through driving the dissociation of negative regulators of the complex.

S-acylation stabilizes ligand-induced receptor kinase complex formation during plant pattern-triggered immune signalling

“…Structural homology modelling of FLS2 indicates that the C 1132,1135 S mutation does not affect FLS2 kinase domain structure (figure S4). Kinase activity is also dispensable for activation of signalling by EFR [32]. The observed effects of the FLS2 C 1132,1135 S and EFR C 975 S mutations on early signalling therefore cannot be readily explained through deleterious effects on kinase activity or structure.…”

Conservation of land plant-specific receptor-like cytoplasmic kinase subfamily XI possessing a unique kinase insert domain

“…RD protein kinases have been shown to require autophosphorylation on the activation loop for regulation (Adams, 2003;Steichen et al, 2010). In contrast, non-RD protein kinases can be regulated through mechanisms other than autophosphorylation (Dardick et al, 2012;Bender et al, 2021). The presence of the KID and the non-RD subdomain VI in RLCK XI may indicate that these protein kinases are regulated through an alternate mechanism from phosphorylation within the activation segment.…”

Molecular evolution of the Pi-d2 gene conferring resistance to rice blast in Oryza

“…Compared to RD kinases, mechanisms for phosphorylation-mediated activation of plant non-RD complexes are largely unknown. A conformation change of non-RD kinase appears to be involved ( Bender et al, 2021 ). A U-box/ARM repeat E3 ligase OsPUB15 was confirmed to interact directly with Pi-d2 and regulate plant cell death and blast disease resistance.…”