Mechanisms of RALF peptide perception by a heterotypic receptor complex

Receptor kinases (RKs) of the Catharanthus roseus RLK1-like (CrRLK1L) family have emerged as important regulators of plant reproduction, growth and responses to the environment1. Endogenous RAPID ALKALINIZATION FACTOR (RALF) peptides2 have been proposed as ligands for several CrRLK1L members1. The mechanistic basis of this perception, however, is unknown. Here, we report that RALF23 induces a complex between the CrRLK1L FERONIA (FER) and LORELEI (LRE)-LIKE GLYCOSYLPHOSPHATIDYLINOSITOL (GPI)-ANCHORED PROTEIN 1 (LLG1) to regulate immune signalling. Structural and biochemical data indicate that LLG1, which is genetically important for RALF23 responses, or the related LLG2, directly binds RALF23 to nucleate the assembly of a RALF23–LLG1/2–FER heterocomplex. A conserved N-terminal region of RALF23 is sufficient for its biochemical recognition by LLG1/2/3, and binding assays suggest that other RALFs sharing this conserved N-terminal region may be perceived in a similar manner. Structural data also show that RALF23 recognition is governed by the conformationally flexible C-terminal sides of LLG1/2/3. Our work reveals an unexpected mechanism of plant peptide perception by GPI-anchored proteins in concert with a phylogenetically unrelated RK, which provides a molecular framework to understand how diverse RALF peptides may regulate multiple processes through perception by distinct heterocomplexes between CrRLK1L RKs and GPI-anchored proteins of the LRE/LLG family.