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.