RALF-FERONIA Signaling: Linking Plant Immune Response with Cell Growth.

Plants perceive various external and internal signals to self-modulate biological processes through members of the receptor-like kinase (RLK) family, among which  receptor-like kinase 1-like (RLK1L) proteins with their ligands, rapid alkalinization factor (RALF) peptides, have attracted considerable interest. FERONIA (FER), a RLK1L member, was initially reported to act as a major plant cell growth modulator in distinct tissues. Subsequently, the RALF-FER pathway was confirmed to function as an essential regulator of plant stress responses, including but not limited to immune responses. Furthermore, the RALF-FER pathway modulates immune responses and cell growth in a context-specific manner, and the vital roles of this pathway are beginning to be appreciated in crop species. The recent remarkable advances in understanding the functions and molecular mechanisms of the RALF-FER pathway have also raised many interesting questions that need to be answered in the future. This review mainly focuses on the roles of FER and other RLK1L members in modulating immune responses in the context of cell growth in response to their RALF peptide ligands and presents a brief outlook for future research.