Understanding the RALF family: a tale of many species

• We concisely collate research on the RALF family of peptides within many plant species.
• Numerous physiological and developmental roles are revealed.
• Differential roles and varied expression patterns highlight the importance of RALFs.
• The first receptor protein for RALF has recently been identified.

Small secreted peptides are gaining importance as signalling molecules in plants. Among the 1000 open reading frames (ORFs) in the Arabidopsis (Arabidopsis thaliana) genome potentially encoding small secreted peptides, the members of the RAPID ALKALINIZATION FACTOR (RALF) family of peptides have been linked to several physiological and developmental processes. Here, we provide a comprehensive overview of current knowledge on the RALF family. Discovered in tobacco (Nicotiana tabacum), the role of RALF peptides has been investigated in numerous plant species. Together, these observations suggest that RALF peptides impact on acidification and cell expansion during growth and development. Although few components of the signalling pathway have been revealed, the recent identification of FERONIA (FER) as a RALF receptor and plasma membrane H+-ATPase 2 as a downstream target provide a major step forward.