Rapid Alkalinization Factor - A cryptide regulating developmental and stress responses

Post-translational modification is a key component that aids in regulated signaling cascades and complex cross talks in the plant cell to eventually facilitate efficient plant growth and development during the transition phases and in response to environment hindrances. Proteolytic processing, being one of the irreversible post-translational modifications, resulting in generation of mature peptide hormones from large precursor proteins, enhance the rapid responses, cell - cell communication including regulation of hormone signaling, basic cellular functions and defense activities. Often most of the proteolytic products are bio-active in nature and such peptides comprises the cryptome of the plant cell. The probability of generation of cryptides would be more via the unfolded or unstructured regions known as Intrinsic disordered regions which has higher sensitivity towards the proteases when compared to the structured regions. Similar to CLE, systemin, PSK (phytosulfokine) and RGF (Root growth factor), RALF, a small secreted cryptic peptide regulates the developmental and stress responses upon proteolytic cleavage. Apart from the identification and in-silico analysis of RALF peptides in different plant species, the role of these peptides in molecular signaling remains largely unknown. This review focuses on the perspective of RALF as a cryptic peptide explains the known roles of the peptide in developmental and stress responses. Further the roles of RALF in terms of molecular signaling are speculated to provide insights into complex growth, development and stress responses.