Processing and release of endogenous immunogenic peptide signals.

In both animals and plants, small secreted peptides known as cytokines and phytocytokines mediate local and systemic communication during immune and stress responses. These signaling molecules are typically synthesized as inactive precursors that require proteolytic processing to become active. They are subsequently perceived by membrane-localized receptors that coordinate defense and developmental signaling pathways. While the maturation and function of cytokines are well understood in animals, the mechanistic understanding of phytocytokine processing and release in plants is limited. Predicting cleavage sites or identifying the responsible proteases remains challenging, and how phytocytokines move as long-distance or systemic signals is a relatively new area of exploration. In here, we discuss the parallels between cytokines and phytocytokines, including their properties as small, secreted peptides activated by proteolysis and perceived by specific receptors to modulate immunity. Finally, we address the divergence and evolution of endogenous peptide signals, emphasizing the molecular arms race between hosts and microbes. Together, these evolutionary dynamics and the functional similarities between cytokines and phytocytokines reveal striking examples of convergence and highlight important gaps in our understanding of how plants process, release, transport, and perceive these immune peptides.