Small Peptides, Smart Plants: The Emerging Significance of SCOOP Signalling

Small secreted peptides have emerged as key regulators of plant growth, development, and stress adaptation. The Serine-rich endogenous peptide (SCOOP) family represents a unique class of phytocytokines that integrates immune signalling with developmental regulation via the receptor-like kinase MIK2. SCOOP peptides illustrate a significant evolutionary advancement in plants, with lineage-specific signals diversifying to strengthen the relationship between growth and immunity, while their microbial analogues SCOOP-like (SCOOPLs) peptides underscore a cross-kingdom signalling interaction perceived by the flexible MIK2 receptor. This review synthesises current knowledge on the SCOOP-MIK2 signalling module, covering PROSCOOP precursor processing by subtilases, ligand-receptor interactions, and downstream pathways including reactive oxygen species (ROS), MAP kinase cascades, and phytohormone networks. Recent findings reveal that different SCOOP peptides may generate different responses through the same receptor, as shown by SCOOP10 and SCOOP12 in leaf senescence, while SCOOP12 additionally regulates root growth via the MIK2-MPK6-PLETHORA-PIN pathway. These findings underscore the SCOOP-MIK2 axis as a central integrator of growth, defence, and environmental response. Understanding the molecular foundation of SCOOP proteolytic activation, ligand specificity, and interspecies functionality may facilitate the strategic tuning of this pathway, opening novel opportunities for developing stress-resilient plants in response to climate change.