Peptide Signalling as a Control Layer for Signal Prioritisation in Plant Stress Networks

Plants in the field rarely encounter single stresses, and simultaneous stresses impose conflicting physiological demands that cannot be satisfied at once. Responses to such combinations are frequently non-additive and cannot be predicted from single-stress responses, a result that crosstalk models struggle to accommodate because they describe which pathways interact without specifying which prevails. This review proposes that peptide signalling functions as a control layer governing signal prioritisation. Under this hypothesis, secreted peptides act above the shared modules of cytosolic calcium, reactive oxygen species, and mitogen-activated protein kinase cascades, and determine which competing input is permitted to dominate their behaviour. Evidence for this proposal spans several peptide families. CEP, CLE and RALF signalling illustrate systemic demand signalling, developmental stabilisation, and receptor complex reorganisation respectively, and the same logic extends to PEP, IDA and PSK signalling across immune, developmental, and drought contexts. Combined drought and nitrogen limitation, resolved through ABA and CEP, provides a worked case in which a global signal sets operating boundaries within which a local signal refines the response, supported by evidence now extending from Arabidopsis into cereals. We derive falsifiable predictions tested against hormone-pathway negative controls and graded drought intensities rather than single treatments, which allows loss of prioritisation to be distinguished from general signalling fragility. We also identify a trade-off constraining crop application, since enhancing peptide-mediated nutrient foraging may reduce drought resilience. Substantial gaps remain in detecting peptide signals directly and in ranking peptide families under field conditions. Reframing plant signalling as a decision system rather than an input-output relay offers a testable foundation for improving stress resilience and nutrient use efficiency in crop breeding.