Peptide signalling in reproductive development, yield determination, and grain quality of cereal crops: From molecular mechanisms to breeding applications

Yield determination in cereal crops is a sequential developmental programme spanning inflorescence meristem activity, floret survival and fertilisation, grain filling, and grain compositional quality. The classical framework for this programme centres on carbon assimilation, source-to-sink ratios, and canonical phytohormones, but does not explain how whole-plant nitrogen status is communicated to reproductive sinks during grain filling, nor how meristem size, floret fate, and nitrogen allocation are coordinated. This review addresses that gap by synthesising evidence for small, secreted peptide signalling across cereal reproductive development, taking wheat as the primary system and drawing on rice and barley evidence. We propose that these peptide systems form an overlapping regulatory network rather than acting independently. The CLV-WUS feedback loop restricts meristem size and sets the upper limit of grain number. RALF peptides enable pollen tube integrity and fertilisation success. Phytosulfokine promotes grain filling through PSKR1 signalling. CEP and its receptor CEPR1 coordinate nitrogen delivery to reproductive sinks, with cereal CEPR1 knockouts showing severe reductions in grain number and weight. IDA signalling regulates floret abscission and grain retention. Nitrogen-related pathways receive emphasis, including NAC-controlled remobilisation via NAM-B1 and CEP-CEPR1-mediated nitrogen demand signalling, given their relevance to the yield-quality trade-off. Abscisic acid integrates nitrogen status with storage protein deposition. Critical gaps include absent direct CEP evidence during grain filling, the uncharacterised CLV3-equivalent peptide at the wheat meristem, and the lack of field-scale biostimulant data. Addressing these gaps will be essential for translating peptide signalling biology into breeding strategies that improve grain number, weight, and protein content.