Hydrogen Peroxide in Cereal Crops Under Abiotic Stress: Mechanistic Modules, Evidence Tiers, and Translational Opportunities.

Hydrogen peroxide (H2O2) is increasingly recognized as a spatially and temporally encoded redox signal, and as a trigger of oxidative stress rather than a by-product of it. In cereal crops, however, the mechanisms that convert H2O2 production into selective physiological outputs remain unevenly resolved. This review examines how H2O2 specificity arises from four interconnected layers: HPCA1-type extracellular perception, aquaporin-mediated membrane transport, thiol peroxidase-based relay chemistry, and reversible oxidative post-translational modifications (oxiPTMs). We emphasize that the functional outcome of H2O2 depends on dose, duration, compartmentation, developmental stage, and antioxidant reset capacity, especially under combined abiotic stresses and where abiotic stress alters the redox context in which pathogen attack is perceived. To distinguish established crop mechanisms from plausible but unvalidated extrapolations, the cereal literature is organized into direct mechanistic evidence, orthology-supported inference, and physiology- or association-based evidence. Rice currently provides the strongest mechanistic anchors, particularly for aquaporin-dependent H2O2 transport and glutathione peroxidase (GPX)-linked redox transduction. Wheat remains dominated by physiological and transcript-associated evidence despite high agronomic relevance, whereas maize offers trait-level priming data with limited validation of upstream perception, transport, and oxiPTM nodes. We propose that cereal H2O2 biology should move from descriptive measurements of reactive oxygen species (ROS) toward threshold-aware, tissue-resolved and evidence-tiered experimentation. Such an approach can guide candidate validation, redox-site prioritization and calibrated priming strategies without assuming that stronger ROS signaling is inherently beneficial.