Cereal crops increasingly encounter drought, salinity and alkalinity, heat, waterlogging, potentially toxic elements, and compound stresses that constrain productivity and grain quality. Secondary metabolites, also referred to as specialized metabolites in the recent literature, represent a dynamic component of cereal stress adaptation rather than a passive catalogue of stress markers. This review aims to synthesize current evidence on how specialized-metabolite pathways contribute to abiotic stress responses in cereals, with particular emphasis on regulatory mechanisms, evidence strength, chemical form, subcellular localization, recovery, and translational relevance. We critically assess evidence from transcriptomics, metabolomics, genetic perturbation, biochemical assays, multi-omics integration, and physiological studies across major cereal crops and evaluate evidence according to causal, associative, and in vitro categories. The evidence reveals that perturbations at the plasma membrane, cell wall, chloroplast, mitochondrion, and endomembrane system levels generate reactive oxygen species, Ca²⁺ signatures, phospholipid signals, and hormone changes that are decoded by CDPK/CPK, CBL–CIPK, SnRK2, and MAPK cascades. These signaling networks converge on MYB–bHLH–WD40, WRKY, NAC, ERF/AP2, HSF, and bZIP regulators and redirect carbon and reducing power through phenylpropanoid, flavonoid, lignin, carotenoid, terpenoid, benzoxazinoid, cyanogenic, and related pathways. Comparative analysis across rice (Oryza sativa L.), wheat (Triticum aestivum L.), maize (Zea mays L.), barley (Hordeum vulgare L.), oat (Avena sativa L.), sorghum (Sorghum bicolor L.), and millets shows conserved regulatory features but substantial crop-, genotype-, tissue-, developmental-stage-, and dose-dependent variation. Functional evidence further indicates that enzyme activity, chemical modification, transport, and subcellular compartmentation can determine whether metabolites contribute to ROS buffering, photosynthetic and membrane protection, cell-wall reinforcement, osmotic or ionic homeostasis, toxic-ion sequestration, or signaling. The review also identifies important limitations in current research, including overreliance on associative omics evidence, insufficient consideration of combined stresses and rehydration, growth–defence trade-offs, and limited field and reproductive-stage validation. To distinguish growth dilution from genuine biosynthetic increases, absolute metabolite content per grain or organ should be measured alongside concentration per unit dry weight. We conclude that improving cereal resilience requires context-dependent and experimentally validated coordination of regulators, biosynthetic enzymes, chemical modification, transport, and compartmentation rather than indiscriminate elevation of total specialized-metabolite concentration.

Drought induces coordinated physiological, biochemical, and molecular responses that extend beyond changes in phytohormone concentrations. Rapid alterations in reactive oxygen species (ROS), apoplastic pH, Ca²⁺ and ionic dynamics, tissue hydration, and cellular redox status provide spatial and temporal information on developing water deficit and regulate stomatal behavior, photosynthesis, water relations, and stress-responsive gene expression. This review explores whether endogenous drought-associated signals can serve as biological triggers for stimulus-responsive metal–organic frameworks (MOFs) designed to regulate abscisic acid (ABA) delivery. We integrate drought-induced changes in redox state, apoplastic pH, ionic composition, and hydration with MOF transformation, host–guest interactions, ABA release, uptake, transport, metabolism, and engagement of the PYR/PYL/RCAR–PP2C–SnRK2 signaling pathway. We propose a framework in which responsive MOFs generate a temporally controlled ABA pulse that follows drought progression, activates protective signaling during water limitation, and attenuates after rewatering to facilitate recovery. Particular emphasis is placed on ROS-Ca²⁺ crosstalk, SnRK2/OST1-dependent guard-cell regulation, hydraulic adjustment, photosynthetic protection, and recovery dynamics. Current studies demonstrate MOF-mediated phytohormone protection, controlled release, and drought-related physiological improvement. Current advances have established the feasibility of MOF-based ABA protection and controlled delivery; however, the integration of drought-responsive carrier activation with receptor-accessible ABA delivery and PYR/PYL/RCAR–PP2C–SnRK2 pathway validation remains an emerging research frontier. We therefore outline a claim-matched validation framework integrating spatial drought-signal analysis, isotope-resolved ABA tracing, carrier localization, phosphorylation-based pathway assays, drought–rewatering experiments, and physiological assessment. This integration of plant stress physiology with responsive nanomaterials provides a conceptual basis for developing more precise ABA delivery strategies for crop resilience under increasingly variable water availability.