Plant Flavonoids: Biosynthesis, Regulation, and Roles in Biotic and Abiotic Stresses

Flavonoids bridge plant defence and acclimation, helping land plants translate UV-B/high light, drought, heat, salinity, and cold into metabolic and physiological change. Recent studies map lineage biases in flavonoid scaffolds and show that core enzymes assemble into endoplasmic reticulum (ER)-associated metabolons, with auxiliary reactions detected at the tonoplast and in the nucleus. After synthesis, cellular pools are set by ABC and MATE transporters, GST ligandins, and vesicle-mediated trafficking. Regulatory layers include MBW-centred transcription-factor networks wired into Ca²⁺, ROS, and JA/SA/ABA signalling, while late tailoring (hydroxylation, glycosylation, O-methylation, and acylation) modulates solubility, stability, localisation, and bioactivity. Under UV, drought, high temperature, salt stress, freezing, nutrient imbalance, and metal toxicity, distinct chemotypes contribute to photoprotection and to biotic defence as phytoalexins and anti-herbivore deterrents. We propose that flavonoids act not only as redox-active, membrane-protective metabolites but also as signals that reset transcriptional and hormonal programmes; pathogens and insects can blunt this interface via detoxification, efflux, and enzymatic breakdown. Key quantitative gaps include in vivo antioxidant weight relative to enzyme cycles, branch-specific flux partitioning, and links between tissue patterning and protection. Priorities are outlined for deploying stress-responsive flavonoid repertoires to boost crop resilience under combined stresses without yield penalties.