Adaptive roles of plant secondary metabolites under drought stress conditions

Drought stress is among the most severe environmental constraints limiting plant growth, productivity and ecosystem stability worldwide. It disrupts cellular homeostasis, impairs photosynthesis, induces oxidative damage and alters metabolite profiles. In response, plants activate coordinated physiological and metabolic responses that reprogram carbon and nitrogen allocation toward protective functions. A major component of this adaptive response is the enhanced production of plant secondary metabolites, which contribute to stress tolerance through integrated biochemical, physiological and ecological mechanisms. This review synthesizes the major classes of drought-responsive secondary metabolites including phenylpropanoids, flavonoids, terpenoids, alkaloids and glucosinolates, and evaluates their biosynthetic pathways, regulatory networks and functional roles in drought adaptation. These metabolites contribute to antioxidant defense, membrane stabilization, osmotic adjustment, lignification and hormonal signaling, maintaining cellular homeostasis under drought stress. Their biosynthesis is coordinated through carbon and nitrogen reallocation via the shikimate, mevalonate, and methylerythritol phosphate pathways, and regulated by transcription factors, abscisic acid signaling and epigenetic modifications. Unlike previous reviews that discuss metabolite classes independently, this review integrates carbon–nitrogen allocation, metabolic trade-offs, regulatory networks and ecological interactions into a systems-level framework for understanding drought adaptation. Therefore, integrating molecular insights with breeding and metabolic engineering approaches facilitates the development of climate-resilient crops adapted to increasingly water-limited environments, supporting global food security and climate adaptation efforts.