γ-Aminobutyric Acid (GABA): Metabolite, Messenger, and Mediator of Stress Adaptation

γ-Aminobutyric acid (GABA), a nonproteinogenic amino acid first identified in biological systems over 70 years ago, has long been recognized as a metabolic intermediate. More recently, GABA has also been acknowledged as a signaling molecule that couples physiological responses to metabolic status. This review presents a conceptual framework for how metabolism sets GABA concentration and localization, which then modulates ion transport and membrane potential dynamics to influence plant growth, development, and adaptation to stress. We explore the emerging network of GABA's interactions with other signaling pathways, highlighting its involvement in environmental sensing and internal regulatory mechanisms via hormones and reactive oxygen species. These interactions influence key physiological processes including stomatal regulation, pathogen and herbivore defense, root growth, and even the modulation of flavor. Collectively, these findings position GABA as a metabolic signal integrator of plant physiological status and responses, with broad implications for enhancing crop stress resilience and food quality.