Various functions have been attributed to γ-aminobutyric acid (GABA), a nonprotein amino acid in a range of organisms, in stress and nonstress conditions. In the GABA shunt, GABA is not only metabolized by bypassing the TCA cycle but can also be synthesized indirectly from proline and polyamines. GABA also serves as a metabolite in nitrogen metabolism and in the biosynthesis of amino acids while its metabolism is a source for carbon skeletons and energy for downstream biosynthetic pathways. In recent decades, in addition to GABA's role as a metabolite in plants, its role as a signaling molecule has been revealed through the identification of putative GABA binding sites and its role in anion flux through aluminum-activated malate transporters. In this chapter, we focus on GABA's role on plant growth, regulation of carbon, nitrogen metabolism, improvement of shelf life, storage quality of products, cytosolic pH homeostasis, and removal of reactive oxygen species, as well as the crosstalk between GABA with hormones, nitric oxide, amino acids, and polyamines. Evidence of GABA's role in signaling during pollen tube growth, regulation of stomatal openings, and stress caused by bacterial infection, is also presented.
