Role of γ‐Aminobutyric Acid in the Mitigation of Abiotic Stress in Plants

Abiotic stress is an important factor that limits the growth and productivity of plants. Plants use several metabolites to overcome such stress; one such metabolite synthesized and utilized by the plants is γ-aminobutyric acid (GABA), discovered quite a long time back. However, its role in plants was not well studied until recently when it is known that GABA not only acts as a signaling molecule but is also a metabolite conferring protective role in plants. GABA is synthesized using α-ketoglutarate, an intermediate of citric acid cycle by a two-step pathway known as GABA shunt. The main enzymes involved in GABA synthesis are glutamate dehydrogenase (GDH) and glutamate decarboxylase (GAD). Several other compounds such as glutamate and polyamines can also synthesize GABA. When externally applied, GABA enhances the endogenous level, ultimately increasing the level of other protective metabolites and enzymes, as well as triggering the expression of genes involved in ameliorating the adverse effects of stressors such as drought, salinity, hypoxia, and heat. Recent studies have shown that when plants are grown under any stress condition, the cytosolic level of Ca2+ increases inducing the activity of Ca2+-calmodulin (CaM)-dependent GAD, ultimately enhancing the endogenous GABA concentration. This chapter mainly deals with the metabolic pathway of GABA synthesis and protective action of GABA in terms of various molecular and biochemical mechanisms evolved by the plants to survive under different abiotic stresses.