GABA shunt: a key-player in mitigation of ROS during stress

Unfavorable environmental conditions such as heat, cold, drought, metal/metalloid toxicity, and pathogens enhance production of intra-and inter-cellular levels of reactive oxygen species (ROS) in plants. ROS, acting as signaling molecules, activate signal transduction pathways in response to various stresses. Alternatively, ROS cause irreversible cellular damage due to lipid peroxidation, oxidation of protein, inactivation of enzymes, DNA damage, and interact with other vital constituents of plant cells through their strong oxidative properties, which drastically alter plant morphological structures, becoming disadvantageous for survival and productivity. Higher plants have complex defense systems to scavenge ROS. Being a central molecule of the defense system, gamma-aminobutyric acid (GABA) is ubiquitous from prokaryotes to eukaryotes cells. GABA helps mitigate ROS in plants and GABA shunt pathway plays a key role either as metabolites or endogenous signaling molecules in several regulatory mechanisms under stress conditions. The GABA transporters (GATs) being activated with the attachment of GABA under environmental stress stimuli facilitate high content of Ca 2+ into the cytosol. Ca 2+ combines with calmodulin (CaM) -binding domain that activates the glutamate decarboxylase (GAD) enzyme for the conversion of glutamate into GABA. This synchronized process regulates GABA shunt gene expressions under stress conditions and improves defense mechanisms in plants. This review highlights the regulatory aspects of GABA shunt pathway for ROS production as well as in the defense mechanism of plants.