Interaction of 4-Aminobutyrate (GABA) with the Tricarboxylic Acid Cycle in Plants Under Salinity Stress

The 4-aminobutyrate (GABA) shunt bypasses 2-oxoglutarate dehydrogenase and succinyl-CoA synthetase in the tricarboxylic acid cycle (TCAC) by diverting 2-oxoglutarate to glutamate and generating GABA via glutamate decarboxylase (GAD), whereas polyamine oxidation generates GABA directly from 4-aminobutanal. During salinity stress, the TCAC switches from a cyclic to a non-cyclic mode of operation probably due to the inhibition of two thiamine pyrophosphate-dependent enzymes, 2-oxoglutarate dehydrogenase and pyruvate dehydrogenase, and increases GAD activity via both transcriptional and post-transcriptional (i.e., elevated cytosolic Ca²⁺/calmodulin, H⁺ or glutamate) processes. Diversion of 2-oxoglutarate may occur via an increase in aminating glutamate dehydrogenase activity, due at least in part to the accumulation of ammonium, resulting from changes in protein synthesis and degradation. Inhibition of diamine oxidase activity by aminoguanidine suggests that polyamine oxidation contributes up to one-third of the salinity-regulated GABA level; however, Arabidopsis thaliana (L.) Heynh. GAD loss-of-function mutants suggest that polyamines account for less. The use of aminoguanidine and/or the GAD inhibitor, 3-mercaptopropionic acid, in combination with GAD or 4-aminobutanal dehydrogenase loss-of-function mutants, offers additional opportunities to understand if both GABA sources give rise to succinate, which can function to restore or partially restore TCAC activity during salinity stress.