Catabolism of 4-aminobutyrate (GABA) and proteinogenic amino acids sustains the tricarboxylic acid cycle in Arabidopsis thaliana (L.) Heynh. plants under salinity stress.

Abstract

Glutamate decarboxylases1,2,4 (GAD1,2,4) are calcium/calmodulin-activated enzymes that produce 4-aminobutyrate (GABA) and eventually lead to succinate (GABA shunt), which can theoretically be incorporated into the tricarboxylic acid cycle (TCAC) during abiotic stress. We tested the hypothesis that the GABA shunt is involved in the adaptive response of the TCAC to salinity stress. Quantitative analyses of amino and organic acids, as well as GAD1,2,4 expression, in shoots and roots of soil-grown, late-vegetative-stage Arabidopsis wild-type plants and GAD1,2,4 single, double and triple loss-of function mutants provided a general understanding of the metabolic response to salinity. Salinity induced similar metabolic changes in shoots and roots, though the magnitude of the response was greater in shoots. Several mechanisms could be responsible for the non-cyclic nature of the TCAC with salinity, but inhibition of 2-oxoglutarate dehydrogenase probably resulted in the diversion of 2-oxoglutarate from the TCAC to the GABA shunt, though activation of this pathway was insufficient to sustain the succinate level observed in the absence of salinity, especially in roots. Both transcriptional and post-transcriptional regulation of GAD could have contributed to activation of the GABA shunt. Inhibition of GAD-mediated GABA production by genetic means was associated with changes in levels of free proteinogenic amino acids, as well as elevated succinate, regardless of salinity stress. Overall, GABA and proteinogenic amino acids serve an important role in ensuring flexible generation of respiratory substrate for TCAC operation and energy production.