Metabolism, enzymology and possible roles of 4-aminobutyrate in higher plants

4-Aminobutyrate is a widely distributed non-protein amino acid and is found in many plants in high amounts. It is formed by the decarboxylation of glutamate by the enzyme, glutamate decarboxylase. Increase in 4-aminobutyrate content and glutamate decarboxylase activity have been observed in many plants under a variety of environmental stress conditions. 4-Aminobutyrate is catabolized in plants by the operation of the 4-aminobutyrate shunt in which it is converted to succinate via succinate semialdehyde by the enzymes, 4-aminobutyrate transaminase and succinate semialdehyde dehydrogenase. Although universal distribution of glutamate decarboxylase among plants is well known, the occurrence of catabolic enzymes is not so well established. Possible physiological roles of 4-aminobutyrate in higher plants are documented and discussed here.