Enzymatic Conversions of Glutamate and γ-Aminobutyric Acid as Indicators of Plant Stress Response

Glutamate plays a central role in amino acid metabolism, in particular, in aminotransferase reactions leading to formation of many other proteinogenic and nonproteinogenic amino acids. In stress conditions, glutamate can be either metabolized to γ-aminobutyric acid (GABA) by glutamate decarboxylase which initiates a GABA shunt bypassing several reactions of the tricarboxylic acid cycle or converted to 2-oxoglutarate by glutamate dehydrogenase. Both reactions direct protein carbon to respiration but also link glutamate metabolism to other cellular pathways, resulting in the regulation of redox level and pH balance. Assays for determination of activities of glutamate dehydrogenase and of the GABA shunt enzymes as the markers of stress response is described in this chapter. These assays are important in the studies of the strategy of biochemical adaptation of plants to changing environmental conditions including elevated CO2, temperature increase, flooding, and other stresses.