Biochemical and Enzymatic Analyses to Understand the Accumulation of γ-Aminobutyric Acid in Wheat Grown under Flooding Stress

Flooding induces low-oxygen stress, which reduces plant growth. The activity of the γ-aminobutyric acid (GABA) shunt is crucial for plant stress adaptation, in which it acts by changing cytosolic pH, limiting reactive oxygen species production, regulating nitrogen metabolism, and bypassing steps in the tricarboxylic acid cycle. GABA accumulates under osmotic stress as well as flooding stress. To clarify the dynamic roles of GABA accumulation in wheat under flooding stress, biochemical and enzymatic analyses were performed using a plant-derived smoke solution (PDSS), which rescued wheat growth from flooding stress. Alcohol dehydrogenase abundance increased under flooding stress; however, under the same conditions, pyruvic acid content increased only following PDSS application. Glutamic acid content increased under flooding stress, but decreased following the application of PDSS after 2 days of flooding. Glutamate decarboxylase abundance and GABA content increased under flooding stress, and further increased after 1 day of application of PDSS. Succinyl semialdehyde dehydrogenase accumulated after 2 days of flooding. These results suggest that flooding stress increases GABA content along with the increase and decrease of glutamate decarboxylase and succinyl semialdehyde dehydrogenase, respectively. Additionally, PDSS increased GABA content along with the increase of glutamate decarboxylase abundance at the initial stage of application.