Abscisic Acid Promotes γ-Aminobutyric Acid Accumulation in Soybean Germinating Under Hypoxia-NaCl Stress

γ-aminobutyric acid is a nonprotein amino acid that accumulates in plants under stress. Abscisic acid is important for stress regulation via modulation of γ-aminobutyric acid function. Our results show that the expression of glutamate decarboxylase but not diamine oxidase gene is upregulated in germinating soybean treating treated with exogenous abscisic acid. There was a concomitant increase in glutamate decarboxylase and diamine oxidase activities and putrescine and spermine contents with a decrease in glutamate. These changes were abrogated by fluridone, an inhibitor of abscisic acid synthesis. In conclusion, abscisic acid treatment increases γ-aminobutyric acid accumulation by upregulating diamine oxidase gene expression and activation of glutamate decarboxylase and diamine oxidase activity in germinating soybean under hypoxia-salt stress.