Accumulation of γ-aminobutyric acid in nodulated soybean in response to drought stress

Nitrogen fixation and nodule permeability to O2 diffusion are decreased by drought stress. Since γ-aminobutyric acid (GABA) synthesis is rapidly stimulated by a variety of stress conditions including hypoxia, it was hypothesized that decreased O2 availability in nodules stimulates glutamate decarboxylase (GAD) activity (EC 4.1.1.15), thereby resulting in GABA accumulation. First, the amino acid composition of xylem sap was determined in plants subjected to soil water deficits. While the xylem sap concentration of several amino acids increased when the plant was subjected to a water deficit, the greatest increase was in GABA. GABA accumulation was examined in response to stress induced by hypoxia or the addition of polyethylene glycol (PEG) to the nutrient solution. The exposure of soybean nodules to hypoxia for 6 h enhanced the GABA concentration by 6-fold, but there was no change in GABA concentration in response to the PEG treatment. No major changes in the in vitro GAD activity were measured in nodule cytosol or bacteroids. The present data do not support the hypothesis that decreased nodule O2 permeability and a resulting O2 deprivation inside nodules may stimulate in vitro GAD activity and thus GABA accumulation. However, the data could indicate a possible effect of hypoxia and drought stress on the in vivo activity of GAD.