Hypoxia treatment on germinating faba bean (Vicia faba L.) seeds enhances GABA-related protection against salt stress

The gamma-aminobutyric acid (GABA) is a non-protein amino acid with some functional properties for human health. Its content is usually lower in plant seeds. Hypoxia or salt (NaCl) stress is an effective way for accumulating GABA during seed germination. However, NaCl stress on GABA accumulation under hypoxia is currently infrequent. The effect of NaCl on GABA accumulation in germinating faba bean (Vicia faba L.) under hypoxia was therefore investigated in this study. Faba bean seeds were steeped in citric acid buffer (pH 3.5) containing NaCl with a final O-2 concentration of 5.5 mg L-1 and germinated for 5 d. Results showed that 60 mmol L-1 NaCl was the optimum concentration for GABA accumulation in germinating faba beans under hypoxia. Germination for 5 d under hypoxia-NaCl stress was less beneficial for GABA accumulation than only hypoxia (control). Polyamine degradation pathway played a more important role for accumulating GABA in germinating faba bean as an adaptive response to NaCl stress. Removing NaCl significantly increased GABA content, while it decreased glutamate decarboxylase (GAD) activity. Simultaneously, polyamine was accumulated, which might be related to the enhancement of physiological activity after recovery. When treated with aminoguanidine (AG) for 3 d, GABA content decreased by 29.82%. These results indicated that the tolerance ability of GABA shunt to NaCl stress was weaker than that of polyamine degradation pathway. The NaCl treatment for 3 d under hypoxia could raise the contribution ratio of polyamine degradation pathway for GABA accumulation. The contribution ratio of polyamine degradation pathway for GABA formation was 29.82% when treated for at least 3 d.