Polyamine Degradation Pathway Regulating Growth and GABA Accumulation in Germinating Fava Bean under Hypoxia-NaCl Stress

In plants, γ-aminobutyric acid (GABA) is synthesized by polyamine degradation pathway besides GABA shunt. Aminoguanidine (AG) is a specific inhibitor of the key enzyme (diamine oxidase, DAO) for GABA formation in polyamine degradation pathway. In this study, AG was applied to study the functions of polyamine degradation pathway on growth and GABA accumulation in germinating fava bean under hypoxia-NaCl stress. The results showed that 5.0 mmol L of AG inhibited DAO activity maximally but not entirely, and inhibited the growth of sprouts simultaneously. Hence, blocking polyamine degradation pathway significantly affected the growth of germinating fava bean. Polyamine degradation pathway provided 26.9 and 29.3% of GABA in cotyledon and embryo, respectively, because DAO activity was not inhibited entirely. Polyamine, especially putrescine (Put), accumulated after polyamine degradation pathway was blocked, indicating that Put was the main substrate of GABA in polyamine degradation pathway.