Polyamines Play a Positive Role in Salt Tolerant Mechanisms by Activating Antioxidant Enzymes in Roots of Vegetable Soybean

Two cultivars contrasting in NaCl tolerance were used to investigate the possible involvement of polyamines in salt tolerant mechanisms in roots of vegetable soybean. ‘Tianfeng’, the salt tolerant cultivar exhibited overall higher contents of free, soluble conjugated and insoluble bound forms of polyamines (putrescine, spermidine and spermine) except for free spermidine over the salt sensitive line during the course of NaCl treatment for 15 days. Consistent with this observation, arginine decarboxylase, the essential enzyme of polyamines biosynthesis and two enzymes catalyzing breakdown of polyamines were differentially regulated between the two cultivars. Antioxidant enzymes downstream to the polyamines signaling globally showed higher activity in ‘Tianfeng’, resulting in lower contents of reactive oxygen species and malondialdehyde subsequently. These results taken together indicate that higher levels of polyamines may be important in conferring enhanced salt tolerance in roots of vegetable soybean by activating antioxidant enzymes and, therefore, attenuating oxidative damage.