| Soybean(Glycine max L.)is rich in macromolecular nutrients such as dietary protein,carbohydrates and fats.During the germination of soybean,endogenous enzymes of soybeans are activated or produced,which macromolecular nutrients are decomposed into small molecular components that are easy absorbed and utilized.Meanwhile,various anti-nutritional factors are degraded,some new functional compounds are accumulated and the nutritional value of soybeans was significantly improved.It is a functional compound with various physiological functions such as blood pressure lowering that y-aminobutyric acid(GABA).During the germination,the GABA synthase of GABA shunt and polyamine degradation pathway after germination,and GABA increased by multiple.However,soybean is a salt-sensitive crop and then the NaCl stress severely inhibits its growth and biological yield.Polyamines(PAs)found to be effective in alleviating the damage of plant growth caused by NaCl stress and promoting plant growth.Among them,Spermidine(Spd)was more effective than other polyamines in alleviating NaCl stress.In addition,Spd is one of the precursors of GABA synthesis and contributes to the synthesis of GABA.In the present study,the mechanism of GABA accumulation in germinating soybeans under NaCl stress and NaCl combining Spd treatment were explored.The main results were as follows:1.When the concentration of exogenous Spd was 0.05～0.15 mM under NaCl stress,the germination length of germinated soybean under 0.10 mM Spd treatment was significantly higher than that of NaCl and 0.05 mM Spd treatment(p<0.05),which was 1.38-and 1.25-fold at 4 d.The fresh weight of germinated soybean under 0.10 mM Spd treatment was significantly higher than that of NaCl and 0.15 mM Spd treatment(p<0.05),which were 1.10-and 1.08-fold at 4 d.Therefore,Spd can significantly alleviate the growth inhibition effect of NaCl on germinated soybean.The GABA content of 0.10 mM Spd treatment under NaCl stress was significantly higher than that of all other treatments(p<0.05),which was 1.37-fold that of NaCl treatment.When 0.10 mM Spd is the optimal concentration to alleviate the stress effect of NaCl on germinated soybean and promote GABA accumulation.The effect of NaCl combining Spd treatment on the physiological metabolism and GABA accumulation in germinating soybeans investigated.Compared with NaCl stress,Spd treatment significantly increased the sprout length and biomass yield of germinated soybean under NaCl stress(p<0.05).At 4 d or 6 d,the sprout length is 1.43-and 1.54-fold that of NaCl treatment.It is 1.14-and 1.15-fold that of NaCl treatment in fresh weight.Free amino acids content decreased significantly while soluble sugar,soluble protein,proline and GABA content increased significantly(p<0.05).At 4 d or 6 d,GABA content was 2.16-and 1.42-fold that of the control and was 1.74-and 1.41-fold that of NaCl treatment.In addition,Spd treatment promoted significantly SOD,CAT,POD and GPX antioxidant enzyme activities and decreased significantly MDA and H2O2 content.2.The effect of NaCl plus Spd treatment accumulation on GABA synthesis and metabolism pathway in Germinated Soybean were investigated.Spd significantly attenuated the growth inhibition effect of NaCl on germinated soybean and promoted GABA accumulation(p<0.05).That is,At 2 d or 4 d,the sprout length of NaCl-Spd treatment was 1.22-and 1.25-fold that of NaCl treatment,the GABA content was 1.16-and 1.92-fold that of NaCl treatment,the GABA content reached 1.30 mg/g DW at 4 days.Compared with the control,NaCl,Spd and NaCl-Spd treatment had no significant effect on the content of Glu in germinated soybean(p>0.05),but could significantly increase GAD,DAO,PAO,AMADH activities(p<0.05),thereby promoting GABA accumulation,NaCl-Spd treatment enhanced the activity of key enzymes of GABA synthesis compared with NaCl treatment and Spd treatment,and the effect of enrichment of GABA was more significant.At 4 d,the contribution rates of polyamine degradation pathways to germinated soybean enrichment of GABA by NaCl,Spd,NaCl-Spd treatment were 80.6%,57.3%and 47.2%.The contribution rate of polyamine degradation pathway to GABA enrichment in germinated soybean decreased on Spd treatment.3.The effect of NaCl plus Spd treatment on the GABA synthesis related enzymes gene expression in germinating soybeans were investigated.At 2 d,compared with the control,NaCl-Spd treatment significantly increased GAD1 expression levels(p<0.05),which was 1.60-fold of the control,and there was no significant change in the expression of other genes in the GADs gene family(p>0.05).There was no significant change in the expression of DAOl and DAO2 genes(p>0.05);There was no significant change in PAO1 gene expression(p>0.05),and PAO2 gene expression was significantly increased(p<0.05),which was 1.87-fold that of the control.There was no significant change in the expression of AMADH1 gene(p>0.05),and the expression of AMADH2 gene was significantly increased(p<0.05),which was 2.44-fold that of the control.At 4 d,compared with the control,the expression of GAD1 and GAD3 genes was significantly increased by NaCl-Spd treatment(p<0.05),which was 2.01-and 1.59-fold of the control;GAD4 gene expression was significantly decreased(p<0.05),which was 65.2%of the control.There was no significant change in the expression levels of other genes in the GADs gene family(p>0.05);There was no significant change in the expression of DAO1 and DA02(p>0.05).The expression of PAO1 gene was significantly decreased(p<0.05),which was 66.4%of the control,and there was no significant change in the expression of PA02 gene(p>0.05).The expression of AMADIII gene was significantly increased(p<0.05),which was 1.97-fold of the control,and there was no significant change in amount of AMADH2 gene expression(p>0.05).The results showed that the effects of different treatments on the expression of key enzymes in GABA synthesis of germinated soybeans were analyzed according to the germination time. |
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