Effects Of Cadmium Contamination In Soil On Soybean Antioxidant System Activities And Resistance Gene Expression

The content of cadmium in soil increases gradually with the extensive use of chemical fertilizers and pesticides as well as the rapid development of industry.In order to study the effects of different concentration of cadmium on antioxidant system activities and resistance gene expression in soybean during the whole growth period,the cultivated and wild soybean were used as experimental materials,and five groups of treatments(0、5、10、20、40 mg·kg^-1)were set in this experiment.The morphological indicators and biomass,leaf chlorophyll content,superoxide radical（O₂.^-）production rate,and hydrogen peroxide（H₂O₂）content as well as the activity of superoxide gasification（SOD）,hydrogen peroxide（H₂O₂）,ascorbate peroxidase（APX）,monodehydroascorbate oxidase（DHAR）,glutathione reductase（GR）and malondialdehyde（MDA）and proline（Pro）contents were measured.We also measured the gene expression levels of SODs,CATs,APX,GR and glutathione thioltransferase（GST）and metallothioneins（MTs）,phytochelatin synthase（PCS）,divalent metal ion transporter（DMT）and iron regulatory transporter（IRT）in cultivated soybean leaves.The results showed that:（1）The chlorophyll content in cultivated soybean leaves increased firstly and then decreased during the whole growth period.The content of chlorophyll a in wild soybean decreased in the later period,and the chlorophyll b content remained high level in the later period.Cadmium stress at 40 mg·kg^-1 significantly decreased the number of branches,leaf width,pod number,hundred-grain weight and biomass of cultivated and wild soybean,while cadmium treatment of 5 mg·kg^-1 significantly increased the biomass of wild soybean.（2）Under cadmium stress,the production rate of O₂.^-in soybean leaf increased during the whole growth period.The SOD activity of cultivated soybean was higher than control in most growth periods,but the SOD activity of wild soybean was not significantly higher than control in most growth periods.The H₂O₂ content in cultivated and wild soybeans did not change significantly during the early growth stage and reached a high level during the physiological maturity period.The activities of APX、DHAR、GR、MDAR in soybean leaves were higher than the control in the whole growth period.The content of MDA in cultivated and wild soybeans reached the highest value at the peak of grains,and Pro content increased during the branching period.The Pro content of cultivated soybean increased at physiological maturity,while the wild soybean maintained a high level during the peak of grain and late grain.（3）The relative expressions of CATs,SODs under cadmium stress were significantly different in seven periods.The relative expression level of APX was up-regulated during the branching period and the late-granule period,while the relative expression level of GR was up-regulated during the branching period and the grain-filling.The relative expression level of APX was up-regulated during the branching period and the late-grain-filling period,while the relative expression level of GR was up-regulated during the branching period and the grain-filling.Cadmium stress induced the expression of PCS in the early growth stage of soybean,while GST showed a higher expression level in the middle period of cadmium treatment.Iron-regulated transporter（IRT）and divalent metal ion transporter（DMT）can transport cadmium in plants.The relative expression of DMT maintained a high level at the flowering stage and the early podding,and it significantly increased compared with the control at the peak of grain-filling.The relative expression of IRT maintained a relatively high level in the early and middle stages.In conclusion,The chlorophyll content of cultivated soybeans decreased after cadmium stress,and 40 mg·kg^-1 treatment significantly reduced the morphological indicators such as chlorophyll content、number of branches、leaf width and biomass of cultivated and wild soybeans,5,10 mg·kg^-1 cadmium treatment has a positive effect on the biomass of wild soybeans.The O₂.^-production rate and enzymes activity of AsA-GSH cycle in the leaves of cultivated and wild soybean were promoted in four concentrations of cadmium stress.The genes involved in antioxidant and phytochelation mechanism play a major role in different periods in cadmium stress.IRT and DMT act as the transport protein of cadmium at different growth period.