Effect Of Cadmium Stress And Sulfur Deficiency On Non-Protein Thiol Content And Glutathione S-Transferase Activity In Rice

China's farmland cadmium (Cd) pollution has caused a serious threat on rice security production. It is important to alleviate the risk associated with Cd contamination and to develop rice varieties with high resistance and low accumulation of Cd. Non-protein thiols (NPT), including glutathione (GSH), phytochelatins (PC), have important roles in alleviating Cd toxicity and reducing Cd transport to shoots. Enzymes associated with GSH metabolism, such as glutathione S transferase (GST), also play a vital role in plant resistance to various environmental stresses. In this paper, a series of hydroponic experiments with Cd treatment were conducted to investigate the dynamic variation of Cd content in different parts of rice seedlings, NPT content and GST activity, and the interactions between S nutrition and Cd uptake, distribution and thiol metabolism in rice. Furthermore, we carried out GST protein electrophoresis analysis in different periods of rice seedlings. The main results are as follows:1 Dynamic variation of NPT and GST in rice under Cd stressThe results showed that Cd contents in the all parts of rice seedling increased with the time. The concentrations of Cd in different parts of rice followed the order:root>root surface>shoot>xylem sap. More Cd accumulated in root than shoot within 5d, but after 5d, shoot contained more Cd than root. The GSH content increased with the stress time. In root, phytochelatin (PC) and NPT contents initially increased, reached to the maximum level at 5d and then declined. The NPT and PC contents in shoot had little changes. The GST activity first increased and then decreased under Cd stress. Thiobarbituric Acid Reactive Substances (TBARS) content in shoot increased with stress time, but in root, TBARS first decreased and then increased. Hydrogen peroxide (H2O2) content under Cd stress decreased firstly, then attained the lowest value after 5d, and then increased to the control level after 7d. In conclusion, rice thiols had an important role in the process of detoxification of Cd, but there were certain limits. At 5d, rice seemed to reach the highest limit of Cd detoxification mechanisms.2 Effect of sulfur deficiency on variation of NPT and GST in rice under Cd stressWe investigated the interactions between S nutrition and Cd uptake, distribution and thiol metabolism in rice. Plants were grown for 7 days in nutrition solution with or without sulfate before Cd treatment were applied. Variations of rice growth, Cd absorption and translocation, NPT content and GST activity in rice seedling were determined. Cadmium clearly inhibited the growth of rice and induced the synthesis of sulfhydryl (-SH) compounds (NPT, GSH, PC). Sulfur-starvation did not significantly increase the Cd toxic effects, although under such condition Cd uptake and translocation increased and the contents of -SH compounds significantly decreased. The GST activity was lower in S-deficient rice shoots and roots. However, upon Cd exposure, the GST activity in rice root significantly increased. The above results suggest that -SH compounds and GST are complementary in the alleviation of Cd toxicity to rice.3 SDS-PAGE analysis of GST of rice seedlings under Cd stressWe carried out SDS-PAGE analysis to analyze GST in the seedling and the germination stages. GST activity of rice roots and shoots in the third leaf stage decreased at 1std, then reached the maximum and declined. In the germination stage, GST activity were first increased, reached the maximum at 2ndd and then gradually decreased. GST activities were higher in the germination stage than the third leaf stage. GST protein content and its activity were not entirely consistent. In the third leaf stage, root GST protein band of the 3rd day is the most intense, but the GST activity was not the highest. The shoot GST protein bands have no difference during the whole treatment period. The variation of GST protein content in germination stage was consistent with the activity. The above results suggest that, GST may be more sensitive to Cd stress in the germination stage than in the third leaf stage. In addition, GST protein content and activity do not follow the same trend of change.