Effect And Mechanisms Of Selenium On Cadmium Toxicity In Rice

Rice (Oryza sativa L.) is one of the most important staple cereals in a large part of the world, especially in China. It’s the daily staple for over half of the global human beings and in China the ratio reached nearly 60%. Cadmium (Cd) is one of the most deleterious heavy metals to both animals and plants, which has no beneficial biological function in the aquatic or terrestrial organism. High accumulation of Cd in plants not only seriously affects crop yield and quality, but also endangers human health via food chain. Rice is regarded as a type of high-accumulation cereals. It’s therefore important to investigate the mechanisms contributing to the process of Cd enrichment and transporting in the soil-rice root system, which is not only beneficial for seeking ways to eliminate Cd from contaminated paddy soils, but also can guarantee our daily staple consumption safe. Recently, the reports about other elements affected Cd entry into rice are not rare, Se is one of the necessary nutrients to human beings, animals and plants, it could impact the absorption of some heavy metals by crops. In this paper, hydroponic experiments were performed to investigate the effects of exogenous selenite (Se(Ⅳ) was added in the nutrient solution) on Cd absorption, translocation and detoxification in rice under different cadmium stress, also analysed the accumulation of Se and Cd both in rice root and shoot. The main results are as follows:1. Effects of Se(Ⅳ) on Cd absorption and translocation in different rice cultivarsWe set different ways of Se(Ⅳ) added(Cd+Se; Se+Cd; Se&Cd) treatment to study the effect of Se on both selenium and cadmium absorption and translocation in different cultivars of rice(K you 818、Ⅱ you 084、N08-13 and N07-63). The results showed that the way of Se adding could not only influence the absorption and translocation of Se and Cd in rice significantly, but also related to rice cultivars. Exogenous Se(Ⅳ) would promote the absorption of Cd by Ⅱ you 084 and N08-13, their concentrations of Cd in the shoot are increased; Se(Ⅳ) could also improve the absorption of Cd by N07-63, but inhibit its translocation to the shoot; the absorption of Cd by K you 818 doesn’t influenced by Se(Ⅳ), while its translocation to the shoot was promoted.2. Effects of Se(Ⅳ) on Cd absorption and translocation in rice under different cadmium stressWe investigate the effects of Se(Ⅳ) on cadmium absorption and translocation in the rice cultivar of K you 818 under two types of Cd stress(low-Cd 0.5 μM and high-Cd 5.0 μM). The results showed that different concentrations of Cd stress could affect the transport of Cd in rice. Transfer rate of high-Cd stress is significantly lower than the low-Cd stress, exogenous Se(Ⅳ) has no influence with transfer rate, but would markedly reduce the Cd content in rice at high-Cd stress, thus inhibit the absorption of Cd by rice. Existence of Cd would increase the uptake of Se(Ⅳ) by plant, and significantly inhibited the transport of Se(Ⅳ) to shoot, then large amounts of Se(Ⅳ) accumulated in the root, maybe combined with Cd and prevent Cd uptake and transport by rice.3. Effects of Se(Ⅳ) on Cd subcellular distribution in rice rootThis part is followed by the last one, explores the impact of Se on Cd subcellular distribution of rice root. The distribution rule is:cell wall components (F1)> cell soluble material component (F3)> organelle components (F2), and this law is not influencd by exogenous Se(Ⅳ). Between the high-Cd stress treatments, Se(Ⅳ) impact the content of Cd in rice root significantly:the distribution ratio of Cd in F1 components increased by Se(Ⅳ), while it decreased in F3 component, and decreased significantly in F2 components. Se(Ⅳ) would increase the Cd adsorption on the cell wall.4. Effects of Se(Ⅳ) on Cd stress in riceThe experiment setting is the same as the last two parts, investigates the effects of Se on NPT in the root and activity of antioxidant system in the shoot induced by Cd stress in rice. The results showed that, though the concentrations of Se and Cd in this experiment are low and the exposure time is not long, the change of the total antioxidant capacity(T-AOC) in the shoot is not obvious, and there is no effect on rice growth. But exogenous Se(Ⅳ) would significantly increase NPT content in the root of rice under Cd stress, promote complexing Cd, keep Cd stay in the root, inhibiting its transport to the shoot.The content of Cd would affect the activity of the antioxidant system, low-Cd stress could increase the activities of SOD、CAT and POD in rice shoot, while under high-Cd stress, the activities of these enzymes were decreased. Exogenous Se(Ⅳ) relieve the hazards in rice induced by high-Cd stress, by increasing the activity of SOD, reducing the content of TBARS and membrane lipid peroxidation, alleviating the hazards of Cd on rice.