Physiological Mechanism Of Alleviating Cadmium Toxicity In Rice By Exogenous Selenium

Cadmium（Cd）,one of non-essential element for plants,harm to plant growth and physiological metabolism and is readily absorbed by roots of plant from contaminated soil and transported to aerial part,which may be tremendously harmful for human health due to Cd accumulates in the food chain.At present,the problem of cadmium pollution in farmland is one of the major environmental and food security problems that need to be solved urgently in China.Therefore,it is particularly important to develop a strategy to reduce the uptake and transport of cadmium in plants,especially crops,reduce the content of cadmium in plants,and achieve safe food production.Selenium（Se）is an essential trace element for animals and humans.If the organism of humans is in the state of long-time insufficient or excessive Se,it is easy to associated with a series of health diseases.A large number of previous studies showed that selenium may inhibit and protect the toxicity of cadmium,and the presence of Cd-Se complex was found in Marine mammals and birds,but,so far,the literature related to Cd-Se complex in plants rarely has been reported.In addition,although many studies have proved that selenium inhibits cadmium uptake and transport in plants,there is still no consensus on its optimal inhibitory conditions,and of which potential physiological and biochemical mechanisms are still unclear.Moreover,there are few reports on which cadmium effects the absorption and transportation of selenium in plants.This study adopts the model of rice plants,Oryza.Sativa L.spp.japonica,as experiment material and designed the different concentration gradients of cadmium and selenium that has various disposal approach through the experiment of indoor hydroponics.The analysis of Se（Cd）impacts on the absorption,transportation and distribution of Cd（Se）in rice in order to find out the optimal condition of Se（Cd）inhibit Cd（Se）in rice.Coupled with some interrelated physiological parameters,to explores underlying physiological and biochemical mechanism of Se mitigate the toxicity of Cd in rice by exogenous application of Se.Which provide theoretical basis and technical support for the development of remediation measures for Cd pollution in farmland.The main research results from this study are as follows:1.There was antagonistic effect between Se and Cd within certain range.There was an inverse relationship between Se（Cd）levels and Cd（Se）levels in various organs of rice（r₁≤﹣0.16（r₂≤﹣0.17）,p<0.01 for all）.A consistently negative correlation was bserved between Se（Cd）levels in solution and Cd（Se）levels in various organs of rice（r₁≤﹣0.39（r₂≤﹣0.22）,p<0.01 for all）.In addition,Se（Cd）concentration of root was significantly negatively correlated with Cd（Se）concentration of above-ground parts（r₁≤﹣0.24（r₂≤﹣0.44）,p<0.01 for all）.Similarly,TFs-Cd（TFs-Se）in different organs of rice decreased with the increase of Se（Cd）contents in solution（r₁≤﹣0.56（r₂≤﹣0.24）,p<0.01 for all）.The molar ratio of Cd/Se in solution was significantly negatively correlated with Cd concentration（r≤﹣0.69,p<0.01 for all）and has an inverse relationship to TFs-Cd in all organs of rice.However,the molar ratio of Cd/Se for solution was only negatively correlated with the Se concentration for various parts of rice（r≤﹣0.36,p<0.01 for all）.2.The distribution of Se（Cd）in all organs of rice had similar patterns（Se:root>leaves≥stem>leaf sheath;Cd:root>stem≈leaf sheath>leaf）.Root of rice could effectively restrain the transport of Se（Cd）to the aboveground parts and reduce the content of Se（Cd）in the aboveground parts.The enrichment of Cd（Se）in rice root was improved by exogenous Se（Cd）application.At the same concentration of Cd in solution,when the concentration of Cd was less than 10μmol/L,it was easier for Se to accumulate Cd in the root of rice than Pre Se.At this time,Se and Cd showed antagonistic effect.However,when the concentration of Cd was equal to 50μmol/L,there is not only antagonistic effect between Se and Cd,but also synergistic effect,Pre Se was more likely to enrich Cd in the root than Se.Furthermore,adding Se could enhance the enrichment of Cd in the leaf sheath of rice.Analogously,under the same Se concentration in solution,Se was more likely to accumulate in the root of rice than Pre Se.3.Under the same concentration of Cd,both Se and Pre Se could improve the contents of As A,GSH,Pro and total protein as well as the activities of seven related antioxidant enzymes（SOD,CAT,APX,GPX,GS,GR and PCS）to mitigate the contents of ROS and MDA in leaves and roots of rice.Moreover,Pre Se of the physiological indicators mentioned above had a larger improvement than Se of those.Interestingly,Se could reduce the content of total chlorophyll,biomass and moisture content of plant,while the Pre Se could increase their levels,which was contrary to previous results in this study that it was easier for Pre Se to reduce Cd concentration in all organs of rice than Se of those.Moreover,the extent of enhancing the contents of As A,GSH and the activity of enzymes by application of exogenous Se related was greater than those without Se.In this study,the main research conclusions were selenium could alleviate the physiological toxicity of cadmium,of which underlying mechanisms might be:（1）Se could inhibit the absorption,transportation and distribution of Cd from in solution to aerial parts of rice.When the molar ratio of Se/Cd was greater than or equal to 1,Se could minimize the concentration of Cd and the transport of Cd to aerial parts in rice.And when the mole ratio of Se/Cd in solution was equal to 1,the possible generation of Cd-Se complex in rice roots/root that was undissolved,relatively low toxicity would tremendously limit the absorption and transportation to above-ground parts of bioavailable Cd（Se）in solution.However,when the molar ratio of Cd/Se in solution was greater than or equal to 20,Cd could maximally reduce the concentration and the transport of Se to aerial parts in rice.（2）The application of exogenous Se could improve the contents（activities）of physiological indicators such as antioxidant system for the cells of rice leaves and roots to reduce the contents of ROS and MDA for those.Thus,Se alleviated the physiological toxic effect of Cd in rice.（3）Application of exogenous Se could promote the rapid progress of As A-GSH cycle to mitigate the toxic effect in rice.