Cadmium Accumulation And Physiological Response In High Cadmium Accumulating Rice Line

Cadmium（Cd）pollution in soil is one of the most serious environmental problems,which seriously threatened the safety of agricultural products and human health.It is urgent to remediate the Cd-contaminated soil.Phytoextraction has been proposed to be an eco-friendly and effective biotechnology for heavy metals removal from soil.However,the lack of knowledge of the mechanism of plant enrichment Cd has limited the application and improvement of this technology.The high Cd accumulating rice line possessed a superior capability of Cd accumulation with large amount of shoot biomass and its cultivation system is well-established and highly mechanized,which suggested that the rice line was a promising line for phytoextraction from Cd-contaminated soils.In this study,both hydroponic experiment and pot experiment were carried out to explore the Cd accumulation and physiological response in the high Cd accumulating rice line（Lu527-8）,in comparision with the normal rice line（Lu527-4）.The results are as follows:（1）Cd stress increased quantities of organic acids,but had no effe ct on composition in root exudates of the two rice lines.In Cd treatments,the contents of every detected organic acid in root exudates of Lu527-8 were 1.76².43 times higher than Lu527-4.Significant and positive correlations between organic acids contents and Cd contents in plants were observed in both rice lines,except that malic acid was only highly relevant to Lu527-8,but not to Lu527-4.Both composition and quantities of amino acids in root exudates changed a lot under Cd stress and this change differed in two rice lines.Alanine,methionine and lysine were induced by Cd stress in root exudates of both rice lines,while phenylalanine was only secreted by Lu527-8.The contents of those four newly secreted amino acids from Lu527-8increased significantly with the increase of Cd dose and each had a high-positive correlation with Cd contents,but the same change did not appear in Lu527-4.（2）Cd contents in xylem saps of Lu527-8 were 1.68².55 times higher than those of Lu527-4 under Cd stress.A high-positive correlation between Cd contents in xylem saps and Cd contents in shoots was observed.Lu527-8 owned a more rapid and effective transport of Cd to above-ground part.By analyzing the relationship between inorganic anions,organic components and Cd contents in xylem saps,the lower H₂PO₄^-and oxalate contents were considered to be related to the higher Cd transport in xylem sap of Lu527-8.As for citrate,tartaric and histidine content,significant increases were observed with the increasing Cd contents in xylem saps of two rice lines,and their contents of Lu527-8 were significantly higher than those of Lu527-4.Citrate,tartaric and histidine could take part in root-to-shoot Cd transport in xylem.（3）Under 2 and 5 mg·L^-1 Cd treatment,Cd was mainly distributed in cell wall and soluble fraction,few contents in organelle and cell membrane in Lu527-8.Cell wall deposition and vacuolar compartmentalization played a critical role in the reduction of Cd toxicity for Lu527-8.The main form of Cd in roots of the two rice lines was ethanol extraction,and in stem and leaf were pectin,protein and insoluble phosphate combinations,which was the important reason for Lu527-8 to own strong transport capacity and tolerance to Cd.By analyzing the composition and contents of organic acids and amino acids in two rice lines roots,the contents of tartaric,malate,citrate and acetic acid in Lu527-8 decreased significantly with the increasing Cd dose,and the decline was greater than Lu527-4,but the contents were always higher than lu527-4.The proline,phenylalanine,histidine,lysine and methionine contents in Lu527-8 increased significantly with the increasing Cd dose,and were significantly higher than those of Lu527-4.In addition,the cell wall polysaccharide composition,Cd binding capacity and pectin modification on cell wall of two rice lines were discussed in this study.The result showed that pectin and hemicellulose 1 in the cell wall of Lu527-8 were the main binding sites of Cd.The immobilization ability of pectin on Cd was significantly enhanced under demethylation catalyzed by pectin methesterase.Under 5 mg·L^-1Cd treatment,pectin methylesterase activity in Lu527-8 increased by 68.64%than in the absence of Cd,which was significantly higher than in Lu527-4.Lu527-8 had more low-methylesterified pectin and more hemicellulose 1 in the root cell wall,indicating the cell wall in Lu527-8 was stronger for Cd binding than Lu527-4.