Cadmium Accumulation Mechanism In Root Of High Cadmium Accumulating Rice Line

Cadmium（Cd）pollution in soil is a serious environmental problem,which threatens the safety of agricultural products and human health.It is urgent to remediate the Cd-contaminated soil.The high Cd accumulating rice line possesses 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,hydroponic experiments were carried out to explore the Cd absorption ability and Cd binding capacity of cell wall in root of the high Cd accumulating rice line（Lu527-8）,in comparision with the normal rice line（Lu527-4）.The results are as follows:（1）Under the ion channel blockers La Cl₃ treatment,the Cd concentrations in the roots of Lu527-8 and Lu527-4 decreased by 57%and 52%,respectively.Under the metabolic inhibitor DNP treatment,the Cd concentrations in the roots of Lu527-8 and Lu527-4 decreased by 64%and 40%,respectively.It proved that rice passively absorbed Cd through non-selective cation channels（NSCCs）,which also actively absorbed Cd through transporters.Under 10μmol·L^-1 Cd treatment,NSCCs played a greater role in Cd absorption in root of Lu527-4,while active absorption played an important role in Lu527-8.NSCCs played no role in transport of Cd to shoot,while the Cd concentrations in shoot under DNP treatment decreased by 73%,indicating that the translocation of Cd from the root to shoot in plants is an active process.In terms of absorption capacity,the maximum absorption rate V_maxof root in Lu527-8was 1.26 times that of Lu527-4,which indicated that the Cd absorption capacity of root in Lu527-8 was higher than Lu527-4.At the same time,the Cd binding capacity of apoplast in Lu527-8 root was significantly higher than that of Lu527-4.（2）More than 80%of the Cd in the two rice lines were accumulated in the roots.About 60%of Cd in roots of the two rice lines was bound to cell walls,indicating that cell wall was the main site for Cd accumulation in roots of the two rice lines.Cd adsorbed on the root cell walls of Lu527-8 was 5%-16%more than that of Lu527-4.The stronger binding capacity of the root cell wall of Lu527-8 may be related to its more-OH,-COOH,N-H,C-O,C-O-C and C=O groups from polysaccharides,amines and proteins.Cd treatment promoted the synthesis of polysaccharides in the cell wall of rice roots,especially Lu527-8.Cd in the root cell wall of rice mainly bound to pectin and HCl.Pectin and HCl of Lu527-8 had stronger Cd binding capacity than Lu527-4.The binding capacity of Cd in the cell wall without pectin significantly reduced.After the pectin was removed,there was no difference in Cd binding capacity on the root cell walls of the two rice lines,indicating that pectin was an important reason for the difference in the Cd binding capacity of the two rice lines.Lu527-8 had more low-methylesterified pectin in the root cell wall,which was one of the reasons for its stronger binding capacity of pectin.（3）Under Cd treatment,the nitric oxide（NO）concentration in the roots of two rice lines was significantly increased.Nitrate reductase（NR）and nitric oxide synthetase（NOS）are NO-related synthetases.Under Cd treatment,the NR activity was significantly increased,and the NOS activity was significantly reduced,indicating that NR may be the main functioning enzyme that caused endogenous NO synthesis.The NO synthesis was stronger than Lu527-4.In the two rice lines,sodium nitroprusside（SNP）treatment alleviated the inhabitation of root elongation and the increase of catalase and superoxide dismutase activities caused by Cd treatment,indicating that NO could alleviate the toxicity of Cd.Moreover,the Cd concentrations of the two rice lines significantly increased under SNP treatment.SNP treatment significantly increased the distribution ratio of Cd in the root cell wall,and reduced the distribution ratio of organelles.