Mechanisms Of Cadmium Retention In The Root Cell Wall Of A Cadmium-safe Rice Line (Oryza Sative L.)

Cadmium?Cd?pollution in farmland,if no control and/or remediation measures taken,can be a permanent menace to human health via food chain.Cultivation of the low grain-Cd-accumulating rice is of realistic significance to ensure food safety.A clear understanding of the physiological mechanism of low Cd accumulation in rice provides a theoretical basis for the cultivation of Cd safe rice cultivars.In this study,hydroponic experiments were carried out to reveal the Cd distribution in root and the physiological responses of a Cd-safe rice line designated D62B compared with a common rice line designated Luhui17.The main results are as follows:?1?Cd accumulated in plant of two rice lines increased with increasing Cd concentrations in hydroponic media.There was 53.00%-80.43%of total Cd in plant retained in roots of two rice lines.D62B accumulated significantly lower Cd in different parts with a lower translocation factor of Cd to shoot compared with Luhui17.There was about 69.99%-80.43%of total Cd in plant retained in root of D62B,suggesting a significant higher proportion of Cd in root compared with Luhui17.D62B showed a superior capacity to retain Cd in root so that less Cd was translocated to shoot.Majority of Cd in root of two rice lines was observed in the soluble fraction and cell wall.The proportion of Cd in root cell wall of D62B was 32.10%-36.87%,higher than that in Luhui17.About 70.90%-83.23%of total Cd in root cell wall of two rice lines was mainly bound to pectin and hemicellulose 1.D62B showed significant lower Cd concentrations in pectin and hemicellulose 2 of root cell wall in comparison with Luhui17.While Cd concentration in hemicellulose 1 of root cell wall of D62B was 1.22-1.46 times higher than that of Luhui17.The amount of Cd per uronic acid in pectin of root cell wall of two rice lines was apparently higher than those in hemicellulose 1 and hemicellulose 2,which was3.13-8.96 times of hemicellulose 1 and 8.44-13.05 times of hemicellulose 2,respectively.The proportion of Cd in pectin of root cell wall in two rice lines increased with increasing Cd concentrations in hydroponic media,which increased more in D62B.The uronic acid concentration of pectin in root cell wall of D62B was 1.18-1.25 times higher than that of Luhui17,showing a higher Cd bind capacity in D62B.With a higher amount of Cd per uronic acid,the hemicellulose 1 in root cell wall of D62B was superior to retain Cd in cell wall compared with Luhui17.?2?The adsorption of Cd on cell wall components of two rice lines increased with increasing Cd concentrations in solution and the extension of adsorption time.The isothermal adsorption characteristics of Cd on cell wall components of two rice lines fitted the improved Langmuir model well with a R² in 0.988-0.998,indicating a process of multimolecular adsorption.Cd adsorption on cell wall fractions occurred in two steps:an initial fast step which lasts for 60 min?shorter time?followed by the slower second phase?longer time?which continued until the equilibrium was reached.Adsorption dynamics analysis indicates that pseudo-second-order model could be described very well to the adsorption of Cd on cell wall fractions of two rice lines,suggesting that the adsorption process was effected by chemical adsorption.Intra-particle diffusion process is not the unique rate-limiting step.Cd adsorption on cell wall fractions decreased with the remove of pectin and hemicellulose 1.Pectin and hemicellulose 1 played a greater role in Cd adsorption on cell wall,in which the capacity of pectin to bind with Cd was superior in pectin.The root cell wall of D62B showed a higher maximal Cd adsorption capacity to Cd in comparison with Luhui17,about 1.32 times higher.Data of FTIR of D62B revealed the involvement of more functional groups including carboxylic groups of galacturonic acid in pectin,oxhydryls of polysaccharide chain,amidogen of protein,sulphates and phosphate in Cd adsorption on cell wall.D62B showed a higher amount of negatively charged functional groups in root cell wall in comparison with Luhui17.As a better capacity of pectin and hemicellulose 1 to bind with Cd in D62B,Cd was quickly adsorbed on the root cell wall of D62B in comparison with Luhui17.?3?Cd treatments promoted the synthesis and components adjustment of pectin in root cell wall of two rice lines.The total polysaccharides concentration and uronic acid concentration of pectin in root cell wall of two rice lines increased significantly with increasing Cd concentrations in hydroponic media.The increase of total polysaccharides concentration of high demethylated pectin was greater than that of low demethylated pectin,and new high demethylated pectin was synthesized in cell wall.The uronic acid concentration of both low and high demethylated pectin in root cell wall of D62B were significantly higher than that of Luhui17,which was 1.05-1.46 times and 1.18-1.27 times,respectively.D62B showed a higher degree of pectin esterification in comparison with Luhui17.Cd promoted the pectin methylesterase activity in root cell wall of two rice lines to increase the demethylation of pectin.The pectin methylesterase activity in root cell wall of D62B was 1.37-1.95 times higher than that of Luhui17.In addition,Cd induced the accumulation of H₂O₂ in root,which would regulate the degree of pectin esterification through PME improvement and non-enzymatic pathway.As significant lower H₂O₂concentration and cell wall POD activities in root of D62B,the non-enzyme regulatory pathway might contribute less to pectin esterification degree in D62B.