The Role Of Pectin In The Accumulation Of Cadmium In The Root Cell Wall Of A High Cadmium-accumulating Rice Line

Root is the first organ of plants when exposed to cadmium（Cd）,the Cd accumulated in root is also the source of Cd accumulation in shoot.Root cell wall is the preferential and major site for Cd binding,in which pectin makes an important contribution.Cd stress induces hydrogen peroxide（H₂O₂）synthesis in rice roots,and H₂O₂ has been demonsdrated to regulate pectin synthesis and modification in plants.Thus,it has important benefits to discuss the role of pectin in Cd accumulation in root and the regulation of H₂O₂ in this process,which can provide a theoretical basis for perfecting the theory construction of Cd accumulation in high Cd-accumulating rice lines and promote the application of the specific rice genotypes in phytoremediation.In this study,the high Cd-accumulating rice Lu527-8was used as the research object,and the normal rice Lu527-4 with similar genetic background was selected as the control to explore the role of pectin and its fraction in the Cd accumulation in root cell wall of the rice.The mechanism of H₂O₂ regulates pectin synthesis and modification induced by Cd stress was further explored through hydroponic experiments.The main research results are as follows:（1）The roots and shoots of Lu527-8 showed stronger Cd accumulation than Lu527-4in both hydroponics and soil-culture experients.The Cd in the root cell wall of Lu527-8 and Lu527-4 accounted for 34%and 32%of the total Cd in the roots under hydroponics,and 37%and 35%in the soil culture,and the Cd concentration in the root cell wall of Lu527-8 was significantly higher than that of Lu527-4.The concentrations of galacturonic acid in the root cell walls of Lu527-8 significantly increased under Cd stress,and the concentrations of xylose,arabinose,galactose,galacturonic acid,glucose,mannose and mannuronic acid in Lu527-8 were significantly higher than that of Lu527-4.The Cd concentration of the cell wall polysaccharide components of the two rice roots was highest in pectin,followed by hemicellulose 1 and hemicellulose 2.The Cd in the pectin of Lu527-8 and Lu527-4accounted for both 82%of the total Cd in the roots cell walls under hydroponics,and 79%and 78%in the soil culture,and the Cd concentration in pectin of Lu527-8 was significantly higher than that of Lu527-4.Cd concentration of different pectin fractions was CDTA-soluble pectin>Alkali-soluble pectin>Water-soluble pectin,and the Cd binding amount of chelating pectin and alkali-soluble pectin in Lu527-8 was significantly higher than that of Lu527-4.（2）The Cd fluorescence intensity and pectin concentration of Lu527-8 roots were significantly higher than those of Lu527-4 when exposed to Cd.Under Cd stress,the expression levels of pectin synthesis genes Os GAUT1,Os GAUT4 and Os QUA1 in the roots of the two rices increased by 91%-226%,and the expression levels in Lu527-8 were 1.33,1.43,and 2.81 times of Lu527-4,respectively.The relative expression levels of the pectin methylesterase activity-related genes Os PME12 and Os PME14 in the two rices were up-regulated by 70%-470%under Cd stress,which significantly enhanced pectin methylesterase activity,increased the concentration of low-esterified pectin.The pectin methylesterase activity and low esterified pectin concentration in Lu527-8 were higher than those in Lu527-4.（3）Cd stress induced H₂O₂ production in rice roots,which in Lu527-8 was significantly higher than Lu527-4.The Cd concentration in the roots of the two rices increased by the addition of exogenous H₂O₂,and the Cd concentration in the roots of Lu527-8 decreased after the addition of 4-hydroxy-TEMPO（H₂O₂ scavenger）,indicating that H₂O₂ participates in the regulation of Cd accumulation in rice roots.At the subcellular level,exogenous H₂O₂significantly increased the Cd concentration of the cell wall and soluble fraction in the two rice roots,while 4-hydroxy-TEMPO decreased the Cd concentration of the cell wall,suggesting that H₂O₂ regulates Cd accumulation in root cell wall.The H₂O₂ concentration,root Cd concentration and cell wall Cd concentration of Lu527-8 were significantly higher than those of Lu527-4 after adding exogenous H₂O₂ and 4-hydroxy-TEMPO.It showed that H₂O₂ induced by Cd stress could increase the abundance of anionic functional groups such as hydroxyl（-OH）and reduce the ester（C-H）functional groups in pectin,and the abundance of-OH functional groups in the root cell wall of Lu527-8 were higher.82%of Cd in cell wall polysaccharides was distributed in pectin,and exogenous H₂O₂ significantly increased the pectin concentration and Cd concentration in root cell wall of the two rice lines,while 4-hydroxy-TEMPO showed the opposite impact.Under Cd stress,H₂O₂ increased pectin methylesterase activity and low-esterified pectin by up-regulating the expression of Os PME14 in two rices and Os PME12 in Lu527-8.The higher H₂O₂ concentration in roots can be an important reason that explain the higher Cd accumulation in Lu527-8.