Objectives We compared the properties of cell wall pectin in roots of cadmium (Cd)-safe and common rice line when exposure to Cd stress, to fully understand the mechanism of Cd retention by root of Cd-safe rice line.

Methods A hydroponic experiment was conducted using a Cd-safe rice line D62B and a common rice line Luhui17 as the comparing material. The Cd concentration of nutrient solution included 0 (CK), 0.5 mg/L (Cd0.5), 1.0 mg/L (Cd1) and 2.0 mg/L (Cd2). At the tillering stage, root samples were collected. The Cd concentrations in cell wall polysaccharides, including pectin, hemicellulose 1, hemicellulose 2 and residue fraction, were determined respectively. Furthermore, the uronic acid concentration of pectin, pectin esterification degree, pectin methyl esterase activity (PME), root hydrogen peroxide (H₂O₂) concentration and cell wall peroxidase (POD) activity were detected, so as to clarify the response characteristics of root cell wall pectin to Cd.

Results 1) Cd treatment promoted the pectin synthesis of root cell wall in both rice lines, and the uronic acid concentrations of low and high esterified pectin in the root cell wall of D62B were higher than those of Luhui17. Compared with CK, the uronic acid concentrations of low esterified and high esterified pectin in the root cell wall of D62B were increased by 13.21%–71.82%, 22.10%–64.27%, and those in Luhui17 were increased by 24.14%–137.86%, 13.12%–41.26%, respectively, under Cd treatment. 2) The pectin esterification degree and PME activity in root cell wall of D62B and Luhui17 were higher than those of CK under Cd1 and Cd2 treatment. The PME activity in root cell wall of D62B was significantly higher than that of Luhui17 under the same Cd stress. 3) Cd induced H₂O₂ accumulation and influenced peroxidase activity in the roots of both rice lines. Under Cd stress, the H₂O₂ concentration and peroxidase activity of D62B roots were lower than those of Luhui17, indicating that pectin methyl esterase had stronger demethylation effect on pectin.

Conclusions When exposed to Cd stress, Cd-safe rice line D62B showed stronger ability of low and high esterified pectin synthesis and had higher PME activity in its root cell wall than that of common material, leading to provide more Cd binding sites and show stronger Cd binding ability, which was conducive to the retention of Cd in D62B roots.