Accumulation and fixation of Cd by tomato cell wall pectin under Cd stress

The plant cell wall acts as a barrier to prevent heavy metal ions from entering the plant cytoplasm. To study the mechanism of Cd fixation by tomato (Solanum lycopersicum) cell wall pectin, the expression levels of pectin synthesis and pectin-modifying enzyme-related genes were determined in this study. Cell wall composition analysis was also carried out to explore the changes in chemical groups related to metal ion binding. The results showed that cell wall pectin plays an important role in heavy metal binding. The expression of the pectin-related genes SlQUA1 and SlPME1 was upregulated by 2.1–6.5-fold and 2.38–5.09-fold with in 6–24 h, respectively. Furthermore, the pectin content and SlQUA1 expression increased in a Cd dose-dependent manner. The pectin methylesterase activity was also greatly upregulated by 121.19%, 187.95% and 227.91% under high concentrations of Cd in roots, stems and leaves, respectively. The esterification and pectinase modification of the root cell wall, stem cell wall and leaf cell wall reduced Cd accumulation by approximately 35–60% (P < 0.05). These results indicate the important role of cell wall pectin in Cd fixation and provide some new insights into the accumulation of heavy metals in tomato cell walls.