Characterization of cadmium accumulation in the cell walls of leaves in a low-cadmium rice line and strengthening by foliar silicon application

Cadmium (Cd) remobilization in leaves is affected by whether Cd is stored in nonlabile subcellular compartments, which might be regulated by silicon (Si) application. However, the underlying mechanism is still far from being completely understood. In this research, the Cd distribution pattern in leaves and a Cd-binding characterization in the cell wall of the low-Cd rice line YaHui2816 were investigated through one hydroponic experiment with 10 μM Cd in solutions. Foliar Si application was further adopted to explore its influence on the Cd accumulation in the cell walls of leaves in YaHui2816. Most of the Cd (69.4%) was distributed in the cell walls of YaHui2816 leaves, whereas the isolated cell walls of leaves from YaHui2816 exhibited a lower capacity for Cd chemisorption than the contrasting line C268A, which was resulted from its fewer relative peak areas of functional groups in the cell wall, such as carboxyl CO and OH stretching. Foliar Si application significantly increased the Cd concentration in leaves and various cell wall fractions (pectin, hemicellulose 1 and residue) by 191% and 137-160%, respectively. RNA-seq analysis revealed that foliar Si application depressed the expression of the metal transporters OsZIP7 and OsZIP8, up-regulated the expression of genes participating in the glutathione metabolism and the cellulose synthesis. Overall, the influence of foliar Si application on Cd-accumulation in the cell wall of leaves in a low-Cd rice line was demonstrated in this research, which inspires further avenues to ensure the food safety of rice grains.