Silicon reduces cadmium accumulation in Moso bamboo (Phyllostachys edulis) root cell sap by sequestering cadmium in hemicellulose 1

Abstract

Moso bamboo is one of the most important economic bamboo species in China, but cadmium (Cd) pollution has become a potential threat of its sustainable development. Silicon (Si) reduces Cd accumulation in many plant species. However, the exact mechanisms of this effect in Moso bamboo are still poorly understood. Here, we investigated the effect of Si on Cd accumulation in Moso bamboo in terms of Cd concentration in roots, Cd cellular and subcellular distribution, root cell morphology, and gene expression. Seedlings (ten days old) were exposed to different concentrations of Cd (0, 1, 5, and 50 μmol L^-1) in a 0.5 mmol L^-1 CaCl₂ solution treated with (+Si) and without (-Si) 1 mmol L^-1 Si (as silicic acid) for two days. The effect of Si on the alleviation of Cd-induced inhibition of root elongation was not obvious, but Si could significantly reduce Cd accumulation in roots at all tested Cd concentrations (1, 5, and 50 μmol L^-1). Cadmium was localized in all cells of roots, but Si application altered the Cd distribution from all cells to distal side of exodermis cells in roots. Semi-quantitative determination of Cd using energy-dispersive X-rays revealed higher Cd concentrations in exodermis, but lower concentrations in the stele when Si was applied. However, Si increased Cd accumulation in root cell wall, but decreased it in cell sap. Moreover, more than 70% of Cd and Si were found in hemicellulose 1 of the cell wall. These results suggested that Si reduced Cd accumulation by sequestering Cd in hemicellulose 1 in the root cell wall at the subcellular level and retaining most of the Cd in the root exodermis at the cellular level in Moso bamboo under short-term Si application.