Root cell wall polysaccharides are involved in cadmium hyperaccumulation in Sedum alfredii

The role of polysaccharide modification in metal accumulation in hyperaccumulators is still unknown. Our aim was to compare the differences in the role of root cell-wall polysaccharides in cadmium (Cd) accumulation between hyperaccumulating (HE) and non-hyperaccumulating ecotype (NHE) of Sedm alfredii. Hydroponic experiments were performed to characterize root-to-shoot Cd translocation, cadmium species and polysaccharide modification in root cell-wall of S. alfredii using stable isotope tracing, X-ray absorption near edge structure and immunofluorescence localization techniques. Cd absorbed was more readily available for transport to the shoots by the HE roots than by the NHE roots, which is confirmed by a 6-fold higher Cd-113 concentration in xylem sap. Root Cd efflux originated mainly from the cell walls. The concentration of cell-wall polysaccharides and activity of pectin methylesterase were higher in the NHE than in the HE in the absence of Cd, and even higher in the presence of Cd. More pectins were methylated in the HE than in the NHE, indicating more free pectic acid residues in the NHE. The cell-wall-bound Cd was retained more tightly in the NHE than in the HE. Cadmium hyperaccumulation by HE of S. alfredii is associated with its enhanced Cd flux into the xylem, which is partly regulated by cell-wall polysaccharide modification in roots.