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Aims: As a vital polysaccharide related to mechanisms of plant resistance to trace metal in the root cell wall, the role of hemicellulose in cadmium (Cd) accumulation in hyperaccumulators is still unknown. We investigated hemicellulose modification in response to Cd in two populations of Sedum alfredii. Methods: Nonhyperaccumulating population (NHP) and hyperaccumulating population (HP) of S. alfredii were grown in nutrient solutions with or without 25 μM Cd for 15 d. Monosaccharide composition of root cell wall hemicellulose and its remolding mechanisms (e.g. enzyme activity and gene expression) were analyzed by using gas chromatography-mass spectrometer (GC-MS), fourier transform infrared spectrometer (FTIR), nuclear magnetic resonance (NMR) and quantitative real-time PCR techniques. Results: In 25 μM Cd treatment, root cell wall hemicellulose in the NHP significantly (P < 0.05) increased and its hemicellulose-bound Cd was nearly 2.5-fold higher than that of HP. In the presence of Cd, xylose and glucose, proved to be the main component of hemicellulose, were higher in the NHP than in the HP owing to the up-regulation of XET/XEH and encoding gene (XTH 31). 113Cd-NMR and FTIR results indicated that hemicellulose with hydroxyl and carboxyl groups of HP retained less Cd than that of NHP. Conclusion: Hemicellulose modification decreased the Cd-binding capacity of the root cell wall and increased the entry of Cd in the shoot of HP S. alfredii. [ABSTRACT FROM AUTHOR]

Background and aims: 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. Methods: 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. Results: 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 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. Conclusions: 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. [ABSTRACT FROM AUTHOR]