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ENVIRONMENTAL STRESS AND ADAPTATION TO STRESS

Cell Wall Polysaccharides Are Specifically Involved in the Exclusion of Aluminum from the Rice Root Apex¹

State Key Laboratory of Plant Physiology and Biochemistry, College of Life Sciences, Zhejiang University, Hangzhou 310058, China (J.L.Y., Y.J.Z., Y.R.W., P.W., S.J.Z.); Ministry of Education Key Laboratory for Environmental Remediation and Ecosystem Health, College of Environmental and Resource Sciences, Zhejiang University, Hangzhou 310029, China (J.L.Y., Y.Y.L.); and Institute of Genetics, College of Life Sciences, Fudan University, Shanghai 200433, China (S.S.Z.)

Rice (Oryza sativa) is the most aluminum (Al)-resistant crop species among the small-grain cereals, but the mechanisms responsible for this trait are still unclear. Using two rice cultivars differing in Al resistance, rice sp. japonica ‘Nipponbare’ (an Al-resistant cultivar) and rice sp. indica ‘Zhefu802’ (an Al-sensitive cultivar), it was found that Al content in the root apex (0–10 mm) was significantly lower in Al-resistant ‘Nipponbare’ than in sensitive ‘Zhefu802’, with more of the Al localized to cell walls in ‘Zhefu802’, indicating that an Al exclusion mechanism is operating in ‘Nipponbare’. However, neither organic acid efflux nor changes in rhizosphere pH appear to be responsible for the Al exclusion. Interestingly, cell wall polysaccharides (pectin, hemicellulose 1, and hemicellulose 2) in the root apex were found to be significantly higher in ‘Zhefu802’ than in ‘Nipponbare’ in the absence of Al, and Al exposure increased root apex hemicellulose content more significantly in ‘Zhefu802’. Root tip cell wall pectin methylesterase (PME) activity was constitutively higher in ‘Zhefu802’ than in ‘Nipponbare’, although Al treatment resulted in increased PME activity in both cultivars. Immunolocalization of pectins showed a higher proportion of demethylated pectins in ‘Zhefu802’, indicating a higher proportion of free pectic acid residues in the cell walls of ‘Zhefu802’ root tips. Al adsorption and desorption kinetics of root tip cell walls also indicated that more Al was adsorbed and bound Al was retained more tightly in ‘Zhefu802’, which was consistent with Al content, PME activity, and pectin demethylesterification results. These responses were specific to Al compared with other metals (CdCl₂, LaCl₃, and CuCl₂), and the ability of the cell wall to adsorb these metals was also not related to levels of cell wall pectins. All of these results suggest that cell wall polysaccharides may play an important role in excluding Al specifically from the rice root apex.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Shao Jian Zheng (sjzheng{at}zju.edu.cn).

www.plantphysiol.org/cgi/doi/10.1104/pp.107.111989

^* Corresponding author; e-mail sjzheng{at}zju.edu.cn.

Received October 29, 2007; accepted December 1, 2007; published December 14, 2007.