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Published on December 14, 2007; 10.1104/pp.107.111989

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Received October 29, 2007
Accepted December 1, 2007

Cell Wall Polysaccharides are Specifically Involved in the Exclusion of Aluminum from the Rice Root Apex

Jian Li Yang , Ya Ying Li , Yue Jiao Zhang , Shan Shan Zhang , Yun Rong Wu , Ping Wu , and Shao Jian Zheng

State Key Laboratory of Plant Physiology and Biochemistry, College of Life Sciences, Zhejiang University, Hangzhou 310058, China; Ministry of Education Key Laboratory for Environmental Remediation and Ecosystem Health, College of Environmental and Resource Sciences, Zhejiang University, Hangzhou 310029, China; and Institute of Genetics, College of Life Sciences, Fudan University, Shanghai 200433, China

Rice (Oryza sativa L.) is the most 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, Oryza sativa subsp. japonica cv. Nipponbare (an Al-resistant cultivar) and Oryza sativa subsp. indica cv. 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 methlyesterase (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 the bound Al was retained more tightly in Zhefu802, which was consistent with the 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 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 rice root apex.






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