Watanabe Toshihiro, Misawa Seiji, Hiradate Syuntaro, Osaki Mitsuru
Plant roots exude various chemical compounds. Mucilage is the gelatinous high molecular weight compound, which consists mainly of polysaccharides, and is exuded from the outer layers of the root cap. One of the suggested roles of mucilage for plant growth is detoxification (fixation) of toxic metal cations, including aluminium (Al) (Horst et al. 1982). Polysaccharides in mucilage contain uronic acids, of which carboxyl groups may adsorb and inactivate the cations in the rhizosphere (Mench et al. 1987). In the present study, characteristics of root mucilage were compared between Melastoma malabathricum (Al accumulator) and Zea mays (Al non-accumulator), with emphasis on Al adsorption. Selectivity for the absorption of Al, La, and Ba in the isolated mucilage was determined to know its characteristics as the cation exchangers. Mucilage of M. malabathricum adsorbed more Al and La (trivalent cation) than Ba (divalent cation). On the other hand, the cation adsorption affinity of Z. mays mucilage was in the following order; Ba>La>Al. The 27Al NMR spectrum of the Al-adsorbed mucilage indicated that Al was concentrated in the mucilage of M. malabathricum, and binds very weakly to the mucilage. These results suggest that roots of M. malabathricum can easily absorb concentrated Al in the mucilage and the mucilage helps Al-hyperaccumulation in this species. In contrast, it has been reported that Al binds very tightly to the mucilage in Z. mays (Li et al. 2000). Total sugar concentrations in the mucilage are significantly higher in M. malabathricum than in Z. mays, and the mucilage of M. malabathricum contained a lower proportion of fucose and arabinose and higher proportion of xylose and glucuronic acid than that of Z. mays. Whereas glucuronic acid (uronic acid) is the primary monosaccharide component of M. malabathricum mucilage, the degree of methylation was significantly lower in Z. mays than in M. malabathricum. These differences may be related to the different characteristics in Al adsorption between M. malabathricum and Z. mays.
References
Horst WJ et al. 1982. Z. Pflanzenphysiol., 109, 95-103
Li XF et al. 2000. Physiol. Plant., 108, 152-160
Mench M et al. 1987. Biol. Fertil. Soils, 3, 165-169