Characterization of root mucilage from Melastoma malabathricum, with emphasis on its roles in aluminum accumulation

Plant roots exude viscous polysaccharides, called mucilage. One of the suggested roles of mucilage is immobilization of toxic metal cations, including aluminum (Al), in the rhizosphere. Mucilage exuded from roots of Melastoma malabathricum (Al accumulator) was characterized in comparison with that of Zea mays (maize; Al nonaccumulator). Removal of mucilage significantly reduced Al accumulation in M. malabathricum. The cation adsorption affinity of M. malabathricum mucilage was higher for Al and lanthanum (La) than for barium (Ba), whereas that of maize mucilage was in the order Ba > La > Al. A Al-27 nuclear magnetic resonance (NMR) spectrum of the Al-adsorbed mucilage and bioassay with alfalfa seedlings indicated that the concentrated Al in the mucilage of M. malabathricum, unlike that of maize, bound very weakly to cation exchange sites of mucilage. The higher charge density in M. malabathricum mucilage, derived from unmethylated uronic acid, is inferred to be related to preferential adsorption of trivalent cation. Not only a higher degree of methylation in the uronic acid (glucuronic acid) but also H+ release from roots to the mucilage appears to be responsible for the loose binding of Al in M. malabathricum mucilage. These characteristics of mucilage may help Al hyperaccumulation in M. malabathricum.