Summary• 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 27 Al 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.

Root mucilage is gelatinous polysaccharide-containing material exuded from the outer layers of the root cap. Although mucilage has been suggested to play several roles in plant growth, its role in mineral uptake has not been well understood. Melastoma malabathricum L. is an aluminum (Al) accumulator growing in tropical acid soils. This species accumulates more than 10 mg Al g -1 DW in leaves and roots. Root mucilage is generally known to immobilize metal cations such as Al in the rhizosphere. However, we found that roots of M. malabathricum exuded large amounts of mucilage. Using the Zea mays L. mucilage as a control, we have recently shown that mucilage of M. malabathricum has unique physical and chemical characteristics, and facilitates Al uptake in this species. Since M. malabathricum cannot grow well in Al-deficient soil (nonacid soils), this species might have developed a mechanism for Al acquisition. We have also discussed the reason for this species' requirement of Al, a nonessential element.Plant roots exude various chemical compounds. Root mucilage is a gelatinous high molecular weight compound, consisting mainly of polysaccharides, and is exuded from the outer layers of the root cap. One of the suggested roles of mucilage in plant growth is detoxification (fixation) of toxic metal cations, including Al in acidic soils. 1 Polysaccharides in mucilage contain uronic acids, 2 the carboxyl groups of which might adsorb and inactivate Al in the rhizosphere. 3 However, Li et al.,4 suggested that the contribution of mucilage to Al inactivation in the rhizosphere of Zea mays L. is almost negligible owing to its small amount.Melastoma malabathricum L. is an Al accumulator growing in tropical acidic soils and is one of the dominant woody species growing in acid sulfate soils in tropical Asia, Australia and Polynesia. This species accumulates more than 10 mg Al g -1 DW in leaves and roots. 5 It is known that the growth of non-essential element-accumulating plants is often enhanced by the application of the accumulated element (e.g., Na in a Na accumulator plant). The growth of M. malabathricum is enhanced by Al application under hydroponic conditions. 6 It has been predicted that this growth enhancement is due to alleviation of iron toxicity 7 or a disorder of organic acid metabolism in the absence of Al. 8 Recently, we observed that the roots of M. malabathricum exuded large amounts of mucilage under "Al deficient" conditions, raising a question about the role of mucilage in this species. Our recent study indicated that M. malabathricum mucilage facilitates Al uptake in this species. 9 Contrastingly, uptake of other cations such as K, Ca and Mg were not affected. To the best of our knowledge, this is the first study to demonstrate that mucilage selectively enhances mineral uptake in plant. To elucidate the factors that were responsible for these characteristics of M. malabathricum mucilage, we compared the physical and chemical properties of M. malabathricum mucilage with that of Z. mays mucilage, which is known t...

Generally, plants that have Al levels of at least 1000 mg·kg–1 in their leaves are defined as Al accumulators. These plants are often found in very acid soils in the tropics. The mechanisms of Al uptake in strong Al accumulators are still unclear. In this study, we investigated the characteristics of Al uptake and accumulation in the roots of Melastoma malabathricum L., an Al-accumulating plant that grows in acidic soils in the tropics. Melastoma malabathricum roots hardly absorb any La, possibly because of lower affinity of the root apoplast to La than to Al. Exposure to La did not affect the concentration of citrate in the roots; however, application of Al increased the citrate level considerably, corresponding with the amount of Al accumulation in the symplast. 27Al NMR analysis revealed that Al complexes with oxalate, but not with citrate, in the roots of M. malabathricum. This investigation revealed that oxalate, which occurs constitutively at high concentrations, is a ligand for Al accumulation in both root and shoot tissue, and that citrate, the synthesis of which is induced by Al application, is a ligand mainly used for Al translocation from the roots to the shoots.

It is well known that lupin forms cluster roots, which help in dissolving insoluble P in soils. In nonleguminous species, cluster roots also appear to contribute to the utilization of organic N in soils. In white lupin ( Lupinus albus L.), however, the characteristics of its organic N utilization have not been studied. Therefore, we examined whether white lupin can utilize organic N in soils. Soybean ( Glycine max (L.) Merr.), which does not form cluster roots, was used as a control plant. Seedlings of lupin and soybean were cultivated in soils with different N sources (non-N, ammonium sulphate, ammonium sulphate plus cattle farmyard manure, or cattle farmyard manure). The rate of glycine uptake by excised roots was determined in a hydroponic experiment to investigate the ability of lupin and soybean to directly utilize amino acids. Nitrogen accumulation in soybean corresponded to the decrease in inorganic N in the soils. In contrast, N accumulation in lupin was higher than the decrease in inorganic N in the soil, especially with the cattle farmyard manure treatment, indicating that lupin derived more N from an organic N source. Wheat ( Triticum aestivum L.) cultivated with lupin in a pot accessed more available N than wheat with soybean or wheat in monoculture, suggesting that lupin roots themselves or the lupin rhizosphere microorganisms were able to decompose organic N in soils. Excised roots of lupin, especially cluster roots, exhibited higher rates of glycine uptake than roots of soybean. In conclusion, lupin decomposed organic N in the rhizosphere and was able to absorb amino acids from decomposition in addition to any inorganic N produced by further microbial decomposition.