Root mucilage enhances aluminum accumulation inMelastoma malabathricum, an aluminum accumulator

Abstract

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...

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