Aluminum accumulation in the roots of Melastoma malabathricum, an aluminum-accumulating plant

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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.