Molecular mechanism of Al tolerance by the small molecules produced in plant.

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Al induced secretion of oxalic acid in buckwheat was investigated. Secretion of oxalic acid was maintained for 8 h by a 3-h pulse treatment with 150muM Al. Oxalic acid was found to be sereted in the region 0 to 10 mm from the root tip. Anion-channel inhibitor, phenylglyoxal (PG), inhibited the secretion of oxalic acid by 50%. Root elongation in buckwheat was not inhibited by 25muM Al or p10mM PG alone but was inhibited by 40% in the presence of Al and PG.Furthermore, Al resistant buckwheat accumulate Al in the leaves and roots. The internal detoxification mechanism was studied. About 90% of Al accumulated in the leaves was found in the cell sap, in which the dominant organic acid was oxalic acid.Purification of the Al complex in the cell sap of leaves resulted in a complex with a ratio of Al to oxalic acid of 1 : 3. Al tolerance in the roots and leaves of buckwheat is achieved by formation of a nonphytotoxic Al-oxalate (1 : 3) complex.Al enhances ferrous ion Fe^<2+>-mediated peroxidation of lipids. Chlorogenic acid (CGA) and caffeic acid (CA) protected the cells from both lipid peroxidation and the loss of viability caused by the combined application of Al and Fe^<2+>, indicating that the phenylpropanoids acted as antioxidant molecules.