Magnesium enhances aluminum-induced citrate secretion in rice bean roots (Vigna umbellata) by restoring plasma membrane H+-ATPase activity

We demonstrated that magnesium (Mg) can alleviate aluminum (Al) toxicity in rice bean Vigna umbellata (Thunb.) Ohwi & Ohashi] more effectively than is expected from a non-specific cation response. Micromolar concentrations of Mg alleviated the inhibition of root growth by Al but not by lanthanum, and neither strontium nor barium at the micromolar level alleviates Al toxicity. Aluminum also induced citrate efflux from rice bean roots, and this response was stimulated by inclusion of 10 microM Mg in the treatment solution. The increase in the Al-induced citrate efflux by Mg paralleled the improvement in root growth, suggesting that the ameliorative effect of Mg might be related to greater citrate efflux. Vanadate (an effective H+-ATPase inhibitor) decreased the Al-induced citrate efflux, while addition of Mg partly restored the efflux. Mg addition also increased the activity of Al-reduced plasma membrane H+-ATPase, as well as helping to maintain the Mg and calcium contents in root apices. We propose that the addition of Mg to the toxic Al treatment helps maintain the tissue Mg content and the activity of the plasma membrane H+-ATPase. These changes enhanced the Al-dependent efflux of citrate which provided extra protection from Al stress.