Aluminium tolerance is achieved by exudation of citric acid from roots of soybean (Glycine max)

Fourteen soybean (Glycine max [L.] Merr.) cultivars were analysed and found to differ considerably in aluminium (Al) resistance. The cultivars Suzunari (Al-resistant) and Shishio (Al-sensitive) were selected for further analysis of physiological mechanisms of Al resistance. The relative root growth of Shishio was 48% compared to 76% for Suzunari in response to 15 mu M Al (24 h). Aluminium accumulation and Al-induced callose formation in root apices were 50 and 25% of that in Suzunari, respectively. Al inhibited both Suzunari and Shishio during the first 6 h of exposure. However, the root growth inhibition was further increased in Shishio but not in Suzunari, suggesting an Al-induced Al-resistant mechanism operating in Suzunari. Organic acid analysis in root exudates of both cultivars revealed that they specifically exuded citrate in response to Al. However, the titrate exudation rate was significantly higher in Suzunari during the 6 h/24 h Al treatment, which was 52/330 compared to Shishio's 26/118 (nmol [g root fresh weight](-1) [6 h](-1)), respectively. This Al-induced citric acid exudation was found to be specific for Al, as several other metals failed to induce citrate exudation in both cultivars. Fourteen days of P deficiency did not elicit citrate excretion in both cultivars, while application of Al to P-deficient plants rapidly induced citrate exudation in both cultivars, confirming the specificity of the response of these soybean cultivars to Al. To our knowledge, this is the first report demonstrating an Al-exclusion mechanism in soybean cultivars, which is conferred by enhanced and specific Al-induced exudation of citrate.