Aluminum regulation of citrate metabolism for Al-induced citrate efflux in the roots of Cassia tora L.

Abstract

Aluminum-induced exudation of organic acid from plant roots has been proposed to be an important mechanism for Al tolerance. In search for the physiological basis of citrate efflux, the activities of several enzymes involved in citrate metabolism and internal citrate accumulation in the root tips of Cassia tora L. were examined. Results indicated that aluminum at 10–50 μM progressively stimulated activities of citrate synthase (CS, EC 4.1.3.7). A time-course experiment revealed that the increase in CS activities occurred within 3 h after the Al exposure, while the substantial citrate exudation began 6 h after Al treatment, suggesting that citrate exudation could be due to the increased CS capacity. By contrast, aconitase (Aco, EC 4.2.1.3) activities in Al-treated root tips were always suppressed compared with controls, indicating that aconitase was possibly involved in the aluminum-induced citrate efflux. The increase in CS activity and decrease in aconitase activity with Al treatments were associated with the enhanced citrate accumulation in root tips. We also examined NADP-isocitrate dehydrogenase (NADP-ICDH, EC 1.1.1.42), malate dehydrogenase (MDH, EC 1.1.1.37) and phosphoenolpyruvate carboxylase (PEPCase, EC 4.1.1.31) in root tips exposed to Al. However, none of the enzymes showed significant alterations in activities. These results suggested that NADP-ICDH, MDH and PEPCase were not directly involved in the Al-induced citrate exudation in the roots of C. tora.