Abstract
In a simple calcium medium containing sucrose, cultured tobacco cells exhibit an increase in soluble sugar content, which is a driving force of water influx and is blocked by aluminum (Al). The soluble sugar content is the balance between sugar uptake and sugar consumption. We have recently found that Al inhibits the uptake of sucrose, while it is well known that Al causes the secretions of organic acids and callose. In this study, the possible contribution of the organic acid and callose secretions to the decrease in soluble sugar content under Al stress was investigated, using a pair of cell lines, V9 and T4 (V9 but over-expression line of wheat Al-tolerant gene ALMT1 encoding Al-activated malate transporter; Sasaki et al. 2004). The amount of soluble sugar decreased under Al stress was more than 100 times higher than the amounts of malate and callose secreted from cells, suggesting that the secretions of malate and callose cannot explain the decrease in soluble sugar content under Al stress. Furthermore, our results suggest that the Al-enhanced malate secretion via ALMT1 inhibits the synthesis of callose and the repression of sugar uptake inside cells.
