Abstract
This article reports on the realtions between the activities of enzymes, such as phenylalanine–ammonia lyase (PAL) and glutathione-s-transferase (GST), and the concentrations of total tannins, total glutathione (TGSH), total phytochelatins (TPC), and lipid peroxidation (MDA) in leaves and roots of sorghum cultivars 132 and 552 that are exposed to four concentrations (0, 10, 20, and 30 mg l−1) of AlCl3 in a mixture of river sand and peat. The Al3+ significantly increased the level of tannins, TGSH, and TPC in the roots and leaves of both cultivars. In the roots of both cultivars, the production of TPC was significantly coupled to decrease in the total TGSH. The concentration of TGSH in the leaves of cultivar 132 was found to be higher than in those of cultivar 552. In the plants treated with 10, 20, and 30 mg l−1 Al3+, the PAL activities in leaves of both cultivars increased (by 73, 44, and 18 %, respectively), the TGSH in the roots of cultivar 552 declined (by 50, 45, and 23 %, respectively), the GST activities in the leaves of both cultivars were higher (90, 98, and 100 %, respectively) than those of the control plants. Al3+ also enhanced levels of MDA in the leaves and roots. These results suggested that the increase in PAL and GST activities might be controlled by antioxidant potentials and different routes of carbon channeling in the leaves. In cultivar 552, antioxidant compounds such as TPC and TGSH with rapid turnover and high accumulation were more effective than tannins for leaves because tannin was low. In cultivar 132, the amount of tannins was high and stable; therefore they do not need high accumulation of TPC in leaves. The depletion of TGSH can be ascribed to the Al3+-induced TPC synthesis in the leaves and roots of cultivar 552. The syntheses of TPC and MDA can be related to changes in TGSH and tannins, suggesting that TGSH and tannins are normally involved in Al3+ sequestration under conditions of subtoxic exposure. The increased TPC in the roots could provide an effective means of restricting Al3+ to these organs by chelating. In cultivar 552, TGSH contents may have been consumed for two strategies: the maintenance of regular redox potential, and the precursor for TPC.
Highlights
Most strategies of plant tolerance to Al3? exposure are based on the reduction, by maintaining the cytosolic concentration of free Al3?
A positive relationship was found between GST activity and TGSH in the roots of two cultivars (Table 3, roots 132; R2 = 0.98; y = 0.0.0026x ? 0.16 and roots 552; R2 = 0.8; y = -0.0021x ? 0.21, P \ 0.005)
Roots of Al3?treated cultivar 552 generally appeared darker than control plants, perhaps Al3? stimulated efflux of phenolic and organic acids, due to the fact that the surface of roots was darker, but the cultivar 132 grown at 30 mg l-1 was low chlorotic, appearing slightly smaller than controls [20], [21], [24] and [32]
Summary
Most strategies of plant tolerance to Al3? exposure are based on the reduction, by maintaining the cytosolic concentration of free Al3?. Most strategies of plant tolerance to Al3? Exposure are based on the reduction, by maintaining the cytosolic concentration of free Al3? In this way, the plant cell avoids accumulation of Al3? By compartmentalization, this distribution is not clearly established yet [17]. Al3?-induced exudation of the flavonoid-type tannins {catch in} and quercetin from 10-mm root tips in an Al3?-resistant maize variety [2, 6]. The Al3?-induced exudation of {catechin} at the rate of 100 lmol per tip h-1 in comparison with citrate, which did not exceed 1 gmol per tip h-1, a rate reported for citrate exudation in maize by other authors [6]
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