Aluminum stress induces up-regulation of an efficient antioxidant system in the Al-tolerant maize line but not in the Al-sensitive line

The response of the antioxidant enzymes, superoxide dismutase (SOD; EC 1.15.1.1) and peroxidase (POD; EC 1.11.1.7), to Aluminum (Al) stress was studied in roots of two inbred lines of maize (Zea mays L.) differing in their tolerance to Al. In addition, the production of malondialdehyde (MDA) was measured to evaluate the level of lipid peroxidation as well as the accumulation of proline (Pro) and carbohydrates under 72 h Al stress. Roots of Al (0, 120, 240, 360 and 480 μM, at pH 4.2) -treated plants were sampled at various times (12, 24, 48, 72 h) after commencement of stress. A major difference in the antioxidant enzymes between the two maize lines associated with Al tolerance was observed after 24 h of Al exposure. A gradual increase in the membrane lipid peroxidation in Al-stressed root of the susceptible maize line was accompanied by decreased activities of the antioxidant enzymes SOD and POD. In contrast, increased activities of the SOD and POD were found in Al-treated roots of the tolerant maize line, in which the level of membrane lipid peroxidation remained almost unchanged. After 72 h exposure to 480 μM Al the accumulation of Al in roots was almost from 90 times (tolerant) to 140 times (sensitive) than the control (without Al), while at the same time Al treatment resulted in 2.2 to 2.5-fold (at 240, 360 and 480 μM Al) increased Pro content in the roots of the tolerant line compared to 0 μM Al. Yet, 72 h exposure to 480 μM Al increased 1.7-fold the carbohydrate concentration in the roots of the Al tolerant maize line VA-22 while in the sensitive line A4/67 remained almost unchanged. These data provide evidence of an internal mechanism of tolerance that increase the antioxidant system activity in order to limit cellular damages and possibly linked to the Al tolerance of the maize line VA-22. Analyses of the 12, 24, 48, and 72 h POD and SOD isoforms showed that in the Al-tolerant maize plants the anionic POD isoforms A1, A3 and A4 and the SOD isoforms SOD1 and SOD2 were induced by increased Al-stress. It seemed that in the Al tolerant maize line, the anionic POD isoforms A1, A3 and A4 and the SOD isoforms SOD1 and SOD2 were required for adaptation as the oxidant level increased by the increased Al stress. Our results suggest that Al toxicity may be mediated by oxidative stress and that the better protection of the Al tolerant maize roots from Al-induced oxidative damage results, at least partially, from the increased activity of their antioxidative system.