An intracellular mechanism of aluminum tolerance associated with high antioxidant status in cultured tobacco cells

An aluminum (Al) tolerance mechanism, together with oxidative stress tolerance, was investigated in an Al tolerant cell line (ALT301) and the parental Al sensitive cell line (SL) of tobacco. During Al exposure in a simple calcium solution for 24 h, Al triggered the evolution of a reactive oxygen species (ROS) in SL much higher than ALT301 [Plant Physiol. 128 (2002) 63]. Under the conditions, Al enhanced comparable rates of citrate secretion from both cell lines to the same extent. Al enhanced the gene expression of manganese superoxide dismutase (MnSOD) in both cell lines, but at a significantly higher rate in SL than in ALT301, and also enhanced the enzyme activity of MnSOD in both cell lines to nearly the same level. These results suggest that the extracellular chelation of Al with organic acids and MnSOD is not involved in the mechanism of Al tolerance of ALT301. ALT301 contained ascorbate (ASA) and glutathione (GSH) levels that were higher than SL under normal growth conditions. During 24 h of post-Al treatment culture in growth medium, but not during 24-h Al exposure in a simple Ca²⁺ solution, lipid peroxidation was enhanced in SL much higher than in ALT301, and the average SL amounts of ASA and GSH were exhausted compared to ALT301. Pre-loading of ASA prior to Al treatment improved the growth of SL during the post-Al treatment culture. ALT301 also exhibited cross-tolerance to H ₂O₂, Fe²⁺ and Cu²⁺. Under these oxidant exposures, ALT301 contained lower levels of intracellular H ₂O₂ or lipid peroxides, and maintained higher amounts of ASA and GSH than SL. Taken together, we conclude that the accumulation of Al in cells enhances the peroxidation of lipids exclusively under growing conditions, and that the higher content of ASA and GSH in ALT301 than in SL seems to be in part responsible for the tolerance mechanism of ALT301 to Al by protecting cells from either lipid peroxidation or H₂O₂ commonly enhanced by Al or other oxidants.