Oxidative Stress Induced by Cadmium in Transgenic Nicotiana tabacum Over-expressing a Plastidial Mn-Superoxide Dismutase

Nicotiana tabacum transgenic plants overexpressing manganese (Mn) superoxide dismutase (SOD) targeted to plastids were studied upon 12.5 M cadmium (Cd) application. No differences in tolerance were observed in comparison to wild type plants. Seed germination inhibition, seedling growth, and plant biomass were measured at different developmental stages. Cd analysis revealed the usual distribution of Cd between roots and shoots, 10-times higher in roots. Lipid peroxidation level, a characteristic parameter of oxidative stress, indicated that little protection was gained by overexpressing MnSOD. However, chlorophyll concentration was not affected upon Cd treatment in 35S:MnSOD transgenic plants, suggesting that a possible protection localised at the chloroplast could indeed occur. Three antioxidant enzymes sensitive to metal stress were also analysed after in gel activity staining: SOD, ascorbate peroxidase (APX) and glutathione reductase (GR). As expected, MnSOD activity was remarkably higher in transgenic N. tabacum plants. Interestingly, Cu/ZnSOD activity decreased upon exposure of plants to 12.5 M Cd. GR and APX activities augmented in metal-treated plants. In particular, APX activity was slightly higher in MnSOD over-expressing plants, which was possibly related with an enhanced generation of H2O2 through higher SOD activity. _____________________________________________________________________________________________________________