FACTORS AFFECTING THE ENHANCEMENT OF OXIDATIVE STRESS TOLERANCE IN TRANSGENIC TOBACCO OVEREXPRESSING MANGANESE SUPEROXIDE-DISMUTASE IN THE CHLOROPLASTS

Two varieties of tobacco (Nicotiana tabacum var PBD6 and var SR1) were used to generate transgenic lines overexpressing Mn-superoxide dismut ase (MnSOD) in the chloroplasts. The overexpressed MnSOD suppresses th e activity of those SODs (endogenous MnSOD and chloroplastic and cytos olic Cu/ZnSOD) that are prominent in young leaves but disappear largel y or completely during aging of the leaves. The transgenic and control plants were grown at different light intensities and were then assaye d for oxygen radical stress tolerance in leaf disc assays and for abun dance of antioxidant enzymes and substrates in leaves. Transgenic plan ts had an enhanced resistance to methylviologen (MV), compared with co ntrol plants, only after growth at high light intensities. In both var ieties the activities of FeSOD, ascorbate peroxidase, dehydroascorbate reductase, and monodehydroascorbate reductase and the concentrations of glutathione and ascorbate (all expressed on a chlorophyll basis) in creased with increasing light intensity during growth. Most of these c omponents were correlated with MV tolerance. It is argued that SOD ove rexpression leads to enhancement of the tolerance to MV-dependent oxid ative stress only if one or more of these components is also present a t high levels. Furthermore, the results suggest that in var SR1 the ov erexpressed MnSOD enhances primarily the stromal antioxidant system.