Two varieties of tobacco (Nicofiana fabacum var PBD6 and var SR1) were used to generate transgenic lines overexpressing Mnsuperoxide dismutase (MnSOD) in the chloroplasts. The overexpressed MnSOD suppresses the activity of those SODs (endogenous MnSOD and chloroplastic and cytosolic Cu/ZnSOD) that are prominent in young leaves but disappear largely or completely during aging of the leaves. The transgenic and control plants were grown at different light intensities and were then assayed for oxygen radical stress tolerance i n leaf disc assays and for abundance of antioxidant enzymes and substrates in leaves. Transgenic plants had an enhanced resistance to methylviologen (MV), compared with control plants, only after growth at high light intensities. I n both varieties the activities of FeSOD, ascorbate peroxidase, dehydroascorbate reductase, and monodehydroascorbate reductase and the concentrations of glutathione and ascorbate (all expressed on a chlorophyll basis) increased with increasing light intensity during growth. Most of these components were correlated with M V tolerance. It is argued that SOD overexpression leads to enhancement of the tolerance to MV-dependent oxidative stress only if one or more of these components is also present at high levels. Furthermore, the results suggest that i n var SR1 the overexpressed MnSOD enhances primarily the stromal antioxidant system.The photosynthetic electron transport chain in higher plant chloroplasts contains, at the acceptor side of PSI, a number of auto-oxidizable enzymes. For example, Fd in the reduced state can react with oxygen, yielding the superoxide anion radical (OJ (Furbank and Badger, 1982;Asada and Takahashi, 1987), and it has been shown that superoxide is also generated in the aprotic membrane interior (Elstner and Frommeyer, 1978;Takahashi and Asada, 1988). Superoxide does not seem to be particularly toxic, although it does inactivate a number of metal-containing enzymes such as Fd-linked nitrate reductase, catalase, and peroxidases

A chimeric gene consisting of the coding sequence for chloroplastic Fe superoxide dismutase (FeSOD) from Arabidopsis thaliana, coupled to the chloroplast targeting sequence from the pea ribulose-1,5-bisphosphate carboxylase/oxygenase small subunit, was expressed in Nicotiana tabacum cv Petit Havana SR1. Expression of the transgenic FeSOD protected both the plasmalemma and photosystem II against superoxide generated during illumination of leaf discs impregnated with methyl viologen. By contrast, overproduction of a mitochondrial MnSOD from Nicotiana plumbaginifolia in the chloroplasts of cv SR1 protected only the plasmalemma, but not photosystem II, against methyl viologen (L. Slooten, K. Capiau, W. Van Camp, M. Van Montagu, C. Sybesma, D. Inze [1995] Plant Physiol 107: 737–750). The difference in effectiveness correlates with different membrane affinities of the transgenic FeSOD and MnSOD. Overproduction of FeSOD does not confer tolerance to H2O2, singlet oxygen, chilling-induced photoinhibition in leaf disc assays, or to salt stress at the whole plant level. In nontransgenic plants, salt stress led to a 2- to 3-fold increase in activity, on a protein basis, of FeSOD, cytosolic and chloroplastic Cu/ZnSOD, ascorbate peroxidase, dehydroascorbate reductase, and glutathione reductase. In FeSOD-overproducing plants under salt stress, the induction of cytosolic and chloroplastic Cu/ZnSOD was suppressed, whereas induction of a water-soluble chloroplastic ascorbate peroxidase isozyme was promoted.

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