ENHANCEMENT OF OXIDATIVE STRESS TOLERANCE IN TRANSGENIC TOBACCO PLANTS OVERPRODUCING FE-SUPEROXIDE DISMUTASE IN CHLOROPLASTS

A chimeric gene consisting of the coding sequence for chloroplastic Fe superoxide dismutase (FeSOD) from Arabidopsis thaliana, coupled to th e chloroplast targeting sequence from the pea ribulose-1,5-bisphosphat e carboxylase/oxygenase small subunit, was expressed in Nicotiana taba cum cv Petit Havana SR1. Expression of the transgenic FeSOD protected both the plasmalemma and photosystem II against superoxide generated d uring illumination of leaf discs impregnated with methyl viologen. By contrast, overproduction of a mitochondrial MnSOD from Nicotiana plumb aginifolia in the chloroplasts of cv SR1 protected only the plasmalemm a, but not photosystem II, against methyl viologen (L. Slooten, K. Cap iau, W. Van Camp, M. Van Montagu, C.Sybesma, D. Inze [1995] Plant Phys iol 107: 737-750). The difference in effectiveness correlates with dif ferent membrane affinities of the transgenic FeSOD and MnSOD. Overprod uction of FeSOD does not confer tolerance to H2O2, singlet oxygen, chi lling-induced photoinhibition in leaf disc assays, or to salt stress a t 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, cyto solic and chloroplastic Cu/ZnSOD, ascorbate peroxidase, dehydroascorba te reductase, and glutathione reductase. In FeSOD-overproducing plants under salt stress, the induction of cytosolic and chloroplastic Cu/Zn SOD was suppressed, whereas induction of a water-soluble chloroplastic ascorbate peroxidase isozyme was promoted.