In order to understand the role of cytosolic
antioxidant enzymes in drought stress protection, transgenic tobacco (Nicotiana tabacum cv. Xanthi) plants overexpressing cytosolic
Cu/Zn-superoxide dismutase (cytsod) (EC 1.15.1.1) or
ascorbate peroxidase (cytapx) (EC 1.11.1.1) alone, or in combination, were produced and tested for tolerance against mild
water stress. The results showed that the simultaneous overexpression of Cu/Znsod and apx or at least apx in the cytosol of transgenic tobacco plants alleviates, to some extent, the damage produced by
water stress conditions. This was correlated with higher water use efficiency and better photosynthetic rates. In general, oxidative stress parameters, such as lipid peroxidation,
electrolyte leakage, and H(2)O(2) levels, were higher in non-transformed plants than in transgenic lines, suggesting that, at the least, overexpression of cytapx protects tobacco membranes from
water stress. In these conditions, the activity of other
antioxidant enzymes was induced in transgenic lines at the subcellular level. Moreover, an increase in the activity of some
antioxidant enzymes was also observed in the chloroplast of transgenic plants overexpressing cytsod and/or cytapx. These results suggest the positive influence of cytosolic
antioxidant metabolism on the chloroplast and underline the complexity of the regulation network of plant
antioxidant defences during drought stress.