DEFENSE RESPONSES TO TETRAPYRROLE-INDUCED OXIDATIVE STRESS IN TRANSGENIC PLANTS WITH REDUCED UROPORPHYRINOGEN DECARBOXYLASE OR COPROPORPHYRINOGEN OXIDASE ACTIVITY

We analyzed the antioxidative defense responses of transgenic tobacco (Nicotiana tabacum) plants expressing antisense RNA for uroporphyrinog en decarboxylase or coproporphyrinogen oxidase. These plants are chara cterized by necrotic leaf lesions resulting from the accumulation of p otentially photosensitizing tetrapyrroles. Compared with control plant s, the transformants had increased levels of antioxidant mRNAs, partic ularly those encoding superoxide dismutase (SOD), catalase, and glutat hione peroxidase. These elevated transcript levels correlated with inc reased activities of cytosolic Cu/Zn-SOD and mitochondrial Mn-SOD. Tot al catalase activity decreased in the older leaves of the transformant s to levels lower than in the wild-type plants, reflecting an enhanced turnover of this photosensitive enzyme. Most of the enzymes of the Ha lliwell-Asada pathway displayed increased activities in transgenic pla nts. Despite the elevated enzyme activities, the limited capacity of t he antioxidative system was apparent from decreased levels of ascorbat e and glutathione, as well as from necrotic leaf lesions and growth re tardation. Our data demonstrate the induction of the enzymatic detoxif ying defense system in several compartments, suggesting a photosensiti zation of the entire cell. It is proposed that the tetrapyrroles that initially accumulate in the plastids leak out into other cellular comp artments, thereby necessitating the local detoxification of reactive o xygen species.