DEFENSE RESPONSES TO TETRAPYRROLE-INDUCED OXIDATIVE STRESS IN TRANSGENIC PLANTS WITH REDUCED UROPORPHYRINOGEN DECARBOXYLASE OR COPROPORPHYRINOGEN OXIDASE ACTIVITY
Hp. Mock et al., DEFENSE RESPONSES TO TETRAPYRROLE-INDUCED OXIDATIVE STRESS IN TRANSGENIC PLANTS WITH REDUCED UROPORPHYRINOGEN DECARBOXYLASE OR COPROPORPHYRINOGEN OXIDASE ACTIVITY, Plant physiology, 116(1), 1998, pp. 107-116
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.