Response of the antioxidative system to photooxidative stress in transgenic tobacco plants overexpressing thylakoid membrane-bound ascorbateperoxidase

Abstract

Ascorbate peroxidase isoenzymes are distributed in stroma and thylakoid membrane (tAPX) in chloroplasts and play an important role in protecting photosynthesis from photooxidative stresses. Recently, we found that the decrease in activities of chloroplastic APX isoenzymes are caused by the depletion of ascorbate (AsA) in tobacco and spinach under potooxidative stress (Plant Physiol., 123, 223-233, 2000; Plant Cell Physiol., 41, 311-320, 2000). Here we evaluated the defense system in chloroplasts in response to photooxidative stress using transgenic tobacco plants (tAPX-12) overexpressing tAPX in chloroplasts. The tAPX activity in tAPX-12 plants was about 20-fold higher than that of the wild-type. There were no significant differences in CO2 fixation, PSII activity, antioxidant levels, and activities of antioxidative enzymes between wild-type and tAPX-12 plants. Paraquat (50 µM) was sprayed on both plants under illumination at 400 µmol m-1 s-2. At 24 h after the paraquat treatment, the destruction of chlorophyll was only observed in wild-type plants. The CO2 fixation and PSII activity in tAPX-12 plants were not influenced. Slight effects on the level of AsA and the activity of tAPX were observed in tAPX-12 plants, while those in wild-type plants significantly decreased. The changes in glutathione level and activities of AsA-regeneration enzymes were similar in both plants. These observations suggest that the enhanced expression of tAPX improves the scavenging capacity of H2O2 in chloroplasts, leading to the maintenance of the level and redox status of AsA in chloroplasts and the protection of photosynthesis from photooxidative stress conditions.