Evaluation of the defense system in chloroplasts to photooxidative stress caused by paraquat using transgenic tobacco plants expressing catalase fromEscherichia coli

We evaluated the defense system in chloroplasts to photooxidative stress im posed by paraquat treatment under illumination in transgenic tobacco plants with increased tolerance to drought stress at a high light intensity produ ced by catalase from Escherichia coli targeted to chloroplasts [Shikanai et al. (1998) FEES Lett. 428: 47]. At 24 h after the paraquat application, Ch i was destroyed in the wild-type plants, but not in transgenic plants. Phot osynthetic activities monitored by CO2 fixation and Chi fluorescence were m uch less affected by the paraquat treatment in transgenic lines. The activi ties of chloroplastic ascorbate peroxidase (APX) isoenzymes decreased in pa rallel with the depletion of ascorbate (AsA) in leaves in both lines. Paraq uat treatment had no effect on the transcript level of chloroplastic APX is oenzymes, while it significantly lowered the level of their proteins. These data suggest that the depletion of AsA in chloroplasts under severe stress conditions inactivates and degrades chloroplastic APX isoenzymes.