H2O2 photoproduced intrinsically in chloroplasts of higher plants inactivates ascorbate peroxidase (APX) -Galdieria partita-APX maintains the activity of the water-water cycle (WWC) in transplastomic tobacco plants-

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We evaluated the scavenging activity of H2O2 in WWC of illuminated intact chloroplasts from wild tobacco leaves. An illumination caused intact chloroplasts to uptake O2 and to accumulate H2O2. With the O2-uptake both APX and GAPDH lost their activities. The loss of APX activity lowered the activity of WWC, as judged from the decrease in ψ(PSII)xPFD. For the verification of these implications, we made a transplastomic tobacco plant where APX originated from red alga, Galdieria partita, was overproduced in chloroplasts. In illuminated intact chloroplasts from transplastomic tobacco, neither O2-uptake nor H2O2-accumulation was observed. Furthermore, both activities of WWC and GAPDH were maintained. The present work is the first report that APX was inactivated by H2O2 intrinsically photoproduced in intact chloroplasts from wild plants. This would be the reason why plants die when plants are exposed to abiotic stress which suppresses photosynthesis.