Ferredoxin limits cyclic electron flow around PSI (CEF-PSI) in higher plants--stimulation of CEF-PSI enhances non-photochemical quenching of Chl fluorescence in transplastomic tobacco.

We tested the hypothesis that ferredoxin (Fd) limits the activity of cyclic electron flow around PSI (CEF-PSI) in vivo and that the relief of this limitation promotes the non-photochemical quenching (NPQ) of Chl fluorescence. In transplastomic tobacco (Nicotiana tabacum cv Xanthi) expressing Fd from Arabidopsis (Arabidopsis thaliana) in its chloroplasts, the minimum yield (F(o)) of Chl fluorescence was higher than in the wild type. F(o) was suppressed to the wild-type level upon illumination with far-red light, implying that the transfer of electrons by Fd-quinone oxidoreductase (FQR) from the chloroplast stroma to plastoquinone was enhanced in transplastomic plants. The activity of CEF-PSI became higher in transplastomic than in wild-type plants under conditions limiting photosynthetic linear electron flow. Similarly, the NPQ of Chl fluorescence was enhanced in transplastomic plants. On the other hand, pool sizes of the pigments of the xanthophyll cycle and the amounts of PsbS protein were the same in all plants. All these results supported the hypothesis strongly. We conclude that breeding plants with an NPQ of Chl fluorescence increased by an enhancement of CEF-PSI activity might lead to improved tolerance for abiotic stresses, particularly under conditions of low light use efficiency.

Photoinactivation of ascorbate peroxidase in isolated tobacco chloroplasts: Galdieria partita APX maintains the electron flux through the water-water cycle in transplastomic tobacco plants.

We evaluated the H2O2-scavenging activity of the water-water cycle (WWC) in illuminated intact chloroplasts isolated from tobacco leaves. Illumination under conditions that limited photosynthesis [red light (>640 nm), 250 micromol photons m(-2) s(-1) in the absence of HCO3-] caused chloroplasts to take up O2 and accumulate H2O2. Concomitant with the O2 uptake, both ascorbate peroxidase (APX) and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) lost their activities. However, superoxide dismutase (SOD), monodehydroascorbate radical reductase (MDAR), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) activities remained unaffected. The extent to which the photosynthetic linear electron flow decreased was small compared with the decline in APX activity. Therefore, the loss of APX activity lowered the electron flux through the WWC, as evidenced by a decrease in relative electron flux through PSII [Phi(PSII)xPFD]. To verify these interpretations, we created a transplastomic tobacco line in which an H2O2-insensitive APX from the red alga, Galdieria partita, was overproduced in the chloroplasts. In intact transplastomic chloroplasts which were illuminated under conditions that limited photosynthesis, neither O2 uptake nor H2O2 accumulation occurred. Furthermore, the electron flux through the WWC and the activity of GAPDH were maintained. The present work is the first report of APX inactivation by endogenous H2O2 in intact chloroplasts.