Plant and Cell Physiology Advance Access originally published online on April 3, 2008
Plant and Cell Physiology 2008 49(5):825-834; doi:10.1093/pcp/pcn055
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Characterization of Factors Affecting the Activity of Photosystem I Cyclic Electron Transport in Chloroplasts
1Graduate School of Agriculture, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka, 812-8581 Japan
2Department of Botany, Graduate School of Science, Kyoto University, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto, 606-8502 Japan
*Corresponding author: E-mail, shikanai{at}pmg.bot.kyoto-u.ac.jp; Fax, +81-75-753-4257.
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Abstract |
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PSI cyclic electron transport is essential for photosynthesis and photoprotection. In higher plants, the antimycin A-sensitive pathway is the main route of electrons in PSI cyclic electron transport. Although a small thylakoid protein, PGR5 (PROTON GRADIENT REGULATION 5), is essential for this pathway, its function is still unclear, and there are numerous debates on the rate of electron transport in vivo and its regulation. To assess how PGR5-dependent PSI cyclic electron transport is regulated in vivo, we characterized its activity in ruptured chloroplasts isolated from Arabidopsis thaliana. The activity of ferredoxin (Fd)-dependent plastoquinone (PQ) reduction in the dark is impaired in the pgr5 mutant. Alkalinization of the reaction medium enhanced the activity of Fd-dependent PQ reduction in the wild type. Even weak actinic light (AL) illumination also markedly activated PGR5-dependent PSI cyclic electron transport in ruptured chloroplasts. Even in the presence of linear electron transport [11 µmol O2 (mg Chl)–1 h–1], PGR5-dependent PSI electron transport was detected as a difference in Chl fluorescence levels in ruptured chloroplasts. In the wild type, PGR5-dependent PSI cyclic electron transport competed with NADP+ photoreduction. These results suggest that the rate of PGR5-dependent PSI cyclic electron transport is high enough to balance the production ratio of ATP and NADPH during steady-state photosynthesis, consistently with the pgr5 mutant phenotype. Our results also suggest that the activity of PGR5-dependent PSI cyclic electron transport is regulated by the redox state of the NADPH pool.
Keywords: Antimycin A - Arabidopsis - Ferredoxin - PGR5 - PSI cyclic electron transport - Redox regulation
Abbreviations: AA, antimycin A; ETR, electron transport rate; Fd, ferredoxin; FNR, ferredoxin-NADP+ reductase; FQR, ferredoxin-plastoquinone reductase; NDH, NAD(P)H dehydrogenase; NPQ, non-photochemical quenching; PGR5, PROTON GRADIENT REGULATION 5; PQ, plastoquinone.
(Received February 20, 2008; Accepted March 31, 2008)
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