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C Wang, H Yamamoto and T Shikanai
In addition to ∆pH formed across the thylakoid membrane, membrane potential contributes to proton motive force (pmf) in chloroplasts. However, the regulation of photosynthetic electron transport is mediated solely by ∆pH. To assess the contribution of two cyclic electron transport pathways around photosystem I (one depending on PGR5/PGRL1 and one on NDH) to pmf formation, electrochromic shift (ECS) was analyzed in the Arabidopsis pgr5 mutant, NDH-defective mutants (ndhs and crr4-2), and their double mutants (ndhs pgr5 and crr4-2 pgr5). In pgr5, the size of the pmf, as represented by ECSt, was reduced by 30% to 47% compared with that in the wild type (WT). A gH(+) parameter, which is considered to represent the activity of ATP synthase, was enhanced at high light intensities. However, gH(+) recovered to its low-light levels after 20min in the dark, implying that the elevation in gH(+) is due to the disturbed regulation of ATP synthase rather than to photodamage. After long dark adaptation more than 2h, gH(+) was higher in pgr5 than in the WT. During induction of photosynthesis, gH(+) was more rapidly elevated in pgr5 than that in the WT. Both results suggest that ATP synthase is not fully inactivated in the dark in pgr5. In the NDH-deficient mutants, ECSt was slightly but significantly lower than in the WT, whereas gH(+) was not affected. In the double mutants, ECSt was even lower than in pgr5. These results suggest that both PGR5/PGRL1- and NDH-dependent pathways contribute to pmf formation, although to different extents. This article is part of a Special Issue entitled: Chloroplast Biogenesis.
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Organelle, Cyanobacteria, Chloroplast, ATP synthase, Cellular respiration, Adenosine triphosphate, Thylakoid, Photosynthesis
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