Role of cyclic electron transport around photosystem I in regulating proton motive force

Abstract 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 ECS t , was reduced by 30% to 47% compared with that in the wild type (WT). A g H + parameter, which is considered to represent the activity of ATP synthase, was enhanced at high light intensities. However, g H + recovered to its low-light levels after 20 min in the dark, implying that the elevation in g H + is due to the disturbed regulation of ATP synthase rather than to photodamage. After long dark adaptation more than 2 h, g H + was higher in pgr5 than in the WT. During induction of photosynthesis, g H + 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, ECS t was slightly but significantly lower than in the WT, whereas g H + was not affected. In the double mutants, ECS t 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.