Abstract
Photosystem I (PSI) cyclic electron transport is essential for photosynthesis and photoprotection. In higher plants, the ferredoxin (Fd) -dependent pathway is the main route in PSI cyclic electron transport. Although a small thylakoid protein, PGR5 (PROTON GRADIENT REGULATION 5), is essential for this pathway, the fuction of the PGR5 protein is still unclear and there are numerous debates on the rate of electron transport in vivo and its regulation.
We evaluated Fd-dependent PQ reduction in ruptured chloroplasts by using a Arabidopsis mutant, pgr5. We detected high activity of PGR5-dependent cyclic electron transport even in the presence of linear electron transport. In the wild type, a competition between NADP+ photoreduction and PGR5-dependent cyclic electron transport was observed. From these results, it is suggested that the rate of PGR5-dependent cyclic electron transport is high enough to explain the mutant phenotype and the redox state of NADP+ regulates the activity of this pathway.
