Regulation of Photosynthesis via PSI Cyclic Electron Transport

Abstract

In higher plants PSI cyclic electron transport consists of PGR5-dependent and NAD(P)H dehydrogenase-dependent pathways. Characterization of the Arabidopsis mutants defective in the pathways indicate that PSI cyclic electron transport is essential for both protecting chloroplasts from photo-oxidative damage and supplying ATP for photosynthesis. Although the physiological function of PSI cyclic electron transport is becoming clearer, the exact routes taken by electrons are still unclear. To clarify the function of PGR5 protein, we characterized the Arabidopsis plants over-accumulating PGR5 in thylakoid membranes. We found that the level of PGR5 is closely related to the rate of PSI cyclic electron transport. Consequently, higher PGR5 level sustains transient NPQ longer under fluctuating light conditions. The PGR5 level may be involved in maintaining the redox balance in chloroplasts to optimize the light absorption during fluctuating light conditions. In contrast, over-accumulation of PGR5 does not affect the electron transport during steady-state photosynthesis. Consistent with this observation, we found that the NADP+ /NADPH ratio regulates the operation of PSI cyclic electron transport in broken chloroplasts.