Abstract
Cyclic electron flow was first discovered based on its coupling with ATP synthesis approximately 50 years ago. However, it has been still matter of debate whether it contributes to photosynthesis. Two redundant pathways including FQR and NDH function in this electron flow. In contrast to the main pathway, FQR, which is essential for ΔpH generation to induce NPQ, NDH possibly functions compensatory to prevent the over-reduction of the stroma. To clarify the physiological function of cyclic electron flow around PSI, we created a double mutant between pgr5 lacking the FQR activity and crr2 affected in the ndhB expression. crr2 pgr5 was completely defective in cyclic activity and cannot grow even under low light conditions. In crr2 pgr5, electron transport was drastically restricted and PSI was highly reduced because of the stromal over-reduction. We conclude that cyclic electron flow is essential for the photosynthesis to maintain the proper ATP/NADPH production ratio.
