NDH-Mediated Cyclic Electron Flow Around Photosystem I is Crucial for C4 Photosynthesis

C-4 photosynthesis exhibits efficient CO2 assimilation in ambient air by concentrating CO2 around ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) through a metabolic pathway called the C-4 cycle. It has been suggested that cyclic electron flow (CEF) around PSI mediated by chloroplast NADH dehydrogenase-like complex (NDH), an alternative pathway of photosynthetic electron transport (PET), plays a crucial role in C-4 photosynthesis, although the contribution of NDH-mediated CEF is small in C-3 photosynthesis. Here, we generated NDH-suppressed transformants of a C-4 plant, Flaveria bidentis, and showed that the NDH-suppressed plants grow poorly, especially under low-light conditions. CO2 assimilation rates were consistently decreased in the NDH-suppressed plants under low and medium light intensities. Measurements of non-photochemical quenching (NPQ) of Chl fluorescence, the oxidation state of the reaction center of PSI (P700) and the electrochromic shift (ECS) of pigment absorbance indicated that proton translocation across the thylakoid membrane is impaired in the NDH-suppressed plants. Since proton translocation across the thylakoid membrane induces ATP production, these results suggest that NDH-mediated CEF plays a role in the supply of ATP which is required for C-4 photosynthesis. Such a role is more crucial when the light that is available for photosynthesis is limited and the energy production by PET becomes rate-determining for C-4 photosynthesis. Our results demonstrate that the physiological contribution of NDH-mediated CEF is greater in C-4 photosynthesis than in C-3 photosynthesis, suggesting that the mechanism of PET in C-4 photosynthesis has changed from that in C-3 photosynthesis accompanying the changes in the mechanism of CO2 assimilation.