Flavodiiron proteins prevent the Mehler reaction in Chlamydomonas reinhardtii

Abstract

Regulation of photosynthetic electron transfer is fundamental for energetic efficiency and coping with changing environmental factors. All plant groups, except angiosperms, use flavodiiron proteins (FDPs) on the acceptor side of photosystem I (PSI), putatively protecting from PSI photoinhibition under fluctuating light. An alternative electron flow during photosynthesis is the direct reduction of O2 with consequential production of reactive oxygen species (ROS; the Mehler reaction), but to what extent FDPs prevent this is unknown. Here, we quantified O2-dependent electron flow, photosystem activity and the Mehler reaction in Chlamydomonas reinhardtii. Near-infra red absorbance measurement of PSI reaction centre (P700) showed that FDPs remain active long after a dark-to-light transition and their activity increased under hyperoxia. Light-induced hydrogen peroxide (H2O2) production, as a marker of the Mehler reaction, was influenced by O2 concentration, and was up to 67 % higher in an FDP-deficient mutant (flvb) than in the wild-type under saturating constant light. In cultures kept under sub-saturating constant light, flvb produced 315 % more H2O2 and had lower PSII efficiency than wild-type. Inhibiting electron transfer out of photosystem II (PSII) with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) only partially blocked H2O2 production, particularly under hyperoxia, indicating that PSII was an additional ROS source. P700+ reduction in the dark in the presence of DCMU was faster in flvb than wild-type, revealing enhanced cyclic electron flow, which may also have led to PSI mediated Mehler reaction. We conclude that FDPs remain active in constant light and can prevent PSI mediated Mehler reaction, with relevance to PSII photoinhibition.