C3 photosynthesis, C4 photosynthesis, chloroplast, cyclic electron flow, mutant, NDH complex, PGRL1, PGR5, photosynthesis
Eukaryotic photosynthesis can be divided into two distinct electron transport pathways: (i) linear electron flow (LEF), which involves the extraction of electrons from water and the production of ATP and NADPH, and (ii) cyclic electron flow (CEF), which contributes to an increase in ATP production by reinjection of electrons from reduced ferredoxin back into the electron transport chain. For many years principle aspects of the mechanisms and physiological significance of CEF have been debated, and only recently some of the controversial aspects could be clarified. Two major CEF pathways exist that can be discriminated based on their sensitivity to the inhibitor antimycin A (AA). The AA-insensitive pathway requires the NAD(P)H dehydrogenase complex, whereas the AA-sensitive pathway involves the two thylakoid proteins PGR5 and PGRL1. Both pathways contribute to establish a trans-thylakoid proton gradient affecting non-photochemical quenching and ATP production. This review provides an overview on the physiological role of CEF and gives an update of the mechanisms and regulation of CEF.