Non-photochemical reduction of thylakoid photosynthetic redox carriers in vitro: Relevance to cyclic electron flow around photosystem I?

Non-photochemical (dark) increases in chlorophyll a fluorescence yield associated with non-photochemical reduction of redox carriers (F-npr) have been attributed to the reduction of plastoquinone (PQ) related to cyclic electron flow (CEF) around photosystem I. In vivo, this rise in fluorescence is associated with activity of the chloroplast plastoquinone reductase (plastid NAD(P)H:plastoquinone oxidoreductase) complex. In contrast, this signal measured in isolated thylakoids has been attributed to the activity of the protein gradient regulation-5 (PGR5)/PGR5-like (PGRL1)-associated CEF pathway. Here, we report a systematic experimentation on the origin of F-npr in isolated thylakoids. Addition of NADPH and ferredoxin to isolated spinach thylakoids resulted in the reduction of the PQ pool, but neither its kinetics nor its inhibitor sensitivities matched those of Notably, F-npr was more rapid than PQ reduction, and completely insensitive to inhibitors of the PSII Q(B) site and oxygen evolving complex as well as inhibitors of the cytochrome b(6)f complex. We thus conclude that Fnpr in isolated thylakoids is not a result of redox equilibrium with bulk PQ. Redox titrations and fluorescence emission spectra imply that F-npr is dependent on the reduction of a low potential redox component (Em about -340 mV) within photosystem II (PSII), and is likely related to earlier observations of low potential variants of Q(A) within a subpopulation of PSII that is directly reducible by ferredoxin. The implications of these results for our understanding of CEF and other photosynthetic processes are discussed. (C) 2014 Elsevier B.V. All rights reserved.