title: Non-photochemical reduction of thylakoid photosynthetic redox carriers in vitro: relevance to cyclic electron flow around photosystem I?
abstract: Non-photochemical (dark) increases in chlorophyll a fluorescence yield associated with non-photochemical reduction of redox carriers (Fnpr) 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
  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 Fnpr 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 Fnpr. Notably, Fnpr was more rapid than PQ reduction, and completely insensitive to inhibitors of the PSII QB site and oxygen evolving complex as well as inhibitors of the cytochrome b6f 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 Fnpr 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 QA 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.
authors.affiliation: MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, MI 48824, USA.
authors.forename: Nicholas
authors.initials: N
authors.lastname: Fisher
authors.affiliation: MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, MI 48824, USA; Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI 48824, USA. Electronic address: kramerd8@msu.edu.
authors.forename: David M
authors.initials: DM
authors.lastname: Kramer
mesh_terms: D000968:Antimycin A / Q000494:pharmacology; D002734:Chlorophyll / Q000737:chemistry / Q000378:metabolism; D002736:Chloroplasts / Q000187:drug effects / Q000378:metabolism; D045346:Cytochrome b6f Complex / Q000037:antagonists & inhibitors / Q000378:metabolism; D004237:Diuron / Q000494:pharmacology; D004579:Electron Transport / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005287:Ferredoxin-NADP Reductase / Q000037:antagonists & inhibitors / Q000378:metabolism; D005288:Ferredoxins / Q000378:metabolism / Q000494:pharmacology; D005453:Fluorescence; D007700:Kinetics; D009243:NAD / Q000378:metabolism / Q000494:pharmacology; D009861:Onium Compounds / Q000494:pharmacology; D010084:Oxidation-Reduction / Q000187:drug effects; D010788:Photosynthesis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism; D045331:Photosystem I Protein Complex / Q000378:metabolism*; D010940:Plant Proteins / Q000378:metabolism; D010971:Plastoquinone / Q000378:metabolism*; D011808:Quinone Reductases / Q000378:metabolism; D013050:Spectrometry, Fluorescence; D018724:Spinacia oleracea / Q000378:metabolism; D020524:Thylakoids / Q000187:drug effects / Q000378:metabolism*
chemical_list: D004791:Enzyme Inhibitors; D005288:Ferredoxins; D009861:Onium Compounds; D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex; D010940:Plant Proteins; D009243:NAD; D002734:Chlorophyll; D000968:Antimycin A; C007517:diphenyleneiodonium; D045346:Cytochrome b6f Complex; D004237:Diuron; D005287:Ferredoxin-NADP Reductase; D011808:Quinone Reductases; C073412:sulfide quinone reductase; D010971:Plastoquinone