title: Cyclic electron flow around photosystem I is essential for photosynthesis.
abstract: Photosynthesis provides at least two routes through which light energy can be used to generate a proton gradient across the thylakoid membrane of chloroplasts, which is subsequently used to synthesize ATP. In the first route, electrons released from water in photosystem II (PSII) are eventually transferred to NADP+ by way of photosystem I (PSI). This linear electron flow is driven by two photochemical reactions that function in series. The cytochrome b6f complex mediates electron transport between the two photosystems and generates the proton gradient (DeltapH). In the second route, driven solely by PSI, electrons can be recycled from either reduced ferredoxin or NADPH to plastoquinone, and subsequently to the cytochrome b6f complex. Such cyclic flow generates DeltapH and thus ATP without the accumulation of reduced species. Whereas linear flow from water to NADP+ is commonly used to explain the function of the light-dependent reactions of photosynthesis, the role of cyclic flow is less clear. In higher plants cyclic flow consists of two partially redundant pathways. Here we have constructed mutants in Arabidopsis thaliana in which both PSI cyclic pathways are impaired, and present evidence that cyclic flow is essential for efficient photosynthesis.
authors.affiliation: Graduate School of Biological Sciences, Nara Institute of Science and Technology, Ikoma, Nara 630-0101, Japan.
authors.forename: Yuri
authors.initials: Y
authors.lastname: Munekage
authors.forename: Mihoko
authors.initials: M
authors.lastname: Hashimoto
authors.forename: Chikahiro
authors.initials: C
authors.lastname: Miyake
authors.forename: Ken-ichi
authors.initials: K
authors.lastname: Tomizawa
authors.forename: Tsuyoshi
authors.initials: T
authors.lastname: Endo
authors.forename: Masao
authors.initials: M
authors.lastname: Tasaka
authors.forename: Toshiharu
authors.initials: T
authors.lastname: Shikanai
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D017360:Arabidopsis / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D002736:Chloroplasts / Q000378:metabolism; D004579:Electron Transport; D005292:Ferricyanides / Q000378:metabolism; D017343:Genes, Plant / Q000235:genetics; D006863:Hydrogen-Ion Concentration; D009154:Mutation / Q000235:genetics; D009249:NADP / Q000378:metabolism; D010100:Oxygen / Q000378:metabolism; D010641:Phenotype; D010788:Photosynthesis; D045331:Photosystem I Protein Complex / Q000378:metabolism*; D010971:Plastoquinone / Q000378:metabolism; D018892:Proton-Motive Force
chemical_list: D005292:Ferricyanides; D045331:Photosystem I Protein Complex; C007931:hexacyanoferrate III; D009249:NADP; D000255:Adenosine Triphosphate; D010971:Plastoquinone; D010100:Oxygen