title: Competition between linear and cyclic electron flow in plants deficient in Photosystem I.
abstract: Photosynthetic electron transport can involve either a linear flow from water to NADP, via Photosystems (PS) II and I or a cyclic flow just involving PSI. Little is known about factors regulating the relative flow through each of these pathways. We have examined photosynthetic electron transport through each system in plants of Arabidopsis thaliana in which either the PSI-D1 or PSI-E1 subunits of PSI have been knocked out. In both cases, this results in an imbalance in the turnover of PSI and PSII, such that PSII electron transport is limited by PSI turnover. Phosphorylation of light-harvesting complex II (LHCII) and its migration to PSI is enhanced but only partially reversible and not sufficient to balance photosystem turnover. In spite of this, cyclic electron flow is able to compete efficiently with PSI across a range of conditions. In dark-adapted leaves, the efficiency of cyclic relative to linear flow induced by far-red light is increased, implying that the limiting step of cyclic flow lies in the re-injection of electrons into the electron transport chain. Illumination of leaves with white light resulted in transient induction of a significant non-photochemical quenching in knockout plants which is probably high energy state quenching induced by cyclic electron flow. At high light and at low CO(2), non-photochemical quenching was greater in the knockout plants than in the wildtype. Comparison of PSI and PSII turnover under such conditions suggested that this is generated by cyclic electron flow around PSI. We conclude that, when the concentration of PSI is limiting, cyclic electron flow is still able to compete effectively with linear flow to maintain a high DeltapH to regulate photosynthesis.
authors.affiliation: Faculty of Life Sciences, University of Manchester, Michael Smith Building, Oxford Road, Manchester, M13 9PT, UK.
authors.forename: Simon
authors.initials: S
authors.lastname: Hald
authors.forename: Mathias
authors.initials: M
authors.lastname: Pribil
authors.forename: Dario
authors.initials: D
authors.lastname: Leister
authors.forename: Patrick
authors.initials: P
authors.lastname: Gallois
authors.forename: Giles N
authors.initials: GN
authors.lastname: Johnson
mesh_terms: D017360:Arabidopsis / Q000378:metabolism* / Q000528:radiation effects; D029681:Arabidopsis Proteins; D002245:Carbon Dioxide / Q000378:metabolism; D002734:Chlorophyll / Q000378:metabolism; D004579:Electron Transport / Q000528:radiation effects; D005453:Fluorescence; D008027:Light; D045342:Light-Harvesting Protein Complexes / Q000378:metabolism; D008564:Membrane Potentials / Q000528:radiation effects; D009154:Mutation / Q000235:genetics; D010766:Phosphorylation / Q000528:radiation effects; D045331:Photosystem I Protein Complex / Q000235:genetics / Q000378:metabolism*; D045332:Photosystem II Protein Complex / Q000378:metabolism*; D018515:Plant Leaves / Q000378:metabolism / Q000528:radiation effects; D021381:Protein Transport / Q000528:radiation effects; D020524:Thylakoids / Q000378:metabolism / Q000528:radiation effects
chemical_list: D029681:Arabidopsis Proteins; D045342:Light-Harvesting Protein Complexes; D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; C412446:psaE1 protein, Arabidopsis; D002734:Chlorophyll; D002245:Carbon Dioxide