title: Inhibition of CO2 fixation by iodoacetamide stimulates cyclic electron flow and non-photochemical quenching upon far-red illumination.
abstract: The Benson-Calvin cycle enzymes are activated in vivo when disulfide bonds are opened by reduction via the ferredoxin-thioredoxin system in chloroplasts. Iodoacetamide reacts irreversibly with free -SH groups of cysteine residues and inhibits the enzymes responsible for CO2 fixation. Here, we investigate the effect of iodoacetamide on electron transport, when infiltrated into spinach leaves. Using fluorescence and absorption spectroscopy, we show that (i) iodoacetamide very efficiently blocks linear electron flow upon illumination of both photosystems (decrease in the photochemical yield of photosystem II) and (ii) iodoacetamide favors cyclic electron flow upon light excitation specific to PSI. These effects account for an NPQ formation even faster in iodoacetamide under far-red illumination than in the control under saturating light. Such an increase in NPQ is dependent upon the proton gradient across the thylakoid membrane (uncoupled by nigericin addition) and PGR5 (absent in Arabidopsis pgr5 mutant). Iodoacetamide very tightly insulates the electron current at the level of the thylakoid membrane from any electron leaks toward carbon metabolism, therefore, providing choice conditions for the study of cyclic electron flow around PSI.
authors.affiliation: Unité Mixte de Recherche 7141 CNRS-Université Paris 6, Institut de Biologie Physico-Chimique, 13, rue Pierre et Marie Curie, 75005 Paris, France. pjoliot@ibpc.fr
authors.forename: Pierre
authors.initials: P
authors.lastname: Joliot
authors.forename: Jean
authors.initials: J
authors.lastname: Alric
mesh_terms: D017360:Arabidopsis / Q000187:drug effects* / Q000502:physiology / Q000528:radiation effects; D029681:Arabidopsis Proteins; D002245:Carbon Dioxide / Q000378:metabolism*; D002736:Chloroplasts / Q000187:drug effects / Q000378:metabolism; D004579:Electron Transport / Q000187:drug effects; D007460:Iodoacetamide / Q000494:pharmacology*; D008027:Light; D008029:Lighting; D010788:Photosynthesis / Q000187:drug effects*; D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D018515:Plant Leaves; D018724:Spinacia oleracea / Q000187:drug effects* / Q000502:physiology / Q000528:radiation effects
chemical_list: D029681:Arabidopsis Proteins; C465062:PGR5 protein, Arabidopsis; D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D002245:Carbon Dioxide; D007460:Iodoacetamide