chlorophyll-a and myxothiazol

A possible implication of the plastid NADH-plastoquinone-oxidoreductase (Ndh) complex in the response against ozone-mediated oxidative stress in barley (Hordeum vulgare L.) leaves was investigated. After a 4 h treatment, exposure of barley seedlings to moderate ozone concentrations produced leaf-age-dependent increases in lipid peroxidation, peroxidase, and Ndh complex activities in the thylakoid membranes. A significant amount and activity of the Ndh complex were detected in mature barley leaves, but not in young barley leaves. In fact, young barley leaves behaved like ndh-deficient leaves in most of the aspects studied. When plants were exposed to photo-oxidative light after ozone fumigation, the recovery of Fv/Fm was lower in young leaves than in mature leaves. Ozone treatment significantly decreased non-photochemical quenching (qN) in young leaves, but not in mature leaves. Mature leaves showed higher levels of the energy (DeltamuH+) dependent (qE) component of qN. Treatment with antimycin A, an inhibitor of cyclic electron flow, increased the decay of qN produced by ozone in young leaves, but not in mature ones. The reduction state of plastoquinone increased after ozone treatment in mature dark-adapted leaves and was strongly quenched by far red light. It is proposed that the function of the Ndh complex helps the maintenance of qN, probably through the poising of the redox steady-state level of the intersystem carriers and then by optimizing the rate of cyclic electron flow. This should constitute an age-dependent early response in barley leaves, by contributing to minimize photoinhibition in the presence of ozone and high light.

Topics: Antimycin A; Chlorophyll; Chlorophyll A; Electron Transport; Fluorescence; Hordeum; Light; Lipid Peroxidation; Methacrylates; Multienzyme Complexes; NADH, NADPH Oxidoreductases; Oxidative Stress; Ozone; Peroxidase; Plant Leaves; Seedlings; Thiazoles; Thylakoids; Time Factors

Interactions between photosynthesis, mitochondrial respiration (mitorespiration), and chlororespiration have been investigated in the green alga Chlamydomonas reinhardtii using flash illumination and a bare platinum electrode. Depending on the physiological status of algae, flash illumination was found to induce either a fast (t(1/2) approximately 300 ms) or slow (t(1/2) approximately 3 s) transient inhibition of oxygen uptake. Based on the effects of the mitorespiratory inhibitors myxothiazol and salicyl hydroxamic acid (SHAM), and of propyl gallate, an inhibitor of the chlororespiratory oxidase, we conclude that the fast transient is due to the flash-induced inhibition of chlororespiration and that the slow transient is due to the flash-induced inhibition of mitorespiration. By measuring blue-green fluorescence changes, related to the redox status of the pyridine nucleotide pool, and chlorophyll fluorescence, related to the redox status of plastoquinones (PQs) in C. reinhardtii wild type and in a photosystem I-deficient mutant, we show that interactions between photosynthesis and chlororespiration are favored when PQ and pyridine nucleotide pools are reduced, whereas interactions between photosynthesis and mitorespiration are favored at more oxidized states. We conclude that the plastid oxidase, similar to the mitochondrial alternative oxidase, becomes significantly engaged when the PQ pool becomes highly reduced, and thereby prevents its over-reduction.

Topics: Animals; Chlamydomonas reinhardtii; Chlorophyll; Darkness; Fluorescence; Light; Light-Harvesting Protein Complexes; Methacrylates; Oxidation-Reduction; Oxygen Consumption; Photosynthesis; Photosynthetic Reaction Center Complex Proteins; Photosystem I Protein Complex; Propyl Gallate; Pyrimidine Nucleotides; Salicylamides; Thiazoles

Tobacco (Nicotiana tabacum var Petit Havana) ndhB-inactivated mutants (ndhB-) obtained by plastid transformation (E.M. Horvath, S.O. Peter, T. Joët, D. Rumeau, L. Cournac, G.V. Horvath, T.A. Kavanagh, C. Schäfer, G. Peltier, P. MedgyesyHorvath [2000] Plant Physiol 123: 1337-1350) were used to study the role of the NADH-dehydrogenase complex (NDH) during photosynthesis and particularly the involvement of this complex in cyclic electron flow around photosystem I (PSI). Photosynthetic activity was determined on leaf discs by measuring CO2 exchange and chlorophyll fluorescence quenchings during a dark-to-light transition. In the absence of treatment, both non-photochemical and photochemical fluorescence quenchings were similar in ndhB- and wild type (WT). When leaf discs were treated with 5 microM antimycin A, an inhibitor of cyclic electron flow around PSI, both quenchings were strongly affected. At steady state, maximum photosynthetic electron transport activity was inhibited by 20% in WT and by 50% in ndhB-. Under non-photorespiratory conditions (2% O2, 2,500 microL x L(-1) CO2), antimycin A had no effect on photosynthetic activity of WT, whereas a 30% inhibition was observed both on quantum yield of photosynthesis assayed by chlorophyll fluorescence and on CO2 assimilation in ndhB-. The effect of antimycin A on ndhB- could not be mimicked by myxothiazol, an inhibitor of the mitochondrial cytochrome bc1 complex, therefore showing that it is not related to an inhibition of the mitochondrial electron transport chain but rather to an inhibition of cyclic electron flow around PSI. We conclude to the existence of two different pathways of cyclic electron flow operating around PSI in higher plant chloroplasts. One of these pathways, sensitive to antimycin A, probably involves ferredoxin plastoquinone reductase, whereas the other involves the NDH complex. The absence of visible phenotype in ndhB- plants under normal conditions is explained by the complement of these two pathways in the supply of extra-ATP for photosynthesis.

Topics: Antimycin A; Carbon Dioxide; Chlorophyll; Electron Transport; Kinetics; Light; Light-Harvesting Protein Complexes; Methacrylates; NADH Dehydrogenase; Nicotiana; Photosynthesis; Photosynthetic Reaction Center Complex Proteins; Photosystem I Protein Complex; Plant Proteins; Plants, Toxic; Plastids; Thiazoles

The reduction of duroquinone (DQ), 2,3-dimethoxy-5-methyl-6-decyl-1,4-benzoquinone (DB), and dichlorophenol indophenol (DCIP) by succinate and NADH was investigated in yeast mitochondria which have no spectrally detectable cytochrome b. Succinate reduces DB in the cytochrome b-deficient mitochondria at rates comparable to that observed in wild-type mitochondria, suggesting that succinate:ubiquinone oxidoreductase is unaffected by the lack of cytochrome b. In the mutant mitochondria, succinate does not reduce DQ or DCIP at significant rates; however, NADH reduces both DQ and DCIP at rates similar to that of the wild-type mitochondria in a myxothiazol, but not antimycin, sensitive reaction. The Ki for myxothiazol in this reaction is close to that for electron transfer through the cytochrome b-c1 complex. In addition, myxothiazol does not inhibit NADH:ubiquinone oxidoreductase. These results confirm our previous suggestion that the cytochrome b-c1 complex is involved in electron transfer from the primary dehydrogenases to DQ and DCIP and suggest that cytochrome b is not the binding site for myxothiazol.

Topics: 2,6-Dichloroindophenol; Antimycin A; Binding Sites; Chlorophyll; Cytochrome b Group; Electron Transport; Electron Transport Complex III; Indophenol; Light-Harvesting Protein Complexes; Methacrylates; Mitochondria; Mutation; NAD; Photosynthetic Reaction Center Complex Proteins; Plant Proteins; Quinones; Succinates; Succinic Acid; Thiazoles; Yeasts