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At least 973 records · Page 54Linked to original sources
PubMed2000

Isolation and characterization of cDNA clones for the e1beta and E2 subunits of the branched-chain alpha-ketoacid dehydrogenase complex in Arabidopsis.

Branched-chain alpha-ketoacid dehydrogenase (BCKDH) has been known in mammals to be a key enzyme of the catabolic pathway of branched-chain amino acids. We have isolated two cDNA clones encoding the E1beta and E2 subunits of BCKDH, respectively, from Arabidopsis thaliana. Proteins encoded in these cDNA sequences had putative mitochondrial targeting sequences and conserved domains reported for their mammalian counterparts. Northern blot and immunoblot analyses showed that transcripts from the respective genes and E2 protein markedly accumulated in leaves kept in the dark. We found that the activity of BCKDH in the leaf extracts also increased when plants were placed in the dark. Addition of sucrose to detached leaves inhibited the accumulation of transcripts, whereas application of a photosynthesis inhibitor strongly induced the expression of these genes even under light illumination. These observations indicate that the cellular sugar level is likely responsible for the dark-induced expression of these genes. The transcript levels of these genes were also high in senescing leaves, in which photosynthetic activity is low and free amino acids from degraded protein are likely to serve as an alternative energy source.

3-Methyl-2-Oxobutanoate Dehydrogenase (Lipoamide)↗
PubMed2001

A novel mechanism of nuclear photosynthesis gene regulation by redox signals from the chloroplast during photosystem stoichiometry adjustment.

Photosynthetic organisms acclimate to long term changes in the environmental light quality by an adjustment of their photosystem stoichiometry to maintain photosynthetic efficiency. By using light sources that predominantly excite either photosystem I (PSI) or photosystem II (PSII), we studied the effects of excitation imbalances between both photosystems on nuclear PSI gene transcription in transgenic tobacco seedlings with promoter::beta-glucuronidase gene fusions. Shifts from PSI to PSII light sources (and vice versa) induced changes in the reduction/oxidation state of intersystem redox components, and acclimation of tobacco seedlings to such changes were monitored by changes in chlorophyll a/b ratios and in vivo chlorophyll a fluorescence. The ferredoxin-NADP(+)-oxidoreductase gene promoter did not respond to these treatments, those from the genes for subunits PsaD and PsaF of PSI are activated by a reduction signal, and the plastocyanin promoter responded to both reduction and oxidation signals. Additional experiments with photosynthetic electron transport inhibitors 3-(3',4'-dichlorophenyl)-1,1'-dimethyl urea and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone demonstrated that the redox state of the plastoquinone pool controls the activity of the plastocyanin promoter, whereas subunit PsaD and PsaF gene transcription is regulated by other photosynthesis-derived signals. Thus, the expression of nuclear-encoded PSI genes is controlled by diverse light quality-dependent redox signals from the plastids during photosystem stoichiometry adjustment.

Binding Sites↗
PubMed2003

Characterization of the stimulating effect of low-dose stressors in maize and bean seedlings.

The effect of some more or less harmful compounds like Cd, Pb, Ni, Ti salts and DCMU at low concentrations on the development of chloroplasts in maize and bean seedlings was investigated. Chlorophyll content, chlorophyll a/b ratio, photosynthetic activity (14CO2 fixation), chlorophyll-protein composition of thylakoid membranes, fluorescence spectra of chloroplasts, fluorescence induction parameters of leaves and electron microscopic structure of maize and bean chloroplasts as well as growth parameters were studied. Stimulation of chlorophyll synthesis and photosynthetic activity was observed at different intervals during all of the treatments, while chlorophyll a/b ratios and fluorescence properties of leaves or chloroplasts did not change considerably except in DCMU treated plants. Heavy metal treatments increased the amount of photosystem I and light-harvesting complex II, while decreased amount of photosystem I and higher amount of light-harvesting complex II was found in DCMU treated thylakoids. Electron microscopy showed only sligth differences in the morphology of chloroplast lamellar system (mostly in DCMU treated plants), while the status of the plasmalemma and tonoplast seemed to be altered as a result of certain metal treatments. Results showed the expression of a cytokinin-like effect on the development of chloroplasts. It is assumed, that these low-dose stressors generate non-specific alarm reactions in plants, which may involve changes of the hormonal balance.

Chlorophyll↗
PubMed2002

Isolation and characterisation of nuclear mutants with enhanced mitochondrial mutability in the fission yeast Schizosaccharomyces pombe.

In this paper we report the isolation and preliminary characterisation of nuclear mutants with increased mitochondrial mutability in fission yeast. Screening of about 2000 clones after nitrosoguanidine mutagenesis led to the isolation of ten mutator mutants. For one of them (mut-1) we show that the mutation is chromosomally encoded. The activity of the mutator is restricted to the mitochondrial genome, since it increases the mutation rate to mitochondrially encoded drug resistance considerably, whereas the mutability of nuclear genes is not altered.

Cell Nucleus↗
PubMed2000

Antagonistic effects of hydrogen peroxide and glutathione on acclimation to excess excitation energy in Arabidopsis.

The redox status of the quinone B (Q(B)) and plastoquinone (PQ) pools plays a key role in the cellular and systemic signalling processes that control acclimatory responses in plants. In this study, we demonstrate the effects of hydrogen peroxide and glutathione on acclimatory responses controlled by redox events in the proximity of the Q(B)-PQ pools. Our results suggest that the chloroplast is a sink for H2O2 and that, paradoxically, high concentrations of H2O2 in the chloroplast protect the photosynthetic apparatus and the plant cell from photoinhibition and photooxidative damage. Excess glutathione, however, caused an effect antagonistic to that observed for high H2O2. An explanation of this apparent paradox and a hypothetical redox-signalling model are suggested.

Adaptation, Physiological↗
PubMed1998

Quenching of chlorophyll fluorescence by quinones.

Quinones caused quenching of Chl a fluorescence in native and model systems. Menadione quenched twofold the fluorescence of Chl a and BChl a in pea chloroplasts, chromatophores of purple bacteria, and liposomes at concentrations of 50-80 microM. To obtain twofold quenching in Triton X-100 micelles and in ethanol, the addition of 1.3 mM and 11 mM menadione was required, respectively. A proportional decrease in the lifetime and yield of Chl a fluorescence in chloroplasts, observed as the menadione concentration increased, is indicative of the efficient excitation energy transfer from bulk Chl to menadione. The decrease in the lifetime and yield of fluorescence was close to proportional in liposomes, but not in detergent micelles. The insensitivity of the menadione quenching effect to DCMU in chloroplasts, and similarity of its action in chloroplasts and liposomes indicate that menadione in chloroplasts interacts with antenna Chl, i.e., nonphotochemical quenching of fluorescence occurs.

Bacterial Chromatophores↗
PubMed1973

Phototaxis in Chlamydomonas reinhardtii.

Parameters which distinguish phototaxis from random motility in Chlamydomonas reinhardtii have been defined with quantitative assays. The phototactic responses in photosynthetic, mixotrophic, and heterotrophic cultures were highest during exponential growth and declined rapidly as the cultures entered stationary phase. In contrast, random motility was relatively constant throughout growth. Phototaxis and motility also differ in their sensitivity to azide and antimycin A. Both of these drugs inhibited phototaxis within 5 min, but motility was unaffected for at least 30 min. Phototaxis and motility have different ion requirements. Optimum motility was observed in the presence of either Ca(++) or Mg(++); phototaxis required Ca(++) and either K(+) or NH(4) (+). Photosynthesis is not required for phototaxis, since phototaxis was not inhibited by dichlorophenyldimethyl urea, and a mutant lacking chlorophyll was phototactic.

Antimetabolites↗
PubMed1980

Regulation of the Chlamydomonas cell cycle by light and dark.

By growing cells in alternating periods of light and darkness, we have found that the synchronization of phototrophically grown Chlamydomonas populations is regulated at two specific points in the cell cycle: the primary arrest (A) point, located in early G1, and the transition (T) point, located in mid-G1. At the A point, cell cycle progression becomes light dependent. At the T point, completion of the cycle becomes independent of light. Cells transferred from light to dark at cell cycle position between the two regulatory points enter a reversible resting state in which they remain viable and metabolically active, but do not progress through their cycles. The photosystem II inhibitor dichlorophenyldimethylurea (DCMU) mimics the A point block induced by darkness. This finding indicates that the A point block is mediated by a signal that operates through photosynthetic electron transport. Cells short of the T point will arrest in darkness although they contain considerable carbohydrate reserves. After the T point, a sharp increase occurs in starch degradation and in the endogenous respiration rate, indicating that some internal block to the availability of stored energy reserves has now been released, permitting cell cycle progression.

Carbon Dioxide↗
PubMed1982

Phosphorylation of chlamydomonas reinhardi chloroplast membrane proteins in vivo and in vitro.

Phosphorylation of thylakoid membrane proteins in the chloroplast of wild-type and mutant strains of Chlamydomonas reinhardi has been studied in vivo and in vitro. Intact cells or purified membranes were labeled with [32P]orthophosphate or [gamma-32P]ATP, respectively, and the presence of phosphorylated polypeptides was detected by autoradiography after membrane fractionation by SDS PAGE. The 32P was esterified to serine and threonine residues. At least six polypeptides were phosphorylated in vitro and in vivo, and corresponded to components of the photosystem II complex contributing to the formation of the light-harvesting-chlorophyll (LHC) a,b-protein complex, the DCMU binding site (32-35 kdaltons), and the reaction center (26 kdaltons). In agreement with previous reports (Alfonzo, et al., 1979, Plant Physiol., 65:730-734; and Bennett, 1979, FEBS (Fed. Eur. Biochem. Soc.) Lett., 103:342-344), the membrane-bound protein kinase was markedly stimulated by light in vitro via a mechanism requiring photosystem II activity. Phosphorylation of thylakoid membrane polypeptides in vivo was, however, completely independent of illumination. Similar amounts of phosphate were incorporated into the photosynthetic membranes of cells incubated in the dark, in white light with or without 3-(3,4-dichlorophenyl-1,1-dimethyl urea (DCMU), or in red or far-red light. Different turnovers of the phosphate were observed in the light and dark, and a phosphoprotein phosphatase involved in this turnover process was also associated with the membrane. Comparison of the amount of esterified phosphate per protein in vivo and the maximum incorporation in isolated membranes revealed that only a small fraction of the available sites could be phosphorylated in vitro. In contrast to the DCMU binding site, the LHC and 26-kdalton polypeptide were not phosphorylated in vivo when the reaction center II polypeptides of 44-54 kdaltons were missing. The finding that all the phosphoproteins appear to be components of the photosystem II complex and are only partially dephosphorylated in vivo suggests strongly that protein phosphorylation might play an important role in the maintenance of the organizational integrity of this complex. The observation that the LHC is not phosphorylated in the absence of the reaction center lends support to this idea.

Chlamydomonas↗
PubMed1982

Phagosome-lysosome fusion inhibited by algal symbionts of Hydra viridis.

Certain species of Chlorella live within the digestive cells of the fresh water cnidarian Hydra viridis. When introduced into the hydra gut, these symbiotic algae are phagocytized by digestive cells but avoid host digestion and persist at relatively constant numbers within host cells. In contrast, heat-killed symbionts are rapidly degraded after phagocytosis. Live symbionts appear to persist because host lysosomes fail to fuse with phagosomes containing live symbionts. Neither acid phosphatase nor ferritin was delivered via lysosomes into phagosomes containing live symbionts, whereas these lysosomal markers were found in 50% of the vacuoles containing heat-killed symbionts 1 h after phagocytosis. Treatment of symbiotic algae before phagocytosis with polycationic polypeptides abolishes algal persistence and perturbs the ability of these algae to control the release of photosynthate in vitro. Similarly, inhibition of photosynthesis and hence of the release of photosynthetic products as a result of prolonged darkness and 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) treatment also abolishes persistence. Symbiotic algae are not only protected from host digestive attack but are also selectively transported within host cells, moving from the apical site of phagocytosis to a basal position of permanent residence. This process too is disrupted by polycationic polypeptides, DCMU and darkness. Both algal persistence and transport may, therefore, be a function of the release of products from living, photosynthesizing symbionts. Vinblastine treatment of host animals blocked the movement of algae within host cells but did not perturb algal persistence: algal persistence and the transport of algae may be initiated by the same signal, but they are not interdependent processes.

Acid Phosphatase↗
PubMed2002

Regulation of rhizosphere acidification by photosynthetic activity in cowpea (Vigna unguiculata L. walp.) seedlings.

In contrast to cereals or other crops, legumes are known to acidify the rhizosphere even when supplied with nitrates. This phenomenon has been attributed to N2 fixation allowing excess uptake of cations over anions; however, as we have found previously, the exposure of the shoot to illumination can cause rhizosphere acidification in the absence of N2 fixation in cowpea (Vigna unguiculata L. Walp). In this study, we examined whether the light-induced acidification can relate to photosynthetic activity and corresponding alterations in cation-anion uptake ratios. The changes of rhizosphere pH along the root axis were visualized using a pH indicator agar gel. The intensity of pH changes (alkalization/acidification) in the rhizosphere was expressed in proton fluxes, which were obtained by processing the images of the pH indicator agar gel. The uptake of cations and anions was measured in nutrient solution. The rhizosphere was alkalinized in the dark but acidified with exposure of the shoots to light. The extent of light-induced acidification was increased with leaf size and intensity of illumination on the shoot, and completely stopped with the application of photosynthesis inhibitor. Although the uptake of cations was significantly lower than that of anions, the rhizosphere was acidified by light exposure. Proton pump inhibitors N,N'-dicyclohexyl carbodimide and vanadate could not stop the light-induced acidification. The results indicate that light-induced acidification in cowpea seedlings is regulated by photosynthetic activity, but is not due to excess uptake of cations.

Anions↗
PubMed2005

Detoxification of dissolved SO2 (bisulfite) by terricolous mosses.

BACKGROUND AND AIMS: The widespread calcifuge moss Pleurozium schreberi is moderately tolerant of SO2, whereas Rhytidiadelphus triquetrus is limited to calcareous soils in regions of the UK that were strongly affected by SO2 pollution in the 20th century. The proposition that tolerance of SO2 by these terricolous mosses depends on metabolic detoxification of dissolved bisulfite was investigated. METHODS: The capacities of the two mosses to accelerate loss of bisulfite from aqueous solutions of NaHSO3 were studied using DTNB [5, 5-dithio-(2-nitrobenzoic acid)] to assay bisulfite, and HPLC to assay sulfate in the incubation solutions. Incubations were performed for different durations, in the presence and absence of light, at a range of solution pH values, in the presence of metabolic inhibitors and with altered moss apoplastic Ca2+ and Fe3+ levels. KEY RESULTS: Bisulfite disappearance was markedly stimulated in the light and twice as great for R. triquetrus as for P. schreberi. DCMU, an inhibitor of photosynthetic electron chain transport, significantly reduced bisulfite loss. CONCLUSIONS: Bisulfite (SO2) tolerance in these terricolous mosses involves extracellular oxidation using metabolic (photo-oxidative) energy, passive oxidation by adsorbed Fe3+ (only available to the calcifuge) and probably also internal metabolic detoxification.

Biodegradation, Environmental↗
PubMed2001

Trehalose becomes the most abundant non-structural carbohydrate during senescence of soybean nodules.

Carbohydrate metabolism and symbiont survival were studied in nodules of soybean (G. max [L.] Merr. cv. Maple Arrow infected with Bradyrhizobium japonicum 61-A-101), induced to senesce simultaneously by application of the photosynthesis inhibitor dichloromethyl urea (DCMU). The plant-borne carbohydrates sucrose and starch started to decline after 2 d and reached background levels after 8 d, in parallel with the decline of nitrogenase. However, the microsymbiont-borne disaccharide trehalose declined only by about 40% and subsequently remained at a constant level of c. 6 mg x g(-1) dry weight up to 14 d, when nodules softened and decayed. The number of re-isolated viable bacteria was not significantly decreased in senescent nodules as compared to control nodules. These results indicate that during terminal senescence of nodules an appreciable part of the bacteria conserve their trehalose pools and survive.

Bradyrhizobium↗
PubMed2002

Light modulation of the gravitropic set-point angle (GSA).

A study has been made of the means by which light influences the gravitropic set-point angle (GSA) of the nodes of Tradescantia and the hypocotyls of the lazy-2 mutant of tomato. In light-grown Tradescantia there is a light-regulated developmental change in the GSA with the magnitude of this change being dependent on the photon flux density of white light. The photosynthetic inhibitor DCMU abolished the effect of white light. Low fluence rates of red light had no significant effect on the GSA of Tradescantia: It was concluded that there is an interaction between photosynthesis and the GSA in Tradescantia: The light-induced reduction of the GSA of the hypocotyl of lazy-2 tomato has previously been assumed to be solely an action of light acting via phytochrome. However, it can be shown that the GSA of hypocotyls of lazy-2 seedlings grown in white light is sensitive to DCMU and norflurazon treatment, hence the light effects on the GSA of an organ can be mediated via both phytochrome and photosynthesis. The implication of these findings to the study of gravitropism is discussed.

Diuron↗
PubMed2004

Internal recycling of respiratory CO2 in pods of chickpea (Cicer arietinum L.): the role of pod wall, seed coat, and embryo.

It has previously been proposed that respiratory CO2 released from the embryo in grain legume pods is refixed by a layer of cells on the inner pod wall. In chickpea this refixation process is thought to be of significance to the seed carbon budget, particularly under drought. In this study it is reported that the excised embryo, seed coat, and pod wall in chickpea are all photosynthetically competent, but the pod wall alone is capable of net O2 evolution over and above respiration. The predominant role of the pod wall in refixation is supported by measurements of fixation of isotopically labelled CO2, which show that more than 80% of CO2 is fixed by this tissue when provided to the pod interior. Chlorophyll concentrations are of the same order for embryo, seed coat, and pod wall tissues in younger pods on both an area and a fresh weight basis, but decline differentially with development from 12-30 d after podding. Imaging of chlorophyll distribution in the pod wall suggests that less than 15% of chloroplasts are located in the inner layer of cells thought to refix CO2 in legumes; this would be sufficient to refix less than 40% of respired CO2. It is concluded that while all tissues of the pod are capable of refixing respiratory carbon, the entire pod wall is responsible for the majority of this process, rather than a specialized layer of cells on the inner epidermis. The role of this fixed carbon in the pod for reallocation to the seed is discussed

Carbon Dioxide↗
PubMed2006

Imaging the production of singlet oxygen in vivo using a new fluorescent sensor, Singlet Oxygen Sensor Green.

Singlet oxygen is known to be produced by cells in response to photo-oxidative stresses and wounding. Due to singlet oxygen being highly reactive, it is thought to have a very short half-life in biological systems and, consequently, it is difficult to detect. A new commercially available reagent (singlet oxygen sensor green, SOSG), which is highly selective for singlet oxygen, was applied to a range of biological systems that are known to generate singlet oxygen. Induction of singlet oxygen production by the addition of myoglobin to liposome preparations demonstrated that the singlet oxygen-induced increases in SOSG fluorescence closely followed the increase in the concentration of conjugated dienes, which is stoichiometrically related to singlet oxygen production. Applications of photo-oxidative stresses to diatom species and leaves, which are known to result in the production of singlet oxygen, produced large increases in SOSG fluorescence, as did the addition of 3-(3',4'-dichlorophenyl)1,1-dimethylurea (DCMU) to these systems, which inhibits electron transport in photosystem II and stimulates singlet oxygen production. The conditional fluorescent (flu) mutant of Arabidopsis produces singlet oxygen when exposed to light after a dark period, and this coincided with a large increase in SOSG fluorescence. Wounding of leaves was followed by an increase in SOSG fluorescence, even in the dark. It is concluded that SOSG is a useful in vivo probe for the detection of singlet oxygen.

Diatoms↗
PubMed1975

Isolation and characterization of a proteinase from the sarcocarp of melon fruit.

A proteinase from the sarcocarp of melon (Cucumis Melo L. var. Prince) was purified by a three-step procedure involving batch-wise treatment with CM-cellulose fibers, column chromatography on CM-cellulose powder and gel filtration on Sephadex G-75. The final enzyme preparation was homogeneous on acrylamide gel electrophoresis. Its molecular weight was estimated by two different methods to be about 50,000. Anlayses indicated tha presence of 475 amino acid residues and at least 7 moles of hexose. The maximum activity was found in the alkaline pH region against casein as a substrate. The optimum temperature against casein was 70 degrees at pH 7.1. The enzyme was strongly inhibited by diisopropyl fluorophosphate, partly inhibited by HgCl2 and not inhibited by EDTA, p-chloromercuribenzoic acid, N-tosyl-L-lysine chloromethyl ketone, N-tosyl-L-phenylalanine chloromethyl ketone, and soybean trypsin inhibitor. The reduced and carboxymethylated insulin B-chain was cleaved at the peptide bonds of Asn3-Gln4, Cm-Cys7-Gly8, Glu13-Ala14, Leu15-Tyr16, Cm-Cys19-Gly20, Phe25-Tyr26, Pro28-Lys29, and Lys29-Ala30 by the enzyme.

Amino Acids↗