Regulation of thiol metabolism as a factor that influences the development and storage capacity of beech seeds

Seeds are the basis of propagation for the common beech (Fagus sylvatica L.), but the seed set of the beech is unsteady, with 5-10 years between abundant crops. Beech seeds are very difficult to store and lose their viability quickly even in optimum storage conditions. To date, it has not been possible to determine factors indicative of the aging process and the loss of viability of beech seeds during storage. To address this important economic challenge and interesting scientific problem, we analyzed the adjustment of the redox state during the development and storage of seeds. Many metabolic processes are based on reduction and oxidation reactions. Thiol proteins control and react to the redox state in the cells. The level of thiol proteins increased during seed maturation and decreased during storage. Gel-based redox proteomics identified 17 proteins in beech seeds during development. The proteins could be assigned to processes like metabolism and antioxidant functions. During storage, the number of proteins decreased to only six, i.e., oxidoreductases, peptidases, hydrolases and isomerases. The occurrence of peroxiredoxins (PRX) as thiol peroxidases and redox regulators indicates an important role of cytosolic 1CysPRX and PRXIIC, mitochondrial PRXIIF, and plastidic PRXIIE, 2CysPRX, and PRXQ in beech seeds during development and storage. Particularly, 2CysPRX was present in beech seeds during development and storage and may perform an important function in regulation of the redox state during both seed development and storage. The role of thiol proteins in the regulation of the redox state during the development and storage of beech seeds is discussed.

The acclimation response to high light is initiated within seconds as indicated by upregulation of AP2/ERF transcription factor network in Arabidopsis thaliana

High light acclimation implicates mechanisms on various molecular levels and time scales. The recently identified small transcription factor network of APETALA 2/ETHYLENE RESPONSE FACTOR (AP2/ERF) transcription factors is triggered upon transfer of Arabidopsis to high light and depends on metabolite export and mitogen activated protein kinase activation. An experimental design was developed consisting of a low light to high light and back to low light illumination. This allowed the determination of the time point of no return post high light transfer which activates transcription of the AP2/ERF network. Within 10 seconds of high light treatment transcript levels of ERF6, ERF104, ERF105 and RRTF were triggered to increase from low to high levels within the next 10 minutes witnessing an ultrafast retrograde pathway with a very early time point of no return. This response differed profoundly from other high light-responsive transcripts such as stromal ascorbate peroxidase (sAPX) which accumulated in a dose-dependent manner or COR47.

The function of the chloroplast 2-cysteine peroxiredoxin in peroxide detoxification and its regulation

The Arabidopsis genome contains nine open reading frames with homology to members of the peroxiredoxin (prx) family: one 1-Cys-prx, two 2-Cys-prx, five type II-prx, and one peroxiredoxin Q. The function of the peroxiredoxins in plant metabolism is only slowly emerging. They are assumed to reduce toxic peroxides to their corresponding alcohols with a rather broad substrate specificity. The 2-Cys peroxiredoxins (2-CP) were recently identified as members of the antioxidant defence system of chloroplasts. Knock-out mutants of Synechocystis and antisense mutants of Arabidopsis have provided insight into the function of 2-CPs in the photosynthetic antioxidant network. This review summarizes present knowledge on the enzymatic mechanism, the physiological context and the genetic regulation of the 2-CPs in plants and cyanobacteria. In addition, an extrapolation on the metabolic role of the chloroplast 2-CP is attempted based on the molecular features of 2-CPs from other organisms.

Redox-regulation of the expression of the peroxide-detoxifying chloroplast 2-cys peroxiredoxin in the liverwort Riccia fluitans

2-Cys peroxiredoxins (2-CPs) are H2O2- and alkyl hydroperoxide-detoxifying enzymes, and occur in animals, fungi, bacteria and higher plants. Here, the cDNA encoding a peroxiredoxin of a multicellular cryptogamic plant was first cloned from the liverwort Riccia fluitans L., and the dependence of its expression on the cellular redox state was analysed. The presence of an N-terminal targeting signal indicates that, like 2-CPs from higher plants, Riccia 2-CP is posttranslationally imported into chloroplasts. Addition of ascorbate and other reductants suppressed 2-CP gene expression and decreased 2-CP protein levels. With ascorbate, the decrease in 2-CP transcript level was fast, concentration dependent, and correlated with the amounts of ascorbate taken up by the tissue. In an approach to identify signaling components, staurosporine was proved to be a highly potent inhibitor of ascorbate-dependent repression of 2-CP-expression. The staurosporine effect indicates that a serine/threonine-kinase is involved in ascorbate-modulated redox regulation of 2-CP expression.

Significance of the V-type ATPase for the adaptation to stressful growth conditions and its regulation on the molecular and biochemical level

Two electrogenic H(+)-pumps, the vacuolar type H(+)-ATPase (V-ATPase) and the vacuolar pyrophosphatase, coexist at membranes of the secretory pathway of plants. The V-ATPase is the dominant H(+)-pump at endomembranes of most plant cells, both in terms of protein amount and, frequently, also in activity. The V-ATPase is indispensable for plant growth under normal conditions due to its role in energizing secondary transport, maintenance of solute homeostasis and, possibly, in facilitating vesicle fusion. Under stress conditions such as salinity, drought, cold, acid stress, anoxia, and excess heavy metals in the soil, survival of the cells depends strongly on maintaining or adjusting the activity of the V-ATPase. Regulation of gene expression and activity are involved in adapting the V-ATPase on long- and short-term bases. The mechanisms known to regulate the V-ATPase are summarized in this paper with an emphasis on their implications for growth and development under stress.

Reversible redox control of plant vacuolar H+-ATPase activity is related to disulfide bridge formation in subunit E as well as subunit A

The plant vacuolar proton pump can be subjected to reversible redox regulation in vitro. The redox-dependent activity change involves disulfide bridge formation not only in Vatp A, as reported for bovine V-ATPase, but also in the stalk subunit Vatp E. Microsomal membranes isolated from barley leaves were analysed for their activity of bafilomycin-sensitive ATP hydrolysis and proton pumping using quinacrine fluorescence quenching in vesicle preparations. ATP hydrolysis and proton pumping activity were inhibited by H2O2. H2O2-deactivated ATPase was reactivated by cysteine and glutathione. The glutathione concentration needed for half maximal reactivation was 1 mmol l-1. The activity loss was accompanied by shifts in electrophoretic mobility of Vatp A and E which were reversed upon reductive reactivation. The redox-dependent shift was also seen with recombinant Vatp E, and was absent following site-directed mutagenesis of either of the two cys residues conserved throughout all plant Vatp E sequences. V-ATPase was also inhibited by oxidized thioredoxin. These results support the hypothesis that tuning of vacuolar ATPase activity can be mediated by redox control depending on the metabolic requirements.

Characterization of plant growth promoting rhizobacteria isolated from polluted soils and containing 1-aminocyclopropane-1-carboxylate deaminase

Fifteen bacterial strains containing 1-aminocyclopropane-1-carboxylate (ACC) deaminase were isolated from the rhizoplane of pea (Pisum sativum L.) and Indian mustard (Brassica juncea L.) grown in different soils and a long-standing sewage sludge contaminated with heavy metals. The isolated strains were characterized and assigned to various genera and species, such as Pseudomonas brassicacearum, Pseudomonas marginalis, Pseudomonas oryzihabitans, Pseudomonas putida, Pseudomonas sp., Alcaligenes xylosoxidans, Alcaligenes sp., Variovorax paradoxus, Bacillus pumilus, and Rhodococcus sp. by determination of 16S rRNA gene sequences. The root elongation of Indian mustard and rape (Brassica napus var. oleifera L.) germinating seedlings was stimulated by inoculation with 8 and 13 isolated strains, respectively. The bacteria were tolerant to cadmium toxicity and stimulated root elongation of rape seedlings in the presence of 300 microM CdCl2 in the nutrient solution. The effect of ACC-utilising bacteria on root elongation correlated with the impact of aminoethoxyvinylglycine and silver ions, chemical inhibitors of ethylene biosynthesis. A significant improvement in the growth of rape caused by inoculation with certain selected strains was also observed in pot experiments, when the plants were cultivated in cadmium-supplemented soil. The biomass of pea cv. Sparkle and its ethylene sensitive mutant E2 (sym5), in particular, was increased through inoculation with certain strains of ACC-utilising bacteria in pot experiments in quartz sand culture. The beneficial effect of the bacteria on plant growth varied significantly depending on individual bacterial strains, plant genotype, and growth conditions. The results suggest that plant growth promoting rhizobacteria containing ACC deaminase are present in various soils and offer promise as a bacterial inoculum for improvement of plant growth, particularly under unfavourable environmental conditions.

Characterization of plant growth promoting rhizobacteria isolated from polluted soils and containing 1-aminocyclopropane-1-carboxylate deaminase

Fifteen bacterial strains containing 1-aminocyclopropane-1-carboxylate (ACC) deaminase were isolated from the rhizoplane of pea (Pisum sativum L.) and Indian mustard (Brassica juncea L.) grown in different soils and a long-standing sewage sludge contaminated with heavy metals. The isolated strains were characterized and assigned to various genera and species, such as Pseudomonas brassicacearum, Pseudomonas marginalis, Pseudomonas oryzihabitans, Pseudomonas putida, Pseudomonas sp., Alcaligenes xylosoxidans, Alcaligenes sp., Variovorax paradoxus, Bacillus pumilus, and Rhodococcus sp. by determination of 16S rRNA gene sequences. The root elongation of Indian mustard and rape (Brassica napus var. oleifera L.) germinating seedlings was stimulated by inoculation with 8 and 13 isolated strains, respectively. The bacteria were tolerant to cadmium toxicity and stimulated root elongation of rape seedlings in the presence of 300 microM CdCl2 in the nutrient solution. The effect of ACC-utilising bacteria on root elongation correlated with the impact of aminoethoxyvinylglycine and silver ions, chemical inhibitors of ethylene biosynthesis. A significant improvement in the growth of rape caused by inoculation with certain selected strains was also observed in pot experiments, when the plants were cultivated in cadmium-supplemented soil. The biomass of pea cv. Sparkle and its ethylene sensitive mutant E2 (sym5), in particular, was increased through inoculation with certain strains of ACC-utilising bacteria in pot experiments in quartz sand culture. The beneficial effect of the bacteria on plant growth varied significantly depending on individual bacterial strains, plant genotype, and growth conditions. The results suggest that plant growth promoting rhizobacteria containing ACC deaminase are present in various soils and offer promise as a bacterial inoculum for improvement of plant growth, particularly under unfavourable environmental conditions.

Salt-induced expression of the vacuolar H+-ATPase in the common ice plant is developmentally controlled and tissue specific

For salinity stress tolerance in plants, the vacuolar type H+-ATPase (V-ATPase) is of prime importance in energizing sodium sequestration into the central vacuole and it is known to respond to salt stress with increased expression and enzyme activity. In this work we provide information that the expressional response to salinity of the V-ATPase is regulated tissue and cell specifically under developmental control in the facultative halophyte common ice plant (Mesembryanthemum crystallinum). By transcript analysis of subunit E of the V-ATPase, amounts did not change in response to salinity stress in juvenile plants that are not salt-tolerant. In a converse manner, in halotolerant mature plants the transcript levels increased in leaves, but not in roots when salt stressed for 72 h. By in situ hybridizations and immunocytological protein analysis, subunit E was shown to be synthesized in all cell types. During salt stress, signal intensity declined in root cortex cells and in the cells of the root vascular cylinder. In salt-stressed leaves of mature plants, the strongest signals were localized surrounding the vasculature. Within control cells and with highest abundance in mesophyll cells of salt-treated leaves, accumulation of subunit E protein was observed in the cytoplasm, indicating its presence not only in the tonoplast, but also in other endoplasmic compartments.

Antisense suppression of 2-cysteine peroxiredoxin in Arabidopsis specifically enhances the activities and expression of enzymes associated with ascorbate metabolism but not glutathione metabolism

The aim of this study was to characterize the effect of decreased 2-cysteine peroxiredoxin (2-CP) on the leaf anti-oxidative system in Arabidopsis. At three stages of leaf development, two lines of transgenic Arabidopsis mutants with decreased contents of chloroplast 2-CP were compared with wild type and a control line transformed with an empty vector. Glutathione contents and redox state were similar in all plants, and no changes in transcript levels for enzymes involved in glutathione metabolism were observed. Transcript levels for chloroplastic glutathione peroxidase were much lower than those for 2-CP, and both cytosolic and chloroplastic glutathione peroxidase were not increased in the mutants. In contrast, the foliar ascorbate pool was more oxidized in the mutants, although the difference decreased with plant age. The activities of thylakoid and stromal ascorbate peroxidase and particularly monodehydroascorbate reductase were increased as were transcripts for these enzymes. No change in dehydroascorbate reductase activity was observed, and effects on transcript abundance for glutathione reductase, catalase, and superoxide dismutase were slight or absent. The results demonstrate that 2-CP forms an integral part of the anti-oxidant network of chloroplasts and is functionally interconnected with other defense systems. Suppression of 2-CP leads to increased expression of other anti-oxidative genes possibly mediated by increased oxidation state of the leaf ascorbate pool.

Phosphate uptake across the tonoplast of intact vacuoles isolated from suspension-cultured cells of Catharanthus roseus (L.) G. Don

Transport of inorganic orthophosphate (Pi) across the tonoplast membrane was studied using intact vacuoles isolated from suspension-cultured cells of Catharanthus roseus. Orthophosphate uptake was strongly stimulated in the presence of Mg-ATP and Mg-pyrophosphate and inhibited by bafilomycin and concanamycin which are potent inhibitors of the vacuolar H+-ATPase. These results indicated that the build-up of an electrochemical gradient by the H - pumps was essential for the uptake of Pi. Potassium thiocyanate, which dissipates the membrane potential across the tonoplast, strongly inhibited the Mg-ATP-stimulated uptake of Pi, while only a weak inhibition was observed in the presence of NH4Cl, which dissipates the pH gradient. These results indicate that, as observed for other anions like malate or chloride, the electrical component is the driving force of Pi uptake, whereas the deltapH plays only a minor role. Possible competitive inhibitors of Pi, MoO4(2-) , VO4(3-) and CrO4(2-) were tested. Among them, CrO4(2-) strongly inhibited Pi uptake into the vacuoles. Various inhibitors of anion transport were also tested. Only 4,4-diisothiocyanostilbene-2,2'-disulfonic acid strongly inhibited Pi uptake into the vacuoles. The function of the vacuolar Pi transporters for cytoplasmic Pi homeostasis is discussed.

Effect of associative bacteria on element composition of barley seedlings grown in solution culture at toxic cadmium concentrations

The response of barley seedlings to inoculation with associative rhizobacteria Azospirillum lipoferum 137, Arthrobacter mysorens 7, Agrobacterium radiobacter 10 and Flavobacterium sp. L30 was studied in hydroponic and quartz sand cultures in the presence of 50 microM CdCl2. Cadmium caused severe inhibition in the growth and uptake of nutrient elements by the plants. Inoculation with the bacteria slightly stimulated root length and biomass of hydroponically grown Cd-treated seedlings. The bacteria increased the content of nutrients such as P, Mg, Ca, Fe, Mn and Na in roots and or shoots of the plants grown in the absence of Cd. Positive changes in the element composition caused by the bacteria were less pronounced in Cd-treated plants, whereas the total amount of nutrients taken by the inoculated plants was generally increased significantly. The content of Cd in the inoculated plants was unchanged, except increased in roots upon addition of A. lipoferum 137. Inoculation did not affect the activity of peroxidase, alpha-mannosidase, phosphodiesterae, alpha-galactosidase, and concentration of sulfhydryl compounds used as biochemical markers of stress in plant roots. The results showed that associative bacteria were capable of decreasing partially the toxicity of Cd for the barley plants through the improvement in uptake of nutrient elements.

Extracellular beta-glucosidase activity in barley involved in the hydrolysis of ABA glucose conjugate in leaves

Abscisic acid conjugate concentrations increased in barley xylem sap under salinity, whereas it remained at a low level in the intercellular washing fluid (IWF) of barley primary leaves (Hordeum vulgare cv. Gerbel). Here it is shown that IWF contains beta-glucosidase activity which releases abscisic acid (ABA) from the physiologically inactive ABA-glucose conjugate pool in the leaf apoplast. The following data support this conclusion and give the first biochemical and physiological characterization of the extracellular glucosidase activity in barley. Free ABA was released by the incubation of ABA glucose ester with IWF. The product exhibited the retention time of authentic ABA upon separation by thin layer chromatography and was identified by ABA-ELISA. p-Nitrophenol-beta-D-glucopyranoside (pNPG) was used as the substrate for beta-glucosidases. The K(M)(pNPG) was 1.8 mmol l(-1). The activity was affected by ABA glucopyranoside in a competitive type of inhibition with a K(I) of 400 micromol l(-1). Various hormone conjugates were compared with respect to their inhibitory effect on beta-glucosidase activity. Inhibition was highest for the ABA glucopyranoside and the zeatin riboside, but insignificant for ABA methyl ester and zeatin-9-beta-D-glucoside. The specific activity of the beta-glucosidase was 16-fold greater in IWF as compared to crude leaf extracts confirming its extracellular compartmentation. The activity of beta-glucosidase was strongly increased after growth in hydroponic medium supplemented with NaCl. The data support the hypothesis that the glucose conjugate is a long-distance transport form of ABA.

Extracellular beta-glucosidase activity in barley involved in the hydrolysis of ABA glucose conjugate in leaves

Abscisic acid conjugate concentrations increased in barley xylem sap under salinity, whereas it remained at a low level in the intercellular washing fluid (IWF) of barley primary leaves (Hordeum vulgare cv. Gerbel). Here it is shown that IWF contains beta-glucosidase activity which releases abscisic acid (ABA) from the physiologically inactive ABA-glucose conjugate pool in the leaf apoplast. The following data support this conclusion and give the first biochemical and physiological characterization of the extracellular glucosidase activity in barley. Free ABA was released by the incubation of ABA glucose ester with IWF. The product exhibited the retention time of authentic ABA upon separation by thin layer chromatography and was identified by ABA-ELISA. p-Nitrophenol-beta-D-glucopyranoside (pNPG) was used as the substrate for beta-glucosidases. The K(M)(pNPG) was 1.8 mmol l(-1). The activity was affected by ABA glucopyranoside in a competitive type of inhibition with a K(I) of 400 micromol l(-1). Various hormone conjugates were compared with respect to their inhibitory effect on beta-glucosidase activity. Inhibition was highest for the ABA glucopyranoside and the zeatin riboside, but insignificant for ABA methyl ester and zeatin-9-beta-D-glucoside. The specific activity of the beta-glucosidase was 16-fold greater in IWF as compared to crude leaf extracts confirming its extracellular compartmentation. The activity of beta-glucosidase was strongly increased after growth in hydroponic medium supplemented with NaCl. The data support the hypothesis that the glucose conjugate is a long-distance transport form of ABA.

Characterization of a small GTP-binding protein of the rab 5 family in Mesembryanthemum crystallinum with increased level of expression during early salt stress

A cDNA encoding a member of the Ypt/Rab family of small GTP-binding proteins was cloned from the facultative CAM plant Mesembryanthemum crystallinum. Mcrab5b includes an open reading frame of 201 amino acids. The deduced amino acid sequence shows 91% similarity to LjRAB5b isolated from Lotus japonicus. The amino acid sequence of McRAB5b provides interesting features suggesting that McRAB5b and its homologue from Lotus japonicus represent a new subclass of Ypt/Rab proteins. The fact that proteins like McRAB5b and LjRAB5b were only found in plants and not in yeast or vertebrates suggests that they have plant-specific functions. The expression of Mcrab5b as investigated by northern blot hybridization and RT-PCR was stimulated under salt stress. After heterologous expression in Escherichia coli an antibody was raised against recombinant McRABSb protein. Western blot analysis revealed that McRAB5b was bound to membranes. It is present in a monomeric and a dimeric form in vitro and in vivo. In vitro only the monomeric protein exhibits a binding capacity for radiolabelled GTP, while the dimer is unable to do so, indicating that the activity may be regulated by monomer/dimer transition.

Subunit C of the vacuolar H+-ATPase of Hordeum vulgare

The molecular cloning of the first subunit C of the plant vacuolar H+-ATPase is reported. Tonoplast vesicles were purified from barley leaves by sucrose gradient centrifugation, and the tonoplast polypeptides were separated by two-dimensional (2-D) gel electrophoresis. Using an anti-ATPase holoenzyme antibody, a polypeptide was recognized in the molecular mass range of 40 kDa with an isoelectric point of about 6.0, and tentatively identified as subunit C. The polypeptide spot was excised from about 50 2-D gels and subjected to endo Lys C proteolysis. Two proteolytic peptides were sequenced and the amino acid sequences were used to design degenerated oligonucleotides, followed by PCR amplification with cDNA template and screening of a cDNA library synthesized from Hordeum vulgare poly A mRNA of epidermis strips. The full length clone of 1.5 kbp contains an open reading frame of 1062 bp encoding a polypeptide of 354 amino acids with a molecular mass of 39,982 Da and an isoelectric point of 6.04. Amino acid identity with sequences of SUC from animals and fungi is in the range of 36.7 to 38.5%. Expression of the cloned gene was demonstrated by Northern blotting and RT-PCR.

Subunit D of the vacuolar H+-ATPase of Arabidopsis thaliana

A 1034 bp cDNA encoding the full length sequence of subunit D of the vacuolar H+-ATPase was cloned from Arabidopsis thaliana. The open reading frame of the cDNA clone vatpD contains 780 bp and codes for a protein of 29.1 kDa with a pI of 9.52. Structural predictions show similarities to subunit gamma of the F-ATP synthases. Identity between subunit D of the vacuolar H+-ATPase of A. thaliana and subunits D from other eukaryotic organisms is in the range of 57% (Bos taurus) to 48% (Candida albicans). Hybridization of genomic DNA with vatpD indicates the existence of one gene copy of subunit D in A. thaliana. Northern blot hybridization and in situ hybridization showed expression of vatpD in all cell types. The expression of subunit D was not modified by salt stress or abscisic acid treatment in A. thaliana.

Protective function of chloroplast 2-cysteine peroxiredoxin in photosynthesis. Evidence from transgenic Arabidopsis

2-Cysteine peroxiredoxins (2-CPs) constitute a ubiquitous group of peroxidases that reduce cell-toxic alkyl hydroperoxides to their corresponding alcohols. Recently, we cloned 2-CP cDNAs from plants and characterized them as chloroplast proteins. To elucidate the physiological function of the 2-CP in plant metabolism, we generated antisense mutants in Arabidopsis. In the mutant lines a 2-CP deficiency developed during early leaf and plant development and eventually the protein accumulated to wild-type levels. In young mutants with reduced amounts of 2-CP, photosynthesis was impaired and the levels of D1 protein, the light-harvesting protein complex associated with photosystem II, chloroplast ATP synthase, and ribulose-1,5-bisphosphate carboxylase/oxygenase were decreased. Photoinhibition was particularly pronounced after the application of the protein synthesis inhibitor, lincomycin. We concluded that the photosynthetic machinery needs high levels of 2-CP during leaf development to protect it from oxidative damage and that the damage is reduced by the accumulation of 2-CP protein, by the de novo synthesis and replacement of damaged proteins, and by the induction of other antioxidant defenses in 2-CP mutants.

Thioredoxin peroxidase in the Cyanobacterium Synechocystis sp. PCC 6803

The amino acid sequence deduced from the open reading frame designated sll0755 in Synechocystis sp. PCC 6803 is similar to the amino acid sequences of thioredoxin peroxidases from other organisms. In the present study, we found that a recombinant SLL0755 protein that was expressed in Escherichia coli was able to reduce H2O2 and tertiary butyl hydroperoxide with thioredoxin from E. coli as the electron donor. Targeted disruption of open reading frame sll0755 in Synechocystis sp. PCC 6803 cells completely eliminated the H2O2-dependent and tertiary butyl hydroperoxide-dependent photosynthetic evolution of oxygen and the electron flow in photosystem II. These results indicate that the product of open reading frame sll0755 is a thioredoxin peroxidase whose activities are coupled to the photosynthetic electron transport system in Synechocystis sp. PCC 6803.

Modulation of the vacuolar H+-ATPase by adenylates as basis for the transient CO2-dependent acidification of the leaf vacuole upon illumination

Using tonoplast vesicles, we have investigated the activity of the vacuolar H+-ATPase which is the dominant proton pump at the tonoplast of mesophyll cells. Bafilomycin-sensitive ATP hydrolysis or acidification of tonoplast vesicles in the presence of ATP were measured at varying ATP, ADP and Pi concentrations, and in the presence of oxidized or reduced glutathione. Increased ATP/ADP ratios as reported for the extrachloroplast cytoplasm during the induction phase of photosynthesis at high or low CO2 (P. Gardeström, Biochim. Biophys. Acta 1183 (1993) 327-332) increased the activity of the V-ATPase in simulation experiments with vesicles. Depending on reported subsequent decreases in cytoplasmic ATP/ADP ratios in the presence of high or low CO2, the ATPase activity of tonoplast vesicles changed in simulation experiments to lower values. More than 10 mM phosphate was required to decrease the ATPase activity in vesicles significantly at ATP/ADP ratios of 3 or higher, indicating that ATPase activity is controlled more by ratios of ATP to ADP than by phosphorylation potentials (ATP)/(ADP)(Pi). Oxidized glutathione was inhibitory. The results permit interpretation of the observation that on illumination of previously darkened leaves the pH of the vacuoles of mesophyll cells decreases indicating energized transport of protons across the tonoplast into acidic vacuoles, and that the extent of vacuolar acidification depends on the CO2 concentration of the surrounding air (Z.-H. Yin, S. Neimanis, U. Heber, Planta 182 (1990) 253-261). We conclude that short term control of tonoplast ATPase activity in leaves during dark/light transients can essentially be understood on the basis of reported changes in cytoplasmic ATP/ADP ratios, with a possible participation of redox modulation.

Na+-ATPase from the plasma membrane of the marine alga Tetraselmis (Platymonas) viridis forms a phosphorylated intermediate

Plasma membranes isolated from the marine unicellular alga Tetraselmis (Platymonas) viridis were phosphorylated by [gamma-32P]ATP, and membrane proteins were then analyzed by PAGE in SDS, under acidic conditions. Three radioactive components with apparent molecular masses of 100 kDa, 76 kDa, and 26 kDa were detected. The phosphorylation of one of them, the 100 kDa polypeptide, was specifically stimulated by Na+. Vanadate almost completely inhibited the Na+-mediated phosphorylation of the peptide. The phosphate bound to this peptide underwent rapid turnover and was discharged by hydroxylamine. The 100 kDa phosphopeptide was sensitive to ADP. The conclusion is drawn that the 100 kDa phosphopeptide is a phosphorylated intermediate of the Na+-transporting ATPase in the T. viridis plasma membrane.

Cadmium leads to stimulated expression of the lipid transfer protein genes in barley: implications for the involvement of lipid transfer proteins in wax assembly

In order to investigate the nature of genes expressed in leaf epidermal cells of higher plants, we have identified the nucleotide sequence of a cDNA designated ltp 7a2b encoding a novel nonspecific lipid transfer protein of barley (Hordeum vulgare L. cv. Gerbel). The cDNA of 755 basepairs contains an open reading frame of 366 nucleotides coding for a 12.3-kDa polypeptide. The first 29 amino acids constitute the putative signal peptide, characteristic for targeting to the secretory pathway. Analysis of mRNA levels by Northern blotting indicated that ltp 7a2b is preferentially expressed in the leaf epidermis. Levels of mRNA decreased during ageing of leaf tissue. Expression of ltp 7a2b was stimulated by a factor of 2 - 3 when the seedlings were grown in the presence of cadmium (10 - 1600 microM). Concomitantly, the primary leaves of Cd-exposed seedlings contained elevated levels of abscisic acid and thicker was layer of the cuticle. At 100 microM Cd in the hydroponic medium, the was cover was increased by 50%. The increase in abscisic acid content, ltp 7a2b mRNA and was coverage was either not seen, or seen much less, in Ni- and Zn-stressed seedlings. The data add circumstantial evidence to the recently proposed hypothesis that nonspecific lipid transfer proteins function in transfer of cutin and/or wax monomers from the site of synthesis in the cell to the cuticle.

The plant 2-Cys peroxiredoxin BAS1 is a nuclear-encoded chloroplast protein: its expressional regulation, phylogenetic origin, and implications for its specific physiological function in plants

2-Cys peroxiredoxins constitute a family of enzymes which catalyze the transfer of electrons from sulfhydryl residues to peroxides and are ubiquitously distributed among all organisms. This paper characterizes the higher plant 2-Cys-peroxiredoxin BAS1. (i) Escherichia coli over-expressing BAS1 exhibit increased tolerance for alkyl hydroperoxides in vivo. This result substantiates the peroxiredoxin function of BAS1. (ii) BAS1 protein is associated with the soluble chloroplast fraction of mesophyll protoplasts. Import and processing of in vitro-transcribed and cell-free translated BAS1 protein into isolated chloroplasts provides conclusive evidence that the plant-specific N-terminal extension of bas1 encodes the chloroplast import signal which targets the pre-form of BAS1 to the chloroplast stroma where it is cleaved to its mature size. (iii) Genomic analysis reveals that the targeting signal is encoded by a separate exon in Arabidopsis thalina. (iv) The amino acid sequence of the BAS1 core protein of higher plants has a higher degree of similarity to open reading frames in the genome of the bluegreen algae Synechochystis PCC sp. 6803 and in the plastome of the red algae Porphyra purpurea than to any other nuclear-encoded 2-Cys peroxiredoxin. Therefore, it is tempting to speculate that the chloroplast import signal was added to an ancestor gene of endosymbiotic origin in the course of plant evolution. (v) The bas1 gene expression is regulated under the control of the cellular redox state which is in accordance with the anti-oxidant function of the enzyme. While oxidative stressors increased expression only slightly, antioxidants such as reduced thiols strongly suppressed the transcript level. The implications of these findings are discussed with respect to the possible physiological functions of BAS1.

Photosynthesis in the basal growing zone of barley leaves

Cell proliferation, elongation, determination and differentiation mainly take place in the basal 5 mm of a barley leaf, the so-called basiplast. A considerable portion of cDNAs randomly selected from a basiplast cDNA library represented photosynthetic genes such as CP29, RUBISCO-SSU and type I-LHCP II. Therefore, we became interested in the role of the basiplast in establishing photosynthesis. (1) Northern blot analysis revealed expression of photosynthetic genes in the basiplast, although at a low level. Analysis of basiplasts at different developmental stages of the leaves revealed maximal expression of photosynthetic genes during early leaf development. The activity of these genes shows that plastid differentiation involves the development of the photosynthetic apparatus even at this early state of leaf cell expansion. (2) This conclusion was supported by the fact that chlorophylls and carotenoids are synthesized in the basiplast. The qualitative pattern of pigment composition was largely similar to that of fully differentiated green leaves. (3) The transition from proplastids to chloroplasts progressed in the basal 5 mm of the leaf, so that the number of grana lamellae per thylakoid stack increased with distance from the meristem from zero to about five. (4) Photosynthetic function was studied by chlorophyll a-fluorescence measurements. In dark-adapted 8-day-old primary leaves, the fluorescence ratio (FP-Fo)/FP was little decreased in basiplasts as compared to leaf blades. During steady state photosynthesis, the ratio (FM'-Fo)/FM' was high in leaf blade (0.5), but low in the sheath (0.25) and in the basiplast (0.18), indicating the existence of functional, albeit low light-adapted chloroplasts in the basiplast. (5) Further on, chlorophyll a fluorescence analysis in relation to seedling age revealed efficient photosynthetic performance in the basiplast of 3- to 6-day-old seedlings which later-on differentiates into leaf blade as compared to the basiplast of 7- to 12-day-old seedlings which develops into leaf sheath and finally ceases to grow. The leaf age dependent changes in basiplast photosynthesis were reflected by changes in pigment contents and LHCP II expression both of which also revealed a maximum in the basiplast of 4-day-old seedlings.

cDNA sequence and expression of subunit E of the vacuolar H(+)-ATPase in the inducible Crassulacean acid metabolism plant Mesembryanthemum crystallinum

A cDNA coding for subunit E of the vacuolar H(+)-ATPase was cloned from Mesembryanthemum crystallinum, a plant which switches from C3-photosynthesis to Crassulacean acid metabolism under saline growth conditions. Sequence homology between the three subunit E-polypeptides of different higher plant species varied between 77.6 and 73.3%; peptide length was between 226 and 230 amino acid residues, 43 of which are invariant in all seven subunit E-polypeptides known so far from animals, fungi and plants. The deduced relative molecular mass of subunit E in Mesembryanthemum crystallinum is 26162 Da. Subunit E is present both in C3- and CAM-plants. mRNA levels increased severalfold in leaves of CAM-induced plants. This was accompanied by a less pronounced increase in subunit E protein. Obviously, expression is stimulated under conditions of increased requirement for tonoplast H(+)-pumping activity.

Primary structure and expression of plant homologues of animal and fungal thioredoxin-dependent peroxide reductases and bacterial alkyl hydroperoxide reductases

Higher plants express genes encoding peroxiredoxins of the two-cysteine type. This is concluded from the isolation of cDNAs from spinach (Spinacia oleracea) and barley (Hordeum vulgare cv. Gerbel) which are homologous to animal, fungal, and bacterial two-cysteine peroxiredoxins. Northern blot analysis indicated the presence of at least one corresponding gene in all angiosperms analyzed suggesting that bas1 is a member of an ubiquitous gene family encoding a protein of fundamental importance in oxidative stress defense also in plants. In barley, expression increased upon application of methyl viologen but was not affected by ozone. mRNA levels increased during deetiolation in the light. Maximal abundance of bas1 transcripts was observed in young developing shoot segments where cell division and elongation take place. Expression was insignificant in roots. The amount of bas1 protein was high in the leaf blade, particularly in etiolated plants, and did not respond to oxidative stress. bas1 protein was not detected in roots. From our data, we suggest that bas1 is an antioxidant enzyme particularly important in the developing shoot and photosynthesizing leaf.

Degradation of glutathione S-conjugates by a carboxypeptidase in the plant vacuole

For plants, glutathione conjugation is a major pathway to detoxify organic xenobiotic. Glutathione S-conjugates (SG-conjugates) are formed in the cytosol, the in vitro transport over the tonoplast has been described and a final storage in the vacuole has been postulated. We show here that alachlor rapidly accumulates as GS-conjugates in the plant vacuole and that the first step of its degradation, the formation of the respective gamma-glutamylcysteinyl-S-conjugate, is catalyzed by a vacuolar carboxypeptidase. These results suggest the glutathione conjugate as a transport form but not a storage form of xenobiotic molecules.

Abscisic acid causes changes in gene expression involved in the induction of the landform of the liverwort Riccia fluitans L

The conversion of the submerged form of Riccia fluitans to the landform either by transfer to a moist solid surface or by treatment with abscisic acid (ABA), is accompanied by the formation of a set of new polypeptides and concomitant down-regulation of other polypeptides. Changes in gene expression were analyzed by two-dimensional separations of proteins and differential screening of a cDNA library. One of the landform-specific proteins might depend on the expression of the newly discovered Ric 1 gene. The deduced amino acid sequence of the isolated Ric 1 cDNA clone codes for a protein with a molecular mass of 30.1 kDa. This polypeptide possesses two amino acid sequences which are repeated five times each and it is largely hydrophilic with the exception of a hydrophobic carboxyl-terminal region. Under ABA treatment the expression of the Ric 1 mRNA had already reached its maximum after 1 h of incubation. Transferring submerged thalli onto an agar surface resulted in a slower induction. The Ric 1 gene product shows homology to an embryo-specific polypeptide of carrot seeds and to the group 3 of late-embryogenesis-abundant (LEA) proteins. Interestingly, ABA treatment improved the desiccation tolerance of the submerged thalli. Additionally, ABA stimulated the synthesis of a protein which is immunologically related to a tonoplast protein. This finding, together with the fact that the ABA-induced landform exhibits an increased activity of several vacuolar enzymes, may indicate a special role of the tonoplast and the vacuole during ABA-induced conversion of the thallus from the submerged to the terrestrial form.

Subunit E of the vacuolar H(+)-ATPase of Hordeum vulgare L.: cDNA cloning, expression and immunological analysis

A tonoplast protein of 31 kDa apparent molecular mass (TpP 31) was isolated from two-dimensional gels. Amino acid sequences were determined from LysC endoproteinase-peptide fragments. Using degenerate oligonucleotides, a corresponding cDNA clone of 1034 bp was isolated from a barley leaf cDNA library. It encodes for subunit E of the vacuolar H(+)-ATPase, the first one identified in plants so far. The open reading frame extends over 681 bp, encoding a gene product of 227 amino acids and a calculated molecular weight of 26,228 g mol-1. Northern and Western blot analysis indicates constitutive expression of subunit E in all plant organs with only small effects of salt stress. Localization of TpP 31 at the tonoplast was confirmed in fractions of purified vacuolar membrane obtained by free-flow electrophoresis. Immunoprecipitation of newly synthesized 35S-labelled membrane proteins with anti-TpP 31 gave two additional bands with apparent molecular masses of about 53 and 62 kDa. Gel filtration after mild solubilization showed co-purification of TpP 31 with the 55 kDa subunit of the H(+)-ATPase. Both results provide evidence beyond the sequence homology that TpP 31 is a structural component of the vacuolar H(+)-ATPase.

Solute transport across the tonoplast of barely mesophyll vacuoles: Mg2+ determines the specificity, and ATP lipophilic amino acids the activity of the amino acid carrier

After stimulation with ATP and in the absence of divalent cations, isolated barely mesophyll vacuoles exhibited massive solute fluxes across the tonoplast, measured either as efflux of endogenous solutes or as uptake of radioactive-labeled compounds. Transported solutes were ions (particularly K+, NO3-, Cl-) and amino acids (for example, ala, arg, asp, gln, leu, met). Addition of Mg2+ in excess of added ATP inhibited fluxes of inorganic ions and of positively charged amino acids, but not, or to a smaller extent, those of neutral amino acids. Thus, Mg2+ increased the specificity of the carrier for amino acids such as alanine and glutamine. All ATP-stimulated transport processes were sensitive towards inhibition by lipophilic amino acids, for example by leucine and phenylalanine. After stimulation with sulfhydryl reagents, the inhibitory properties of Mg2+ and lipophilic amino acids were lost. These data concur with the hypothesis of a single transporter which exhibits a channel-like structure with a low degree of substrate selectivity in the absence of Mg2+, and which functions as a neutral amino acid carrier in the presence of Mg2+.

Continuous-wave laser pump light sources: new concepts

The relatively short lifetime of rare-gas discharge pump light sources for high-power cw solid-state lasers is caused mainly by vaporized cathode emitter material reacting with the thermally highly loaded quartz wall. The introduction of new bipolar electrodes without special emitter materials mounted on hot-ended molybdenum-cup seals, together with 60-kHz ac operation, makes possible the application of a tungsten halide cleaning cycle within the lamp bulb. This new type of high-power rare-gas pump light source exhibits 20% better radiation efficiency, 20% lower Nd:YAG laser threshold, and considerably extended lifetime.

Regulatory protein phosphorylation of phosphoenolpyruvate carboxylase in the facultative crassulacean-acid-metabolism plant Mesembryanthemum crystallinum L

Phosphoenolpyruvate PyrP carboxylase (PyrPC) and PyrPC kinase were copurified from dark-adapted leaves of the common ice plant Mesembryanthemum crystallinum L. with crassulacean-acid metabolism (CAM). Purification by (NH4)2SO4 fractionation, chromatography on Fractogel-DEAE and hydroxylapatite resulted in a PyrPC preparation with a specific activity of 23-25 U/mg protein and a protein kinase activity of 255 mumol Pi.mol-1 PyrPC.s-1. After in vitro phosphorylation, the most prominently phosphorylated polypeptide was identified as PyrPC by immunoblotting and sequencing. Phosphorylation of PyrPC in vitro by incubation with 400 microM MgATP decreased its sensitivity towards malate. When purified in the absence of the protease inhibitor chymostatin, PyrPC lost an N-terminal sequence of 128 amino acids. Although the carboxylation reaction was unaffected, the truncated PyrPC could neither be phosphorylated in vitro nor inhibited by malate. This result and data obtained by limited proteolysis concur with the hypothesis [Jiao, J.A. & Chollet, R. (1989) Arch. Biochem. Biophys. 283, 300-305] that Ser11 is the phosphorylation site of the CAM PyrPC of M. crystallinum. At pH 7.0, the Km for ATP of the protein kinase was 25 microM; phosphorylation of PyrPC was maximal after 30 min at pH 7.0. The kinase showed also activity with histone III-S but not with dephosphorylated casein. It was inhibited by malate. The results show, that reversible protein phosphorylation is an important factor in the regulation of PyrPC in the facultative CAM plant M. crystallinum, similar to C4 and constitutive CAM plants.

Identification of an essential histidine residue at the active site of the tonoplast malate carrier in Catharanthus roseus cells

The involvement of a histidyl residue in the binding or translocation step was investigated in the malate carrier at the tonoplast of Catharanthus roseus cells. The transport rate was strongly stimulated when the pH of the incubation medium was decreased from pH 7.0 to 5.0. The histidine-specific reagent diethylpyrocarbonate (DEPC) efficiently inhibited the activity of the malate carrier. Inhibition developed rapidly and was completed after 5 min at a concentration of 2 mM DEPC. The original substrate, malate, partially protected the carrier from inactivation by DEPC. Other organic acids (citrate, quinate) which are known to affect the malate transport of isolated vacuoles or tonoplast vesicles also showed protective properties. Inhibition of malate transport on tonoplast vesicles can also be achieved by photooxidation in the presence of the dye Rose Bengal. Malate also proved to protect against inactivation. The results strongly support the notion that a histidyl residue(s) is involved either in the binding or translocation of malate and that the protonation of the histidyl residue is essential to provide a high rate of malate transport.

Characterization of the epidermis from barley primary leaves : I. Isolation of epidermal protoplasts

A method is described for isolating epidermal protoplasts from the primary leaves of barley (Hordeum vulgare L.). Epidermal protoplasts are lighter than mesophyll protoplasts because of their smaller ratio of cytoplasm to vacuole, and can be separated from the latter by density-gradient centrifugation after complete digestion of the leaves. We have started a basic characterization of the epidermal protoplast fraction in comparison with mesophyll protoplasts. Epidermal protoplasts had a mean diameter of 63.5 μm, whereas that of mesophyll protoplasts was 35.7 μm. Their respiratory oxygen consumption was not influenced by light. They contained acid hydrolases and cytoplasmic enzymes in relative activities different from those of mesophyll protoplasts. Their polypeptide pattern as judged from two-dimensional separations was, in principle, similar to that of mesophyll cells after elimination of the plastids from the latter by the preparation of vacuoplasts. However, in addition, a considerable number of epidermis-specific polypeptides were observed. Isolated epidermal protoplasts were viable and efficiently incorporated [(35)S]methionine into newly synthesized proteins. The results show that epidermal protoplasts are suitable for the investigation of the physiological and molecular properties of epidermal cells in leaves.

Characterization of the epidermis from barley primary leaves : II. The role of the epidermis in ion compartmentation

The cation and anion distribution between the epidermis and mesophyll of primary leaves of 10-d-old barley (Hordeum vulgare L.) seedlings was studied in relation to growth conditions. A new method was employed to isolate epidermal protoplasts. The following observations were made: (i) Under standard hydroponic growth conditions, K(+) was the dominant cation and NO 3 (3-) the predominant anion, both in epidermal and mesophyll protoplasts. (ii) Levels of Cl(-) and particularly of Ca(2+) were specifically increased in the epidermis when growth conditions stimulated uptake of these ions into the plants. (iii) Epidermal PO 4 (3-) was maintained at a very low level even in the presence of 50 mM phosphate in the rooting medium, whereas the phosphate concentration in the mesophyll was high, with phosphate being accumulated in the mesophyll vacuoles. (iv) Sulphate and NO 3 (-) accumulated to a similar degree in the mesophyll and the epidermis when K2SO4 or KNO3 in the rooting medium caused salt stress to the plants. (v) Epidermal protoplasts took up (35)SO 4 (2-) and (36)Cl(-) from the surrounding medium. A comparison of the uptake rates indicates that specific ion deposition into the epidermis may partially be the consequence of differential uptake of ions from the transpiration stream. The results indicate an important function of the epidermis for ion compartmentation in barley leaves.

Spectroscopic measurements of electron densities and gas temperatures in deuterium lamps

Plasma diagnostics results of electron densities and gas temperatures in commercial low-pressure deuterium lamps at current densities from 38 A/cm(-2) to 306 A/cm(-2) are presented. These plasma parameters are obtained by comparing measured and calculated Balmer line profiles, taking into account Stark broadening, Doppler broadening, and the influence of fine structure and the apparatus profile. Results for the plasma spot show a steady increase of electron density from 1.6 x 10(13) cm(-3) to 5.0 x 10(13) cm(-3) and of gas temperature from 1800 to 6800 K.

Partial purification of a potassium channel with low permeability for sodium from tonoplast membranes of Hordeum vulgare cv. Gerbel

A potassium-specific tonoplast channel was identified by reconstitution of tonoplast polypeptides into planar lipid bilayer membranes. Highly purified tonoplast membranes were solubilized in Triton X-100-containing buffer and fractionated by size-exclusion chromatography. The protein fractions were assayed for ion channel activity in a planar bilayer system, and the potassium channel was routinely recovered in specific fractions corresponding to an apparent molecular mass of 80 kDa. In symmetrical electrolyte solutions of 100 mM potassium chloride, the potassium channel had a single-channel conductance of 72 pS. Substates of the channel with conductances of 17, 33 and 52 pS were frequently observed. After identification of the channel in low or high KCl, addition of sodium acetate or sodium chloride caused only insignificant conductance changes. This result suggested that the channel was not or little permeable for sodium or chloride, whereas it had similar single-channel conductance for rubidium and caesium ions as compared with potassium ions. The channel is presumably responsible for the equilibration of potassium between the vacuole and the cytosol. The role of the channel in the physiology of the barley cell under salt stress is discussed.

Reconstitution of vacuolar ion channels into planar lipid bilayers

Vacuolar ion channels were characterized after reconstitution into planar lipid bilayers. (1) Channel activity was observed after incorporation of tonoplast-enriched microsomal membranes, purified tonoplast membranes or of solubilized tonoplast proteins. (2) Channels of varying single-channel conductances were detected after reconstitution. In symmetrical 100 mmol l-1 KCl, conductances between 1 and 110 pS were frequently measured; the largest number of independent reconstitution events was seen for single-channel conductances of 16-25 pS (28 experiments), 30-42 pS (26), 49-56 pS (15) and 64-81 pS (15). Channel current usually increased linearly with voltage. (3) In asymmetrical solutions, cation-, non-selective and, for the first time for the tonoplast, anion-selective channels were detected. Ca(2+)-dependent regulation of channel opening was not observed in our reconstitution system. (4) Permeability was also observed for Cl-, NO3-, SO4(2-) and phosphate. (5) After fractionation of tonoplast proteins by size exclusion chromatography, ion channel activity was recovered in specific fractions. (6) Some of these fractions catalyzed sulfate transport after reconstitution into liposomes. The results suggest that different channels are active at the tonoplast membrane at a larger number than has been concluded from previous work.

Immunological characterization of two dominant tonoplast polypeptides

At least 14 distinct polypeptides reside in the tonoplast of barley (Hordeum vulgare) mesophyll vacuoles. Two of the polypeptides were isolated from two-dimensional separations of vacuoplast membrane proteins and used for immunization. With the antisera, the localization on the membrane and the distribution of the polypeptides in the plant kingdom and in various tissues of barley plants was studied. The polypeptides have an apparent molecular mass of 31 and 40 kilodaltons. After freeze-thaw cycles or washing of the membranes with 4.5 millimolar NaCl, the polypeptides were still sedimented with the membranes, suggesting an intrinsic localization. The antiserum against the 31-kilodalton polypeptide bound to the outer surface of isolated intact vacuoles. In chromatographic separations of Triton X-100-solubilized membrane fractions, the residual activities of various acid hydrolases eluted distinct from the 31- and 40-kilodalton polypeptides. Both polypeptides tend to form larger aggregates, however smaller than the tonoplast ATPase. Cross-reactive polypeptides were present in higher and lower plants (the green alga Chara corallina and the liverwort Conocephalum) and in liver tissue from rat and beef, but were not detected in other animal tissues tested so far. The results indicate a wide distribution of these tonoplast polypeptides in vacuole-containing organisms.

Reconstitution of the tonoplast amino-acid carrier into liposomes : Evidence for an ATP-regulated carrier in different species

The tonoplast amino-acid transporter of barley (Hordeum vulgare L.) mesophyll cells was functionally reconstituted by incorporating solubilized tonoplast membranes, vacuoplast membranes or tonoplast-enriched microsomal vesicles into phosphatidylcholine liposomes. (i) Time-, concentration- and ATP-dependence of amino-acid uptake were similar to results with isolated vacuoles. Although the orientation of incorporation could not be controlled, the results indicate that the transporter functions as a uniport system which allows regulated equilibration by diffusion between the cytosolic and vacuolar amino-acid pools. (ii) The ATP-modulated amino-acid carrier was also successfully reconstituted from barley epidermal protoplasts and Valerianella or Tulipa vacuoplasts, indicating its general occurrence. (iii) Fractionation of solubilized tonoplasts by size-exclusion chromatography followed by reconstitution of the fractions for glutamine transport gave two activity peaks: the first eluted in the region of high-molecular-mass vesicles and the second at a size of 300 kDa for the Triton-protein micelle.

Transport of arginine and aspartic Acid into isolated barley mesophyll vacuoles

The transport of arginine into isolated barley (Hordeum vulgare L.) mesophyll vacuoles was investigated. In the absence of ATP, arginine uptake was saturable with a K(m) of 0.3 to 0.4 millimolar. Positively charged amino acids inhibited arginine uptake, lysine being most potent with a K(i) of 1.2 millimolar. In the presence of free ATP, but not of its Mg-complex, uptake of arginine was drastically enhanced and a linear function of its concentration up to 16 millimolar. The nonhydrolyzable adenylyl imidodiphosphate, but no other nucleotide tested, could substitute for ATP. Therefore, it is suggested that this process does not require energy and does not involve the tonoplast ATPase. The ATP-dependent arginine uptake was strongly inhibited by p-chloromercuriphenylsulfonic acid. Furthermore, hydrophobic amino acids were inhibitory (I(50) phenylalanine 1 millimolar). Similar characteristics were observed for the uptake of aspartic acid. However, rates of ATP-stimulated aspartic acid transport were 10-fold lower as compared to arginine transport. Uptake of aspartate in the absence of ATP was negligible.

Light-dependent pH changes in leaves of C3 plants : III. Effect of inhibitors of photosynthesis and of the developmental state of the photosynthetic apparatus on cytosolic and vacuolar pH changes

Etiolated leaves and the inhibitors of photosynthesis 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and DL-glyceraldehyde were used to study the relationship between thylakoid energization, photosynthesis, the light-dependent alkalization of the cytosol of mesophyll cells and the acidification of mesophyll vacuoles. No light-dependent pH changes were observed in etiolated leaves. As chloroplasts developed in the light and became photosynthetically competent, mesophyll vacuoles became more acidic when the leaves were illuminated in CO2-free air. Acidification was suppressed and even replaced by a small light-dependent alkaline shift during photoassimilation of CO2. In green leaves, DCMU and DL-glyceraldehyde inhibited the cytosolic and vacuolar pH responses to illumination. Inhibition was also observed under anaerobiosis. In the absence of DCMU and glyceraldehyde, the extent of the light-dependent vacuolar acidification corresponded closely to the extent of thylakoid energization by light. Because, in contrast to DCMU, glyceraldehyde did not inhibit thylakoid energization while inhibiting the extrachloroplast pH responses, it is concluded that the signal transfer from the chloroplasts to the cytosol which results in increased vacuolar acidification in the light involves metabolites of the Calvin cycle. The observations do not support the view that the cytosolic energy state is increased in the light by the mitochondrial oxidation of the NADH generated during the oxidation of photorespiratory glycine in the mitochondria.

Carbon metabolism in spinach leaves as affected by leaf age and phosphorus and sulfur nutrition

Spinach (Spinacea oleracea) plants were grown either continuously on complete nutrient solutions or for 2 weeks on media deficient in phosphate or sulfate. To characterize leaf carbohydrate metabolism, levels of phosphorylated intermediates, activities of enzymes involved in photosynthetic carbon metabolism, contents of soluble and acid hydrolyzable sugars were measured in leaves differing in age and mineral status and related to leaf rates of photosynthesis and assimilate partitioning. Concentrations of metabolites-particularly those which are preferentially compartmented in the cytosol-decreased from young to old leaves and were lowest in old phosphate starved leaves. Nutrient deficiency showed comparable effects on stromal and cytosolic intermediates. Whole leaf ATP to ADP ratios were dependent on the growth regime, but did not much change with leaf age. The assimilatory force increased in all leaves suffering from mineral deficiency; the assimilatory force was low when photosynthesis was high and vice versa. Sugars accumulated although enzyme activities were decreased under deficiency. The results show that growth of P- and S-starved plants is not limited by photosynthetic reactions.

ATP dependence of anion uptake by isolated vacuoles: requirement for excess Mg2+

Vacuoles were isolated from barley mesophyll protoplasts. [14C]Malate or 36Cl- were taken up from the surrounding medium. Uptake was only slightly increased in the presence of equimolar levels of ATP and Mg2+ (as magnesium gluconate). In the presence of excess Mg2+ in the medium, ATP-stimulated uptake of malate and chloride increased several-fold. Stimulation by excess Mg2+ was not observed for ATP-stimulated amino acid uptake by isolated vacuoles. Stimulation of uptake by excess Mg2+ was observed at all malate concentrations upto 10 mmol.l-1. The content of Mg2+ needed for half-maximum stimulation was about 3.5 mmol.l-1 in the presence of 1 mmol.l-1 ATP. The increase in Mg2+ concentration had no effect on the tonoplast ATPase activity.

Amino Acid Transport across the Tonoplast of Vacuoles Isolated from Barley Mesophyll Protoplasts : Uptake of Alanine, Leucine, and Glutamine

Mesophyll protoplasts from leaves of well-fertilized barley (Hordeum vulgare L.) plants contained amino acids at concentrations as high as 120 millimoles per liter. With the exception of glutamic acid, which is predominantly localized in the cytoplasm, a major part of all other amino acids was contained inside the large central vacuole. Alanine, leucine, and glutamine are the dominant vacuolar amino acids in barley. Their transport into isolated vacuoles was studied using (14)C-labeled amino acids. Uptake was slow in the absence of ATP. A three- to sixfold stimulation of uptake was observed after addition of ATP or adenylyl imidodiphosphate an ATP analogue not being hydrolyzed by ATPases. Other nucleotides were ineffective in increasing the rate of uptake. ATP-Stimulated amino acid transport was not dependent on the transtonoplast pH or membrane potential. p-Chloromercuriphenylsulfonic acid and n-ethyl maleimide increased transport independently of ATP. Neutral amino acids such as valine or leucine effectively decreased the rate of alanine transport. Glutamine and glycine were less effective or not effective as competitive inhibitors of alanine transport. The results indicate the existence of a uniport translocator specific for neutral or basic amino acids that is under control of metabolic effectors.

Phosphate transport across biomembranes and cytosolic phosphate homeostasis in barley leaves

Barley (Hordeum vulgare L.) plants were grown hydroponically with or without inorganic phosphate (Pi) in the medium. Leaves were analyzed for the intercellular and the intracellular distribution of Pi. Most of the leaf Pi was contained in mesophyll cells; Pi concentrations were low in the xylem sap, the apoplast and in the cells of the epidermis. The vacuolar concentration of Pi in mesophyll cells depended on Pi availability in the nutrient medium. After infiltrating the intercellular space of leaves with solutions containing Pi, Pi was taken up by the mesophyll at rates higher than 2.5 μmol· (g fresh weight)(-1) · h(-1). Isolated mesophyll protoplasts did not possess a comparable capacity to take up Pi from the medium. Phosphate uptake by mesophyll protoplasts showed a biphasic dependence on Pi concentration. Uptake of Pi by Pi-deficient cells was faster than uptake by cells which had Pi stored in their vacuoles, although cytoplasmic Pi concentrations were comparable. Phosphate transport into isolated mesophyll vacuoles was dependent on their Pi content; it was stimulated by ATP. In contrast to the vacuolar Pi concentration, and despite different kinetic characteristics of the uptake systems for pi of the plasmalemma and the tonoplast, the cytoplasmic pi concentration was regulated in mesophyll cells within narrow limits under very different conditions of Pi availability in the nutrient medium, whereas vacuolar Pi concentrations varied within wide limits.

Characterization of vacuolar polypeptides of barley mesophyll cells by two-dimensional gel electrophoresis and by their affinity to lectins

Vacuoles were isolated from primary leaves of barley (Hordeum vulgare L.) by mechanical breakage of protoplasts, and their polypeptide composition analyzed by two-dimensional gel electrophoresis. Vacuoplasts which consist of the vacuole, a portion of the plasmalemma and of the cytoplasma were prepared from protoplasts by ultracentrifugation. By comparing the vacuolar polypeptide pattern with polypeptide patterns of isolated chloroplasts and of vacuoplasts, vacuolar polypeptides could clearly be distinguished from polypeptides derived from cross-contaminating cell compartments. At least 14 polypeptides of apparent molecular mass between 12 and 76 kilodaltons and an isoelectric point between 4.5 and 7.6 could be attributed to the tonoplast fraction of the vacuole, and 35 polypeptides to the soluble fraction of the vacuole. Several lectins with different specificity were employed to characterize the degree and nature of glycosylation of vacuolar polypeptides. Concanavalin A bound to a large number of polypeptides. Three out of the 14 tonoplast polypeptides exhibited detectable carbohydrate moieties and almost two-thirds of the surveyed soluble polypeptides were glycosylated.

Fractional control of photosynthesis by the QB protein, the cytochrome f/b 6 complex and other components of the photosynthesic apparatus

In order to obtain information on fractional control of photosynthesis by individual catalysts, catalytic activities in photosynthetic electron transport and carbon metabolism were modified by the addition of inhibitors, and the effect on photosynthetic flux was measured using chloroplasts of Spinacia oleracea L. In thylakoids with coupled electron transport, light-limited electron flow to ferricyanide was largely controlled by the QB protein of the electron-transport chain. Fractional control by the cytochrome f/b 6 complex was insignificant under these conditions. Control by the cytochrome f/b 6 complex dominated at high energy fluence rates where the contribution to control of the QB protein was very small. Uncoupling shifted control from the cytochrome f/b 6 complex to the QB protein. Control of electron flow was more complex in assimilating chloroplasts than in thylakoids. The contributions of the cytochrome f/b 6 complex and of the QB protein to control were smaller in intact chloroplasts than in thylakoids. Thus, even though the transit time for an electron through the electron-transport chain may be below 5 ms in leaves, oxidation of plastohydroquinone was only partially responsible for limiting photosynthesis under conditions of light and CO2 saturation. The energy fluence rate influenced control coefficients. Fractional control of photosynthesis by the ATP synthetase, the cytochrome f/b 6 complex and by ribulose-1,5-bisphosphate carboxylase increased with increasing fluence rates, whereas the contributions of the QB protein and of enzymes sensitive to SH-blocking agents decreased. The results show that the burdens of control are borne by several components of the photosynthetic apparatus, and that burdens are shifted as conditions for photosynthesis change.