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  • Oxford Journals (Oxford University Press)  (61)
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  • 1
    In: Journal Of Experimental Botany, 2014, Vol. 65(20), pp.5711-5724
    Description: Metabolite flux analyses support our understanding of physiological processes. An integrating approach is required combining metabolite fluxes at different levels of complexity from cells via tissues and organs to the whole plant. Understanding the dynamics of physiological process in the systems biology era requires approaches at the genome, transcriptome, proteome, and metabolome levels. In this context, metabolite flux experiments have been used in mapping metabolite pathways and analysing metabolic control. In the present review, sulphur metabolism was taken to illustrate current challenges of metabolic flux analyses. At the cellular level, restrictions in metabolite flux analyses originate from incomplete knowledge of the compartmentation network of metabolic pathways. Transport of metabolites through membranes is usually not considered in flux experiments but may be involved in controlling the whole pathway. Hence, steady-state and snapshot readings need to be expanded to time-course studies in combination with compartment-specific metabolite analyses. Because of species-specific differences, differences between tissues, and stress-related responses, the quantitative significance of different sulphur sinks has to be elucidated; this requires the development of methods for whole-sulphur metabolome approaches. Different cell types can contribute to metabolite fluxes to different extents at the tissue and organ level. Cell type-specific analyses are needed to characterize these contributions. Based on such approaches, metabolite flux analyses can be expanded to the whole-plant level by considering long-distance transport and, thus, the interaction of roots and the shoot in metabolite fluxes. However, whole-plant studies need detailed empirical and mathematical modelling that have to be validated by experimental analyses.
    Keywords: Compartmentation ; Membrane Transport ; Metabolite Flux Analyses ; Modelling ; Sulphur Flux ; Sulphur Reduction And Assimilation.
    ISSN: 0022-0957
    E-ISSN: 1460-2431
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  • 2
    In: Journal of Experimental Botany, 2010, 2009, Vol. 61(4), pp.1065-1074
    Description: Glutathione (GSH) and ascorbate (ASC) are important antioxidants that are involved in stress defence and cell proliferation of meristematic root cells. In principle, synthesis of ASC and GSH in the roots as well as ASC and GSH transport from the shoot to the roots by phloem mass flow is possible. However, it is not yet known whether the ASC and/or the GSH level in roots depends on the supply from the shoot. This was analysed by feeding mature leaves with [ 14 C]ASC or [ 35 S]GSH and subsequent detection of the radiolabel in different root fractions. Quantitative dependency of root ASC and GSH on shoot-derived ASC and GSH was investigated with poplar ( Populus tremula × P. alba ) trees interrupted in phloem transport. [ 35 S]GSH is transported from mature leaves to the root tips, but is withdrawn from the phloem along the entire transport path. When phloem transport was interrupted, the GSH content in root tips halved within 3 d. [ 14 C]ASC is also transported from mature leaves to the root tips but, in contrast to GSH, ASC is not removed from the phloem along the transport path. Accordingly, ASC accumulates in root tips. Interruption of phloem transport disturbed the level and the ASC redox state within the entire root system. Diminished total ASC levels were attributed mainly to a decline of dehydroascorbate (DHA). As the redox state of ASC is of particular significance for root growth and development, it is concluded that phloem transport of ASC may constitute a shoot to root signal to coordinate growth and development at the whole plant level.
    Keywords: Ascorbate ; Glutathione ; Phloem Transport ; Poplar ; Redox State ; Root Growth
    ISSN: 0022-0957
    E-ISSN: 1460-2431
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  • 3
    In: Journal of Experimental Botany, 2010, 2009, Vol. 61(2), pp.609-622
    Description: Sulphate assimilation provides reduced sulphur for the synthesis of cysteine, methionine, and numerous other essential metabolites and secondary compounds. The key step in the pathway is the reduction of activated sulphate, adenosine 5′-phosphosulphate (APS), to sulphite catalysed by APS reductase (APR). In the present study, [ 35 S]sulphur flux from external sulphate into glutathione (GSH) and proteins was analysed to check whether APR controls the flux through the sulphate assimilation pathway in poplar roots under some stress conditions and in transgenic poplars. (i) O -Acetylserine (OAS) induced APR activity and the sulphur flux into GSH. (ii) The herbicide Acetochlor induced APR activity and results in a decline of GSH. Thereby the sulphur flux into GSH or protein remained unaffected. (iii) Cd treatment increased APR activity without any changes in sulphur flux but lowered sulphate uptake. Several transgenic poplar plants that were manipulated in sulphur metabolism were also analysed. (i) Transgenic poplar plants that overexpressed the γ-glutamylcysteine synthetase ( γ-ECS ) gene, the enzyme catalysing the key step in GSH formation, showed an increase in sulphur flux into GSH and sulphate uptake when γ-ECS was targeted to the cytosol, while no changes in sulphur flux were observed when γ-ECS was targeted to plastids. (ii) No effect on sulphur flux was observed when the sulphite oxidase ( SO ) gene from Arabidopsis thaliana , which catalyses the back reaction of APR, that is the reaction from sulphite to sulphate, was overexpressed. (iii) When Lemna minor APR was overexpressed in poplar, APR activity increased as expected, but no changes in sulphur flux were observed. For all of these experiments the flux control coefficient for APR was calculated. APR as a controlling step in sulphate assimilation seems obvious under OAS treatment, in γ -ECS and SO overexpressing poplars. A possible loss of control under certain conditions, that is Cd treatment, Acetochlor treatment, and in APR overexpressing poplar, is discussed.
    Keywords: Aps Synthetase ; Flux Measurements ; Glutathione ; Poplar ; Roots ; Sulphate Uptake ; Sulphite Oxidase
    ISSN: 0022-0957
    E-ISSN: 1460-2431
    Source: Oxford University Press
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  • 4
    In: Tree Physiology, 2011, Vol. 31(2), pp.196-207
    Description: Beech seedlings originating from 11 German provenances with different climatic conditions were grown in pots and cultivated in a greenhouse. The composition of macro- and microelements in roots, axes and leaves was measured after half of the seedlings were subjected to a simulated summer drought. The recently described sensitivity of these provenances to drought was compared with drought-mediated changes in the elemental and ionic composition in organs of the seedlings; in addition, partitioning between roots and shoots was evaluated. A number of element concentrations were decreased in roots due to drought (K 94% of control, Mg 94%, Mn 75% and Zn 85%). However, chloride concentration increased in all organs (115–125%) and was the only element affected in leaves. Some changes in ionome can be related to sensitivity of provenances, but it is difficult to decide whether these changes are a result of, or a reason for, drought tolerance or sensitivity. Observed increases in chloride concentration in all plant parts of drought-treated beech seedlings can be explained by its function in charge balance, in particular since the level of phosphate was reduced. As a result of chloride accumulation, the sum of added charges of anions (and cations) in water extracts of leaf and root material was similar between drought and control plants. Since only the partitioning of Ca and Al (both only in axis) as well as Mn was affected and other elements (together with previously observed effects on C, N, S and P) remained unaffected by drought in all provenances, it can be concluded that direct effects by means of mass flow inhibition in xylem and phloem are unlikely. Secondary effects, for example on the pH of transport sap and the apoplastic space, cannot be excluded from the present study. These effects may affect partitioning between the apoplast and symplast and therefore may be significant for drought sensitivity.
    Keywords: Anions ; Beech ; Cations ; Plant Nutrition ; Water Stress
    ISSN: 0829-318X
    E-ISSN: 1758-4469
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  • 5
    In: Journal Of Experimental Botany, 2013, Vol. 64(5), pp.1317-1332
    Description: Symbiotic nitrogen fixation (SNF) involves global changes in gene expression and metabolite accumulation in both rhizobia and the host plant. In order to study the metabolic changes mediated by leaf–root interaction, photosynthesis was limited in leaves by exposure of plants to darkness, and subsequently gene expression was profiled by real-time reverse transcription–PCR (RT–PCR) and metabolite levels by gas chromatography–mass spectrometry in the nodules of the model legume Lotus japonicus . Photosynthetic carbon deficiency caused by prolonged darkness affected many metabolic processes in L. japonicus nodules. Most of the metabolic genes analysed were down-regulated during the extended dark period. In addition to that, the levels of most metabolites decreased or remained unaltered, although accumulation of amino acids was observed. Reduced glycolysis and carbon fixation resulted in lower organic acid levels, especially of malate, the primary source of carbon for bacteroid metabolism and SNF. The high amino acid concentrations together with a reduction in total protein concentration indicate possible protein degradation in nodules under these conditions. Interestingly, comparisons between amino acid and protein content in various organs indicated systemic changes in response to prolonged darkness between nodulated and non-nodulated plants, rendering the nodule a source organ for both C and N under these conditions.
    Keywords: Carbon Starvation ; 〈Kwd〉〈Italic〉Lotus Japonicus〈/Italic〉〈/Kwd〉 ; Metabolomic Analysis ; Nodule ; Symbiosis ; Transcript Profile.
    ISSN: 0022-0957
    E-ISSN: 1460-2431
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  • 6
    In: Journal of Experimental Botany, 2011, Vol. 62(8), pp.2959-2971
    Description: During symbiotic nitrogen fixation (SNF), the nodule becomes a strong sink for photosynthetic carbon. Here, it was studied whether nodule dark CO 2 fixation could participate in a mechanism for CO 2 recycling through C 4 -type photosynthesis. Differences in the natural δ 13 C abundance between Lotus japonicus inoculated or not with the N-fixing Mesorhizobium loti were assessed. 13 C labelling and gene expression of key enzymes of CO 2 metabolism were applied in plants inoculated with wild-type or mutant fix – (deficient in N fixation) strains of M. loti , and in non-inoculated plants. Compared with non-inoculated legumes, inoculated legumes had higher natural δ 13 C abundance and total C in their hypergeous organs and nodules. In stems, 13 C accumulation and expression of genes coding for enzymes of malate metabolism were greater in inoculated compared with non-inoculated plants. Malate-oxidizing activity was localized in stem xylem parenchyma, sieve tubes, and photosynthetic outer cortex parenchyma of inoculated plants. In stems of plants inoculated with fix – M. loti strains, 13 C accumulation remained high, while accumulation of transcripts coding for malic enzyme isoforms increased. A potential mechanism is proposed for reducing carbon losses during SNF by the direct reincorporation of CO 2 respired by nodules and the transport and metabolism of C-containing metabolites in hypergeous organs.
    Keywords: C Labelling ; Dark Co Fixation ; Δc Abundance ; Malate Metabolism ; Symbiotic N Fixation
    ISSN: 0022-0957
    E-ISSN: 1460-2431
    Source: Oxford University Press
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  • 7
    In: Tree Physiology, 2010, Vol. 30(9), pp.1047-1049
    Keywords: Mobilization ; Nitrogen ; Nutrient Cycle ; Nutrient Losses ; Phosphorus ; Storage
    ISSN: 0829-318X
    E-ISSN: 1758-4469
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  • 8
    Description: Sulphate assimilation is an essential pathway being a source of reduced sulphur for various cellular processes and for the synthesis of glutathione, a major factor in plant stress defence. Many reports have shown that sulphate assimilation is well co-ordinated with the assimilation of nitrate and carbon. It has long been known that, during nitrate deficiency, sulphate assimilation is reduced and that the capacity to reduce nitrate is diminished in plants starved for sulphate. Only recently, however, was it shown that adenosine 5′ phosphosulphate reductase (APR), the key enzyme of sulphate assimilation, is regulated by carbohydrates. In plants treated with sucrose or glucose APR was induced, whereas the activity was strongly reduced in plants grown in CO 2 -free air. The availability of cysteine is a crucial factor in glutathione synthesis, but an adequate supply of glutamate and glycine are also important. The molecular mechanisms for the co-ordination of S, N, and C assimilation are not known. O -acetylserine, a precursor of cysteine, was proposed to be the signal regulating sulphate assimilation, but most probably is not the outgoing signal to N and C metabolism. cDNA arrays revealed the induction of genes involved in auxin synthesis upon S-starvation, pointing to a possible role of phytohormones. Clearly, despite significant progress in understanding the regulation of sulphate assimilation and glutathione synthesis, their co-ordination with N and C metabolism achieved, and several potential signal molecules identified, present knowledge is still far from being sufficient.
    Keywords: Glutathione Synthesis; Nitrate Deficiency; Nitrate Reduction; Sulphate Assimilation; Regulation
    ISSN: 0022-0957
    E-ISSN: 14602431
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  • 9
    In: Tree Physiology, 2010, Vol. 30(2), pp.244-256
    Description: Nut yield is highly variable in commercial macadamia production, and to ensure that nitrogen (N) supply does not limit yield, high rates of N fertilizer are generally applied. To elucidate N source and sink relations in mature Macadamia integrifolia Maiden et Betche trees, we traced 15 N label after injection into individual branches and, after soil application, analysed xylem sap and examined the effects of hedging on tree N relations. Xylem sap N and sugar composition and concentration changed in relation to phenology and tree management. Canopy position did not affect xylem sap N concentration but sampling date had a significant effect. Hedging in spring was associated with a rapid and dramatic reduction of the concentration of xylem sap N until the following autumn, but unhedged trees were not available to unequivocally assess the significance of the results. Following 15 N-branch injection in winter, most 15 N label was incorporated into flushing leaves and into bark. After 15 N injection in spring, flushing leaves and flowers were most strongly 15 N-labelled. In late spring, 15 N label was equally incorporated by developing nuts that were retained or later abscised. Soil 15 N application in summer resulted in 15 N-labelling of outer and mid-canopy leaves. In the following spring, 15 N label was translocated to flushing leaves, flowers and developing nuts. The results indicate that outer and mid-canopy leaves are the main N sink for soil-derived N during the vegetative phase and a N source for developing tissues during the reproductive phase. Our study provides evidence that N supply to developing nuts is not a primary cause for nut abscission, supporting the notion that high N fertilizer application rates do not improve nut retention. We propose that current orchard design and hedging practices should be reviewed in context of the role of outer canopy leaves as a source of N for reproductive tissues.
    Keywords: Amino Acids ; Macadamia ; N - Labelling ; Nitrogen Remobilization ; Nitrogen Storage ; Nut Retention ; Xylem Sap
    ISSN: 0829-318X
    E-ISSN: 1758-4469
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  • 10
    In: Tree Physiology, 2011, Vol. 31(1), pp.3-15
    Description: This review discusses how understanding of functional relationships between parasitic plants and their woody hosts have benefited from a range of approaches to their study. Gross comparisons of nutrient content between infected and uninfected hosts, or parts of hosts, have been widely used to infer basic differences or similarities between hosts and parasites. Coupling of nutrient information with additional evidence of key processes such as transpiration, respiration and photosynthesis has helped elucidate host–parasite relationships and, in some cases, the anatomical nature of their connection and even the physiology of plants in general. For example, detailed analysis of xylem sap from hosts and parasites has increased our understanding of the spatial and temporal movement of solutes within plants. Tracer experiments using natural abundance or enriched application of stable isotopes ( 15 N, 13 C, 18 O) have helped us to understand the extent and form of heterotrophy, including the effect of the parasite on growth and functioning of the host (and its converse) as well as environmental effects on the parasite. Nutritional studies of woody hosts and parasites have provided clues to the distribution of parasitic plants and their roles in ecosystems. This review also provides assessment of several corollaries to the host–parasite association.
    Keywords: Carbon Assimilation ; Ecosystem Function ; Hemiparasite ; Mistletoe ; Nutrient Uptake ; Water Relations
    ISSN: 0829-318X
    E-ISSN: 1758-4469
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