bluuebunny/crossref_metadata_2025 huggingface dataset:
Regulation of glutamate dehydrogenase activity by manipulation of nucleotide supply in Daucus carota suspension cultures
Authors: Athwal Gurdeep S. , Pearson John , Laurie Sophie
Published: May 2006

The enzyme glutamate dehydrogenase (GDH, EC 1.4.1.2) is ubiquitous in plant species. It is now generally accepted that the primary role of this enzyme is not assimilation of ammonium and it has been suggested that GDH may be important in provision of carbon skeletons under conditions of carbon limitation. In carrot (Daucus carota L. Chantenay) cell suspension cultures carbon starvation results in de‐repression of GDH activity. The regulation of this de‐repression has not been investigated. This paper examines the possibility that the availability of adenosine nucleotides is instrumental in the regulation of GDH activity. In repressed cultures the adenosine nucleotides cAMP (0.2 mM), AMP (0.2 mM) and ADP (0.4 mM) caused an increase in GDH activity of 61, 33 and 7%, respectively. ATP (0.2 mM) had the opposite effect in maintaining repression of GDH. Under de‐repressed conditions only cAMP (0.2 mM) enhanced GDH activity (14%). Inhibition of oxidative phosphorylation using a range of inhibitors resulted in de‐repression of GDH and stimulation of respiration. The results from this work indicate that exogenously applied adenosine nucleotides and electron transport inhibitors alter the GDH repression/de‐repression status. Addition of these compounds alters or disrupts ATP levels, mimicking carbon depletion. This causes an increase in GDH activity, supporting the idea that GDH may provide carbon skeletons for carbon metabolism and suggesting that ATP status is important in regulation of the enzyme activity.
The effect of methionine, ethylene and polyamine catabolic intermediates on polyamine accumulation in detached soybean leaves
Authors: Turano Frank J. , Kramer George F. , Wang Chien Y.
Published: May 2006

In the present study we determined the effects of methionine, intermediates of polyamine catabolic pathways and inhibitors of either ethylene biosynthetic or polyamine catabolic pathways on polyamine accumulation in soybean leaves. Inhibitors to SAM decarboxylase and spermidine synthase, methylglyloxal‐bis‐(guanylhy‐drazone) and cyclohexylamine, respectively, suggest that methionine may provide aminopropyl groups for the synthesis of polyamine via S‐adenosylmethionine (SAM). Results from experiments that utilized a combination of compounds which altered either ethylene or polyamine biosynthesis, namely, aminoethoxyvinyl glycine, CoSO4, 2,5‐norbornadiene, and CuSO4, suggest the two pathways compete for a common precursor. However, exogenous addition of ethylene (via ethephon treatments) had little or no effect on polyamine biosynthesis. Likewise, polyamine treatments had little or no effect on ethylene biosynthesis. These data suggest that there are few or no inhibitory effects from the end products of one pathway on the synthesis of the other. Data from leaves treated with metabolic intermediates in the catabolic pathway of polyamines and inhibitors of enzymes in the catabolic pathway, i.e. aminoguanidine, hydroxyethyldrazine and gabaculine, suggest that the observed increases in polyamine titers were not due to decreased catabolism of the polyamines. One catabolic intermediate, γ‐aminobutyric acid (GABA), elevated putrescine, spermidine and spermine by 12‐, 1.4‐, and 2‐fold, respectively, Ethylene levels decreased (25%) in GABA‐treated leaves. This small decrease in ethylene could not account for such large increase in putrescine titers. Further analysis demonstrated that the GABA‐mediated polyamine accumulation was inhibited by difluoromethylarginine, an inhibitor of arginine decarboxylase, but not by difluoromethylornithine, an inhibitor of ornithine decarboxylase. These data suggest that GABA directly or indirectly affects the biosynthesis of polyamines via arginine decarboxylase.
Pullulanase in mung bean cotyledons. Purification, some properties and developmental pattern during and following germination
Authors: Morinaga Kozaburo , Honda Eiichi , Morohashi Yukio , Matsushima Hisashi
Published: May 2006

Starch debranching enzyme was purified from mung bean (Vigna radiata) cotyledons to investigate its properties and developmental pattern during and following germination. A debranching enzyme was purified up to the step where only a doublet of polypeptides with molecular masses of 99 and 101 kDa, respectively, was detected by SDS‐PAGE. The enzyme is thought to be a single chain monomer, as the molecular mass of the enzyme determined by gel filtration was 72 kDa. Monoclonal antibodies raised against the purified preparation recognized the doublet, indicating that the two polypeptides have immunological homology to each other. The enzyme preparation showed a high activity with pullulan as a substrate, low activity with soluble starch and amylopectin, and no activity with glycogen. These substrate specificities indicate that the debranching enzyme from mung bean cotyledons is of the pullulanase type. Immunoblotting profiles revealed that the enzyme is present in dry seeds and decreases gradually after imbibition, suggesting the possibility that the pullulanase plays a role in developing mung bean cotyledons.
Salt‐induced changes in protein composition in light‐grown callus of Mesembryanthemum crystallinum
Authors: Yen H. Emilie , Zhang Dianzhong , Lin Jeng‐Horng , Edwards Gerald E. , Ku Maurice S. B.
Published: May 2006

The halophyte Mesembryanthemum crystallinum (ice plant) has been suggested as a model for salt‐tolerance in higher plants. To investigate salt‐induced changes in polypeptide patterns at the cellular level, a light‐grown callus of M. crystallinum with substantial chlorophyll content, was established and the effect of NaCl on the composition of phenol‐extracted protein was examined by SDS‐ and 2D‐polyacrylamide gel electrophoresis (PAGE). SDS‐PAGE showed the accumulation of five polypeptides with estimated molecular masses of 40, 34, 32, 29 and 14 kDa was enhanced by the addition of 200 mM NaCl to the culture media. The addition of ABA (10 μM) or mannitol (400 mM) did not elicit the same degree of accumulation of these salt‐specific proteins. These polypeptides were classified into two groups according to their course of induction: early responsive (40, 34, 29 kDa) and late‐responsive (32, 14 kDa) proteins. In addition, two polypeptides (20, 18 kDa) were transiently accumulated during salt treatment. Further separation of soluble proteins by 2‐D gel electrophoresis, either isoelectric focusing (IEF) or non‐equilibrium pH‐gradient electrophoresis (NEPHGE) followed by SDS‐PAGE, showed more alterations in accumulation of polypeptides by NaCl than 1‐D gel electrophoresis. Overall, levels of more than 30% of basic polypeptides, detected by NEPHGE/SDS‐PAGE, were altered by 200 mM NaCl treatment, while only 10% of neutral and acidic polypeptides, detected by IEF/SDS‐PAGE, were changed. The enhanced expression of these proteins by salt in cultured cells is most likely related to the cellular responses to salinity, and not to the mechanism of CAM induction in this facultative halophyte.
Sugar uptake by protoplasts isolated from tomato fruit tissues during various stages of fruit growth
Authors: Brown M. M. , Hall J. L. , Ho L. C.
Published: May 2006

The uptake of radioactive glucose and sucrose by protoplasts isolated from pericarp and placenta tissues of tomato (Lycopersicon esculentum cv. Counter) fruit was investigated in relation to the dry matter accumulation rates of these tissues. Uptake of glucose by protoplasts isolated from pericarp tissue was highest in fruit of around 20 g fresh weight or 25 days after anthesis. Sucrose uptake by pericarp protoplasts was lower than that of glucose and did not show a peak of uptake. The maximum rate of glucose uptake by protoplasts from the pericarp was at the time when the tomato fruit was accumulating dry matter at the highest rate. Glucose uptake by placenta protoplasts was lower and at a similar level as sucrose.

Protoplast uptake of glucose, but not of sucrose, was partially inhibited by (1) p‐chloromercuribenzene sulphonic acid, a sulphydryl group modifier; (2) erythrosin B, an H+‐ATPase inhibitor; and (3) valinomycin, a K+‐ionophore, suggesting that membrane transport of glucose by tomato fruit sink cells may be a carrier‐mediated, energy‐dependent process.

The main route of carbohydrate accumulation by tomato fruit during the period of rapid fruit growth may be by cleavage of sucrose by apoplastic acid invertase prior to hexose transport across the plasma membrane.
Temporal accumulation and ultrastructural localization of dehydrins in Zea mays
Authors: Egerton‐Warburton Louise M. , Balsamo Ronald A. , Close Timothy J.
Published: May 2006

Immunolocalization using polyclonal antibodies raised against a conserved dehydrin amino acid sequence was used to establish the temporal and spatial patterns of dehydrin accumulation in embryo tissue of Zea mays L. (var. Ohio 43) kernels imbibed in the presence of abscisic acid. The temporal pattern of accumulation indicated an increase in dehydrins over time (particularly between 15 and 30 h) and with maximum levels detected 48 h after the onset of imbibition. Dehydrins were first evident, and also the most concentrated, in the cytosol throughout the accumulation period suggesting that the primary function of dehydrins involves the cytosol and the structures contained therein. Only after an accumulation of dehydrins in the cytosol was there an increase in the abundance of nuclear dehydrins. In addition, dehydrins were also observed in association with the proteinaceous matrix of protein bodies and membranes of protein and lipid bodies; these findings have not been reported previously. The observed localization at a number of sites indicates that the specific biochemical roles of dehydrins are likely to be diverse.
Characterization of purified leaf cytosolic pyruvate kinase from the C4 plant Cynodon dactylon
Authors: Zervoudakis George , Georgiou Christos D. , Mavroidis Manolis , Kokolakis George , Angelopoulos Kostas
Published: May 2006

Cytosolic pyruvate kinase (EC 2.7.1.40) from leaves of the C4 plant Cynodon dactylon (L.) Pers. was purified 56‐fold to apparent homogeneity by polyethylene glycol fractionation and column chromatography including Q‐Sepharose anion exchanger, ADP‐Agarose and gel filtration. Nondenaturing PAGE of the final preparation resulted in a single protein band that co‐migrated with the pyruvate kinase activity. Gel filtration and SDS‐PAGE (± DTT) showed that this enzyme has a molecular mass of 200 kDa and is a homotetramer with a subunit molecular mass of 50 kDa. The subunits are not associated to each other with S‐S bonds. The enzyme has a pH optimum of 6.2 and is heat stable. Typical Michaelis‐Menten kinetics was obtained for both substrates, PEP and ADP, with Km values of 64 and 235 μM, respectively. Initial velocity studies indicated a sequential binding of the substrates to the enzyme.
Effects of canopy shade on the lipid composition of soybean leaves
Authors: Burkey Kent O. , Wilson Richard F. , Wells Randy
Published: May 2006

The effect of canopy shade on leaf lipid composition was examined in soybeans (Glycine max cv. Young) grown under field conditions. Expanding leaves were tagged at 50, 58 and 65 days after planting (DAP) in plots with either a high (10 plants m−1 row) or low (1 plant m−1 row) plant density. At 92 DAP, light conditions ranged from a pho‐tosynthetic photon flux density (PPFD) of 87% of full sun with a far‐red/red (735 nm/645 nm) ratio of 0.9 at upper canopy leaves to extreme shade where the PPFD was 10% of full sun with a far‐red/red ratio greater than 6. Highly shaded leaves in the high plant density treatment accumulated triacylglycerol (TG) up to 25% of total leaf lipid, a 2.4‐fold increase in TG on a chlorophyll basis compared to leaves in the upper canopy. Although total polar lipid content was reduced up to 50% in shaded leaves, shade had little affect on the lipid content or composition of thylakoid membranes. Shade did not affect leaf chlorophyll content. Therefore, the changes in leaf lipid composition were not related to senescence. These findings suggest that conditions of low irradiance and/or a high FR/R ratio cause a shift in carbon metabolism toward the accumulation of TG, a storage lipid. Eighteen‐carbon fatty acid desaturation was also affected in highly shaded leaves where a reduction in linolenic acid (18:3) content was accompanied by a proportional increase in oleic (18:1) and linoleic (18:2) acids.
Gibberellins and the photoperiodic control of leaf growth in Poa pratensis
Authors: Junttila Olavi , Heide Ola M. , Lindgård Bente , Ernstsen Arild
Published: May 2006

The role of gibberellins (GAs) in photoperiodic control of leaf elongation in Poa pratensis was studied by both application of exogenous GAs and analysis of endogenous GAs. Leaf elongation was strongly increased under long day (LD, 24 h) conditions at both 9 and 21°C, leaf length at 9°C LD being similar to that in plants grown in short days (SD, 8 h) at 21°C. However, even at 21°C leaf elongation was enhanced by LD. Exogenous GA1 could completely compensate for LD at both 9 and 21°C. Gibberellins A20, A19 and A44 could also partly replace LD, but they were significantly less active than GA1, GA53 was inactive when applied to plants grown at 9°C in SD and exhibited only marginal activity at 9°C LD and 21°C SD. The total level of GAs of the early 13‐hydroxylation pathway (A53, A44, A19, A20 and A1) increased rapidly when plants were transferred from SD to LD at 9°C. After transfer from 9 to 21°C, there was an increase in GA levels at both LD and SD, followed by a decrease under LD conditions. In all cases, GA19 was the predominant GA, accounting for 60 to 80% of the analysed GAs. Levels of the bioactive GA1 were low and increased transiently by LD four days after transfer from SD to LD. At both temperatures, the ratio GA19 to GA20 and GA20 to GA1 at 9°C was enhanced by LD compared with SD. Taken together, these results support the hypothesis that photoperiodic regulation of leaf elongation in Poa pratensis is GA‐mediated, and they indicate a photoperiodic control of oxidation of GA53 to GA44 and GA19 to GA20, and perhaps also of 3β‐hydroxylation of GA20 to GA1.
Differential effect of ammonium on the induction of nitrate and nitrite reductase activities in roots of barley (Hordeum vulgare) seedlings
Authors: Aslam Muhammad , Travis Robert L. , Rains D. William , Huffaker Ray C.
Published: May 2006

The effect of exogenous NH4+ on the induction of nitrate reductase activity (NRA; EC 1.6.6.1) and nitrite reductase activity (NiRA; EC 1.7.7.1) in roots of 8‐day‐old intact barley (Hordeum vulgare L.) seedlings was studied. Enzyme activities were induced with 0.1, 1 or 10 mM NO3+ in the presence of 0, 1 or 10 mM NH4+, Exogenous NH4+ partially inhibited the induction of NRA when roots were exposed to 0.1 mM, but not to 1 or 10 mM NO3+, In contrast, the induction of NiRA was inhibited by NH4+ at all NO3+ levels. Maximum inhibition of the enzyme activities occurred at 1.0 mM NH4+ Pre‐treatment with NH4+ had no effect on the subsequent induction of NRA in the absence of additional NH4+ whereas the induction of NiRA in NH4+‐pretreated roots was inhibited in the absence of NH4+ At 10 mM NO3+ L‐methionine sulfoximine stimulated the induction of NRA whether or not exogenous NH4+ was present. In contrast, the induction of NiRA was inhibited by L‐methionine sulfoximine irrespective of NH4+ supply. During the postinduction phase, exogenous NH4+ decreased NRA in roots supplied with 0.1 mM but not with 1mM NH3+ whereas, NiRA was unaffected by NH4+ at either substrate concentration. The results indicate that exogenous NH4+ regulates the induction of NRA in roots by limiting the availability of NO3+. Conversely, it has a direct effect, independent of the availability of NO3+, on the induction of NiRA. The lack of an NH4+ effect on NiRA during the postinduction phase is apparently due to a slower turnover rate of that enzyme.
Chlorophyll organization in dark‐grown and light‐grown pine (Pinus brutia) and barley (Hordeum vulgare)
Authors: Raskin Victor I. , Marder Jonathan B.
Published: May 2006

A study was conducted comparing the organization of chlorophyll during development of the photosynthetic apparatus in dark‐grown and light‐grown pine and barley. The rationale was that gymnosperms, but not angiosperms, have a capacity to synthesize chlorophyll in darkness. Seedlings of Pinus brutia were germinated and grown in darkness or under photoperiodic (day/night) conditions. The low‐temperature (77 K) fluorescence spectra of newly‐emerging dark‐grown seedlings exhibited a single fluorescence band peaking at 678–679 nm, which decayed primarily with a ∼5.5 ns lifetime. Over the first few days of growth, the emission shifted to longer wavelengths and a subnanosecond lifetime component became prevalent. After several days of dark growth the emission spectrum and lifetime profile of the low temperature fluorescence came to resemble those of light‐grown pine and barley. At room temperature, dark‐grown pine showed little variable fluorescence, though addition of DCMU caused a substantial fluorescence rise. Illumination with moderate light for a few hours was sufficient to ‘photoinduce’ the appearance of normal variable fluorescence. At 77 K, DCMU‐treated samples clearly showed a very long‐lived (∼40 ns) fluorescence lifetime component in light‐grown pine and barley. This component was undetectable in dark‐grown pine. If, however, dark‐grown samples were illuminated either before or after DCMU addition and then frozen to 77 K, the ∼40 ns lifetime component appeared at a fluorescence intensity similar to that in light‐grown samples. These results are explained primarily in terms of photoactivation of the photosystem II (PSII) donor side. The temporary maintenance of PSII in an inactive, highly‐quenched state is suggested to provide an available, yet protected precursor for active PSII.
Intracellular resistance correlates with initial stage of frost hardening in willow (Salix viminalis)
Authors: Repo Tapani , Hiekkala Panu , Hietala Timo , Tahvanainen Liisa
Published: May 2006

Distributed model parameters of shoots of five clones of willow (Salix viminalis) were examined with electrical impedance analysis at the end of the growing season and with cold acclimation. The parameters were compared with regard to frost hardiness, linoleic (18:2) and linolenic (18:3) fatty acids, unsaturation/saturation ratio of fatty acids and dry weight content. The intracellular resistance (ri) correlated best with changes in frost hardiness. The value of ri rose from 1–2 Ω m in non‐hardened to about 12 Ω m in hardened samples. In the initial stages of frost hardening, a linear relationship was found between ri and frost hardiness and levels of 18:2 fatty acid, and an inverse relationship between ri and levels of 18:3 fatty acid. The unsaturation/saturation ratio of fatty acids rose fairly rapidly in the initial stage of hardening. The dry weight content increased stepwise during the experimental period, and less steadily than ri. In addition, equivalent circuit parameters changed in the prehardening phase, and were probably connected with cell differentiation and lignification. Frost hardiness by the visual method and by extracellular resistance, determined after controlled freezing tests, correlated well in the initial stages of hardening until about − 10°C but deviated upon further hardening.
Two independent biochemical pathways for isopentenyl diphosphate and isoprenoid biosynthesis in higher plants
Authors: Lichtenthaler Hartmut K. , Rohmer Michel , Schwender Jörg
Published: May 2006

In the early times of isoprenoid research, a single pathway was found for the formation of the C5 monomer, isopentenyl diphosphate (IPP), and this acetate/mevalonate pathway was supposed to occur ubiquitously in all living organisms. Now, 40 years later, a totally different IPP biosynthesis route has been detected in eubacteria, green algae and higher plants. In this new pathway glyceraldehyde 3‐phosphate (GAP) and pyruvate are precursors of isopentenyl diphosphate, but not acetyl‐CoA and mevalonic acid. In green tissues of three higher plants it was shown that all chloroplastbound isoprenoids (β‐carotene, phytyl chains of chlorophylls and nona‐prenyl chain of plastoquinone‐9) are formed via the GAP/pyruvate pathway, whereas the cytoplasmic sterols are formed via the acetate/mevalonate pathway. Also, isoprene, emitted by various plants at high light conditions by action of the plastid‐bound isoprene synthase, is formed via the new GAP/pyruvate pathway. Thus, in higher plants, there exist two separate and biochemically different IPP biosynthesis pathways: (1) the novel alternative GAP/pyruvate pathway apparently bound to the plastidic compartment and (2) the classical cytoplasmic acetate/mevalonate pathway. This new GAP/pyruvate pathway for IPP formation allows a reasonable interpretation of previous odd results concerning the biosynthesis of chloroplast isoprenoids, which, so far, had mainly been interpreted assuming compartmentation differences. The novel GAP/pyruvate pathway for IPP formation in plastids appears as a heritage of their prokaryotic, endosymbiotic ancestors.
Cytokinin biosynthesis and interconversion
Authors: Chen Chong‐maw
Published: May 2006

To maintain hormone homeostasis, the rate of cytokinin biosynthesis, interconversion, and degradation is regulated by enzymes in plant cells. Cytokinins can be synthesized via direct (de novo) or indirect (tRNA) pathways. In the de novo pathway, a cytokinin nucleotide is synthesized from 5′‐AMP and isopentenyl pyrophosphate; a key enzyme which catalyzes this synthesis has been isolated from plant tissues, slime mold, and some microorganisms. Studies on the in vitro synthesis of the isopentenyl side chain of cytokinin in tRNA demonstrated that the isopentenyl group was derived from mevalonate, and turnover of the cytokinin‐containing tRNA may serve as a minor source of free cytokinins in plant cells. The interconversion of cytokinin bases, nucleosides and nucleotides is a major feature of cytokinin metabolism; and enzymes that regulate the interconversion have been identified. The N6‐side chain and purine moiety of cytokinins are often modified and some of the enzymes involved in the modifications have been isolated. Most of the cytokinin metabolites have been characterized but very few enzymes regulating their metabolism have been purified to homogeneity. It remains a significant challenge to isolate plant genes involved in the regulation of cytokinin biosynthesis, interconversion and degradation.
Regulation of cytokinin content in plant cells
Authors: Kamínek Miroslav , Motyka Václav , Vaňková Radomíra
Published: May 2006

Cytokinin levels in plant cells are dependent on cytokinin biosynthesis and/or uptake from extracellular sources, metabolic interconversions, inactivation and degradation. Cytokinin conversion to compounds differing in polarity seems to be decisive for their entrapment within the cell and intracellular compartmentation, which affects their metabolic stability. Increase in cytokinin levels, resulting either from their uptake or intracellular biosynthesis, may promote further autoinductive accumulation of cytokinins which may function in the induction of cytokinin‐initiated physiological processes. Accumulated cytokinins are capable of inducing cytokinin oxidase which consequently decreases cytokinin levels. This seems to be the mechanism of re‐establishment and maintenance of cytokinin homeostasis required for further development of physiological events induced by transient cytokinin accumulation. Auxin may influence cytokinin levels by down regulation of cytokinin biosynthesis and/or by promotion of cytokinin degradation. A model of the regulation of cytokinin levels in plant cells based on these phenomena is presented and its physiological role(s) is discussed.
Regulation and cellular localization of ent‐kaurene synthesis
Authors: Sun Tai‐ping , Kamiya Yuji
Published: May 2006

The two‐step cyclization reaction of ent‐kaurene synthesis from geranylgeranyl diphosphate is the first committed step in the biosynthetic pathway of the plant hormone gibberellin. Recent molecular cloning and characterization of the genes encoding the two corresponding enzymes, copalyl diphosphate synthase (CPS) and ent‐kau‐rene synthase (KS), have demonstrated that ent‐kaurene synthesis is localized in the plastids and is highly regulated in specific tissues and cell types during plant development. In addition to occurring in actively growing tissues, ent‐kaurene synthesis also takes place in fully expanded leaves. Therefore mature leaves may produce gibberellin intermediates or bioactive gibberellins for transport to responsive tissues. DNA sequence analyses have revealed a conserved aspartate‐rich motif, D(I/V)DDTA among CPS and other protonation‐initiated terpene cyclases, while KS contains a highly conserved DDXXD motif which was proposed to function as a divalent metal ion‐diphos‐phate complex binding site in ionization‐initiated terpene cyclases and prenyltrans‐ferases.
The oxidases of gibberellin biosynthesis: Their function and mechanism
Authors: Hedden Peter
Published: May 2006

Gibberellins (GAs) are biosynthesised from the diterpene ent‐kaurene by a series of oxidative reactions catalysed by two classes of enzymes. The early steps, involving transformations of highly hydrophobic substrates, are carried out by membrane‐associated monooxygenases, probably involving cytochrome P450, whereas the later reactions are catalysed by soluble 2‐oxoglutarate‐dependent dioxygenases. Some reactions involving substrates, such as GA12 and GA12‐aldehyde, that have intermediate polarity are catalysed by enzymes in both classes. The monooxygenases and dioxygenases catalyse the same types of reactions: hydroxylation, desaturation, alcohol and aldehyde oxidation. For both enzyme classes, the oxidant is thought to be an oxyferryl species, depicted as Felv=O, that is derived from molecular oxygen by different mechanisms, the reducing power being supplied by NADPH in the case of cytochrome P450 monooxygenases and by the decarboxylation of 2‐oxoglutarate to succinate for the dioxygenases. The recent availability of cDNA clones for several of the dioxygenases and the ability to prepare active enzymes by heterologous expression of cDNAs in Escherichia coli have provided new opportunities for examining the function of these enzymes. They have relatively low substrate specificity and, in many cases, are multifunctional. Consequently, fewer enzymes than expected are required to produce the large number of GA structures encountered in higher plants. In the present review, the major oxygenases of GA biosynthesis are described and their reactions are discussed in an attempt to rationalise this multifunctionality.
Ethylene and fruit ripening
Authors: Lelièvre Jean‐Marc , Latchè Alain , Jones Brian , Bouzayen Mondher , Pech Jean‐Claude
Published: May 2006

The latest advances in our understanding of the relationship between ethylene and fruit ripening are reviewed. Considerable progress has been made in the characterisation of genes encoding the key ethylene biosynthetic enzymes, ACC synthase (ACS) and ACC oxidase (ACO) and in the isolation of genes involved in the ethylene signal transduction pathway, particularly those encoding ethylene receptors (ETR). These have allowed the generation of transgenic fruit with reduced ethylene production and the identification of the Nr tomato ripening mutant as an ethylene receptor mutant. Through these tools, a clearer picture of the role of ethylene in fruit ripening is now emerging. In climacteric fruit, the transition to autocatalytic ethylene production appears to result from a series of events where developmentally regulated ACO and ACS gene expression initiates a rise in ethylene production, setting in motion the activation of autocatalytic ethylene production. Differential expression of ACS and ACO gene family members is probably involved in such a transition. Finally, we discuss evidence suggesting that the NR ethylene perception and transduction pathway is specific to a defined set of genes expressed in ripening climacteric fruit and that a distinct ETR pathway regulates other ethylene‐regulated genes in both immature and ripening climacteric fruit as well as in non‐climacteric fruit. The emerging picture is one where both ethylene‐dependent and ‐independent pathways coexist in both climacteric and non‐climacteric fruits. Further work is needed in order to dissect the molecular events involved in individual ripening processes and to understand the regulation of the expression of both ethylene‐dependent and ‐independent genes.
Jasmonic acid in wound signal transduction pathways
Authors: Seo Shigemi , Sano Hiroshi , Ohashi Yuko
Published: May 2006

Wounding induces expression of genes encoding defense‐related proteins involved in wound healing. An intensive survey has been carried out to clarify the initial signal transduction pathways that mediate this stress to expression of genes. In this context, signal molecules that intermediate in the wound signal to cellular response have been actively searched for. Jasmonic acid (JA) has been considered to be a key signal molecule in this pathway. Systemin, ABA, ethylene, and electrical current have been suggested to function by transmitting the wound signal to JA. A mitogen‐activated protein kinase has been shown to respond rapidly to wounding, and proposed to function as one of the key enzymes involved in JA biosynthesis. Transgenic plants overexpressing a gene encoding a Rab‐type, small GTP‐binding protein contained 6‐fold higher levels of cytokinins than wild‐type plants, and responded to wounding by rapidly producing JA and, uncommonly, accumulating salicylic acid (SA), a pathogenic signal. These phenomena observed in the transgenic plants were reproduced when wild‐type plants were wounded in the presence of the synthetic cytokinin, benzylaminopurine, suggesting that cytokinins are indispensable in the control of endogenous levels of JA and SA.
Measurement of leaf epidermal transmittance of UV radiation by chlorophyll fluorescence
Authors: Bilger Wolfgang , Veit Markus , Schreiber Lukas , Schreiber Ulrich
Published: May 2006

In higher plants one of the important functions of the leaf epidermis is the effective screening of ultraviolet‐B (280–320 nm, UV‐B) radiation, due mostly to phenolic compounds. The assessment of the contribution of this function is necessary for an evaluation of the impact of increasing UV‐B radiation. A method is proposed to estimate epidermal transmittance on the basis of chlorophyll fluorescence measurements. Fluorescence of chlorophyll induced by UV‐A (320–400 nm, measuring beam centered at 366 nm, half band width 32 nm) or UV‐B (measuring beam centered at 314 nm, half band width 18 nm) is compared to that induced by a blue‐green measuring light (475 nm, half band width 140 nm). It is shown that the ratios of UV‐and blue‐green (BG)‐induced fluorescence, F(UV‐A)/F(BG) and F(UV‐B)/F(BG), are relatively constant among leaf samples of various species (Vicia faba, Spinacia oleracea, Rumex scutatus) from which the epidermis was removed. In epidermis‐free leaves no significant differences were found between adaxial and abaxial leaf sides, suggesting that leaf structure has negligible influence on the F(UV)/F(BG) ratios. On the other hand, fluorescence excitation ratios varied over a vast range when intact leaves from different species and habitats were investigated. Ratios were low in sun leaves and relatively high in shade‐ and greenhouse‐grown leaves. By relating these results to those obtained with epidermis‐free leaves, epidermal transmittances for UV‐B radiation could be estimated, with values ranging between 1 and 45%. The data demonstrate a large adaptability of epidermal UV‐A and UV‐B transmittance in higher plants. The proposed method may prove a versatile and relatively simple tool for investigating epidermal UV transmittance complementing established methods.
Nitrogen distribution in young Norway spruce (Picea abies) trees as affected by pedospheric nitrogen supply
Authors: Stoermer Heike , Seith Bettina , Hanemann Ulrike , George Eckhard , Rennenberg Heinz
Published: May 2006

In numerous locations in Europe spruce trees are exposed to high loads of nitrogen. The present study was performed to characterize the distribution of nitrogen compounds under these conditions. For this purpose Norway spruce (Picea abies [L.] Karst.) trees were cultivated under close‐to‐natural conditions of a forest understory in soil from an apparently nitrogen‐limited field site in the Black Forest either with, or without supplementation of nitrogen as ammonium nitrate. After 11 and 20 months, growth, total nitrogen contents of the biomass, and total soluble non‐proteinogenic nitrogen compounds (TSNN, i.e. nitrate, ammonium, soluble proteinogenic and non‐proteinogenic amino compounds) in needles, xylem sap and phloem exudate were analysed. After 20 months of growth, N‐fertilization had slightly enhanced the biomass of current‐, but not of 1‐year‐old shoots. At both harvests, total N‐content of 1‐year‐old needles was increased by N‐fertilization, whereas current‐year needles were not significantly affected. By contrast, TSNN was elevated by N‐fertilization in both current‐year and 1‐year‐old needles. The increase in TSNN was mainly attributed to an accumulation of arginine. Xylem sap analysis showed that the increase in TSNN of the needles was a consequence of enhanced nitrogen assimilation of the roots rather than the shoot. Since also TSNN in phloem exudates was enhanced, it appears that N‐fertilization elevates the cycling pool of amino compounds in young Norway spruce trees. However, this pool seems to be subject to metabolic interconversion, since mainly glutamine and aspartate are transported in the xylem from the roots to the shoot, but arginine accumulated in the needles and the phloem.
Root nutation modelled by two ion flux‐linked growth waves around the root
Authors: Shabala Sergey N. , Newman Ian A.
Published: May 2006

A new model of root circumnutation is proposed, based on the correlation between nutation and ion flux oscillations in the elongation zone. The model considers that, in a small region on one side of the root and within the elongation zone, the growth rate is different from the average. This local disturbance to growth may have been caused by mechanical, chemical or other stimuli, or it may be merely an effect of existing nutation, tropisms or other asymmetric growth. If this disturbance is a reduction in the local growth rate, the root will be bending towards that side. The model provides that the region of growth reduction enlarges and spreads as a wave along and around the root. As the wave of growth reduction moves away, the initially disturbed region recovers. Viewing the root in the cross section containing the original region, the growth reduction moves from that region with different speeds in the two opposite directions around the circumference of the root. The mathematical formulation for the movement of the growth disturbance is given. The resulting bending of the root can be quite complex. The proposed model allows us to describe typical features of nutating roots reported in the literature and observed in our experiments. Results of simulation show a high correspondence with experimental observations. Some components of a possible physiological mechanism for the model are discussed.
Water and salt stress‐induced alterations in proline metabolism of Triticum durum seedlings
Authors: Mattioni C. , Lacerenza N. G. , Troccoli A. , De Leonardis A. M. , Di Fonzo N.
Published: May 2006

Many plants accumulate proline as a non‐toxic and protective osmolyte under saline or dry conditions. Its accumulation is caused by both the activation of its biosynthesis and inactivation of its degradation. We report here on the alterations induced by water and salt stress in the proline metabolism and amino acid content of 5‐day‐old seedlings of Triticum durum cv. Simeto. Most of the amino acids showed an increase with the induction of either stress, but proline increased more markedly than did other amino acids. We also measured the activities of two enzymes, Δ1‐pyrroline‐5‐carboxylate (P5C) reductase (EC 1.5.1.2) and proline dehydrogenase (EC 1.5.1.2), which are involved in proline biosynthesis and catabolism, respectively. The activity of P5C reductase was enhanced during both water and salt stress, while proline dehydrogenase was inhibited only during salt stress. The results indicate that synthesis de novo is the predominant mechanism in proline accumulation in durum wheat. Use of a cDNA clone that encodes P5C‐reductase from Arabidopsis thaliana, showed no differences in the gene expression between controls and stressed plants, implying that the increase in enzyme activity is unrelated to the expression of this gene.
Influence of cadmium on the production of γ‐glutamylcysteine peptides and enzymes of nitrogen assimilation in Zea mays seedlings
Authors: Ju Grace C. , Li Xui‐Zhen , Rauser Wilfried E. , Oaks Ann
Published: May 2006

We examined the effect of cadmium (Cd) additions on a GDH1‐null line of maize and its wild‐type isogenic sibling. Addition of Cd increases the synthesis of metallothioneines which are glutamate‐ and cysteine‐rich peptides. We predicted a reduced synthesis of γ‐glutamylcysteine (γEC) peptides in the mutant relative to the wild type if glutamate dehydrogenase (GDH) was limiting the drainage of carbon from the tricar‐boxylic acid cycle (TCAC). In our experiments there were similar increases in levels of γEC peptides in both mutant and wild‐type seedlings in response to Cd. There was a marked increase in the phosphoenolpyruvate carboxylase (PEPcase) polypeptide and in one of the polypeptide bands of glutamine synthetase in both mutant and wild‐type seedlings. However, no change was seen in the polypeptide levels of GDH or glutamate synthase (GOGAT). Thus, in contrast to PEPcase, an enhanced carbon drain from the TCAC in response to Cd exposure does not require enhanced levels of either GDH or GOGAT polypeptides.
Inhibition of fructokinase and sucrose synthase by cytosolic levels of fructose in young tomato fruit undergoing transient starch synthesis
Authors: Schaffer Arthur A. , Petreikov Marina
Published: May 2006

Immature tomato fruit are characterized by a transient period of starch accumulation. Sucrose synthase (EC 2.4,1.13) and fructokinase (EC 2.7,1.4) are two of the initial enzymes in the sucrose to starch synthetic pathway. Both enzymes in tomato fruit are significantly inhibited by fructose at concentrations physiological to young tomato fruit. Compartmental analysis of immature fruit pericarp indicates that fructose is not specifically compartmentalized in the vacuole and that physiological cytosolic concentrations of fructose in young tomato fruit are above 30 mM. Such physiological levels of fructose significantly inhibit sucrose synthase cleavage activity as well as the activity of a partially purified fructokinase. These data suggest a mechanism of a coordinated, in vivo regulation of tomato sucrose synthase and fructokinase activity, which may be potentially limiting to starch accumulation in young tomato fruit.
Ageing‐induced changes in glutathione system of sunflower seeds
Authors: Torres M. , De Paula M. , Pérez‐Otaola M. , Darder M. , Frutos G. , Martínez‐Honduvilla C. J.
Published: May 2006

The glutathione system is thought to be involved in defence mechanisms present in plant tissues. The efficacy of this system was evaluated in large seeds of sunflower (Helianthus annuus L. cv. Peredovik) in response to accelerated ageing (43°C/75% relative humidity from 1 to 11 days). Differences between the embryo axis and cotyledons in relation to the glutathione system were also investigated. Additionally, lipid peroxidation was determined by measuring the malondialdehyde (MDA) content. All assays were performed using dry seeds and seeds subsequently hydrated by imbibition in distilled water for 12 h at 25°C. Accelerated ageing caused a marked decrease in seed viability, accompanied by an increase in mean germination time. There were no changes in total glutathione in dry seeds. However, the distribution in its reduced (GSH) and oxidized (GSSG) forms revealed that ageing produced a slow conversion from GSH to GSSG. As the ageing period increased, this effect was accompanied by a decrease in glutathione reductase (GR, EC 1.6.4.2) activity. The results also indicated that the GSH system exerts a different response in the embryo axis as compared with the cotyledon: (1) the GSH levels decreased less in the cotyledons than in axes of aged seeds, and (2) the GSSG level in cotyledons was independent of ageing, while its amount increased in aged embryo axes. These different responses, in conjunction with the lower MDA levels in large as compared with small seeds, indicate a possible protective role of the reserve lipids. The efficacy of the GSH system in aged seeds was associated with seed viability, as revealed by multiple regression analysis. Upon imbibition, aged seeds were able to restore their GSH levels, reaching values approximating those of unaged seeds.
Carbon metabolism in germinating Ricinus communis cotyledons. Purification, characterization and developmental profile of NADP‐dependent malic enzyme
Authors: Colombo S. L. , Andreo C. S. , Podestá F. E.
Published: May 2006

The possible implication of NADP‐dependent malic enzyme (NADP‐ME; L‐malate:NADP oxidoreductase [oxaloacetate‐decarboxylating], EC 1.1.1.40) in fatty acid synthesis was examined in Ricinus communis L. cotyledons, NADP‐ME catalyses the conversion of L‐malate to pyruvate and NADPH, potential substrates for fatty acid synthesis. NADP‐ME activity and protein levels were monitored during germination, up to 20 days postimbibition. The developmental profile showed a peak in activity (6 times with respect to the basal value) and immunoreactive protein (a single 72‐kDa band using anti‐maize NADP‐ME antibodies) around day 7. The enzyme was partially purified (41‐fold) and its kinetics characterized. The optimum pH was around 7.1. Km values for L‐malate and NADP+ were 0.68 mM and 8.2 μM respectively. The enzyme used Mg2+ or Mn2+ as essential cofactors. Several metabolites were assayed as potential enzyme modulators. Succinate, CoA, acetyl‐CoA and palmitoyl‐CoA were activators of NADP‐ME, at saturating or sub‐saturating substrate concentrations, K2 values for CoA and derivative compounds were in the micromolar range (i.e., 0.8 μM for acetyl‐CoA). No significant effects were obtained with other Krebs cycle intermediates and amino acids (i.e. 2‐oxoglutarate, glutamate, glutamine, fumarate). The activity was 29 times higher in the forward (decarboxylating) direction compared to the reverse direction. These results hint at cotyledon NADP‐ME behaving as a regulatory enzyme in R. communis. Its activity is responsive to metabolites of the fatty acid synthesis pathway, and thus a role in this metabolism is suggested.
Accumulation of the chalcone isosalipurposide in primary leaves of barley flavonoid mutants indicates a defective chalcone isomerase
Authors: Reuber Sebastian , Jende‐Strid Barbro , Wray Victor , Weissenböck Gottfried
Published: May 2006

Mutants defective in flavonoid biosynthesis have become increasingly useful in elucidating the potential functions of these compounds in plants. To define the role of flavonoids as UV‐B protectants in barley, we have screened part of the collection of proanthocyanidin‐free barley mutants at the Carlsberg Research Laboratory, Copenhagen, Denmark. The four mutants ant 30–245, ant 30–272, ant 30–287 and ant 30–310 showed drastically reduced flavonoid levels in the primary leaf as compared to their corresponding parent varieties, and in addition accumulated a new mutant‐specific phenolic compound which was identified as the chalcone glucoside isosalipurposide. Results from diallelic crosses indicate that all four mutants belong to the same new complementation group, which is designated as the Ant 30 locus. This gene has not earlier been described in barley. The data presented suggest a defective chalcone isomerase gene for the observed flavonoid pattern in leaves of ant 30 mutants.
A peroxidase/phenolics/ascorbate system can scavenge hydrogen peroxide in plant cells
Authors: Takahama Umeo , Oniki Takayuki
Published: May 2006

The function of a peroxidase/phenolics/ascorbic acid system in plant vacuoles has not yet been well elucidated. We wished to study the redox reactions among hydrogen peroxide, phenolics and ascorbic acid (AA) in the presence of horseradish peroxidase. Horseradish peroxidase oxidized rutin and chlorogenic acid (CGA), compounds present in many kinds of plant. The oxidation was inhibited by AA. As a result of the inhibition. AA was oxidized and when almost all of it had been oxidized, oxidation of the phenolics commenced. Monodehydroascorbic acid (MDA) radical was detected during the oxidation of AA, suggesting that the inhibition of oxidation of rutin and CGA was due to reduction of phenoxyl radicals by AA. By comparison of time courses of changes in levels of AA and MDA radicals, and by kinetic calculation, it is suggested that in addition to AA, MDA radicals may also reduce phenoxyl radicals. It is proposed that the peroxidase/phenolics/AA system can function as a hydrogen peroxide scavenging system.
Export of amino acids to the phloem in relation to N supply in wheat
Authors: Caputo Carla , Barneix Atilio J.
Published: May 2006

The effect of different N supply on amino acid export to the phloem was studied in young plants of wheat (Triticum aestivum L. cv. Klein Chamaco), using the exudation in EDTA technique. Plants were grown in a growth cabinet in pots with sand, and supplied with nutrient solutions of different NO3− concentrations. When plants were grown for 15 days with nutrient solutions containing 1.0, 3.0, 5.0, 10.0, 15.0 or 20.0 mM KNO3, the exudation rate of sugars from the phloem was unaffected by N supply, but sugars accumulated in the leaf tissue when the N supply was limiting for growth. On the other hand, the rate of exudation of amino acids was proportional to the NO3− concentration in the nutrient solution. When the supply of N to plants grown for 15 days with 15.0 mM NO3− was interrupted, the exudation of sugars was again unaffected, but there was a fast decrease in the amount of amino acids exudated, and of the concentration of amino acids and nitrogen in the tissues. Also, when 10‐day‐old plants grown without N were supplied with 15.0 mM NO3−, there was a sharp increase in the exudation of amino acids, without changes in the amount of sugar exudated. The rate of exudation of amino acids to the phloem was independent of the concentration of free amino acids in the leaves in all three types of experiment.

Asp was the most abundant amino acid in the leaf tissue, while Glu was the one most abundant in the phloem exudate. Asp and Ala were exported to the phloem at a rate lower than expected from their leaf tissue concentrations, indicating some discrimination. On the contrary, Glu showed a preferential export at low N supply.

It is concluded that the rate of amino acid export from the leaf to the phloem is dependent on the N available to the plant. This N is used for synthesis of leaf protein when the supply is low, exported to the phloem when supply is adequate, and accumulated in the storage pool when supply is above plant demand.