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1.
Freshly cut discs from growing potato tubers were incubated for 3 h with 10 mM orotate or 10 mM uridine. Control discs incubated
without precursors showed a 30–40% decrease of uridine nucleotides, but not of adenine nucleotides. Orotate- and uridine-feeding
led to a 1.5- to 2-fold increase in the levels of uridine nucleotides compared with control discs, and a 15–30% increase compared
with the original values in intact tubers, but did not alter the levels of adenine nucleotides. Between 70–80% of the uridine
nucleotides were present as UDPglucose, 15–25% as UTP, and 2–3% as UDP. The increase of uridine nucleotides involved a similar
relative increase of UDPglucose, UTP and UDP. It was accompanied by a slight stimulation of the rate of [14C]sucrose uptake, a 2-fold stimulation of the rate at which the [14C]sucrose was subsequently metabolised, a small increase in the levels of hexose phosphates, glycerate-3-phospate and ADPglucose,
and a 30% shift in the allocation of the metabolised label in favour of starch synthesis, resulting in a 2.4-fold stimulation
of the rate of starch synthesis. Orotate led to a similar increase of uridine nucleotide levels in the presence of [14C]glucose, but did not significantly alter the rate of glucose uptake and metabolism to starch, nor did it increase the rate
of sucrose resynthesis. The levels of uridine nucleotides were high in tubers on 6 to 10-week-old potato plants, and declined
in tubers on 12 to 15-week-old plants. Comparison with the effect of the uridine nucleotide level in discs shows that the
high levels of uridine nucleotides in tubers on young plants will play an important role in determining the rate at which
sucrose can be converted to starch, and that the level of uridine nucleotides is probably co-limiting for sucrose-starch conversions
in tubers on older plants.
Received: 25 September 1998 / Accepted: 29 December 1998 相似文献
2.
Regulation of sucrose and starch metabolism in potato tubers in response to short-term water deficit 总被引:24,自引:0,他引:24
Peter Geigenberger Ralph Reimholz Michael Geiger Lucia Merlo Vittoria Canale Mark Stitt 《Planta》1997,201(4):502-518
To investigate the effect of water stress on carbon metabolism in growing potato tubers (Solanum tuberosum L.), freshly cut and washed discs were incubated in a range of mannitol concentrations corresponding to external water potential
between 0 and −1.2 MPa. (i) Incorporation of [14C]glucose into starch was inhibited in water-stressed discs, and labeling of sucrose was increased. High glucose overrode
the changes at low water stress (up to −0.5 MPa) but not at high water stress. (ii) Although [14C]sucrose uptake increased in water-stressed discs, less of the absorbed [14C]sucrose was metabolised. (iii) Analysis of the sucrose content of the discs confirmed that increasing water deficit leads
to a switch, from net sucrose degradation to net sucrose synthesis. (iv) In parallel incubations containing identical concentrations
of sugars but differing in which sugar was labeled, degradation of [14C]sucrose and labeling of sucrose from [14C]glucose and fructose was found at each mannitol concentration. This shows that there is a cycle of sucrose degradation and
resynthesis in these tuber discs. Increasing the extent of water stress changed the relation between sucrose breakdown and
sucrose synthesis, in favour of synthesis. (v) Analysis of metabolites showed a biphasic response to increasing water deficit.
Moderate water stress (0–200 mM mannitol) led to a decrease of the phosphorylated intermediates, especially 3-phosphoglycerate
(3PGA). The decrease of metabolites at moderate water stress was not seen when high concentrations of glucose were supplied
to the discs. More extreme water stress (300–500 mM mannitol) was accompanied by an accumulation of metabolites at low and
high glucose. (vi) Moderate water stress led to an activation of sucrose phosphate synthase (SPS) in discs, and in intact
tubers. The stimulation involved a change in the kinetic properties of SPS, and was blocked␣by protein phosphatase inhibitors.
(vii) The amount of ADP-glucose (ADPGlc) decreased when discs were incubated on 100 or 200 mM mannitol. There was a strong
correlation between the in vivo levels of ADPGlc and 3PGA when discs were subjected to moderate water stress, and when the
sugar supply was varied. (viii) The level of ADPGlc increased and starch synthesis was further inhibited when discs were incubated
in 300–500 mM mannitol. (ix) It is proposed that moderate water stress leads to an activation of SPS and stimulates sucrose
synthesis. The resulting decline of 3PGA leads to a partial inhibition of ADP-glucose pyrophosphorylase and starch synthesis.
More-extreme water stress leads to a further alteration of partitioning, because it inhibits the activities of one or more
of the enzymes involved in the terminal reactions of starch synthesis.
Received: 26 August 1996 / Accepted: 5 November 1996 相似文献
3.
The short-term changes in metabolism that occurred after adding glucose or sucrose to freshly cut discs from growing potato
(Solanum tuberosum L.) tubers were investigated. (i) When glucose was supplied, there was a marked increase in glycolytic metabolites, and respiration
was stimulated. When sucrose was supplied, amounts of glycolytic metabolites including hexose phosphates and 3-phosphoglycerate
(3PGA) were similar to or lower than in control discs incubated without sugars, and respiration did not rise initially above
that in control discs. This different response to sucrose and glucose was found across the concentration range 5–200 mM. A
larger proportion of the metabolised 14C was converted to starch when [14C] sucrose was supplied than when [14C] glucose was supplied. The different effect on metabolite levels, respiration and starch synthesis was largest after 20–30 min,
and decreased in longer incubations. (ii) When 5 or 25 mM sucrose was added in the presence of [14C] glucose, it led to a decrease in hexose phosphates and 3PGA, and a small increase in the rate of starch synthesis compared
to discs incubated with glucose in the absence of sucrose. These differences were seen in a 30-min pulse and a 2-h pulse.
Whereas ADP-glucose levels after adding sucrose resembled those in control discs, glucose led to a decrease in ADP-glucose.
This decrease did not occur when 5 or 25 mM sucrose was added with the glucose. (iii) To check the relevance of these experiments
for intact tubers, water or 100 mM mannitol, sucrose or glucose were supplied through the stolon to intact tubers for 24 h.
A 0.2 mM solution of [14C] glucose was then introduced into the tubers, and its metabolism investigated during the next 30 min. Labelling of starch
was increased after preincubation with sucrose, and significantly inhibited after preincubation with glucose. (iv) It is concluded
that glucose and sucrose have different effects on tuber metabolism. Whereas glucose leads to a preferential stimulation of
respiration, sucrose preferentially stimulates starch synthesis via a novel mechanism that allows stimulation of ADP-glucose
pyrophosphorylase even though the levels of hexose phosphates and the allosteric activator 3PGA decrease.
Received: 9 October 1997 / Accepted: 3 February 1998 相似文献
4.
Water stress stimulates sucrose synthesis and inhibits starch and cell-wall synthesis in tissue slices of growing potato
(Solanum tuberosum L. cv. Desirée) tubers. Based on the analysis of fluxes and metabolites, Geigenberger et al. (1997, Planta 201: 502–518)
proposed that water deficits up to −0.72 MPa stimulate sucrose synthesis, leading to decreased starch synthesis as a result
of the resulting decline of phosphorylated metabolite levels, whereas more-severe water deficits directly inhibit the use
of ADP-glucose. Potato plants with decreased expression of adenosine 5′-diphosphoglucose pyrophosphorylase (AGPase) have been
used to test the prediction that the contribution of AGPase to the control of starch synthesis should decrease in severely
water-stressed tuber material. Freshly cut slices from wild-type and antisense tubers were incubated at a range of mannitol
concentrations (20, 300 and 500 mM) and the metabolism of [14C]glucose was analysed. A 86–97% reduction of AGPase activity led to a major but non-stoichiometric inhibition of starch accumulation
in intact growing tubers attached to the plant (40–85%), and an inhibition of starch synthesis in non-stressed tuber slices
incubated in 20 mM mannitol (60–80%). The inhibition of starch synthesis was accompanied by a 2- to 8-fold increase in the
levels of sugars in intact tubers and a 2- to 3-fold stimulation of sucrose synthesis in tuber slices, whereas respiration
and cell-wall synthesis were not significantly affected. The strong impact of AGPase on carbon partitioning in non-stressed
tubers and tuber slices was retained in slices subjected to moderate water deficit (300 mM mannitol, corresponding to −0.72 MPa).
In discs incubated in 500 mM mannitol (corresponding to −1.2 MPa) this response was modified. A 80–97% reduction of AGPase
resulted in only a 0–40% inhibition of starch synthesis. Further, the water stress-induced stimulation of sucrose synthesis
was abolished in the transformants. The results provide direct evidence that the contribution of AGPase to the control of
starch synthesis can be modified by environmental factors, leading to a lower degree of control during severe water deficits.
There was also a dramatic decrease in the labelling of cell-wall components in wild-type tuber slices incubated with 300 or
500 mM mannitol. The water stress-induced inhibition of cell-wall synthesis occurred independently of AGPase expression and
the accompanying changes in starch and sucrose metabolism, indicating a direct inhibition of cell-wall synthesis in response
to water stress.
Received: 24 February 1999 / Accepted: 28 May 1999 相似文献
5.
Jutta Verscht Bernhard Kalusche Jutta Köhler Walter Köckenberger Alexander Metzler Axel Haase Ewald Komor 《Planta》1998,205(1):132-139
The sucrose concentration was measured at 70-min intervals in the phloem of individual bundles of the hypocotyl of Ricinus seedlings by 1H nuclear magnetic resonance (NMR) spectroscopic imaging. The sucrose concentration stayed fairly constant in all bundles
for more than 7 h if the cotyledons were embedded in the endosperm or excised and incubated in 100 mM sucrose. If, however,
the sucrose solution was replaced by sucrose-free buffer solution, the sucrose levels in the phloem decreased with a kinetic
depending on the seedling: in some cases there was a smooth decline, in some a decline followed by a slight recovery and in
some cases a clear-cut oscillation. The sucrose concentration was often not identical in the phloem of the individual bundles.
The oscillations were larger in the phloem at the apex of the hypocotyl than in the phloem at the base of the hypocotyl. Cutting
the petiole of one cotyledon led to a decrease in sucrose not only in the four bundles directly connected to the severed petiole
but in all eight bundles of the hypocotyl. Cutting the petiole and dividing the vascular ring at the cotyledonary node and
at the root crown did not prevent the decline of sucrose in all eight bundles. Therefore, a functional equilibration of translocated
solutes between the eight bundles may occur within the 1-h measuring interval by radial diffusion through the parenchyma of
the hypocotyl.
Received 4 July 1997 / Accepted: 4 October 1997 相似文献
6.
To investigate the importance of the overall size of the total adenine nucleotide pool for the regulation of primary metabolism
in growing potato tubers, freshly cut discs were provided with zero or 2 mM adenine in the presence of 1 or 100 mM [U-14C]glucose or 100 mM [U-14C]sucrose in the presence and absence of 20 mM orthophosphate (Pi). Adenine led to a 150–250% increase of the total adenine
nucleotide pool, which included an increase of ADP, a larger increase of ATP and an increase of the ATP:ADP ratio. There was
a 50–100% increase of ADP-glucose (ADPGlc), and starch synthesis was stimulated. Respiratory oxygen uptake was stimulated,
and the levels of glycerate-3-phosphate, phosphoenolpyruvate and α-ketoglutarate decreased. The response to adenine was not
modified by Pi. It is proposed that increased ATP stimulates ADPGlc pyrophosphorylase, leading to a higher rate of starch
synthesis. The impact on starch synthesis is constrained, however, because increased ADP can lead to a stimulation of respiration
and decline of glycerate-3-phosphate, which will inhibit ADPGlc pyrophosphorylase. The quantitative impact depends on the
conditions. In the presence of 1 mM glucose, the levels of phosphorylated intermediates and the rate of starch synthesis were
low. Adenine led to a relatively large stimulation of respiration, but only a small stimulation of starch synthesis. In the
presence of 100 mM glucose, discs contained high levels of phosphorylated intermediates, low ATP:ADP ratios (<3) and low rates
of starch synthesis (<20% of the metabolised glucose). Adenine led to marked increase of ATP and 2- to 4-fold stimulation
of starch synthesis. Discs incubated with 100 mM sucrose already had high ATP:ADP ratios (>8) and high rates of starch synthesis
(>50% of the metabolised sucrose). Adenine led to a further increase, but the stimulation was less marked than in high glucose.
These results have implications for the function of nucleotide cofactors in segregating sucrose mobilisation and respiration,
and the need for energy conservation during sugar-starch conversions.
Received: 9 February 2000 / Accepted: 9 June 2000 相似文献
7.
For the production of α-D-glucose-1-phosphate (G-1-P), α-1,4-D-glucan phosphorylase from Thermus caldophilus GK24 was partially purified to a specific activity of 13 U mg−1 and an enzyme recovery of 15%. The amount of G-1-P reached maximum (18%) when soluble starch was used as substrate, and the
smallest substrate for G-1-P formation was maltotriose. The structure of purified G-1-P was confirmed by comparison to 13C-NMR data for an authentic sample. In addition to G-1-P, glucose-6-phosphate (12%) was simultaneously produced when 10 mM
maltoheptaose was used as substrate. Journal of Industrial Microbiology & Biotechnology (2000) 24, 89–93.
Received 12 May 1999/ Accepted in revised form 29 August 1999 相似文献
8.
A method was developed for the purification of main intermediates and storage products of leaves and tubers of potato for
analysis of their 13C content. The method was tested for recovery of metabolites and carbon isotope discrimination during the purification process.
Leaf metabolite δ13C values showed an enrichment of starch relative to sucrose and citrate. This result is in agreement with previous findings
in other higher plants and indicates the existence of isotope discrimination steps during transport and metabolism of triose-phosphates
in potato leaf mesophyll cells. Active anaplerotic replenishment of the tricarboxylic acid cycle in the leaves of the plants
investigated was also deduced from the significant 13C enrichment of malate relative to citrate and asparagine/aspartate relative to glutamine/glutamate. Analysis of tuber metabolite
δ13C values showed no difference between starch and sucrose. However, tuber sucrose appeared significantly enriched compared
with leaf sucrose and also relative to tuber citrate and malate. This finding suggests the existence of sites of isotopic
discrimination during sucrose processing in developing tubers. It also confirms that metabolic cycles of sucrose synthesis
and breakdown and of hexose-phosphate/triose-phosphate interconversion, which have been described in excised tuber tissue,
also occur in intact organs. The δ13C values were also used to estimate the metabolic rate of carbon oxidation in developing tubers on the assumption that pyruvate
dehydrogenase is the main site of isotopic discrimination in the tuber cells. The result obtained was in agreement with the
available literature, suggesting that analyses of natural isotopic distribution in plant products may be a useful tool for
the study of metabolic processes and sink-source relationships in intact plants.
Received: 21 May 1998 / Accepted: 10 July 1998 相似文献
9.
10.
Targeting of auxin carriers to the plasma membrane: differential effects of brefeldin A on the traffic of auxin uptake and efflux carriers 总被引:1,自引:1,他引:0
Monensin and brefeldin A (BFA), inhibitors of Golgi-mediated protein secretion, rapidly perturb the transport catalytic activity
of specific plasma membrane-associated efflux carriers for indole-3-acetic acid (IAA) and inhibit polar transport of IAA.
To determine if these responses result solely from perturbation of the efflux carrier or whether specific auxin uptake carrier
function is also affected, the influence of BFA on the cellular transport of a range of auxins with contrasting affinities
for specific auxin uptake and efflux carriers was investigated in zucchini (Cucurbita pepo L.) hypocotyl tissue. In-flight addition of BFA (3 · 10−5 mol · dm−3) caused a rapid (lag < 10 min) and substantial (fourfold) increase in the rate of [1-14C]IAA net uptake by zucchini hypocotyl tissue. In the presence of the specific auxin efflux carrier inhibitor N-1-naphthylphthalamic acid (NPA; 3 · 10−6 mol · dm−3), BFA slightly reduced the rate of [1-14C]IAA net uptake. Stimulation of [1-14C]IAA net uptake by BFA was concentration-dependent. In the absence of BFA, net uptake of [1-14C]IAA exhibited the characteristic biphasic response to increasing concentrations of competing cold IAA but in the presence
of BFA, [1-14C]IAA uptake decreased smoothly with increase in concentration of competing unlabelled IAA, indicating a loss of auxin efflux
carrier activity but retention of functional uptake carriers. The half-time for mediated efflux of [1-14C]IAA from preloaded zucchini tissue was substantially increased by BFA (t1/2 = 51 min, controls; 107 min, BFA-treated). Treatment with BFA and/or NPA did not significantly affect the net uptake by,
or efflux from, zucchini tissue of [1-14C]2,4-dichlorophenoxyacetic acid ([1-14C]2,4-D), a substrate for the auxin uptake carrier but not the auxin efflux carrier. Uptake of [1-14C]2,4-D declined smoothly with increasing concentrations of competing unlabelled IAA whether or not BFA was included in the
uptake medium, confirming the failure of BFA to perturb auxin uptake carrier function. Transport of 1-[4-3H]naphthaleneacetic acid (1-NAA) exhibited little response to BFA or NPA, confirming that it is only a weakly transported
substrate for the efflux carrier in zucchini cells.
Received: 12 November 1997 / Accepted: 27 January 1998 相似文献
11.
The role of gibberellins (GAs) in the regulation of shoot elongation is well established but the phytohormonal control of
dry-matter production is poorly understood. In the present study, shoot elongation and dry-matter production were resolved
by growing Brassica napus L. seedlings under five light intensities (photon flux densities) ranging from 25 to 500 μmol m−2 s−1. Under low light, plants were tall but produced little dry weight; as light intensity was increased, plants were progressively
shorter but had increasing dry weights. Endogenous GAs in stems of 16- and 17-d-old plants were analyzed by gas chromatography-selected
ion monitoring with [2H2] internal standards. The contents of GAs increased dramatically with decreasing light intensity: GA1, GA3, GA8 and GA20 were 62, 15, 16 and 32 times higher, respectively, under the lowest versus highest light intensities. Gibberellin A19 was not measured at 25 μmol m−2 s−1 but was 9␣times greater in the 75 compared to 500 μmol m−2 s−1 treatment. Shoot and hypocotyl lengths were closely positively correlated with (log) GA concentration (for example: r
2 = 0.93 for GA1 and hypocotyl length) but shoot dry matter was negatively correlated with GA concentration. The application of gibberellic
acid (GA3) produced elongation of plants grown under high light, indication that their low level of endogenous GA was limiting shoot
elongation. Although endogenous GA20 showed the greatest influence of light treatment, metabolism of [3H]GA20 and of [3H]GA1 was only slightly influenced by light intensity, suggesting that neither 2β- nor 3β-hydroxylation were points of metabolic
regulation. The results of this study indicate that GAs control shoot elongation but are not directly involved in the regulation
of shoot dry weight in Brassica. The study also suggests a role of GAs in photomorphogenesis, serving as an intermediate between light condition and shoot
elongation response.
Received: 18 June 1998 / Accepted: 29 July 1998 相似文献
12.
Double-stranded DNA in many bacterial viruses (phage) is strongly confined, which results in internal genome pressures of
tens of atmospheres. This pressure is strongly dependent on local ion concentration and distribution within the viral capsid.
Here, we have used electron energy loss spectroscopy (EELS), energy-filtered TEM (EFTEM) and X-ray energy dispersive spectroscopy
to provide such chemical information from the capsid and the phage tail through which DNA is injected into the cell. To achieve
this, we have developed a method to prepare thin monolayers of self-supporting virus/buffer films, suitable for EELS and EFTEM
analysis. The method is based on entrapment of virus particles at air–liquid interfaces; thus, the commonly used method of
staining by heavy metal salts can be avoided, eliminating the risk for chemical artifacts. We found that Mg2 + concentration was approximately 2–4 times higher in the DNA-filled capsid than in the surrounding TM buffer (containing 10 mM
Mg2 + ). Furthermore, we also analyzed the DNA content inside the phage tail by mapping phosphorus and magnesium. 相似文献
13.
The soluble acid invertase (β-D-fructofuranoside fructohydrolase, EC 3.2.1.26) from potato (Solanum tuberosum L. cv. Kennebec) tubers was located in the vacuoles. Although the functionality of this invertase in the vacuoles has been
assumed, the activity of the enzyme has never been shown within isolated vacuoles. Vacuoles were prepared by gentle osmotic
shock from free protoplasts obtained by enzymic digestion of tuber tissues. The mean volume of these vacuoles, (0.26 ± 0.05) × 10−2 μl, was estimated by optical microscopy. Sucrose, glucose and fructose concentrations were calculated to be 100 mM, 20 mM
and 40 mM, respectively, in the vacuoles. Sucrose hydrolysis and the increase in glucose and fructose concentrations within
the vacuoles were measured during vacuolar incubations. An almost identical pattern of sucrose hydrolysis by invertase was
found by an in-vitro assay reproducing the vacuolar conditions. In view of the determinations of internal vacuolar pH (5.2),
the possibility of spontaneous hydrolysis of sucrose was disregarded. Vacuoles were shown to be free from proteinaceous inhibitors,
confirming the extravacuolar location of these inhibitors. The vacuolar hydrolytic pattern of sucrose confirms the regulatory
role of the reaction products previously proposed for in-vitro assays.
Received: 21 July 1997 / Accepted: 31 August 1997 相似文献
14.
Efficacy of silver thiosulfate (STS) in reducing ethylene-induced culture abnormalities during minimal growth conservation
of microplants was studied in seven potato (Solanum tuberosum L.) genotypes. Different concentrations of STS (0, 1.5, 3.0, 4.5, 6.0, 7.5 and 9.0 μg ml–1) were tested in minimal growth medium based on MS medium supplemented with 20 g l–1 mannitol and 40 g l–1 sucrose. STS improved the microplant growth and reduced the culture abnormalities during prolonged maintenance of potato
shoot cultures in vitro. The beneficial effect of STS was most prominent for number of green leaves per microplant and leaf
senescence. After 16 months of storage, desirable microplant growth was observed in cultures conserved in medium containing
6.0–9.0 μg ml–1 STS. The profile of the peroxidase isozymes of conserved cultures did not show any apparent genetic variation due to the
presence of STS in the conservation medium.
Received: 2 September 1998 / Revision received: 20 November 1998 / Accepted: 12 December 1998 相似文献
15.
Maple sap, an abundant natural product especially in Canada, is rich in sucrose and thus may represent an ideal renewable
feedstock for the production of a wide variety of value-added products. In the present study, maple sap or sucrose was employed
as a carbon source to Alcaligenes latus for the production of poly-β-hydroxybutyrate (PHB). In shake flasks, the biomass obtained from both the sap and sucrose were
4.4 ± 0.5 and 2.9 ± 0.3 g/L, and the PHB contents were 77.6 ± 1.5 and 74.1 ± 2.0%, respectively. Subsequent batch fermentation
(10 L sap) resulted in the formation of 4.2 ± 0.3 g/L biomass and a PHB content of 77.0 ± 2.6%. The number average molecular
weights of the PHB produced by A. latus from maple sap and pure sucrose media were 300 ± 66 × 103 and 313 ± 104 × 103 g/mol, respectively. Near-infrared, 1H magnetic resonance imaging (MRI), and 13C-MRI spectra of the microbially produced PHB completely matched those obtained with a reference material of poly[(R)-3-hydroxybutyric
acid]. The polymer was found to be optically active with [α]25
D equaled to −7.87 in chloroform. The melting point (177.0°C) and enthalpy of fusion (77.2 J/g) of the polymer were also in
line with those reported, i.e., 177°C and 81 J/g, respectively. 相似文献
16.
During cassava starch production, large amounts of cyanoglycosides were released and hydrolysed by plant-borne enzymes, leading
to cyanide concentrations in the wastewater as high as 200 mg/l. For anaerobic degradation of the cyanide during pre-acidification
or single-step methane fermentation, anaerobic cultures were enriched from soil residues of cassava roots and sewage sludge.
In a pre-acidification reactor this culture was able to remove up to 4 g potassium cyanide/l of wastewater at a hydraulic
retention time (t
HR) of 4 days, equivalent to a maximal cyanide space loading of 400 mg CN− l−1 day−1. The residual cyanide concentration was 0.2–0.5 mg/l. Concentrated cell suspensions of the mixed culture formed ammonia and
formate in almost equimolar amounts from cyanide. Little formamide was generated by chemical decay. A concentration of up
to 100 mmol ammonia/l had no inhibitory effect on cyanide degradation. The optimal pH for cyanide degradation was 6–7.5, the
optimal temperature 25–37 °C. At a pH of 5 or lower, cyanide accumulated in the reactor and pre-acidification failed. The
minimal t
HR for continuous cyanide removal was 1.5 days. The enriched mixed culture was also able to degrade cyanide in purely mineralic
wastewater from metal deburring, either in a pre-acidification reactor with a two-step process or in a one-step methanogenic
reactor. It was necessary to supplement the wastewater with a carbon source (e.g. starch) to keep the population active enough
to cope with any possible inhibiting effect of cyanide.
Received: 29 April 1998 / Received revision: 8 June 1998 / Accepted: 14 June 1998 相似文献
17.
Water conductance of the cuticular membrane (CM) of mature sweet cherry fruit (Prunus avium L. cv. Sam) was investigated by monitoring water loss from segments of the outer pericarp excised from the cheek of the fruit.
Segments consisted of epidermis, hypodermis and several cell layers of the mesocarp. Segments were mounted in stainless-steel
diffusion cells with the mesocarp surface in contact with water, while the outer cuticular surface was exposed to dry silica
(22 ± 1 °C). Conductance was calculated by dividing the amount of water transpired per unit area and time by the difference
in water vapour concentration across the segment. Conductance values had a log normal distribution with a median of 1.15 × 10−4 m s−1 (n=357). Transpiration increased linearly with time. Conductance remained constant and was not affected by metabolic inhibitors
(1 mM NaN3 or 0.1 mM carbonylcyanide m-chlorophenylhydrazone) or thickness of segments (range 0.8–2.8 mm). Storing fruit (up to 42 d, 1 °C) used as a source of
segments had no consistent effect on conductance. Conductance of the CM increased from cheek (1.16 ± 0.10 × 10−4 m s−1) to ventral suture (1.32 ± 0.07 × 10−4 m s−1) and to stylar end (2.53 ± 0.17 × 10−4 m s−1). There was a positive relationship (r2=0.066**; n=108) between conductance and stomatal density. From this relationship the cuticular conductance of a hypothetical astomatous
CM was estimated to be 0.97 ± 0.09 × 10−4 m s−1. Removal of epicuticular wax by stripping with cellulose acetate or extracting epicuticular plus cuticular wax by dipping
in CHCl3/methanol increased conductance 3.6- and 48.6-fold, respectively. Water fluxes increased with increasing temperature (range
10–39 °C) and energies of activation, calculated for the temperature range from 10 to 30 °C, were 64.8 ± 5.8 and 22.2 ± 5.0 kJ
mol−1 for flux and vapour-concentration-based conductance, respectively.
Received: 23 March 2000 / Accepted: 28 July 2000 相似文献
18.
Richard N. Trethewey Jörg W. Riesmeier Lothar Willmitzer Mark Stitt Peter Geigenberger 《Planta》1999,208(2):227-238
Fluxes were investigated in growing tubers from wild-type potato (Solanum tuberosum L. cv. Desiree) and from transformants expressing a yeast invertase in the cytosol under the control of the tuber-specific
patatin promoter either alone (EC 3.2.1.26; U-IN2-30) or in combination with a Zymomonas mobilis glucokinase (EC 2.7.1.2; GK3-38) by supplying radiolabelled [14C]sucrose, [14C]glucose or [14C]fructose to tuber discs for a 90-min pulse and subsequent chase incubations of 4 and 12 h, and by supplying [14C]fructose for 2 h and 4 h to intact tubers attached to the mother plant. Contrary to the expectation that this novel route
for sucrose degradation would promote starch synthesis, the starch content decreased in the transgenic lines. Labelling kinetics
did not reveal whether this was due to changes in the fluxes into or out of starch. However, they demonstrated that glycolysis
is enhanced in the transgenic lines in comparison to the wild type. There was also a significant stimulation of sucrose synthesis,
leading to a rapid cycle of sucrose degradation and resynthesis. The labelling pattern indicated that sucrose phosphate synthase
(SPS; EC 2.4.1.14) was responsible for the enhanced recycling of label into sucrose. In agreement, there was a 4-fold and
6-fold increase in the activation status of SPS in U-IN2-30 and GK3-38, respectively, and experiments with protein phosphatase
inhibitors indicated that this activation involves enhanced dephosphorylation of SPS. It is proposed that this activation
of SPS is promoted by the elevated glucose 6-phosphate levels in the transgenic tubers. These results indicate the pitfalls
of metabolic engineering without a full appreciation of the metabolic system and regulatory circuits present in the tissue
under investigation.
Received: 21 July 1998 / Accepted: 5 December 1998 相似文献
19.
Mannose is an unusable carbon source for many plants. In our study we compared the effects of mannose and sucrose on growth
and sucrose levels in azuki bean (Vigna angularis) cells grown in liquid media and in solid media. The suspension cells grew actively in a liquid medium containing 90 mM sucrose
but not in that containing 90 mM mannose, where the intracellular sucrose levels were reduced to 20% or less of those in sucrose-grown
cells. These results suggested that the limited conversion of mannose to sucrose resulted in cell growth inhibition. When
sucrose-grown suspension cells (1 × 105) were transferred onto agar medium containing mannose, they grew little initially, but, after a month lag period, they started
to form many callus colonies at a high apparent variation rate (1.3 × 10−3). Time-course studies for sugar and enzyme analysis revealed that the mannose-accommodated cells were capable of converting
mannose to sucrose, with enhanced phosphomannose isomerase activity. The mannose-accommodated cells actively grew in liquid
medium with sucrose but lost their ability to grow with mannose again, suggesting a specific trait of callus culture for mannose
utilization. The possible differences in the metabolic activities and other physiological characteristics are discussed between
callus and suspension cells.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
20.
In order to clearly establish the properties of the enzymes responsible for hexose phosphorylation we have undertaken the
separation and characterization of these enzymes present in tomato fruit (Martinez-Barajas and Randall 1996). This report
describes the partial purification and characterization of glucokinase (EC. 2.7.1.1) from young green tomato fruit. The procedure
yielded a 360-fold enrichment of glucokinase. Tomato fruit glucokinase is a monomer with a molecular mass of 53 kDa. Glucokinase
activity was optimal between pH 7.5 and 8.5, preferred ATP as the phosphate donor (K
m = 0.223 mM) and exhibited low activity with GTP or UTP. The tomato fruit glucokinase showed highest affinity for glucose
(K
m = 65 μM). Activity observed with glucose was 4-fold greater than with mannose and 50-fold greater than with fructose. The tomato
fruit glucokinase was sensitive to product inhibition by ADP (K
i = 36 μM). Little inhibition was observed with glucose 6-phosphate (up to 15 mM) at pH 8.0; however, at pH 7.0 glucokinase
activity was inhibited 30–50% by physiological concentrations of glucose 6-phosphate.
Received: 4 October 1997 / Accepted: 10 January 1998 相似文献