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1.
A quantitative study of indole-3-acetic acid (IAA) turnover, and the contribution of tryptophan-dependent and tryptophan-independent
IAA-biosynthesis pathways, was carried out using protoplast preparations and shoot apices obtained from wild-type and transgenic,
IAA-overproducing tobacco (Nicotiana tabacum L.) plants, during a phase of growth when the level of endogenous IAA was stable. Based on the rate of disappearance of [13C6]IAA, the half-life of the IAA pool was calculated to be 1.1 h in wild-type protoplasts and 0.8 h in protoplasts from the
IAA-overproducing line, corresponding to metabolic rates of 59 and 160 pg IAA (μg Chl)−1 h−1, respectively. The rate of conversion of tryptophan to IAA was 15 pg IAA (μg Chl)−1 h−1 in wild-type protoplasts and 101 pg IAA (μg Chl)−1 h−1 in protoplasts from IAA-overproducing plants. In both instances, IAA was metabolised more rapidly than it was synthesised
from tryptophan. As the endogenous IAA pools were in a steady state, these findings indicate that IAA biosynthesis via the
tryptophan-independent pathway was 44 pg IAA (μg Chl)−1 h−1 and 59 pg IAA (μg Chl)−1 h−1, respectively, in the wild-type and transformed protoplast preparations. In a parallel study with apical shoot tissue, the
presumed site of IAA biosynthesis, the rate of tryptophan-dependent IAA biosynthesis exceeded the rate of metabolism of [13C6]IAA despite the steady state of the endogenous IAA pool. The most likely explanation for this anomaly is that, unlike the
protoplast system, injection of substrates into the apical tissues did not result in uniform distribution of label, and that
at least some of the [2H5]tryptophan was metabolised in compartments not normally active in IAA biosynthesis. This demonstrates the importance of using
experimental systems where labelling of the precursor pool can be strictly controlled.
Received: 18 January 2000 / Accepted 24 February 2000 相似文献
2.
Michel Edmond Ghanem Cristina Mart��nez-And��jar Alfonso Albacete Hana Posp��?ilov�� Ian C. Dodd Francisco P��rez-Alfocea Stanley Lutts 《Journal of Plant Growth Regulation》2011,30(2):144-157
Mixed nitrate/ammonium fertilization can partially alleviate the negative effects of salinity on growth of some plant species
compared to all-nitrate or all-ammonium fertilization. To gain insights about the mechanisms involved, tomato (Solanum lycopersicum L. cv Moneymaker) plants were grown hydroponically for 3 weeks with two NO3
−/NH4
+ fertilization regimes (6/0.5 and 5/1.5; Ntotal = 6.5 mM) in the absence (control) or presence of salt stress (100 mM NaCl). Ammonium enrichment had no effect on growth
and other parameters under control conditions. Under salinity, however, ammonium enrichment improved shoot and root biomass
by 20% and maintained leaf PSII efficiency close to control levels. These changes were related to higher leaf K+, NO3
−, and NH4
+ concentrations and activities of the N-assimilatory enzymes glutamate synthase (GOGAT) and glutamine synthase (GS) in the
leaves. Ammonium enrichment also attenuated the salt-induced increase in leaf abscisic acid (ABA) concentration and decrease
in leaf concentrations of indole 3-acetic acid (IAA) and the cytokinins trans-zeatin (tZ) and trans-zeatin riboside (tZR). Enhanced cytokinin status was probably due to maintenance of root-to-shoot cytokinin transport and decreased leaf induction
of the cytokinin-degrading enzyme cytokinin oxidase/dehydrogenase (CKX) under ammonium-enriched conditions. It is concluded
that nitrogen form modifies salinity-induced physiological responses and that these modifications are associated with changes
in plant hormone status. 相似文献
3.
Analytical electron microscopical investigations on the apoplastic pathways of lanthanum transport in barley roots 总被引:5,自引:0,他引:5
In transmission electron microscopy studies, lanthanum ions have been used as electron-opaque tracers to delineate the apoplastic
pathways for ion transport in barley (Hordeum vulgare L.) roots. To localize La3+ on the subcellular level, e.g. in cell walls and on the surface of membranes, electron-energy-loss spectroscopy and electron-spectroscopic
imaging were used. Seminal and nodal roots were exposed for 30 min to 1 mM LaCl3 and 10 mM LaCl3, respectively. In seminal roots, possessing no exodermis, La3+ diffusion through the apoplast was stopped by the Casparian bands of the endodermis. In nodal roots with an exodermis, however,
La3+ diffusion through the cortical apoplast had already stopped at the tight junctions of the exodermal cell walls resembling
the Casparian bands of the endodermis. Therefore, we conclude that in some specialized roots such as the nodal roots of barley,
the physiological role of the endodermis is largely performed by the exodermis.
Received: 28 July 1999 / Accepted: 24 February 2000 相似文献
4.
The tryptophan auxotroph mutant trp3-1 of Arabidopsis thaliana (L.) Heynh., despite having reduced levels of l-tryptophan, accumulates the tryptophan-derived glucosinolate, glucobrassicin and, thus, does not appear to be tryptophan-limited.
However, due to the block in tryptophan synthase, the mutant hyperaccumulates the precursor indole-3-glycerophosphate (up
to 10 mg per g FW). Instability of indole-3-glycerophosphate leads to release of indole-3-acetic acid (IAA) from this metabolite
during standard workup of samples for determination of conjugated IAA. The apparent increase in “conjugated IAA” in trp3-1 mutant plants can be traced back entirely to indole-3-glycerophosphate degradation. Thus, the levels of neither free IAA
nor conjugated IAA increase detectably in the trp3-1 mutant compared to wild-type plants. Precursor-feeding experiments to shoots of sterile-grown wild-type plants using [2H]5-l-tryptophan have shown incorporation of label from this precursor into indole-3-acetonitrile and indole-3-acetic acid with
very little isotope dilution. It is concluded that Arabidopsis thaliana shoots synthesize IAA from l-tryptophan and that the non-tryptophan pathway is probably an artifact.
Received: 1 March 2000 / Accepted: 10 April 2000 相似文献
5.
To test the hypothesis that the contribution of phosphoribulokinase (PRK) to the control of photosynthesis changes depending
on the light environment of the plant, the response of transgenic tobacco (Nicotiana tabacum L.) transformed with antisense PRK constructs to irradiance was determined. In plants grown under low irradiance (330 μmol m−2 s−1) steady-state photosynthesis was limited in plants with decreased PRK activity upon exposure to higher irradiance, with a
control coefficient of PRK for CO2 assimilation of 0.25 at and above 800 μmol m−2 s−1. The flux control coefficient of PRK for steady-state CO2 assimilation was zero, however, at all irradiances in plant material grown at 800 μmol m−2 s−1 and in plants grown in a glasshouse during mid-summer (alternating shade and sun 300–1600 μmol m−2 s−1). To explain these differences between plants grown under low and high irradiances, Calvin cycle enzyme activities and metabolite
content were determined. Activities of PRK and other non-equilibrium Calvin cycle enzymes fructose-1,6-bisphosphatase, sedoheptulose-1,7-bisphosphatase
and ribulose-1,5-bisphosphate carboxylase-oxygenase were twofold higher in plants grown at 800 μmol m−2 s−1 or in the glasshouse than in plants grown at 330 μmol m−2 s−1. Activities of equilibrium enzymes transketolase, aldolase, ribulose-5-phosphate epimerase and isomerase were very similar
under all growth irradiances. The flux control coefficient of 0.25 in plants grown at 330 μmol m−2 s−1 can be explained because low ribulose-5-phosphate content in combination with low PRK activity limits the synthesis of ribulose-1,5-bisphosphate.
This limitation is overcome in high-light-grown plants because of the large relative increase in activities of sedoheptulose-1,7-bisphosphatase
and fructose-1,6-bisphosphatase under these conditions, which facilitates the synthesis of larger amounts of ribulose-5-phosphate.
This potential limitation will have maintained evolutionary selection pressure for high concentrations of PRK within the chloroplast.
Received: 15 November 1999 / Accepted: 27 January 2000 相似文献
6.
High-efficiency induction of soybean hairy roots and propagation of the soybean cyst nematode 总被引:8,自引:0,他引:8
Cotyledon explants of 10 soybean [Glycine max (L.) Merr.] cultivars were inoculated with Agrobacterium rhizogenes strain K599 with and without binary vectors pBI121 or pBINm-gfp5-ER possessing both neomycin phosphotransferase II (nptII) and β-glucuronidase (gus) or nptII and green fluorescent protein (gfp) genes, respectively. Hairy roots were produced from the wounded surface of 54–95% of the cotyledon explants on MXB selective
medium containing 200 μg ml−1 kanamycin and 500 μg ml−1 carbenicillin. Putative individual transformed hairy roots were identified by cucumopine analysis and were screened for transgene
incorporation using polymerase chain reaction. All of the roots tested were found to be co-transformed with T-DNA from the
Ri-plasmid and the transgene from the binary vectors. Southern blot analysis confirmed the presence of the 35S-gfp5 gene in the plant genomes. Transgene expression was also confirmed by histochemical GUS assay and Western blot analysis for
the GFP. Attempts to induce shoot formation from the hairy roots failed. Infection of hairy roots of the soybean cyst nematode
(Heterodera glycines Ichinohe)-susceptible cultivar, Williams 82, with eggs of H. glycines race 1, resulted in the development of mature cysts about 4–5 weeks after inoculation. Thus the soybean cyst nematode could
complete its entire life cycle in transformed soybean hairy-root cultures expressing GFP. This system should be ideal for
testing genes that might impart resistance to soybean cyst nematode.
Received: 13 July 1999 / Accepted: 8 August 1999 相似文献
7.
Batch cultures of photoautotrophic cell suspensions of Chenopodiumrubrum L., growing in an inorganic medium on CO2 under a daily balanced light–dark regime of 16 : 8 h could be maintained for approximately 100 d without subcultivation.
The long-lived cultures showed an initial cell division phase of 4 weeks, followed by a stationary phase of another 4 weeks,
after which ageing and progressive cell death reduced the number of living cells and the cultures usually expired after another
3–4 weeks. These developmental phases of the cell culture were characterised with respect to photosynthetic performance, dark
respiration, content of phytohormones and capacity of cell division. Cell division of the majority of the cells finished in
the G1- or G0-phase of the cell cycle, caused by a pronounced decline in the endogenous levels of auxin and cytokinins. Supply
of these growth factors to resting cells resulted in resumption of cytokinesis, at least by some of the cells. However, responsiveness
to the phytohomones declined during the stationary phase, and subcultivation was no longer possible beyond day 60 when the
phases of ageing and death commenced. Ageing was characterised by a further decline in the photosynthetic capacity of the
cells, by a climacteric enhancement of dark respiration, but also by a slight increase in the level of IAA and cytokinins
concomitant with a decrease in ethylene. Similarities and differences between the development of batch-cultured photoautotrophic
cells of C. rubrum and that of a leaf are discussed with respect to using the cell culture as a model for a leaf.
Received: 30 April 1999 / Accepted: 21 August 1999 相似文献
8.
Abscisic acid and hydraulic conductivity of maize roots: a study using cell- and root-pressure probes 总被引:31,自引:0,他引:31
Using root- and cell-pressure probes, the effects of the stress hormone abscisic acid (ABA) on the water-transport properties
of maize roots (Zea mays L.) were examined in order to work out dose and time responses for root hydraulic conductivity. Abscisic acid applied at
concentrations of 100–1,000 nM increased the hydraulic conductivity of excised maize roots both at the organ (root Lpr: factor of 3–4) and the root cell level (cell Lp: factor of 7–27). Effects on the root cortical cells were more pronounced
than at the organ level. From the results it was concluded that ABA acts at the plasmalemma, presumably by an interaction
with water channels. Abscisic acid therefore facilitated the cell-to-cell component of transport of water across the root
cylinder. Effects on cell Lp were transient and highly specific for the undissociated (+)-cis-trans-ABA. The stress hormone ABA facilitates water uptake into roots as soils start drying, especially under non-transpiring conditions,
when the apoplastic path of water transport is largely excluded.
Received: 26 February 2000 / Accepted: 17 August 2000 相似文献
9.
Isolation and characterization of nitrite-reductase-deficient mutants of Chlorella sorokiniana (strain 211-8k) 总被引:1,自引:0,他引:1
A method is presented to isolate mutants of Chlorella sorokiniana with defects in NO3
− metabolism. Three nitrite-reductase (NIR; E.C.1.7.7.1)-deficient mutants were obtained from 500 pinpoint-colony-forming clones.
The final screening was performed using NO3
−, NO2
− or NH+
4 as N-source. The mutants isolated absorb NO3
− with rates close to those measured for the wild type and they excrete NO2
− into the medium. The ratio between NO3
− uptake and NO2
− excretion was 1:1. The sensitivity of NO3
− uptake to NH+
4 was reduced in the mutant strains as it was in the N-starved wild type of Chlorella. Nitrate reductase (NR; EC 1.6.6.1) expression and NR activity were slightly reduced compared to the wild type due to feedback
regulation in the mutant strains. No NIR protein was found in the three mutants. However, NIR activity was obtained (50% of
the wild-type) for one mutant strain. The NIR-deficient mutants and the already available NR-deficient mutants will be promising
tools for investigations of the nitrate assimilation pathway on the molecular level and for studies searching for signaling
of C and N metabolism by inorganic N-compounds.
Received: 8 October 1999 / Accepted: 25 January 2000 相似文献
10.
Somatic embryogenesis induced by the simple application of abscisic acid to carrot (Daucus carota L.) seedlings in culture 总被引:3,自引:0,他引:3
Seedlings of carrot (Daucus carota L. cv. Red Cored Chantenay) formed somatic embryos when cultured on medium containing abscisic acid (ABA) as the sole source
of growth regulator. The number of embryos per number of seedlings changed depending on the concentration of ABA added to
the medium, with a maximum embryo number at 1 × 10−4 M ABA. Seedling age was critical for response to exogenous ABA; no seedling with a hypocotyl longer than 3.0 cm was able to
form an embryo. Removal of shoot apices from seedlings completely inhibited the embryogenesis induced by application of exogenous
ABA, suggesting that the action of ABA requires some substance(s) that is translocated basipetally from shoot apices through
hypocotyls. Histologically, somatic embryos shared common epidermal cells and differentiated not through the formation of
embryogenic cell clumps, but directly from epidermal cells. These morphological traits are distinct from those of embryogenesis
via formation of embryogenic cell clumps, which has been found in embryogenic carrot cultures established using 2,4-dichlorophenoxyacetic
acid or other auxins. These results suggest that ABA acts as a signal substance in stress-induced carrot seedling somatic
embryogenesis.
Received: 22 April 2000 / Accepted: 8 June 2000 相似文献
11.
Fast track to the trichome: induction of N-acyl nornicotines precedes nicotine induction in Nicotiana repanda 总被引:3,自引:0,他引:3
Nicotiana repanda Wildenow ex Lehmann acylates nornicotine in its trichomes to produce N-acyl-nornicotine (NacNN) alkaloids which are dramatically
more toxic than nicotine is to the nicotine-adapted herbivore, Manduca sexta. These NacNNs, like nicotine, were induced by methyl jasmonate (MeJA) and wounding, but the 2-fold increase in NacNN pools
was much faster (within 6 h) than the MeJA-induced increase in nornicotine pools (24 h to 4 d), its parent substrate. When
15NO3
− pulse-chase experiments with intact and induced plants were used to follow the incorporation of 15N into alkaloids in different plant parts over the plant's lifetime, it was found that the root nicotine pool was most rapidly
labeled, followed by the shoot nornicotine and NacNN pools. After 3 d, 3.12% of 15N acquired was in nicotine (0.93%), nornicotine (0.32%) and NacNNs (1.73%) while only 0.14% was in anabasine. Once NacNNs
are externalized to the leaf surface, they are not readily re-distributed within the plant and are lost with senescing leaves.
The wound- and MeJA-induced N-acylation of nornicotine is independent of induced changes in nornicotine pools and the rapidity
of the response suggests its importance in defense against herbivores.
Received: 3 July 1999 / Accepted: 17 September 1999 相似文献
12.
Summary. The aim of the present study was to measure MPO activity in PMN leukocytes after endotoxin administration, and to compare
the levels of NO2
− competing with taurine for reaction with HOCl. Furthermore we aimed to determine TauCl levels, a product of MPO–H2O2–Halide system, and to evaluate anti-inflammatory properties of PMN in endotoxemia. In addition, our second objective was
to investigate the effect of taurine, an antioxidant amino acid, on anti-bactericidal and anti-inflammatory functions of PMN
after administration of endotoxin together with taurine.
All experiments were performed with four groups (control, taurine, endotoxemia, and taurine plus endotoxin) of ten guinea
pigs. After endotoxin administration (4 mg/kg), MPO activities increased and taurine levels decreased. Therefore levels of
TauCl, NO2
•− increased. We observed the effects of taurine as conflicting. When taurine was administrated alone (300 mg/kg), all of these
parameters decreased.
Consequently, we suggested that taurine is influential in infected subjects but not on healthy ones as an antioxidative amino
acid. In addition, we believe that in vivo effects of taurine may differ from those in vitro depending on its dosage. 相似文献
13.
Infiltrating detached maize (Zeamays L.) leaves with L-galactono-1,4-lactone (L-GAL) resulted in a 4-fold increase in the content of leaf ascorbate. Upon exposure to high irradiance (1000 μmol photons m−2 s−1) at 5 °C, L-GAL leaves de-epoxidized the xanthophyll-cycle pigments faster than the control leaves; the maximal ratio of de-epoxidized
xanthophyll-cycle pigments to the whole xanthophyll-cycle pool was the same in both leaf types. The elevated ascorbate content,
together with the faster violaxanthin de-epoxidation, did not affect the degree of photoinhibition and the kinetics of the
recovery from photoinhibition, assayed by monitoring the maximum quantum efficiency of photosystem II primary photochemistry
(Fv/Fm). Under the experimental conditions, the thermal energy dissipation seems to be zeaxanthin-independent since, in contrast
to the de-epoxidation, the decrease in the efficiency of excitation-energy capture by open photosystem II reaction centers (Fv′/Fm′) during the high-irradiance treatment at low temperature showed the same kinetic in both leaf types. This was also observed
for the recovery of the maximal fluorescence after stress. Furthermore, the elevated ascorbate content did not diminish the
degradation of pigments or α-tocopherol when leaves were exposed for up to 24 h to high irradiance at low temperature. Moreover,
a higher content of ascorbate appeared to increase the requirement for reduced glutathione.
Received: 20 May 1999 / Accepted: 29 October 1999 相似文献
14.
Regulation by irradiance level of the mechanism for dissolved inorganic carbon (DIC) acquisition was examined in the red
macroalga Gracilaria tenuistipitata Zhang et Xia. For this purpose, affinity for external DIC, carbonic anhydrase (CA; EC 4.2.1.1) activity and content of ribulose-1,5-bisphosphate
carboxylase/oxygenase (Rubisco; EC 4.1.1.39) were determined in thalli grown at 45 and 500 μmol photons m−2 s−1. Oxygen evolution rates declined by 50% when the medium pH was changed from 8.1 to 8.7, and the pH compensation point attained
was ca. 9.2. These characteristics were unaffected by the light treatments. In contrast, photosynthetic conductance for DIC
at pH 8.7 was doubled in thalli grown at high irradiance compared with those grown at low irradiance (to 0.74 × 10−6 from 0.33 × 10−6 m s−1). Photosynthetic rates at saturating DIC concentration were also higher by 60% in thalli grown at high irradiance. These
differences could not be attributed to changes in the use of external DIC, since external CA activity did not vary. Although
the irradiance level did not modify the pool size of Rubisco, Rubisco content expressed on a chlorophyll a basis was almost doubled at high irradiance. These results likely indicate that the internal transport of DIC towards the
active-site of Rubisco, rather than the external use of DIC, is enhanced in the thalli grown at high irradiance.
Received: 7 June 1999 / Accepted: 16 October 1999 相似文献
15.
Summary. Glutathione (reduced form GSH and oxidized form GSSG) constitutes an important defense against oxidative stress in the brain,
and taurine is an inhibitory neuromodulator particularly in the developing brain. The effects of GSH and GSSG and glycylglycine,
γ-glutamylcysteine, cysteinylglycine, glycine and cysteine on the release of [3H]taurine evoked by K+-depolarization or the ionotropic glutamate receptor agonists glutamate, kainate, 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate
(AMPA) and N-methyl-D-aspartate (NMDA) were now studied in slices from the hippocampi from 7-day-old mouse pups in a perfusion system.
All stimulatory agents (50 mM K+, 1 mM glutamate, 0.1 mM kainate, 0.1 mM AMPA and 0.1 mM NMDA) evoked taurine release in a receptor-mediated manner. Both
GSH and GSSG significantly inhibited the release evoked by 50 mM K+. The release induced by AMPA and glutamate was also inhibited, while the kainate-evoked release was significantly activated
by both GSH and GSSG. The NMDA-evoked release proved the most sensitive to modulation: L-Cysteine and glycine enhanced the
release in a concentration-dependent manner, whereas GSH and GSSG were inhibitory at low (0.1 mM) but not at higher (1 or
10 mM) concentrations. The release evoked by 0.1 mM AMPA was inhibited by γ-glutamylcysteine and cysteinylglycine, whereas
glycylglycine had no effect. The 0.1 mM NMDA-evoked release was inhibited by glycylglycine and γ-glutamylcysteine. In turn,
cysteinylglycine inhibited the NMDA-evoked release at 0.1 mM, but was inactive at 1 mM. Glutathione exhibited both enhancing
and attenuating effects on taurine release, depending on the glutathione concentration and on the agonist used. Both glutathione
and taurine act as endogenous neuroprotective effectors during early postnatal life.
Authors’ address: Prof. Simo S. Oja, Brain Research Center, Medical School, FI-33014 University of Tampere, Finland 相似文献
16.
Apoplastic transport of abscisic acid through roots of maize: effect of the exodermis 总被引:22,自引:0,他引:22
The exodermal layers that are formed in maize roots during aeroponic culture were investigated with respect to the radial
transport of cis-abscisic acid (ABA). The decrease in root hydraulic conductivity (Lpr) of aeroponically grown roots was stimulated 1.5-fold by ABA (500 nM), reaching Lpr values of roots lacking an exodermis. Similar to water, the radial flow of ABA through roots (JABA) and ABA uptake into root tissue were reduced by a factor of about three as a result of the existence of an exodermis. Thus,
due to the cooperation between water and solute transport the development of the ABA signal in the xylem was not affected.
This resulted in unchanged reflection coeffcients for roots grown hydroponically and aeroponically. Despite the well-accepted
barrier properties of exodermal layers, it is concluded that the endodermis was the more effective filter for ABA. Owing to
concentration polarisation effects, ABA may accumulate in front of the endodermal layer, a process which, for both roots possessing
and lacking an exodermis, would tend to increase solvent drag and hence ABA movement into the xylem sap at increased water
flow (JVr). This may account for the higher ABA concentrations found in the xylem at greater pressure difference.
Received: 26 January 1999 / Accepted: 26 May 1999 相似文献
17.
Inactivation of DNA replication origins by the cell cycle regulator, trigonelline, in root meristems of Lactuca sativa 总被引:4,自引:0,他引:4
The effects of trigonelline (TRG) on the cell cycle in root meristems of Lactuca sativa L. were examined in the knowledge that TRG is a cell cycle regulator that causes cell arrest in G2, and prevents ligation
of replicons in S-phase. The hypothesis was tested that continuous exposure to TRG would perturb DNA replication which, in
turn, would lengthen the cell cycle and impair root elongation. Using DNA fibre autoradiography, mean replicon size was 31
and 13 μm in the TRG (3 mM) and control treatments, respectively. Trigonelline also resulted in a lengthening of both S-phase
and the cell cycle and a decrease in primary root elongation. Hence, replicon inactivation was responsible for the protracted
S-phase. Trigonelline treatment also resulted in a 1.6-fold increase in fork rate (13.8 μm h−1) compared with the control (8.4 m h−1). The faster fork rate in the larger replicons is in accord with the highly significant positive relationship already established
between fork rate and replicon size for various unrelated higher plants.
Received: 11 October 1999 / Accepted: 23 December 1999 相似文献
18.
Maize (Zea mays L.) cell cultures incorporated radioactivity from [14C]cinnamate into hydroxycinnamoyl-CoA derivatives and then into polysaccharide-bound feruloyl residues. Within 5–20 min, the
CoA pool had lost its 14C by turnover and little or no further incorporation into polysaccharides then occurred. The system was thus effectively a
pulse–chase experiment. Kinetics of radiolabelling of diferulates (also known as dehydrodiferulates) varied with culture age.
In young (1–3 d) cultures, polysaccharide-bound [14C]feruloyl- and [14C]diferuloyl residues were both detectable within 1 min of [14C]cinnamate feeding. Thus, feruloyl residues were dimerised <1 min after their attachment to polysaccharides. For at least
the first 2.3 h after [14C]cinnamate feeding, polysaccharide-bound [14C]diferuloyl residues remained almost constant at ≈7% of the total polysaccharide-bound [14C]ferulate derivatives. Since feruloyl residues are attached to polysaccharides <1 min after the biosynthesis of the latter,
and >10 min before secretion, the data show that extensive feruloyl coupling occurred intra-protoplasmically. Exogenous H2O2 (1 mM) caused little additional feruloyl coupling; therefore, wall-localised coupling may have been peroxidase-limited. In
older (e.g. 4 d) cultures, less intraprotoplasmic coupling occurred: during the first 2.5 h, polysaccharide-bound [14C]diferuloyl residues were a steady 1.4% of the total polysaccharide-bound [14C]ferulate derivatives. In contrast to the situation in younger cultures, exogenous H2O2 induced a rapid 4- to 6-fold increase in all coupling products, indicating that coupling in the walls was H2O2-limited. In both 2- and 4-d-old cultures, polysaccharide-bound 14C-trimers and larger coupling products exceeded [14C]diferulates 3- to 4-fold, but followed similar kinetics. Thus, although all known dimers of ferulate can now be individually
quantified, it appears to be trimers and larger products that make the major contribution to cross-linking of wall polysaccharides
in cultured maize cells. We argue that feruloyl arabinoxylans that are cross-linked before and after secretion are likely
to loosen and tighten the cell wall, respectively. The consequences for the control of cell expansion and for the response
of cell walls to an oxidative burst are discussed.
Received: 19 January 2000 / Accepted: 13 April 2000 相似文献
19.
Atomic force microscopy (AFM) enables the topographical structure of cells and biological materials to be resolved under
natural (physiological) conditions, without fixation and dehydration artefacts associated with imaging methods in vacuo. It
also provides a means of measuring interaction forces and the mechanical properties of biomaterials. In the present study,
AFM has been applied for the first time to the study of the mechanical properties of a natural adhesive produced by a green
plant cell. Swimming spores of the green alga Enteromorpha linza (L.) J. Ag. (7–10 μm) secrete an adhesive glycoprotein which provides firm anchorage to the substratum. Imaging of the adhesive in its
hydrated state revealed a swollen gel-like pad, approximately 1 μm thick, surrounding the spore body. Force measurements revealed
that freshly released adhesive has an adhesion strength of 173 ± 1.7 mN m−1 (mean ± SE; n=90) with a maximum value for a single adhesion force curve of 458 mN m−1. The adhesive had a compressibility (equivalent to Young's modulus) of 0.54 × 106 ± 0.05 × 106 N m−2 (mean ± SE; n=30). Within minutes of release the adhesive underwent a progressive `curing' process with a 65% reduction in mean adhesive
strength within an hour of settlement, which was also reflected in a reduction in the average length of the adhesive polymer
strands (polymer extension) and a 10-fold increase in Young's modulus. Measurements on the spore surface itself revealed considerably
lower adhesion-strength values but higher polymer-extension values than the adhesive pad, which may reflect the deposition
of different polymers on this surface as a new cell wall is formed. The study demonstrates the value of AFM to the imaging
of plant cells in the absence of fixation and dehydration artefacts and to the characterisation of the mechanical properties
of plant glycoproteins that have potential utility as adhesives.
Received: 22 February 2000 / Accepted: 20 April 2000 相似文献
20.
Alena Gaudinová Petre I. Dobrev Blanka Šolcová Ondřej Novák Miroslav Strnad David Friedecký Václav Motyka 《Journal of Plant Growth Regulation》2005,24(3):188-200
Cytokinin metabolism in plants is very complex. More than 20 cytokinins bearing isoprenoid and aromatic side chains were identified
by high performance liquid chromatography-mass spectrometry (HPLC-MS) in pea (Pisum sativum L. cv. Gotik) leaves, indicating diverse metabolic conversions of primary products of cytokinin biosynthesis. To determine
the potential involvement of two enzymes metabolizing cytokinins, cytokinin oxidase/dehydrogenase (CKX, EC 1.5.99.12) and
zeatin reductase (ZRED, EC 1.3.1.69), in the control of endogenous cytokinin levels, their in vitro activities were investigated in relation to the uptake and metabolism of [2−3H]trans-zeatin ([2−3H]Z) in shoot explants of pea. Trans-zeatin 9-riboside, trans-zeatin 9-riboside-5′-monophosphate and cytokinin degradation products adenine and adenosine were detected as predominant
[2−3H]Z metabolites during 2, 5, 8, and 24 h incubation. Increasing formation of adenine and adenosine indicated extensive degradation
of [2−3H]Z by CKX. High CKX activity was confirmed in protein preparations from pea leaves, stems, and roots by in vitro assays. Inhibition of CKX by dithiothreitol (15 mM) in the enzyme assays revealed relatively high activity of ZRED catalyzing
conversion of Z to dihydrozeatin (DHZ) and evidently competing for the same substrate cytokinin (Z) in protein preparations
from pea leaves, but not from pea roots and stems. The conversion of Z to DHZ by pea leaf enzyme was NADPH dependent and was
significantly inhibited or completely suppressed in vitro by diethyldithiocarbamic acid (DIECA; 10 mM). Relations of CKX and ZRED in the control of cytokinin levels in pea leaves
with respect to their potential role in establishment and maintenance of cytokinin homeostasis in plants are discussed. 相似文献