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1.
Hieracium is a member of the Asteraceae family, and contains sexual species in addition to apomictic species that reproduce by apospory
and produce seed without fertilization. A homologue of the floral organ-identity gene DEFICIENS (DEF) was isolated from an apomictic line of Hieracium piloselloides (Vill.) following differential display between mature ovules and those initiating autonomous embryogenesis. The gene termed
HPDEF has 93% amino acid identity with GDEF2, a DEF homologue isolated from Gerbera hybrida (D. Yu et al., 1999, Plant J. 17: 51–62), another member of the Asteraceae. In-situ analysis showed that early in floral
development HPDEF is expressed in stamen and petal primordia, indicating expected B-function activity, according to the ABC model of floral
organ identity (J. L. Bowman et al., 1991, Development 112: 1–20; E. S. Coen and E. M. Meyerowitz, 1991, Nature 353: 31–37).
However, HPDEF expression was also observed in ovule primordia and expression continued in developing ovules until anthesis, indicating
that this gene may have a role in ovule development. Expression of HPDEF was not detected in megaspore mother cells, or in sexual or aposporous embryo sacs. In sexual Hieracium, HPDEF was uniformly expressed throughout the ovule integument until anthesis. In most ovules of the apomict, however, HPDEF expression was transiently down-regulated in a specific zone in the chalazal region where cells initiating aposporous embryo
sac formation differentiate. Uniform low-level HPDEF expression was subsequently observed prior to anthesis in ovules from sexual and apomictic plants. HPDEF may be down-regulated as a consequence of apomictic initiation and/or its down-regulation may facilitate progression of apomictic
events.
Received: 15 September 1999 / Accepted: 12 October 1999 相似文献
2.
The actin cytoskeletal organization and nuclear behavior of normal and indeterminate gametophyte1 (ig1) embryo sacs of maize were examined during fertilization. After pollination, during degeneration of one of the synergids
and before arrival of the pollen tube, the cytoskeletal elements undergo dramatic changes including formation of the actin
coronas at the chalazal end of the degenerating synergid and at the interface between the egg cell and central cell. The actin
coronas are present only for a limited period of time and their presence is coordinated with pollen tube arrival and fusion
of the gametes; they disappear before the zygote divides. This allows us to estimate the frequency of fertilized ovules along
the ear. Up to 88% of the ovules on an ear contain actin coronas in the embryo sacs when observed 16–19 h after pollination,
indicating the high frequency of fertilizing kernels along the ear at this stage. In the ig embryo sacs, two or more degenerated synergids containing actin coronas at their chalazal ends receive multiple pollen tubes
for gametic fusion and can consequently give rise to twin or polyembryos. These findings with the monocot maize are consistent
with previous reports on the dicots Plumbago and Nicotiana, suggesting that the formation of actin coronas in the embryo sac during fertilization is a universal phenomenon in angiosperms
and is part of a mechanism of interaction between gametic signaling and actin cytoskeleton behavior which appears to precisely
position and facilitate the access of male gametes to the egg cell and central cell for fusion.
Received: 15 May 1998 / Revision accepted: 17 August 1998 相似文献
3.
Various membrane-impermeable, water-soluble fluorescent tracers with different molecular weights were microinjected into
the central cell of the embryo sac of Torenia fournieri Lind. before and during fertilization. Before anthesis, there was high symplastic permeability between the central cell and
the egg apparatus cells. In this stage, fluorescent tracers up to 10 kDa could pass from the central cell into the egg apparatus
cells, whereas those with larger molecular weight remained in the central cell. As the embryo sac matured, symplastic permeability
decreased such that 2 d after anthesis only tracers less than 3 kDa could spread from the central cell into the egg cell.
There appeared to be no symplastic permeability between the primary endosperm and zygote after fertilization, since tracers
as small as 521 Da could not pass into the zygote in about half of the microinjected embryo sacs. This is the first report
of a change in cell-to-cell communication among the cells of the female germ unit before and after fertilization.
Received: 16 December 1999 / Accepted: 4 February 2000 相似文献
4.
Induced single fertilization in maize 总被引:1,自引:1,他引:0
Akio Kato 《Sexual plant reproduction》1997,10(2):96-100
Bicellular pollen with one vegetative nucleus and one diploid arrested generative cell (”monospermic” pollen) was induced
by trifluralin treatment of diploid maize plants at 7–9 days before flowering. The arrested generative cell (seemingly a diploid
sperm cell) fused with the central cell of diploid plants and produced shriveled endosperm resembling that of a 2n×4n cross
in maize. Dual pollination experiments with a purple embryo marker revealed single fertilization events in which the union
of one sperm cell with the egg occurs but there is no union of a second sperm cell with the central cell. Singly fertilized
ovules survived at least 4 days. Furthermore, many viable triploid plants were obtained. This technique therefore appears
to have the potential for manipulating ploidy level in crops and may become useful in investigating fertilization mechanisms
of angiosperms.
Received: 1 October 1996 / Revision accepted: 8 January 1997 相似文献
5.
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
6.
Cytological details of endosperm development after pollination with irradiated pollen were studied in Actinidia deliciosa (kiwifruit) cultivar Hayward. Pollinations were carried out involving five different sources of pollen (Matua, Tomuri, Burt,
Berryman, and fruiting male) irradiated with gamma rays at doses of 700 and 900 Gy. Non-irradiated crosses were used as controls.
Irradiated pollen induced development of approximately 25–30% of the ovules. Two types of ovules were observed: (1) with both
embryo and endosperm and (2) with endosperm only. No mitotic abnormalities were found in control or irradiated endosperms.
Mitotic divisions were regular and nuclei spherical and evenly spaced. However, the cells of irradiated endosperm usually
contained low amounts of storage products. Ploidy level of the endosperms was evaluated by nuclear size (volume) with the
use of image analyzis. Mean nuclear size in control and irradiated endosperms was 1598.3 and 750.9 μm3, respectively. It is concluded that endosperm produced after pollination with irradiated pollen is autonomous and represents
the 2n level.
Received: 14 October 1998 / Revision accepted: 10 March 1999 相似文献
7.
Haploid induction in onion can, to date, be induced only via gynogenesis by culturing unfertilized flowers, ovaries or ovules.
The process of haploid embryo induction has been macroscopically well studied, but only limited data exist from microscopic
examination of ovule development status at the inoculation stage and of the origin of gynogenic embryos. Microscopic studies
were carried out using individual donor plants with relatively high embryo induction frequencies (45.9 embryos formed per
100 flowers, on average, for 2 years). Ovaries from flower bud culture were fixed at 1 week intervals up to the 7th week of
culture. These were compared with pollinated ovaries at 1 or 2 weeks after pollination. In total, 1428 unfertilized embryo
sacs were examined. The results indicate that, at the time of inoculation, ovules within ovaries 2.0–3.0 mm in diameter contained
two- or four-nucleate embryo sacs in the smallest ovaries to mature embryo sacs in the largest ovaries. It seems likely that
the embryos are actually induced from ovaries cultured at the immature stage. After 1 or 2 weeks in culture, the egg apparatus
primarily consisted of distinctly enlarged synergids and the egg cell, which was often detached from the micropylar pole.
But free nuclear endosperm was also formed. From the 2nd to 7th week in culture, formation of haploid embryos (from globular
to the almost mature cylindrical stage) was detected in 5.7% of the ovules. Their origin, for several reasons, was most likely
the egg cell. In addition, ovules containing endosperm only (3.6%) and ovules containing the egg apparatus (0.5%) or both
endosperm and embryo (0.4%) were detected. This observation is probably unique and has not yet been reported in other species
studied.
Received: February 2001 / Revision accepted: 20 April 2001 相似文献
8.
Microtubule organization plays an important role in plant morphogenesis; however, little is known about how microtubule arrays
transit from one organized state to another. The use of a genetically incorporated fluorescent marker would allow long-term
observation of microtubule behavior in living cells. Here, we have characterized a Nicotiana tabacum L. cv. Bright Yellow 2 (BY-2) cell line that had been stably transformed with a gfp-mbd construct previously demonstrated to label microtubules (J. Marc et al., 1998, Plant Cell 10: 1927–1939). Fluorescence levels
were low, but interphase and mitotic microtubule arrays, as well as the transitions between these arrays, could be observed
in individual gfp-mbd-transformed cells. By comparing several attributes of transformed and untransformed cells it was concluded that the transgenic
cells are not adversely affected by low-level expression of the transgene and that these cells will serve as a useful and
accurate model system for observing microtubule reorganization in vivo. Indeed, some initial observations were made that are
consistent with the involvement of motor proteins in the transition between the spindle and phragmoplast arrays. Our observations
also support the role of the perinuclear region in nucleating microtubules at the end of cell division with a progressive
shift of these microtubules and/or nucleating activity to the cortex to form the interphase cortical array.
Received: 2 June 1999 / Accepted: 13 August 1999 相似文献
9.
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
10.
Different pH-dependences of K+ channel activity in bundle sheath and mesophyll cells of maize leaves
The isolation of bundle sheath protoplasts from leaves of Zea mays L. for patch clamp whole-cell experiments presents special problems caused by the suberin layer surrounding these cells.
These problems were overcome by the isolation technique described here. Two different types of whole-cell response were found:
a small response caused by MB-1 (maize bundle sheath conductance type 1) which was instantaneously activated, and another
caused by MB-2 (maize bundle sheath conductance type 2) consisting of an instantaneous response (maize bundle sheath K+ instantaneous current type 2; MB-KI2) similar to but stronger than the current through MB-1 plus a small time-dependent outward
rectifying component (maize bundle sheath activated outward rectifying current; MB-AOR) with voltage-dependent delayed activation.
The occurrence of MB-AOR was often accompanied by a smaller contribution from an inward rectifying channel at negative potentials.
Activation of MB-2 required ATP. It is suggested that MB-1 and MB-2 are related to bundle sheath cells with and without direct
contact with the xylem vessels. In mesophyll cells, only one type of response caused by MM-2 (maize mesophyll conductance
type 2) was found with an instantaneous (maize mesophyll K+ instantaneous current type 2, MM-KI2) and a voltage-dependent delayed component (maize mesophyll activated outward rectifying
current, MM-AOR). The most striking difference between bundle sheath and mesophyll cells was the pH dependence of K+ uptake. At pH 7.2, uptake of K+ by MB-2 was identical to that by MM-2 over the whole voltage range. However, acidification stimulated K+ conductance in bundle sheath cells, whereas a decrease was found for MM-2. At pH 6.15, the bundle sheath channel MB-2 had
more than a 10-fold higher K+ uptake at positive and negative potentials than MM-2. The channel MB-1, too, was stimulated by low pH. This seems to indicate
a putative role for MB-1 and MB-2 in charge balance during uptake of nutrients via cotransport from the xylem into the symplasm.
Received: 23 April 1999 / Accepted: 19 July 1999 相似文献
11.
Summary. A randomised, double blind, placebo-controlled study was performed giving 0.5 g · kg−1 · day−1 of undiluted alanyl-glutamine (20%) or saline in a peripheral vein during 4 hours in ICU patients (n = 20). During the infusion
period a steady state in plasma concentration was reached for alanyl-glutamine, but not for alanine, glutamine or glutamate.
On the other hand there was no accumulation of any of the amino acids, as the pre-infusion concentrations were reached within
8 hours after the end of infusion. The half-life of the dipeptide was 0.26 hours (range, 0.15–0.63 h). The distribution volume
of alanyl-glutamine was larger than the extracellular water volume, indicating a rapid hydrolysis of the dipeptide. There
was no detectable alanyl-glutamine in the urine of any of the patients. All patients had excretion of small amounts of amino
acids in urine, but the renal clearance of alanine, glutamine and glutamate were not different between the two groups. 相似文献
12.
Autonomous endosperm was found in unfertilized ovules of V. odorata L. cultured on MS medium supplemented with 2,4-D as a sole growth regulator or on media with 2,4-D and BAP or kinetin. Frequency
of endosperm induction was approximately 9% in ovules analyzed. The induction rate depended mainly on genotype of the donor
plant, and to lesser degrees, on floral stage, flower series and medium type. Multinuclear endosperms consisting of 10–37
nuclei were found in ovules after as few as 4 days of culture. In some ovules at this stage, the egg cell and two polar nuclei
were present. The process of endosperm degeneration began after 3 weeks of culture. In some ovules, degenerating autonomous
endosperm was observed up to the 7th week. Parthenogenetic development of egg cells or apogamy did not accompany autonomous
endosperm, supporting the hypothesis of independent pathways for embryo and endosperm development.
Received: 1 December 1998 / Revision accepted: 6 April 1999 相似文献
13.
The dye FM1-43 was used alone or in combination with measurements of the membrane capacitance (Cm) to monitor membrane changes in protoplasts from Viciafaba L. guard cells. Confocal images of protoplasts incubated with FM1-43 (10 μM) at constant ambient osmotic pressure (πo) revealed in confocal images a slow internalisation of FM1-43-labelled membrane into the cytoplasm. As a result of this process
the relative fluorescence intensity of the cell interior (fFM,i) increased with reference to the total fluorescence (fFM,t) by 7.4 × 10−4 min−1. This steady internalisation of dye suggests the occurrence of constitutive endocytosis under constant osmotic pressure.
Steady internalisation of FM1-43 labelled membrane caused a prominent staining of a ring-like structure located beneath the
plasma membrane. Abrupt elevation of πo by 200 mosmol kg−1 caused, over the first minutes of incubation, a rapid internalisation of FM1-43 fluorescence into the cytoplasm concomitant
with a decrease in cell perimeter. Within the first 5 min the cell perimeter decreased by 7.9%. Over the same time fFM,i/fFM,t increased by 0.13, reflecting internalisation of fluorescent label into the cytoplasm. Combined measurements of Cm and total fluorescence of a protoplast (fFM,p) showed that an increase in πo evoked a decrease in Cm but no change in fFM,p. This means that surface contraction of the protoplast is due to retrieval of excess membrane from the plasma membrane and
internalisation into the cytoplasm. Further inspection of confocal images revealed that protoplast shrinking was only occasionally
associated with internalisation of giant vesicles (median diameter 2.7 μm) with FM1-43-labelled membrane. But, in all cases,
osmotic contraction was correlated with a diffuse distribution of FM1-43 label throughout the cytoplasm. From this, we conclude
that endocytosis of small vesicles into the cytoplasm is the obligatory process by which cells accommodate an osmotically
driven decrease in membrane surface area.
Received: 4 May 1999 / Accepted: 19 August 1999 相似文献
14.
Summary. 3-Hydroxynorvaline (HNV; 2-amino-3-hydroxypentanoic acid), a microbial L-threonine analogue, is toxic to mammalian cells and
displays antiviral properties. In view of this, we investigated the toxicity and/or potential teratogenicity of HNV in developing
chicken and mouse embryos. HNV was administered to chicken embryos (in ovo; dose 75–300 μmole/egg; 48 h post-incubation) and pregnant Hanover NMRI mice (per os; total dose 900–1800 mg/kg body mass; gestation days 7–9). Control animals received sterile saline solutions. Harvested embryos
(chicken embryos, 10 days post-incubation; mouse embryos; gestation day 18) were fixed in glutaraldehyde and stereomicroscopically
inspected for signs of dysmorphogenesis. Body mass, body and toe length and mortality of chicken embryos, and the body mass
and mortality of mouse embryos were recorded. HNV exposure significantly increased the incidence of embryotoxic (growth retardation,
toxic mortality) and congenital defects in both chicken and mouse embryos. All the observed effects were dose-dependent. In
conclusion, HNV is an embryotoxic and teratogenic compound, which caused significant developmental delay and congenital defects
in developing chicken and mouse embryos. 相似文献
15.
Mesophyll cells (MCs) and bundle-sheath cells (BSCs) of leaves of the C4 plant maize (Zea mays L.) were separated by cellulase digestion to determine the relative proportion of the glutamine synthetase (GS; EC 6.3.1.2)
or the NADH-glutamate dehydrogenase (GDH; EC 1.4.1.2) isoforms in each cell type. The degree of cross-contamination between
our MC and BSC preparations was checked by the analysis of marker proteins in each fraction. Nitrate reductase (EC 1.6.6.1)
proteins (110 kDa) were found only in the MC fraction. In contrast, ferredoxin-dependent glutamate synthase (Fd-GOGAT; EC
1.4.7.1) proteins (160 kDa) were almost exclusively present in the BSC fraction. These results are consistent with the known
intercellular distribution of nitrate reductase and Fd-GOGAT proteins in maize leaves and show that the cross-contamination
between our MC and BSC fractions was very low. Proteins corresponding to cytosolic GS (GS-1) or plastidic GS (GS-2) were found
in both the MC and BSC fractions. While equal levels of GS-1 (40 kDa) and GS-2 (44 kDa) polypeptides were present in the BSC
fraction, the GS-1 protein level in the MC fraction was 1.8-fold higher than the GS-2 protein pool. Following separation of
the GS isoforms by anion-exchange chromatography of MC or BSC soluble protein extracts, the relative GS-1 activity in the
MC fraction was found to be higher than the relative GS-2 activity. In the BSC fraction, the relative GS-1 activity was very
similar to the relative GS-2 activity. Two isoforms of GDH with apparent molecular weights of 41 kDa and 42 kDa, respectively,
were detected in the BSC fraction of maize leaves. Both GDH isoenzymes appear to be absent from the MC fraction. In the BSCs,
the level of the 42-kDa GDH isoform was 1.7-fold higher than the level of the 41-kDa GDH isoform. A possible role for GS-1
and GDH co-acting in the synthesis of glutamine for the transport of nitrogen is discussed.
Received: 25 January 2000 / Accepted: 30 March 2000 相似文献
16.
Summary. Structurally diverse amino acids were prepared as versatile synthons for combinatorial chemistry. Using an optimized solid-phase
synthesis by Strecker-three-component-reaction (S-3CR), two different polymer linker constructs carrying piperazine were investigated.
(a) Acrylate derived base-labile linker yielded α-aminonitriles with N-alkylated piperazines via Hofmann elimination after
quarternisation with an alkyl halide. The crude product purities were in the range of 54–87%. (b) A urethane type linker yielded
α-aminonitriles with the free piperazine nitrogen when cleaved with acid and the product purities were 72–93%. The α-aminonitriles
were easily converted to novel Nɛ – Fmoc-protected α-amino acids with α-(1-piperazinyl) and α-phenyl substituents. 相似文献
17.
Torenia fournieri Lind. has a naked embryo sac that protrudes from the micropyle. The precise time course of the entire process of double fertilization
and the kinetics of fertilization events were determined in this species by the following methods: (i) without squashing,
pollen tubes on the torn stylar canal were observed by fluorescence microscopy after staining with both 4′,6-diamidino-2-phenylindole
(DAPI) and aniline blue; and (ii) large numbers of living embryo sacs were observed directly by differential interference
microscopy before and after fertilization. The pollen began to germinate 5 min after pollination and extruded pollen tubes
which elongated at a constant rate of 2.3 mm · h−1. At 4.0 h after pollination, the mitotic index of the generative cell within the pollen tube reached 88% and the two sperm
cells were formed. Pollen tubes began to arrive at ovules 8.9 h after pollination and directly entered one of two synergids
in the naked embryo sac. The time required for transport of sperm cells in the degenerated synergid was estimated statistically
to be 1.9 ± 1.8 min for transport of the first cell and 7.4 ± 1.6 min for the second. In the nucleus of the fertilized egg
cell, the male nucleolus began to emerge 10 h after pollination and the female nucleolus often decreased in size. The two
nucleoli fused together prior to elongation of the zygote, which began 28 h after pollination. In the central cell, the secondary
nucleus migrated to a region adjacent to the egg apparatus after pollination but prior to the arrival of the pollen tube.
The primary endosperm nucleus rapidly returned to the inner region after fertilization. Prior to embryogenesis, the first
division of the primary endosperm began about 15 h after pollination, at a defined site, to form the chalazal haustorium.
Received: 24 October 1996 / Accepted: 13 March 1997 相似文献
18.
Physiological elevations in cytoplasmic free calcium by cold or ion injection result in transient closure of higher plant plasmodesmata 总被引:7,自引:0,他引:7
The concentration of cytoplasmic free calcium ([Ca2+]cyt) required to close higher plant plasmodesmata was investigated using corn (Zea mays L. cv. Black Mexican Sweet) suspension-culture cells. Physiological elevations of [Ca2+]cyt were applied by cold treatment, and ion injection was also used to increase [Ca2+]cyt, by diffusion (for small increases) or by iontophoresis (for larger increases). The impact of such treatments on [Ca2+]cyt was measured by ratiometric ion imaging. Intercellular communication during treatments was monitored using our recently developed
electrophysiological technique that allows the electrical resistance of plasmodesmata and the plasma membranes of a sister-cell
pair to be measured. A 4-fold increase in the calculated resistance of single plasmodesmata was observed in response to cold
treatment that caused a 2-fold increase in average [Ca2+]cyt (from 107 to 210 nM). In response to iontophoresis of Ca2+, plasmodesmata were observed to go from “open” (low resistance) to “shut” (high resistance) and then back “open” within 10 s.
Our results thus indicate that higher plant plasmodesmata respond quickly to physiological changes in [Ca2+]cyt.
Received: 2 June 1999 / Accepted: 16 July 1999 相似文献
19.
Pectin methyltransferase (PMT) catalyzing the transfer of the methyl group from S-adenosyl-L-methionine (SAM) to the C-6 carboxyl group of galactosyluronic acid residues in pectin was found in a membrane preparation
of etiolated hypocotyls from 6-d-old soybean (Glycinemax Merr.). The enzyme was maximally active at pH 6.8 and 35–40 °C, and required 0.5% (w/v) Triton X-100. The incorporation of
the methyl group was significantly enhanced by addition of a pectin with a low (22%) degree of methyl-esterification (DE)
as exogenous acceptor substrate. The apparent Michaelis constants for SAM and the pectin (DE22) were 0.23 mM and 66 μg · ml−1, respectively. Attachment of the methyl group to the carboxyl group of the pectin via ester linkage was confirmed by analyzing
radiolabeled product from incubation of the enzyme with [14C]methyl SAM and the acceptor pectin. Size-exclusion chromatography showed that both enzymatic hydrolysis with a pectin methylesterase
and a mild alkali treatment (saponification) led to the release of radioactive methanol from the product. Enzymatic hydrolysis
of the product with an endopolygalacturonase degraded it into small pectic fragments with low relative molecular mass, which
also supports the idea that the methyl group is incorporated into the pectin. The soybean hypocotyls were fractionated into
their cell wall components by successive extraction with water, EDTA, and alkali treatment. Among the resulting polysaccharide
fractions, high PMT activity was observed when a de-esterified polysaccharide derived from the EDTA-soluble fraction (the
pectic fraction) was added as an alternative acceptor substrate, indicating that the enzyme may be responsible for producing
methyl-esterified pectin in vivo.
Received: 10 September 1999 / Accepted: 11 October 1999 相似文献
20.
Intracellular chloroplast photorelocation in the moss Physcomitrella patens is mediated by phytochrome as well as by a blue-light receptor 总被引:3,自引:0,他引:3
The light-induced intracellular relocation of chloroplasts was examined in red-light-grown protonemal cells of the moss Physcomitrella patens. When irradiated with polarized red or blue light, chloroplast distribution in the cell depended upon the direction of the
electrical vector (E-vector) in both light qualities. When the E-vector was parallel to the cross-wall (i.e. perpendicular
to the protonemal axis), chloroplasts accumulated along the cross-wall; however, no accumulation along the cross-wall was
observed when the E-vector was perpendicular to it (i.e. parallel to the protonemal axis). When a part of the cell was irradiated
with a microbeam of red or blue light, chloroplasts accumulated at or avoided the illumination point depending on the fluence
rate used. Red light of 0.1–18 W m−2 and blue light of 0.01–85.5 W m−2 induced an accumulation response (low-fluence-rate response; LFR), while an avoidance response (high-fluence-rate response;
HFR) was induced by red light of 60 W m−2 or higher and by blue light of 285 W m−2. The red-light-induced LFR and HFR were nullified by a simultaneous background irradiation of far-red light, whereas the
blue-light-induced LFR and HFR were not affected at all by this treatment. These results show, for the first time, that dichroic
phytochrome, as well as the dichroic blue-light receptor, is involved in the chloroplast relocation movement in these bryophyte
cells. Further, the phytochrome-mediated responses but not the blue-light responses were revealed to be lost when red-light-grown
cells were cultured under white light for 2 d.
Received: 7 September 1999 / Accepted: 15 October 1999 相似文献