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1.
The unloading of sucrose in the apical part of the hypocotyl of Ricinus communis L. seedlings was measured by 13C-nuclear magnetic resonance (NMR) spectroscopy. The cotyledons of the seedling were immersed in 5 mM Mes buffer containing
100 mM 13C-labeled sucrose. At intervals of 70–90 min, 13C-NMR spectra with broadband decoupling and nuclear Overhauser enhancement were acquired in vivo. The spectra showed growing 13C-resonances of the labeled positions in the sucrose molecule reaching steady-state labeling within 7–8 h. The specific 13C labeling of sucrose in the G1-position changed from 0.38 in the supplied sucrose solution to 0.16 in the sucrose extracted from the hypocotyl piece at
the end of the experiment (13 h). Labeling of starch (and other insolubles) in the hypocotyl piece was ca. 0.10. It is proposed
that the decreased specific labeling of unloaded sucrose is mostly due to the separate local pools of sucrose in the cortex
and pith parenchyma, respectively, and less to continuous starch degradation and conversion to sucrose. The report gives an
example of the application of 13C-NMR spectroscopy in assimilate allocation studies.
Received: 10 October 1998 / Accepted: 31 December 1998 相似文献
2.
External sucrose, supplied by the endosperm in vivo, is the physiological source of sucrose for Ricinus communis L. seedlings. It is taken up by the cotyledons and exported via the sieve tubes to the growing hypocotyl and root. Two parallel
pathways of external sucrose to the sieve tubes, directly via the apoplasm and indirectly after transit through the mesophyll,
have already been established (G. Orlich and E. Komor, 1992). In this study, we analysed whether a symplasmic flow of sucrose
contributes to phloem loading. Uptake of external sucrose into the mesophyll and into the sieve tubes, and export of total
sucrose were measured with intact and exuding seedlings in the presence of p-chloromercuribenzenesulfonic acid (PCMBS). Sucrose uptake into the mesophyll and into the sieve tubes was inhibited by 80–90%.
Consequently, export of total sucrose slowed down. However, after the addition of PCMBS, sucrose was transiently exported
in such a high amount that could not be accounted for by the residual uptake activity nor by the amount of sucrose confined
to the sieve element-companion cell complex (seccc). From the results, we conclude that most of the sucrose exported transiently
had moved to the sieve tubes from a symplasmic domain larger than the seccc, comprising at least all the cells of the bundle
including the bundle sheath. We suggest that the symplasmic flow of sucrose observed is a mass flow driven by a turgor pressure.
As a structural prerequisite for a symplasmic flow, plasmodesmata interconnect all the cells from the bundle sheath to the
sieve tubes and also occur between the bundle sheath and the mesophyll. The phloem loading pathway of Ricinus cotyledons can thus be classified as a combination of three different routes.
Received: 17 October 1997 / Accepted: 9 March 1998 相似文献
3.
Gabriele Orlich 《Planta》1998,206(2):266-271
The aim of this study was to reveal the factors determining sucrose export and volume flow through the sieve tubes in Ricinus communis L. seedlings. The cotyledons take up sucrose from the apoplasm in vivo, and export most of it to the growing sinks, hypocotyl
and root. This simple source-sink system allowed sucrose uptake and export to be studied under controlled conditions with
respect to apoplasmic sucrose concentrations. From the additional knowledge of the sucrose concentrations in the mesophyll
and the sieve tubes, transmembrane concentration differences were calculated. The volume flow rate along the sieve tubes could
be calculated from the export rate and the sucrose concentration in the sieve tubes. While the export rate exhibited saturation
kinetics, the volume flow rate decreased at high external sucrose concentrations. The export rate correlated with the sucrose
uptake rate, the volume flow rate correlated with the sucrose concentration (osmotic pressure) difference across the sieve-tube
plasma membrane, the driving force for transmembrane water flux. From these data it can be concluded that sucrose export and
the volume flow through the sieve tubes are determined by activities of the source. Export out of Ricinus cotyledons was considerably higher than export out of green source leaves of different species. The concomitant comparatively
low sucrose concentration in the sieve-tube sap of the seedlings can thus be attributed to a very high water flux into and
along the sieve tubes associated with the high sucrose flux.
Received: 28 November 1997 / Accepted: 4 April 1998 相似文献
4.
Expression of the phloem lectin is developmentally linked to vascular differentiation in cucurbits 总被引:8,自引:0,他引:8
Joanne M. Dannenhoffer Alexander Schulz Megan I. Skaggs Dwight E. Bostwick Gary A. Thompson 《Planta》1997,201(4):405-414
The conducting elements of phloem in angiosperms are a complex of two cell types, sieve elements and companion cells, that
form a single developmental and functional unit. During ontogeny of the sieve element/companion cell complex, specific proteins
accumulate forming unique structures within sieve elements. Synthesis of these proteins coincides with vascular development
and was studied in Cucurbita seedlings by following accumulation of the phloem lectin (PP2) and its mRNA by RNA blot analysis, enzyme-linked immunosorbent
assay, immunocytochemistry and in␣situ hybridization. Genes encoding PP2 were developmentally regulated during vascular differentiation
in hypocotyls of Cucurbita maxima Duch. Accumulation of PP2 mRNA and protein paralleled one another during hypocotyl elongation, after which mRNA levels decreased,
while the protein appeared to be stable. Both PP2 and its mRNA were initially detected during metaphloem differentiation.
However, PP2 mRNA was detected in companion cells of both bundle and extrafascicular phloem, but never in differentiating
sieve elements. At later stages of development, PP2 mRNA was most often observed in extrafascicular phloem. In developing
stems of Cucurbita moschata L., PP2 was immunolocalized in companion cells but not to filamentous phloem protein (P-protein) bodies that characterize
immature sieve elements of bundle phloem. In contrast, PP2 was immunolocalized to persistent ␣ P-protein bodies in sieve elements
of the extrafascicular phloem. Immunolocalization of PP2 in mature wound sieve elements was similar to that in bundle phloem.
It appears that PP2 is synthesized in companion cells, then transported into differentiated sieve elements where it is a component
of P-protein filaments in bundle phloem and persistent P-protein bodies in extrafascicular phloem. This differential accumulation
in bundle and extrafascicular elements may result from different functional roles of the two types of phloem.
Received: 31 July 1996 / Accepted: 27 August 1996 相似文献
5.
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
6.
Compartmentation fluxes of carbohydrates along the phloem path were
analysed in the petiole of Cyclamen persicum (L.)
Mill. Sucrose represented the dominant fraction (58-75% of soluble
carbohydrates in the vascular symplast). Planteose (12-22%), glucose (3-8%)
and fructose (3-13%) occurred in lower amounts (data from liquid
chromatography, percentages of the total peak area). Starch was not
detectable. Upon feeding leaves with 14CO2, 98% and
90% of radiolabel was recovered as sucrose in the vascular symplast after 3
h and 24 h, respectively. Thus, sucrose appeared to be the exclusive
transport sugar in Cyclamen. Experiments with
asymmetrically labelled sucrose revealed that there was no metabolism of
translocated sucrose. Analysis of six consecutive petiole segments (each 2
cm in length) showed a homogeneous longitudinal distribution of these
sugars differed markedly. On average, the sucrose concentration amounted to
4.7 and 0.4 mg g-1 FM in the vascular apoplast and
petiole parenchyma, respectively. Sucrose was unloaded with out hydrolysis
and stored in the periphery of the phloem path. Planteose was identified as
another storage saccharide. Sucrose synthesis by sucrose phosphate synthase
occurred when isolated vascular bundles were incubated with
[14C]glucose or
[14C]fructose. These data suggest that the phloem
path is characterized by both source and sink like
activity. 相似文献
7.
Xiaolei Sui Jing Nie Huan Liu Tao Lin Xuehui Yao Robert Turgeon 《The Plant journal : for cell and molecular biology》2021,106(4):1163-1176
Cucurbit phloem is complex, with large sieve tubes on both sides of the xylem (bicollateral phloem), and extrafascicular elements that form an intricate web linking the rest of the vasculature. Little is known of the physical interconnections between these networks or their functional specialization, largely because the extrafascicular phloem strands branch and turn at irregular angles. Here, export in the phloem from specific regions of the lamina of cucumber (Cucumis sativus L.) was mapped using carboxyfluorescein and 14C as mobile tracers. We also mapped vascular architecture by conventional microscopy and X-ray computed tomography using optimized whole-tissue staining procedures. Differential gene expression in the internal (IP) and external phloem (EP) was analyzed by laser-capture microdissection followed by RNA-sequencing. The vascular bundles of the lamina form a nexus at the petiole junction, emerging in a predictable pattern, each bundle conducting photoassimilate from a specific region of the blade. The vascular bundles of the stem interconnect at the node, facilitating lateral transport around the stem. Elements of the extrafascicular phloem traverse the stem and petiole obliquely, joining the IP and EP of adjacent bundles. Using pairwise comparisons and weighted gene coexpression network analysis, we found differences in gene expression patterns between the petiole and stem and between IP and EP, and we identified hub genes of tissue-specific modules. Genes related to transport were expressed primarily in the EP while those involved in cell differentiation and development as well as amino acid transport and metabolism were expressed mainly in the IP. 相似文献
8.
Careful cutting of the hypocotyl of Ricinus communis L. seedlings led to the exudation of pure sieve-tube sap for 2–3 h. This offered the possibility of testing the phloem-loading system qualitatively and quantitatively by incubating the cotyledons with different solutes of various concentrations to determine whether or not these solutes were loaded into the sieve tubes. The concentration which was achieved by loading and the time course could also be documented. This study concentrated on the loading of sucrose because it is the major naturally translocated sieve-tube compound. The sucrose concentration of sieve-tube sap was approx. 300 mM when the cotyledons were buried in the endosperm. When the cotyledons were excised from the endosperm and incubated in buffer, the sucrose concentration decreased gradually to 80–100 mM. This sucrose level was maintained for several hours by starch breakdown. Incubation of the excised cotyledons in sucrose caused the sucrose concentration in the sieve tubes to rise from 80 to 400 mM, depending on the sucrose concentration in the medium. Thus the sucrose concentration in the sieve tubes could be manipulated over a wide range. The transfer of labelled sucrose to the sieve-tube sap took 10 min; full isotope equilibration was finally reached after 2 h. An increase of K+ in the medium or in the sieve tubes did not change the sucrose concentration in the sievetube sap. Similarly the experimentally induced change of sucrose concentration in the sieve tubes did not affect the K+ concentration in the exudate. High concentrations of K+, however, strongly reduced the flow rate of exudation. Similar results were obtained with Na+ (data not shown). The minimum translocation speed in the sieve tubes in vivo was calculated from the growth increment of the seedling to be 1.03 m·h-1, a value, which on average was also obtained for the exudation system with the endosperm attached. This comparison of the in-vivo rate of phloem transport and the exudation rate from cut hypocotyls indicates that sink control of phloem transport in the seedlings of that particular age was small, if there was any at all, and that the results from the experimental exudation system were probably not falsified by removal of the sink tissues.Abbreviations PTS
3-hydroxy-5,8, 10-pyrenetrisulfonate 相似文献
9.
Chalcone synthase activity and polyphenolic compounds of shoot tissues from adult and rejuvenated walnut trees 总被引:3,自引:0,他引:3
Changes in the metabolism of naphthoquinone and flavonoids during the growth of half-sib adult and rejuvenated walnut shoots
(Juglans nigra × Juglans regia L.) were studied at the tissue level for two years after pruning. Moreover, the role of chalcone synthase (CHS; EC 2.3.1.74)
in the regulation of flavonoid biosynthesis was investigated at the level of enzyme activity. The end products of walnut flavonoid
biosynthesis, myricitrin and quercitrin, which accumulated in the bark and phloem at the end of growth, did not inhibit the
biosynthetic process at concentrations of up to 100 μM each. There was no evidence of CHS regulation by feedback or similar
mechanisms which might modulate enzyme activity. Mathematical correlation of CHS activity and flavonoid accumulation during
shoot growth, however, indicated that CHS is the rate-limiting enzyme of the pathway in bark and phloem and that flavonoids
seem to be transported from phloem to bark where they accumulated mainly during growth. In defoliated shoots, naphthoquinone
metabolism appeared to be a marker of the walnut rejuvenation stage in the medulla, phloem and buds immediately after cutting
and thereafter mainly in buds one year after cutting. Chalcone synthase and flavonoid contents appeared to be markers of the
adult stage in the phloem.
Received: 30 September 1996 / Accepted: 17 March 1997 相似文献
10.
Ultrastructural effects in potato leaves due to antisense-inhibition of the sucrose transporter indicate an apoplasmic mode of phloem loading 总被引:8,自引:0,他引:8
To study the export of sugars from leaves and their long-distance transport, sucrose-proton/co-transporter activity of potato
was inhibited by antisense repression of StSUT1 under control of either a ubiquitously active (CaMV 35S ) or a companion-cell-specific (rolC) promotor in transgenic plants. Transformants exhibiting reduced levels of the sucrose-transporter mRNA and showing a dramatic
reduction in root and tuber growth, were chosen to investigate the ultrastructure of their source leaves. The transformants
had a regular leaf anatomy with a single-layered palisade parenchyma, and bicollateral minor veins within the spongy parenchyma.
Regardless of the promoter used, source leaves from transformants showed an altered leaf phenotype and a permanent accumulation
of assimilates as indicated by the number and size of starch grains, and by the occurrence of lipid-storing oleosomes. Starch
accumulated throughout the leaf: in epidermis, mesophyll and, to a smaller degree, in phloem parenchyma cells of minor veins.
Oleosomes were observed equally in mesophyll and phloem parenchyma cells. Companion cells were not involved in lipid accmulation
and their chloroplasts developed only small starch grains. The similarity of ultrastructural symptoms under both promotors
corresponds to, rather than contradicts, the hypothesis that assimilates can move symplasmically from mesophyll, via the bundle
sheath, up to the phloem. The microscopical symptoms of a constitutively high sugar level in the transformant leaves were
compared with those in wild-type plants after cold-girdling of the petiole. Inhibition of sugar export, both by a reduction
of sucrose carriers in the sieve element/companion cell complex (se/cc complex), or further downstream by cold-girdling, equally
evokes the accumulation of assimilates in all leaf tissues up to the se/cc complex border. However, microscopy revealed that
antisense inhibition of loading produces a persistently high sugar level throughout the leaf, while cold-girdling leads only
to local patches containing high levels of sugar.
Received: 4 March 1998 / Accepted: 7 April 1998 相似文献
11.
Jozef Šamaj Simon Hawkins Virginie Lauvergeat Jacqueline Grima-Pettenati Alain Boudet 《Planta》1998,204(4):437-443
Cinnamyl alcohol dehydrogenase 2 (CAD 2) localization and the cell-specific activity of the eucalyptus CAD 2 promoter were
investigated by CAD 2 immunogold localization and promoter β-glucuronidase (GUS) histochemistry in apical and mature parts
of stable transformed poplar (Populus tremula × P. alba) stems. Both CAD 2 protein and GUS activity were found to be confined in the same types of cells in the shoot apices, particularly
in the determined meristematic cells in leaf axils and shell zones, procambium and developing tracheids. Within mature stems,
CAD 2 and GUS were also identified in cambium and in fully or partially lignified cells derived from it (young xylem, developing
phloem fibres, chambered parenchyma cells around phloem). Additionally, GUS activity was found in the scale leaves of apical
shoot buds and in the roots (namely in the procambium, cambium, phellogen, young xylem, pericycle) of transformed plants.
By employing immunogold cytochemistry, CAD 2 was shown to be localized in the cytoplasm within cambial, ray and young xylem
cells in stems, the gold particles being randomly attached to endoplasmic reticulum and Golgi-derived vesicles. These results
support a crucial role for CAD 2 in lignification and indicate a new role for this enzyme in branching events within the shoot
apex and during lateral root formation.
Received: 24 April 1997 / Accepted: 17 July 1997 相似文献
12.
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
13.
Microautoradiography was used to follow the translocation pathways of 14C-labeled photosynthate from mature source leaves, through the stem, to immature sink leaves three nodes above. Translocation occurred in specific bundles of the midveins and petioles of both the source and sink leaves and in the interjacent internodes. When each of six major veins in the lamina of an exporting leaf was independently spot-fed 14CO2, label was exported through specific bundles in the petiole associated with that vein. When the whole lamina of a mature source leaf was fed 14CO2, export occurred through all bundles of the lamina, but acropetal export in the stem was confined to bundles serving certain immature sink leaves. Cross-transfer occurred within the stem via phloem bridges. Leaves approaching maturity translocated photosynthate bidirectionally in adjacent subsidiary bundles of the petiole. That is, petiolar bundles serving the lamina apex were exporting unlabeled photosynthate while those serving the lamina base were simultaneously importing labeled photosynthate. The petioles and midveins of maturing leaves were strong sinks for photosynthate, which was diverted from the export front to differentiating structural tissues. The data support the idea of bidirectional transport in adjacent bundles of the petiole and possibly in adjacent sieve tubes within an individual bundle.Abbreviations C
central leaf trace
- L
left leaf trace
- LPI
leaf plastochron index
- R
right leaf trace 相似文献
14.
A. Krastanov 《Applied microbiology and biotechnology》1997,47(5):476-481
A novel immobilized biocatalyst with invertase activity was prepared by adhesion of yeast cells to wool using glutaraldehyde.
Yeast cells could be immobilized onto wool by treating either the yeast cells or wool or both with glutaraldehyde. Immobilized
cells were not desorbed by washing with 1 M KCl or 0.1 M buffers, pH 3.5–7.5. The biocatalyst shows a maximum enzyme activity
when immobilized at pH 4.2–4.6 and 7.5–8.0. The immobilized biocatalyst was tested in a tubular fixed-bed reactor to investigate
its possible application for continuous full-scale sucrose hydrolysis. The influence of temperature, sugar concentration and
flow rate on the productivity of the reactor and on the specific productivity of the biocatalyst was studied. The system demonstrates
a very good productivity at a temperature of 70 °C and a sugar concentration of 2.0 M. The increase of the volume of the biocatalyst
layer exponentially increases the productivity. The productivity of the immobilized biocatalyst decreases no more than 50%
during 60 days of continuous work at 70 °C and 2.0 M sucrose, but during the first 30 days it remains constant. The cumulative
biocatalyst productivity for 60 days was 4.8 × 103kg inverted sucrose/kg biocatalyst. The biocatalyst was proved to be fully capable of continuous sucrose hydrolysis in fixed-bed
reactors.
Received: 8 November 1996 / Received revision: 31 January 1997 / Accepted: 31 January 1997 相似文献
15.
A panel of monoclonal antibodies (MAC204, MAC236, MAC265) which recognise extracellular matrix glycoproteins implicated in
plant-microbe interactions has been used to study glycoprotein antigens in petioles of turnip (Brassica campestris L.). While MAC204 recognised two glycoproteins (gp120 and gp45) with apparent Mr 120 000 and 45 000 in petiole extracts made with 2-amino-2-(hydroxymethyl)-1,3-propanediol (Tris) buffer containing sodium
dodecyl sulfate, MAC236 recognised gp120 but not gp45, and MAC265 gave no or only weak reactivity. Tissue dissection studies
established that gp120 was predominantly associated with the vascular bundle whereas gp45 was largely associated with the
pith. This was consistent with results from tissue prints probed with MAC204 and MAC236 which also suggested a vascular localisation
for gp120. Immunoelectronmicroscopy showed that MAC204 and MAC236 both labelled three-way junctions between cells of the phloem
and sclerid fibres. Both gp120 and gp45 were shown to carry epitopes in common with known hy`droxyproline-rich glycoproteins.
Unlike gp45, gp120 could be extracted from petioles with Tris buffer alone and then isolated from this extract by trichloroacetic
acid treatment (which left gp120 soluble), followed by size-exclusion and ion-exchange chromatography. Amino acid analysis
revealed gp120 to be a novel glycoprotein, particularly rich in proline, lysine, valine and threonine but relatively poor
in hydroxyproline. The most abundant sugars were arabinose and galactose. The potential role of this very basic cell surface
glycoprotein in plant defence against microbes is discussed.
Received: 25 November 1996 / Accepted: 12 December 1996 相似文献
16.
When a vine petiole is carrying labelled sucrose away from thelamina, the quantity of labelled carbon dioxide lost from thepetiole bears a constant relation to the quantity of labelledsucrose inside the petiole. Sucrose is virtually the only labelledsugar in the petiole, and the labelled sucrose is confined topart of the phloem. Calculations based on these measurementsand some assumptions suggest that the rate of breakdown of thetranslocated sucrose is about 0·5 mg. per c.c. of phloemper hour. The bearings of these findings on the problem of energysupply to translocation are discussed. 相似文献
17.
Moni Brauer Wen-Jun Zhong Till Jelitto Christian Schobert Dale Sanders Ewald Komor 《Planta》1998,206(1):103-107
Changes in free Ca2+ in sieve-tube sap have been proposed to be important in the regulation of phloem transport, and Ca2+-activated protein kinase activity has been described in phloem exudate (S.A. Avdiushko et al. 1997 J Plant Physiol 150: 552–559).
Using atomic absorption spectrometry, we have determined that the total Ca2+ concentration in sieve-tube sap from Ricinus seedlings containing the endosperm is about 100 μM (range 80–150 μM). We used
three independent methods to determine the free calcium ion concentration in the phloem sap ([Ca2+]p). The first method was to calculate [Ca2+]p from the total Ca2+ concentration, in combination with the binding constants and concentrations of the ionic solutes in phloem sap. The resultant
estimate of [Ca2+]p was 63 μM. The second method used the Ca-specific fluorescent dye 2-[2-(5-carboxy)oxazole]-5-hydroxy-6-aminobenzofuran-N,N,O-triacetic-acid
(FURAPTRA) on exuded sieve-tube sap. Although the sap interfered severely with the fluorescence properties of the dye, Ca2+ titrations enabled a value of [Ca2+]p = 20 μM to be deduced. The third method used Ca2+-selective microelectrodes on exuded sap samples, which gave an average value for [Ca2+]p = 13 μM. No significant change in this value was observed during the sap exudation period. The Ca2+ buffer capacity was determined and the result of about 0.6 mmol · l−1 · pCa−1 displayed excellent agreement with the measured values of free and total Ca2+ concentration in sieve-tube sap. Since the measured values for free Ca2+ are 20- to 100-fold higher than those usually reported for the cytosol of a range of plant cells in resting conditions, it
is concluded that either regulation of [Ca2+]p is of limited physiological importance, or that the Ca2+-dependent proteins respond only to relatively high [Ca2+]p. The implications for regulation of cytosolic free Ca2+ in symplastically connected companion cells is discussed.
Received: 15 February 1998 / Accepted: 14 March 1998 相似文献
18.
We have investigated the regulation of ferredoxin–glutamate synthase (Fd-GOGAT) in leaves of barley (Hordeum vulgare L. cv. Maris Mink) at the mRNA, protein and enzyme activity levels. Studies of the changes in Fd-GOGAT during plant development
showed that the activity in shoots increases rapidly after germination to reach a maximum (on a fresh-weight basis) at day
10 and then declines markedly to less than 50% of the maximal activity by day 30, this decline being correlated with an equivalent
loss of Fd-GOGAT protein. Growing the plants in darkness reduced the maximum activity attained in the shoots, but did not
affect the overall pattern of the changes or their timing. The activity of Fd-GOGAT increased two- to three-fold within 48 h
when etiolated leaves were exposed to light, and Northern blots indicated that the induction occurred at the mRNA level. However,
whilst a carbon source could at least partially substitute for light in the induction of nitrate reductase activity, no induction
of Fd-GOGAT activity was seen when etiolated leaves were treated with either sucrose or glucose. Interestingly, the levels
of Fd-GOGAT mRNA and activity remained high up to a period of 16 h or 72 h darkness, respectively. Compared with plants grown
in N-free medium, light-grown plants supplied with nitrate had almost two-fold higher Fd-GOGAT activities and increased Fd-GOGAT
mRNA levels, but nitrate had no effect on the abundance of the enzyme or its mRNA in etiolated plants, indicating that light
is required for nitrate induction of barley Fd-GOGAT.
Received: 23 April 1997 / Accepted: 28 May 1997 相似文献
19.
Klaus Schmitz 《Planta》1970,92(3):208-221
Summary The petiole of Pelargonium zonale is traversed by 17 bundles, whose arrangement and form are typical for this plant. The bundles of the petiole are connected with the conducting system of the axis and with the main nerves by a system of phloem anastomoses in the leaf base and in the junction between the petiole and the leaf blade (Fig. 2). The anatomical findings were confirmed and extended by a study of the translocation of K-fluorescein and 14C. It could be shown that the metaphloem of the central petiole bundle is composed of phloem subunits, each of which is connected with the phloem of one certain main nerve only (Fig. 4). Accordingly, if fluorescein or 14CO2 is applied to one main nerve, the dye or 14C-material is translocated exclusively in a small phloem area of the central bundle. Autoradiograms of the petioles indicate that the 14C-labelled assimilates (sucrose, glucose, fructose and amino acids) are translocated exclusively in the phloem. A lateral movement of the labelled material within the petiole was not observed. The metaphloem of the central petiole bundle of Pelargonium zonale revealed a functional organization of phloem subunits.
Teil einer Dissertation unter der wissenschaftlichen Leitung von Prof. Dr. J. Willenbrink. 相似文献
Teil einer Dissertation unter der wissenschaftlichen Leitung von Prof. Dr. J. Willenbrink. 相似文献
20.
Single cell sap sampling and analysis were used to measure the longitudinal and radial distribution of sucrose, glucose and fructose in the apical cell division zone and in the basal, elongated zone of the Ricinus hypocotyl. Sucrose and hexose increased in concentration from the apex to the base of the seedling axis. In the cell division zone low hexose and sucrose concentrations prevailed in cortex and pith, with a slightly higher hexose concentration in pith cells. The sucrose concentrations in sieve tubes and in phloem were much higher than in the cortex and pith cells. In the basal zone of the hypocotyl high levels of sucrose in phloem, cortex and pith were found, therefore radial, diffusional sucrose flow away from the phloem was considered unlikely. It is proposed that radial flow of growth-water to the hypocotyl periphery together with the down-regulation of a sucrose transporter at the phloem leads to a preferential sucrose flow to the expanding cortex. The pith cells, which do not experience flow of growth-water, are probably insufficiently supplied with sucrose from the phloem resulting eventually in cell death as the plant grows. Shortage of sucrose supply, experimentally achieved by removal of the endosperm, led to sucrose hydrolysis in the pith. The sucrose levels in the other tissues decreased less. It appears that the hydrolysis to hexose was initiated to maintain the osmotic value in the pith cell sap. It is speculated that high hexose levels in the cells are indicative of insufficient sucrose supply via the phloem and that the pith cells are confronted with that situation during early seedling development. 相似文献