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Plant Physiol, August 2000, Vol. 123, pp. 1217-1226
UPDATE ON INTRACELLULAR TRANSPORT
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INTRODUCTION |
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 TOP
 INTRODUCTION
 DIRECT TRANSPORT OF VACUOLAR...
TRANSPORT OF SOLUTES OF...
OPPOSING VIEWS
POSSIBLE ALTERNATIVES
CONCLUSIONS AND FUTURE...
LITERATURE CITED
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The traditional view of the plant
cell vacuole as a storage compartment has been replaced in recent years
to that of a multipurpose organelle by the realization that the vacuole
carries out numerous metabolic functions (Wink, 1993
). For instance,
under certain physiological conditions, in specific cell types, or at
determined developmental stages, the vacuole participates in the export
of a variety of solutes ranging from simple sugars and organic acids to
amino acids and mineral ions. Mobilization of vacuolar components is a
prominent function of many cells especially those comprising the
storage organs of seeds during germination and of tubers, hypocotyls,
roots, rhizomes, and corms during resumption of vegetative and/or
reproductive growth. In photosynthetic cells, the vacuole is an
important exporter of Suc during dark periods and at times of low
photosynthetic activity. The vacuole also appears to serve as an
interim location in the process of sugar secretion in nectaries, ion
secretion by salt glands, and for the final protein-processing steps of
several secreted enzymes in cell suspension cultures (e.g.
-mannosidase, class I chitinase, and class I 
-1,3-glucanase; Wink, 1994; Kunze et al., 1998). Finally, metabolite efflux from the
vacuole is intimately involved in the rapid volume changes associated
with stomatal closure and with the loss of turgor in the pulvinar motor
cells that drive leaf movements in Mimosa pudica (MacRobbie, 1999).
The basic question regarding how solutes are mobilized from the vacuole
and secreted outward across the plasmalemma has been infrequently
addressed and therefore, has yet to receive a definitive answer. However, by analogy to other intracellular transport systems, solute trafficking from the vacuole involves one or both of two fundamentally distinct pathways: carrier- or vesicle-mediated transport. Carrier-mediated transport refers to the movement of individual solute particles across biological membranes whether assisted by a membrane-bound carrier, by a pump, or simply by diffusion
through specific channels. In contrast, vesicle-mediated transport refers to the collective movement of numerous solute particles enclosed within small membrane vesicles across the cytosol. In most cases of vesicles-mediated transport, it is believed that the
membrane of the secretion vesicle fuses and becomes incorporated with
the plasmalemma as the secreted material is deposited in the
extracytoplasmic space (Kronestedt-Robards and Robards, 1992; Battey et al., 1999). These two processes of secretion are not exclusionary, and both may occur in parallel in specific secretory systems.
The present review describes existing evidence supporting the presence
of a vesicle-mediated pathway for solute movement from the vacuole to
the apoplast in plant cells. It should be noted that the bulk of
evidence in support of a vesicle-mediated vacuolar export system in
plant cells is derived primarily from cytological and anatomical
studies, many of which contain little or no physiological data.
Although efforts were made to cover the most recent developments in
this area, most of the direct data come from studies conducted around
the 1970s when the relationship between structure and function received
significant attention. References to more recent reports that
indirectly support the existence of vesicle-mediated intracellular transport from the vacuole are also included, even though the primary
intention of such studies was unrelated to vacuole exporting mechanisms. Finally, opposing evidence and possible alternatives are
presented. The reader is referred to reviews by Battey et al. (1999)
and Fahn (1988) for information on granulocrine secretions (exocytosis)
by plant cells and secreting organs, respectively, mostly originating
directly from Golgi and endoplasmic reticulum (ER).
Involvement of the vacuole in the secretion process of certain
secretory cells has been inferred from compartmental analyses that
indicate a higher concentration of secreted salts (Berry, 1970) and
sugars in the exudate than in the secreting cells or in the
translocation sap. In view of the vacuole as the location where sugar
and salts accumulate, implication of the vacuole in these exporting
systems is warranted. Finally, vesicle transport is also inferred from
the inability of membrane-bound carrier transport systems to explain
ion accumulation and excretion in cells such as those of Nitella
translucens and in the fast moving guard and pulvinar cells
(MacRobbie, 1999).
In organizing the available data, it became evident that any structure of classification would be fundamentally artificial. The peculiarities inherent in each system invariably make each case a unique process. Therefore, after much thought, I decided on a classification based on the content of secreted solutes. In the first group, secreted metabolites are those commonly found in plant vacuoles and presumably transported directly to the exterior of the cell. The second group consists of systems in which the secretions also contain metabolites known to originate from ER and/or the dictyosome, implying a coordinated endomembrane network of vesicle trafficking.
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DIRECT TRANSPORT OF VACUOLAR SOLUTES |
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TOP
INTRODUCTION
DIRECT TRANSPORT OF VACUOLAR...
TRANSPORT OF SOLUTES OF...
OPPOSING VIEWS
POSSIBLE ALTERNATIVES
CONCLUSIONS AND FUTURE...
LITERATURE CITED
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Vesicle Transport in Salt-Secreting Trichomes
The existence of a vesicle-mediated system for metabolite
transport from the vacuole to the cell membrane was initially advanced by Ziegler and Luttge (1967) in their studies of the secretory salt
glands of Limonium vulgare. They observed that
salt-secreting cells contained numerous and well-defined mitochondria
and were rich in ER, but lacked a conspicuous large central vacuole.
Instead, the cells contained a series of smaller "vacuole-like"
membrane vesicles, many of which were seen in close proximity to the
cell membrane (Fig. 1). More important,
the tonoplast of the "vacuole-like" vesicles often appeared to fuse
with the plasmalemma (Fig. 1) in agreement with a vesicle-mediated
secretion process. Comparable observations were concurrently made by
Thomson and Liu (1967) in their studies of the salt-secreting gland
cells of Tamarix aphylla (Fig.
2) and later by Cardale and Field (1971)
and Shimony et al. (1973) for salt glands of the mangrove species
Aegiceras corniculatum and Avicenia marina,
respectively. During salt secretion in T. aphylla,
numerous "microvacuoles" (similar to those of L. vulgare) appear at the periphery of the secreting cells and
often in close association with wall projections (Fig. 2A; Thomson and Liu, 1967). At higher magnification, direct contact between the tonoplast and the plasmalemma was also noted (Fig. 2, B and C). From
these observations Thomson et al. (1969) concluded that, as the salt
accumulates in the glands cells, it becomes compartmentalized within
the microvacuoles and is secreted when the microvacuoles fuse with the
plasmalemma.
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Some indirect anatomical and physiological considerations support the
direct involvement of the vacuole and subsequent tonoplast vesicle-mediated system of intracellular transport to the plasmalemma. For example, secretory "gland cell complexes" are completely
enclosed by an impermeable cuticular envelope opening sporadically to
the underlying cells to allow symplastic connections. Across these "transfusion areas" (Thomson and Liu, 1967) the secretory cells make contact with the surrounding mesophyll through abundant
plasmodesmata (Thomson and Platt-Aloia, 1985). Compartmental analysis
confirmed symplastic movement of ions from the mesophyll and into the
gland cells (Hill, 1970). As the concentration of secreted salts reach exceedingly high levels (1 M Cl
in
Aegitalis annulata; Atkinson et al., 1967) a
concentrating mechanism must occur within a compartment inside the
salt-secreting cells or at the plasmalemma if extrusion were to occur
by an active pump.
Two independent observations in A. marina indicated
that the active salt-concentrating step occurs at the tonoplast. First, salt continues to be secreted during the night hours when transpiration rates in this mangrove species have virtually ceased. This observation led Fitzgerald et al. (1982) to suggest that salt accumulated in the
vacuoles during the day hours is later extruded at night. Subsequent
x-ray maps of secreting cells confirmed this observation (Fitzgerald et
al., 1982; Robards and Oates, 1986). Second, using ion selective
electrodes, Shimony et al. (1973) noted a much lower salt concentration
in the cytosol of secreting cells than in the xylem sap, suggesting
accumulation of salts at some intracellular compartment. The
experiments of Thomson and Liu (1969), however, offer the most
convincing evidence for the vacuole as the site for ion accumulation in
secreting cells. When T. aphylla plants were grown in
a solution containing rubidium, electron dense accumulations appeared
in their "microvacuoles" (Fig. 2B). The darkly stained vesicles
were visible in micrographs, and in many instances their membranes
appear fusing with the plasmalemma (Fig. 2C). Taken together these data
indicated that salts initially accumulate within the vacuoles prior to
secretion by vesicle fusion. More recent evidence has demonstrated the
vacuole as the site of salt accumulation and the existence of ion
antiports, pumps, and channels at the tonoplast of plant cells
(Blumwald and Gelli, 1997).
Vesicle Involvement in Organic Acid and Suc Secretion
Two studies involving the salt and organic acid-secreting
trichomes of chickpeas (Cicer arietinum; Lazzaro and
Thomson, 1992a, 1992b, 1996) and the mobilizing parenchyma cells of red
beet (Beta vulgaris) hypocotyls (Echeverría and
Achor, 1999) offer additional evidence in support of a direct vesicular
transport of solutes from the vacuole to the plasmalemma. In the
multicellular trichome of chickpea, the highly acidic secretion
contains a variety of solutes including malic, oxalic, hydrochloric
acids, and calcium ions. With a pH of 0.5 to 1.0, the secreted solution
must be compartmentalized, otherwise it could cause irreparable damage
to normal cytosolic operations. Using x-ray microanalysis, fluorescent
dye lucifer yellow-CH, non-toxic levels of lanthanum,
antimonate-calcium precipitation, and confocal and
Nomarski-differential interference contrast microscopy, Lazzaro and
Thomson (1996) identified the intracellular location of secreted
solutes to be the vacuole. The vacuole of chickpea trichome cells,
however, consists of an unusual network of tubular compartments that
extend along the entire multicellular structure. This vacuolar
continuum traverses through plasmodesmata and
terminates in the secreting head cells. Calcium deposits were also
localized in the numerous smaller "vesicles" located at the
periphery of secretory head cells, some of which fuse with the
plasmalemma. Figure 3A demonstrates the
proliferation of small "vacuole-like" vesicles proximal to the
secreting cell wall, calcium deposits within the small vacuoles (Fig.
3B), and the fusion of a vesicle with the plasmalemma (Fig. 3C). It is
pertinent to note at this point that tubular vacuoles are common in
filamentous fungi and are intimately involved in intracellular
transport (Cole et al., 1998).
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Further evidence for direct vesicle-mediated solute transport from the
vacuole comes from the mobilizing parenchyma cells of red beet
hypocotyls during resumption of growth (Echeverría and Achor,
1999). After a dormant over-wintering period, hypocotyl cells begin a
process of reserve mobilization to support the growth of leaves, roots,
and reproductive organs. Unlike previous examples of cell secretions,
mobilized solutes from storage parenchyma are not secreted to the plant
surface but rather are deposited in either the apoplast or symplast for
long distance transport. To supply their own metabolic demands, storage
cells possess a tonoplast-bound ATP-dependent Suc carrier
(Echeverría and Gonzalez, 2000) for the direct transport
of Suc from the vacuole to the cytosol.
Electron and light microscopic observations of mobilizing red beet hypocotyl parenchyma cells established the presence of an array of single membrane vesicles located scattered throughout the cytoplasm. In contrast to mobilizing cells, dormant cells possess a clear and homogeneous cytoplasm with no apparent vesiculation. Most vesicles in mobilizing cells ranged in size from one to 10 µm, although larger vesicles are frequently observed. When protoplasts from the same tissues were examined under light microscopy, the vesicles appeared red in color, revealing their content of the soluble pigment betacyanin, and more importantly, identifying them as vacuolar in origin (Fig. 4A). As the tonoplast vesicles become smaller, the intensity of the red color subsides appearing light pink in the smallest visible vesicles. The decrease in red intensity in the smaller vesicles is likely due to two factors: a decrease in depth area resulting from reduced volumes and/or changes in the content composition as the internal solution is modified prior to secretion. In electron micrographs, vesicles are seen fusing with the plasmalemma (Fig. 4B), and both membranes (tonoplast and plasmalemma) appear contiguous. Vesicle movement of vacuolar solutes is advantageous in that large amounts of metabolites can be transported while being protected from enzymatic attack by cytosolic enzymes.
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Vesicle Involvement in the Movement of Solutes from Guard and Motor Cells
In a recent analysis on the mechanisms of ion uptake into vacuoles
of N. translucens, MacRobbie (1999) carefully
described kinetic irregularities that could not be reconciled by a
process of single ion transport at the plasmalemma and tonoplast. The anomalies in ion transport are more consistent with a transport process
of salt-filled vesicles to and from the vacuole than with a static
system of protein carriers mediating single ion molecules. MacRobbie
(1999) speculated that the same mechanism may be responsible for the
rapid loss of solutes from the vacuole of guard cells and the motor
cells responsible for the slow sleep movements of leaf pulvini in
Samanea sp. and the fast leaflet closures of M. pudica (the sensitive plant). In M. pudica, leaf movement follows a mechanical disturbance of
the sensitive leaflets and the ensuing propagation of the stimulus to
the pulvinus. Upon its arrival to the pulvinus, the action potential
initiates a cascade of events starting with a massive loss of solutes
from the vacuole and the cell, followed by structural collapse, and
finally leaf closure. Although initially thought to involve ion fluxes
alone, work by Fromm and Eschrich (1988) showed that stimulation of the
action potential in motor cells also involves release of Suc. Suc
release seems to be involved with turgor lost from the stomatal guard cells as well. The simultaneous release of ions and Suc strongly suggest a common transport mechanism difficult to explain by movement through membrane channels alone. In all cases, motor cells cycle between a turgid state with one single vacuole (open stomata and open
leaf blades) and a shrunken state of much reduced vacuolar volume with
many small vacuoles (during stomatal closure and folded leaf blades).
Microscopic observations revealing the presence of microvacuoles during
the dramatic shrinkage of M. pudica pulvini associated with excitation (Weintraub, 1951) argues in favor of a
vesicle-mediated solute transport from the vacuole.
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TRANSPORT OF SOLUTES OF COMBINED VACUOLAR AND NON-VACUOLAR ORIGIN |
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INTRODUCTION
DIRECT TRANSPORT OF VACUOLAR...
TRANSPORT OF SOLUTES OF...
OPPOSING VIEWS
POSSIBLE ALTERNATIVES
CONCLUSIONS AND FUTURE...
LITERATURE CITED
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Vesicle Transport in Nectar Secretion
Evidence for a direct vesicle-mediated transport system between
the vacuole and the plasmalemma is less apparent from cytological studies in sugar-secreting nectary cells, although in a few instances claims of such have appeared (Findlay and Mercer, 1971). Instead of a
direct transport route of tonoplast vesicles from the vacuole to the
plasmalemma, a more complex vesicle-mediated system is envisioned when
one integrates the extensive cytological and anatomical observations of
secreting nectaries with our current knowledge of Suc accumulation into
intracellular compartments (Leigh, 1997). Nectaries are multicellular
surface glands specialized in secreting sugar solutions often
containing different amounts of other hydrophilic substances such as
oligosaccharides, amino acids, organic acids, ions, vitamins, etc.
(Fahn, 1979; Caldwell and Gerhardt, 1986). The high concentration of
the secreted sugar and the differences in concentration between the
nectar and the phloem sap (Pate et al., 1985) indicate that at some
point selective transport has occurred against a concentration
gradient. As in salt-secreting glands, prenectar enters the
secretory cells by the symplast (Sawidis, 1991). During
nectar secretion, the cellular architecture of the nectar-secreting
cells is overwhelmingly dominated by an extensive network of ER (Fig.
5). This observation has been reported in nearly all species examined including Tropaeolum majus
(Rachmilevitz and Fahn, 1975), Achillea millefolium
(Figuereido and Pais, 1994), and Cucurbita pepo (Nepi et
al., 1996). Vesicles apparently derived from the edges of the ER
cisternae are believed to be associated with the secretion process as
they frequently fuse with the plasmalemma (Fig. 5). Autoradiographic
evidence showing accumulation of supplied radiolabeled sugars in the ER
(Fahn and Rachmilevitz, 1975), ion accumulation in the
"secretory reticulum" (Robards and Oates, 1986), and the
conspicuous swelling of ER cisternae during secretion (Christ and
Schnepf, 1985; Fahn, 1988) support the contention that the ER is the
site of sugar location prior to secretion (Sawidis, 1987).
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In accordance with our present understanding of intracellular
compartmentation, the vacuole constitutes the site for sugar accumulation within the cell (Leigh, 1997). However, vacuoles are not
prominent in nectar-secreting cells and only sparsely present in
micrographs. If we assume sugars are to be concentrated in the small
vacuoles and the secretion later modified in the ER, a sequential
multi-step vesicle transport system must occur in which different
nectar components are sequentially added as solutes pass from one
organelle to another and finally excreted by ER or dictyosomes-derived
vesicles. Although not yet reported for higher plants, retrograde
transport from the vacuole to the endosomal compartment has been
demonstrated in yeast and from the lysosome to ER in animal cells
(Bryant et al., 1998). An alternative route for nectar secretion could
occur if different components are secreted separately by vesicles
arising from several sources and subsequently mixed after being
secreted to the exterior (Bosabalidis and Thomson, 1984).
The difficulties encountered in analyzing a dynamic process such as
nectar secretion by two-dimensional microscopy is reflected in the lack
of a coherent view concerning the involvement of secretory vesicles and
their exact origin. In this context, the observations of Verbelen and
Tao (1998) should be given special consideration. Using confocal laser
scanning microscopy, they demonstrated the presence of conspicuous
ripples along the vacuole surface, which often gave the appearance of
ER. The unusual ripples resemble tubules with a width of approximately
2 µm and lengths ranging from 5 to over 100 µm. Their comparison of
fixed material to that made under differential interference microscopy
made apparent that the arrays of tubular/spherical structures observed
in vivo were disrupted upon fixation and became profusely vesiculated. In considering these findings, it is possible that the structures identified as ER in the many nectaries may well be corrugated vacuoles.
Alternatively, the ER may have the capacity to accumulate sugars in
special circumstances such as in nectary cells, although this
possibility has not yet been reported.
Vesicle Secretions of Sugars and Adhesive Polysaccharides by Mistletoe Seeds
Developing mistletoe (Phthirusa pyrifolia) seeds present a peculiar example of tonoplast vesicle involvement in intracellular transport. Mistletoe seeds are surrounded by viscin, an elastic, mucilaginous and sticky tissue that adheres the falling seeds to branches and stems. The characteristic elastic seed walls are synthesized from large amounts of polysaccharides secreted by the wall cells. In addition, the cells secrete adhesive polysaccharides and sugars that function as attractants of dispersal agents. Active viscin cells contain abundant ER cisternae and Golgi bodies. Both structures are seen at the periphery of the cells and in close proximity to the distinctively large vacuoles. The fibrillar character of the vacuolar contents and their positive staining with toluidine blue reveal that at least some of the vacuolar secretions are pectin in nature. The pectic nature of the vacuolar contents suggests the inclusion of secretions originating from the Golgi and ER and the possible coalescing of their vesicles with the larger vacuoles. Figure 6 shows two vacuoles (approximately 5 µm in diameter) merging with the plasmalemma and releasing their contents to the apoplastic space.
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OPPOSING VIEWS |
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TOP
INTRODUCTION
DIRECT TRANSPORT OF VACUOLAR...
TRANSPORT OF SOLUTES OF...
OPPOSING VIEWS
POSSIBLE ALTERNATIVES
CONCLUSIONS AND FUTURE...
LITERATURE CITED
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Despite all the cytological and anatomical evidence in support of
a vesicle-mediated system for solute transport between the vacuole and
the plasmalemma and the apparent feasibility of such system, strong
objections to this process have been raised (Hill and Hill, 1976;
Kronestedt-Robards and Robards, 1992). These objections are based
primarily on the incompatibility between the rates of membrane turnover
and the volumes secreted by nectaries and gland cells. For example, the
rates of vesicle fusion necessary to sustain the volumes of nectar
secreted by Abutilon striatum trichomes range from
750,000 vesicles s1 (for 50-nm vesicles) to 750 vesicles s1 (for 500-nm vesicles;
Kronestedt-Robards and Robards, 1992). These values are many orders of
magnitude higher than reported values of 650 to 850 vesicles
s1 for oat coleoptile and of 50 to 85 vesicles
s1 for Tradescantia
virginiana-germinating pollen tubes (Picton and Steer,
1981). In A. striatum trichome cells, the estimated rates of vesicle fusion necessary to accommodate the levels of secreted
nectar would result in the turnover of the entire plasmalemma every
minute. This rate is 10-fold higher than rates determined for
nonexpanding secretory cells of one cell surface turnover every 10 min
(Steer, 1988). In Strelitzia reginae flowers (bird of
paradise), however, rates of vesicle fusion with the plasmalemma during
nectar secretion are much lower, ranging from 3,400 to 3.4 vesicles
s1 per cell for 50- and 500-nm vesicles,
respectively (Kronestedt and Robards, 1987).
The above calculations are based on the assumption that the
concentration of secreted metabolites (mostly sugars) are identical in
the vesicles as in the secreted nectar, implying that the volume of
nectar is entirely secreted by vesicle fusion. This may not be the case
if secreted solutes are more concentrated and movement of water follows
osmotically. Water movement following the highly concentrated salt and
sugar secretion could reduce the requirement for high rates of vesicle
fusion necessary to reach the high volumes of exudate (Hill and Hill,
1976).
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POSSIBLE ALTERNATIVES |
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TOP
INTRODUCTION
DIRECT TRANSPORT OF VACUOLAR...
TRANSPORT OF SOLUTES OF...
OPPOSING VIEWS
POSSIBLE ALTERNATIVES
CONCLUSIONS AND FUTURE...
LITERATURE CITED
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Although there is compelling anatomical and cytological evidence
supporting the existence of a vesicle-mediated transport system between
the vacuole and the plasmalemma, in view of the lack of adequate
physiological and biochemical data we cannot ignore the likelihood that
some calculated rates of secretion are impossibly high to be accounted
for by vesicle transport to the plasmalemma. Furthermore,
vesicle-mediated transport to the plasmalemma would involve a constant
recycling of membrane to accommodate the increasing amount of
vesicle fusion. Some alternative mechanisms of vesicle-mediated
secretion delivery to the plasmalemma may account for the
inconsistencies with the abnormally high rates of membrane fusion. The
proposed routes for vacuolar nectar secretion in A. striatum (Robards and Stark, 1988) and for the salt glands of L. vulgare (Vassilyev and Stepanova, 1990) are
appealing in that the need for membrane recycling is substantially
reduced or totally eliminated. In A. striatum, rapid
freezing of the secretory trichomes followed by freeze-fracture
replication revealed an extensive "secretory reticulum" within all
the trichome cells. The membranous "secretory reticulum" appeared
closely associated with the plasmalemma. This and other observations
from similar freeze-substituted cells led Robards and Stark (1988) to
propose a mechanism in which the prenectar solutes are actively
transported into the "secretory reticulum" of all trichome cells.
The ensuing increase in hydrostatic pressure induces the opening of
"sphincters" that connect the cysternal space of the reticulum with
the plasmalemma. Following the opening of the sphincter, the contents
are released in a process reminiscent of the "water expulsion
vacuoles" in zoospores of Oomycetes sp. (Mitchell
and Hardham, 1999). Opening and resealing of vesicle pores after
content discharge has been reported for mouse mast cells (Alvarez de
Toledo et al., 1993) and for protoplasts from maize coleoptile (Thiel
et al., 1998).
The model presented by Vassilyev and Stepanova (1990) is based on the
lack of visible membrane fusion between secretory "vacuolar vesicles" and the plasmalemma in secreting glands of L. vulgare. In their proposal, "secretory vesicles" dock
onto the plasmalemma without fusion (Fig.
7, A and B). The two membranes (tonoplast and plasmalemma) form "junctional complexes" where channels on both
membranes connect allowing passage of solutes from the vesicle to the
exterior. In the proposed system, active accumulation of solutes occurs
across the tonoplast and into the "micro-vacuoles." From the
vacuoles to the periplasmic space, transport occurs by diffusion
through channels in the contacting area of the tonoplast and
plasmalemma (Fig. 7C). Very similar secretory systems have been
demonstrated for muscle cells engaged in Ca+2
release in which the empty vesicle undergoes a process of recycling several times (Murthy and Stevens, 1998). For nonexpanding plant cells,
a mechanism for cargo discharge without membrane incorporation (such as
those described above) would be useful in view of the energy costs
involved in generating endocytotic recovery vesicles in the
presence of high turgor pressure (Gradman and Robinson, 1989).
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CONCLUSIONS AND FUTURE PERSPECTIVES |
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TOP
INTRODUCTION
DIRECT TRANSPORT OF VACUOLAR...
TRANSPORT OF SOLUTES OF...
OPPOSING VIEWS
POSSIBLE ALTERNATIVES
CONCLUSIONS AND FUTURE...
LITERATURE CITED
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Based on our present knowledge, it is difficult to draw firm
conclusions regarding the transport of solutes from the vacuole to the
plasmalemma. However, from the available information, it is possible to
visualize the likelihood of a vesicle-mediated system for solute
transport from the vacuole to the plasmalemma independent of vesicle
fusion (Fig. 8). This system would facilitate the bulk movement
of solutes to the exterior of the cell whether to be transported to
distant organs, excreted to the plant surface, or simply temporarily
deposited in the apoplast. It would seem that mobilized reserves are
delivered more efficiently to the apoplast in protected compartments.
Protection of transported solutes at high concentrations would be
advantageous since deleterious effects are avoided and labile solutes
are protected from enzymatic attack. Transport of specific solutes from
the vacuole most likely involves the selective transformation of the
milieu, assuming the vacuole contents are homogeneously distributed. In
this respect, the tonoplast pumps, carrier and channel proteins may
play a dominant role. Alternatively, cells may contain different
vacuoles with specific organ-related functions as already demonstrated
(Swanson et al., 1998). The use of modern techniques of biochemistry,
microscopy, and cell biology and the availability of plants with
mutations in the transport process should add substantial information
needed to create a definite picture of intracellular transport systems originating from the vacuole.
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ACKNOWLEDGMENTS |
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I thank Drs. Michael Salvucci, Bruce Darby, and James Syvertsen for their critical review of the manuscript. I also thank Brandon D. Hockema for his artwork.
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FOOTNOTES |
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Received January 10, 2000; accepted March 15, 2000.
1 This paper is Florida Agricultural Experiment Station Journal Series No. R-07286.
* E-mail eje{at}lal.ufl.edu; fax 863-956-4631.
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LITERATURE CITED |
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TOP
INTRODUCTION
DIRECT TRANSPORT OF VACUOLAR...
TRANSPORT OF SOLUTES OF...
OPPOSING VIEWS
POSSIBLE ALTERNATIVES
CONCLUSIONS AND FUTURE...
LITERATURE CITED
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-mannosidase, class I chitinase and class I -1,3-glucanase in suspension cultures of tobacco cells.
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W. Van den Ende, S. Clerens, R. Vergauwen, D. Boogaerts, K. Le Roy, L. Arckens, and A. Van Laere Cloning and functional analysis of a high DP fructan:fructan 1-fructosyl transferase from Echinops ritro (Asteraceae): comparison of the native and recombinant enzymes J. Exp. Bot., March 1, 2006; 57(4): 775 - 789. [Abstract] [Full Text] [PDF]
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E. Etxeberria and P. Gonzalez Evidence for a tonoplast-associated form of sucrose synthase and its potential involvement in sucrose mobilization from the vacuole J. Exp. Bot., May 1, 2003; 54(386): 1407 - 1414. [Abstract] [Full Text] [PDF]
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