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Plant Physiol. (1998) 116: 709-714
Phloem Transport of Fructans in the Crassulacean Acid Metabolism
Species Agave deserti1
Ning Wang2 and
Park S. Nobel*
Department of Biology, University of California, Los Angeles,
California 90095-1606
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ABSTRACT |
Four
oligofructans (neokestose, 1-kestose, nystose, and an un-identified
pentofructan) occurred in the vascular tissues and phloem sap of
mature leaves of Agave deserti. Fructosyltransferases (responsible for fructan biosynthesis) also occurred in the vascular tissues. In contrast, oligofructans and fructosyltransferases were
virtually absent from the chlorenchyma, suggesting that fructan biosynthesis was restricted to the vascular tissues. On a molar basis,
these oligofructans accounted for 46% of the total soluble sugars in
the vascular tissues (sucrose [Suc] for 26%) and for 19% in the
phloem sap (fructose for 24% and Suc for 53%). The Suc concentration
was 1.8 times higher in the cytosol of the chlorenchyma cells than in
the phloem sap; the nystose concentration was 4.9 times higher and that
of pentofructan was 3.2 times higher in the vascular tissues than in
the phloem sap. To our knowledge, these results provide the first
evidence that oligofructans are synthesized and transported in the
phloem of higher plants. The polymer-trapping mechanism proposed for
dicotyledonous C3 species may also be valid for
oligofructan transport in monocotyledonous species, such as A. deserti, which may use a symplastic pathway for phloem loading
of photosynthates in its mature leaves.
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INTRODUCTION |
Plant growth depends on the supply of photosynthates via the
phloem to sink organs. The process of photosynthate delivery from
photosynthetic cells to the phloem of source organs (phloem loading) is
an important determinant of such growth, and numerous studies have been
conducted to understand its mechanism (Giaquinta, 1983 ; van Bel, 1993).
In species for which Suc is the only transported sugar, phloem loading
may occur by co-transport of Suc and protons from the apoplast (i.e.
apoplastic loading; Giaquinta, 1983; van Bel, 1993; Grusak et al.,
1996). For other species, however, RFO and other carbohydrates in
addition to Suc are transported in the phloem (Turgeon et al., 1975;
Ziegler, 1975; Fisher, 1986; Flora and Madore, 1993). In many of these
species, such as Coleus blumei, Cucurbita pepo,
and Olea europaea, phloem loading is not sensitive to the
inhibitor of active Suc transport,
p-chloromercuriphenylsulfonic acid; therefore, such a
mechanism of apoplastic phloem loading may not apply to such species.
Because the Suc concentration in the photosynthetic cells is often
lower than or similar to that in the phloem for these species,
symplastic phloem loading of Suc via plasmodesmata would also not
occur.
To resolve this dilemma of symplastic phloem loading, Turgeon (1991)
has proposed a polymer-trapping model for loading of Suc and RFO, in
which Suc in the photosynthetic cells diffuses via plasmodesmata down
its concentration gradient to the bundle-sheath cells and then to
intermediary cells (specialized companion cells), where raffinose and
stachyose are synthesized. Raffinose and stachyose then diffuse from
the intermediary cells to sieve tubes down their concentration
gradients but cannot diffuse back into bundle-sheath cells because the
channel size of plasmodesmata between intermediary cells and
bundle-sheath cells is too small for their passage. This
polymer-trapping model is supported by ultrastructural studies, the
concentration of RFO in the intermediary cells, immunolocalization of
enzymes responsible for RFO synthesis, and other physiological evidence
(van Bel, 1993; Haritatos and Turgeon, 1996). However, it is not clear
whether such a model can also apply to species that transport
oligosaccharides other than RFO (such as fructans) or to
monocotyledonous species.
Fructans are soluble polymers of Fru with a terminal Glc residue. They
function as the main storage carbohydrates in 15% of flowering plant
species, including many economically important crops (Pollock and
Cairns, 1991; Pilon-Smits et al., 1996; Wiemken et al., 1996). Fructans
also play roles in osmoregulation during drought (Spollen and Nelson,
1994; Wiemken et al., 1996) and can act as protectants against
dehydration imposed by drought or freezing (Wiem-ken et al., 1996).
Despite the important functions and wide distribution of oligofructans
in flowering plants, virtually no information is available concerning
oligofructan transport in higher plants.
Fructans are synthesized and stored in the stems of agaves (Aspinall
and Gupta, 1959; Dorland et al., 1977; Bhatia and Nandra, 1979). The
main function of fructans in the stems of such CAM plants is storage,
as for C3 and C4 plants,
and they may also act as osmoprotectants during drought. However, it is
unknown whether fructans occur in the phloem of agaves. When we studied source-sink photosynthate partitioning for the CAM species Agave deserti, preliminary observations indicated that oligofructans occur in the phloem sap and vascular tissues of mature leaves. Such findings raise important questions about where these oligofructans are synthesized and how they are loaded into the phloem of mature leaves. For example, is the phloem loading of photosynthate
apoplastic or symplastic?
The present study was initiated to determine the cellular site(s) for
oligofructan biosynthesis and distribution and to test whether the
polymer-trapping model (Turgeon, 1991) can also explain phloem loading
of oligofructans in mature source leaves of the monocotyledon A. deserti. Thus, the distribution of mono- and oligosaccharides
among different tissues of mature source leaves of A. deserti was examined to see whether oligofructans are specifically located in the phloem. The activities of enzymes responsible for fructan biosynthesis and hydrolysis were analyzed to determine the
site(s) of fructan biosynthesis. The concentration gradients of mono-
and oligosaccharides along the phloem-loading pathways from
photosynthetic cells to the sieve tubes/companion cells were estimated
to establish a possible loading mechanism for photosynthates in mature
leaves of A. deserti.
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MATERIALS AND METHODS |
Twenty plants of Agave deserti Engelm. (Agavaceae) with
10 to 12 unfolded leaves averaging 28 cm in length were maintained in
14-L pots in a greenhouse at the University of California, Los Angeles.
The mean total daily PPFD was 38 mol m 2
d1, corresponding to a mean instantaneous value
of 800 µmol m2 s1,
and the daily maximum/minimum air temperatures averaged 28/16°C, respectively (North and Nobel, 1995). The plants were watered twice
weekly with 0.1-strength Hoagland solution. Two months before the
experiments, the plants were transferred to Conviron E-15 environmental
growth chambers (Controlled Environments, Pembina, ND) with daily
maximum/minimum air temperatures of 25/15°C, respectively. The
photoperiod was 12 h, with a total daily PPFD of about 35 mol
m2 d1. The plants were
again watered twice weekly with 0.1-strength Hoagland solution. Such
conditions are near the optimum for the growth of A. deserti
(Nobel, 1988).
Tissue Harvest and Phloem Sap Collection
Mature leaves of A. deserti have chlorenchyma layers
about 1 mm thick on both the upper and lower surfaces, with 1 to 2 mm of water-storage parenchyma in between, which allows for ready separation and collection of these two tissues. To assess diurnal changes of carbohydrates and malate in the chlorenchyma and in the
water-storage parenchyma, 0.1 to 0.5 g of each tissue was harvested individually at various times of the day from the middle of
mature source leaves of A. deserti using a razor blade.
One-half of the harvested chlorenchyma or water-storage parenchyma was immediately frozen using dry ice and stored at  70°C, and the other
half was dried at 80°C for 48 h to determine water content.
To obtain the vascular tissues, which in mature leaves occur as
longitudinal veins extending from the leaf base to the tip without
direct lateral vascular connections, sections with a width of about 1 mm were cut transversely across the middle of mature leaves. These
sections, which contained the chlorenchyma, water-storage parenchyma,
and vascular tissues, were frozen on dry ice and then dehydrated in a
freeze-drying system (Labconco, Kansas City, MO) prior to storage at
70°C. A syringe needle of about 0.35 mm in diameter was inserted
into the vascular strand area, which is readily visible under a
stereomicroscope. Cores of vascular tissues, which were contaminated
with a small amount of chlorenchyma and water-storage parenchyma, were
collected by repeating such insertions and were weighed with a
microbalance (ATI Cahn, Boston, MA). The dissected vascular tissues
were stored at 70°C for measurements of metabolites and enzyme
activities.
Phloem sap was collected with severed stylets of the scale insect
Ovaticoccus californicus McKenzie, which naturally infests mature leaves of A. deserti, using a method similar to that
developed for Opuntia ficus-indica (Wang and Nobel, 1995).
Colonies of the insects that infested the middle region of mature
leaves were gently wiped away with tissue paper soaked in 80% ethanol,
leaving the severed stylets protruding from the leaf surface, and
mineral-oil-filled wells were constructed covering the areas containing
the severed stylets. Phloem sap was collected once every 2 to 3 h
to minimize microorganism contamination. In many cases, phloem sap
exuded naturally from the leaf surfaces that may have been previously punctured by the insects; such droplets of semidry phloem sap were also
collected.
Chemical Analysis of Metabolites
To extract solutes, the frozen samples of the chlorenchyma or
water-storage parenchyma were pulverized together with dry ice, ground
in 0.5 mL of methanol:chloroform:water (12:5:3, v/v), and extracted at
25°C with 5 mL of distilled water. After the samples were heated to
95°C for 3 min to inactivate enzymes and then centrifuged, the
decanted supernatant was passed through C18
sample preparation cartridges (Alltech Associates, Deerfield, IL)
presaturated with distilled water to remove lipophilic materials; the
eluate was further cleaned by passage through a 0.2-µm nylon filter.
The final filtrate was collected for metabolite analysis. To measure starch content (Wang and Nobel, 1996), the insoluble portions of tissue
extract were resuspended and then centrifuged three times with
methanol:chloroform:water (12:5:3, v/v) and twice with distilled water
to remove pigments, lipids, and remaining solutes. The remaining
precipitates were mixed with 2 mL of distilled water and then heated at
100°C for 2 h to suspend the starch, which was hydrolyzed with
amyloglucosidase (EC 3.2.1.3) at 55°C overnight; the released Glc was
quantified using Glc oxidase (EC 1.1.3.4; Sturgeon, 1990). Soluble
proteins in the chlorenchyma or water-storage parenchyma were also
extracted at 0 to 2°C, according to procedures developed for O. ficus-indica (Wang and Nobel, 1996).
The metabolites in the dissected vascular tissues of freeze-dried
samples were extracted at 60°C with 0.2 mL of HPLC grade water for 30 min. The supernatant extracted from the sections was then passed
through 50 µL of the C18 sample-preparation
cartridge material and a 0.2-µm nylon filter. Soluble proteins in the
vascular tissues were extracted at 0 to 2°C with 0.2 mL of 50 mm Mops-KOH (pH 7.5), 2 mm DTT, 2 mm EDTA, 20 mg mL1
polyvinylpolypyrrolidone (insoluble), 0.5% (v/v) Triton X-100, and 1 mm PMSF (Wang and Nobel, 1996).
Oligofructans and other sugars were separated by HPLC at 24.0 ± 0.5°C using a Microsorb Amino column (Rainin Instrument, Emeryville, CA) with acetonitrile:water (70:30, v/v) as the mobile phase and were
detected by differential refractometry (Frehner et al., 1984), which
can separate oligofructans up to a DP of 8. Individual sugars and
fructans were identified and quantified by comparing them with known
concentrations of Fru, Glc, Suc, 1-kestose, and nystose (1-kestose and
nystose were obtained from TCI America, Portland, OR; other reagents
were from Sigma) and with published standard chromatograms (Cairns and
Pollock, 1988; Pollock and Lloyd, 1994). The malate concentration was
quantified spectrophotometrically (Wang and Nobel, 1996) using malic
dehydrogenase (EC 1.1.1.37). The soluble proteins were quantified by
the method of Bradford (1976) using BSA as a standard, with a slight
modification for microscale analysis. The osmolality of the phloem sap
collected under mineral oil was measured with a vapor pressure
osmometer (model 5500, Wescor, Logan, UT), and the concentrations of
sugars, oligofructans, and malate of the phloem sap were determined as described above.
Measurement of Enzyme Activities
To measure fructosyltransferase activity, 0.5 to 1.0 g of frozen
chlorenchyma or 50 mg of dissected vascular tissues was ground at 0 to
2°C with 1 to 2 mL of 50 mm citric acid, 5 mm
DTT, 5 mm ascorbic acid, 2 mm EDTA, and 1 mm PMSF at pH 5.5 (Lüscher and Nelson, 1995). After
the sample was centrifuged, ammonium sulfate was added to 35%
saturation to the decanted supernatant for 15 min to precipitate
proteins. After the sample was centrifuged a second time, the
supernatant was decanted and ammonium sulfate was added to 60%
saturation for 20 min to precipitate fructosyltransferases. Again, the
sample was centrifuged and the enzyme pellet was resuspended in 50 to
100 µL of the solution used for extraction and dialyzed overnight at
2 to 4°C against 20 mm His (pH 5.5). After dialysis, 20 µL of the dialyzed protein solution, free of sugars, was transferred to microcentrifuge tubes containing 80 µL of 25 mm Mes
(pH 5.5) and 125 mm Suc or 63 mm 1-kestose. The
sample was incubated at 27°C for 2 to 3 h, and the reaction was
stopped by heating to 90°C for 3 min. The sample was centrifuged
again, proteins in the supernatant were removed with a membrane filter,
and the solution was injected onto an HPLC column to separate and
quantify mono- and oligosaccharides. The fructosyltransferase activity
was calculated as the amount of oligofructans produced per minute per
milligram of protein.
To measure the activity of fructan hydrolases, about 20 mg of semidry
phloem exudate collected without using mineral oil wells was dissolved
in 20 µL of 50 mm Mes (pH 5.5) and dialyzed at 2 to 4°C
overnight against the Mes buffer. After dialysis, the desalted phloem
exudate was transferred to microcentrifuge tubes containing 80 µL of
50 mm Mes (pH 5.5) and 63 mm 1-kestose or
nystose. After the sample was incubated at 30°C for 3 h (Pontis,
1990), the reaction was stopped by heating to 90°C for 3 min. The
sample was centrifuged, proteins in the supernatant were removed by
passage through a membrane filter, and the solution was analyzed by
HPLC to separate and quantify mono- and oligosaccharides. The activity
of fructan hydrolases was calculated as the amount of Fru, Suc, and
1-kestose released per minute per milligram of protein.
Suc synthase (EC 2.4.1.13) and acid and alkaline invertases (EC
3.2.1.26) in the chlorenchyma and the vascular tissues were extracted
with 5 mm DTT, 2 mm EDTA, 5 mg
mL1 BSA, and 1 mm PMSF, and their
activities were determined as for O. ficus-indica (Wang and
Nobel, 1996). For phloem sap, the enzyme activities were determined
after the sap was dialyzed against 50 mm Mops-KOH (pH 7.2)
plus 2 µm leupeptin and 1 mm PMSF. The activity of alkaline invertase was measured by quantifying the amount
of Glc released after 50 µL of the enzyme solution (extracted from
the chlorenchyma or the vascular tissues) and dialyzed phloem sap was
mixed with 100 µL of 50 mm Mops-KOH (pH 7.5) plus 150 mm Suc at 25°C for 30 min. All data are presented as
means ± se (n = 4 plants, unless
specified otherwise).
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RESULTS |
Suc was the predominate sugar in the chlorenchyma of mature green
(source) leaves of A. deserti and only one fructan occurred, neokestose (DP 3), which was barely detectable (Fig.
1A). On a molar basis, Suc accounted for
66%, Glc plus Fru accounted for 32%, and neokestose accounted for
only 2% of total soluble sugars in the chlorenchyma (Table
I). Neokestose was also only barely detectable in the water-storage parenchyma (Fig. 1B). On a molar basis,
it accounted for only 3% of total soluble sugars for the water-storage
parenchyma, whereas Suc accounted for 44%. The concentrations of
individual sugars, particularly Suc, and the osmolality were lower in
the water-storage parenchyma than in the chlorenchyma; also, total
soluble sugars accounted for 22% of the osmolality in the chlorenchyma
and 9% in the water-storage parenchyma (Table I).
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| Figure 1.
HPLC profiles of mono- and oligosaccharides in the
various tissues (A, chlorenchyma; B, water-storage parenchyma; and C,
vascular) and in the phloem sap (D) of mature leaves of A. deserti. F, Fru; G, Glc; and S, Suc. Numbers indicate the DP:
3a, neokestose; 3b, 1-kestose; 4, nystose; and 5, an unidentified
pentofructan.
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Table I.
Concentrations of mono- and oligosaccharides in
different tissues and in the phloem sap of mature leaves of A. deserti
Samples were harvested 4 to 5 h after the beginning of the light
period. Data are means ± se; n = 4 plants.
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At least four oligofructans occurred in the vascular tissues of mature
leaves of A. deserti in addition to the sugars Glc, Fru, and
Suc (Fig. 1C). On a molar basis, neokestose, 1-kestose (DP 3), nystose
(DP 4), and an unidentified pentofructan together accounted for 46% of
total soluble sugars in such tissues, almost twice as high as the Suc
concentration (Table I). Total soluble sugars in the vascular tissues
accounted for 45% of the osmolality. These four oligofructans also
occurred in the phloem sap of mature leaves (Fig. 1D), where they
accounted for 19% of total soluble sugars; Fru accounted for 24%, and
Suc accounted for 53% (Table I). Total soluble sugars in the phloem
sap accounted for about 78% of its osmolality. The concentration of
Suc was 3.6 times lower, that of nystose was 4.9 times higher, and that
of pentofructan was 3.2 times higher in the vascular tissues than in
the phloem sap (Table I).
Suc and malate concentrations changed diurnally in a reciprocal manner.
Specifically, the Suc concentration in the chlorenchyma gradually
increased during the daytime, reaching a maximum of 112 mm
at dusk, and then gradually decreasing to a minimum of 45 mm immediately after darkness (Fig.
2A). The malate concentration in the
chlorenchyma gradually decreased throughout the daytime, reaching a
minimum of 30 mm, and then increased to a maximum of 247 mm at the end of the night (Fig. 2B).
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| Figure 2.
Daily changes of Suc concentration (A) and malate
concentration (B) in the chlorenchyma of A. deserti.
Hatched bars indicate nighttime. The data are means ± se for n = 4 plants.
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The activity of fructosyltransferases was not detected in the
chlorenchyma but was substantial in the vascular tissues (Table II). On a total soluble protein basis,
the activity of fructan hydrolases was 18 times higher in the phloem
sap compared with that of fructosyltransferases in the chlorenchyma.
The total soluble protein content in mature leaves averaged 0.097 ± 0.010 mg g1 phloem sap (n = 4 plants) compared with 5.40 ± 0.02 mg g1
fresh weight for the chlorenchyma (n = 5 plants). Thus,
on a fresh weight basis, the fructan hydrolase activity was about 3 times higher in the chlorenchyma than in the phloem sap, accounting for
less than 5% of the Suc hydrolyzed in the phloem. For the chlorenchyma, Suc synthase is the major enzyme responsible for cytosolic Suc breakdown, because its activity was about 3.5 times higher than that of acid invertase plus alkaline invertase (Table II).
No acid or alkaline invertase activity was detected in the phloem sap.
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DISCUSSION |
Fructan biosynthesis was apparently restricted to the vascular
tissues in mature leaves of A. deserti. In particular,
fructans were virtually absent from the chlorenchyma and the
water-storage parenchyma but accumulated in the vascular tissues.
Coincident with the high concentrations of oligofructans in the
vascular tissues was the substantial activity of fructosyltransferases (responsible for fructan biosynthesis), whereas fructosyltransferase activity was not detected in the chlorenchyma. The fact that
oligofructans accumulated only in the vascular tissues and were nearly
absent from both the chlorenchyma and the water-storage parenchyma
suggests that back diffusion of these fructans from the vascular
tissues (such as from phloem parenchyma cells) to the
photosynthetic cells is extremely low despite a substantial
concentration gradient, consistent with the polymer-trapping hypothesis
(Turgeon, 1991).
Suc was the major carbon source for nocturnal malate production in
mature leaves of A. deserti. The nocturnal decrease of Suc
was 66 mm, which sustained the nocturnal malate production of 217 mm (equivalent to 54 mm Suc, because one
Suc can be used to synthesize four malates; Carnal and Black, 1989).
Nocturnal production of other organic acids, such as citric acid (Kluge and Ting, 1978), can also utilize Suc, suggesting that little extra Suc
is available for fructan synthesis, consistent with the small amount of
fructans in the chlorenchyma.
The particular cell type in the vascular tissues that is responsible
for fructan biosynthesis in mature leaves of A. deserti is
unclear. In Turgeon's polymer-trapping model the sites for the
synthesis of RFO in Coleus blumei and Cucurbita
pepo (dicotyledonous species using the C3
photosynthetic pathway) are intermediary cells (specialized companion
cells with numerous plasmodesmata connected to the bundle-sheath cells;
Turgeon, 1991). For A. deserti, a monocot using the CAM
pathway, intermediary cells as found in dicots are unlikely. Moreover,
for the minor veins of C. blumei and C. pepo, the
bundle-sheath cells are directly associated with the intermediary cells
(Turgeon et al., 1975; Fisher, 1986), but no such bundle-sheath cells
were observed in the vascular strands of A. deserti (N. Wang
and P.S. Nobel, unpublished observations). Therefore, the sites for
oligofructan synthesis and accumulation in mature leaves of A. deserti may be phloem parenchyma cells or other types of vascular
cells to be identified in the future.
The fructans in the phloem sap of A. deserti were apparently
not from contamination with microorganisms. When the phloem sap of
O. ficus-indica is contaminated with microorganisms, the activity of invertases often lead to equal amounts of Glc and Fru
after Suc is hydrolyzed (N. Wang and P.S. Nobel, unpublished observations). Also, 6-kestose, a common end product of acid invertase action on oligofructans (Cairns, 1993; Lüscher and Nelson, 1995), was virtually absent from the phloem sap of A. deserti,
consistent with the lack of invertase activity there. On a whole-tissue
basis, the Suc concentration in the chlorenchyma (mesophyll cells)
averaged about 70 mm over 24 h. The volume of cytosol
in mesophyll cells is about 5% of the total cell volume for
C3 and CAM species (Lüttge et al., 1982;
Winter et al., 1994; Haritatos et al., 1996), and about 26% of the
cell Suc pool is in the cytosol for Suc-storage species (Winter et al.,
1994) such as A. deserti, the mature leaves of which
contained virtually no starch (N. Wang and P.S. Nobel, unpublished
observations). Thus, the Suc concentration in the cytosol of mesophyll
cells of mature leaves of A. deserti could be about 350 mm, about 1.8 times higher than in the phloem sap, similar
to the 1.5 times higher Suc concentration in the cytosol of mesophyll
cells in mature leaves of Cucumis melo than in their sieve
tubes/intermediary cells (Haritatos et al., 1996).
The vascular tissues of A. deserti collected with a fine
syringe needle were always contaminated with small amounts of
chlorenchyma and water-storage parenchyma (which had little detectable
oligofructans); therefore, the concentration of oligofructans in
individual phloem parenchyma cells or other types of vascular cells is
higher than the average measured concentration in the vascular tissues.
If we assume that the total soluble sugars in the phloem parenchyma cells or other types of vascular cells also accounted for about 78% of
the osmolality, just as for the phloem sap, sugar concentrations in
these cells can be estimated from the sugar concentrations in the
vascular tissues by multiplying by 1.75 (78/45%). Based on such a
calculation, sugar concentration gradients along the phloem-loading
pathway from the chlorenchyma to the sieve tubes/companion cells can be
approximated. In particular, Suc, 1-kestose, nystose, a pentofructan,
and Glc could diffuse from the chlorenchyma or from phloem parenchyma
cells or other types of vascular cells into the sieve tubes/companion
cells, whereas the situation for Fru is unclear because the Fru
concentration in the cytosol of chlorenchyma cells in mature leaves of
A. deserti is unknown. In any case, such findings are in
contrast with those for plants with apoplastic loading pathways, such
as Zea mays (Bush, 1993), in which Suc is the only sugar in
the phloem sap (Ohshima et al., 1990). However, such findings are
similar to those from plants with symplastic loading pathways,
including the CAM species Xerosicyos danguyi, in which
substantial amounts of oligosaccharides, such as raffinose and
stachyose, are present in the companion cells in addition to Suc
(Madore et al., 1988; van Bel, 1993).
Therefore, like the cucurbit X. danguyi, A. deserti may use a polymer-trapping mechanism (Turgeon, 1991) for
symplastic loading of photosynthates in its mature leaves. Suc in
photosynthetic cells may diffuse via plasmodesmata to vascular
parenchyma cells and then to phloem parenchyma cells or other types of
vascular cells. After Suc enters such cells, it may be converted by
fructosyltransferases to 1-kestose and higher DP fructans such as
nystose and pentofructan. Because the channel size of plasmodesmata
between the vascular parenchyma and the phloem parenchyma cells or
other types of vascular cells may pass Suc but restrict the diffusion
of oligofructans such as 1-kestose, nystose, and a pentofructan,
oligofructan concentrations can increase in the phloem parenchyma cells
or other types of vascular cells. These oligofructans may then diffuse
from such cells to the sieve tubes/companion cells via
plasmodesmata. To our knowledge, this provides the first evidence
that oligofructans are synthesized and transported in the phloem of
mature leaves of higher plants. The polymer-trapping mechanism that
operates in dicotyledonous C3 species may also be
valid for oligofructan transport in monocotyledonous species such as
A. deserti.
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FOOTNOTES |
1
This research was supported by the Office of
Health and Environmental Research, U.S. Department of Energy, Program
for Ecosystem Research (grant no. DE-FG03-93ER61686).
2
Present address: DuPont Central Research and
Development, Experimental Station, P.O. Box 80328, Wilmington, DE
19880-0328.
*
Corresponding author; e-mail psnobel{at}biology.ucla.edu; fax
1-310-825-9433.
Received August 1, 1997;
accepted October 30, 1997.
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ABBREVIATIONS |
Abbreviations:
DP, degree of polymerization.
RFO, raffinose-family oligosaccharides.
| |
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Top
Abstract
Introduction