Download An Important Pool of Sucrose Linked to Starch Biosynthesis is Taken

Plant Cell Physiol. 47(4): pcj011–00 (2006)
doi:10.1093/pcp/pcj011, available online at www.pcp.oupjournals.org
JSPP © 2006
An Important Pool of Sucrose Linked to Starch Biosynthesis is Taken up by
Endocytosis in Heterotrophic Cells
Edurne Baroja-Fernandez 1, Ed Etxeberria 2, Francisco José Muñoz 1, María Teresa Morán-Zorzano 1,
Nora Alonso-Casajús 1, Pedro Gonzalez 2 and Javier Pozueta-Romero 1, *
1
Agrobioteknologia Instituta, Nafarroako Unibertsitate Publikoa, Gobierno de Navarra and Consejo Superior de Investigaciones Científicas,
Mutiloako etorbidea zenbaki gabe, 31192 Mutiloabeti, Nafarroa, Spain
2
University of Florida, IFAS, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred, FL 33850, USA
;
trophic organ to attract photoassimilates is strongly dependent
on the capacity of individual cells to import, metabolize and
store sucrose, ultimately determining plant productivity and
crop yield (Ho 1988).
In organs where apoplastic sucrose unloading occurs, different routes can be envisaged for the subsequent uptake into
storage cells: (i) cleavage of sucrose by an apoplastic invertase
and subsequent uptake of glucose and fructose (Shannon 1972,
Lalonde et al. 1999, Fernie et al. 2000, Oliveira et al. 2002,
Weschke et al. 2003); and/or (ii) import of sucrose by plasmalemma-bound carriers (Tegeder et al. 1999, Barker et al. 2000,
Noiraud et al. 2000, Weschke et al. 2000, Manning et al. 2001,
Aoki et al. 2002, Kühn et al. 2003) (Fig. 1 and Supplementary
Fig. 1). In addition, and as evidenced by recent studies using
citrus juice cells and suspension-cultured cells of sycamore
(Acer pseudoplatanus L.), sucrose can also be taken up by
endocytosis and transported to the central vacuole (Etxeberria
et al. 2005a, Etxeberria et al. 2005b). Subsequent conversion of
internalized sucrose to starch involves a series of enzymatic
reactions wherein sucrose synthase (SuSy) (EC 2.4.1.13) predominantly controls sucrose breakdown and production of a C6 starch precursor molecule entering the amyloplast (Keeling et
al. 1988, Appeldoorn et al. 1997, Hatzfeld and Stitt 1990,
Baroja-Fernández et al. 2003). Because of its importance in the
sucrose–starch conversion process, SuSy is considered to be a
major determinant of sink strength in heterotrophic organs
(Geigenberger and Stitt 1993, Martin et al. 1993, Fu and Park
1995, Zrenner et al. 1995, Ruan et al. 2003)
In addition to its role as the major transported assimilate,
there is now compelling evidence that sucrose is an important
mobile signal that regulates a variety of different genes and
processes in various plant organs (Chiou and Bush 1998,
Sokolov et al. 1998, Lalonde et al. 1999, Barker et al. 2000,
Smeekens 2000, Yang et al. 2004). In heterotrophic cells, for
instance, numerous genes and metabolic pathways involved in
starch biosynthesis are induced or stimulated by sucrose (Fu et
al. 1995, Geiger et al. 1998, Müller-Röber et al. 1990, Fernie et
al. 2001), as is the endocytic system for sucrose uptake
(Etxeberria et al. 2005a).
The existence of two entry points for sucrose into storage
cells may represent the channeling of sucrose to different meta-
We have recently shown the occurrence of endocytic
sucrose uptake in heterotrophic cells. Whether this mechanism is involved in the sucrose–starch conversion process
was investigated by comparing the rates of starch accumulation in sycamore cells cultured in the presence or absence
of the endocytic inhibitors wortmannin and 2-(4-morpholynyl-)-8-phenyl-4H-1 benzopyran-4-1 (LY294002). These
analyses revealed a two-phase process involving an initial
120 min wortmannin- and LY294002-insensitive starch
accumulation period, followed by a prolonged phase that
was arrested by the endocytic inhibitors. Both wortmannin
and LY294002 led to a strong reduction of the intracellular
levels of both sucrose and the starch precursor molecule,
ADPglucose. No changes in maximum catalytic activities of
enzymes closely linked to starch and sucrose metabolism
occurred in cells cultured with endocytic inhibitors. In
addition, starch accumulation was unaffected by endocytic
inhibitors when cells were cultured with glucose. These
results provide a first indication that an important pool of
sucrose incorporated into the cell is taken up by endocytosis
prior to its subsequent conversion into starch in heterotrophic cells. This conclusion was substantiated further by
experiments showing that sucrose–starch conversion was
strongly prevented by both wortmannin and LY294002 in
both potato tuber discs and developing barley endosperms.
Keywords: Acer pseudoplatanus — Hordeum vulgare — Sink
strength — Solanum tuberosum, Sucrose–starch conversion —
Sucrose synthase.
Abbreviations: LatB, latrunculin B; LY, Lucifer yellow-CH;
LY294002, 2-(4-morpholynyl-)-8-phenyl-4H-1 benzopyran-4-1; SuSy,
sucrose synthase.
Introduction
The arrival of sucrose at terminal sink organs is a dynamic
process that can take a symplastic and/or an apoplastic route
depending on the type of organ and developmental state
(Patrick 1997). In either case, the overall ability of the hetero*
Corresponding author: E-mail, [email protected]; Fax: +34-948232191.
1
2
Endocytic sucrose uptake and starch biosynthesis
Fig. 1 Suggested metabolic pathways for the conversion of apoplastic sucrose to starch in heterotrophic cells involving plasmalemma-bound
carriers as well as vesicle-mediated sucrose transport between the apoplast and the vacuole (Etxeberria et al. 2005a). Apoplastic sucrose can be
converted into glucose and fructose by means of apoplastic invertase. Hexoses enter the cell by plasmalemma-bound carriers and/or diffusion
(Stubbs et al. 2004), and are converted into sucrose (Shannon 1972, Geiger et al. 1998, Fernie et al. 2000, Fernie et al. 2002). Alternatively,
sucrose can be taken up from the apoplast by means of plasmalemma-bound sucrose carriers and/or endocytosis to be transported to the cytosol or
to the vacuole, respectively. Vacuolar sucrose can be exported to the cytosol by means of a tonoplast-bound sucrose transporter (Getz 1991, Keller
1992). Cytosolic sucrose is then converted into ADPglucose and fructose by means of soluble SuSy or membrane-bound SuSy (Amor et al. 1995,
Carlson and Chourey 1996, Baroja-Fernández et al. 2003, Etxeberria and Gonzalez 2004, Muñoz et al. 2005). According to this suggested model,
SuSy catalyzes the production of cytosolic ADPglucose entering the amyloplast by means of an ADPglucose translocator (Pozueta-Romero et al.
1991, Shannon et al. 1996, Shannon et al. 1998, Patron et al. 2004), whereas ADPglucose pyrophosphorylase is involved in the scavenging of
starch breakdown products (Pozueta-Romero et al. 1999, Baroja-Fernández et al. 2004, Baroja-Fernández et al. 2005, Muñoz et al. 2005). Membrane recycling takes place essentially as described by Echeverria (2000). A model involving the production of UDPglucose from sucrose by
SuSy and its stepwise conversion into hexose phosphates and ADPglucose is illustrated in Supplementary Fig. 1. Whether sucrose occurring in
vesicles can be transported to the cytosol by means of hypothetical membrane-bound sucrose carriers is being currently investigated.
bolic events, with both mechanisms contributing separately to
the overall sink strength. As a first step to investigate to what
extent endocytic sucrose uptake is involved in the control of
both sink strength and starch biosynthesis in heterotrophic
cells, we employed the well characterized sycamore cultured
cells (Rébeillé et al. 1985, Frehner et al. 1990, Pozueta-Romero
et al. 1991, Etxeberria et al. 2005a), potato (Solanum
tuberosum L.) tuber slices (Viola and Davies 1991, Geiger et
al. 1998, Fernie et al. 2001) and developing barley (Hordeum
vulgare) endosperms. In addition, we employed the potent
endocytic inhibitors wortmannin (Emans et al. 2002) and 2-(4morpholynyl-)-8-phenyl-4H-1 benzopyran-4-1 (LY294002)
(Vlahos et al. 1994), which do not exert any inhibitory effect
on electrogenic sucrose transport (Etxeberria et al. 2005a), and
latrunculin B (LatB). The results presented in this work provide
a first indication that an important pool of apoplastic sucrose
must be taken up by endocytosis prior to its subsequent conversion into starch in heterotrophic organs of both mono- and
dicotyledonous plants.
Endocytic sucrose uptake and starch biosynthesis
3
Results and Discussion
Starch accumulation is strongly reduced by endocytic inhibitors
in sycamore cells cultured with sucrose
By employing both sycamore cultured cells and citrus
juice cells, as well as the potent endocytic inhibitors wortmannin and LY294002, we recently provided strong evidence that
endocytosis is an important mechanism for sucrose uptake in
heterotrophic cells (Etxeberria et al. 2005a). Time course analyses of sucrose accumulation in walled cells revealed a twophase process involving an initial 120 min period of carriermediated sucrose uptake, followed by a prolonged phase of
rapid sucrose accumulation that was greatly inhibited by the
endocytic inhibitors wortmannin and LY294002 (cf. Fig. 1,
Etxeberria et al. 2005a).
To investigate whether sucrose taken up by endocytosis is
involved in the starch biosynthetic process, a time course analysis of starch accumulation was performed using cultured cells
of sycamore previously starved for 24 h prior to re-introduction of sucrose both in the presence and in the absence of either
wortmannin or LY294002. The rationale behind this experimental approach was that, if endocytic uptake of sucrose is
unimportant in the overall sucrose–starch conversion process,
the rate of starch accumulation in cells cultured in the presence
of endocytic inhibitors would not be affected when compared
with control cells. Conversely, if sucrose linked to starch biosynthesis enters via endocytosis, cells cultured in the absence
of endocytic inhibitors will accumulate starch more rapidly
than cells cultured in the presence of endocytic inhibitors.
As illustrated in Fig. 2, the rate of starch accumulation in
cells cultured with 50 mM sucrose was approximately 3 µmol
of glucose transferred to starch g–1 FW during the initial 90 min
of culture, followed by an approximately 30 min asymptotic
phase of low starch accumulation. Similar to the time course
pattern of sucrose incorporation in sycamore cells (cf. Fig. 1,
Etxeberria et al. 2005a), starch accumulation during this initial
120 min culture period was insensitive to endocytic inhibitors,
indicating that sucrose uptake linked to the initial stages of
starch biosynthesis in starved cells does not take place by endocytosis but presumably by plasmalemma-bound carriers. Then
starch accumulation entered a second prolonged phase where
starch accumulated at a rate of 1.5 µmol glucose transferred to
starch g–1 FW h–1. This phase was strongly repressed by endocytic inhibitors, the rate of starch accumulation declining to
250 nmol glucose transferred to starch g–1 FW h–1 in the presence of either 500 nM wortmannin or LY294002. The overall
data thus suggest that, after a 2 h culture period, an important
pool of sucrose linked to starch biosynthesis enters the cell via
endocytosis in starved cells. Strongly substantiating these
observations was the sharp reduction in the rate of starch accumulation when wortmannin was added after 90 min of culture,
a time when sucrose starts entering the cell by endocytosis (cf.
Fig. 1, Etxeberria et al. 2005a).
Fig. 2 Time course of the starch accumulated by sycamore cells cultured with 50 mM sucrose in the absence (filled squares) or in the
presence of either 500 nM wortmannin or 500 nM LY294002. Open
squares and open circles represent starch accumulated by cells grown
in the continuous presence of wortmannin and LY294002, respectively. Open triangles represent starch accumulated by cells to which
wortmannin was added after 90 min of culture. In every case, 4-dayold cultured cells were placed in starving medium for 24 h prior to
addition of sucrose. After sucrose addition, cell aliquots were taken at
the indicated times, washed, extracted in ethanol, and analyzed for
starch content. Values represent the difference between starch content
at the indicated culture time and the starch content at zero time of culture. Starch content at zero time was 12.5 µmol glucose g–1 FW.
Results are given as the mean ± SE of four analyses per time.
Endocytic inhibitors lead to reduced intracellular levels of
ADPglucose, but not of hexose phosphates
We measured the intracellular levels of metabolites
involved in the metabolism of both starch and sucrose in cells
cultured with sucrose both in the presence and in the absence of
500 nM LY294002. As shown in Table 1, analyses of the intracellular levels of sugars after 5 h of culture revealed no significant effect of LY294002 on the levels of hexoses and hexose
phosphates. Subsequent time course analysis confirmed that
LY294002 does not exert any significant effect on the intracellular levels of hexoses and hexose phosphates during 6 h of
culture (not shown). In clear contrast, however, intracellular
levels of sucrose, starch, UDPglucose and the starch precursor
molecule ADPglucose in cells cultured with LY294002 were
significantly lower than those of control cells.
We then performed a time course analysis of sucrose accumulated and intracellular ADPglucose content in cells previously starved for 24 h prior to re-introduction of sucrose both
in the presence and in the absence of LY294002. As illustrated
in Fig. 3, this analysis revealed that, similarly to sucrose, ADPglucose accumulation follows a two-phase process involving
an initial 120 min LY294002-insensitive period, followed by a
prolonged phase of ADPglucose accumulation which was
inhibited by LY294002. This result strongly indicates that
ADPglucose biosynthesis is intimately linked to the metabolism of sucrose taken up by endocytosis. Essentially similar
4
Endocytic sucrose uptake and starch biosynthesis
Table 1 Metabolite levels in sycamore cells cultured with
50 mM sucrose in the presence and in the absence of 500 nM
LY294002
Metabolite
(nmol g–1 FW)
ADPglucose
UDPglucose
Glucose-6-phosphate
Glucose-1-phosphate
Fructose-6-phosphate
Glucose
Fructose
Accumulated sucrose
Starch
Accumulated starch
Control
Treatment
+LY294002
8.0 ± 0.2
481 ± 20.9
343 ± 20.5
126 ± 20.0
120 ± 18.9
3,880 ± 410
5,300 ± 1,170
19,600 ± 940
22,500 ± 1,600
8,600 ± 530
5.0 ± 0.8
401 ± 7.0
338 ± 19.9
152 ± 21.0
95 ± 23.5
3,400 ± 480
4,980 ± 1,270
10,800 ± 720
15,700 ± 1,400
1,900 ± 470
Cells were harvested after 5 h of culture. The results are given as the
mean ± SE of extracts from four cultures. Metabolite levels are given
as nmol g–1 FW. Starch content and starch accumulated after 5 h of
culture are given as nmol glucose g–1 FW. ‘Accumulated starch’ is the
difference between starch content after 5 h of culture and the starch
content at zero time. Starch content at zero time (upon addition of
sucrose to the culture medium) was 13,900 ± 1,300 nmol glucose
g–1 FW. ‘Accumulated sucrose’ is the difference between sucrose
content after 5 h of culture and the sucrose content at zero time. Sucrose
content at zero time was 9,000 nmol g–1 FW. Values significantly
different from those of control cells are indicated in bold.
results were obtained when investigating the effect of
LY294002 on the intracellular levels of UDPglucose. However, as illustrated in Supplementary Fig. 2, reduction of
UDPglucose levels by LY294002 was less pronounced than
that of ADPglucose intracellular levels.
The fact that hexose phosphate levels are normal in cells
cultured in the presence of LY294002, whereas sucrose, ADPglucose, UDPglucose and starch levels are significantly lower
in cells cultured in the presence of LY294002 than in control
cells indicates that (i) metabolism of sucrose taken up by endocytosis is closely connected to that of both ADPglucose and
UDPglucose, most probably by means of SuSy; (ii) the hexose
phosphate pool is not directly connected to that of sucrose
taken up by endocytosis, but most probably it is connected to
the pool of sucrose and/or hexoses taken up by plasmalemmabound carriers; and (iii) hexose phosphates are not intermediates in the sucrose–starch conversion process (BarojaFernández et al. 2003). In addition, the fact that the effect of
LY294002 on the UDPglucose levels was less pronounced than
on ADPglucose levels suggests that the UDPglucose pool is
not solely connected to sucrose taken up by endocytosis, but
also to hexose phosphates, presumably by means of UDPglucose pyrophosphorylase (EC 2.7.7.9).
Fig. 3 Time course of (A) accumulated sucrose and (B) intracellular
ADPglucose content in sycamore cells cultured with 50 mM sucrose in
the absence (filled squares) or in the presence (open circles) of 500 nM
LY294002. ‘Accumulated sucrose’ is the difference between sucrose
content at the indicated culture time and the sucrose content at zero
time of culture. Sucrose content at zero time was 9 µmol g–1 FW. The
time course of UDPglucose is shown in Supplementary Fig. 2. Experimental conditions are the same as those of Fig. 2. Results are given as
the mean ± SE of four analyses per time point.
Reduction of both ADPglucose levels and rate of starch accumulation by endocytic inhibitors is not accompanied by
changes in activities of enzymes directly connected to starch
and sucrose metabolism
Whether reduction of both ADPglucose and starch levels
by inhibitors of endocytic sucrose uptake was a secondary
effect of arrested enzyme activities was investigated by measuring maximum catalytic activities of a broad range of
enzymes closely connected to starch and sucrose metabolism.
As shown in Table 2, these analyses revealed no significant
changes in ADPglucose pyrophosphorylase (EC 2.7.7.27),
alkaline pyrophosphatase (EC 3.6.1.1), UDPglucose pyrophosphorylase, phosphoglucomutase (EC 2.7.5.1), SuSy,
sucrose phosphate synthase (EC 2.3.1.14), acid invertase (EC
3.2.1.26), total starch synthase (EC 2.4.1.21), starch phosphorylase (EC 2.4.1.1) and total amylolytic activity between cells
cultured in the presence or absence of wortmannin and
Endocytic sucrose uptake and starch biosynthesis
5
Table 2 Enzyme activities in sycamore cells after 5 h of culture with 50 mM sucrose in the presence and in the absence of 500 nM of endocytic inhibitors
Enzyme
Control
+ Wortmannin
+ LY294002
ADPglucose pyrophosphorylase
UDPglucose pyrophosphorylase
Phosphoglucomutase
Total amylolitic activity
Starch phosphorylase
Acid invertase
Starch synthase
ADPglucose producing SuSy
Alkaline pyrophosphatase
Sucrose phosphate synthase
27.8 ± 1.4
131.8 ± 11.5
350 ± 15
3,070 ± 363
26.3 ± 1.4
317 ± 22
10.8 ± 0.9
392 ± 22
1,857 ± 170
1,640 ± 111
29.3 ± 1.0
124 ± 5.9
382 ± 29
3,030 ± 346
22.0 ± 3.1
289 ± 18
9.1 ± 1.4
353 ± 41
1,669 ± 209
1,192 ± 214
23.1 ± 1.3
117 ± 6.9
320 ± 28.9
3,101 ± 422
30.9 ± 3.7
301 ± 36
10.1 ± 1.1
401 ± 23
1,844 ± 61
1,834 ± 214
The results are given as the mean ± SE of extracts from four cultures per treatment. Enzyme activities are
given in mU g–1 FW.
LY294002. Therefore, the reduction of both ADPglucose and
starch levels in cells cultured with sucrose in the presence of
wortmannin and LY294002 is ascribable to the inhibition of the
endocytic uptake of sucrose and not to a reduction in the activities of enzymes of sucrose–starch conversion.
Starch accumulation from glucose is unaffected by endocytic
inhibitors
Glucose enters the heterotrophic cell by either carriermediated transport, diffusion or endocytosis (Stubbs et al.
Fig. 4 Time course of the starch accumulation in sycamore cells cultured with 50 mM glucose in the absence of endocytic inhibitors (filled
squares) or in the presence of either 500 nM wortmannin (open
squares) or 500 nM LY294002 (open circles). Four-day-old cultured
cells were placed in starving medium for 24 h prior to addition of glucose. After glucose addition, cell aliquots were taken at the indicated
times, washed, extracted in ethanol and analyzed for starch content.
Values represent the difference between starch content at the indicated
culture time and the starch content at zero time. Starch content at zero
time was 12.5 µmol glucose g–1 FW. Results are given as the mean ±
SE of four analyses per time point.
2004, Etxeberria et al. 2005c). Subsequent conversion of the
internalized glucose to starch involves a series of enzymatic
reactions in the cytosol and the amyloplast which are common
to those involved in the sucrose–starch conversion (Geiger et
al. 1998, Sokolov et al. 1998, Fernie et al. 2001). To investigate further whether reduction of both ADPglucose and starch
levels by inhibitors of endocytic sucrose uptake was a secondary effect of arrested gluconeogenic machinery, and to investigate whether an important pool of glucose destined to starch
production is also taken up by endocytosis, we compared rates
of starch accumulation in cells cultured with glucose both in
the presence and in the absence of either 500 nM wortmannin
or LY294002.
As illustrated in Fig. 4, sycamore cells cultured with glucose accumulated starch at a rate comparable with cells cultured with sucrose (Fig. 2). In addition, the glucose–starch
conversion process was unaffected by the endocytic inhibitors,
indicating that, in contrast to sucrose, the bulk of glucose destined to starch biosynthesis enters the cell by plasmalemmabound carriers and/or diffusion (Stubbs et al. 2004). Furthermore, the data confirm that endocytic inhibitors do not negatively affect the enzymatic machinery involved in starch
biosynthesis.
Rate of starch accumulation is highly reduced by endocytic
inhibitors in potato tuber discs and developing barley endosperms
Having shown that a large proportion of apoplastic
sucrose linked to starch biosynthesis is taken up by endocytosis in sycamore cells, we proceeded to investigate whether
this process also occurs in other species. Towards this end, we
performed a time course analysis of starch production in both
potato tuber discs and developing barley endosperms cultured
with radiolabeled sucrose in the presence and in the absence of
endocytic inhibitors as indicated in Materials and Methods.
6
Endocytic sucrose uptake and starch biosynthesis
tion of apoplastic sucrose unloading has been underlined by the
facts that (i) expression of a yeast invertase in the apoplasm of
potato tubers is accompanied by important metabolic changes
(Sonnewald et al. 1997, Fernie et al. 2000); and (ii) overexpression of sucrose transporters in both developing potato
tubers and pea cotyledons results in marked shifts in metabolite levels and/or growth (Rosche et al. 2002, Leggewie et al.
2003).
Fig. 5 Time course of the starch accumulation in developing barley
endosperms and potato tuber discs cultured with 100 mM [U14
C]sucrose (5×105 dpm/µl), in the absence of endocytic inhibitors
(filled squares) or in the presence of either 500 nM wortmannin (open
squares) or 500 nM LY294002 (open triangles). Aliquots were taken at
the indicated times, washed, extracted in ethanol, and incorporated
radioactivities were measured as described in Materials and Methods.
Results are given as the mean ± SE of four analyses per time point.
As illustrated in Fig. 5, control rates of starch accumulation in developing barley endosperms and potato tuber discs
were 65 and 5 nmol glucose transferred to starch g–1 FW h–1,
respectively. Remarkably, and essentially identically to the case
of cultured cells of sycamore (Fig. 2), rates of starch accumulation were markedly reduced when endocytic inhibitors were
included in the culture medium. The overall data are thus a first
indication that a sizable pool of sucrose linked to starch biosynthesis is taken up by endocytosis in different heterotrophic
organs of both mono- and dicotyledonous species.
Our results are consistent with the idea that cells incorporate sucrose from the apoplasm for its subsequent conversion
into starch. Although there is evidence that sucrose unloading
into parenchyma cells of some sink tissues proceeds symplastically (Patrick 1997, Viola et al. 2001), an important contribu-
Additional remarks
Whereas much has been written about the enzymatic pathways involved in both sink strength and the sucrose–starch
conversion process in heterotrophic organs, the possible
involvement of different mechanisms of apoplastic sucrose
transport into the cell has not been considered as part of the
overall regulatory system. Our data offer the first direct evidence indicating that, essentially as illustrated in Fig. 1, and as
inferred from the results of Table 1 and Fig. 2 and 5 showing
that endocytic inhibitors prevent the sucrose–starch conversion
process, apoplastic sucrose linked to starch biosynthesis enters
the cell through two entirely different routes: by means of
plasmalemma-bound carriers and by endocytosis. That both
wortmannin and LY294002 strongly inhibit the sucrose–starch
conversion process (Fig. 2) is consistent with previous reports
showing that, although apoplastic glucose can be converted
into starch (Geiger et al. 1998) (Fig. 4), cleavage of sucrose by
apoplastic invertase and subsequent import of glucose into the
cell plays a minor role in the overall starch biosynthetic process
of some heterotrophic organs (Ross et al. 1994, Appeldoorn et
al. 1997, Hajirezaei et al. 2000).
The results presented in this work are consistent with the
view that, at least in some heterotrophic organs, SuSy predominantly controls sucrose breakdown and production of precursor(s) for starch biosynthesis (Zrenner et al. 1995), whereas
apoplastic invertase catalyzes the hydrolytic breakdown of
sucrose necessary for processes such as phloem unloading, cell
growth and differentiation, osmoregulation, defense response,
etc. (Miller and Chourey 1992, Sonnewald et al. 1997,
Tauberger et al. 1999, for a review see Sturm and Tang 1999).
In addition, and in agreement with the idea that apoplastic sucrose may act as a signal that regulates carbohydrate metabolism (Lalonde et al. 1999, Smeekens 2000, Fernie et al. 2001),
combined data of Fig. 2 and 4 strongly indicate that conversion of apoplastic glucose into starch is prevented by sucrose.
It is important to point out that, as discussed by Etxeberria
et al. (2005a), endocytic uptake of apoplastic sucrose and its
subsequent conversion to starch does not conflict with transport
through membrane-bound carriers. Our data using starved sycamore cells indicate that carrier-mediated sucrose import may
play a crucial role in the overall sucrose–starch conversion
process during the initial period of sucrose unloading (i.e.
during the initial part of the day), whereas endocytic uptake
may play an important role in transporting the bulk of sucrose
to the vacuole after a prolonged period of sucrose unloading.
Endocytic sucrose uptake and starch biosynthesis
7
Tonoplast-bound sucrose carriers (Keller 1992) and SuSy
(Etxeberria and Gonzalez 2004) would subsequently mediate
export of vacuolar sucrose and conversion to starch intermediates. Whether endocytic vesicles are equipped to export
sucrose to the cytosol is still a matter of conjecture, which is
presently under investigation.
Repression of starch biosynthesis by inhibitors of endocytic sucrose uptake was not limited to compounds interfering
with phosphatidylinositol 3-kinase such as wortmannin and
LY294002 (Li et al. 1995, Brunskill et al. 1998). The F-actindepolymerizing drug LatB, which blocks the internalization of
fluid-filled endocytic vesicles (Baluska et al. 2004), markedly
reduced the endocytic uptake of both Lucifer yellow- CH (LY)
(Fig. 6A) and sucrose (Fig. 6B) in starved sycamore cells. Most
importantly, and as illustrated in Fig. 6C, sucrose–starch conversion was repressed after 2 h of culture with 5 µM LatB. The
overall data thus further strengthen the view that a sizable pool
of sucrose linked to starch biosynthesis is taken up by endocytosis.
The fact that endocytic inhibitors lead to reduced levels
of UDPglucose (Table 1 and Supplementary Fig. 2) appears
to indicate that, in addition to its involvement in the sucrose–
starch conversion process, endocytic sucrose uptake may be
an important determinant of UDPglucose-dependent gluconeogenic processes such as synthesis of cell wall polysaccharides, glycoproteins, etc. (Amor et al. 1995, Muñoz et al.
1996, Buckeridge et al. 1999, Delmer 1999, Ruan et al. 2003,
Konishi et al. 2004). Further studies will be necessary to
confirm this idea.
Materials and Methods
Fig. 6 LatB inhibits the endocytic uptake of sucrose linked to starch
biosynthesis. (A) Time course of incorporation of the endocytic
marker LY in sycamore cells cultured in the absence (filled squares) or
in the presence (open circles) of 5 µM LatB. (B) Time course of
sucrose incorporation in sycamore cells cultured in the absence (filled
squares) or in the presence (open circles) of 5 µM LatB. (C) Time
course of the starch accumulated by sycamore cells cultured in the
absence (filled squares) or in the presence (open circles) of 5 µM
LatB. Cells were cultured in starving medium prior to the simultaneous addition of both 50 mM sucrose and 1 mM LY. Aliquots were
taken at the indicated times and protoplasts were prepared for LY
measurement. Results are given as the mean ± SE of three analyses per
time point.
Plant material
Sycamore (A. pseudoplatanus L) cells were cultured in a medium
supplemented with 50 mM sucrose as described by Frehner et al.
(1990) in continuously agitated 250 ml flasks (200 rpm at 28°C). The
cells were thoroughly washed with ‘starving medium’ (culture
medium without sucrose) and placed back in 250 ml of the same starving solution. After 24 h starvation, cells were transferred to culture
medium containing either 50 mM sucrose or 50 mM glucose in the
presence or absence of 0.5 µM wortmannin (W-1628, Sigma-Aldrich,
St Louis, MO, USA) or 0.5 µM LY294002 (L-9908, Sigma-Aldrich).
Cell aliquots were taken at the indicated times, washed with starving
medium, filtered through Miracloth and ground to a fine powder in liquid nitrogen. Barley (cv. Scarlett) plants were grown in the fields of
the Public University of Nafarroa. Potato (cv. Desirée) plants were cultivated in growth chambers in individual pots under a 16 h light
(21°C)/8 h dark (16°C) regime.
Enzyme assays
All enzymatic reactions were performed at 37°C. Cultured cells
of sycamore were harvested, washed with starving medium, filtered
through Miracloth and immediately freeze-clamped and ground to a
fine powder in liquid nitrogen with a pestle and mortar. To assay
enzyme activity, 1 g of the frozen powder was resuspended at 4°C in
5 ml of 100 mM HEPES (pH 7.5), 2 mM EDTA and 5 mM dithiothreitol (DTT). The suspension was then desalted and assayed for
8
Endocytic sucrose uptake and starch biosynthesis
enzymatic activities. The following enzymes were assayed as
described by Baroja-Fernández et al. (2004): ADPglucose pyrophosphorylase, ADPglucose producing SuSy, acid invertase, UDPglucose pyrophosphorylase, alkaline pyrophosphatase, total starch
synthase, starch phosphorylase, total amylolytic activity, phosphoglucomutase and sucrose phosphate synthase. One unit of enzyme activity is defined as the amount of enzyme that catalyzes the production of
1 µmol of product min–1.
Determination of sugars
Cultured cells of sycamore were harvested and immediately
ground to a fine powder in liquid nitrogen with a pestle and mortar. A
0.5 g aliquot of the frozen powder was resuspended in 0.4 ml of 1.4 M
HClO4, left at 4°C for 2 h and centrifuged at 10,000×g for 5 min. The
supernatant was neutralized with K2CO3 and centrifuged at 10,000×g.
ADPglucose content in the supernatant was determined by using either
one of the following methods: (i) assay A by HPLC on a system
obtained from P.E. Waters and Associates fitted with a Partisil-10SAX column as described by Sweetlove et al. (1996); or (ii) assay B
by HPLC with pulsed amperometric detection on a DX-500 system
(Dionex) fitted to a CarboPac PA10 column.
To confirm further that measurements of ADPglucose were correct, ADPglucose eluted from either the Partisil-10-SAX or CarboPac
PA10 columns was enzymatically hydrolyzed with purified Escherichia
coli ADP-sugar pyrophosphatase (Moreno-Bruna et al. 2001) and
assayed for conversion into AMP and glucose-1-phosphate as
described by Muñoz et al. (2005).
Glucose, sucrose, fructose, glucose-1-phosphate, fructose-6phosphate and glucose-6-phosphate were determined by HPLC with
pulsed amperometric detection on a DX-500 system as described by
Muñoz et al. (2005). Starch was extracted and measured by using an
amyloglucosydase-based test kit as described by Baroja-Fernández et
al. (2004).
Labeling experiments
Tuber discs (diameter of 8 mm, thickness of 2 mm) were cut
directly from growing tubers attached to the mother plant and washed
three times in a 20-fold excess of medium containing 10 mM HEPESKOH buffer (pH 7.0) and 100 mM sucrose (Viola and Davies 1991).
Both tuber discs and barley endosperms at nearly 15 d after pollination (eight discs and 30 endosperms in a volume of 5 ml in a 100 ml
Erlenmeyer flask) were incubated in medium including [U-14C]sucrose
(5×105 dpm/µl) (Amersham, UK), shaken at 90 rpm, harvested at the
indicated incubation periods, washed three times in abundant medium
to remove unabsorbed sucrose and debris, frozen in liquid nitrogen and
homogenized in a mortar. The powder thus obtained was washed
thoroughly with abundant 75% methanol/1% KCl, and 14C incorporation into starch was determined as described by Viola and Davies
(1991). In all cases, >90% of the radiolabel released after digesting
starch with amyloglucosydase was recovered as glucose following
HPLC separation.
Uptake of LY
Four-day-old walled sycamore cells were starved for 24 h. At
this time, the culture medium was replaced with fresh medium containing 50 mM sucrose and 1 mg/ml LY (Molecular Probes, Eugene,
OR, USA) both in the presence and in the absence of 5 µM LatB (Calbiochem, Germany). Aliquots were taken at the indicated times and
protoplasts prepared as described by Frehner et al. (1990). From the
protoplast preparation, aliquots were taken and LY was analyzed
fluorometrically in a Fluoroskan Neonat (Labsystems, Hampshire,
UK) with an excitation/emission of 425/550 nm, respectively.
Supplementary material
Supplementary material mentioned in the article is available to
online subscribers at the journal website www.pcp.oupjournals.org.
Acknowledgments
This research was partially supported by the grant BIO200401922 from the Comisión Interministerial de Ciencia y Tecnología and
Fondo Europeo de Desarrollo Regional (Spain). M.T.M.-Z. acknowledges the Spanish Ministry of Culture and Education for a pre-doctoral
fellowship. E.E. expresses his gratitude to the Spanish Ministry of
Culture and Education, to the Consejo Superior de Investigaciones
Científicas and to the Public University of Nafarroa for their support.
We express our gratitude to Beatriz Zugasti for technical assitance and
help.
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(Received December 5, 2005; Accepted January 17, 2006)