Plant Physiol. (1999) 119: 267-276
Modification of Carbon Partitioning, Photosynthetic Capacity, and
O2 Sensitivity in Arabidopsis Plants with Low
ADP-Glucose Pyrophosphorylase Activity1
Jindong Sun,
Thomas W. Okita, and
Gerald E. Edwards*
Institute of Biological Chemistry (J.S., T.W.O., G.E.E.),
Department of Botany (J.S., G.E.E.), Washington State University,
Pullman, Washington 99164
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ABSTRACT |
Wild-type Arabidopsis plants, the
starch-deficient mutant TL46, and the near-starchless mutant TL25 were
evaluated by noninvasive in situ methods for their capacity for net
CO2 assimilation, true rates of photosynthetic
O2 evolution (determined from chlorophyll fluorescence
measurements of photosystem II), partitioning of photosynthate into
sucrose and starch, and plant growth. Compared with wild-type plants,
the starch mutants showed reduced photosynthetic capacity, with the
largest reduction occurring in mutant TL25 subjected to high light and
increased CO2 partial pressure. The extent of stimulation
of CO2 assimilation by increasing CO2 or by
reducing O2 partial pressure was significantly less for the starch mutants than for wild-type plants. Under high light and moderate
to high levels of CO2, the rates of CO2
assimilation and O2 evolution and the percentage inhibition
of photosynthesis by low O2 were higher for the wild type
than for the mutants. The relative rates of
14CO2 incorporation into starch under high
light and high CO2 followed the patterns of photosynthetic
capacity, with TL46 showing 31% to 40% of the starch-labeling rates
of the wild type and TL25 showing less than 14% incorporation.
Overall, there were significant correlations between the rates of
starch synthesis and CO2 assimilation and between the rates
of starch synthesis and cumulative leaf area. These results indicate
that leaf starch plays an important role as a transient reserve, the
synthesis of which can ameliorate any potential reduction in
photosynthesis caused by feedback regulation.
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INTRODUCTION |
Because plant productivity is governed by photosynthetic activity
and sink activity for utilizing photosynthate (see Zamski and Schaffer,
1996
), it is important to understand the environmental and genetic
factors affecting these processes. In general, photosynthesis is
limited mainly by light harvesting and assimilatory power under low
light and by carboxylation and photorespiration under low CO2. Under saturating light and
CO2, however, photosynthesis may be controlled by
processes that convert triose-P into starch and Suc (Sage, 1990, 1994;
Stitt, 1996). Thus, the capacity to utilize triose-P for carbohydrate
synthesis can establish an upper limit for the maximum rate of
photosynthesis under CO2- and light-saturated conditions (Sage, 1990; Sharkey et al., 1995). This is clearly demonstrated under certain conditions by the response of photosynthesis of C3 plants to low O2. In
many instances, the increase in CO2 assimilation
attributable to the reduction in photorespiration under low
O2 can be predicted accurately based on the known
kinetic properties of Rubisco. However, when the extent of stimulation of photosynthesis by C3 plants under
subatmospheric O2 is less than predicted, or when
there is reversed O2 sensitivity, photosynthesis is considered to be feedback limited as a result of restrictions on
triose-P utilization (Sharkey, 1985; Leegood and Furbank, 1986; Sage
and Sharkey, 1987; Hanson, 1990; Sun et al., 1997). Limitations on
triose-P utilization have also been suggested to be responsible for the
low CO2 saturation response and decreased electron
transport under high light (Sharkey et al., 1988; Peterson and Hanson,
1991; Eichelmann and Laisk, 1994).
The bulk of the photosynthetically fixed carbon in mature leaves is
partitioned between Suc and starch. Based on experimental results and
biochemical models, the events controlling the partitioning of
photosynthate between Suc and starch synthesis occur in the cytoplasm
(Eichelmann and Laisk, 1994; Stitt, 1996). Hence, triose-P may be
converted preferentially into Suc at lower rates of triose-P production, with increased partitioning to starch synthesis occurring as Suc synthesis reaches saturation. Alternatively, carbon partitioning may be programmed so that a portion of the photosynthate is allocated for starch synthesis.
Efforts have been made to increase Suc production by the manipulation
of Suc-P synthase activity, which catalyzes one of the key regulatory
steps in the cytoplasm. Elevation of this enzyme activity by
overexpressing the maize Suc-P synthase in tomato leads to an increase
in Suc synthesis and in the rate of photosynthesis at high
CO2 and high light (Galtier et al., 1993, 1995;
Micallef et al., 1995). However, under some circumstances, the capacity for Suc synthesis may be restricted by limitations on phloem loading, transport, or unloading. Some transport studies have shown that the
export rate of photosynthate from leaves does not increase when plants
are shifted to a higher irradiance (Silvius et al., 1979) or to a
CO2-enriched environment (Ho, 1977; Huber et al., 1984).
A portion of the fixed carbon is also allocated to formation of starch
in many plants. AGPase is an important regulatory enzyme controlling
starch biosynthesis. Mutations reducing the activity of the enzyme lead
to starch deficiency, as demonstrated in Arabidopsis leaf
adg2 (TL46) and adg1 (TL25) mutants (Lin et al.,
1988a, 1988b) and maize endosperm shrunken-2 and
brittle-2 mutants (Tsai and Nelson, 1966; Dickinson and
Preiss, 1969). Neuhaus and Stitt (1990) reported that a reduction in
AGPase activity in leaves of the Arabidopsis mutant TL46 resulted
in a reduction in starch synthesis, but with differential effects on
Suc biosynthesis and photosynthesis, depending on the light intensity.
Under low-light conditions fixed carbon was partitioned mainly into
Suc, with no significant effect on photosynthesis between the mutant
and the wild type. Under high light, however, Suc synthesis, as well as
starch synthesis and photosynthesis, were inhibited in the mutant.
These studies were conducted with excised leaf tissue in an
O2 electrode chamber under saturating
CO2, conditions that are unnatural compared with those in situ.
In the present study a new approach was used to investigate the
photosynthetic properties of wild-type Arabidopsis and two starch
mutants, a starch-deficient mutant, TL46, and a near-starchless mutant,
TL25, using a special plant chamber/gas-exchange system that is capable
of measuring A and 
PSII values by
fluorescence measurements on intact plants (Donahue et al., 1997). With
this system, 14CO2 labeling
(and rates of partitioning into starch and Suc), A, and PSII
activity can be determined simultaneously under varying levels of
CO2 and O2. We present
evidence using noninvasive methods that partitioning of fixed carbon
into starch plays a far more prominent role in dictating the overall
photosynthetic potential than previously thought. Our finding suggests
that leaf starch serves as a transient reserve that can accommodate
relatively large increases in triose-P production by the C3
pathway, thereby preventing potential feedback of this primary
assimilatory process.
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MATERIALS AND METHODS |
Plant Growth
Plant materials used in this study were Arabidopsis Heynh cv
Columbia wild type, TL46, and TL25. These lines were obtained from the
Arabidopsis Repository at Ohio State University (Columbus). The mutant
TL46 contains a missense mutation of the adg2 gene, which
codes the large subunit structural gene of AGPase (Wang et al., 1997).
The mutant TL25 contains a mutation of the adg1 gene, which
codes the small subunit structural gene of AGPase (Lin et al., 1988b).
Plants were grown in controlled environmental growth chambers with a
12-h photoperiod and a PPFD of either 100 (low light) or 350 (high
light) µmol m
2 s1
provided by fluorescent lights. Day and night temperatures were 24°C ± 1°C and 18°C ± 1°C, respectively. RH in the
growth chambers was maintained above 70%.
For photosynthetic studies, the Arabidopsis plants were planted in a
soil mixture containing 60% peat, 20% pumice, 20% sand, and 4.8 kg
m3 agricultural lime in 50-mL polypropylene
centrifuge tubes (model 25325-50, Corning Inc., Corning, NY). The tubes
were painted a dark color to deter algae growth in the soil and
perforated at the bottom for water drainage. After 1 to 2 weeks, plants
were thinned to one plant per tube. Plants were irrigated daily with Hoagland solution (Hoagland, 1950). Healthy, 5- to 6-week-old plants
were used for experiments. Plants were used at the beginning of the
bolting stage so that the wild-type and mutant plants were at similar
developmental stages.
Enzyme Extraction and Assay
Arabidopsis leaves were collected about 2 h into the light
period and stored in liquid nitrogen until analysis. The leaves were
extracted and analyzed on the day they were sampled. The leaves were
extracted in buffer solution containing 50 mM Mops-NaOH, pH
7.5, 15 mM MgCl2, 1 mM
EDTA, 0.1% Triton X-100, 2.5 mM DTT, and 2.5% glycerol.
The homogenate was centrifuged in a microcentrifuge (model 235, Fisher
Scientific) at maximum speed (approximately 12,000g) for 3 min at 4°C. The supernatant was desalted through a small Sephadex
G-25 (superfine) column before assaying for AGPase activity, according
to the method of Sowokinos (1976). AGPase activities of the desalted
leaf extracts were assayed at 37°C in a reaction mixture containing
80 mM glycylglycine, pH 7.5, 5 mM MgCl2, 10 mM NaF, 2.5 mM DTT, 0.5 mM NADP, 1 unit of phosphoglucomutase, 1 unit of
Glc-6-P dehydrogenase, 2 mM ADP-Glc, 4 mM 3-PGA, and 1.5 mM sodium
pyrophosphate. The absorption of the formed NADPH at 340 nm (absorption
coefficient, 6.22 mM1) was
recorded without the addition of sodium pyrophosphate (control) and
with the addition of sodium pyrophosphate in a Perkin-Elmer 552A
spectrophotometer. Protein content was determined using the Bradford
procedure with BSA as the standard (Bradford, 1976).
Gas Exchange
Rates of CO2 assimilation on whole plants
were measured with a Bi-2-dp mini cuvette controller (Bingham, Hyde
Park, UT), an MK3-225 IR gas analyzer (ADC, Hoddesdon, Hertfordshire,
UK), and data were obtained with a linear chart recorder (Tekmar,
Cincinnati, OH). Gas exchange was measured by CO2
depletion in the differential mode with an open system in which a given
gas mixture flowed through the reference cell and the sample cell (in
line with the plant enclosed in a cuvette). The threaded tubes in which
plants were grown were inserted into a threaded port in the bottom of
the special laboratory-built leaf chamber (Donahue et al., 1997) and sealed with modeling clay. The gas-flow rate in the leaf chamber was 1 L min1. CO2 depletion in
the leaf chamber was about 10% to 15% under high light. The plant
cuvette contained a copper-constantan thermocouple, which was placed in
contact with the lower epidermis of a leaf to monitor plant
temperature. Water vapor leaving the chamber was measured with a
digital hygrometer (Fisher Scientific). The leaf chamber had a
temperature-controlled water jacket connected to a water bath. PPFD was
measured with a quantum sensor (model 185, Li-Cor, Lincoln, NE). RH was
maintained at 60% to 80% in the leaf chamber. Some control tests were
made on gas exchange after the aerial portion of the plant was removed,
and showed that the CO2 exchange from roots and soil was
negligible. The soil surface area was small (diameter of tube, 1 inch)
and the bottom hole in the tube was sealed by modeling clay during the assay.
Chlorophyll Fluorescence
Chlorophyll fluorescence was measured with a PAM 101 fluorometer
(Heinz-Walz, Effeltrich, Germany) simultaneously with gas-exchange measurements, while Arabidopsis plants were enclosed in the
gas-exchange chamber, as described above. The fiber-optic bundle of the
fluorometer was positioned on the top corner of the leaf chamber
at an inclined angle (45o) to minimize
shading of the plant from the actinic light source. The distance
between the fluorometer sensor and the plant was about 4 cm. The plant
area, which was covered by the saturating pulse of light and from which
the fluorescence signal was received by the sensor, was about 6 cm2 (covering approximately 30% of the plant
canopy). Fs was monitored continuously, and
for periodic determination of Fm
,
saturating pulses (1-s duration) of white light (10,000 µmol
m2 s1) were applied by
a PAM 103 trigger-control unit (Heinz-Walz). PSII was calculated as
(Fm 
Fs)/Fm, as
described by Genty et al. (1989). JO2 was
calculated as (PSII × Ia × F)/4 (Genty et al., 1989;
Edwards and Baker, 1993), where Ia,
the light absorbed, was assumed to be 0.8 × PPFD, and
F, a factor for the partitioning of photons between incident
PSII and PSI, was assumed to be 0.5 (Donahue et al., 1997).
14CO2 Feeding and Assays of Starch and Suc
Synthesis
14CO2 released by
acidifying a NaH14CO3
solution was collected in a vacuumed, steel gas cylinder and then a
specific composition of gases was added to acquire the desired
concentration of 14CO2 for
feeding. An intact Arabidopsis plant grown in a 50-mL polypropylene
tube was enclosed in the leaf chamber (see "Gas Exchange") and
illuminated to obtain steady-state photosynthesis. The plants were then
exposed to 14CO2 gas
(specific activity, 0.1 Ci mol1) for 10 min and
chased for 10 min for measurement of partitioning into starch and Suc.
A 1-min chase was also tested; the 10-min chase gave similar trends but
less in the ionic fraction and more in starch and Suc without
significant export from the leaf. A test for degree of retention of
label in leaves showed that
14CO2 partitioning into
roots and stems was about 5% of the total CO2 incorporation after a
10-min pulse and a 10-min chase (data not shown). Leaves were extracted
several times by addition of hot 80% ethanol until the extract was
colorless, and separated into soluble and insoluble fractions
(Angelov et al., 1993). The ethanol-soluble fractions from each
sample were pooled, dried under dry air at 50°C, resolubilized in 1.5 mL of distilled water, and frozen until analysis. The soluble fraction
was passed through a Dowex 50 H+ column and then
through a Dowex 1 Cl resin column. The soluble
neutral fraction eluted from the two columns by water was taken as the
Suc fraction. Amino acids were eluted from the Dowex 50 H+ column by 5 M
NH4OH, and phosphorylated intermediates were
eluted from the Dowex 1 Cl column by 2 M HCl (accounting for 10%-30% of the total
14CO2 fixed; partitioning
into these fractions not shown). The residue (insoluble fraction) was
homogenized with 1 mL of water to determine the extent of
14C incorporation into starch. The radioactivity
in each compound was determined with a liquid-scintillation counter
(model LS7000, Beckman).
Leaf Area
Preliminary tests showed that the total leaf area of the plant
determined by summation of the detached individual leaves was about 5%
greater than the plant leaf area determined on intact rosettes. This
was similar among the different genotypes and the 5% difference
represents the degree of self-shading. In this study the plant leaf
area was determined from a photocopy of the intact rosette using a
computerized NIH image system (National Institutes of Health, Bethesda,
MD) or a Li-3000 leaf area meter (Li-Cor).
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RESULTS |
AGPase Activity
AGPase activities of the wild type and the TL46 and TL25 mutants
grown at high light and low light were determined (Table I). AGPase kinetics exhibited two phases
of activity: an initial slope, which lasted for 1 to 2 min, followed by
a lower steady-state slope, which was linear for at least 15 min. There
was a similar reduction in AGPase activity in both low-light- and
high-light-grown TL46, with the initial activity being 29% to 30% of
that in the wild type and the steady-state activity being about 11% of
that in the wild type, respectively. There was lower activity of AGPase in the mutant TL25, with initial activity in both low-light- and high-light-grown plants being 16% to 19% of that in the wild type. The steady-state activity in TL25 in low-light-grown plants was less
than 3% of that in the wild type, whereas the value of
high-light-grown TL25 plants was about 10% of that in the wild type.
In general, the degree of reduction of AGPase activities in mutants
compared with the wild type was similar to that previously reported for line TL46 (Lin et al., 1988a; Neuhaus and Stitt, 1990) and line TL25
(Lin et al., 1988b).
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Table I.
AGPase activities in the leaves of the wild type and
TL46 and TL25 mutants grown at low light (LL) (100 µmol
m2 s1) or high light (HL) (350 µmol
m2 s1)
For replication, n = 2. Values are given as
±SE.
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CO2 Assimilation
The rates of CO2 assimilation under various
PPFDs and CO2 partial pressures were studied in
intact wild-type plants and AGPase mutants enclosed in a special
chamber (Fig. 1). Whereas there was
little or no difference in A between the wild type and the starch mutants under a lower PPFD and at atmospheric
CO2 (31 Pa), there were larger differences under
saturating PPFD and CO2 levels. At a PPFD of 800 µmol m2 s1 and a
CO2 partial pressure of 80 Pa, A was
significantly higher in the wild type than in line TL46 (60% of that
in the wild type), which in turn was higher than in line TL25 (50% of
that in the wild type) (Fig. 1). Under these conditions plants grown in
high light showed higher A than low-light-grown plants.
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| Figure 1.
The A under various PPFD values
(estimated absorbed values by multiplying incident light times 0.8) and
CO2 partial pressures in Arabidopsis wild type (WT), TL46,
and TL25. The plants were grown under a PPFD of either 100 ( ,  )
or 350 ( ,  ) µmol m2 s1.
CO2 assimilation was conducted under various PPFD regimes
at 25°C, an O2 partial pressure of 20 kPa, and a
CO2 partial pressure of either 31 Pa (, ) or 80 Pa
(, ). Symbols represent means of measurements of two plants. No
experiments were conducted with low-light-grown TL25. SD
values are omitted for clarity. The average SD was 0.6 µmol m2 s1.
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Similarly, these Arabidopsis lines showed different responses when
subjected to higher CO2 partial pressures. When
the CO2 partial pressure was shifted from 31 to
80 Pa, wild-type plants had significantly higher A, with the
high-light-grown plants exhibiting a 94% increase and the
low-light-grown plants showing a somewhat smaller increase of 86%.
CO2 assimilation rates were also stimulated by
CO2 enrichment in high-light-grown TL46 plants,
but only by about one-third as much as in wild-type plants. TL46 plants
grown under low light showed very little increase in A at
the elevated CO2 partial pressures. In
high-light-grown TL25, A was saturated at atmospheric
CO2 levels; an increase in
CO2 partial pressure from 31 to 80 Pa did not
significantly increase A (Fig. 1).
The starch mutants also showed differences in their response to light
saturation (Fig. 1). In wild-type plants at 80 Pa
CO2, A continued to increase up to 800 µmol m2 s1 PPFD,
whereas in lines TL46 and TL25, rates were near saturation at about 400 µmol m2 s1 PPFD.
Electron Transport
Electron-transport rates were measured simultaneously with
A. Figure 2 shows that the
JO2 values, as determined from chlorophyll fluorescence measurements of PSII yield, correlated very well with
A. The rates of CO2 assimilation at a
given JO2 were higher under 80 Pa
CO2 than under 31 Pa CO2 for both
high- and low-light-grown wild-type plants. This pattern was expected,
because high CO2 suppresses photorespiration and
increases the A/JO2 ratio. In TL46, A at a given JO2 was
higher under 80 Pa CO2 than under 31 Pa
CO2 in high-light-grown plants but not in
low-light-grown plants. In high-light-grown TL25, however, A
at a given JO2 was essentially identical at
80 and 31 Pa CO2. Overall, these results indicate that modification of starch metabolism significantly affects
A and electron transport through feedback regulation (see
below).
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| Figure 2.
The relationship (initial slope) between
JO2 and the A in Arabidopsis
wild type (WT), TL46, and TL25. The plants were grown under a PPFD of
either 100 (, ) or 350 (, ) µmol m2
s1. CO2 assimilation was conducted under
various PPFD regimes at 25°C, an O2 partial pressure of
20 kPa, and a CO2 partial pressure of either 31 Pa (,
) or 80 Pa (, ). Symbols represent means of measurements of
two plants.
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The increase in JO2 with increasing PPFD in
high-light-grown wild-type, TL46, and TL25 plants at 80 Pa
CO2 is shown in Figure 3. JO2 values
were similar for the wild-type, TL46, and TL25 plants below a PPFD of
300 µmol m2 s1. Above
300 µmol m2 s1,
however, JO2 was significantly higher in
the wild type than in TL46, which was higher than in TL25 (Fig. 3).
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| Figure 3.
JO2 at 80 Pa
CO2 at 25°C and various PPFD regimes in Arabidopsis
wild-type (), TL46 (), and TL25 ( ) plants grown at high light
(350 µmol m2 s1). Symbols represent means
of measurements of two plants.
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Stimulation of CO2 Assimilation by 2 kPa O2
The effect of photorespiration on A was determined by
reducing the O2 levels in the leaf chamber from
20 to 2 kPa and assessing the percentage stimulation of
CO2 assimilation by low O2
[(A2/A20 1) × 100]. Under 70 Pa CO2 and a PPFD of 800 µmol
m2 s1, line TL46
exhibited reversed O2 sensitivity (inhibition of
CO2 assimilation by 2 kPa
O2), whereas the wild type still exhibited partial stimulation of CO2 assimilation by low
O2 (Fig. 4). Under 35 Pa CO2 and a PPFD of 800 µmol
m2 s1, line TL46
exhibited less stimulation of CO2 assimilation
than the wild type when O2 levels were reduced
from 20 to 2 kPa (data not shown). The loss of O2
sensitivity in the mutant is consistent with the loss of
CO2 sensitivity described above (Fig. 1), indicating that
the mutants are more feedback inhibited as a result of reduction in
capacity for starch synthesis.
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| Figure 4.
Stimulation of CO2 assimilation by 2 kPa O2 in Arabidopsis wild type (open bars) and TL46
(striped bars). CO2 assimilation was conducted
under a PPFD of 800 µmol m2 s1 at 25°C
and CO2 partial pressure of 70 Pa. LL, Plants were grown at
low light (100 µmol m2 s1); HL, plants
were grown at high light (350 µmol m2
s1). Symbols represent means of measurements of two
plants. A2, CO2 assimilation at 2 k Pa
O2; A20, CO2
assimilation at 20kPa O2. Bars indicate SD.
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14CO2 Partitioning into Starch and Suc
The steady-state levels of in vivo starch synthesis were assessed
in a special leaf chamber designed to feed
14CO2 to whole Arabidopsis
plants while simultaneously monitoring A with an IR
CO2 gas analyzer. After a steady-state rate of
CO2 assimilation was attained, the leaf chamber
was gassed with 14CO2 for
10 min under a PPFD of 800 µmol m2 s1 at
31 or 80 Pa CO2, and then chased for 10 min.
Overall, in wild-type plants the rate of starch and Suc synthesis was
higher in high-light-grown than in low-light-grown plants. Similarly,
carbohydrate synthesis was higher at 80 Pa CO2
than at 31 Pa CO2 (Table
II). However, the relative partitioning
of 14CO2 incorporation into
starch and Suc differed at these two CO2 concentrations. At 80 Pa, the incorporation of
14CO2 into starch was about 2-fold higher than
14CO2 incorporation into Suc. At the lower
CO2 concentration, there was a large decrease in the ratio
of 14CO2 incorporation into
starch versus Suc in the wild-type plants.
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Table II.
Carbon partitioning into starch and Suc after a
10-min pulse and a 10-min chase
14CO2 feeding was conducted under a PPFD of 800 µmol m2 s1 at a leaf temperature of
25°C. Plants were assayed near initiation of bolting; the average age
was 31 and 36 d for plants grown under a PPFD of 350 and 100 µmol m2 s1, respectively;
n = 2. WT, Wild type; HL, high light; LL, low light.
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The starch mutants showed lower rates of
14CO2 incorporation into
starch, especially under saturated light and CO2
conditions, compared with the wild type (Table II). Under 80 Pa
CO2, starch-labeling rates by high-light-grown
TL46 and TL25 were about 40% and 14%, respectively, of the wild-type
levels. It is interesting that the reduced incorporation of
14CO2 into starch by the
mutants was partially compensated for by increased incorporation into
Suc. This is clearly evident for the starch-deficient mutant TL46, but
less so for the near-starchless mutant TL25. Low-light-grown
wild-type and TL46 plants showed similar patterns of partitioning
of 14CO2 into Suc and starch, although
absolute incorporation levels were reduced, particularly for Suc.
Under 31 Pa CO2, there was a 10% to 20%
reduction in net CO2 uptake in the mutants
compared with the wild type. Again, there was evidence for the
reduction in partitioning into starch being partially compensated for
by increased partitioning into Suc. The
14CO2 partitioning patterns
into starch and Suc were similar between plants pulsed for 10 min and
then chased for 1 or 10 min (data not shown).
There were significant correlations between A and total
14CO2 incorporation into
leaves (Fig. 5A) and between A
and the rate of starch synthesis (Fig. 5B). In contrast, there was no
correlation between A and the rate of Suc synthesis (P > 0.05) (Fig. 5C). Starch synthesis increased more rapidly with
increasing A (slope = 0.6) than Suc synthesis
(slope = 0.3). These results suggested that starch synthesis would
become very low if A was less than 7 µmol
m2 s1, which agrees
with a previous report with bean (Sharkey et al., 1985).
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| Figure 5.
The relationships between CO2
assimilation rate, total 14C incorporation, starch, and Suc
synthesis in Arabidopsis. CO2 assimilation was measured
under a PPFD of 800 µmol m2 s1 at 25°C
and various CO2 partial pressures in wild type (), TL46
(), and TL25 ( ). Different points for each genotype are the
results of different CO2 and light levels (see Table II).
Asterisks indicate P < 0.01. Bars indicate SD.
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Cumulative Leaf Area
Leaf-area development as an indicator of growth was examined under
the two PPFD growth regimes. Under a 12-h light/12-h dark photoperiod,
both wild-type and TL46 plants exhibited similar increases in leaf area
during the growth period (Fig. 6). In
contrast, the near-starchless mutant, TL25, grew much more slowly.
Also, under high light TL25 took 10 d more to initiate flowering
compared with the wild type and TL46.
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| Figure 6.
Cumulative leaf area during growth of Arabidopsis
wild- type (), TL46 (), and TL25 () plants. The plants were
grown under a PPFD of either 100 (A) or 350 (B) µmol m2
s1 and a photoperiod of 12/12 h (light and dark,
respectively). Symbols represent means of measurements of three plants.
Bars indicate SD.
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There was a significant correlation between the rate of starch
synthesis and the cumulative leaf area per plant (Fig.
7). These results suggest that there is a
minimum requirement for starch synthesis for normal plant growth and
development.
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| Figure 7.
The relationship between the rate of starch
synthesis and total leaf area per plant 31 to 36 d after planting
in Arabidopsis wild type (), TL46 (), and TL25 (). LL, Plants
were grown at low light (100 µmol m2 s1);
HL, plants were grown at high light (350 µmol m2
s1). Data for starch synthesis are from Table II.
Asterisks indicate P < 0.01; bars indicate SD.
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DISCUSSION |
Deficiencies in Starch Synthesis Result in Feedback of
Photosynthesis
The results presented in this study demonstrate clearly that leaf
starch metabolism has a very significant effect on the photosynthetic capacity of the plant. Wild-type Arabidopsis plants have higher A than the starch mutants TL46 and TL25 under high
CO2 over a wide range of PPFD conditions (Fig.
1). Although these differences in A were more readily
evident in high-light-grown plants under saturating
CO2, a similar trend was also observed for
low-light-grown plants.
The rates of JO2 and A were
strongly correlated (Fig. 2). At a given
JO2, enhancement of A at
elevated levels of CO2 attributable to inhibition
of photorespiration was higher in the wild type than in TL46, which in
turn was higher than in TL25. At atmospheric levels of
CO2 and O2,
photorespiration contributes significantly to Pi recycling in
C3 plants (Harley and Sharkey, 1991; Eichelmann and Laisk, 1994). When CO2 levels increase,
photorespiration, which acts as an electron acceptor, declines and
starch synthesis becomes important for Pi recycling, as indicated by
the large increase in starch synthesis under high
CO2. The wild type had a higher capacity for
starch synthesis and, thus, retained higher Pi recycling, electron
transport, and higher CO2 assimilation than the
starch mutants.
At a PPFD above 300 µmol m2
s1, JO2 was
significantly higher in the wild type than in TL46, which in turn was
higher than in TL25 (Fig. 3). These results indicate that there is
feedback regulation of electron transport by deficiencies in starch
synthesis. Similar observations were described for the tobacco plastid
phosphoglucomutase mutant, which is defective in starch synthesis and
has a lower rate of electron transport (Peterson and Hanson, 1991;
Eichelmann and Laisk, 1994).
O2 Sensitivity Is a Good Indicator of Feedback
Inhibition
CO2 assimilation is often stimulated by
subatmospheric levels of O2 in
C3 plants because of the reduced oxygenase
activity of the bifunctional enzyme Rubisco (Sage and Sharkey, 1987).
However, in some cases, such as at low temperatures (Leegood and
Furbank, 1986; Sage and Sharkey, 1987; Sun et al., 1997) or at high
CO2 levels (Viil et al., 1977; Sharkey, 1985),
C3 plants exhibit
O2-insensitive photosynthesis or even reversed
O2 sensitivity. Reversed O2
sensitivity is also seen in a Flaveria mutant that contains
reduced levels of the cytosolic Fru bisphosphatase (Sharkey et al.,
1995) and in a tobacco starchless mutant with phosphoglucomutase
deficiency (Hanson, 1990; Eichelmann and Laisk, 1994). The occurrence
of O2-insensitive photosynthesis has been
suggested to be caused by limitations in triose-P utilization, which
causes a Pi limitation and its concomitant effects on reducing ATP and
ribulose-1,5-bisphosphate regeneration (Sharkey et al., 1995). Reversed
O2 sensitivity (i.e. inhibition of
CO2 fixation by low O2) is
poorly understood, but according to one hypothesis it is associated
with the loss of a photorespiratory Pi-generating mechanism in
photosynthesis under low O2, when the capacity for
utilization of triose-P is limiting (Harley and Sharkey, 1991). Perhaps
the limitation on conversion of triose-P to starch biosynthesis in the
Arabidopsis mutants and the associated release of Pi could result in
its having a greater Pi deficiency when photorespiration is eliminated
under low O2. It has also been suggested that
reversed O2 sensitivity may occur by accumulation
of 3-PGA and its inhibition of starch synthesis by inhibition of
phosphoglucoisomerase (Sharkey and Vassey, 1989); however, this would
not account for the mutant, which cannot make starch, being more
susceptible to reversed sensitivity than the wild type. Eichelmann and
Laisk (1994) suggested that the reversal of O2
sensitivity in a tobacco mutant impaired in starch synthesis is caused
by Pi depletion, but they were unable to explain this effect with the
hypotheses described above.
Our results support the view that the loss of O2
sensitivity observed in starch synthesis mutants denotes feedback
limitation attributable to decreased potential to utilize additional
triose-P for carbohydrate synthesis when shifting from 20 to 2 kPa
O2. Under atmospheric CO2
levels, TL46 exhibited less stimulation of CO2
assimilation at 2 kPa O2 than did the wild type.
Under 70 Pa CO2, TL46 exhibited reversed
O2 sensitivity (inhibition of
CO2 assimilation by reduction from 20 to 2 kPa
O2), whereas the wild type showed partial
stimulation of CO2 assimilation by low
O2 (Fig. 4). This inability to utilize additional
triose-P for carbohydrate synthesis causes Pi cycling to become limited for ATP and ribulose-1,5-bisphosphate regeneration, which reduces both
electron transport and CO2-assimilation rates
(Figs. 1-3).
Partitioning of 14CO2 Assimilation between
Suc and Starch
Rates of 14CO2
incorporation into Suc and starch were dependent on
CO2 levels and the previous growth conditions of
the plants. At atmospheric CO2 (31 Pa), almost
one-half of the total 14CO2
incorporated was partitioned into Suc, with a smaller proportion (34%
of the total) allocated into starch. Although the total
14CO2-incorporation rate
was lower in TL46 than in the wild type, TL46 showed a similar
distribution between Suc and starch as the wild type. In TL25 the bulk
of the 14CO2 was
incorporated into Suc with very little labeling into starch, a pattern
consistent with its starchless phenotype.
At higher CO2 levels (80 Pa), total
14CO2-incorporation rates
were increased for the wild type and TL46. In the wild type most of
this increase in 14CO2
incorporation was partitioned into starch, with little increase in Suc.
These results indicate that Suc biosynthesis is saturated or close to
being saturated at atmospheric CO2 and high light levels in
the wild type (see also Fig. 5). Conversely, a different pattern was
evident for TL46 under these same conditions. The bulk of the limited
increase in 14CO2
incorporation was partitioned into Suc, with smaller amounts into
starch. Hence, with the restricted capacity of TL46 to synthesize starch, there was increased shunting of triose-P into Suc under high
CO2, although the rate of photosynthesis was
still limited by triose-P utilization.
TL46 showed a 14CO2 incorporation rate into
starch that was 60% to 80% of that in the wild type at atmospheric
levels of CO2, or 30% to 40% of that in the wild type at
80 Pa CO2 (Table II). The 14CO2
incorporation rates were consistent with the levels of starch (40%-50% of the wild-type levels) reported for TL46 when grown at
atmospheric levels of CO2 and a PPFD either at
120 µmol m2 s1 (Lin
et al., 1988a) or 600 µmol m2
s1 (Schulze et al., 1991). The
14CO2 incorporation rates reported here and the
steady-state starch levels reported elsewhere (Lin et al., 1988a;
Schulze et al., 1991), however, are not consistent with the low
activity of AGPase (7% of that in the wild type) reported for TL46
(Lin et al., 1988a; Neuhaus and Stitt, 1990). Results from recent
biochemical and genetic studies on the structure function of AGPase can
account for this apparent discrepancy. AGPase is composed of two
subunit types, a large subunit (regulatory) and a small subunit
(catalytic) (Okita et al., 1990; Ballicora et al., 1995). The large
subunit is unable to form a functional enzyme by itself, whereas the
small subunit is capable of forming an active enzyme
even in the absence of the large subunit, albeit with
reduced sensitivity to allosteric regulation. TL46 contains a missense
mutation in the large subunit (Wang et al., 1997), resulting in the
formation of an active small subunit enzyme that requires much higher
levels of 3-phosphoglyceric acid for maximum enzyme activity and is
much more sensitive to inhibition by Pi than the wild-type enzyme (Li
and Preiss, 1992). Because limiting amounts of 3-PGA (1-2
mM) were used to measure AGPase activities (Lin et al.,
1988a; Neuhaus and Stitt, 1990), the activities are likely
underestimated.
The relative 14CO2 incorporation rates into
starch and Suc we determined for TL46 differ substantially from those
reported earlier by Neuhaus and Stitt (1990). In their study, under
high CO2 TL46 displayed only 9% of the wild-type
rate of 14CO2 incorporation
into starch, compared with the 30% rate reported here. Moreover,
whereas an increase in
14CO2 incorporation into
Suc was observed at higher CO2 levels in TL46
compared to the wild type (Table II), they reported a reduction in Suc
synthesis. Several factors may account for the discrepancy between
these results. First, Neuhaus and Stitt (1990) used a detached
leaf-disc assay of photosynthesis with an O2
electrode under very high CO2, whereas we used a
nondestructive whole-plant assay. When leaves are cut, Suc accumulation
and wounding may occur, impairing photosynthesis and carbon
partitioning. Therefore, Suc transport out of the leaves is prevented,
as in girdling of the leaf petiole by hot wax (Goldschmidt and Huber,
1992) or cold girdling (Krapp and Stitt, 1995). Evidence in support of
this view is that A during
14CO2 labeling was about
two to three times higher in our study than in that of Neuhaus and
Stitt (1990), and we have observed lower
CO2-saturated rates of photosynthesis in
Arabidopsis using the leaf-disc system than with intact plants (M. Poulson and G. Edwards, unpublished data). A second major factor is
that a mature leaf (used in the leaf-disc assay) versus the whole plant
(used in the present study) can differ in carbon partitioning. For
example, in cassava the Suc content is very similar in leaves of
different ages, whereas starch levels are much higher (up to
severalfold) in young leaves than in old leaves (Angelov et al., 1993).
This results in the whole plant having a higher starch-to-Suc ratio than the mature leaves alone. Third, because long photoperiods can
reduce partitioning into starch (Chatterton and Silvius, 1979), plants
grown under the longer photoperiods (16/8 h) used by Neuhaus and Stitt
(1990) may have lower starch synthesis than the plants in the current
study (12/12 h).
Growth light also affected partitioning into Suc and starch.
High-light-grown wild-type plants had higher AGPase activity and
high rates of CO2 fixation, with a larger
increase in partitioning into starch under high
CO2 (80 Pa), than the low-light-grown plants. However, high-light-grown TL46 had an AGPase activity similar to that
of low-light-grown plants, and the higher rate of photosynthesis in
high-light-grown plants under high CO2 was
largely accounted for by increased partitioning into Suc.
Starch Synthesis Is Regulated and Related to Photosynthetic
Capacity
Eichelmann and Laisk (1994) and Stitt (1996) have suggested that
starch is an "overflow" product when photosynthesis is high. Based
on calculations from a biochemical model, Eichelmann and Laisk (1994)
showed that when photosynthesis increases, Suc synthesis saturates
first and then the capacity of starch synthesis is triggered. There is
also evidence for this in C4 plants, since the
ratio of 14CO2
incorporation into starch/Suc increases with increasing PPFD (Lunn and
Hatch, 1997). Other studies have indicated that starch synthesis is
"programmed." For example, a transient increase (3-4 d) in starch
level in the leaves when shifting to shorter days has been seen as a
programmed response meeting the increased need for carbon during the
lengthened night (Geiger et al., 1985). The ability of leaves to
synthesize starch when photosynthetic rates are low, such as under low
light and low CO2 (Silvius et al., 1979; Lin et
al., 1988b; Schulze et al., 1991; Huber and Hanson, 1992), further
supports the programmed model for starch synthesis.
The programmed and the overflow models are not mutually exclusive, and
both mechanisms may operate depending on the environmental conditions.
The overflow model is operative in Arabidopsis, as indicated by the
results from the 14CO2 labeling study. When
CO2 levels were elevated from 31 to 80 Pa, Suc synthesis
increased only marginally (18%), whereas starch synthesis increased by
almost 3-fold in the wild type (Table II). Hence, increased triose-P
production by increased CO2 assimilation at 80 Pa
CO2 is mainly partitioned into starch, a process
that is facilitated by the allosteric activation of AGPase by increased 3-PGA and decreased Pi levels (Preiss, 1982).
The programmed model is likely constitutive and important for plant
growth during a day/night diurnal regime. Cytosolic
Fru-1,6-bisphosphatase regulation through Fru-2,6-bisP (Stitt, 1996)
and Suc-P synthase phosphorylation (Huber and Huber, 1992) likely play
important roles in programmed starch synthesis. To some extent the
programmed model may apply to the present results with Arabidopsis,
because there was significant starch synthesis at atmospheric levels of CO2 (Table II), a level that was limiting for
photosynthesis (Fig. 1).
There is a strong correlation between rates of starch synthesis and
CO2 assimilation when measurements are made under
varying CO2 levels with plants grown at different
light levels and having different AGPase activities (Fig. 5). These
observations indicate the importance of starch synthesis in
photosynthesis. In the wild type, the rate of starch synthesis was far
from saturation at atmospheric levels of CO2, and
increased by about 3-fold when shifting to 80 Pa
CO2, whereas in TL25 and TL46 plants starch synthesis was near saturation at the lower level of
CO2 (Table II). In addition, reduction in starch
synthesis in AGPase mutants was only compensated for to a small degree
by increased Suc synthesis (Table II). Thus, the wild type had a higher
capacity to utilize triose-P to recycle Pi and prevent feedback
inhibition and, thus, to accommodate the potential for increased
CO2 fixation in the C3
cycle.
Growth under a Day/Night Diurnal Regime Is Starch Dependent
Cumulative leaf area as a measure of growth under a 12-h
photoperiod was significantly reduced in line TL25, whereas there was
only an apparent slight reduction in TL46, compared with the wild type
(Fig. 6). CO2 assimilation rates under atmospheric levels of CO2 were only slightly lower in line TL25 than in the
wild-type or TL46 plants (Table II). Thus, lower starch synthesis in
TL25 (Table II) can be considered to account for its slower growth. Starch is an important carbon source for growth at night, when most
leaf expansion occurs (see Huber and Hanson, 1992). Typically, the
starch level in the leaves increases linearly with time during the day
and decreases linearly during the night, and starch synthesized during
the day equals starch degraded during the night (Lin et al., 1988a,
1988b). At 800 µmol m2
s1 PPFD, TL46 had a rate of starch synthesis
60% to 80% of the wild type at atmospheric levels of
CO2 and a rate of starch synthesis 30% to 40%
of the wild type at 80 Pa CO2. Because TL46 grown
under atmospheric levels of CO2 at 100 or 350 µmol m2 s1 PPFD had a
cumulative leaf area only slightly lower than that of the wild type,
this suggests that it produced sufficient starch under those
conditions.
| |
FOOTNOTES |
1
This research was supported by grants from the
U.S. Department of Agriculture (USDA) (no. 95-37306-2195 to T.W.O. and
G.E.E.) and the Department of Energy (no. DE-FG0687ER136 to T.W.O.).
This research study falls under the purview of Hatch Project 0119, College of Agriculture and Home Economics, Washington State University, and of the USDA North Central 142 Regional Project.
*
Corresponding author; e-mail edwards{at}wsu.edu; fax
1-509-335-3517.
Received June 1, 1998;
accepted September 26, 1998.
| |
ABBREVIATIONS |
Abbreviations:
A, CO2 assimilation
rate.
AGPase, ADP-Glc pyrophosphorylase.
Fm, maximal yield of fluorescence from a
saturating flash of white light.
Fs, steady-state fluorescence.
JO2, gross rate
of O2 evolution.
3-PGA, 3-phosphoglycerate.
PSII, quantum yield of PSII.
| |
ACKNOWLEDGMENT |
The authors thank Sandy Edwards for helpful comments on the
manuscript.
| |
LITERATURE CITED |
Angelov MN,
Sun J,
Byrd GT,
Brown RH,
Black CC
(1993)
Novel characteristics of cassava, Manihot esculenta Crantz, a reputed C3-C4 intermediate photosynthesis species.
Photosynth Res
38:
61-72
[CrossRef]
Ballicora MA,
Laughlin MJ,
Fu Y,
Barry G,
Okita TW,
Preiss J
(1995)
ADP-glucose pyrophosphorylase from potato tuber. Significance of the N terminus of the small subunit for heat stability and phosphate inhibition.
Plant Physiol
109:
245-251
[Abstract]
Bradford MM
(1976)
A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding.
Anal Biochem
72:
248-254
[CrossRef][ISI][Medline]
Chatterton NJ,
Silvius JE
(1979)
Photosynthate partitioning into starch in soybean leaves. I. Effects of photoperiod versus photoperiod duration.
Plant Physiol
64:
749-753
[Abstract/Free Full Text]
Dickinson DB,
Preiss J
(1969)
ADP-glucose pyrophosphorylase in shrunken-2 and brittle-2 mutants of maize endosperm.
Plant Physiol
44:
1058-1062
[Abstract/Free Full Text]
Donahue RA,
Poulson ME,
Edwards GE
(1997)
A method for measuring whole plant gas exchange in Arabidopsis thaliana.
Photosynth Res
52:
263-269
[CrossRef]
Edwards GE,
Baker NR
(1993)
Can CO2 assimilation in maize leaves be predicted accurately from chlorophyll fluorescence analysis?
Photosynth Res
37:
89-102
Eichelmann H,
Laisk AH
(1994)
CO2 uptake and electron transport rates in wild-type and a starchless mutant of Nicotiana sylvestris. The role and regulation of starch synthesis at saturating CO2 concentrations.
Plant Physiol
106:
679-687
[Abstract]
Galtier N,
Foyer CH,
Huber J,
Voeker TA,
Huber SC
(1993)
Effects of elevated sucrose phosphate synthase activity on photosynthesis, assimilate partitioning, and growth in tomato (Lycopersicon esculentum var UC82B).
Plant Physiol
101:
535-543
[Abstract]
Galtier N,
Foyer CH,
Murchie E,
Quick P,
Voelker TA,
Thepenier C,
Lasceve G,
Betsche T
(1995)
Effects of light and atmospheric carbon dioxide enrichment on photosynthesis and carbon partitioning in the leaves of tomato plants over-expressing sucrose phosphate synthase.
J Exp Bot
46:
1335-1344
Geiger DR, Jablonski LM, Ploeger BJ (1985) Significance of carbon
allocation to starch in growth of Beta vulgaris. In RL
Heath, J Preiss, eds, Regulation of Carbon Partitioning in
Photosynthetic Tissue. American Society of Plant Physiologists,
Rockville, MD, pp 289-308
Genty B,
Briantais J,
Baker NR
(1989)
The relationship between the quantum yield of photosynthetic electron transport and quenching of chlorophyll fluorescence.
Biochim Biophys Acta
990:
87-92
Goldschmidt EE,
Huber SC
(1992)
Regulation of photosynthesis by end-product accumulation in leaves of plants storing starch, sucrose, and hexose sugars.
Plant Physiol
99:
1443-1448
[Abstract/Free Full Text]
Hanson KR
(1990)
Steady-state and oscillating photosynthesis by a starchless mutant of Nicotiana sylvestris.
Plant Physiol
93:
1212-1218
[Abstract/Free Full Text]
Harley PC,
Sharkey TD
(1991)
An improved model of C3 photosynthesis at high CO2: reversed O2 sensitivity explained by lack of glycerate reentry into the chloroplast.
Photosynth Res
27:
169-178
Ho LC
(1977)
Effects of CO2 enrichment on the rates of photosynthesis and translocation in tomato leaves.
Ann Appl Biol
87:
191-200
Hoagland DR, Arnon DI, revised by Arnon DI (1950) The
water-culture method for growing plants without soil. In
California Agricultural Experiment Station Circular 347, revised
January 1950. The College of Agriculture, University of California,
Berkeley
Huber SC,
Hanson KR
(1992)
Carbon partitioning and growth of a starchless mutant of Nicotiana sylvestris.
Plant Physiol
99:
1449-1454
[Abstract/Free Full Text]
Huber SC,
Huber JL
(1992)
Role of sucrose-phosphate synthase in sucrose metabolism in leaves.
Plant Physiol
99:
1275-1278
[Abstract/Free Full Text]
Huber SC,
Rogers HH,
Israel DW
(1984)
Effects of CO2 enrichment on photosynthesis and carbon partitioning in soybean (Glycine max) leaves.
Physiol Plant
62:
95-101
Krapp A,
Stitt M
(1995)
An evaluation of direct and indirect mechanisms for the `sink-regulation' of photosynthesis in spinach: changes in gas exchange, carbohydrates, metabolites, enzyme activities and steady-state transcript levels after cold-girdling source leaves.
Planta
195:
313-323
[ISI]
Leegood RC,
Furbank RT
(1986)
Stimulation of photosynthesis by 2% oxygen at low temperatures is restored by phosphate.
Planta
108:
84-93
Li L,
Preiss J
(1992)
Characterization of ADPglucose pyrophosphorylase from a starch-deficient mutant of Arabidopsis thaliana L.
Carbohydr Res
227:
227-239
[CrossRef]
Lin T-P,
Caspar T,
Somerville CR,
Preiss J
(1988a)
Isolation and characterization of a starchless mutant of Arabidopsis thaliana (L.) Heynh lacking ADPglucose pyrophosphorylase activity.
Plant Physiol
86:
1131-1135
[Abstract/Free Full Text]
Lin T-P,
Caspar T,
Somerville CR,
Preiss J
(1988b)
A starch deficient mutant of Arabidopsis thaliana with low ADPglucose pyrophosphorylase activity lacks one of the two subunits of the enzyme.
Plant Physiol
88:
1175-1181
[Abstract/Free Full Text]
Lunn JE,
Hatch MD
(1997)
The role of sucrose-phosphate synthase in the control of photosynthate partitioning in Zea mays leaves.
Aust J Plant Physiol
24:
1-8
Micallef BJ,
Haskins KA,
Vanderveer PJ,
Roh K-S,
Shewmaker CK,
Sharkey TD
(1995)
Altered photosynthesis, flowering, and fruiting in transgenic tomato plants that have an increased capacity for sucrose synthesis.
Planta
196:
327-334
Neuhaus EH,
Stitt M
(1990)
Control analysis of photosynthate partitioning: impact of reduced activity of ADP-glucose pyrophosphorylase or plastid phosphoglucomutase on the fluxes to starch and sucrose in Arabidopsis thaliana (L.) Heynh.
Planta
182:
445-454
[CrossRef]
Okita TW,
Nakata PA,
Anderson JM,
Sowokinos J,
Morell M,
Preiss J
(1990)
The subunit structure of potato tuber ADP-glucose pyrophosphorylase.
Plant Physiol
93:
785-790
[Abstract/Free Full Text]
Peterson RB,
Hanson KR
(1991)
Changes in photochemical and fluorescence yields in leaf tissue from normal and starchless Nicotiana sylvestris with increasing irradiance.
Plant Sci
76:
143-151
[CrossRef]
Preiss J
(1982)
Regulation of the biosynthesis and degradation of starch.
Annu Rev Plant Physiol
33:
431-454
[ISI]
Sage RF
(1990)
A model describing the regulation of ribulose-1,5-bisphosphate carboxylase, electron transport, and triose phosphate use in response to light intensity and CO2 in C3 plants.
Plant Physiol
94:
1728-1734
[Abstract/Free Full Text]
Sage RF
(1994)
Acclimation of photosynthesis to increasing atmospheric CO2: the gas exchange perspective.
Photosynth Res
39:
351-368
[CrossRef]
Sage RF,
Sharkey TD
(1987)
The effect of temperature on the occurrence of O2- and CO2-insensitive photosynthesis in field grown plants.
Plant Physiol
84:
658-664
[Abstract/Free Full Text]
Schulze W,
Stitt M,
Schulze E-D,
Neuhaus HE,
Fichtner K
(1991)
A quantification of the significance of assimilatory starch for growth of Arabidopsis thaliana L. Heynh.
Plant Physiol
95:
890-895
[Abstract/Free Full Text]
Sharkey TD
(1985)
O2-insensitive photosynthesis in C3 plants. Its occurrence and a possible explanation.
Plant Physiol
78:
71-75
[Abstract/Free Full Text]
Sharkey TD,
Berry JA,
Raschke K
(1985)
Starch and sucrose synthesis in Phaseolus vulgaris as affected by light, CO2, and abscisic acid.
Plant Physiol
77:
617-620
[Abstract/Free Full Text]
Sharkey TD,
Berry JA,
Sage RF
(1988)
Regulation of photosynthetic electron-transport as determined by room-temperature chlorophyll a fluorescence in Phaseolus vulgaris L.
Planta
176:
415-424
[CrossRef]
Sharkey TD,
Laporte MM,
Micallef BJ,
Shewmaker CK,
Oakes JV
(1995)
Sucrose synthesis, temperature, and plant yield.
In
P Mathis,
eds, Photosynthesis: From Light to Biosphere, Vol V.
Kluwer Academic Publishers, Dordrecht, The Netherlands, pp 635-640
Sharkey TD,
Vassey TL
(1989)
Low oxygen inhibition of photosynthesis is caused by inhibition of starch synthesis.
Plant Physiol
90:
385-387
[Abstract/Free Full Text]
Silvius JE,
Chatterton NJ,
Kremer DF
(1979)
Photosynthate partitioning in soybean leaves at two irradiance levels. Comparative response of acclimated and unacclimated leaves.
Plant Physiol
64:
872-875
[Abstract/Free Full Text]
Sowokinos JR
(1976)
Pyrophosphorylases in Solanum tuberosum. I. Changes in ADP-glucose and UDP-glucose phosphorylase activities associated with starch biosynthesis during tuberisation, maturation and storage of potatoes.
Plant Physiol
57:
63-68
[Abstract/Free Full Text]
Stitt M
(1996)
Metabolic regulation of photosynthesis.
In
NR Baker,
eds, Photosynthesis and the Environment.
Kluwer Academic Publishers, Dordrecht, The Netherlands, pp 151-190
Sun J,
Edwards GE,
Okita TW
(1997)
Transient feedback inhibition of photosynthesis and electron transport when shifting to low O2 in rice plants. (abstract no. 1043).
Plant Physiol
114:
S-208
Tsai CY,
Nelson OE
(1966)
Starch-deficient maize mutant lacking adenosine diphosphate glucose pyrophosphorylase activity.
Science
151:
341-343
[Abstract/Free Full Text]
Viil J,
Laisk AH,
Oja VM,
Parnik T
(1977)
Enhancement of photosynthesis caused by oxygen under saturating irradiance and high CO2 concentrations.
Photosynthetica
11:
251-259
Wang S-M,
Chu B,
Lue W-L,
Yu T-S,
Eimert K,
Chen J
(1997)
adg2-1 represents a missense mutation in the ADPG pyrophosphorylase large subunit gene of Arabidopsis thaliana.
Plant J
11:
1121-1126
[Medline]
Zamski E, Schaffer AA (1996) Photoassimilate Distribution in
Plants and Crops: Source-Sink Relationships. Marcel Dekker, New
York
