Journal of Experimental Botany, Vol. 67, No. 14 pp. 4067–4077, 2016
doi:10.1093/jxb/erv484 Advance Access publication 19 November 2015
OPINION PAPER
The glucose 6-phosphate shunt around the
Calvin–Benson cycle
Thomas D. Sharkey* and Sean E. Weise
Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI, 48824, USA
* Correspondence: [email protected]
Received 5 September 2015; Accepted 20 October 2015
Editor: Christine Raines, University of Essex
Abstract
It is just over 60 years since a cycle for the regeneration of the CO2-acceptor used in photosynthesis was proposed.
In this opinion paper, we revisit the origins of the Calvin–Benson cycle that occurred at the time that the hexose
monophosphate shunt, now called the pentose phosphate pathway, was being worked out. Eventually the pentose
phosphate pathway was separated into two branches, an oxidative branch and a non-oxidative branch. It is generally
thought that the Calvin–Benson cycle is the reverse of the non-oxidative branch of the pentose phosphate pathway
but we describe crucial differences and also propose that some carbon routinely passes through the oxidative branch
of the pentose phosphate pathway. This creates a futile cycle but may help to stabilize photosynthesis. If it occurs it
could explain a number of enigmas including the lack of complete labelling of the Calvin–Benson cycle intermediates
when carbon isotopes are fed to photosynthesizing leaves.
Key words: Calvin–Benson cycle, cyclic electron flow, energetics, oxidative pentose phosphate pathway, reductive pentose
phosphate pathway, starch synthesis.
Introduction
Photosynthesis in plants involves the storage of energy from
sunlight, captured by photosynthetic electron transport reactions, as reduced carbon. Carbon is reduced in the Calvin–
Benson cycle, which is a combination of gluconeogenic
reactions, most of the non-oxidative branch of the pentose
phosphate pathway, and several unique reactions: Rubisco,
sedoheptulose bisphosphatase, and phosphoribulokinase
(except in some Archaea which make RuBP by an alternate
mechanism). Photosynthetic electron transport and the photosynthetic carbon reduction cycle interact through NADPH
and ATP.
The reactions of the Calvin–Benson cycle were discovered in the early 1950s and culminated in Benson’s dismissal
(Benson, 2010) and Calvin winning the Nobel Prize. The core
pathway of reactions, first shown in 1954 (Bassham et al.,
1954) (Fig. 1), is well accepted. The paper by Bassham et al.
(1954) has many interesting features. For example, Figure 1
of that paper shows an apparatus for the short-term exposure
of algae to 14CO2 called a quenched-flow system rather than
the ‘lollipop’ that is seen in textbooks. The paper also has a
description of the thioctic acid (now called lipoic acid) theory
of Calvin that is clearly out of place in that paper. The thioctic
acid theory proved embarrasingly wrong. Benson worked on
the intermediates of the cycle and the carboxylation enzyme
while Calvin devoted his time to the ill-fated thioctic acid
theory causing tension that, in part, led to Benson’s dismissal
(Benson, 2010). Finally, the last paragraph of that paper
refers to papers presented at scientific meetings that reported
the metabolism of pentose phosphates that appeared to be the
reverse of the reactions being proposed for the regeneration
© The Author 2015. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved.
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4068 | Weise et al.
Fig. 1. The original cycle. ‘Reprinted with permission from Bassham JA, Benson AA, Kay LD, Harris AZ, Wilson AT, Calvin M. 1954. The path of carbon
in photosynthesis. XXI. The cyclic regeneration of carbon dioxide acceptor. Journal of the American Chemical Society 76, 1760–1770. Copyright (1954),
American Chemical Society.’ The original legend was ‘Proposed cycle for carbon reduction in photosynthesis. Heavy lines indicate transformations of
carbon compounds, light lines the path of conversion of radiant energy to chemical energy and the subsequent use of this energy stored momentarily in
some compound (E), to form a reducing agent [H] and oxygen from water.’
of the carbon dioxide acceptor. This work (Horecker et al.,
1954) described the non-oxidative branch of of the pentose
phosphate pathway. Most of the reactions of the two pathways are similar except for the action of the enzyme called
transaldolase, which is absent in the Calvin–Benson cycle.
Horecker et al. (1954) was submitted for publication 17 days
after the Bassham et al. paper. Thus, the non-oxidative
branch of the pentose phosphate pathway is the reverse of
the Calvin–Benson cycle, not the other way around, at least
based on the date the manuscripts were submitted.
However, many observations indicate that there are alterations in the canonical Calvin–Benson pathway that may serve
important functions, such as stabilization and regulation, especially in stochastic environments. For example, a ‘Rubisco shunt’
was shown to increase significantly the carbon efficiency of the
conversion of sugars to lipids in developing seeds (Schwender
et al., 2004). There are also puzzling observations such as the
observation that the Calvin–Benson cycle does not become fully
labelled when 14CO2 (Canvin, 1979) or 13CO2 (Delwiche and
Sharkey, 1993, Szecowka et al., 2013) is fed to photosynthesizing
leaves and the finding that the cause of a high cyclic photosynthetic electron flow phenotype results from a lack of the Calvin–
Benson cycle enzyme fructose bisphosphatase (Livingston et al.,
2010). We hypothesize that alternative pathways related to the
Calvin–Benson cycle are important for regulating and stabilizing photosynthetic carbon metabolism and electron transport.
Specifically, we postulate that the oxidative branch of the pentose phosphate pathway is nearly always slightly active and,
under some conditions, can become a significant fraction of the
non-oxidative branch that leads to net carbon fixation.
Taken literally, this results in a cycle as shown in Fig. 2, left
panel (Sharkey and Weise, 2012), although some alternatives
to the non-oxidative branch of the pentose phosphate pathway
have been proposed [discussed in Kruger and von Schaewen
(2003) but not considered further here]. In the reverse of the
pentose phosphate pathway, here called the transaldolase
cycle, fructose bisphosphatase (FBPase) works twice per three
CO2 molecules fixed. Transketolase transfers two carbons
from F6P (see figure legends for abbreviations) to GAP to
make XuBP and E4P while transaldolase transfers three carbons from F6P to E4P to make S7P and GAP. Transketolase
acts again so that R5P and another Xu5P are formed. In this
theoretical cycle, SBP and sedoheptulose-1,7-bisphosphatase
(SBPase) do not occur. This metabolism was shown to occur
in seeds importing sugar and making lipids in the Rubisco
shunt (Schwender et al., 2004). The accepted Calvin–Benson
cycle differs in that it does not use transaldolase (Fig. 2 right
panel). Instead, E4P is converted to SBP by the addition of
DHAP by aldolase and then a novel activity, SBPase, converts
SBP to S7P. The condensation of E4P with DHAP was proposed by Bassham et al. (1954) based on enzymatic activity
described by Horecker and Smyrniotis (1952) who went on to
discover and name transaldolase (Horecker and Smyrniotis,
1953). The requirement for an SBPase was not recognized at
the time. In the Calvin–Benson cycle, FBPase acts only once
per per three CO2 molecules fixed and all F6P is acted on by
transketolase. In the transaldolase cycle FBPase acts twice and
SBPase activity is not required. The original depiction of the
Calvin–Benson cycle (Fig. 1) did not consider the phosphates.
The Calvin–Benson cycle
A second bisphosphatase provides an important
regulatory point in the cycle
The Calvin–Benson cycle is widely regarded as the reverse of
the non-oxidative branch of the pentose phosphate pathway.
In prokaryotes, SBPase activity is found on the same protein
as FBPase activity but, over the course of evolution, these
The G6P shunt of photosynthesis | 4069
Fig. 2. A carbon reduction cycle that is the reverse of the non-oxidative branch of the pentose phosphate pathway (left panel) and the accepted
Calvin–Benson cycle (right panel) drawn assuming fixation of three CO2. PGA, phosphoglyceric acid; GAP, glyceraldehyde 3-phosphate; DHAP,
dihydroxyacetone phosphate; FBP, fructose 1,6-bisphosphate;, F6P, fructose 6-phosphate; E4P, erythrose 4-phosphate; S7P, sedoheptulose
7-phosphate; R5P, ribose 5-phosphate; XuBP, xylulose 5-phosphate; Ru5P, ribulose 5-phosphate; RuBP, ribulose 1,5-bisphosphate; SBP, sedoheptulose
1,7-bisphosphate.
activities have separated; the enzymes involved in green
algae and plants are only distantly related (Martin et al.,
2000). This allows modulation of the concentration of F6P
and E4P. Changes in SBPase activity has been shown to be
especially effective among Calvin–Benson cycle enzymes in
affecting photosynthesis (Raines et al., 1999, Harrison et al.,
2001, Poolman et al., 2001, Lawson et al., 2006, Feng et al.,
2007, Rosenthal et al., 2011; Liu et al., 2012 ). The Calvin–
Benson cycle also requires triose phosphates in more reactions than does the transaldolase cycle. Four of the five triose
phosphates required to regenerate RuBP are used to make
FBP and then F6P in the transaldolase cycle while just two
are used this way in the Calvin–Benson cycle. If triose phosphates were to be depleted, for example during a brief period
of shading, the Calvin–Benson cycle could not proceed from
F6P to RuBP because three additional triose phosphates are
required. On the other hand the transaldolase cycle requires
just one triose phosphate molecule beyond the aldolase reaction that results in FBP.
Genomic studies indicate that transaldolase is present in
chloroplasts (reviewed by Kruger and von Schaewen, 2003)
and there is nearly as much mRNA in leaves as for FBPase
(when visualized at BAR Arabidopsis eFP Browser, University
of Toronto, based on Schmid et al., 2005). However, there
seems to be little or no transaldolase protein in autotrophic
leaves except after pathogen attack (Caillau and Quick,
2005). The possible presence of transaldolase in chloroplasts
is incompatible with an exclusive role for SBPase in regulating flux through the Calvin–Benson cycle (Woodrow and
Walker, 1983, Harrison et al., 2001, Ölçer et al., 2001, Feng
et al., 2007, Zhu et al., 2007; Rosenthal et al., 2011 ). Gibbs
wrote that, ‘The rigorous proof of whether transaldolase, or
aldolase and sedoheptulose diphosphatase, functions in the
reductive pentose-P cycle is still lacking’ (Gibbs, 1966). It is
now clear that transaldolase plays only a small part, if any,
in healthy photosynthesizing tissue. The evolution of a new
enzyme, SBPase, suggests that SBPase may help to optimize
photosynthetic function in plants. We suggest that SBPase
regulation is important because it partitions carbon flow
between the Calvin–Benson cycle and an alternative carbon
flow (the G6P shunt) that creates a futile cycle that stabilizes
photosynthesis. We hypothesize that understanding the regulation of the outputs from the Calvin–Benson cycle provides
an explanation for why the Calvin–Benson cycle, with SBPase
activity but lacking transaldolase, evolved.
Outputs from the Calvin–Benson cycle
The Calvin–Benson cycle provides a number of outputs but
the two major products are sucrose and starch. The pathways
are similar except for (i) the cellular location of the enzymes,
and (ii) the last synthesis steps (Fig. 3). Triose phosphates are
exported to the cytosol for sucrose synthesis in a phosphateneutral exchange (antiporter) (Flügge and Heldt, 1991). Then
both pathways follow a series of reactions involving aldolase, FBPase, phosphoglucoisomerase (PGI), phosphoglucomutase (PGM), and then a pyrophosphorylase to make a
nucleotide sugar. These reactions occur in the stroma (starch)
and cytosol (sucrose). Despite the common intermediates, the
pathways usually remain isolated. Once FBPase acts, carbon
is committed to the stroma or cytosol. A G6P transporter
in plants (GPT2) is expressed under some conditions but is
normally not present in photosynthesizing leaves (Kammerer
et al., 1998), keeping starch and sucrose metabolism separate.
The separate hexose phosphate metabolism in the stroma
and cytosol allows for differences in regulation. Among the
more interesting observations is that the G6P/F6P ratio is
usually in PGI-equilibrium (3.31, Dyson and Noltmann,
1968) in the cytosol but not the stroma (Dietz, 1985, Gerhardt
et al., 1987, Schleucher et al., 1999). The G6P concentration
in the stroma is generally much less than expected based
on the amount of F6P present. A major reason may be the
regulation of PGI by PGA (Dietz, 1985). PGI is also inhibited by 6-phosphogluconate, DHAP, and E4P (Backhausen
et al., 1997). The effect of PGA on the starch synthesis pathway is complex. PGA is an inhibitor of PGI but activator
of ADPglucose pyrophosphorylase (AGPase). Therefore,
high PGA limits the production of G6P from F6P while
4070 | Weise et al.
Fig. 3. Reactions and locations of starch and sucrose synthesis. ADPG,
ADPglucose; UDPG, UDPglucose.
simultaneously stimulating its consumption for starch synthesis. The effect of PGA, E4P, and DHAP limiting PGI keeps
the G6P concentration in the stroma low, much lower than in
the cytosol (Gerhardt et al., 1987). Increases in these metabolites, and increased restriction at PGI, has been invoked to
explain the decline in photosynthesis at high CO2 that is
sometimes seen in A/Ci curves (Sharkey and Vassey, 1989).
Hypothesis
The G6P shunt
One reason it might be advantageous to keep the concentration of G6P low is to restrict G6P dehydrogenase
(G6PDH) activity in the stroma. Plastids have both the
non-oxidative and oxidative branches of the pentose phosphate pathway (PPP) while only the oxidative branch is
present in the cytosol (Schnarrenberger et al., 1973, 1995).
There are three genes encoding functional plastid-localized G6PDH (Wakao and Benning, 2005) (Table 1). One
is redox-regulated and two are not. The redox-regulated
gene is expressed more in leaves than the redox-insensitive
genes, but all three show some expression in leaves. We
are proposing that G6PDH is not completely inactivated
in the light and that the oxidative branch of the pentose
phosphate pathway occurs simultaneously with the Calvin–
Benson cycle. A number of observations can be explained
by a shunt around the Calvin–Benson cycle that begins
with glucose 6-phosphate and follows the oxidative branch
of the pentose phosphate pathway (Fig. 4, redrawn from
Sharkey and Weise, 2012, Fig. 26.9). We estimate (see section on incomplete labelling) that this shunt could routinely
be responsible for 10–20% of the activity of Rubisco and
much higher amounts in some mutant lines.
Most of the plastid-localized G6PDH activity is redox
regulated and controlled by thioredoxin f (Née et al., 2009).
Normally, light regulation of stromal enzymes results in
activation in the light but in the case of G6PDH, light
causes deactivation. However, unlike some enzymes in
which the redox regulation results in a complete lack of
activity, G6PDH retains some activity in its off (reduced)
state. A number of studies have measured whole-leaf
G6PDH activity and called the activity in the presence of
DTT all cytosolic. However, when the plastidial enzyme was
expressed in E. coli and purified, it was found to have some
activity in the reduced state (Née et al., 2009). The reduced
form has the same kcat but higher Km than the oxidized
form (Scheibe et al., 1989; Hauschild and von Schaewen,
2003). The inhibition of activity caused by reduction can
be reversed in bacteria (Cossar et al., 1984) by low mM
concentrations of G6P. These observations indicate that
G6PDH activity in chloroplasts may be extremely sensitive to the concentration of G6P, possibly showing biphasic
kinetics if G6P activates the enzyme while also serving as
a substrate. Thiol groups that might be responsible for the
redox sensitivity have been identified and it has been shown
that cytosolic forms are not redox sensitive (Wenderoth
et al., 1997).
The G6P shunt requires 6-phosphogluconolactonase
(6PGLase). There is one gene in Arabidopsis that appears to code
for a plastid-localized 6PGLase (Kruger and von Schaewen,
2003) (Table 1). Loss of the plastid enzyme is lethal (Xiong et al.,
2009). The product of G6PDH is δ-6-phosphoguconolactone
(pyranose form), which hydrolyses spontaneously. However,
it has been shown that it also spontaneously converts to γ-6phosphoglucanolactone (furanose form), which can accumulate and is toxic (Miclet et al., 2001). 6PGLase can keep the
concentration of δ-6-phosphoguconolactone so low that γ-6phosphoglucanolactone does not accumulate (Miclet et al.,
2001).
The third enzyme required is 6-phosphogluconate dehydrogenase (6PGDH). This enzyme results in reducing power but
decarboxylates and so undoes the carbon fixation that occurs
at Rubisco. There are two genes that appear to code for plastid-localized 6PGDH. Although the 6PGDH genes do not
indicate the presence of a transit peptide, it is believed that
the products of two of the genes in the Arabidopsis genome
are localized to the chloroplast (Krepinsky et al., 2001). One
of them shows coexpression with a gene coding for a protein
required for NDH-dependent cyclic electron flow (Table 1) –
a hint of some relationship between the G6P shunt and cyclic
electron flow. The G6P shunt, if normally active at 10–20% of
the rate of the non-oxidative Calvin–Benson cycle reactions
and more active whenever G6P accumulates in the stroma,
has significant explanatory power.
The G6P shunt may be responsible for incomplete
labelling of the Calvin–Benson cycle
It has been observed that the Calvin–Benson cycle does not
become fully labelled when 14CO2 (Canvin, 1979) or 13CO2
(Delwiche and Sharkey, 1993; Szecowka et al., 2013) is fed
to photosynthesizing leaves. This observation provides an
estimate of the rate of the G6P shunt. The carbon used by
photosynthesizing leaves to make isoprene comes directly
from the Calvin–Benson cycle and shows the same labelling
The G6P shunt of photosynthesis | 4071
Table 1. Genes of interest in the G6P shunt
For selected genes the Arabidopsis thaliana CoEXpression server (@CoEX) (Atias et al., 2009) was used to find gene neighbours that are
coexpressed in a similar manner to our query gene.
Gene
At#
Location
cDNA (bp)
Other information
Glucose-6-phosphate dehydrogenase
G6PDH1
At5g35790
Plastid
1731
P1 isoform, redox sensitive, @CoEX neighbors: lots and lots of things
G6PDH2
At5g13110
Plastid
1791
P2 isoform, redox insensitive, @CoEX neighbors: G6PDH3, 6PGDH3
G6PDH3
At1g24280
Plastid
1800
P2 isoform, redox insensitive, @CoEX neighbors 6PGDH3 and G6PDH2
G6PDH4
At1g09420
Plastid
non functional, involved in alternative targeting of G6PDH1 to peroxisome, @ CoEx not in database
G6PDH5
At3g27300
Cytosol
G6PDH6
At5g40760
Cytosol
Wakao S, Benning C. 2005. Genome-wide analysis of glucose-6-phosphate dehydrogenases in Arabidopsis. The Plant Journal 41, 243–256.
6-Phosphogluconolactonase
6PGL1
At1g13700
6PGL2
At3g49360
6PGL3
At5g24400
6PGL4
At5g24410
Cytosol
Cytosol
Plastid
Cytosol
KO is lethal, can be targeted to peroxisome, @CoEX not in database
6PGL5
At5g24420
Cytosol
Xiong Y, DeFraia C, Williams D, Zhang X, Mou Z. 2009. Characterization of Arabidopsis 6-phosphogluconolactonase T-DNA insertion mutants reveals an essential
role for the oxidative section of the plastidic pentose phosphate pathway in plant growth and development. Plant and Cell Physiology 50, 1277–1291.
6-Phosphogluconate dehydrogenase
6PGDH1
At1g64190
Plastid
@CoEx not in database
6PGDH2
At3g02360
Plastid
@CoEx neighbors novel subunit of NADPH dehydrogenase complex involved in cyclic electron flow
6PGDH3
At5g41670
Cytosol
There are also three in tomato: Tanksley S, Kuehn G. 1985. Genetics, subcellular localization, and molecular characterization of 6-phosphogluconate
dehydrogenase isozymes in tomato. Biochemical Genetics 23, 441–454.
Transaldolase
TAL1
At5g13420
Plastid
1317
recombinant protein had activity, @CoEX neighbors were PGK, enolase, and ferredoxin: NADPH
oxidoreductase, no KO available
TAL2
At1g12230
Plastid
1284
recombinant protein had no activity, @CoEX not in database, no KO available
Caillau M, Quick Wp. 2005. New insights into plant transaldolase. The Plant Journal 43, 1–16.
kinetics as carbon in the cycle (Delwiche and Sharkey, 1993).
Using isoprene as a window on Calvin–Benson cycle labelling, it was shown that incomplete labelling occurred in
leaves in low oxygen and so photorespiratory intermediates
were not responsible for the input of carbon from a slowly
labelling pool (Delwiche and Sharkey, 1993). Another possibility is a shunt of carbon around the Calvin–Benson cycle
involving cytosolic reactions and the XPT transporter. If the
hexose phosphates in the cytosol label slowly and if some are
converted to Xu5P and reimported, then this could be the
slowly labelling pool. This would provide a way of exporting reducing power as NADPH from the chloroplast to the
cytosol.
However, a more likely scenario is that the plastidial G6P
shunt (Fig. 4) provides carbon from a slow-to-label pool.
The low activity of PGI would allow for isotopic disequilibrium between F6P and G6P as has been demonstrated
(Schleucher et al., 1999). The glucose phosphates in the plastid may be in isotopic equilibrium with a larger pool of glucose molecules that are part of the starch synthesis process.
Starch is different from other polyglucans such as glycogen
because of its crystalline structure. The crystalline structure
results from a double helix structure of A chains of maltodextrin that make up amylopectin. During starch synthesis it
is believed that many A chains form but then are trimmed by
debranching enzyme or α-amylase to make crystallizationcompetent starch (Myers et al., 2000; Delatte et al., 2005;
Wu et al., 2014). If maltodextrins resulting from debranching enzyme activity or α-amylase were acted on by starch
phosphorylase, this could result in an effective pool of glucose phosphates that would be relatively large (Fig. 5). If this
pool of glucose phosphates fed into the G6P shunt, it could
account for the slow labelling of some of the carbon in the
Calvin–Benson cycle.
Assuming that the shunt is responsible for the slow labelling pool of Calvin–Benson cycle intermediates, the magnitude of the shunt can be estimated. The slow labelling pool
was as much as 20% of the total in several studies feeding
either 14CO2 (Canvin, 1979) or 13CO2 (Delwiche and Sharkey,
1993; Szecowka et al., 2013).
Refilling the Calvin–Benson cycle
Stromal PGI activity is inhibited by PGA, meaning that when
PGA is low PGI activity can be high. This provides a route
for injecting carbon into the Calvin–Benson cycle from hexose phosphates. This can be useful when photosynthesis first
begins and when photosynthesis undergoes a rapid change
that depletes the Calvin–Benson cycle of phosphorylated
intermediates. But if TPs are low, hexose phosphates cannot
4072 | Weise et al.
in chloroplasts in the light. In order for starch to provide
carbon to the Calvin–Benson cycle, as happens under photorespiratory conditions (Weise et al., 2006), it is likely that
the carbon must go through the G6P shunt to make pentose
phosphates that are carboxylated and eventually converted to
GAP to refill the Calvin–Benson cycle.
Regulation of the shunt and GPT2
Fig. 4. The G6P shunt uses ATP but is redox neutral. 6PG(L),
6-phosphogluconate (lactone).
Fig. 5. A possible role for maltodextrins involved in starch synthesis as the
slow-to-label pool of carbon. Maltodextrins, resulting from debranching
enzyme activity needed to make the branching of starch regular, are acted
on by starch phosphorylase. The G1P made by starch phosphorylase can
be converted to G6P, which can enter the G6P shunt.
refill the Calvin–Benson cycle because TPs are needed to
make SBP and XuBP. Sixty per cent of the requirement for
TPs occurs downstream of F6P (Fig. 2). Hexose phosphates
could be converted to TPs if there were phosphofructokinase
(PFK) activity inside chloroplasts, but this activity would
become a futile cycle with FBPase. Arabidopsis has one gene
that codes for an apparent plastid-localized PFK (Table 1)
and there is a report that this protein is imported into chloroplasts (Mustroph et al., 2007). There is also a report that
PFK is inactivated in the chloroplast in the light (Heuer et al.,
1982). Our current hypothesis is that there is no PFK activity
The effect of 6PG on stromal PGI (Dietz, 1985) provides negative feedback. If too much carbon is entering the shunt, 6PG
will accumulate and reduce the activity of PGI and so reduce
the flow of carbon into the shunt. However, the regulation at
PGI can be bypassed if GPT2 is present, allowing G6P from
the cytosol to enter the stroma. Current thinking is that GPT2
is not normally expressed in photosynthetic tissue (Kammerer
et al., 1998) but expression has been found in a number of
situations. Expression of GPT2 is induced when leaves are
fed sugar or when sugar accumulates, when leaves lack PGM,
and at high CO2 (Lloyd and Zakhleniuk, 2004; Leakey et al.,
2009; Kunz et al., 2010 ). When plants are moved from low
light to high light, GPT2 is transiently expressed (Dyson et al.,
2015). GPT2 knockout plants do not acclimate to higher light
very well (Athanasiou et al., 2010; Dyson et al., 2015). Plants
lacking stromal FBPase (hcef1; Livingston et al., 2010) also
express the gene for GPT2. Thus, GPT2 expression is generally low in C3 plants but small variations in expression are
critical for adaptation to changes in growth light intensity and
photosynthetic capacity, as determined by CO2 exchange. On
the other hand, GPT2 is strongly expressed in CAM plants
and is induced when facultative CAM plants are induced to
carry out CAM (Neuhaus and Schulte, 1996; Cushman et al.,
2008). It is also expressed in plants grown in elevated CO2
(Leakey et al., 2009). Starch accumulation during the day and
breakdown at night is much greater in CAM plants than in
C3 plants. Expression of GPT2 may be important for carbon
import into plastids during the day (Dyson et al., 2015) to
support high rates of starch synthesis in the face of a kinetically limited stromal PGI. At the same time, by exporting
G6P, CAM chloroplasts can be more efficient at night, when
the supply of ATP can consume a great deal of the plant’s
resources. Conversion of starch in plastids to sucrose in the
cytosol requires just two ATP per sucrose molecule when
GPT2 is present while other potential export pathways require
more ATP per sucrose (Fig. 6). However, GPT2 expression
appears to be a critical control point and highly variable, but
relatively little is known about how its expression is controlled.
Operation of the shunt induces cyclic electron flow
The G6P shunt is a futile cycle in which ATP is consumed
(3 per turn of the cycle) but no CO2 is fixed and there is no
net change in NADPH. However, unlike the FBPase/PFK
futile cycle, the G6P shunt has a number of properties that
may make it useful during photosynthesis. It is well accepted
that normal Calvin–Benson cycle requirements of 3 ATP per
2 NADPH exceeds the normal ratio of ATP produced per
NADPH in linear electron flow and that the ATP deficit is
The G6P shunt of photosynthesis | 4073
Therefore, the flux of electrons required for fixing one CO2
would be 4 through PSII and 4.57 through PSI so that the
proportion of photons through PSII would be 4/8.57=0.467.
If 20% of the carbon fixed by Rubisco ended up in the G6P
shunt then 20%×3 ATP per G6P in the shunt=0.6 additional
ATP would be required per CO2 fixed. Now an extra 2.8 protons are needed and so 0.57+0.8 electrons would have to go
through the NDH complex making the required electron flux
4 through PSII and 5.37 through PSI so the proportion of
photons to PSII would have to be 4/9.37=0.426. When multiplied by a typical absorptance of 85%, the predicted slope
of electron transport versus light intensity would be 0.363
instead of 0.397 in the absence of the G6P shunt. The uncertainty in absorptance of leaves is generally greater than the
extra energy consumed in the proposed G6P shunt.
Fig. 6. ATP per sucrose required during starch to sucrose conversion.
made up by cyclic electron flow (Joliot and Johnson, 2011).
These authors also point out that cyclic electron flow can
help induce a large ΔpH regulating electron generation in
PSII because of both the protonation of PsbS and the formation of zeaxanthin (Takizawa et al., 2007). This helps control
the generation of energetic electrons when carbon metabolism does not keep pace with the capacity for electron transport. The G6P shunt increases the ATP/NADPH required
in photosynthesis (photorespiration on the other hand has
a relatively small effect on the ATP/NADPH ratio; Sharkey,
1988). If 20% of Rubisco activity is devoted to the G6P shunt
then the ATP/NADPH ratio required becomes 3.6 ATP per
2 NADPH. This more than doubles the ATP deficit of linear
electron flow. Recent observations indicate that this deficit
is made up by a stimulation of NDH-dependent cyclic photophosphorylation (Strand et al., 2015).
Energetics
The initial slope of electron transport versus light intensity
is typically in the range of 0.3–0.45 mol of electrons transported per mol of photons. This reflects the absorptance of
the leaf (typical value 0.85 but this can vary significantly) and
the proportion of electrons going to PSII, often taken to be
0.5. However, our current understanding of the stoichiometry
of ATP to NADPH production and consumption requires
some additional source of ATP even in the absence of the
G6P shunt. Assuming that 14 protons are required for three
ATP, and that each electron transported requires two photons
and transports three protons (fully functional Q cycle), then
2 NADPH requires four electrons which transport 12 protons to make 3 × 12/14=2.57 ATP. This leaves a deficit of 0.43
ATP that would require 0.43 × 14/3=two additional protons
to be pumped by cyclic electron flow. We assume the most
efficient of the cyclic electron flow pathways, the NADPH
dehydrogenase complex (NDH) (Burrows et al., 1998; Suorsa
et al., 2009). Two protons could be translocated as the electron moves through the complex and two in the Q cycle for
a total of four protons per electron but there is evidence
the number is closer to 3.5 (Wikström and Hummer, 2012).
A stromal FBPase knock-out has high cyclic electron
flow and GPT2 expression
An Arabidopsis mutant selected for its high cyclic electron
flow (hcef1) is deficient in stromal FBPase (Livingston et al.,
2010). We hypothesized that to bypass the stromal FBPase,
a path of carbon was established in which triose phosphates
were exported to the cytosol, converted to G6P, reimported to
the stroma, and converted to F6P to feed the Calvin–Benson
cycle (Fig. 7). This pathway had been suggested earlier
(Kossmann et al., 1994). In order for this pathway to function, the glucose 6-phosphate transporter would have to be
induced. We have tested this and found that mRNA for GPT2
is much higher in hcef1 plants than in the wild type (Fig. 8).
There is a kinetic limitation at PGI in the direction of G6P
to F6P (PGI Km for F6P is 300 μM but for G6P it is 8 mM;
Schnarrenberger and Oeser, 1974) and so a high concentration of G6P would be required to supply carbon to the
Calvin–Benson cycle by this pathway. This hypothesized high
stromal G6P concentration of hcef1 plants could cause significant rates of carbon flux through the G6P shunt requiring a significant rate of cyclic photophosphorylation. Strand
et al. (2015) have shown that one of the components of the
transduction pathway is H2O2 and that it is the NDH cyclic
pathway that is induced. This is inefficient and plants lacking stromal FBPase do not grow well, but they do grow. We
hypothesize that they grow by allowing G6P from the cytosol
to enter the chloroplast to allow some Calvin–Benson cycle
activity. Hydrogen-peroxide-induced cyclic electron flow
makes up the ATP lost in the G6P shunt. Loss of aldolase
could be overcome by a similar mechanism and plants lacking plastidial aldolase also have a high rate of cyclic electron
flow (Gotoh et al., 2010). Both the FBPase- and aldolaselacking mutants were shown to have elevated NDH-pathway
cyclic electron flow while abiotic stresses such as heat stress
and water stress may, instead, stimulate ferredoxin quinone
reductase (FQR)-dependent cyclic electron flow.
The G6P shunt may interact with SBPase
SBPase is well situated to control the balance between the
Calvin–Benson cycle and the G6P shunt (Fig. 9). SBPase
4074 | Weise et al.
Fig. 7. The stromal FBPase can be bypassed if GPT2 is expressed. The
limitation at PGI will cause G6P to build up, leading to significant flow
through the G6P shunt consuming ATP, but some Calvin–Benson cycle
can occur.
Fig. 9. The relationship between SBPase and the G6P shunt.
to have just the right amount of SBPase for optimal flux
through the G6P shunt. In constant conditions, reducing
flux through the G6P shunt might lead to faster growth, but
there could be a cost in highly variable conditions. Reducing
SBPase activity might increase the G6P shunt leading to less
efficient photosynthesis.
The G6P shunt can relieve excess light energy at
photosystem I
Fig. 8. Transcripts of GPT2 in hcef1 plants. Plants were grown in a 12 h
photoperiod and were 7 weeks old at the time of harvest. Leaf material
was taken in the middle of the day when GPT2 transcript amounts were
previously observed to be the highest. RNA was extracted using a Qiagen
RNeasy plant kit and cDNA was synthesized using Invitrogen SuperScript
II reverse transcriptase, both according to the manufacturer’s directions.
The resulting cDNA sample was split in two and Quantitative PCR was
conducted using primers specific for Actin2 (AT3G18780) or GPT2
(At1g61800). SYBR green PCR master mix from Applied Biosystems was
used. Values are mean ±SE, n=4. (Unpublished data of AL Preiser, DD
Strand, SE Weise, and TD Sharkey)
is one of only three enzymes unique to the Calvin–Benson
cycle. It is distantly related to FBPase. Both genetic and modelling studies have shown that SBPase can have a large role in
determining the rate of photosynthesis (Harrison et al., 2001;
Poolman et al., 2001; Lawson et al., 2006; Feng et al., 2007,
Zhu et al., 2007; Rosenthal et al., 2011; Liu et al., 2012 ). It is
unclear why this enzyme should be in limiting amounts when
most other Calvin–Benson cycle enzymes (except Rubisco)
generally limit very little so that available resources such as
light and CO2 can be used as efficiently as possible. The activity of SBPase could be important in determining the balance
between the Calvin–Benson cycle and the G6P shunt. If some
low flux through the shunt is advantageous to stabilize carbon
metabolism in the chloroplast, then it could be advantageous
A major connection between the regulation of carbon metabolism and electron transport works through phosphate. When
carbon metabolism begins to limit the overall rate of photosynthesis, phosphate may become limiting for ATP synthesis,
possibly because of the affinity of ATP synthase for phosphate but more likely through its effect on the effective ΔG
of formation of ATP. This causes the proton motive force to
increase and the pH component of the PMF results in a low
pH in the thylakoid lumen, protonating PsbS and stimulating zeaxanthin formation, and eventually slowing electron
flow at the cytochrome b6/f complex (Takizawa et al., 2007).
These two effects will cause non-photochemical quenching of
energy arriving at PSII and so reduce electron flow allowing
PSI to remain oxidized. Oxidized PSI is an effective quencher
of incoming light energy. However, it is hypothesized that PSI
sometimes cannot remain as oxidized as needed to quench
incoming energy. Excess light energy arriving at reduced PSI
can cause significant damage to the PSI reaction centre proteins (Sonoike and Terashima, 1994). While PSII has an active
repair cycle, PSI does not. Damage to PSI is frequently seen
in response to chilling (Terashima et al., 1994; Govindachary
et al., 2007) and the induction of cyclic electron flow around
PSI may reduce the damage (Li et al., 2006; Wang et al., 2006;
Govindachary et al., 2007). The cyclic electron flow may help
The G6P shunt of photosynthesis | 4075
to maintain a low luminal pH and so enhance non-photochemical quenching at PSII, but it could also dissipate energy
coming into PSI by using ATP in the G6P shunt. This would
allow energy arriving at reduced PSI centres to be used in
cyclic flow and the protons pumped in this way to be used to
make ATP that is dissipated in the shunt.
Summary
The core reactions leading to a net reduction of CO2 are well
known. However, there are alternatives to the Calvin–Benson
cycle and, in some cases, it is not clear why carbon does not
follow these alternative pathways. On the other hand, it is
very possible that a significant amount of carbon does follow an alternative pathway, the oxidative branch of the pentose phosphate pathway, leading to a futile cycle. This can
be compensated by cyclic photophosphorylation and, given
the energetic efficiency of the NDH-dependent cyclic electron
flow, the overall requirement for photons for fixing CO2 is not
greatly increased. This alternative carbon flow can explain
several previously puzzling observations, especially a lack
of complete labelling of the Calvin–Benson cycle intermediates when carbon isotopes are fed and high cyclic electron
flow phenotypes of mutants in Calvin–Benson cycle enzymes.
Study of this alternative carbon path may help to elucidate
the mechanisms by which photosynthesis is stabilized and
regulated, leading to opportunities for altering the regulation
when that would lead to more production of food, fuel, and
fibre.
Cossar JD, Rowell P, Stewart WDP. 1984. Thioredoxin as a modulator
of glucose-6-phosphate dehydrogenase in a N2-fixing cyanobacterium.
Journal of General Microbiology 130, 991–998.
Cushman JC, Tillett RL, Wood JA, Branco JM, Schlauch KA.
2008. Large-scale mRNA expression profiling in the common ice plant,
Mesembryanthemum crystallinum, performing C3 photosynthesis and
Crassulacean acid metabolism (CAM). Journal of Experimental Botany 59,
1875–1894.
Delatte T, Trevisan M, Parker ML, Zeeman SC. 2005. Arabidopsis
mutants Atisa1 and Atisa2 have identical phenotypes and lack the same
multimeric isoamylase, which influences the branch point distribution of
amylopectin during starch synthesis. The Plant Journal 41, 815–830.
Delwiche CF, Sharkey TD. 1993. Rapid appearance of 13C in biogenic
isoprene when 13CO2 is fed to intact leaves. Plant, Cell and Environment
16, 587–591.
Dietz K-J. 1985. A possible rate-limiting function of chloroplast
hexosemonophosphate isomerase in starch synthesis of leaves.
Biochimica et Biophysica Acta 839, 240–248.
Dyson BC, Allwood JW, Feil R, Xu YUN, Miller M, Bowsher CG,
Goodacre R, Lunn JE, Johnson GN. 2015. Acclimation of metabolism
to light in Arabidopsis thaliana: the glucose 6-phosphate/phosphate
translocator GPT2 directs metabolic acclimation. Plant, Cell & Environment
38, 1404–1417.
Dyson JED, Noltmann EA. 1968. The effect of pH and temperature
on the kinetic parameters of phosphoglucose isomerase. The Journal of
Biological Chemistry 243, 1401–1414.
Feng L, Han Y, Liu G, An B, Yang J, Yang G, Li Y, Zhu Y. 2007.
Overexpression of sedoheptulose-1,7-bisphosphatase enhances
photosynthesis and growth under salt stress in transgenic rice plants.
Functional Plant Biology 34, 822–834.
Flügge UI, Heldt HW. 1991. Metabolite translocators of the chloroplast
envelope. Annual Review of Plant Physiology and Plant Molecular Biology
42, 129–144.
Gerhardt R, Stitt M, Heldt HW. 1987. Subcellular metabolite levels in
spinach leaves. Regulation of sucrose synthesis during diurnal alterations
in photosynthetic partitioning. Plant Physiology 83, 399–407.
Gibbs M. 1966. Intermediary metabolism and pathology. In: Sterward FC.
ed. Plant physiology, Vol. IVB . New York: Academic Press, 3–115.
Acknowledgements
Our work on photosynthesis is funded by the US Department of Energy
grant DE-SCOOO8509 and by USDA in support of TDS’ salary.
References
Athanasiou K, Dyson BC, Webster RE, Johnson GN. 2010. Dynamic
acclimation of photosynthesis increases plant fitness in changing
environments. Plant Physiology 152, 366–373.
Atias O, Chor B, Chamovitz D. 2009. Large-scale analysis of
Arabidopsis transcription reveals a basal co-regulation network. BMC
Systems Biology 3, 86.
Backhausen JE, Jöstingmeyer P, Scheibe R. 1997. Competitive
inhibition of spinach leaf phosphoglucose isomerase isoenzymes by
erythrose 4-phosphate. Plant Science 130, 121–131.
Bassham JA, Benson AA, Kay LD, Harris AZ, Wilson AT, Calvin M.
1954. The path of carbon in photosynthesis. XXI. The cyclic regeneration
of carbon dioxide acceptor. Journal of the American Chemical Society 76,
1760–1770.
Benson AA. 2010. Last days in the old radiation laboratory (ORL),
Berkeley, California, 1954. Photosynthesis Research 105, 209–212.
Burrows PA, Sazanov LA, Svab Z, Maliga P, Nixon PJ. 1998.
Identification of a functional respiratory complex in chloroplasts through
analysis of tobacco mutants containing disrupted plastid ndh genes.
EMBO Journal 17, 868–876.
Caillau M, Quick WP. 2005. New insights into plant transaldolase. The
Plant Journal 43, 1–16.
Canvin DT. 1979. Photorespiration: comparison between C3 and C4
plants. In: Gibbs M, Latzko E, eds.. Encyclopedia of plant physiology, New
Series, Vol 6. Photosynthesis II . Berlin: Springer, 368–396.
Gotoh E, Matsumoto M, Ogawa Ki, Kobayashi Y, Tsuyama M.
2010. A qualitative analysis of the regulation of cyclic electron flow
around photosystem I from the post-illumination chlorophyll fluorescence
transient in Arabidopsis: a new platform for the in vivo investigation of the
chloroplast redox state. Photosynthesis Research 103, 111–123.
Govindachary S, Bigras C, Harnois J, Joly D, Carpentier R. 2007.
Changes in the mode of electron flow to photosystem I following chillinginduced photoinhibition in a C3 plant, Cucumis sativus L. Photosynthesis
Research 94, 333–345.
Harrison EP, Olcer H, Lloyd JC, Long SP, Raines CA. 2001. Small
decreases in SBPase cause a linear decline in the apparent RuBP
regeneration rate, but do not affect Rubisco carboxylation capacity.
Journal of Experimental Botany 52, 1779–1784.
Hauschild R, von Schaewen A. 2003. Differential regulation of glucose6-phosphate dehydrogenase isoenzyme activities in potato. Plant
Physiology 133, 47–62.
Heuer B, Hansen MJ, Anderson LE. 1982. Light modulation of
phosphofructokinase in pea leaf chloroplasts. Plant Physiology 69,
1404–1406.
Horecker BL, Gibbs M, Klenow H, Smyrniotis PZ. 1954. The
mechanism of pentose phosphate conversion to hexose monophosphate.
I. With a liver enzyme preparation. Journal of Biological Chemistry 207,
393–404.
Horecker BL, Smyrniotis PZ. 1952. The enzymatic formation of
sedoheptulose phosphate from pentose phosphate. Journal of the
American Chemical Society 74, 2123.
Horecker BL, Smyrniotis PZ. 1953. Transaldolase: the formation of
fructose-6-phosphate from sedoheptulose-7-phosphate. Journal of the
American Chemical Society 75, 2021–2022.
Joliot P, Johnson GN. 2011. Regulation of cyclic and linear electron flow
in higher plants. Proceedings of the National Academy of Sciences, USA
108, 13317–13322.
4076 | Weise et al.
Kammerer B, Fischer K, Hilpert B, Schubert S, Gutensohn M, Weber
A, Flügge UI. 1998. Molecular characterization of a carbon transporter in
plastids from heterotrophic tissues: the glucose 6-phosphate phosphate
antiporter. The Plant Cell 10, 105–117.
Kossmann J, Sonnewald U, Willmitzer L. 1994. Reduction of the
chloroplastic fructose-1,6-bisphosphatase in transgenic potato plants
impairs photosynthesis and plant growth. The Plant Journal 6, 637–650.
Krepinsky K, Plaumann M, Martin W, Schnarrenberger C.
2001. Purification and cloning of chloroplast 6-phosphogluconate
dehydrogenase from spinach. European Journal of Biochemistry 268,
2678–2686.
Kruger NJ, von Schaewen A. 2003. The oxidative pentose phosphate
pathway: structure and organisation. Current Opinion in Plant Biology 6,
236–246.
Kunz HH, Hausler RE, Fettke J, Herbst K, Niewiadomski P, Gierth
M, Bell K, Steup M, Flugge UI, Schneider A. 2010. The role of plastidial
glucose-6-phosphate/phosphate translocators in vegetative tissues of
Arabidopsis thaliana mutants impaired in starch biosynthesis. Plant Biology
(Stuttgart) 12 Supplement 1, 115–128.
Lawson T, Bryant B, Lefebvre S, Lloyd JC, Raines CA. 2006.
Decreased SBPase activity alters growth and development in transgenic
tobacco plants. Plant, Cell & Environment 29, 48–58.
Leakey ADB, Xu F, Gillespie KM, McGrath JM, Ainsworth EA, Ort
DR. 2009. Genomic basis for stimulated respiration by plants growing
under elevated carbon dioxide. Proceedings of the National Academy of
Sciences, USA 106, 3597–3602.
Li X-g, Zhao J-p, Xu P-l, Meng J-j, He Q-w. 2006. Effects of cyclic
electron flow inhibitor (antimycin A) on photosystem photoinhibition of
sweet pepper leaves upon exposure to chilling stress under low irradiance.
Agricultural Sciences in China 5, 506–511.
Liu X-L, Yu H-D, Guan Y, Li J-K, Guo F-Q. 2012. Carbonylation and
loss-of-function analyses of SBPase reveal its metabolic interface role in
oxidative stress, carbon assimilation, and multiple aspects of growth and
development in Arabidopsis. Molecular Plant 5, 1082–1099.
Livingston AK, Cruz JA, Kohzuma K, Dhingra A, Kramer DM. 2010.
An Arabidopsis mutant with high cyclic electron flow around photosystem
I (hcef) involving the NADPH dehydrogenase complex. The Plant Cell 22,
221–233.
Lloyd JC, Zakhleniuk OV. 2004. Responses of primary and secondary
metabolism to sugar accumulation revealed by microarray expression
analysis of the Arabidopsis mutant, pho3. Journal of Experimental Botany
55, 1221–1230.
Martin W, Scheibe R, Schnarrenberger C. 2000. The Calvin cycle
and its regulation. In: Leegood RC, Sharkey TD, von Caemmerer S. eds.
Advances in photosynthesis, Vol. 9. Photosynthesis: physiology and
metabolism. Dordrecht: Kluwer Academic Publishers: 9–51.
Miclet E, Stoven V, Michels PA, Opperdoes FR, Lallemand
JY, Duffieux F. 2001. NMR spectroscopic analysis of the first two
steps of the pentose-phosphate pathway elucidates the role of
6-phosphogluconolactonase. The Journal of Biological Chemistry 276,
34840–34846.
Mustroph A, Sonnewald U, Biemelt S. 2007. Characterisation of
the ATP-dependent phosphofructokinase gene family from Arabidopsis
thaliana. FEBS Letters 581, 2401–2410.
Myers AM, Morell MK, James MG, Ball SG. 2000. Recent progress
toward understanding biosynthesis of the amylopectin crystal. Plant
Physiology 122, 989–997.
Née G, Zaffagnini M, Trost P, Issakidis-Bourguet E. 2009. Redox
regulation of chloroplastic glucose-6-phosphate dehydrogenase: a new
role for f-type thioredoxin. FEBS Letters 583, 2827–2832.
Neuhaus HE, Schulte N. 1996. Starch degradation in chloroplasts
isolated from C3 or CAM (crassulacean acid metabolism)-induced
Mesembryanthemum crystallinum L. Biochemical Journal 318, 945–953.
Ölçer H, Lloyd JC, Raines CA. 2001. Photosynthetic capacity is
differentially affected by reductions in sedoheptulose-1,7-bisphosphatase
activity during leaf development in transgenic tobacco plants. Plant
Physiology 125, 982–989.
Poolman MG, Ölçer H, Lloyd JC, Raines CA, Fell DA. 2001.
Computer modelling and experimental evidence for two steady states in
the photosynthetic Calvin cycle. European Journal of Biochemistry 268,
2810–2816.
Raines CA, Lloyd JC, Dyer TA. 1999. New insights into the structure
and function of sedoheptulose-1,7-bisphosphatase; an important but
neglected Calvin cycle enzyme. Journal of Experimental Botany 50, 1–8.
Rosenthal DM, Locke AM, Khozaei M, Raines CA, Long SP,
Ort DR. 2011. Over-expressing the C3 photosynthesis cycle enzyme
sedoheptulose-1-7 bisphosphatase improves photosynthetic carbon gain
and yield under fully open air CO2 fumigation (FACE). BMC Plant Biology
11, 123.
Scheibe R, Geissler A, Fickenscher K. 1989. Chloroplast glucose-6phosphate dehydrogenase: Km shift upon light modulation and reduction.
Archives of Biochemistry and Biophysics 274, 290–297.
Schleucher J, Vanderveer P, Markley JL, Sharkey TD. 1999.
Intramolecular deuterium distributions reveal disequilibrium of chloroplast
phosphoglucose isomerase. Plant, Cell & Environment 22, 525–533.
Schmid M, Davison TS, Henz SR, Pape UJ, Demar M, Vingron M,
Scholkopf B, Weigel D, Lohmann JU. 2005. A gene expression map of
Arabidopsis thaliana development. Nature Genetics 37, 501–506.
Schnarrenberger C, Flechner A, Martin W. 1995. Enzymatic evidence
for a complete oxidative pentose phosphate pathway in chloroplasts and
an incomplete pathway in the cytosol of spinach leaves. Plant Physiology
108, 609–614.
Schnarrenberger C, Oeser A. 1974. Two isoenzymes of
glucosephosphate isomerase from spinach leaves and their intracellular
compartmentation. European Journal of Biochemistry 45, 77–82.
Schnarrenberger C, Oeser A, Tolbert NE. 1973. Two enzymes each
of glucose-6-phosphate dehydrogenase and 6-phosphogluconate
dehydrogenase in spinach leaves. Archives of Biochemistry and
Biophysics 154, 438–448.
Schwender J, Goffman F, Ohlrogge JB, Shachar-Hill Y. 2004. Rubisco
without the Calvin cycle improves the carbon efficiency of developing
green seeds. Nature 432, 779–782.
Sharkey TD. 1988. Estimating the rate of photorespiration in leaves.
Physiologia Plantarum 73, 147–152.
Sharkey TD, Vassey TL. 1989. Low oxygen inhibition of photosynthesis
is caused by inhibition of starch synthesis. Plant Physiology 90, 385–387.
Sharkey TD, Weise SE. 2012. Autotrophic carbon dioxide fixation.
In: Eaton-Rye JJ, Tripathy B, Sharkey TD. eds. Photosynthesis: plastid
biology, energy conversion and carbon assimilation . Dordrecht: Springer
Academic Publications, 649–672.
Sonoike K, Terashima I. 1994. Mechanism of photosystem-I
photoinhibition in leaves of Cucumis sativus L. Planta 194, 287–293.
Strand DD, Livingston AK, Satoh-Cruz M, Froehlich JE, Maurino
VG, Kramer DM. 2015. Activation of cyclic electron flow by hydrogen
peroxide in vivo. Proceedings of the National Academy of Sciences, USA
112, 5539–5544.
Suorsa M, Sirpiö S, Aro E-M. 2009. Towards characterization of
the chloroplast NAD(P)H dehydrogenase complex. Molecular Plant 2,
1127–1140.
Szecowka M, Heise R, Tohge T, et al. 2013. Metabolic fluxes in an
illuminated Arabidopsis rosette. The Plant Cell Online 25, 694–714.
Takizawa K, Cruz JA, Kanazawa A, Kramer DM. 2007. The thylakoid
proton motive force in vivo. Quantitative, non-invasive probes, energetics,
and regulatory consequences of light-induced pmf. Biochimica et
Biophysica Acta 1767, 1233–1244.
Tanksley S, Kuehn G. 1985. Genetics, subcellular localization, and
molecular characterization of 6-phosphogluconate dehydrogenase
isozymes in tomato. Biochemical Genetics 23, 441–454.
Terashima I, Funayama S, Sonoike K. 1994. The site of photoinhibition
in leaves of Cucumis sativus L. at low temperatures is photosystem I, not
photosystem II. Planta 193, 300–306.
Wakao S, Benning C. 2005. Genome-wide analysis of glucose-6phosphate dehydrogenases in Arabidopsis. The Plant Journal 41,
243–256.
Wang P, Duan W, Takabayashi A, Endo T, Shikanai T, Ye JY, Mi H.
2006. Chloroplastic NAD(P)H dehydrogenase in tobacco leaves functions
in alleviation of oxidative damage caused by temperature stress. Plant
Physiology 141, 465–474.
Weise SE, Schrader SM, Kleinbeck KR, Sharkey TD. 2006. Carbon
balance and circadian regulation of hydrolytic and phosphorolytic
breakdown of transitory starch. Plant Physiology 141, 879–886.
The G6P shunt of photosynthesis | 4077
Wenderoth I, Scheibe R, von Schaewen A. 1997. Identification of the
cysteine residues involved in redox modification of plant plastidic glucose6-phosphate dehydrogenase. Journal of Biological Chemistry 272,
26985–26990.
Wikström M, Hummer G. 2012. Stoichiometry of proton translocation by
respiratory complex I and its mechanistic implications. Proceedings of the
National Academy of Sciences, USA 109, 4431–4436.
Woodrow IE, Walker DA. 1983. Regulation of stromal sedoheptulose1,7-bisphosphatase activity and its role in controlling the reductive pentose
phosphate pathway of photosynthesis. Biochimica et Biophysica Acta
722, 508–516.
Wu AC, Ral J-P, Morell MK, Gilbert RG. 2014. New perspectives on the role
of α- and β-amylases in transient starch synthesis. PLoS ONE 9, e100498.
Xiong Y, DeFraia C, Williams D, Zhang X, Mou Z. 2009.
Characterization of Arabidopsis 6-phosphogluconolactonase T-DNA
insertion mutants reveals an essential role for the oxidative section of the
plastidic pentose phosphate pathway in plant growth and development.
Plant and Cell Physiology 50, 1277–1291.
Zhu X-G, de Sturler E, Long SP. 2007. Optimizing the distribution of
resources between enzymes of carbon metabolism can dramatically
increase photosynthetic rate: a numerical simulation using an evolutionary
algorithm. Plant Physiology 145, 513–526.