Download The enigmatic contribution of mitochondrial function in photosynthesis

Journal of Experimental Botany, Vol. 59, No. 7, pp. 1675–1684, 2008
doi:10.1093/jxb/ern002 Advance Access publication 5 February, 2008
SPECIAL ISSUE REVIEW PAPER
The enigmatic contribution of mitochondrial function in
photosynthesis
Adriano Nunes-Nesi*, Ronan Sulpice, Yves Gibon and Alisdair R. Fernie
Max-Planck-Institut für Molekulare Pflanzenphysiologie, Am Mühlenberg 1, D-14476 Potsdam-Golm, Germany
Received 27 September 2007; Revised 8 November 2007; Accepted 13 November 2007
Abstract
Introduction
Considerable cumulative evidence has accrued suggesting a vital role for mitochondrial function in
optimizing photosynthesis. Both pharmacological
approaches using respiratory inhibitors and reverse
genetic approaches have recently underscored the
high degree of interconnection between photosynthesis and respiration—the major pathways of energy
production which are largely confined to the plastid
and mitochondria, respectively. Here recent studies
into the nature of these interactions are reviewed, with
particular focus on (i) the recently described link
between the mitochondrial electron transport chain
activity, ascorbate biosynthesis, and photosynthesis;
and (ii) the contribution of mitochondrial metabolism
to the photorespiratory process. Whilst there is
increasing evidence of a role for ascorbate in coordinating the rates of respiration and photosynthesis,
some data are presented here for plants grown under
extreme environmental conditions that suggest that
this relationship is not absolute. It thus seems likely
that interactions between these compartments are
perhaps more numerous and complicated than previously thought. This observation suggests that although the elucidation of the genetic bases of both
photorespiration and the Wheeler–Smirnoff pathway of
ascorbate biosynthesis has recently been completed,
much further research is probably necessary in order
to understand fully how energy metabolism is coordinated in the illuminated leaf.
Mitochondrial oxidative metabolism has been postulated
to be a prerequisite for the maintenance of high rates of
photosynthesis (Padmasree et al., 2002). Depending on
the developmental stage and/or the environment, reducing
equivalents generated by the photochemical reactions
accumulate in the chloroplast stroma, causing the overreduction of the photosynthetic electron chain and the
generation of reactive oxygen species (ROS), leading to
photoinhibition (Foyer and Noctor, 2000; Allen, 2002). It
has been proposed that this excess of reducing equivalents
generated in the chloroplasts can be dissipated by their
export from the chloroplast to the mitochondria, via the
malate valve (Krömer, 1995; Scheibe, 2004; Scheibe
et al., 2005). Within the mitochondria, the reducing
equivalents are subsequently oxidized by the respiratory
electron transport chain (Noctor et al., 2007), thus
allowing continued high rates of photosynthesis. That
said, recent studies have revealed that mitochondria–
chloroplast interactions are by no means limited to this
example, with considerable evidence suggesting roles for
the tricarboxylic acid (TCA) cycle, both cytochrome and
alternative respiration, and the uncoupling protein in the
mediation of this cross-talk. Since the roles of the
mitochondrial electron transport chain have been extensively reviewed previously (Bartoli et al., 2005; NunesNesi et al., 2007a; Sweetlove et al., 2007), they will not
be documented here, but rather attention will be focused
on the mechanisms by which modulation of the activities
of TCA cycle enzymes and uncoupling protein exert their
effects.
Key words: Ascorbate biosynthesis, mitochondrial function,
photorespiration, photosynthesis.
A role for the TCA cycle in photosynthesis
The TCA cycle is a fundamental component of
mitochondrial respiration, linking glycolysis and/or
* To whom correspondence should be addressed. E-mail: [email protected]
ª The Author [2008]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved.
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1676 Nunes-Nesi et al.
extramitochondrial malate synthesis to the mitochondrial
electron transport chain (Fernie et al., 2004; Millar et al.,
2004). As part of an ongoing project, a wide range of
transgenics and mutants with deficient expression of
enzymes from the TCA cycle have been analysed.
Interestingly, modifications in the photosynthetic rate
were found in a subset of these genotypes. Thus, tomato
plants with less fumarase activity showed a decrease in
photosynthesis (Nunes-Nesi et al., 2007b). In contrast,
tomato plants with reduced expression of aconitase or
malate dehydrogenase showed an enhanced photosynthetic performance (Carrari et al., 2003; Nunes-Nesi et al.,
2005). Finally, in contrast to these effects, tomato plants
with less succinyl CoA ligase (Studart-Guimarães et al.,
2007) or less mitochondrially localized isocitrate dehydrogenase (Lemaitre et al., 2007) have no change in
photosynthesis. While the reasons why decreased succinyl
CoA ligase and isocitrate dehydrogenase are not essential
to photosynthesis are currently unclear, the reduced
photosynthetic activity found in fumarase antisense transgenics could be shown to be related to the impairment of
stomatal function (Nunes-Nesi et al., 2007b). However,
this phenotype is very specific to manipulation of
fumarase activity and lies outside the scope of this article.
Furthermore, a comprehensive molecular and metabolic
characterization of lines with altered aconitase and malate
dehydrogenase (MDH) activities revealed a specific increase in ascorbate in leaves (Carrari et al., 2003; NunesNesi et al., 2005), coupled to a general up-regulation of
levels of transcripts encoding genes associated with
photosynthesis (Urbanczyk-Wochniak et al., 2006). These
transgenesis experiments hint at an important role for
ascorbate in the co-ordination of major pathways of
energy metabolism in the leaf. Moreover, they are in
keeping with the current thinking that redox signals
emanating from the mitochondria are important in setting
the cellular machinery to maintain the overall redox
balance (Sweetlove et al., 2007).
To summarize, recent studies have revealed that
photosynthetic performance can be modulated by modifications in mitochondrial pathways, such as the electron
transport chain (Bartoli et al., 2005) or TCA cycle (Carrari
et al., 2003; Nunes-Nesi et al., 2005). These studies have
suggested ascorbate as a linkage between mitochondrial
and chloroplast metabolism. Recently, another mitochondrial protein, an uncoupling protein (UCP-1), has been
found to be involved in a novel mechanism contributing
to photosynthesis through efficient oxidation of glycine
produced via photorespiration (Sweetlove et al., 2006).
The aim of this article is to review the current understanding of the contribution of the mitochondrial
metabolism to photosynthesis, with a focus on the
mitochondrial components of ascorbate metabolism and
photorespiration. In the next sections, the intimate relationship between ascorbate metabolism and general
mitochondrial metabolism is described in order to provide
a possible context for the results described above.
Ascorbate metabolism in mitochondria
Ascorbate is an abundant antioxidant present at millimolar
concentrations in plant leaves and in storage organs, which
contributes significantly to the cellular redox state (Noctor
and Foyer, 1998; Smirnoff, 2000; Smirnoff et al., 2001;
Ishikawa et al., 2006). Ascorbate has also been reported to
be involved in a range of other cellular processes such as
photoprotection (Kiddle et al., 2003), establishment of
whole-plant morphology, root architecture, and development (Olmos et al., 2006; Horemans et al., 2003).
Furthermore, its oxidized form, i.e. dehydroascorbate
(DHA), has been found to be involved in the regulation of
the cell cycle (Potters et al., 2000, 2004) and in the control
of stomatal closure in tobacco leaves (Chen and Gallie,
2004). Recently, the last missing intermediate steps of the
Smirnoff–Wheeler pathway of ascorbate synthesis (Fig. 1)
have been elucidated (Laing et al., 2007; Linster et al.,
2007). A link between mitochondria and ascorbate biosynthesis has been acknowledged for many years, with
the enzymatic conversion of L-galactono-c-lactone to
L-ascorbate, the final reaction of the Smirnoff–Wheeler
pathway, being demonstrated in the 1950s (Mapson et al.,
1953; Mapson and Breslow, 1958). However, direct
evidence that L-galactono-1,4-lactone dehydrogenase
(GalLDH) is a membrane-intrinsic protein linked to the
inner membrane of mitochondria was provided only
recently (Siendones et al., 1999; Bartoli et al., 2000).
Thus, in intact potato mitochondria, cytochrome c was
found to be the electron acceptor for the reaction catalysed
by GalLDH (Bartoli et al., 2000). It was also demonstrated
that the activity of cytochrome c oxidase can modulate
ascorbate synthesis. More recently, studies performed on
mitochondria isolated from Arabidopsis leaves suggested
that ascorbate synthesis is coupled to the cytochrome c
pathway (Millar et al., 2003). However, rotenone, which
inhibits the transfer of electrons from the Fe–S centres of
complex I to ubiquinone, was found to inhibit the rate of
ascorbate synthesis to the same extent as KCN, which
blocks electron transport at the level of the cytochrome
oxidase (Millar et al., 2003). Interestingly, when electrons
entered the electron transport chain via complex II
(succinate dehydrogenase) and without complex I being
engaged, the addition of rotenone had no effect on the rate
of ascorbate synthesis (Millar et al., 2003). Similarly,
when electrons were introduced at cytochrome c via the
oxidation of L-galactono-c-lactone as the sole substrate,
rotenone exerted no effect on the rate of ascorbate
synthesis (Millar et al., 2003). These observations suggest
that the electron flow through complex I affects the rate of
ascorbate synthesis, and thus implies that GalLDH is
Contribution of mitochondrial metabolism to photosynthesis 1677
physically associated with complex I in Arabidopsis.
Reverse genetics approaches have also been undertaken
with this enzyme. Thus, the reduction of its expression in
BY2 tobacco cells resulted in an impairment of cell
division and growth (Tabata et al., 2001). More recently,
an RNA interference (RNAi) approach was used in tomato
to reduce the expression of GalLDH, in order to evaluate
the effects of the modification of ascorbate pools on plant
and fruit development (Alhagdow et al., 2007). Up to 80%
reduction of the activity of GalLDH led to a strong
reduction of leaf size, as a result of reduced cell
expansion. Intriguingly, this was accompanied by an
altered mitochondrial respiration (Alhagdow et al., 2007).
Ascorbate recycling in mitochondria
Ascorbate becomes rapidly oxidized to DHA via reactions
involving oxidative species, and must therefore be
efficiently recycled to prevent the degradation of DHA
Fig. 1. Simplified representation of the Smirnoff–Wheeler pathway.
HXK, hexokinase; PGI, phosphoglucose isomerase; PMI, phosphomannose isomerase; PMM, phosphomannose mutase; GMPP, GDPmannose pyrophosphorylase; GME, GDP-mannose-3,5-epimerase; GGP,
GDP-L-galactose phosphorylase; L-GalP, L-galactose-1-phosphatase;
L-GalDH, L-galactose dehydrogenase; GalLDH, L-galactono-1,4-lactone
dehydrogenase.
via ring opening (Winkler, 1987). Ring opening can
deplete ascorbate pools within minutes; however, it is
believed to be a non-enzymatic process and the physiological function of its product is currently unknown. That
said, in plants, ascorbate turnover is relatively fast, for
example, 13% per hour in pea seedlings (Pallanca and
Smirnoff, 2000) and 40% in a 22 h period in Arabidopsis
leaves (Conklin et al., 1997). In both plants and animals,
mitochondria are also responsible for the regeneration of
ascorbate from its oxidized form (Jiménez et al., 1997; Li
et al., 2002; Chew et al., 2003; Szarka et al., 2007).
DHA can be reduced to ascorbate via two mechanisms
involving small electron carriers such as the reduced form
of glutathione, GSH (Jiménez et al., 1999; Chew et al.,
2003), or lipoic acid (Xu and Wells, 1996), or by the
respiratory electron transport chain (Szarka et al., 2007).
In tobacco BY2 cells, the regeneration of ascorbate was
found to be stimulated by succinate and abolished by
malonate, a competitive inhibitor of the reaction catalysed
by succinate dehydrogenase, complex II (Szarka et al.,
2007). It has also been suggested that the reduction of
DHA is coupled to complex II and may contribute to the
redox homeostasis of the mitochondria, with the transport
of DHA through the inner mitochondrial membrane
potentially playing a crucial role in this process (Szarka
et al., 2007). However, exact details regarding the
mechanism of its transport are currently lacking, and
further work is required to substantiate these claims.
It has also recently been demonstrated in both tobacco
and maize leaves that ascorbate levels can be increased by
enhancing the recycling of ascorbate via the overexpression of dehydroascorbate reductase, DHAR (Chen et al.,
2003). DHAR catalyses the reduction of DHA to
ascorbate in a GSH-dependent reaction (Fig. 2). The
implication of this enzyme in regulating the ascorbate
redox state has also been associated with the regulation of
the foliar ROS level and subsequent photosynthetic
activity during leaf development and ageing (Chen and
Fig. 2. Simplified representation of the ascorbate–glutathione cycle in
mitochondria. GSSG, glutathione disulphide; GSH, reduced glutathione;
AsA, ascorbate; MDHA, monodehydroascorbate, DHA, dehydroascorbate; MDHAR, MDHA reductase; APX, ascorbate peroxidase; DHAR,
DHA reductase; GR, glutathione reductase. The dashed black arrow
indicates the spontaneous and irreversible hydrolytic ring rupture of
DHA.
1678 Nunes-Nesi et al.
Gallie, 2006). In addition, the GSH-dependent DHAR
together with other enzymes of the ascorbate–GSH cycle
(Fig. 2) has been found in purified Arabidopsis (Chew
et al., 2003) and pea (Jiménez et al., 1997) mitochondria.
Furthermore, in vitro import experiments showed dual
targeting of ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), and glutathione reductase
(GR) proteins to mitochondria and chloroplasts (Chew
et al., 2003). In the same study, APX was reported to be
located on the outside of the inner membrane, while the
three enzymes involved in the ascorbate–GSH cycle
system, i.e. MDHAR, GSH-dependent DHAR, and GR,
were largely found in the matrix. The authors proposed
that the association of APX with the inner membrane
allows the coupling with GalLDH, in order to produce
MDHA and DHA, which are needed in the matrix. The
location of MDHAR, DHAR, and GR largely in the
matrix is then proposed to ensure the recycling of NADH
and NADPH (Chew et al., 2003). Whilst this theory is
highly attractive, further evidence is required to support it.
Nevertheless, the mitochondrial location of the ascorbate–
glutathione cycle suggests that an interaction of key
importance is taking place between mitochondrial and
extramitochondrial metabolism in illuminated leaves.
Interaction of ascorbate metabolism, respiration,
and photosynthesis
There are now several lines of correlative evidence that
link the rates of ascorbate biosynthesis and respiration. It
has been observed that plants exhibiting a down-regulation
of mitochondrially localized enzymes, and displaying up
to 50% reduction in dark respiration, contained increased
levels of total ascorbate (Carrari et al., 2003; Nunes-Nesi
et al., 2005; Urbanczyk-Wochniak et al., 2006). Similar
results were obtained in studies on tomato mitochondria,
wherein intact mitochondria isolated from green tomato
fruits of plants with reduced activity of mitochondrial
MDH were assayed. These experiments revealed no difference in the rate of oxygen consumption in the presence
of malate and pyruvate, but a significantly enhanced rate of
oxygen consumption when L-galactono-lactone and NADH
were provided as respiratory substrates (Nunes-Nesi et al.,
2005), suggesting that GalLDH can effectively act as an
alternative electron donor in cases where flux through the
TCA cycle is impaired.
Clear and direct evidence that light and respiration
affect the capacity for ascorbate synthesis was provided
recently (Bartoli et al., 2006). The authors also demonstrated that the relative capacities of the cytochrome and
alternative oxidase (AOX) pathways, as well as the overall
capacity for leaf respiration, also influence the extent of
leaf ascorbate accumulation. It has been further postulated
that ascorbate biosynthesis is influenced by AOX activity,
suggesting that ascorbate production is intimately linked
to the mitochondrial electron transport chain (Foyer and
Noctor, 2005). In light of previous work it is additionally
tempting to speculate that ascorbate is the key link
between light quality, respiration, and photosynthesis.
However, recent data, shown in Fig. 3, reveal that this
link is not absolute. Growing plants deficient in the
expression of mitochondrial MDH under short-day conditions resulted in a dwarf phenotype (potentially indicative of less efficient photosynthesis; Fig. 3A–C),
although displaying elevated levels of ascorbate and
alteration in the redox state of this pool (Fig. 3D–F).
These data thus indicate that despite a clear role for
ascorbate, under many environmental conditions, other
currently undefined mechanisms can prevent the upregulation of photosynthesis if the prevailing conditions
are not suitable to support it. The most likely explanation
for this observation is that the TCA cycle is, at least to
a limited extent, inhibited in the light (Hanning and Heldt,
1993; Tovar-Mendez et al., 2003; Tcherkez et al., 2005)
whereas in the dark it becomes the only functional
pathway of energy metabolism. It could perhaps, therefore, be anticipated that extension of the dark period
would produce the observed phenotype. That said, the fact
that under optimal conditions inhibition of TCA cycle
enzymes can result in improved plant performance,
presumably by an effect on ascorbate biosynthesis,
remains of great interest. The exact mechanism by which
this effect is mediated is currently unclear, since although
photosynthetic gene expression is clearly affected in the
transgenics, ascorbate is capable of exerting a multiplicity
of other effects on photosynthesis (Smirnoff and Wheeler,
2000). Indeed, results of a previous study demonstrated
that loading of leaves with ascorbate for a mere 2 h in the
dark resulted in an increase in carbon assimilation,
suggesting that the effect seen in the transformants may
be mediated by the metabolite directly. Genetic screens
are currently being set up in Arabidopsis in the hope of
being able to dissect this process (and the factors that
prevent it from functioning under short-day conditions).
The role of UCP in photosynthesis
As described above, the multiplicity of mechanisms
linking mitochondrial and plastidial function serve to
reinforce the high degree of metabolic co-ordination
inherent in the plant cell. Another mechanism that
highlights the essentiality of the mitochondria for plant
photosynthetic metabolism has been recently proposed
that extends the concept of metabolic co-ordination to the
cytosol and peroxisome. Biochemical and physiological
analyses of a T-DNA insertional mutant of Arabidopsis
deficient in the expression of the uncoupling protein
AtUCP-1 revealed a specific inhibition of photorespiration
Contribution of mitochondrial metabolism to photosynthesis 1679
Fig. 3. Effect of down-regulation of antisense mitochondrial MDH tomato plants under short-day conditions. The left-hand panel shows the growth
phenotype of transgenic plants under short-day conditions (6 h light). (A) Photograph showing representative plants at 3 weeks growth. (C) Rate of
growth over the developmental period. (E) Biomass accumulation in various plant organs at 4 weeks growth. The right-hand panel shows the
ascorbate content of fully expanded leaves from plants at 4 weeks growth. (B) Total ascorbate content; (D) reduced ascorbate content; (F) reduced
ascorbate/DHA ratio. Samples were taken from mature source leaves in the middle of a light period. Total ascorbate and reduced ascorbate were
measured as described by Kampfenkel et al. (1995). DHA was calculated as the difference between total ascorbate and reduced ascorbate. Values are
presented as mean 6SE of determinations on six individual plants per line; an asterisk indicates values that were determined by the t-test to be
significantly different (P <0.05) from the control plants.
(Sweetlove et al., 2006). Uncoupling proteins are integral
to the inner mitochondrial membrane and catalyse proton
conductance across this membrane (Fig. 4), dissipating the
mitochondrial proton gradient as heat (Krauss et al.,
2005). It has been postulated that this is important when
the demand for oxidation of NADH is high, and may
facilitate high TCA cycle flux (Smith et al., 2004).
Consistent with this suggestion, the ucp1 mutants displayed a reduced photosynthetic carbon assimilation rate
that was linked to a reduced rate of photorespiratory
glycine oxidation (Sweetlove et al., 2006). This finding
thus strongly corroborates both pharmacological and
genetic evidence that mitochondrial respiratory processes
play an important role in the co-ordination of metabolism
in the illuminated leaf and introduces the much studied
photorespiratory pathway as another area worthy of
discussion here.
Mitochondrial reactions of photorespiration
Environmental stresses can affect the efficiency of the
Calvin cycle in consuming NAD(P)H, and result in
changes in the redox state of the chloroplasts and in
a rapid increase of free radicals and ROS, a response that
has to be tightly regulated to allow optimal functionality
of photosynthesis (Foyer and Noctor, 2000; Noctor et al.,
2007). A number of metabolic responses may provide
protection against this oxidative process, including the
process of photorespiration that is believed to mitigate
1680 Nunes-Nesi et al.
Fig. 4. Simplified representation of the photorespiratory pathway and the relationship between UCP and photosynthesis. RuBP, ribulose-1,5bisphosphate; PGP, phosphoglycolate phosphatase; GOX, glycolate oxidase; SGAT, serine/glyoxylate aminotransferase; GDC/SHMT, glycine
decarboxylase/serine hydroxymethyltransferase; HPR; hydroxypyruvate reductase; GK, glycerate kinase; GS, glutamine synthetase; GOGAT,
glutamate/oxoglutarate aminotransferase; Fdred, reduced ferredoxin; Fdox, oxidized ferredoxin; Glu, glutamate; Gln, glutamine; 2-OG, 2-oxoglutarate;
OAA, oxaloacetate. Black arrows indicate chemical reactions, while grey dashed arrows show membrane transport.
photooxidative damage by functioning as an electron sink
to prevent the over-reduction of the photosynthetic
electron transport chain and photoinhibition (Kozaki and
Takeba, 1996; Wingler et al., 2000; Noctor et al., 2002).
Photorespiration is effectively a salvage pathway that
returns some of the carbon lost to phosphoglycolate
(produced under atmospheric conditions by the oxygenase
reaction of Rubisco) to the Calvin cycle. This pathway,
which involves both light-dependent carbon dioxide
evolution and oxygen uptake, occurs concurrently with
photosynthesis in the green leaves of C3 plants. Photorespiration restricts photosynthesis under conditions that lead
to a high oxygenation of Rubisco such as high light, low
external CO2 concentrations, high temperatures, and stress
conditions that lead to stomatal closure and reduction in
intercellular CO2 concentrations (Wingler et al., 2000).
The reactions of the photorespiratory pathway are spatially separated across three distinct subcellular compartments: the chloroplast, mitochondrion, and peroxisomes
(Fig. 4). The photorespiratory pathway is, however,
commonly regarded to be highly inefficient given the loss
of CO2 inside the mitochondria. This loss of carbon has
been estimated as between 17% and 25% (Sharkey, 1988;
Cegelski and Schaefer, 2006) of that fixed during
photosynthesis. However, from the pathway itself, it
would be postulated that from the carbon used for oxygen
fixation, two out of five atoms enter photorespiration, and
one out of four of these carbon atoms is released.
Irrespective of the exact amount of carbon atoms lost, it
is clearly a ‘wasteful’ process and as such photorespiration has been the issue of many studies which aim to
alleviate the negative effects of this pathway. A good
example of this is the work performed by Kebeish et al.
(2007) showing that photorespiratory losses in Arabidopsis
thaliana can be reduced by introducing into chloroplasts a bacterial pathway for the catabolism of the
photorespiratory substrate, glycolate. This manipulation
resulted in stunning increases in biomass which are
presumably at least in part due to the bypass of the
mitochondrial reactions of photorespiration. The exact
Contribution of mitochondrial metabolism to photosynthesis 1681
reason, from an evolutionary perspective, why the plant
photorespiratory pathway was retained, is of great interest,
and the fact that the pathway is spread across the cell is
both important and of general interest with respect to
interorganellar co-ordination of metabolism. Within the
mitochondrial matrix, the glycine decarboxylase–serine
hydroxymethyltransferase (GDC–SHMT) complex convert two molecules of glycine to one molecule of serine
with the simultaneous evolution of carbon dioxide and
ammonium, and the production of NADH (Douce et al.,
2001). GDC is a mitochondrial multienzyme complex,
which consists of four different component enzymes, the
P-, H-, T-, and L-proteins, involved in two pathways,
photorespiration and one-carbon metabolism, in all photosynthesizing organs and in all biosynthetically active
tissues (Douce et al., 2001; Bauwe and Kolukisaoglu,
2003). Whether GDC is obligatory for processes other
than photorespiration was recently studied by Engel et al.
(2007) who isolated and characterized T-DNA insertion
mutants of both Arabidopsis P-protein genes, which
encode the glycine decarboxylating subunit (Bauwe and
Kolukisaoglu, 2003). The authors concluded that the
independent knockout of either of the two genes did not
significantly alter growth. However, when both P-protein
knockouts were combined, seedling development was
arrested at the cotyledon stage even under non-photorespiratory conditions, suggesting that GDC activity is
required for processes other than photorespiration (Engel
et al., 2007). A second mitochondrial enzyme which cooperates with GDC in photorespiration is SHMT, which
converts glycine to serine (Douce et al., 2001). SHMT
mutants in Arabidopsis, previously named as smt mutants,
later renamed as shm, and recently termed shm1-1 (Voll
et al., 2006), were among the first photorespiratory
mutants described (Somerville and Ogren, 1981). Early
studies of the shm1-1 mutant revealed that it contained no
mitochondrial SHMT activity and exhibited 40-fold
increases in the foliar level of glycine with respect to the
wild-type control (Somerville and Ogren, 1981). More
recently, Moreno et al. (2005) showed that SHMT activity
plays a role in controlling cell damage caused by abiotic
stress, probably because photorespiration forms part of the
dissipation mechanisms that minimize production of ROS
in the chloroplast and then mitigate oxidative damage.
A novel mitochondrially targeted protein, glycolate dehydrogenase (AtGDH), has been identified in Arabidopsis
(Bari et al., 2004). This enzyme has no homology on
the protein level and differs in important enzymatic
properties from the peroxisomal isoform (Bari et al.,
2004). AtGDH transcripts are present in all tissues, but
Fig. 5. Schematic representation of the proposed shuttle systems for ammonium transport in the form of amino acids between mitochondria and
plastids. Dashed red arrows indicate the citrulline–ornithine shuttle, and the black pathway indicates the glutamate–glutamine shuttle. CPS,
carbamoylphosphate synthetase; Cit, citrulline; Orn, ornithine; CarbP, carbamoylphosphate; OCT, Orn carbamoyltransferase; CK, carbamate kinase.
1682 Nunes-Nesi et al.
accumulate preferentially in illuminated leaves, with
higher accumulation under photorespiratory conditions,
implying its contribution to this process (Niessen et al.,
2007). However, T-DNA insertion lines for AtGDH,
showing drastic reduction in mitochondrial GDH activity
and photorespiratory CO2 release from glycolate, interestingly display no clear phenotype under ambient growth
conditions (Niessen et al., 2007), suggesting that mitochondrial glycolate oxidation might not be essential for
photorespiration. Although the physiological role of
mitochondrial glycolate oxidation remains unclear, it has
been reasonably postulated that it may act in combination
with peroxisomal glycolate metabolism to provide the
plant with flexibility to adapt to environmental conditions
(Niessen et al., 2007).
A further complexity of photorespiration was brought to
light by the recent demonstration by Taira et al. (2004)
that Arabidopsis GLN2 encodes GS2 precursors that can
be translocated to both leaf mitochondria and chloroplasts.
This surprising finding could potentially change the
previously held view that NH+4 ammonia is generated in
the mitochondria during photorespiration and subsequently actively or passively transported to plastids to be
assimilated into glutamate by the glutamine synthetase
2 (GS2) and Fd-dependent glutamate synthetase (FdGOGAT) system (Fig. 5). These authors postulated an
alternative model for this process in which a citrulline–
ornithine shuttle between chloroplasts and mitochondria is
proposed. This model would transfer not only ammonia
from the mitochondria to the chloroplast but also carbon
dioxide. This finding is largely in keeping with the
observation that transgenic potato plants expressing
SHMT in the antisense orientation displayed a lower
photosynthetic capacity and accumulate glycine in the
light (Schjoerring et al., 2006). Surprisingly, the nocturnal
serine synthesis and concomitant release of postphotorespiratory NH+4 were increased in the SHMT
transgenic plants. Also increased were the GS2 and FdGOGAT protein and activity levels (Schjoerring et al.,
2006). However, it should be borne in mind that subcellular compartmentalization was not considered in this
study. Furthermore, such a function can only be provided
at the cost of higher energy consumption (Linka and
Weber, 2005). Additional models have recently been put
forward in which a less costly alternative glutamate–
glutamine shuttle between mitochondria and chloroplasts
is proposed (Linka and Weber, 2005) (Fig. 5), and
therefore it seems prudent that more in-depth experimentation is required to resolve this issue.
Conclusions
Although the final genes encoding components of the
photorespiratory (Boldt et al., 2005) and Smirnoff–
Wheeler (Dowdle et al., 2007; Laing et al., 2007; Linster
et al., 2007) pathways have recently been cloned, our
understanding of the interaction of these processes and
those of photosynthesis and respiration remains far from
complete. The combined works reviewed here provide
a strong case for the essentiality of mitochondrial function
in the maintenance of efficient photosynthesis and,
furthermore, begin to define the mechanisms by which
this is mediated. Many open questions remain regarding
the finer details of these interactions, and, as illustrated in
the cases that are presented here, little is known concerning the hierarchical control, if any, that ensures that
metabolism and function are effectively co-ordinated in
the illuminated leaf. Amongst these questions are: ‘Exactly
how are respiration and photosynthesis co-ordinated?’ ‘Is
this at the level of a signal metabolite such as ascorbate,
and, if so, what is the nature of the signal transduction?’
and ‘At which enzyme reactions does metabolic control
occur within the pathways of photorespiration and ascorbate biosynthesis?’ Once answered, these questions will
bring us closer to full understanding of the enigmatic
contribution of mitochondrial function in photosynthesis.
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