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 inphotosynthesis

Adriano Nunes-Nesi*, Ronan Sulpice, Yves Gibon and Alisdair R. Fernie

Max-Planck-Institut fur Molekulare Pflanzenphysiologie, Am Muhlenberg 1, D-14476 Potsdam-Golm, Germany

Received 27 September 2007; Revised 8 November 2007; Accepted 13 November 2007

Abstract

Considerable cumulative evidence has accrued sug-

gesting 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 photosynthe-

sis 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 co-

ordinating 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 pre-

viously thought. This observation suggests that al-

though 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 co-

ordinated in the illuminated leaf.

Key words: Ascorbate biosynthesis, mitochondrial function,

photorespiration, photosynthesis.

Introduction

Mitochondrial oxidative metabolism has been postulatedto be a prerequisite for the maintenance of high rates ofphotosynthesis (Padmasree et al., 2002). Depending onthe developmental stage and/or the environment, reducingequivalents generated by the photochemical reactionsaccumulate in the chloroplast stroma, causing the over-reduction of the photosynthetic electron chain and thegeneration of reactive oxygen species (ROS), leading tophotoinhibition (Foyer and Noctor, 2000; Allen, 2002). Ithas been proposed that this excess of reducing equivalentsgenerated in the chloroplasts can be dissipated by theirexport from the chloroplast to the mitochondria, via themalate valve (Kromer, 1995; Scheibe, 2004; Scheibeet al., 2005). Within the mitochondria, the reducingequivalents are subsequently oxidized by the respiratoryelectron transport chain (Noctor et al., 2007), thusallowing continued high rates of photosynthesis. Thatsaid, recent studies have revealed that mitochondria–chloroplast interactions are by no means limited to thisexample, with considerable evidence suggesting roles forthe tricarboxylic acid (TCA) cycle, both cytochrome andalternative respiration, and the uncoupling protein in themediation of this cross-talk. Since the roles of themitochondrial electron transport chain have been exten-sively reviewed previously (Bartoli et al., 2005; Nunes-Nesi et al., 2007a; Sweetlove et al., 2007), they will notbe documented here, but rather attention will be focusedon the mechanisms by which modulation of the activitiesof TCA cycle enzymes and uncoupling protein exert theireffects.

A role for the TCA cycle in photosynthesis

The TCA cycle is a fundamental component ofmitochondrial respiration, linking glycolysis and/or

* To whom correspondence should be addressed. E-mail: nunesnesi@mpimp-golm.mpg.de

ª The Author [2008]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved.For Permissions, please e-mail: journals.permissions@oxfordjournals.org

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extramitochondrial malate synthesis to the mitochondrialelectron transport chain (Fernie et al., 2004; Millar et al.,2004). As part of an ongoing project, a wide range oftransgenics and mutants with deficient expression ofenzymes from the TCA cycle have been analysed.Interestingly, modifications in the photosynthetic ratewere found in a subset of these genotypes. Thus, tomatoplants with less fumarase activity showed a decrease inphotosynthesis (Nunes-Nesi et al., 2007b). In contrast,tomato plants with reduced expression of aconitase ormalate dehydrogenase showed an enhanced photosyn-thetic performance (Carrari et al., 2003; Nunes-Nesi et al.,2005). Finally, in contrast to these effects, tomato plantswith less succinyl CoA ligase (Studart-Guimaraes et al.,2007) or less mitochondrially localized isocitrate dehydro-genase (Lemaitre et al., 2007) have no change inphotosynthesis. While the reasons why decreased succinylCoA ligase and isocitrate dehydrogenase are not essentialto photosynthesis are currently unclear, the reducedphotosynthetic activity found in fumarase antisense trans-genics could be shown to be related to the impairment ofstomatal function (Nunes-Nesi et al., 2007b). However,this phenotype is very specific to manipulation offumarase activity and lies outside the scope of this article.Furthermore, a comprehensive molecular and metaboliccharacterization of lines with altered aconitase and malatedehydrogenase (MDH) activities revealed a specific in-crease in ascorbate in leaves (Carrari et al., 2003; Nunes-Nesi et al., 2005), coupled to a general up-regulation oflevels of transcripts encoding genes associated withphotosynthesis (Urbanczyk-Wochniak et al., 2006). Thesetransgenesis experiments hint at an important role forascorbate in the co-ordination of major pathways ofenergy metabolism in the leaf. Moreover, they are inkeeping with the current thinking that redox signalsemanating from the mitochondria are important in settingthe cellular machinery to maintain the overall redoxbalance (Sweetlove et al., 2007).

To summarize, recent studies have revealed thatphotosynthetic performance can be modulated by modifi-cations in mitochondrial pathways, such as the electrontransport chain (Bartoli et al., 2005) or TCA cycle (Carrariet al., 2003; Nunes-Nesi et al., 2005). These studies havesuggested ascorbate as a linkage between mitochondrialand chloroplast metabolism. Recently, another mitochon-drial protein, an uncoupling protein (UCP-1), has beenfound to be involved in a novel mechanism contributingto photosynthesis through efficient oxidation of glycineproduced via photorespiration (Sweetlove et al., 2006).The aim of this article is to review the current un-derstanding of the contribution of the mitochondrialmetabolism to photosynthesis, with a focus on themitochondrial components of ascorbate metabolism andphotorespiration. In the next sections, the intimate re-lationship between ascorbate metabolism and general

mitochondrial metabolism is described in order to providea possible context for the results described above.

Ascorbate metabolism in mitochondria

Ascorbate is an abundant antioxidant present at millimolarconcentrations in plant leaves and in storage organs, whichcontributes significantly to the cellular redox state (Noctorand Foyer, 1998; Smirnoff, 2000; Smirnoff et al., 2001;Ishikawa et al., 2006). Ascorbate has also been reported tobe involved in a range of other cellular processes such asphotoprotection (Kiddle et al., 2003), establishment ofwhole-plant morphology, root architecture, and develop-ment (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 ofthe cell cycle (Potters et al., 2000, 2004) and in the controlof stomatal closure in tobacco leaves (Chen and Gallie,2004). Recently, the last missing intermediate steps of theSmirnoff–Wheeler pathway of ascorbate synthesis (Fig. 1)have been elucidated (Laing et al., 2007; Linster et al.,2007). A link between mitochondria and ascorbate bio-synthesis has been acknowledged for many years, withthe enzymatic conversion of L-galactono-c-lactone toL-ascorbate, the final reaction of the Smirnoff–Wheelerpathway, being demonstrated in the 1950s (Mapson et al.,1953; Mapson and Breslow, 1958). However, directevidence that L-galactono-1,4-lactone dehydrogenase(GalLDH) is a membrane-intrinsic protein linked to theinner membrane of mitochondria was provided onlyrecently (Siendones et al., 1999; Bartoli et al., 2000).Thus, in intact potato mitochondria, cytochrome c wasfound to be the electron acceptor for the reaction catalysedby GalLDH (Bartoli et al., 2000). It was also demonstratedthat the activity of cytochrome c oxidase can modulateascorbate synthesis. More recently, studies performed onmitochondria isolated from Arabidopsis leaves suggestedthat ascorbate synthesis is coupled to the cytochrome cpathway (Millar et al., 2003). However, rotenone, whichinhibits the transfer of electrons from the Fe–S centres ofcomplex I to ubiquinone, was found to inhibit the rate ofascorbate synthesis to the same extent as KCN, whichblocks electron transport at the level of the cytochromeoxidase (Millar et al., 2003). Interestingly, when electronsentered the electron transport chain via complex II(succinate dehydrogenase) and without complex I beingengaged, the addition of rotenone had no effect on the rateof ascorbate synthesis (Millar et al., 2003). Similarly,when electrons were introduced at cytochrome c via theoxidation of L-galactono-c-lactone as the sole substrate,rotenone exerted no effect on the rate of ascorbatesynthesis (Millar et al., 2003). These observations suggestthat the electron flow through complex I affects the rate ofascorbate synthesis, and thus implies that GalLDH is

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physically associated with complex I in Arabidopsis.Reverse genetics approaches have also been undertakenwith this enzyme. Thus, the reduction of its expression inBY2 tobacco cells resulted in an impairment of celldivision and growth (Tabata et al., 2001). More recently,an RNA interference (RNAi) approach was used in tomatoto reduce the expression of GalLDH, in order to evaluatethe effects of the modification of ascorbate pools on plantand fruit development (Alhagdow et al., 2007). Up to 80%reduction of the activity of GalLDH led to a strongreduction of leaf size, as a result of reduced cellexpansion. Intriguingly, this was accompanied by analtered mitochondrial respiration (Alhagdow et al., 2007).

Ascorbate recycling in mitochondria

Ascorbate becomes rapidly oxidized to DHA via reactionsinvolving oxidative species, and must therefore beefficiently recycled to prevent the degradation of DHA

via ring opening (Winkler, 1987). Ring opening candeplete ascorbate pools within minutes; however, it isbelieved to be a non-enzymatic process and the physio-logical function of its product is currently unknown. Thatsaid, in plants, ascorbate turnover is relatively fast, forexample, 13% per hour in pea seedlings (Pallanca andSmirnoff, 2000) and 40% in a 22 h period in Arabidopsisleaves (Conklin et al., 1997). In both plants and animals,mitochondria are also responsible for the regeneration ofascorbate from its oxidized form (Jimenez et al., 1997; Liet al., 2002; Chew et al., 2003; Szarka et al., 2007).

DHA can be reduced to ascorbate via two mechanismsinvolving small electron carriers such as the reduced formof glutathione, GSH (Jimenez et al., 1999; Chew et al.,2003), or lipoic acid (Xu and Wells, 1996), or by therespiratory electron transport chain (Szarka et al., 2007).In tobacco BY2 cells, the regeneration of ascorbate wasfound to be stimulated by succinate and abolished bymalonate, a competitive inhibitor of the reaction catalysedby succinate dehydrogenase, complex II (Szarka et al.,2007). It has also been suggested that the reduction ofDHA is coupled to complex II and may contribute to theredox homeostasis of the mitochondria, with the transportof DHA through the inner mitochondrial membranepotentially playing a crucial role in this process (Szarkaet al., 2007). However, exact details regarding themechanism of its transport are currently lacking, andfurther work is required to substantiate these claims.

It has also recently been demonstrated in both tobaccoand maize leaves that ascorbate levels can be increased byenhancing the recycling of ascorbate via the overexpres-sion of dehydroascorbate reductase, DHAR (Chen et al.,2003). DHAR catalyses the reduction of DHA toascorbate in a GSH-dependent reaction (Fig. 2). Theimplication of this enzyme in regulating the ascorbateredox state has also been associated with the regulation ofthe foliar ROS level and subsequent photosyntheticactivity during leaf development and ageing (Chen and

Fig. 1. Simplified representation of the Smirnoff–Wheeler pathway.HXK, hexokinase; PGI, phosphoglucose isomerase; PMI, phosphoman-nose isomerase; PMM, phosphomannose mutase; GMPP, GDP-mannose 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-lactonedehydrogenase.

Fig. 2. Simplified representation of the ascorbate–glutathione cycle inmitochondria. GSSG, glutathione disulphide; GSH, reduced glutathione;AsA, ascorbate; MDHA, monodehydroascorbate, DHA, dehydroascor-bate; MDHAR, MDHA reductase; APX, ascorbate peroxidase; DHAR,DHA reductase; GR, glutathione reductase. The dashed black arrowindicates the spontaneous and irreversible hydrolytic ring rupture ofDHA.

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Gallie, 2006). In addition, the GSH-dependent DHARtogether with other enzymes of the ascorbate–GSH cycle(Fig. 2) has been found in purified Arabidopsis (Chewet al., 2003) and pea (Jimenez et al., 1997) mitochondria.Furthermore, in vitro import experiments showed dualtargeting of ascorbate peroxidase (APX), monodehydro-ascorbate reductase (MDHAR), and glutathione reductase(GR) proteins to mitochondria and chloroplasts (Chewet al., 2003). In the same study, APX was reported to belocated on the outside of the inner membrane, while thethree enzymes involved in the ascorbate–GSH cyclesystem, i.e. MDHAR, GSH-dependent DHAR, and GR,were largely found in the matrix. The authors proposedthat the association of APX with the inner membraneallows the coupling with GalLDH, in order to produceMDHA and DHA, which are needed in the matrix. Thelocation of MDHAR, DHAR, and GR largely in thematrix is then proposed to ensure the recycling of NADHand NADPH (Chew et al., 2003). Whilst this theory ishighly attractive, further evidence is required to support it.Nevertheless, the mitochondrial location of the ascorbate–glutathione cycle suggests that an interaction of keyimportance is taking place between mitochondrial andextramitochondrial metabolism in illuminated leaves.

Interaction of ascorbate metabolism, respiration,and photosynthesis

There are now several lines of correlative evidence thatlink the rates of ascorbate biosynthesis and respiration. Ithas been observed that plants exhibiting a down-regulationof mitochondrially localized enzymes, and displaying upto 50% reduction in dark respiration, contained increasedlevels of total ascorbate (Carrari et al., 2003; Nunes-Nesiet al., 2005; Urbanczyk-Wochniak et al., 2006). Similarresults were obtained in studies on tomato mitochondria,wherein intact mitochondria isolated from green tomatofruits of plants with reduced activity of mitochondrialMDH were assayed. These experiments revealed no dif-ference in the rate of oxygen consumption in the presenceof malate and pyruvate, but a significantly enhanced rate ofoxygen consumption when L-galactono-lactone and NADHwere provided as respiratory substrates (Nunes-Nesi et al.,2005), suggesting that GalLDH can effectively act as analternative electron donor in cases where flux through theTCA cycle is impaired.

Clear and direct evidence that light and respirationaffect the capacity for ascorbate synthesis was providedrecently (Bartoli et al., 2006). The authors also demon-strated that the relative capacities of the cytochrome andalternative oxidase (AOX) pathways, as well as the overallcapacity for leaf respiration, also influence the extent ofleaf ascorbate accumulation. It has been further postulatedthat ascorbate biosynthesis is influenced by AOX activity,

suggesting that ascorbate production is intimately linkedto the mitochondrial electron transport chain (Foyer andNoctor, 2005). In light of previous work it is additionallytempting to speculate that ascorbate is the key linkbetween light quality, respiration, and photosynthesis.However, recent data, shown in Fig. 3, reveal that thislink is not absolute. Growing plants deficient in theexpression of mitochondrial MDH under short-day con-ditions resulted in a dwarf phenotype (potentially in-dicative of less efficient photosynthesis; Fig. 3A–C),although displaying elevated levels of ascorbate andalteration in the redox state of this pool (Fig. 3D–F).These data thus indicate that despite a clear role forascorbate, under many environmental conditions, othercurrently undefined mechanisms can prevent the up-regulation of photosynthesis if the prevailing conditionsare not suitable to support it. The most likely explanationfor this observation is that the TCA cycle is, at least toa 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 functionalpathway of energy metabolism. It could perhaps, there-fore, be anticipated that extension of the dark periodwould produce the observed phenotype. That said, the factthat under optimal conditions inhibition of TCA cycleenzymes can result in improved plant performance,presumably by an effect on ascorbate biosynthesis,remains of great interest. The exact mechanism by whichthis effect is mediated is currently unclear, since althoughphotosynthetic gene expression is clearly affected in thetransgenics, ascorbate is capable of exerting a multiplicityof other effects on photosynthesis (Smirnoff and Wheeler,2000). Indeed, results of a previous study demonstratedthat loading of leaves with ascorbate for a mere 2 h in thedark resulted in an increase in carbon assimilation,suggesting that the effect seen in the transformants maybe mediated by the metabolite directly. Genetic screensare currently being set up in Arabidopsis in the hope ofbeing able to dissect this process (and the factors thatprevent it from functioning under short-day conditions).

The role of UCP in photosynthesis

As described above, the multiplicity of mechanismslinking mitochondrial and plastidial function serve toreinforce the high degree of metabolic co-ordinationinherent in the plant cell. Another mechanism thathighlights the essentiality of the mitochondria for plantphotosynthetic metabolism has been recently proposedthat extends the concept of metabolic co-ordination to thecytosol and peroxisome. Biochemical and physiologicalanalyses of a T-DNA insertional mutant of Arabidopsisdeficient in the expression of the uncoupling proteinAtUCP-1 revealed a specific inhibition of photorespiration

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(Sweetlove et al., 2006). Uncoupling proteins are integralto the inner mitochondrial membrane and catalyse protonconductance across this membrane (Fig. 4), dissipating themitochondrial proton gradient as heat (Krauss et al.,2005). It has been postulated that this is important whenthe demand for oxidation of NADH is high, and mayfacilitate high TCA cycle flux (Smith et al., 2004).Consistent with this suggestion, the ucp1 mutants dis-played a reduced photosynthetic carbon assimilation ratethat was linked to a reduced rate of photorespiratoryglycine oxidation (Sweetlove et al., 2006). This findingthus strongly corroborates both pharmacological andgenetic evidence that mitochondrial respiratory processesplay an important role in the co-ordination of metabolismin the illuminated leaf and introduces the much studied

photorespiratory pathway as another area worthy ofdiscussion here.

Mitochondrial reactions of photorespiration

Environmental stresses can affect the efficiency of theCalvin cycle in consuming NAD(P)H, and result inchanges in the redox state of the chloroplasts and ina rapid increase of free radicals and ROS, a response thathas to be tightly regulated to allow optimal functionalityof photosynthesis (Foyer and Noctor, 2000; Noctor et al.,2007). A number of metabolic responses may provideprotection against this oxidative process, including theprocess of photorespiration that is believed to mitigate

Fig. 3. Effect of down-regulation of antisense mitochondrial MDH tomato plants under short-day conditions. The left-hand panel shows the growthphenotype of transgenic plants under short-day conditions (6 h light). (A) Photograph showing representative plants at 3 weeks growth. (C) Rate ofgrowth over the developmental period. (E) Biomass accumulation in various plant organs at 4 weeks growth. The right-hand panel shows theascorbate content of fully expanded leaves from plants at 4 weeks growth. (B) Total ascorbate content; (D) reduced ascorbate content; (F) reducedascorbate/DHA ratio. Samples were taken from mature source leaves in the middle of a light period. Total ascorbate and reduced ascorbate weremeasured as described by Kampfenkel et al. (1995). DHA was calculated as the difference between total ascorbate and reduced ascorbate. Values arepresented as mean 6SE of determinations on six individual plants per line; an asterisk indicates values that were determined by the t-test to besignificantly different (P <0.05) from the control plants.

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photooxidative damage by functioning as an electron sinkto prevent the over-reduction of the photosyntheticelectron transport chain and photoinhibition (Kozaki andTakeba, 1996; Wingler et al., 2000; Noctor et al., 2002).Photorespiration is effectively a salvage pathway thatreturns some of the carbon lost to phosphoglycolate(produced under atmospheric conditions by the oxygenasereaction of Rubisco) to the Calvin cycle. This pathway,which involves both light-dependent carbon dioxideevolution and oxygen uptake, occurs concurrently withphotosynthesis in the green leaves of C3 plants. Photores-piration restricts photosynthesis under conditions that leadto a high oxygenation of Rubisco such as high light, lowexternal CO2 concentrations, high temperatures, and stressconditions that lead to stomatal closure and reduction inintercellular CO2 concentrations (Wingler et al., 2000).The reactions of the photorespiratory pathway are spa-tially separated across three distinct subcellular compart-ments: the chloroplast, mitochondrion, and peroxisomes(Fig. 4). The photorespiratory pathway is, however,

commonly regarded to be highly inefficient given the lossof CO2 inside the mitochondria. This loss of carbon hasbeen estimated as between 17% and 25% (Sharkey, 1988;Cegelski and Schaefer, 2006) of that fixed duringphotosynthesis. However, from the pathway itself, itwould be postulated that from the carbon used for oxygenfixation, two out of five atoms enter photorespiration, andone out of four of these carbon atoms is released.Irrespective of the exact amount of carbon atoms lost, itis clearly a ‘wasteful’ process and as such photorespira-tion has been the issue of many studies which aim toalleviate the negative effects of this pathway. A goodexample of this is the work performed by Kebeish et al.(2007) showing that photorespiratory losses in Arabidopsisthaliana can be reduced by introducing into chloro-plasts a bacterial pathway for the catabolism of thephotorespiratory substrate, glycolate. This manipulationresulted in stunning increases in biomass which arepresumably at least in part due to the bypass of themitochondrial reactions of photorespiration. The exact

Fig. 4. Simplified representation of the photorespiratory pathway and the relationship between UCP and photosynthesis. RuBP, ribulose-1,5-bisphosphate; PGP, phosphoglycolate phosphatase; GOX, glycolate oxidase; SGAT, serine/glyoxylate aminotransferase; GDC/SHMT, glycinedecarboxylase/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.

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reason, from an evolutionary perspective, why the plantphotorespiratory pathway was retained, is of great interest,and the fact that the pathway is spread across the cell isboth important and of general interest with respect tointerorganellar co-ordination of metabolism. Within themitochondrial matrix, the glycine decarboxylase–serinehydroxymethyltransferase (GDC–SHMT) complex con-vert two molecules of glycine to one molecule of serinewith the simultaneous evolution of carbon dioxide andammonium, and the production of NADH (Douce et al.,2001). GDC is a mitochondrial multienzyme complex,which consists of four different component enzymes, theP-, H-, T-, and L-proteins, involved in two pathways,photorespiration and one-carbon metabolism, in all photo-synthesizing organs and in all biosynthetically activetissues (Douce et al., 2001; Bauwe and Kolukisaoglu,2003). Whether GDC is obligatory for processes otherthan photorespiration was recently studied by Engel et al.(2007) who isolated and characterized T-DNA insertionmutants of both Arabidopsis P-protein genes, whichencode the glycine decarboxylating subunit (Bauwe andKolukisaoglu, 2003). The authors concluded that theindependent knockout of either of the two genes did notsignificantly alter growth. However, when both P-proteinknockouts were combined, seedling development was

arrested at the cotyledon stage even under non-photo-respiratory conditions, suggesting that GDC activity isrequired for processes other than photorespiration (Engelet al., 2007). A second mitochondrial enzyme which co-operates with GDC in photorespiration is SHMT, whichconverts glycine to serine (Douce et al., 2001). SHMTmutants in Arabidopsis, previously named as smt mutants,later renamed as shm, and recently termed shm1-1 (Vollet al., 2006), were among the first photorespiratorymutants described (Somerville and Ogren, 1981). Earlystudies of the shm1-1 mutant revealed that it contained nomitochondrial SHMT activity and exhibited 40-foldincreases in the foliar level of glycine with respect to thewild-type control (Somerville and Ogren, 1981). Morerecently, Moreno et al. (2005) showed that SHMT activityplays a role in controlling cell damage caused by abioticstress, probably because photorespiration forms part of thedissipation mechanisms that minimize production of ROSin the chloroplast and then mitigate oxidative damage.

A novel mitochondrially targeted protein, glycolate de-hydrogenase (AtGDH), has been identified in Arabidopsis(Bari et al., 2004). This enzyme has no homology onthe protein level and differs in important enzymaticproperties 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 andplastids. 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.

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accumulate preferentially in illuminated leaves, withhigher 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 activityand photorespiratory CO2 release from glycolate, interest-ingly display no clear phenotype under ambient growthconditions (Niessen et al., 2007), suggesting that mito-chondrial glycolate oxidation might not be essential forphotorespiration. Although the physiological role ofmitochondrial glycolate oxidation remains unclear, it hasbeen reasonably postulated that it may act in combinationwith peroxisomal glycolate metabolism to provide theplant with flexibility to adapt to environmental conditions(Niessen et al., 2007).

A further complexity of photorespiration was brought tolight by the recent demonstration by Taira et al. (2004)that Arabidopsis GLN2 encodes GS2 precursors that canbe translocated to both leaf mitochondria and chloroplasts.This surprising finding could potentially change thepreviously held view that NH4

+ ammonia is generated inthe mitochondria during photorespiration and subse-quently actively or passively transported to plastids to beassimilated into glutamate by the glutamine synthetase2 (GS2) and Fd-dependent glutamate synthetase (Fd-GOGAT) system (Fig. 5). These authors postulated analternative model for this process in which a citrulline–ornithine shuttle between chloroplasts and mitochondria isproposed. This model would transfer not only ammoniafrom the mitochondria to the chloroplast but also carbondioxide. This finding is largely in keeping with theobservation that transgenic potato plants expressingSHMT in the antisense orientation displayed a lowerphotosynthetic capacity and accumulate glycine in thelight (Schjoerring et al., 2006). Surprisingly, the nocturnalserine synthesis and concomitant release of post-photorespiratory NH4

+ were increased in the SHMTtransgenic plants. Also increased were the GS2 and Fd-GOGAT protein and activity levels (Schjoerring et al.,2006). However, it should be borne in mind that sub-cellular compartmentalization was not considered in thisstudy. Furthermore, such a function can only be providedat the cost of higher energy consumption (Linka andWeber, 2005). Additional models have recently been putforward in which a less costly alternative glutamate–glutamine shuttle between mitochondria and chloroplastsis proposed (Linka and Weber, 2005) (Fig. 5), andtherefore it seems prudent that more in-depth experimen-tation is required to resolve this issue.

Conclusions

Although the final genes encoding components of thephotorespiratory (Boldt et al., 2005) and Smirnoff–

Wheeler (Dowdle et al., 2007; Laing et al., 2007; Linsteret al., 2007) pathways have recently been cloned, ourunderstanding of the interaction of these processes andthose of photosynthesis and respiration remains far fromcomplete. The combined works reviewed here providea strong case for the essentiality of mitochondrial functionin the maintenance of efficient photosynthesis and,furthermore, begin to define the mechanisms by whichthis is mediated. Many open questions remain regardingthe finer details of these interactions, and, as illustrated inthe cases that are presented here, little is known concern-ing the hierarchical control, if any, that ensures thatmetabolism and function are effectively co-ordinated inthe illuminated leaf. Amongst these questions are: ‘Exactlyhow are respiration and photosynthesis co-ordinated?’ ‘Isthis 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 controloccur within the pathways of photorespiration and ascor-bate biosynthesis?’ Once answered, these questions willbring us closer to full understanding of the enigmaticcontribution of mitochondrial function in photosynthesis.

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