Role of L-ascorbate in alleviating abiotic stresses in crop plants PDF
Preview Role of L-ascorbate in alleviating abiotic stresses in crop plants
VenkateshandParkBotanicalStudies2014,55:38 http://www.as-botanicalstudies.com/content/55/1/38 REVIEW Open Access Role of L-ascorbate in alleviating abiotic stresses in crop plants Jelli Venkatesh and Se Won Park* Abstract L-ascorbic acid (vitamin C) is a major antioxidant in plants and plays a significant rolein mitigation of excessive cellular reactive oxygen species activities caused by numberof abiotic stresses. Plant ascorbate levels change differentially in response to varying environmental stress conditions, depending on thedegree ofstressand species sensitivity. Successful modulation of ascorbate biosynthesis through genetic manipulation of genes involved in biosynthesis, catabolism and recycling of ascorbate has been achieved. Recently, roleof ascorbate inalleviating number ofabiotic stresses has been highlighted in crop plants. In this article, wediscuss thecurrentunderstanding ofascorbate biosynthesis and its antioxidant role inorder to increaseour comprehension of how ascorbatehelps plants to counteract or cope with various abiotic stresses. Keywords: Abiotic stress; Antioxidant; L-ascorbate; Reactive oxygen species; Transgenics Review cell signaling modulator in numerous cellular processes Introduction including cell division, cell expansion and cell wall Adverse environmental factors such as excessive cold, growth (Liso et al. 1984; Conklin and Barth 2004; heat, drought and salinity stresses result in a consider- Woluckaetal.2005;Zhangetal.2007).Itisacofactorfor able yield loss of crop plants all over the world. These thenumberofenzymessuchasviolaxanthinde-epoxidase abiotic stresses elicit complex cellular responses in the (VDE,xanthophyllcycle),1-aminocyclopropane-1-carboxylic plant system, resulting in the production of excessive re- acid(ACC)oxidase(ethylenebiosynthesis)and2-oxoacid- active oxygen species (ROS) such as hydrogen peroxide dependent dioxygenases (ABA and GA biosynthesis) (H O ), hydroxyperoxyl (HO ·), superoxide (O−) and (Eskling et al. 1997; Davey et al. 2000; Smirnoff 2000). 2 2 2 2 singlet oxygen (1O ) radicals. Excessive ROS generated Plants withlowascorbatebiosynthesisarerathersensitive 2 inplantcellstends tointeract with different macromole- to various environmental stress conditions affecting their cules resulting in oxidation of proteins, membrane lipids growth and development (Müller-Moulé et al. 2004; andnucleicacidsandcausescellulardamage,ultimatelyaf- Huang et al. 2005; Alhagdow et al. 2007; Gao and Zhang fecting the growth and productivity of plants (Wang et al. 2008). Recently, it has been reported that ascorbate plays 2003). To protect themselves from adverse conditions, a crucial role in protection against various environmental plants have evolved a number of cellular defense mecha- stressessuchas,drought(Hemavathietal.2011;Fotopoulos nismsincludingantioxidantssuchasascorbate,glutathione et al. 2008), salinity (Kwon et al. 2003; Huang et al. 2005; and tocopherols as well as ROS-detoxifying enzymes such Wang et al. 2005; Sun et al. 2010a; Zhang et al. 2011; as superoxide dismutases, peroxidases and catalases (Inzé Venkateshetal.2012),ozone(Zhengetal.2000;Sanmartin andVanMontagu1995;NoctorandFoyer1998). et al. 2003; Feng et al. 2010), low/high temperatures Among the plant antioxidants, L-ascorbate is a major (Kwon et al. 2003; Larkindale et al. 2005) and high light antioxidant playing a vital role in the mitigation of ex- intensity (Müller-Moulé et al. 2004; Talla et al. 2011). cessive ROS activity through enzymatic as well as non- These studies on mutant and/or transgenic plants (sum- enzymatic detoxification (Mittler 2002). It also acts as a marized in the Table 1) with altered endogenous ASA levels proved that ascorbate plays a significant role in plantgrowthanddevelopmentaswellasabioticstresstol- *Correspondence:[email protected] DepartmentofMolecularBiotechnology,KonkukUniversity,1, erance. In this article, an attempt has been made to Hwayang-dong,Gwangjin-gu,Seoul,KoreaRepublic ©2014VenkateshandPark;licenseeSpringer.ThisisanOpenAccessarticledistributedunderthetermsoftheCreative CommonsAttributionLicense(http://creativecommons.org/licenses/by/4.0),whichpermitsunrestricteduse,distribution,and reproductioninanymedium,providedtheoriginalworkisproperlycredited. Table1Roleofascorbateinplantgrowthanddevelopmentandabioticstresstolerance hV ttpen Enzyme/protein pTalarngtet Gene Genesource Tmyapneipouflagteionnetic Ascorbatecontent Phenotypicchanges Reference ://wkate ws GDP-mannose Tobacco GMPase Tomato Overexpression 2.0–4.0-foldincrease Increasedtolerancetotemperaturestress Wangetal.2011 w.ahan PIpPshhyoroomosspepphrhhaooossemmphaannonnryoolamseseutase TAorabbaicdcoopsis PNPMMbPII12MM AT–orabbaicdcoopsis VRT-INGDASNiAknockout U0N.po47tc–oh0a.36n.50g--effoollddddeeccrreeaassee c–Noonpdhiteionnostyinpibcocthhanmguetsanutnsdernormalgrowth MQiaarnuteateatl.a2l.0200708 s-botanicalstudiedParkBotanical Arabidopsis NAtbPPMMMM TAorabbaicdcoopsis VOVvMerEeExpression 00..225––00.5.3-f3o-lfdolidncinrecarseease –IncreasedtolerancetoMVstress s.com/co2Studies n0 VTC4/Myoinositol TAorabbaicdcoopsis PVMTCM4 –Acerola OT-vDeNreAxpkrneoscskioonut 02..601-f–o0ld.75in-fcoreldasdeecrease –22.4%–34%decreasesinmyoinositolcontent TBoadraebjoineetjaadl.e2t0a0l9.2009 tent/514,55 monophosphatase(IMP) 5/1:38 Slowseedgerminationundercontrolconditions /3 8 SlightlyhypersensitivetoABAandNaClduring seedgermination GDP-L-galactose Arabidopsis vtc5-1and Arabidopsis T-DNAknockout 0.2-folddecrease Plantgrowthretardationandbleachingof Dowdleetal.2007 phosphorylase vtc5-2 thecotyledons L-Galactose Tobacco L–GalLDH Tobacco Overexpression 1.5–2.0-foldincrease Highermitoticindexincells Tokunagaetal.2005 dehydrogenase (BY–2cells) Reducedbrowningandcellsdeathofcultures IncreasedtolerancetoMV L-galactono-1,4-lactone Tobacco GLDH Tobacco Antisense 0.30-folddecrease Adverselyeffectedplantcelldivision,growth Tabataetal.2001 dehydrogenase (BY–2cells) downregulation andstructureofplantcell Tobacco RrGalLDH Rosaroxburghii Overexpression 2.1-foldincrease Enhancedtolerancetosaltstress Liuetal.2013a Monodehydroascorbate Tobacco AtMDAR1 Arabidopsis Overexpression Upto2.2-foldincrease Enhancedtolerancetoozone,saltandPEGstresses Eltayebetal.2007 reductase Tobacco Am-MDAR Avicenniamarina Overexpression Upto2.0-foldincrease Increasedtolerancetosaltstress Kavithaetal.2010 Tobacco MDAR-OX Arabidopsis Overexpression Upto1.1-foldincrease NochangeinAluminiumtolerance Yinetal.2010 Dehydroascorbate Tobacco DHAR-OX Arabidopsis Overexpression Upto1.3-foldincrease IncreasedtolerancetoAlstress Yinetal.2010 reductase Tobacco DHAR Arabidopsis Overexpression 1.9–2.1-foldincrease Enhancedtolerancetoozone,droughtandsalinity Eltayebetal.2006 Tobacco DHAR Wheat Overexpression 2.1-foldincrease IncreasedozonetoleranceandNPR ChenandGallie2005 Tobacco Antisense 0.29-folddecrease SubstantiallyreducedstomatalareaandlowNPR downregulation Tobacco DHAR Human Overexpression Nosignificantchange EnhancedtolerancetolowtemperatureandNaCl Kwonetal.2003 P a g e 2 o f 1 9 Table1Roleofascorbateinplantgrowthanddevelopmentandabioticstresstolerance(Continued) hV ttpen Ascorbateperoxidase Tobacco tAPx Tobacco Overexpression Nochange uInncdreearsleigdhttocleornadnictieontosMVandchillingstresses Yabutaetal.2002 ://wwkates wh Tobacco/Spinach Antisense – Plantsfailedtogrow .aan Arabidopsis HvAPX1 Barley Odovwernerxepgreuslastioionn – Increasedtolerancetosaltstress Xuetal.2008 s-botandPark Arabidopsis OsAPXaand Rice Overexpression – Increasedtolerancetosaltstress Luetal.2007 icals Bota Tobacco COAsAPPOXAb1 Pepper Overexpression – IInnccrreeaasseeddptolalenrtangcroewtothMVstress Sarowaretal.2005 tudies.com/c nicalStudies Tobacco cAPX Arabidopsis Antisense Nochange Increasedtoleranceagainstheatandsaltstresses Ishikawaetal.2005 o2 n0 BToYb-2accceolls StAPX Tomato Odovwernerxepgreuslastioionn – Improvedseedgermination Sunetal.2010a tent/514,55 Increasedtolerancetosaltandosmoticstresses 5/1/3:38 8 Rice Apx1/Apx2 Rice RNAi(Apx1+Apx2) Upto1.5-folddecrease Nochangeinplantgrowthanddevelopment Rosaetal.2010 Increasedtolerancetoaluminium RNAi(Apx1orApx2) – Producedsemi-dwarfphenotype Rice OsAPx-R Rice RNAi – Delayedplantdevelopment Lazzarottoetal.2011 Rice OsAPXa Rice Overexpression – Increasedspikeletfertilityundercoldstress Satoetal.2011 Rice Osapx2 Rice Overexpression – Enhancedstresstolerance Zhangetal.2013 Sensitivetoabioticstresses – T-DNAknockout – Semi-dwarfseedlings,yellow-greenleaves,leaf lesion-mimicandseedsterility Alfalfa Osapx2 Rice Overexpression – Increasedsaltresistance Guanetal.2012 Tomato cAPX Pea Overexpression – EnhancedtolerancetoUV-Bandheatstresses Wangetal.2006 Tomato cAPX Pea Overexpression – Enhancedtolerancetochillingandsaltstresses Wangetal.2005 Tomato LetAPX Tomato Antisense Nosignificantchange Transgenicplantsphotosyntheticallylessefficient Duanetal.2012b downregulation andsensitivetochillingstress Ascorbateoxidase Tobacco AAO Cucumber Overexpression Nochange Plantsbecomesusceptibletoozone Sanmartinetal.2003 Tobacco AAO Cucumber Overexpression Nochange Increaseddroughttoleranceduetoreduced Fotopoulosetal.2008 stomatalconductance Tobacco AAO Pumpkin Overexpression 2.0-foldincreasein Numberofsmallerflowerssignificantlyincreased Pignocchietal.2003 apoplasticASA 6%to14%reductionofinseedweight Tobacco Antisense 2.0-foldincreasein Nosignificantchanges downregulation apoplasticASA P Tobacco AAO Tobacco Overexpression – Severeinhibitionofgerminationandseedyield Yamamotoetal.2005 ag e underhighsalinity 3 o f 1 9 Table1Roleofascorbateinplantgrowthanddevelopmentandabioticstresstolerance(Continued) hV ttpen Tobacco AAO Tobacco Adonwtisnernesgeulation – Increasedtolerancetosaltstress Yamamotoetal.2005 ://wwkates wh – Increasedseedyieldundersaltstress .aan MASyAoimnoasnitnoolsoexypgaethnwasaey AARirrcaaebbiiddooppssiiss AOAAMsMORI1OX –R–ice OTT--vDDeNNreAAxpkknrneoosccskkiooonuutt 2N.o0–c3h.0a-nfogledincrease IIIInnnnccccrrrreeeeaaaasssseeeeddddtdsooezroleoedunrageynhitcetoeldtleotroulaennrsadacneletcrsestareltssstress DZhuaanngeettaal.l.22001029a s-botanicalstudiedParkBotanical ArePgxu-Rla,AtoPrX1-related;CAPOA1,Capsicumannuumascorbateperoxidase-like1gene;MV,methylviologen;NPR,netphotosyntheticrate;PEG,polyethyleneglycol;RNAi,RNAinterference;VIGS,Virus-inducedgenesilencing; s.com/c Studies VVMEE,Viral-vector-mediatedectopic-expression. o2 n0 tent/514,55 5/1:38 /3 8 P a g e 4 o f 1 9 VenkateshandParkBotanicalStudies2014,55:38 Page5of19 http://www.as-botanicalstudies.com/content/55/1/38 illustratetheroleofascorbateinvariousabioticstressesin theessentialroleofenzymesinvolvedinthebiosynthesis cropplantsbyexploringtransgenictechnology. of L-ascorbate (Conklin et al. 1996; Conklin et al. 2000; Huang et al. 2005; Conklin et al. 2006; Müller-Moulé Overview:ascorbicacidbiosynthesis,transportation, 2008). Now it is well known that in higher plants, recyclinganddegradationprocessesinplants ascorbate biosynthesis occurs through well-characterized In plants, the accumulation or steady level of ascorbate D-mannose/L-galactosepathway(Smirnoff-Wheelerpath- is maintained in homeostasis through the rate of synthe- way), where D-mannose is converted to L-galactose via sis, recycling and degradation, as well as intra- and GDP-sugarintermediates(Wheeleretal.1998)(Figure1). inter-cellular transport (Horemans et al. 2000a; Pallanca L-galactose is further oxidized to L-galactono-1,4-lactone, andSmirnoff2000;GreenandFry 2005). which is converted intoascorbate, by L-galactono-1,4-lac- tone dehydrogenase (L-GalLDH), located on the inner Biosynthesis mitochondrialmembrane(Siendonesetal.1999;Smirnoff Characterization of low ascorbate producing mutants 2001). All of the genes that are involved in this pathway (vtc) of Arabidopsis has helped us to better understand havebeenwell-characterized;theseincludegenesencoding Figure1L-ascorbicacidbiosynthesispathwaysinplants(modifiedafterHemavathietal.2010):(1)Smirnoff-Wheelerpathway, (2)L-gulosepathway,(3)Myoinositol-basedpathway,(4)D-galacturonicacidpathway. VenkateshandParkBotanicalStudies2014,55:38 Page6of19 http://www.as-botanicalstudies.com/content/55/1/38 GDP-D-mannose pyrophosphorylase (Conklin et al. membrane (reviewed in Horemans et al. 2000a); how- 1999), GDP-D-mannose-3,’5’-epimerase (Wolucka and Van ever, the specific mechanisms by which they transport Montagu 2003; Watanabe et al. 2006), GDP-L-galactose ASAorDHAhavenotbeenwell elucidated. phosphorylase (L-galactose guanylyltransferase) (Dowdle Ascorbate biosynthesis occurs in almost all plant cells et al. 2007; Linster and Clarke 2008), L-galactose-1-phos- and tissues. However, its level is generally high in photo- phate phosphatase (Laing et al. 2004a), L-galactose de- synthetic tissues, meristematic tissues, flowers, young hydrogenase (Gatzek et al. 2002; Laing et al. 2004b) and fruits, root tips, and apices of stolons or tubers (Gest L-GalLDH (Imai et al. 1998; Siendones et al. 1999; do et al. 2013). In certain fruits, such as Ribes nigrum (by Nascimento et al. 2005; Tokunaga et al. 2005; Alhagdow galactose pathway, Hancock et al. 2007) and strawberry etal.2007). (by D-galacturonic acid pathway, Agius et al. 2003), in- In addition to the Smirnoff-Wheeler pathway, three creased accumulation of ascorbate occurs by a combin- other potential pathways of ascorbate biosynthesis have ation of long-distance transport and in situ biosynthesis. been identified in plants. It was demonstrated that in High ascorbate demand in developing sink tissues is addition to production of GDP-L-galactose, GDP-D- probably because it is critical for cell cycle and cell div- mannose-3’,5’-epimerase can also produce GDP-L-gulose ision/growth, which cannot be met entirely by sink tis- (Davey et al. 1999; Wolucka and Van Montagu 2003). sue alone (Smirnoff 2000; Franceschi and Tarlyn 2002). Moreover, exogenous L-gulose and L-gulono-1,4-lactone Ascorbate accumulation in sink tissue is controlled to were shown to serve as direct precursors of ascorbate in some extent by ascorbate biosynthesis in source tissues Arabidopsis cell cultures (Davey et al. 1999). These ob- (Franceschi and Tarlyn 2002; Tedone et al. 2004). servations led to a proposal for an alternative L-gulose Franceschi and Tarlyn (2002), demonstrated that the pathway in which L-gulose and L-gulono-1,4-lactone are long-distance transport of ASA in plants occurs via important intermediates (Wolucka and Van Montagu phloem, where L-ascorbate was found to be loaded into 2003). However, the intermediate steps in this pathway the phloem of source leaves and transported to sink tis- have not yet been characterized in plants. D-galacturonic sues. In addition, ascorbate biosynthesis, which occurs acid pathway involves the conversion of D-galacturonic in phloem tissue via the D-Man/L-Gal pathway could acid,aproductofthedegradationofcellwallpectinstoL- alsocontributetoASAaccumulationinplant storageor- ascorbate via L-galactono-1,4-lactone (Agius et al. 2003; gans (Hancocketal. 2003). Cruz-Rus et al. 2011; Badejo et al. 2012) (Figure 1). Fol- In mammals, sodium-dependent ascorbate transporters lowing the cloning of Arabidopsis myoinositol oxygenase (SVCT1 and SVCT2), which belong to the nucleobase- (MIOX) gene by Lorence et al. (2004), a myoinositol- ascorbate transporter (NAT) family, have been identified based pathway (animal-like pathway) was proposed and well characterized as an active ascorbate transport (Figure 1). MIOX converts myoinositol to D-glucuronate system(Daruwalaetal.1999;Tsukaguchietal.1999;Ishi- and plants can catalyze the conversion of D-glucuronate kawa et al. 2006). Although numerous NATs have been intoL-gulonicacid.However,recently,EndresandTenha- identified in plants (Li and Schultes 2002; Maurino et al. ken (2009), proved that the MIOX is involved mainly in 2006), their role in ASA transportation has not been themodulation ofthe metabolite level ofmyoinositol and established. Further studies are required to determine the playsanegligibleroleintheplantascorbatebiosynthesis. definitiveroleinplantascorbatetransportation. Ascorbatetransport Ascorbaterecycling Once the ascorbate is synthesized on the inner mito- ASA pool in cells is maintained through synthesis, re- chondrial membrane, it is transported to different cellu- cycling and transportation, and plays an important role lar compartments including the apoplast. Both the in adaptation of plant to various stresses (Stevens et al. ascorbate and DHA transport is mainly mediated by fa- 2008). Ascorbate takes part in several enzymatic and cilitated diffusion or active transport systems (Ishikawa non-enzymatic mechanisms for elimination of deleteri- et al. 2006). In contrast to ascorbate, DHA tends to be ous ROS (Asada and Takahashi 1987), and as a result, more efficiently transported across plant membranes MDHA and DHA accumulates in the cells. The two en- with a higher affinity and capacity (Horemans et al. zymes involved in the oxidation of ascorbate are ascor- 1998; Szarka et al. 2004). It was proposed that specific bate oxidase (AAO) and ascorbate peroxidase (APX). plasma membrane transporters transport ASA or DHA AAO is an apoplastic enzyme that catalyzes the oxida- in plants (Horemans et al. 2000b). However, either the tion of ASA to MDHA using oxygen and is associated protein or the gene associated with this transport and with cell wall metabolism and cell expansion (Smirnoff the nature of the mechanisms driving these carrier pro- 1996). Ascorbate peroxidase (APX) is a class I peroxid- teins are still inconclusive. Several other putative ascor- ase catalyzesthe conversion ofH O intoH O,usingas- 2 2 2 bate transporters are associated with the plant plasma corbate as a specific electron donor, thus resulting in the VenkateshandParkBotanicalStudies2014,55:38 Page7of19 http://www.as-botanicalstudies.com/content/55/1/38 accumulation of MDHA as a by-product (Teixeira et al. Genetic modulation of plant ascorbate pathway has be- 2004). comefeasiblewithadvancementsmadeinplantgenomics The ASA pool size is dependent, on both the rate of and genetic engineering. Several possible strategies have synthesis and the rate of reduction of MDHA and DHA been followed to increase ascorbate production in plants back to ascorbate. MDHA and DHA produced as a re- viageneticengineeringofenzymesinvolvedinthebiosyn- sult of activities of APX and AAO, respectively, should thesis and recycling of ascorbate. Several transgenes, be efficiently recycled to maintain the reduced pool of which are of plant and animalorigins, havebeensuccess- ASA. MDHA is reduced back to ASA by MDAR using fully used for increasing biosynthesis of ascorbic acid. NADH/NADPH as electron donors. In addition, plant Mouse L-gulono-c-lactone oxidase (GLOase) gene in PM cytb 561 (plasma membrane b-type cytochrome c) is tobacco, lettuce and potato (Jain and Nessler 2000; alsoassociatedwiththerecyclingofASAfromMDHAon Hemavathietal.2010),humandehydroascorbate(DHAR) thecytoplasmicsideoftheplasmamembrane(Trostetal. gene in tobacco (Kwon et al. 2003), wheat DHAR gene in 2000; Asard et al. 2001; Pignocchi and Foyer 2003). DHA tobacco and maize (Chen et al. 2003; Naqvi et al. 2009), isreducedtoASAbydehydroascorbatereductase(DHAR) Arabidopsis MDAR gene (AtMDAR1) in tobacco, straw- using reduced glutathione (GSH) as an electron donor or berryD-galacturonicacidreductase(GalUR)geneinAra- by the electron-transport chain (ETC.) electron carriers bidopsis and potato (Agius et al. 2003; Hemavathi et al. (Szarka et al. 2007). Thus, DHAR and MDAR are crucial 2009) and rice L-GalLDH gene in rice (Liu et al. 2011) components in the maintenance of the reduced pool of havebeensuccessfullyclonedandexpressed(summarized ASAandareofprimeimportanceinoxidativestresstoler- intheTable2). ance(Eltayebetal.2006). Role of ascorbate in photosynthesis as a Ascorbatedegradation photoprotectant Although the pathway of ascorbate synthesis is distrib- A high concentration of ascorbate in chloroplasts would uted between the cytosol and the mitochondrion (Foyer imply its central role in photosynthesis (Smirnoff 1996). 2004; Smirnoff et al. 2004), the ascorbate degradation Ascorbate plays a crucial roles in scavenging the dele- pathway appears to reside in the apoplast (Green and terious ROS that are generated as by-products of photo- Fry 2005). In most plants, ascorbate degradation can synthesis and as a key component in excess photonic occur via dehydroascorbate, yielding oxalate (OxA) and energy dissipation mechanisms, such as the water-water L-threonate (ThrO). However, in some plants (Vitaceae cycle (WWC) (Neubauer and Yamamoto 1992; Asada eg. grape), ascorbate can also be degraded via L-idonate 1999) and the xanthophyll cycle (Müller-Moulé et al. to L-threarate (L-tartrate) (Green and Fry 2005). A deg- 2002; Yabuta et al. 2007). WWC, which is also known as radation pathway for ASA/DHA catabolism in plants Mehler peroxidase reaction, is one of the most important has been reported recently (Simpson and Ortwerth detoxification systems functioning in intact chloroplasts 2000; Parsons and Fry 2012). Ascorbate degradation (Asada 1994, 1999, 2006). It involves the photoreduction pathwayinvolvesenzymicand/ornon-enzymicoxidation of O by PSI to a superoxide radical, followed by the dis- 2 to dehydroascorbic acid (DHA), which may irreversibly mutation of superoxide radical by superoxide dismutase hydrolyze to 2,3-diketogulonate (DKG). However, many (SOD) to hydrogen peroxide and oxygen (Müller-Moulé of the enzymes involved in the degradation pathway of etal.2002).Thehydrogenperoxideisreducedtowaterby ASA are not well characterized inplants. Both DHA and ascorbate, catalyzed by ascorbate peroxidase (APX), and DKG prone to further oxidation under the same physio- the resulting by-product monodehydroascorbate (MDA) logical conditions as that of apoplast (Parsons and Fry is directly reduced to ascorbate either by reduced ferre- 2012). DHA can be oxidized by H O non-enzymatically doxin of PSI (Miyake and Asada 1992; Miyake and Asada 2 2 toamonoanion(cyclic-oxalyl-threonate;cOxT)andadia- 1994;Asada1999)orbyNAD(P)H-dependentchloroplas- nion (oxalyl-threonate [OxT] isomers, 3-OxTand 4-OxT) ticMDHAreductaseusingNADHorNADPHaselectron independently through formation of a reactive intermedi- donor (Sano et al. 2005). MDHA can spontaneously dis- ate cyclic-2,3-O-oxalyl-L-threonolactone (Parsons et al. proportionate to ascorbate and dehydroascorbate (DHA) 2011). In the absence of H O , DKG is relatively stable, (Asada 1999). DHA is unstable at the physiological pH 2 2 however slowly generates a range of products, such as 2- andirreversiblydegradeto2,3diketo-1-gulonicacidifnot carboxy-l-xylonolactone, 2-carboxy-l-lyxonolactone and recycled back to ascorbate. To preserve the ascorbate 2-carboxy-l-threo-pentonate (Parsons et al. 2011). In the pool, DHA should be rapidly reduced back to ascorbate. presence of apoplastic plant esterases or prolonged non- DHA is recycled back to ascorbate via the ascorbate- enzymatic incubations, substantial hydrolysis of cOxT to glutathione cyclebyreducedglutathione(GSH),catalyzed OxTand then OxT to OxA and ThrO would take place by DHAR (Shimaoka et al. 2003). Finally, glutathione re- (Parsonsetal.2011). ductase (GR) converts glutathione disulfide (GSSG) back Table2TransgenicapproachesforoverproductionofL-ascorbateinplants hV ttpen Enzyme Targetplant Gene Genesource mTyapneipouflagteionnetic Ascorbatecontent Phenotypicchange Reference ://wkate ws GDP-l-galactose Tomato GGP/VTC2 Actinidiachinensis Overexpression 3.0–6.0-foldincreaseinfruits – Bulleyetal.2012 w.ahan phosphorylases SPtortaawtoberry Potato/ OOvveerreexxpprreessssiioonn U2.p0-ftoold3.0in-cforeldasiencinreafrsueitisntuber –– s-botandPark Arabidopsis icals Bota GpyDrPo-pmhaonspnhooserylase Potato GMPase Potato Adonwtisnernesgeulation 0.88–1.44-foldreductioninleaves Darkspotsonleafveinsandstems Kelleretal.1999 tudie nical GDP-Mannose Tomato SlGME1 Tomato Overexpression 0U.p56t-ofo1ld.42re-fdouldctiionncreinasteubinerlseaves EImarplyrosveendestcoelerancetovariousabiotic Zhangetal.2011 s.com/co2Studies 3’,5’-epimerase Upto1.60-foldincreaseinfruits stressessuchascold,saltandMV ntent/5014,55 SlGME2 Overexpression Upto1.37-foldincreaseinleaves 5/1:38 /3 Upto1.24-foldincreaseinfruits 8 L-galactose Tobacco GalT Kiwifruit Transientexpression Upto3.0-foldincrease – Laingetal.2007 guanyltransferase (leaves) L-Galactose Tobacco L-GalDH Arabidopsis Overexpression Nochange – Gatzeketal.2002 dehydrogenase Arabidopsis Arabidopsis Antisense 0.7-folddecrease – downregulation L-galactono-1,4-lactone Rice L-GalLDH Rice RNAi 0.6–0.87-folddecrease Slowplantgrowthrateandpoorseedset Liuetal.2011 dehydrogenase Rice Overexpression Upto1.48-foldincrease IncreasedNPRandhigherseedset Tomato SlGalLDH Tomato RNAi Nochange Slowplantgrowthrate Alhagdowetal.2007 Strongreductioninleafandfruitsize Rice L-GalLDH Rice RNAi 0.3–0.5-folddecrease Slowgrowthrate,reducedtillernumber, Liuetal.2013b decreasedNPRandprematuresenescence L-gulono-c-lactone Arabidopsis GLOase Rat Overexpression Upto2.0–3.0-foldincrease – Radzioetal.2003 oxidase Lettuce GLOase Rat Overexpression 4.0–7.0-foldincrease – JainandNessler2000 Tobacco Overexpression Upto7.0-foldincrease – Tomato GLOase Rat Overexpression 1.5-foldincreaseinfruits EnhancedtolerancetoMV,NaCl, Limetal.2012 andmannitol Potato GLOase Rat Overexpression Upto1.41-foldincrease EnhancedtolerancetoMV,NaCl, Hemavathietal.2010 andmannitol D-galacturonicacid Arabidopsis GalUR Strawberry Overexpression 2.0–3.0-foldincrease – Agiusetal.2003 reductase Potato GalUR Strawberry Overexpression 1.6–2.0-foldincrease EnhancedtolerancetoMV,NaCl, Hemavathietal.2009 andmannitol P a g Tomato GalUR Strawberry Overexpression 2.5-foldincrease Highgrowthrate WevarOlleretal.2009 e 8 (HairyRoots) o f Tomato LeMDAR Tomato Overexpression Upto1.18-foldincrease Lietal.2010 19 Table2TransgenicapproachesforoverproductionofL-ascorbateinplants(Continued) hV ttpen Mredonucotdaesheydroascorbate E(lnohwa/nhcigehd)taonledraMnVcesttroestseemsperature ://wwkates wh HighNPR .aan Adonwtisnernesgeulation Upto1.3-folddecrease Susceptibletovariousabioticstresses s-botandPark Tomato MDAR Tomato Overexpression 0.7-foldreducedinfruits – Haroldsenetal.2011 icals Bota Nochangeinleaves tudie nical Dreedhuycdtaroseascorbate Tomato DHAR Tomato Overexpression 1N.o6-fcohldanignecreinasleeaivnesfruits – Haroldsenetal.2011 s.com/c Studies o2 MMaaiizzee(Kernels) DDHHAARR WWhheeaatt OOvveerreexxpprreessssiioonn 61U..p09–-ftfooolld1d.8in(k-cfeorrelndaes(lelse)aivnecsre)aasned –– NChaqenvieettaall..22000039 ntent/55/1014,55:38 Tobacco DHAR Wheat Overexpression 2.2–3.9-foldincrease – Chenetal.2003 /3 8 Tobacco DHAR Rice Overexpression Upto1.6-foldincrease Enhancedtolerancetosaltandcoldstresses LeMartretetal.2011 Tobacco DHAR Human Overexpression 1.1-foldincrease IncreasedSODandAPXactivitiesin Leeetal.2007 (chloroplasts) conjunctionviatriplegeneconstruct IncreasedtolerancetoMVandNaCl inducedstress Potato DHAR Sesame Overexpression 1.1–1.3-foldincreaseintuber – Gooetal.2008 withpatatinpromoter Overexpression 1.5-and1.6-foldincreasein 1.5-and1.6-foldincreaseinleavesand leavesandtuberrespectively, tuberrespectively,withCaMV35Spromoter withCaMV35Spromoter Potato StDHAR1 Potato Overexpression Upto0.69-foldincreaseinleaves – Qinetal.2011 (Cytosol) Upto0.29-foldincreaseintubers Upto0.50-foldincreaseinleaves – StDHAR2 Overexpression Nosignificantchangeintubers (Chloroplast) Arabidopsis DHAR1 Rice Overexpression >1.4-foldincrease Enhancedtolerancetosaltstress Ushimaruetal.2006 Arabidopsis DHAR Arabidopsis Overexpression 2.0–4.25-foldincrease Enhancedtolerancetohigh–lightand Wangetal.2010 high–temperaturestress Myoinositoloxygenase Arabidopsis miox4 Arabidopsis Overexpression 2.0–3.0-foldincrease – Lorenceetal.2004 MV,methylviologen;NPR,netphotosyntheticrate;RNAi,RNAinterference. P a g e 9 o f 1 9 VenkateshandParkBotanicalStudies2014,55:38 Page10of19 http://www.as-botanicalstudies.com/content/55/1/38 into GSH using NAD(P)H as a reducing agent (Figure 2). oxidative stress (Mittler 2002; Mullineaux and Karpinski Recently, Huang et al. (2008), reported that thioredoxin 2002). Several isoforms of APX have been found in many h2 (Trx h2) having both DHA reductase (in the presence plantspeciesincludingbothmonocotsanddicots,andare of glutathione) and MDA reductase (in the presence of localized to various subcellular compartments. In Arabi- NADH) activity may also involve in the regeneration of dopsis, nine APX genes were described (Panchuk et al. ascorbatefromDHAandMDHA,respectively. 2002; Mittler et al. 2004; Narendra et al. 2006; Kousse- Increased biosynthesis of ascorbate in high light ex- vitzky et al. 2008): two cytosolic, two microsomal, three posed plants and enhanced photoinhibition and oxida- chloroplastic,onemitochondrial,andonedual-targetedto tive damage inascorbate-deficientplantswould implyits mitochondria and chloroplasts (Chew et al. 2003). In to- role in excess light energy dissipation (Smirnoff 2000; mato,APXgenefamilycomprisesofsevengenesencoding Müller-Mouléetal.2004;Yabutaetal.2007).Itwasprevi- three cytosolic, two peroxisomal, and two chloroplastic ously reported that high light stress results in the induc- APXs(Najamietal.2008).Whereas,inrice,theAPXgene tion of the cytosolic APX and protects the cytosol and familyconsists ofeight genes encodingtwo cytosolic, two other cellular compartments from high light induced peroxisomal, and three chloroplastic isoforms and one is Figure2MultiplefunctionsofL-ascorbateinplants.Duringabioticstressconditions,scavengingofROSbyAPXincreasesMDAcontentin bothapoplastandsymplast.IftheMDAisnotrapidlyreducedbacktoascorbatebyMDAR,spontaneouslydisproportionateintoascorbateand DHA.CytoplasmicDHARcanreduceDHAbacktoascorbateusingGSH,andtheresultingGSSGisregeneratedbacktoGSHthroughtheactionof GRinaNADPHdependentreaction.Furthermore,duringoxidativestressconditions,L-ascorbateactsasacofactorforviolaxanthinde-epoxidase fortheformationofzeaxanthinandalsoinvolvesintheregenerationofα-tocopherolfromtocotrienoxylradicals.
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