ABSTRACT
The production of reactive oxygen species (ROS) is an unavoidable consequence of aerobic metabolism.ROSarehighlyreactiveproductsthataregeneratedduetothestepwisereductionofmolecular oxygen(O2) by high-energy exposure or as a result of electron-transfer chemical reactions. ROS includefreeradicalsaswellasthenonradicalmoleculesofhighreactivity,suchassuperoxideanion (O2•−), hydroxyl radical (•OH), hydrogen peroxide (H2O2), singlet oxygen (1O2) etc. Dual roles for ROSinplantshavebeensuggested,astheyserveaskeyregulatorsofgrowth,development,and defensepathways,aswellasatexcessivelevelstheycauseoxidativedamagetobiomoleculesleading totoxicityinthecell(Mittleretal.,2004).Stressfulconditionsoftheenvironmentsuchasdrought, saltstress,chilling,heatshock,excesslevelsofheavymetals,anaerobiosis,UV-Bradiation,gaseous pollutants lead to the enhanced generation of ROS in plants due to the disruption of cellular homeostasis(Yanetal.,1996;Pellinenetal.,1999;Shahetal.,2001;Mittler,2002;Pasqualinietal., 2002;SharmaandDubey,2005;SuzukiandMittler,2006;Esfandiarietal.,2008;Huetal.,2008; Hanetal.,2009;Kumuthaetal.,2009;MaheshwariandDubey,2009;Sairametal.,2009;Tanou etal.,2009).TheenhancedproductionofROSduringabioticstressescanposeathreattocellsby causing the peroxidation of lipids, oxidation of proteins, damage to nucleic acids, enzyme inhibition,activationofprogrammedcelldeath(PCD)pathwayandultimatelycelldeath(Shahetal., 2001;Mittler,2002;Pasqualinietal.,2003;VermaandDubey,2003;Merigaetal.,2004;Sharma andDubey,2005,2007;MaheshwariandDubey,2009).ROSareviewedasthecellularindicators ofstressastheyserveassecondarymessengersinthestressresponsesignaltransductionpathway. ROS also play an important role in lignication and other cross-linking processes in the cell wall (Bradleyetal.,1992;Ogawaetal.,1997;Desikinetal.,2001;Mittler,2002).BecauseofthemultifunctionalrolesofROS,itisnecessaryforthecellstocontrolthelevelofROStightlytoavoidany oxidativeinjuryandnottoeliminatethemcompletely.ScavengingordetoxicationofexcessROS isachievedbyanefcientantioxidativesystemcomprisingofthenonenzymicaswellasenzymic antioxidants (Noctor and Foyer, 1998). Ascorbate (AsA), glutathione (GSH), carotenoids, tocopherols,andphenolicsserveaspotentnonenzymicantioxidantswithinthecell.Theenzymicantioxidantsincludesuperoxidedismutase(SOD),catalase(CAT),guaiacolperoxidase(GPX),enzymesof ascorbate-glutahione(AsA-GSH)cyclesuchasascorbateperoxidase(APX),monodehydroascorbatereductase(MDHAR),dehydroascorbatereductase(DHAR),andglutathionereductase(GR) (Noctor and Foyer, 1998). However, the equilibrium between production and scavenging of ROS isperturbedunderstressfulconditions;thereforethemodulationofantioxidantlevelsconstitutes animportantadaptiveresponsetowithstandadverseenvironmentalconditions(Foyeretal.,1994; Noctoretal.,2002;PitzschkeandHirt,2006).Variousworkershavereportedincreasedactivities ofmanyenzymesoftheantioxidantdefensesysteminplantstocombatoxidativestressinduced by drought (Rajamickam et al., 2005; Sharma and Dubey, 2005), soil salinity (Lin and Kao, 2000; Tsaietal.,2005;Tanouetal.,2009),hightemperature(Rijstenbiletal.,1994;Almeselmanietal., 2006;Yinetal.,2008),chilling(Fryeretal.,1998;BafeelandIbrahim,2008;Radyuketal.,2009), metaltoxicity(Shahetal.,2001;VermaandDubey,2003;SharmaandDubey,2007;Maheshwari andDubey,2009),anaerobiosis(Linetal.,2008;Kumuthaetal.,2009),gaseouspollutants(Ranieri et al., 1997; Scebba et al., 2003), and UV-B radiation (Santos et al., 1999; Agarwal and Shaheen, 2007).ThemaintenanceofahighantioxidantcapacitytoscavengethetoxicROShasbeenlinked toincreasedtoleranceofplantstotheseabioticstresses(AgarwalandShaheen,2007;Zaefyzadeh et al., 2009).
