Metabolic Processes During Seed Germination - IntechOpen

Physiological and biochemical changes followed by morphological changes during germination are strongly related to seedling survival rate ... Home>Books>AdvancesinSeedBiologyOpenaccesspeer-reviewedchapterMetabolicProcessesDuringSeedGerminationWrittenByAwatifS.AliandAlaaeldinA.ElozeiriSubmitted:March20th,2017Reviewed:August21st,2017Published:December6th,2017DOI:10.5772/intechopen.70653DOWNLOADFORFREEShareCiteCitethischapterTherearetwowaystocitethischapter:1.ChoosecitationstyleSelectstyleVancouverAPAHarvardIEEEMLAChicagoPlaceholderCopytoclipboard2.ChoosecitationstyleSelectformatBibtexRISDownloadcitationIntechOpenSeedBiologyEditedbyJoseCarlosJimenez-LopezFromtheEditedVolumeSeedBiologyEditedbyJoseC.Jimenez-LopezBookDetailsOrderPrintChaptermetricsoverview6,242ChapterDownloadsViewFullMetricsDOWNLOADFORFREEShareCiteCitethischapterTherearetwowaystocitethischapter:1.ChoosecitationstyleSelectstyleVancouverAPAHarvardIEEEMLAChicagoPlaceholderCopytoclipboard2.ChoosecitationstyleSelectformatBibtexRISDownloadcitationImpactofthischapterIntechOpenDownloads6,242TotalChapterDownloadsonintechopen.comCitationsCitations29CitationsAdvertisementAdvertisementAbstractSeedgerminationiscrucialstageinplantdevelopmentandcanbeconsideredasadeterminantforplantproductivity.Physiologicalandbiochemicalchangesfollowedbymorphologicalchangesduringgerminationarestronglyrelatedtoseedlingsurvivalrateandvegetativegrowthwhichconsequentlyaffectyieldandquality.Thisstudyisaimedtofocusonproceedingofthemostvitalmetabolicprocessesnamelyreservemobilization,phytohormonalregulation,glyoxylatecycleandrespirationprocessundereitherstressfulornon-stressfulconditionsthatmaybeledtosuggestandconductthemoresuccessfulexperimentalimprovements.Seedimbibitiontriggeredtheactivationofvariousmetabolicprocessessuchassynthesisofhydrolyticenzymeswhichresultedinhydrolysisofreservefoodintosimpleavailableformforembryouptake.Abioticstressespotentiallyaffectseedgerminationandseedlingestablishmentthroughvariousfactors,suchasareductioninwateravailability,changesinthemobilizationofstoredreserves,hormonalbalancealterationandaffectingthestructuralorganizationofproteins.Recentstrategiesforimprovingseedqualityinvolvedclassicalgenetic,molecularbiologyandinvigorationtreatmentsknownasprimingtreatments.H2O2accumulationandassociatedoxidativedamagestogetherwithadeclineinantioxidantmechanismscanberegardedasasourceofstressthatmaysuppressgermination.Seedprimingwasaimedprimarilytocontrolseedhydrationbyloweringexternalwaterpotential,orshorteningthehydrationperiod.KeywordsreservemobilizationproteolysisglyoxylatecyclephyticacidseedprimingstresstolerancemechanismsAuthorInformationShow+AwatifS.AliBotanyDepartment,FacultyofScience,KafrElsheikhUniversity,KafrElsheikh,EgyptAlaaeldinA.Elozeiri*EnvironmentalEngineeringDepartment,ZewailCityofScienceandTechnology,Cairo,Egypt*Addressallcorrespondenceto:awatifali95@yahoo.com1.IntroductionSeedgerminationisvitalstageinplantdevelopmentandcanbeconsideredasadeterminantforplantproductivity.Itbeginsbywaterimbibition,mobilizationoffoodreserve,proteinsynthesisandconsequenceradicleprotrusion[1].Tosustainagoodseedlingdevelopment,seedstoresafoodreservemainlyasproteins,lipidsandcarbohydrates[2].Proteinandoilbodiesarethemajorreserveinoilseedwhichrepresentasourceforeachofenergy,carbon,andnitrogenduringseedlingestablishment[3].Becausethephysiologyofreservemobilizationduringgerminationandpost-germinationeventsisstillpoorlyunderstood,extensivestudiesmustbeperformedtoknowthemetabolicmechanismsofreservefoodmobilizationprovidinginsightsintotheabilitytousesuchseedsasplantingmaterial[4].Enzymatichydrolysisofprotein,lipidandcarbohydrate,andtransportationofmetabolitesisdependentmainlyonwateravailability[5].Physiologicalandbiochemicalchangesfollowedbymorphologicalchangesduringgerminationarestronglyrelatedtoseedlingsurvivalrateandvegetativegrowthwhichaffectyieldandquality.Foodreserveofstarchandproteinaremainlystoredintheendosperm.Ingeneral,germinationprocesscanbedistinguishedintothreephases:phaseI,rapidwaterimbibitionbyseed;phaseII,reactivationofmetabolism;andphaseIII,radicleprotrusion[6].ThemostcriticalphaseisphaseIIwhereas,theessencephysiologicalandbiochemicalprocessessuchashydrolysis,macromoleculesbiosynthesis,respiration,subcellularstructures,andcellelongationarereactivatedresultingininitiationofgermination[7].Waterimbibitionbyreservesubstancesingerminatingwheatseedstimulatestheembryotoproducephytohormonesmainlygibberellicacid(GA)whichcandiffusetoaleuronelayerandinitiateasignalingcascaderesultinginthesynthesisofα-amylasesandotherhydrolyticenzymes.Then,hydrolyticenzymessecreteintotheendospermandhydrolyzedfoodreserve[8,9].Germinationisconsideredaresponseincludesbidirectionalinteractionsbetweentheembryoandendospermsincetheendospermcansecretesignalstocontrolembryogrowth[10].Previousstudieswereinvestigatedtheactivityofsomekeyenzymesinglycolysis,pentosephosphatepathway(PPP),thetricarboxylicacidcycle(TCAcycle),andaminoacidmetabolismduringgermination[11].Seedgerminationisparticularlyvulnerabletoenvironmentalstressencounteredconditions,specificallysaltandwaterwhicharewidespreadproblemaroundtheworld[12].Highsaltanddroughttoleranceseedsmightbeshowedrapidgerminationresultinginagoodseedlingestablishmentandhenceexpectedtomaintainhighyieldproductivity[13].Waterandsaltstressconditionsaffectseedgerminationwithreducinggerminationrateanddelayintheinitiationofgermination[14].Underwaterstress,enzymesactivitysuchasα-amylaseinCicerarietinumcotyledons[15]orα-andβ-amylaseinMedicagosativagerminatingseeds[16]werereduced.Incontrast,waterstressconditionsledtoanincreaseintheactivityofα-amylaseinHordeumvulgareseedlings[17],β-amylaseinCucumissativuscotyledons[18],cytosolicglyceraldehyde-3-phosphatedehydrogenaseinCraterostigmaplantagineumplants[19]andproteaseinOryzasativaseedlings[20].Saltstresscausesiontoxicity,osmoticstressandreactiveoxygenspecies(ROS)stress[21].ROSreactswithcellmacromolecules[22]andlipids[23],anddisruptdiversephysiologicalandbiochemicalprocesses,suchashormonalimbalanceandreduceduseofreserves[24].PlantsdevelopROS-scavengingmechanismsincludeenzymaticandnon-enzymaticantioxidantsystems[25]thatprotectplantsagainstoxidativedamage.Therefore,improvementtheactivityofantioxidantenzymesinplantsorgansisnecessaryforincreasingplant’ssalttolerance.Speciesandvarieties/cultivarsvariedintheirabilityforsalttolerancemechanism.Comparingwithadultplant,themechanismsofstresstoleranceingerminatingphasearepoorlyinterpretedandmightberelatedtoaseriesoffactorsthatareinherenttothespeciesandenvironment[26,27].Phytohormoneshaveessentialroleininducingplantacclimatizationtochangeinenvironmentalconditionsbymediatinggrowth,development,source/sinktransitions,andnutrientallocation[28].Phytohormonesareconsideredthemostimportantendogenoussubstancesformodulatingphysiologicalandmolecularresponses[28].Theyincludeauxin(IAA),cytokinins(CKs),abscisicacid(ABA),ethylene(ET),gibberellins(GAs),salicylicacid(SA),brassinosteroids(BRs),andjasmonates(JAs).Thestrigolactone(SL)arerelativelynewphytohormones.GeneticallyandphysiologicalstudieshavebeendemonstratedtheeffectiverolesoftheplanthormonesABAandGAsinregulationofdormancyandgermination[29].Tocounteracttheadverseeffectsofabioticstress,seedprimingmethodshavebeenappliedtoimprovegermination,uniformity,improveseedlingestablishmentandstimulatevegetativegrowthinmorefieldcrops[30,31].Wheatseedswereprimingtoincreasegerminationcharacteristicsandstresstolerance.Asseedsimbibewater,metabolicprocessesinitiatewithanincreaseinrespirationrate[7].Earlydevelopmentalstagesofseedlingrequirefuelingenergybeforeitbecomesautotrophic[32].Seedsstoremineralnutrientsassucroseoraminoacidswhicharesynthesizedintostarchorproteinsduringdevelopmenttobeusedinearlyseedlingemergence.Phosphorusistakenupbyplantsasphosphateandtranslocatetodevelopedseedswhereitisstoredinphyticacidformmainly(about75%).Advertisement2.TheroleofhydrolyticenzymesinseedgerminationOnseedhydration,separateintercellularbodiesofseedstoredcarbohydrates,proteins,lipidandphosphateactasenergysourceandcarbonskeleton[33].Seedimbibitiontriggeredmanymetabolicprocessessuchasactivationorfreshlysynthesisofhydrolyticenzymeswhichresultedinhydrolysisofstoredstarch,lipid,proteinhemicellulose,polyphosphatesandotherstoragematerialsintosimpleavailableformforembryouptake.Also,consumptionofanelevatedlevelofoxygenmaybeinducedactivation/hydrationofmitochondrialenzymes,involvedintheKrebscycleandelectrontransportchain[34,35].2.1.HydrolysisofstorageseedproteinsProteolyticenzymeshavethemainroleinusingstoredproteininmetabolismofgerminatingseedswhichproceedthroughmanystages[36].AccordingtoGepstinandIlan[37],proteolyticactivityingerminatingbeansincreasedduringthefirst7 dayswhichpartiallydependentontheembryonicaxis.Proteasesandpeptidaseshavebeendetectedinmanyseedsduringgerminationwhereas;plantproteaseandamylaseinhibitorswhichareproteinaceousinnaturearebeingdisappeared[38].Antitrypticandantichymotrypticactivitieswereobservedtobemarkedlyreducedintheendospermoffingermilletongerminationwhichmightbeattributedtotheproteolyticactivityinhydrolysisoftheinhibitoryproteins[39].Hydrolysisofstoredproteinsproducedfreeaminoacids,whichsupportproteinsynthesisinendospermandembryoandsoproceedingofgerminationprocess[40].Schlerethet al.[41]recordedaninitiallittledecreaseinfreeaminoacidsatthebeginningofvetchseedsimbibitionwhichisattributedtoleakagefromtheaxis,butremainwithoutchangeduringlategerminationstage.AdisulfideproteometechniquewasdevelopedbyYanoet al.[42]tovisualizeredoxchangesinproteins.Thistechniquewasusedtoanalyzericebranresultinginidentificationofembryo-specificprotein2(ESP2),dienelactonehydrolase,putativeglobulin,andglobulin-1S-likeproteinasputativetargetofthioredoxin,whichsupportthehypothesisthatthioredoxinactivatescysteineproteasewithaconcurrentunfoldingofitssubstrateduringgermination[43].Inbuckwheatseeds,themainstorageproteinconstituentabout16%oftotalseedproteinisthe13Sglobulinwithmolecularmassofabout300kDaandconsistsofacidandbasicsubunitswithmolecularmassesrangingfrom57.5to23.5 kDa[44].Duringseedgermination,13Sglobulinishydrolyzedbyproteolyticenzymesthroughstagesandtheproductsareusedbythegrowingseedling.Thefirststageofthe13Sglobulindegradationresultedfromalimitedproteolysisactivityofmetalloproteinasewiththecleavageofabout1.5%ofpeptidebonds.Thisstageproceedsduringthefirst3 daysofgermination.Ittakesplaceduringthefirst3 daysofgermination[45].Metalloproteinaseactivityiscontrolledbyaproteinaceousinhibitor(Mr—10 kDa),presentindrybuckwheatseedsinacomplexwiththeenzymewhichdissociatedbybivalentcationsliberatedfromphytinhydrolysisprocess.Phytinispresentinbuckwheatseedsinsufficientamountintheformofgloboidsdisposedinproteinbodies[46].Duringthesecondstageof13Sglobulindegradation;theproductsofmetalloproteinaseproteinactivityhydrolyzedintosmallpeptidesandaminoacidsatacidpH(5.6)bycysteineproteinaseandcarboxypeptidasewhichappearingerminatingseeds[47].Itwasclearthatcysteineproteinaseisabletohydrolyzeonlythemodifiedl3Sglobulinbutnotthenative.Theroleofcarboxypeptidaseistofacilitatetheflowofstorageproteinhydrolysisandworksincooperationwithcysteineproteinase.AtlateststagewhenpHbecomesmoreacidic(5.0)inthevacuoles,asparticproteinasewhichispresentindryseedsisinvolvedintothecourseofhydrolysisproteinbodies.2.2.HydrolysisofstorageseedstarchCarbohydratesrepresentthemoststoragefoodconstituentincerealgrains,whereasitcontainsabout70–80%starch,about15%protein,lessthan5%lipids,mineralsandvitamins.Incereals,mosthydrolysisenzymesareproducedinthealeuroneorscutelluminresponsetogerminationsignals.Severalmodifiedseedsystemswereusedtodetecttheinductionprocessandidentifypotentialfactorscontrollingenzymeinductioninabsenceoftheembryo[48].ChrispeelsandVarner[49]observedthatisolatedaleuronefailedtosynthesizeα-amylaseinamannerquantitativelysimilartodistalhalfseedsledtocorrectionbyaddingcalciumtothemedium.Theroleofcalciummightbeexpectedtoinvolveamylasestability,andtohaveamuchmorecomplexinvolvementinregulatingenzymeactivities[50].Becauseofde novoamylasesynthesisduringseedgerminationtostimulatethestoredstarchmobilizationforprovidingyoungplanttillphotosynthesiswillbeinitiate,amylasehasbeenshowedhighactivity[51].Paryset al.[52]showedthattheamylaseactivityisregulatedbytheconcentrationofreducingsugarsin vivoinbothcotyledonsandaxis.Atthetime,theamylaseactivityinthecotyledonsincreasedgraduallyandreachedamaximumonthe5thdayofgerminationprocess,whilethestarchdecreasedandsolublesugarsincreased[53].Manystudieswhichconcernedwithstudyingtheessentialityofα-amylaseactivityduringseedgerminationunderdroughtstressandcouldbesummarizedasfollows;thepromotionofdroughtstressedgerminatingseedsisaresultofhighα-amylaseactivitydirectlybut,itmightberelatedtoadaptivestrategytowaterdeficitsinceitsactivityisrequiredforsolutesaccumulationanddecreaseosmoticpotential[54,55].Inaddition,α-amylasesynthesisinhibitionmightbenotamechanismbywhichdroughtpreventsthegerminationofAgropyrondesertorumseeds[54].GAscanalleviatethedroughtstress-causedinhibitionofseedgerminationthroughregulationofα-amylase[19].2.3.HydrolysisofstorageseedlipidsGenerallyoilseedscomposedoftwoparts,thekernelwhichismainpartandtheseedcoveringthatenclosedthekernelandcalledthehuskortegument.Thekernelcomprisedtwopartswhicharetheembryoandtheendosperm.Lipaseactivityisinvestigatedduringseedgerminationwhereitismaximumvalue[56,57].Triacylglycerolsisstoredinoleosomesandcompriseinrangefrom20to50%ofdry.Asgerminationproceeds,triacylglycerolsarehydrolyzedtoproduceenergywhichrequiredforthesynthesisofsugars,aminoacids(mainlyasparagine,aspartate,glutamineandglutamate)andcarbonchainsrequiredforembryonicgrowth[58].Lipidlevelandlipaseactivitywerestudiedinvariousgerminatingseeds.Itwasshowedthatβ-oxidationtakesplace4 daysaftergerminationofCastorbeenseeds[59].Themajorhydrolyticenzymesconcernedwiththelipidmetabolismduringgerminationarethelipaseswhichcatalyzethehydrolysisofestercarboxylatebondsandreleasingfattyacidsandorganicalcohols[60,61]andthereversereaction(esterification)orevenvarioustransesterificationreactions[62].Theabilityoflipasestocatalyzethesereactionswithgreatefficiency,stabilityandversatilitymakestheseenzymeshighlyattractivefromacommercialpointofview.Villeneuve[63]andothersclassifiedlipasesspecificitiesintothreemaingroups;the1stgroupissubstratespecificityinwhichglycerolestersrepresentthenaturalsubstrates,the2ndgroupiscalledregioselectiveandinvolvesthesubgroupsnon-specificlipasesthathydrolyzethetriacylglycerolsintofattyacidsandglycerolinarandomwaywithproductionofmono-anddiacylglycerolsasintermediateproducts(Figure 1);specific1.3lipaseswhichcatalyzethehydrolysisatC1andC3glycerolbondsintriacylglycerolswithliberatingoffattyacidsandunstableintermediates2-monoacylglycerolsand1.2-or2.3-diacylglycerolsandspecificorselectivetypefattyacidthathydrolyzetheesterbondofaspecificfattyacidoraspecificgroupoffattyacidsatanypositionoftriacylglycerol.The3rdgroupenantioselectivecouldidentifyenantiomersinaracemicmixture.Theenantiospecificitiesoflipasesdependonthetypeofsubstrate[64].Figure 1.Regioselective:non-specificand1,3specificlipasescatalyzethehydrolysisoftriglyceridesindifferentmannerswiththeproductionoffattyacids.Theinductionoflipaseactivityduringgerminationmightbedependentonfactorsfromembryo[65].EarlystudyofShoshiandReevers[66]showedthepresenceoftwolipasesintheendospermofCastorbeenseed,acidlipaseindryseedandalkalinelipaseduringgermination.Ontheotherhand,storagetissuesofalltheoilseedsexceptCastorbeancontainedonlylipaseactivitywhichincreasedduringgermination[67].Becauseofsucroseisthesubstrateforlipidbiosynthesisindevelopingseedandtheendproductoflipiddegradation,itmightbeprimarilyconsideredasregulatoryfactorinstudyingthemechanismsoflipidmetabolism[58,68].Inaddition,asparagineandnitrateareconsideredregulatoryfactorsinlipidmetabolismoflupine[69].Inlupingerminatingseeds,thelevelofasparaginecanreach30%ofdrymatter,anditisamaintransportformofnitrogenfromsourcetosinktissues[70].Boreket al.[71]reportedthatasparaginecontrolsthemetabolismofcarbohydrateasitcausedasignificantdecreaseinsolublesugarsandincreaseinstarchinorgansofgerminatinglupinseed.Incontrast,nitrateisnotafavorablesourceofnitrogeninproteinmetabolisminlupinseeds[72]andratherdoesnotinfluencethecarbohydratemetabolism[71].NitratesimilarlyasNsucrose,isregardedasafactorwhichcanregulateplantmetabolismbychangesintheexpressionofsomegenes[73].Storagelipidmobilizationingerminatingseedsbeginswithhydrolysisoftriacylglycerolsinoleosomesbylipasesintofreefattyacidsandglycerol.Thenfattyacidsundergoβ-oxidationinperoxisomes.Next,glyoxylatecyclewillproceedpartiallyintheperoxisomeandpartiallyinthecytoplasm.Threeofthefiveenzymesoftheglyoxylatecycle(citratesynthase,isocitratelyaseandmalatesynthase)arelocatedinperoxisomes,whiletwootherenzymes(aconitaseandmalatedehydrogenase)operateinthecytoplasm[74].SuccinatetransportedfromperoxisometomitochondriaandhereisconvertedtomalateviatheKrebscycle.Malateinturn,aftertransporttothecytoplasm,isconvertedtooxaloacetate.Finally,gluconeogenesisandthesynthesisofsugarsaretheprocesseswhichareaformofcarbontransportespeciallyingerminatingseedsproceed[58,75].2.4.HydrolysisofphyticacidduringseedgerminationThegreateststorageformoftotalphosphorus(about50–80%)isphyticacid(C6H18O24P6)andalsoknownasinositolhexophosphate(IP6)inlegumesandcerealsseeds[76].Phyticisregardedasantinutrientbecauseithastheabilitytoformcomplexeswithproteinsandbindwithcations(especiallyFe,Ca,K,Mn,Mg,Zn)viaionicassociationtoformamixedsaltcalledphytinorphytatewiththereductionoftheirdigestiveavailability[77].Ontheotherhand,phytatemayplayanimportantroleasanantioxidantbyformingironcomplexthatcauseadecreaseinfreeradicalgenerationandtheperoxidationofmembranes,andmayalsoactasananticarcinogen,providingprotectionagainstcoloncancer[78].Becauseofitwasregardedasantioxidant,anticarcinogenorvitaminlikesubstance,itisessentialtomeasureandmanipulatephytatecontentinfoodgrainssuchasbeans[79,80].Oneofthemajorbreedingobjectivesisthedevelopmentofcropcultivarswithlowseedphytincontent.Itwasfoundthattheincreaseinmyo-inositolandreducedamountsofmyo-inositolphosphateintermediatesintheseedsofmaizemutantswithaphenotypeofreducedphyticacidhadalittleeffectonplantgrowthanddevelopment[81].Thesefindingsmightsuggestthatahighlevelofstoredphytateisnotnecessaryforseedviabilityandgerminationorseedlingsgrowth.Phytinismainlystoredinproteinbodiesinseedscalledgloboidsinthealeuronelayerandscutellumcellsofmostgrains.Phyticacidhasastrongabilitytochelatemultivalentmetalions,speciallyzinc,calcium,ironandaswithproteinresidue.Seedphytatecontentdependmainlyontheenvironmentalmainlyplantphosphorusfertilization[82].Ithasbeenshowntheimportantgeneticvariabilityinthephytatecontentofbeansanditappearstobeatraitcontrolledbyseveralgenes[83].Also,acorrelationbetweenphytateandproteincontentswasfound[84],sotheproteincontentofgrainscanbeconsideredanotherfactorthatregulatesphytatecontent.Phytiningerminatingseedsishydrolyzedbyanacidphosphataseenzymecalledphytase[85],withreleasingofphosphate,cations,andinositolwhichareutilizedbytheseedlings.ItwasfoundlittlechangesinextractiblePiinhazelseedsduringchillingaccompaniedwithIP6mobilizationthatmightbesuggestedtherapidconversionofPiintoorganicform[86].Theseresultswerediscussedasevidenceofactivemetabolismingerminatingseed[87].Inagreement,phytaseisstronglyandcompetitivelyinhibitedbyPi,whilethedecreaseinphytaseactivitycoincidedwithmaximalIP6turnover[88].Itwasfoundthatabout87%ofIP6isdigestedduringthefirst6 daysofgermination[89].Inthisrespect,Ogawaet al.[90]postulatedthattheearlyaxiferousIP6digestionisessentialformetabolicactivityoftherestingtissueviasupplyingPiandmineralsforphysiologicalandmetabolicrequirements,forexample,enzymesofstarchmetabolism.Inaddition,IP6relatedcompoundssuchaspyrophosphate-containinginositolphosphates(PP-IP)playapotentialroleinprovidingPiforATPsynthesisduringtheearlystagesofgerminationbeforecompletedependenceonaerobicmitochondrialrespirationthemainlysourceofATPproduction[91].Instressedseeds,manyvitalprocessessuchasgermination,growth,respirationandotherrelatedprocessesareaffectedwhichconsequentlycantriggerothereffectsonmetabolicactivitiesparticularlytheenzymesofphosphatemetabolismthatplayanimportantroleingerminationandseeddevelopment[92].Phosphatemetabolismisoneofnegativelyaffectedprocessesunderdifferentstressfulconditions[93].Understressfulconditions,therestrictionofgrowthandphosphorusavailabilityresultinginenhancementtheactivityofphosphatasestoproducePibyhydrolysistheinsolublephosphateformthatmodulatemechanismoffreephosphateuptake.Inagreement,OlmosandHellin[94]reportedthatacidphosphatasesactivityincreasedtosustainPilevelwhichenablesittobeco-transportedwithH+downaprotonmotiveforcegradient.Advertisement3.EffectofabioticstressonmetabolicactivitiesduringseedgerminationAbioticstressesincludingsalt,drought,heavymetals,pollutants,heat,etc.,potentiallyaffectseedgerminationandseedlinggrowth.Dependingonthestressintensityandgeneticbackground,germinationisdelayedorsuppressed.Plantshavedevelopeduniquestrategiesincludingatightregulationofgerminationensuringspeciessurvival[95].ItwaswellknownthatstressexposurewouldproduceearlysignalssuchaschangeinintracellularCa2+,secondarysignalingmoleculessuchasinositolphosphateandROSaswellasactivationofkinasecascades.Seedimbibitiontriggersmanybiochemicalandcellularprocessesassociatedwithgerminationinvolvethereactivationofmetabolism,theresumptionofcellularrespirationandthebiogenesisofmitochondria,thetranslationand/ordegradationofstoredmRNAs,DNArepair,thetranscriptionandtranslationofnewmRNAs,andtheonsetofreservemobilization[7,96].TheseprocessesarefollowedbyROS(mostlyH2O2)accumulationasaresultofapronouncedincreaseintheintracellularandextracellularproductionduringearlystages[97,98].ROSfunctionascellularmessengersortoxicmoleculesonseedhydration[99].ROScausedseeddamageaccompaniedwithalossofseedvigorandasarepercussionofaging[100].ThehighlyactivityofrespirationduringgerminationresultsinsuperoxideanionproductionduringelectronleakagefromthemitochondrialelectrontransportchainfollowedbydismutationtoH2O2.OthersourcesofROSareNADPHoxidasesoftheplasmamembrane,extracellularperoxidases,β-oxidationpathwayinglyoxysomes[97].H2O2isalong-livedROSthatcandiffuseeasilythroughmembranesandthatcanreachtargetsfarfromproductionsites,andisrecognizedasanimportantsignalingmolecule[101].H2O2isconsideredasstrongoxidizingagent,itcouldinteractwithmostbiomoleculesresultinginoxidativestressthatcausescellulardamage.Itcauseslipidperoxidationwhichinturnaffectspolyunsaturatedfattyacids(PUFAs)foundinmembranesorreservelipids.Also,H2O2causeoxidationofnucleicacids(DNA,RNA)andproteins[97].InductionofDNAoxidationbyH2O2resultedintheaccumulationof7,8-dihydro-8-oxoguanine(8-oxo-dG),whichhasbeenshowntocausetheaccumulationofdouble-strandbreaksingenomeanddeleteriouseffectsoncellviability[102].DNAoxidationbyROSisconsideredamainsourceofDNAdamageduringseedstorageandgermination.KongandLin[103]haveshownthatmRNAismuchmoresensitivetooxidativedamagethanDNA,mainlyduetoitscellularlocalization,singlestrandedstructureandlackofrepairmechanisms.GuanineisthemostfrequentlyoxidizedbaseinRNAleadstotheaccumulationof8-hydroxyguanosine(8-OHG).OxidativedamagetomRNAresultsintheinhibitionofproteinsynthesisandinproteindegradation[104].OxidationofproteinbyROSresultinalterationofproteinfunctionsduetoenzymaticandbindingpropertiesmodifications[105].H2O2accumulationandassociatedoxidativedamagestogetherwithadeclineinantioxidantmechanismscanberegardedasasourceofstressthatmaysuppressgermination.Ontheotherhand,Barba-Espinet al.[106]reportedthattheselectiveoxidationofproteinsandmRNAscanactasapositiveregulatorofseedgermination.UsingofcalciumsensorscalledCa2+bindingproteinsrevealedanincreaseinintracellularcalciumconcentrationunderabiotic-stressconditions[107].Thisisaccompaniedwithenhancementofcalcium-dependentproteinkinases(CDPKs),calcium/calmodulin-dependentproteinkinases(CCaMKs)orphosphataseswhichstimulatethephosphorylation/ordephosphorylationofspecifictranscriptionfactors,resultinginanincreaseofstress-responsivegenesexpression[108].However,activatedCa2+sensorsregulatestress-responsivegeneseitherbybindingtocis-elementsinthepromotersorbyinteractingwithDNA-bindingproteinsofgenesthatledtogeneactivationorsuppression.Stressed-germinatingwheatseedsdevelopapowerfulregulatormechanisminresponsetostresseswhichiscalreticulin-likeprotein(M16andM13)andabundantCa2+-bindingproteinpredominantlylocatedintheendoplasmicreticulum(ER)ofhigherplants[109].Itsexpressiontrendwasmainlyup-regulated,especiallyinthelastperiodofgerminationwhichhintsthatwheatseedmayencounterstressinlategermination[110].Anotherregulatormechanismwithpeptidyl-prolylcis-transisomeraseactivitywhichinvolvedinsignaltransduction,cellapoptosis,andproteinfoldingcalledcyclophilin(M51)wasdetectedinstressedgerminatingwheatseeds[111].Becauseofthecellularstructureisnotcompleteinearlygermination,M51increasedslowlyinfirstthreegerminationstagesbutincreasedsharplyinthelaststage[109].Oneofthemosteffectivefactorsonseedimbibitionandgerminationisthetemperature.Itaffectswateruptakeandreactivationofmetabolicprocesses[7].Manyphysiological,biochemicalandmoleculardisturbancewilloccurwithtemperaturedeviationawayfromoptimaltosustaincellularhomeostasis[112].Advertisement4.TheroleofphytohormonesduringgerminationPlantsarecharacterizedbyproducingvarioustypesofgrowthregulatorsthatdifferedintheirchemicalstructureandphysiologicalaction.Theyincludeauxins,cytokinins(CK),gibberellins(GA),abscisicacid(ABA),ethylene(ET),salicylicacid(SA),jasmonates(JA),brassinosteroids(BR)andstrigolactones.EachofABA,SA,JAandETisfoundtoplayanessentialroleinmediatingplantdefenseresponseagainststresses[113].Duringtheearlyphaseofseedgermination,adecreaseinJAandSAcontentsandanincreasedlevelofauxinswererecordedinArabidopsisseeds[114].BothJAandSAwereshowntoactasnegativeregulatorsofseedgermination[115].AuxinsareconsideredtoberegulatorsoftheseedgerminationprocessinacrosstalkwithGAs,ABA,andET[116].ThebrassinosteroidssignalcouldstimulategerminationbydecreasingthesensitivitytoABA[117].Avarietyofcellularprocessesinplantsareundercontrolofphytohormoneswhichplaykeyrolesandcoordinatevarioussignaltransductionpathwaysduringabiotic-stressresponse[118].SeedimbibitionsresultedinanactivationofGAbiosynthesisandresponsepathwayswiththeproductionofthebioactiveGAs.Then,GAsstimulatedthegenesencodingforenzymessuchasendo-β-1,3glucanase[119],β-1,4mannanendohydrolase[120]whichhydrolyzetheendospermandalleviatetheinhibitoryeffectsofABAonembryogrowthpotential[121].TheseresultsareindicatedtheantagonisticrelationbetweeneachofABAandGAwhichinterpretthepresenceofhighGAandlowABAlevelsinseedsunderfavorableenvironmentalconditionsandareverserationunderunfavorableconditions.Thus,thecrosstalkrelationbetweenseeddormancyandgerminationisbalancedbyGA-ABAration,akeymechanismforcopeearlyabiotic-stressconditions.ABAinhibitswateruptakebypreventingcellwalllooseningoftheembryoandtherebyreducesembryogrowthpotential[122].GAsareinvolvedindirectenhancementthegrowthoftheembryoduringlatephase[123].GAsrepressivetheABAeffectduringtheearlyandthelatephasesofgerminationthroughstimulationofgenesexpressionencodingcellwalllooseningthatresultinremodelingenzymessuchasα-expansinsinearlyphaseofgermination.LightandcoldacttogethertobreakdormancyofimbibedseedsandtopromoteseedgerminationbyincreasingGAslevels.ArapiddecreaseofABAendogenouscontentduringPhaseIIisoneofmanyfactorsthatinfluencethesuccessfulcompletionofgermination[124].Highlyleakageofcellularsolutesduetoinitialimbibitionindicatescellularmembranesdamagecausedbyrehydration.Inaddition,dryingandrapidseeddehydrationprocessesinfluenceDNAintegrity[125].Seedshavedevelopedanumberofrepairmechanismsduringseedgermination,includingtherepairofmembranes,aswellasproteinsandDNA[126].Understressconditions,phytohormonesplayacrucialroleviaresponsiveproteinmediatedstress.ItwasfoundC1-(cysteinerichproteinfamily)domaincontainingproteinsthatplayapartinplanthormone-mediatedstressresponses[127].Inaddition,72responsiveproteinsmediatedstressareidentifiedinArabidopsisthatcontainedallthreeuniquesignaturedomains.ManyhydrolyticenzymesbiosynthesisandactivityareinfluencedbyGA3inwheatandbarley.CatalaseandascorbateperoxidaseactivityshowedasignificantimprovementinwheatSA-andGA-primedwheatseedscomparedtotheunprimed[128,129].Advertisement5.Primingstrategytoimproveseedgerminationunderstressfulornon-stressfulconditionsUndervariousconditions,thepotentialofseedsforrapiduniformemergenceanddevelopmentundervariousconditionsisdeterminedmostlybyseedvigortrait[130].Recentstrategiesforimprovementseedqualityinvolvedclassicalgenetic,molecularbiologyandinvigorationtreatmentsknownasprimingtreatments.Seedprimingwasaimedprimarilytocontrolseedhydrationbyloweringexternalwaterpotential,orshorteningthehydrationperiod,becauseofmostseedsarepartiallyhydratedafterprimingprocessandreachapre-germinatestagewithoutradicleprotrusion[131].Itwasreportedthatprimedseedsshowedimprovedgerminationrateanduniformityunderbothoptimalandadverseenvironmentsinwheat[132].Thecellularmechanismofprimingasitrelatestoimprovedstresstoleranceingerminatingseedsisstillrequiredmorestudy.Currentlyseedprimingtechniquesincludeosmopriming(soakingseedsinosmoticsolutionsasPEGorinsaltsolutions),hydropriming(soakingseedsinpredeterminedamountsofdistilledwaterorlimitingimbibitionperiods),andhormonepriming(seedaretreatedwithplantgrowthregulators)whicharemorecommonlystudiedinlaboratoryconditions,andthermopriming(itisaphysicaltreatmentachievedbypre-sowingofseedsatdifferenttemperaturethatimprovegerminationvigorunderadverseenvironmentalconditions)andmatricpriming(mixingseedswithorganicorinorganicsolidmaterialsandwaterindefiniteproportionsandinsomecasesaddingchemicalorbiologicalagents)[130,133].Hydroprimingandosmoprimingwithlarge-sizedprimingmoleculescannotpermeatecellwall/membranesowaterinfluxwouldbetheonlyexternalfactoraffectingpriming.Thedeterminationofsuitableprimingtechniqueisdependentmainlyonplantspecies,seedmorphologyandphysiology.Ontheotherhand,saltsandhormoneprimingaffectnotonlytheseedhydrationbutalsoothergermination-relatedprocessesduetoabsorptionofexogenousions/hormones,consequentlyconfusingtheeffectsofimbibitionversusthatofions/hormones.Improvementgerminationperformanceofprimedseedsmaybeconsideredaresultofadvancedmetabolismprocesses[134]includingenhancementeachoftheefficiencyofrespiration[135]andantioxidantactivity[136],initiationofrepairingprocesses[137]andalterationphytohormonalbalance[138].Also,improvementofgerminationperformancemaybelinkedtohigherexpressionsofgenesandproteinsinvolvedinwatertransport,cellwallmodification,cytoskeletalorganization,andcelldivisionandincreasesinproteinsynthesispotential,post-translationalprocessingcapacity,andtargetedproteolysishavebeenlinkedtotheadvancedgerminationofprimedseeds[139].Seedgerminationprocessisregulatedbyanetworkoftranscriptionfactorsthathavebothconfusedandseparatefunctions.Inordertomaintainorbreaktheperiodofarrestedgerminationandtocompletegerminationunderstressconditions,differentmetabolicpathwaysincludingphytohormonesbiosynthesisandsignaltransductionpathways,chromatinmodifications,andmicroRNAposttranscriptionalregulation,areinvolved[140].Manyeffectsonmetabolicprocesses,germinationperformanceandseedlingestablishmentduetoseedprimingwithH2O2wereobservedalthoughseedsoakingfollowedbydehydrationhaveanimportantroleincontrollinggeneexpressionandbiosynthesisofproteins[141].Seedprimingwithauxin,cytokinin,GA,andethylene(ET)resultedinimprovementofgerminationofpigeonpeaseedsunderbothcontrolandCd-stressconditions[142].ABApretreatedseedsshowedareductioningerminationthatmaybeattributedtometabolicdeviation,limitingtheavailableenergyandchangesinmetabolomicsormaybeattributedtomodulatetheendogenousABAlevel[143].Oncontrary,GA3seedtreatmenthasnotaffectseedgerminationsubstantially.ItisdocumentedthatGA3haveastimulatoryeffectsongerminationandassociatedenzymes[144].Also,auxinnamelyIAAisdocumentedtoregulateseeddormancyandplantshadeavoidancesyndromethatadverselyaffectsseedlingdevelopmentandcropyield[145].CytokininpretreatmentmayactasauxinsinpromotingseedgerminationbyantagonizingtheinhibitoryeffectofABAongerminationprocess.However,itwasfoundthatcytokininantagonizetheinhibitoryeffectofABAonpost-germinatinggrowthofArabidopsisthroughthestimulationofABI5proteindegradation[146].RecentlypublisheddatasupporttheexistenceofinteractionsbetweenROSandphytohormonesignalingnetworksthatmodulategeneexpressionandcellularredoxstatus[147].InteractionbetweenphytohormonesandH2O2canbeantagonisticorsynergistic.SignalingprocessestriggerinteractionsarenotdevelopedonlybetweenparticularphytohormonesbutalsobetweenphytohormonesandothersignalingmoleculessuchasNO[148],H2S[149],·OH[150]andH2O2[151],whichisbelievedtoplayacentralroleinsignalingprocessesduringplantdevelopmentandstressresponses[152].GAtreatmentenhancedROSproductionnamelysuperoxideandH2O2inradishplants[153]andArabidopsis[154].Ontheotherhand,exogenousapplicationofH2O2doesnotinfluenceABAbiosynthesisandsignalingbutithasamorepronouncedeffectonGAsignaling,resultinginthemodulationofhormonalbalanceandinsubsequentgerminationinitiation[154].ItwasshowedthatH2O2diminishedtheinhibitoryeffectsofABAonendospermdamage.Mülleret al.[155]showedthatH2O2abolishesinhibitoryeffectsofABAonendospermrupture.AssuggestedpreviouslybyLariguetet al.[154],H2O2regulatestheexpressionofgeneencodingenzymehydrolyzingthetestaandendosperm,whichfacilitateArabidopsisgerminationbyreleasingtheembryofromthecontroloftheseedenvelope.Advertisement6.TherespiratoryreactivationduringseedgerminationTheinitialliberationofseedstoredfoodatthebeginningofgerminationismainlybyanaerobicrespiration.Anaerobicrespirationiscatalyzedbytheactivityofenzymeswhicharenotrequiredaerobicconditionssuchasdehydrogenases[156].Dehydrogenasefacilitatingthetransportofelectronsfromsubstratestooxygenthroughelectrontransportchainusingnicotinamideadeninedinucleotide(NAD+),nicotinamideadeninedinucleotidephosphate(NADP+)orriboflavinascofactor[157].Activitiesofdehydrogenaseshavebeenshowntoinvolvetheactivitiesofalcoholdehydrogenase,lactatedehydrogenaseandsuccinatedehydrogenase[158]whichmediatedtheconversionofstoragelipidandcarbohydratesthroughtheanaerobicrespiration.Succinatedehyrogenase,acomplexenzymetightlyboundtotheinnermitochondrialmembraneoxidizessuccinatetofumarate[159].LactatedehydrogenasecatalyzesthereversibleoxidationoflactatetopyruvateusingNAD+asaco-enzyme.Anaerobicrespirationwasrecordedtotakeplaceduringrestingstagesofseedsandtheinitialstagesofseedgermination[160].Itwasshowedthatthereactivityofdehydrogenasescoveredthefirst3daysofcowpeaseedgerminations[161].Theincreaseinrespiratoryrateingerminatingseedsisassociatedwiththeincreaseinglycolyticactivity.TheintermediatesofglycolysisaretransferredtotheOPPPpathwaywhichfeedsitsproductsbackintoglycolysis,sotheactivityofthispathwayisalsoimportantindeterminingthefluxthroughglycolysis[162].Duringgermination,seedsusesugarsandothermoleculesasasubstrateforrespiration.α-amylaseandβ-amylaseareinvolvedindegradationofendospermstarch.Starchhydrolysisintoglucoseiscatalyzedbyactionofα-andβ-amylases,debranchingenzymeandα-glucosidases(maltase)[163].So,importanceofamylasesisrelatedtotheirabilitytoprovidegrowingembryowithrespiratorysubstratesforproducingenergyandcarbonsourceuntiltheestablishedseedlingcanphotosynthesize.Inaddition,embryogrowthfromquiescentstagetoactivephasedependingmainlyontheutilizationofstoredATPandstoragelipidbreakdownproducts[164].Seedgerminationrepresentsagoodperiodformitochondriadevelopmentstudy.Resultsobtainedfromprevioustranscriptomestudiesrecordedasubstantialincreaseinmitochondrialtranscriptsencodingproteinsandproteincontentaccompaniedwithchangesintheirfunctionsduringearly3 hofseedimbibitions[165].Duringthefirst48 hofseedimbibitions,56differentiallyexpressedproteinsweredetectedwhichincludetheoutermembranechannelTOM40andtheinnermembraneTIM17/22/23families,comparedtodryseed.Theinterpretationofsuggestionthatimportpathwaycapacityisabsolutelydependentonthepresenceofoxygen(aerobicrespiration)isrelatedtothesignificantdecreaseincapacityofthegeneralimportpathwayinmitochondriaunderanaerobicconditions,comparedtounderaerobicconditions.Insupportingforthissuggestion,threeproteinsfromtheTIM17/22/23familywerefoundtobe6–14foldsup-regulatedunderanaerobicconditions[166]andadeclineinproteinsinvolvingimportapparatuswasdetectedinthematuremitochondriathatmightbesuggestedthattheaccumulationoftheseimportproteinsinthedryseedcouldoperatefunctionsafter2 himbibition,andthenserveasdonorsofTCAcycleandelectrontransportchaincomponents[167].Advertisement7.TheroleofglyoxylatecycleinoilseedgerminationGlyoxylatecyclehasbeenknowntoplayacrucialroleinlipiddegradationinoilseeds,whereasstoredlipidisconvertedintoglucosethemainrespiratorysubstrateduringgerminationandhenceseedlingestablishment[168].Seedimbibitiontriggershighlyincreaseinoxygenconsumptionwhichreflectstheenhancementofoxidationofproducedcarbohydratesfromtheglyoxylatecycle[169].Alongsidetoglyoxylatecycle,theOPPPoperateswhereanumberofenzymesandintermediatesparticipatethetwopathways[170].ItfunctionstoprovidethecellwithNADPHforbiosyntheticreactionsandappearstobeimportantintheregulationofgermination[171].Theactionofthetwoglyoxylatecycleenzymesisocitratelyase(ICL)andmalatesynthase(MS)thatbypassthedecarboxylationstepsoftheTCAcycleareessentialinoilseedgermination.Whereas,twomolesofacetyl-CoAareintroducedwitheachturnofthecycle,resultinginthesynthesisofonemoleofthefour-carboncompoundsuccinatethataretransportedfromtheglyoxysomeintothemitochondrionandconvertedintomalateviaTCAcycle.Thismalateisthenexportedtocytosolinexchangeforsuccinateandisconvertedtooxalacetate.PEP-CKcatalyzestheconversionofoxaloacetatetophosphoenolpyruvateandthisfuelsthesynthesisofsolublecarbohydratesnecessarytogermination[169].Advertisement8.ConclusionUnderstressfulconditions,oxidativedamagetomRNAresultsintheinhibitionofproteinsynthesisandinproteindegradationwhichcauseddisturbanceinproteinfunctionsduetoenzymaticandbindingpropertiesmodification.Consequently;seedgerminationmaydelayorsuppress.Theprimingtechniquesimprovestressacclimationmechanismsduringgerminationbutthecellularmechanismofprimingisstillrequiresmorestudying.Inresponsetoabioticstresses,activityofacidphosphatasesincreasedtomatchadefinitelevelofinorganicphosphatewhichcanbeco-transportedwithH+downprotonmotiveforcegradient.Thesignalinginteractionsamongmultiplephytohormonesarerathercommonincontrollingvariousgrowthanddevelopmentalprocesses.Hormonalsignalingcoordinationmayberegulatedthroughcontrollingbiosynthesisofcertainphytohormone,bymodifyingtheavailablepoolofhormonemoleculesorbyelaborateregulationofthesignalingprocess.However;seedpretreatmentwitheachofGAs,auxinsorcytokininpromoteseedgerminationnotonlythroughstimulationofhydrolyzingenzymesbutalsobyantagonizingtheinhibitoryeffectofABAongerminationprocess.Phytohormonesignalcrosstalkwillpresentvaluablenewavenuesforgeneticimprovementofcropplantsneededtomeetthefuturefoodproductiontargetsinthefaceofglobalclimatechange.Surprising;seedprimingwithH2O2resultedinimprovementgerminationprocessandseedlingestablishment.ThismayberesultedfromitseffectonGAsignalingandmodulationofhormonalbalancethatpromoteinitiationofseedgermination.Inaddition;H2O2diminishedtheinhibitoryeffectsofABAonendospermdamagethroughexpressionofgeneencodingenzymehydrolyzingthetestaandendospermwiththereleasingofembryo.References1.HasanuzzamanM,NaharK,AlamMM,RoychowdhuryR,FujitaM. Physiological,biochemical,andmolecularmechanismsofheatstresstoleranceinplants.InternationalJournalofMolecularSciences.2013;14:9643-96842.BorgherettiF,FerreiraAG. Germinaçãodobásicoaoaplicado.PortoAlegre:Artmed;2000.222p3.ZienkiewiczZ,ZienkiewiczAK,RejonJD,AlcheJD,CastroAJ,Rodringuez-GarciaMI. Oliveseedproteinbodiesstoredegradingenzymesinvolvedinmobilizationofoilbodies.JournalofExperimentalBotany.2014;65:103-115.DOI:10.1093/jxb/ert3554.GonçalvesJFC,et al.AspectosfisiológicosbioquímicosdeplantasdaAmazônia.ProjetoJacarandaFaseII:PesquisasFlorestaisnaAmazôniaCentral.Manaus:INPA;2003.p.89-1015.BewleyJD,BlackM. PhysiologyofDevelopmentandGermination.2nded.New York:PlenumPress;1994445p6.BewleyJD. Seedgerminationanddormancy.ThePlantCell.1997;9:1055-10667.BewleyJD,BradfordK,HilhorstH,NonogakiH. Seeds:PhysiologyofDevelopment,GerminationandDormancy.3rded.New York:Springer;20138.JacobsenJV,ChandlerPM. Gibberellinandabscisicacidingerminatingcereals.In:D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