Uptake system of silicon in different plant species Mitani and Ma.docx
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Uptake system of silicon in different plant species Mitani and Ma.docx
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UptakesystemofsiliconindifferentplantspeciesMitaniandMa
Abstract
Theaccumulation(累积)ofsilicon(Si)intheshoots(嫩枝)variesconsiderablyamongplantspecies,butthemechanismresponsibleforthisvariationispoorlyunderstood.TheuptakesystemofSiwasinvestigatedintermsoftheradialtransportfromtheexternalsolutiontotherootcortical(皮层的)cellsandthereleaseofSifromthecortical(皮层的)cellstothexylem(木质部)inrice,cucumber,andtomato,whichdiffergreatlyinshootSiconcentration.Symplasmicsolutionsoftheroottipswereextractedbycentrifugation.TheconcentrationsofSiintheroot-cellsymplast(共质体)inallspecieswerehigherthanthatintheexternalsolution,althoughtheconcentrationinricewas3-and5-foldhigherthanthatincucumberandtomato,respectively.AkineticstudyshowedthattheradialtransportofSiwasmediatedbyatransporterwithaKmvalueof0.15mMinallspecies,butwithdifferentVmaxvaluesintheorderofrice>cucumber>tomato.Inthepresenceofthemetabolic(变化的)inhibitor(抑制剂)2,4-dinitrophenol,andatlowtemperature,theSiconcentrationintheroot-cellsymplast(共质体)decreasedtoalevelsimilartothatoftheapoplasmicsolution.Theseresultssuggestthatbothtransporter-mediatedtransportandpassivediffusionofSiareinvolvedintheradialtransportofSiandthatthetransporter-mediatedtransportisanenergy-dependentprocess.TheSiconcentrationofxylem(木质部)sapinricewas20-and100-foldhigherthanthatincucumberandtomato,respectively.Incontrasttorice,theSiconcentrationinthexylem(木质部)sapwaslowerthanthatintheexternalsolutionincucumberandtomato.Akinetic(运动的)studyshowedthatxylem(木质部)loadingofSiwasalsomediatedbyakindoftransporterinrice,butbypassivediffusionincucumberandtomato.Theseresultsindicatethatahigherdensityoftransporterforradialtransportandthepresenceofatransporterforxylem(木质部)loadingareresponsibleforthehighSiaccumulation(累积)inrice.
Keywords:
Passivediffusion,radialtransport,silicon,transporter,uptake,xylem(木质部)loading.
Introduction
Silicon(Si)isabeneficialelementforplantgrowth.Siliconhelpsplantstoovercomemultiple(多重的)stressesincludingbioticandabioticstresses(forarecentreview,seeMa,2004).Forexample,Siplaysanimportantroleinincreasingtheresistanceofplantstopathogens(病原体)suchasblastonrice(Datnoffetal.,1997)andpowderymildewoncucumber(MiyakeandTakahashi,1982a,b).Siliconiseffectiveinpreventinglodginginricebyincreasingthethicknessoftheculmwallandthesizeofthevascularbundles(Shimoyama,1958),therebyenhancingthestrengthofthestems(茎).Siliconalsoalleviatestheeffectsofotherabioticstressesincludingsaltstress,metaltoxicity,droughtstress,radiationdamage,nutrientimbalance,hightemperature,andfreezing(Epstein,1999;MaandTahakashi,2002;Ma,2004).ThesebeneficialeffectsaremostlyexpressedthroughSideposition(沉积物)intheleaves,stems(茎),andhulls,althoughothermechanismshavealsobeenproposed(Ma,2004).Therefore,theSieffectischaracterizedbyalargereffectassociatedwithagreaterSiaccumulation(累积)intheshoots(嫩枝).
However,Siaccumulation(累积)intheshoots(嫩枝)variesconsiderablyamongplantspecies,rangingfrom0.1%to10%Siinthedryweight(MaandTakahashi,2002).TakahashiandcoworkersmadeanextensivesurveyontheSiconcentrationsofnearly500plantspeciesfromBryophytatoAngiospermae,grownundersimilarsoilconditions(forthesummary,seeMaandTakahashi,2002).TheresultsshowedthatthereisacharacteristicdistributionofSiaccumulation(累积)intheplantkingdom.Inhigherplants,onlyplantsinGramineaeandCyperaceashowhighSiaccumulation(累积).PlantsinCucurbitales,Urticales,andCommelinaceaeshowintermediateSiaccumulation(累积),whereasmostotherplantsspeciesshowlowSiaccumulation(累积).ThedifferenceinSiaccumulation(累积)hasbeenattributedtotheabilityoftherootstotakeupSi(Takahashietal.,1990).Siliconistakenupintheformofanunchargedmolecule,silicicacid(TakahashiandHino,1978).ThreedifferentmodesofSiuptakehavebeenproposedforplantshavingdifferentdegreesofSiaccumulation(累积),thatis,active,passive,andrejectiveuptake(Takahashietal.,1990).PlantswithanactivemodeofuptaketakeupSifasterthanwater,resultinginadepletionofSiintheuptakesolution.PlantswithapassivemodeofuptaketakeupSiataratethatissimilartotheuptakerateofwater;thus,nosignificantchangesintheconcentrationofSiintheuptakesolutionareobserved.Bycontrast,plantswitharejectivemodeofuptaketendtoexcludeSi,whichisdemonstratedbytheincreasingconcentrationofSiintheuptakesolution.However,themechanismsinvolvedinthedifferentuptakemodesarenotunderstood.Theobjectiveofthisstudywasto0examinetheuptakesystems(茎)ofSiinrice,cucumber,andtomato,whichrepresenthigh,intermediate,andlowSiaccumulation(累积),respectively.各自地
Materialsandmethods
Plantmaterialsandgrowthcondition
Seedsofrice(OryzasativaL.cv.Oochikara)weresoakedinwaterovernightat258Cinthedark.Theseedswerethentransferredtoanetfloatedon0.5mMCaCl2solutioninaplasticcontainer.Onday7,theseedlingsweretransferredtoa3.5lplasticpotcontaininghalf-strengthKimuraBsolution.Thecompositionofthisnutrientsolutionisreportedinthepreviouspaper(Maetal.,2001).ThepHofthissolutionwas5.6andthenutrientsolutionwasrenewedevery2d.
Seedsofcucumber(CucumissativusL.cv.Suyo)andtomato(LycopersiconesculentumMill.cv.Oogatahukujyu)weresoakedinwaterforabout1handthenplacedinarefrigeratorat48Covernight.Theseedsweretransferredtoanetfloatedon0.5mMCaCl2solutioninaplasticcontainer.Onday5,theseedlingsweretransferredtoa3.5lplasticpotcontainingone-fifthHoagland’ssolution.Thenutrientsolutioncontainedthemacronutrients1.0mMKNO3,1mMCa(NO3)2,1.4mMMgSO4,and0.2mMKH2PO4,andthemicro-nutrients20μmFe-EDTA,3μmH3BO3,1.0μm(NH4)6Mo7O24,0.5μmMnCl2,0.4μmZnSO4,and0.2μmCuSO4.ThepHofthissolutionwasadjustedto6.0andthenutrientsolutionwasrenewedevery3d.Allexperimentswereconductedatatemperatureof258Cinacontrolledgrowthchamberwithnaturallights.
Siuptakeexperiment
Short-termSiuptakebyrice,cucumber,andtomatowasexaminedwiththreereplicates.Twoseedlings(30-d-old)wereplacedina180mlblackbottlecontaininghalf-strengthKimuraBsolution(forrice)orone-fifthHoagland’ssolution(forcucumberandtomato)with0.5mMSiassilicicacid.Silicicacidwaspreparedbypassingpotassiumsilicatethroughcation-exchangeresin(AmberliteIR-120B,H+form,Organo,Tokyo)(Maetal.,2001).
AtdifferenttimepointsindicatedinFig.1,0.5mlaliquotsofuptakesolutionweretakenforthedeterminationofSiconcentration.Transpiration(waterloss)wasalsorecordedateachsamplingtime.Attheendoftheexperiment,rootsandshoots(嫩枝)wereharvestedseparatelyandtheirfreshanddryweightswererecorded.
Collectionofapoplasmicandsymplasmicsolutions
ApoplasmicandsymplasmicsolutionswereextractedbycentrifugationwithslightmodificationsofthemethodsofYuetal.(1999).Foratime-courseexperiment,theroottips(0–1.5cm)wereexcisedfromseedlingsofrice(5-d-old)orcucumberandtomato(6-d-old)attimepointsindicatedinFig.2afterexposureto0.5mMSisolutionat25℃.Foreachsample,40rootsforriceand80rootseachforcucumberandtomatowereused.Thecutendswerewashedindistilledwaterquicklyandblotteddry.Thetipswereplacedina0.22μmfilterunit(ULTRAFREER-MC,Millipore)withthecutendsfacingdownandcentrifugedat2000gfor15minat48Ctoobtaintheapoplasticsolution.Aftercentrifugation,rootsegmentswerefrozenat-80℃for2handthenthawedatroomtemperature.Thesymplasmicsolutionwaspreparedfromthefrozen–thawedtissuesbycentrifugationat2000gfor15minat4℃.TheSiconcentrationsintheapoplasmicandsymplasmicsolutionsweredeterminedimmediatelyasdescribedbelow.Inapreliminaryexperiment,itwasconfirmedthatthefreeze–thawprocessdidnotaffecttheSiconcentration.
Tocheckthepurityoftheapoplasmicsolution,theactivityofmalicdehydrogenase(MDH)intheapoplasmicandsymplasmicsolutionswasdeterminedaccordingtoBergmeyerandBernt(1974).TheactivityofMDHintheapoplasticsolutionwasbelow1/20–1/40thatofthesymplasmicsolution.
Inakinetic(运动的)study,seedlingsofrice(5-d-old),cucumberandtomato(6-d-old)wereculturedinnutrientsolutionscontainingvariousSiconcentrations.Afteran8hculture,theapoplasmicandsymplasmicsolutionswereextractedasdescribedabove.Allexperimentswereconductedwiththreereplicates.
Inhibitor(抑制剂)andlow-temperatureexperiments
Toinvestigatetheeffectofmetabolic(变化的)inhibitor(抑制剂)sandlowtemperatureonSiconcentrationintheapoplasmicandsymplasmicsolutions,seedlingsofrice(5-d-old),cucumberandtomato(6-d-old)wereexposedtoanutrientsolutioncontaining0.5mMsilicicacidinthepresenceorabsenceof0.05mMHgCl2or0.5mM2,4-dinitrophenol(DNP).2,4-Dinitrophenolwasdissolvedinethanolbeforebeingaddedtothenutrientsolution,withafinalethanolconcentrationof0.3%(v/v).PreliminaryexperimentsshowedthatthisconcentrationofethanolhadnoeffectonSiuptake.Thetreatmentperiodwas6h.
Forthelow-temperatureexperiment,seedlingswereexposedtoanutrientsolutioncontaining0.5mMsilicicacidthathadbeenprecooledat48C.After6h,Siconcentrationintheapoplasmicandsymplasmicsolutionswasdet
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