Bt蛋白对农田中的钻柱螟虫具有毒杀作用，因此Bt转基因抗虫玉米防治玉米螟的效果十分显著，二十多年来为广大种植者带来了巨大收益。最近，科学家却发现了一个奇怪的现象：类胡萝卜素会显著降低Bt蛋白对玉米螟的作用，这是怎么回事？
4Bt转基因玉米能够组成型表达Bt蛋白Cry1Ac，HC玉米能够在种子胚乳中特异表达玉米psy1和菠萝泛菌crtI基因，从而产生较高含量的类胡萝卜素。研究人员通过杂交聚合方法得到转基因玉米4BtxHC，期望该玉米既具有高含量的类胡萝卜素，同时具有抗虫性。然而，虽然4BtxHC中叶片和籽粒中的Bt蛋白含量与亲本4Bt差不多（叶片约47μg/g鲜重，籽粒约35μg/g鲜重），但是致死率、化蛹成功率等杀虫活性指标却降低了（最大达到20%）。包含4Bt玉米籽粒的人工饲料添加类胡萝卜素的体外昆虫生测试验也得到了同样的结果。为了研究这一现象内在的生化机制，研究人员测定了昆虫体内三类具解毒功能的酶活性，分别是过氧化氢酶CAT、超氧化物歧化酶SOD和谷胱甘肽-S-转移酶GST。结果显示，在无类胡萝卜素的情况下，Cry1Ac蛋白的表达可以显著增加SOD和CAT的酶活性；而在类胡萝卜素存在的条件下，Cry1Ac蛋白的表达却可以显著降低CAT的酶活性。因此，在昆虫体内对Cry1Ac蛋白进行解毒的过程中，类胡萝卜素可能充当着抗氧化剂的作用，清除ROS活性氧的产生，从而增强了欧洲玉米螟对Cry1Ac蛋白的抵抗力。
该研究揭示了类胡萝卜素和Bt蛋白在昆虫体内的相互作用关系，为Bt转基因作物的推广应用带来了新的启示。
PLoSONE,10July
CarotenoidsmoderatetheeffectivenessofaBtgeneagainsttheEuropeancornborer,Ostrinianubilalis
Author
DanielaZanga,GeorginaSanahuja,MatildeEizaguirre*,RamonAlbajes,PaulChristou,TeresaCapell,PaulFraser,ChrisGerrisch,CarmenLópez
DepartmentofCropandForestSciences,UniversityofLleida-AgrotecnioCenter,Spain
Abstract
Weassessedtheeffectivenessofabiofortifiedmaizeline(4BtxHC)whichaccumulateshighlevelsofantioxidantcarotenoidsthatalsoexpressedtheinsecticidalCry1AcBacillusthuringiensis(Bt)geneagainsttheEuropeancornborerOstrinianubilalis.Thislinehadbeenpreviouslyengineeredtoaccumulatecarotenoidsspecificallyintheseedendosperm,whereastheBtgenewasexpressedconstitutively.TheconcentrationsofBttoxin(Cry1Ac)intheleavesofthe4Btand4BtxHClineswerenotsignificantlydifferentat47±6μg/goffreshweight(FW);neitherweretheyinthekernelsofbothlines(35±3μg/gFW).ThekernelsandleavesweretoxictothelarvaeofO.nubilalis.However,theinsecticidalactivitywassubstantiallylower(ca.20%)thanthatoflinesthatexpressedonlyBtinspitethatthetwolinesshowedaquantityoftoxinnotsignificantlydifferentinkernelsorinleaves.AlthoughthereducedeffectivenessofCry1Acinkernelsmaynotbeentirelysurprising,theobservationofthesamephenomenoninvegetativetissueswasunexpected.Whensemi-artificialdietscontainingkernelsfrom4Btsupplementedwithdifferentlevelsofβ-carotenewereusedininsectbioassays,theβ-carotenemoderatedtheeffectivenessoftheBtsimilarlytotheplantmaterialwithcarotenoidenrichment.ToelucidatethebiochemicalbasisofthereducedeffectivenessofBttoxininthecarotenoid-enrichedplants,wemeasuredtheactivityofthreeenzymesknowntobeimplicatedinthedetoxificationdefence,namely,catalase,superoxidedismutaseandglutathioneS-transferase.WhereasCry1AcexpressionsignificantlyincreasedSODandCATenzymaticactivityintheabsenceofcarotenoids,carotenoids,eitherin4BtxHCorinartificialdietsenrichedwithβ-carotene,significantlyloweredCATactivity.CarotenoidscanthereforemoderatethesusceptibilityofthemaizeborerO.nubilalistoCry1Ac,andwehypothesizethattheirroleasantioxidantscouldexplainthisphenomenonviatheirscavengingofreactiveoxygenspeciesproducedduringCry1Acdetoxificationinthelarvae.TheinvolvementofthismechanisminthedecreasedmortalitycausedbyCry1Acwhencarotenoidsarepresentinthedietisdiscussed.
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