Chemistry Reference
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sugarbeets(James2008).Approximately80%ofthetotalareadevotedtoGMcropshasbeensofar
plantedwithHRcrops,virtuallyallbeingglyphosate-resistant(GR)crops.Thus,asinglegenetic
trait—glyphosate[
N
-(phosphonomethyl)glycine]resistance—accountsformostofthesuccessof
transgeniccropsatthistime.ThewidespreadadoptionofGRcropsandglyphosatehashadsignii-
cant economic effects in agriculture,from the replacement of previous herbicide markets to cost
savingsforfarmersinweedmanagement,andhasgenerallyreducedtheadverseenvironmentaland
healthimpactsofweedmanagement.However,theoveruseofthissingleweedmanagementtech-
nologyisjeopardizingthissafe,highlyeffective,andeconomicaltoolduetotheemergenceofnew
weedspeciesthatareonlypoorlycontrolledbyglyphosate(Owen2008)andtheevolutionofGR
weeds.ThenumberandrangeofGRweedshasbeenincreasingintheU.S.sincethecommercial-
izationofRoundupReady
®
(RR)crops(Heap2009).Thus,beneitsofHRcropsaremultifaceted,
aredificulttoquantify,andmaybethreatenedbytheevolutionofweedsthatareresistanttoher-
bicidesusedintheprocessoftheircultivation.ThesustainableuseofHRcrops,therefore,requires
strategiestodelaytheevolutionofHRweeds.Thereisanincreasingneedtoidentifyproduction
practicestoreducetheriskofweedresistancetoglyphosateandotherherbicidestomaintainthe
beneitsofGRandotherherbicide-tolerantcrops.Proposedpracticesincludetheincorporationof
a residual herbicide into the GR weed management program and the rotation of herbicides from
onegrowingseasontothenext.Usingaresidualherbicidewithglyphosatehelpsmitigateorreduce
theriskofresistance,becauseresidualherbicidescontrolweedsthatescapeglyphosatecontroland
preventthemfromsettingseed.Rotatingbetweenherbicideswithdifferentmodesofactionhelps
reducetheriskofweedresistancetoherbicidesbyreducingtheselectionpressureforresistanceto
anyoneherbicide(Hurleyetal.2009).
One of the leastunderstood areasof the environmental riskassessment of GM crops istheir
impactonsoil-andplant-associatedmicrobialcommunities.Canmicroorganismstakeupgenetic
materialfromplantsandintegrateitintotheirgenomes?Couldthisbeawayforgenesfromgeneti-
callyengineeredplantstospreadintheenvironment?Safetyresearchhasbeenlookingintothese
questions for almost 20 years (
www.gmo-compass.org
)
, as minor alterations in the diversity of
themicrobialsoilcommunitycouldaffectsoilhealthandecosystemfunctioning.Theimpactthat
plantvarietymayhaveonthedynamicsoftherhizospheremicrobialpopulationand,inturn,plant
growthandhealthandecosystemsustainabilityrequiresspecialattention.
Themovementofgeneticmaterialbetweenunrelatedspeciesiscalledhorizontalgenetransfer,
aphenomenonknowntooccurinbacteria.Onewaybywhichthiscanhappenisthedirectincor-
porationoffreeDNAbybacterialcells.Thisdirectformofgenetransfer,forinstance,inthesoil
or in the digestive tract of animals, is the most commonly predicted scenario for the transfer of
geneticmaterialfromtransgenicplantstomicroorganisms.Sofar,horizontalgenetransfercanonly
bedemonstratedunderoptimizedlaboratoryconditions.Still,itoccursextremelyinfrequentlyand,
thus,dificulttodetect.Ongoingsafetyresearchisstudyingtransgenicplantstoseeifhorizontal
genetransfertomicroorganismsispossibleorcommonorifitwouldcauseanyconsiderablecon-
sequences(DunieldandGermida2004;Giovannettietal.2005).Oneexampleforsuchaconcern
is with antibiotic resistance genes used as marker genes in the development of transgenic plants
helpingscientiststoindoutwhichcellssuccessfullyincorporatedthegeneofinterest.Ifantibiotic-
resistant genes from GM foods are taken up by bacteria in the gut during digestion, it would be
verydificulttotreatbacterialinfections.Untilnow,accordingtonumerousscientiicstudiesinthe
last15years,thereisnorealproofthattheantibioticresistancegenesinGMplantsposeathreat.
Regardless,asaprecautionarymeasure,somestillbelievethatantibioticresistancemarkersshould
notbeusedatall,becauseifGMplantswithantibiotic-resistantgenesareplantedoveraverylarge
area,therareeventofhorizontalgenetransfercouldbecomesigniicant.Accordingly,intheEU,
GMplantswithcertainantibiotic-resistantmarkershavebeengivenonlylimitedauthorizationfor
releaseintotheenvironment.Severalcriteriaareconsideredwhenevaluatingthesegenes(EFSA
GMO panel 2009; Sparrow2010), the most important ones being the medical importance of the
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