Agriculture Reference
In-Depth Information
might.suppress.the.host.resistance.provided.by.the.host.transgene,.thereby.allowing.
the. host. to. be. infected. by. the. original. virus. toward. which. their. resistance. had.
been.targeted.
Though.each.of.these.mechanisms.could.lead.to.a.potential.breakdown.in.host.
resistance,. it. is. important. to. note. that. the. risk. of. resistance. breakdown. is. not.
unique. to. virus-resistant. transgenic. crops. nor. is. it. unique. to. transgenic. crops..
Resistance.breakdown.is.also.an.issue.associated.with.the.deployment.of.resis-
tance. genes.derived.via. traditional. breeding.. Pathogens.capable. of.overcoming.
deployed. resistance. genes. have. been. extensively. documented;
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. likewise,. the.
deployment. of. new. resistance. genes. has. been. shown. to. alter. pathogen. popula-
tions.to.overcome.that.resistance.
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.Initial.infection.by.a.virus.not.targeted.by.a.
traditional.breeding-derived.resistance.gene.has.also.been.shown.to.lead.to.a.break-
down.of.the.resistance.to.the.virus.targeted.by.the.resistance.gene
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—analogous.to.
the.breakdown.of.resistance.due.to.a.co-suppressor.of.RNA.silencing..Therefore,.
it. is. unclear. how. the. risks. of. breakdown. (with. transgenic. virus. resistance). are.
substantially.different.from.those.associated.with.the.use.of.resistance.genes.in.
traditional.breeding.
Speciicity of Engineered Virus Resistance
Another.concern.unique.to.virus-resistant.transgenic.technology.is.the.so-called.
speciicity. of. resistance. provided. by. the. transgene.. As. has. been. shown. previ-
ously,
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. the. resistance. provided. by. the. transgene. might. only. be. speciic. to. the.
virus.isolate.from.which.it.was.derived.and.a.few.closely.related.isolates..Though.
previously.not.well.understood,.the.current.understanding.of.RNA.silencing.sug-
gests.that.this.may.be.due.to.the.sequence.speciicity.of.the.resistance.mechanism.
itself..Since.the.RNA.silencing.mechanism.relies.on.the.alignment.of.cleaved.frag-
ments.of.the.target.RNA.in.the.search.for.invading.RNA.sequences,.divergence.
at.the.sequence.level.(over.∼10%).can.lead.to.an.apparent.breakdown.in.resistance.
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.
However,. this. has. not. been. shown. to. be. the. case. with. all. of. the. virus-resistant.
transgenic.plants,
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.as.a.single.transgene.is.able.to.confer.resistance.to.challenge.
from. numerous. isolates. of. the. same. virus.
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. Also,. it. is. important. to. note. that.
similar.speciicity.has.been.shown.with.resistance.genes.derived.from.traditional.
breeding,
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.and.it.is.unclear.how.the.resistance.provided.in.the.transgenic.case.is.
substantially. more. speciic. than. the. resistance. provided. by. the. resistance. genes.
derived.by.traditional.breeding.
EXAMINATION OF RISKS ASSOCIATED WITH
COMMERCIALIZED TRANSGENIC HORTICULTURAL CROPS
Once.safety.issues.associated.with.transgenic.horticultural.crops.are.identiied,.how.
are.risks.assessed?.How.does.one.examine.the.signiicance.of.risk.assessment.data?.
When.is.there.enough.evidence.to.start.drawing.conclusions.on.the.safety.of.trans-
genic. horticultural. crops?. From. risk. assessment. conclusions,. are. there. any. safety.
issues.that.need.to.be.examined.further?.Or,.are.there.any.that.can.be.put.to.rest,.so.
to.speak?.In.the.following.section,.we.address.these.questions.with.regard.to.virus-
resistant.transgenic.horticultural.crops.
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