Virus-Resistant Transgenic Horticultural Crops: Safety Issues and Lessons from Risk Assessment Studies - Transgenic Horticultural Crops: Challenges and Opportunities - page 279

in.the.mid-1990s..Safety.issues.have.been.expressed.during.their.development.and.
release,.but.most.of.these.risks.are.the.same.or.similar.to.those.posed.by.tradition-
ally.bred.plants.with.host.resistance..Since.the.commercialization.of.virus-resistant.
transgenic.squash.in.1996,.considerable.data.have.been.gathered.from.many.parts.
of.the.world.on.the.effects.on.the.environment.and.human.health.of.virus-resistant.
and. Bt .horticultural.crops..From.these.studies,.some.general.trends.have.emerged..
Commercialized.summer.squash,.tomato,.sweet.pepper,.and.papaya.expressing.viral.
coat.protein.genes,.and. Bt .sweet.corn.have.effectively.controlled.viruses.and.species.
of. Lepidoptera,.respectively.. Also,.extensive.research.has. been.published.showing.
that.these.transgenic.crops.have.little.to.no.impact.on.the.environment.and.human.
health.beyond.those.of.virus-infected.plants.in.natural.settings,.in.traditional.agricul-
ture,.and.exposed.to.conventional.pest.management.strategies. 2,14,45,62,121,122
PERSPECTIVES
I ntroductIon
Since.the.initial.discovery.of.engineered.resistance.to.viruses.via.expression.of.the.
TMV.coat.protein.gene.in.plants, 16 .pathogen-derived.resistance 10 .and.coat.protein-
mediated.resistance.have.proven.to.be.effective.tools.to.control.viruses.in.horticul-
tural.crops. 17 .As.discussed,.PRSV,.ZYMV,.CMV,.and.WMV.have.been.effectively.
controlled.in.commercial.settings.of.transgenic.papaya,.summer.squash,.tomato,.and.
sweet. pepper. with. the. use. of. coat. protein. genes.. In. addition,. in. 2007. alone,. more.
than.25.ield.trial.permits.for.resistance.against.other.viruses.were.granted.by.the.
USDA-APHIS. 123 .Target.viruses. included. Tomato spotted wilt virus ,. Beet necrotic
yellow vein virus ,. Sorghum mosaic virus ,. Grapevine fanleaf virus ,. Grapevine
leafroll-associated virus 2 ,. Grapevine leafroll-associated virus 3 ,. Citrus tristeza
virus ,. Cassava mosaic virus ,. Sugarcane mosaic virus ,. Sugarcane yellow leaf virus ,.
Papaya leaf distortion mosaic virus ,.PPV,.and.PVY. 123
In. recent. years,. new. knowledge. of. the. mechanism. behind. engineered. virus.
resistance. has. been. gained.. This. has. greatly. expanded. the. potential. for. utilizing.
the.antiviral.pathways.of.RNA.silencing.to.control.plant.viral.diseases..Though.all.
the.currently.available.commercial.virus-resistant.transgenic.crops.utilize.the.coat.
protein-mediated.resistance,.recent.publications,.patents,.and.ield.trial.data.reveal.
that. there. are. many. alternative. approaches. currently. in. the. pipeline. to. engineer.
virus.resistance.in.plants..Transgenes.incorporating.short.fused.sequences.derived.
from.different.viral.strains.have.been.engineered.successfully.to.provide.resistance.
to. several. virus. species. 124 . Other.methods.which,. unlike. the. RNA. silencing-based.
approaches,.do.seem.to.depend.on.protein.production.have.also.been.shown.to.have.
some.promise,.including.the.use.of.defective.viral.movement.protein.and.replicase.
genes. 25 .The.risks.that.these.gene.constructs.pose.are.not.yet.fully.determined.and.
are.likely.different.from.those.risks.already.discussed.with.regard.to.the.viral.coat.
protein.gene..Though.the.risk.assessment.of.the.coat.protein.technology.has,.as.dis-
cussed,.alleviated.most.of.the.concerns.of.this.technology,.questions.remain.about.
the.new.technologies.that.are.becoming.available..Do.they.alleviate.any.of.the.prob-
lems.of.the.existing.technology?.Do.they.raise.new.safety.issues.that.may.pose.their.