Virus-Resistant Transgenic Horticultural Crops: Safety Issues and Lessons from Risk Assessment Studies - Transgenic Horticultural Crops: Challenges and Opportunities - page 267

that.the.parent.viruses.heretofore.had.not.come.into.contact.with..Transencapsidation.
has. been. shown. experimentally. 11 . However,. many. of. the. concerns. with. respect.
to. transencapsidation. have. been. alleviated. by. our. current. understanding. of. RNA.
silencing..This.is.true.because.when.RNA.silencing.is.active.(as.is.expected.if.the.
transgenic.plants.exhibit.viral.resistance),.the.expression.of.viral-derived.proteins.is.
regulated.posttranscriptionally,.with.little.to.no.detectable.capsid.protein.produced..
In. the. case. of. untranslatable. coat. protein. transgenes,. this. concern. is. alleviated.
altogether..Also,.since.the.interactions.between.viral.proteins.or.between.capsid.pro-
teins.and.viral.genomes.is.often.very.speciic, 31 .it.seems.unlikely.that.interactions.
necessary. for. a. viral.coat.protein.to. aid. in. the.replication.or.movement. of. its.cor-
responding.viral.genome.would.take.place.in.coordination.with.genomic.material.or.
proteins.from.a.heterologous.virus..Finally,.it.is.unclear.how.these.risks.are.substan-
tially.different.from.the.risks.already.present.when.a.host.plant.is.infected.with.two.
distinct. viruses—which. has. been.shown.to. lead.to. transencapsidation 11 —and. how.
transencapsidation.would.be.any.more.likely.to.occur.in.the.transgenic.case.than.in.
the.non-transgenic.multiple.infection.scenario. 2,11,12
Even. if. transencapsidation. did. occur,. it. is. questionable. how. it. could. result. in.
viruses. with. permanently. altered. properties,. since. it. is. not. conceivable. how. the.
movement.of.a.transencapsidated.virus.to.a.new.host.would.be.anything.other.than.
a.so-called.dead.end,.since.(due.to.the.lack.of.a.coat.protein.from.the.heterologous.
virus. in. the. new. host). all. new. viruses. produced. in. the. new. host. plant. would. be.
encapsidated.within.their.own.capsid.protein. 2,11-13 .An.exception.would.be.if.the.
new.host.were.transgenic.and.expressing.the.coat.protein.gene.of.a.closely.related.
heterologous. virus.. In. any. event,. if. problems. with. transencapsidation. did. arise,.
the.problem.could.be.eliminated.by.ending.the.cultivation.of.the.transgenic.crop.
in.question. 25
r ecombInatIon
Another.potential.risk.of.virus-resistant.transgenic.crop.plants.expressing.viral.genes.
is. that. of. recombination. between. viral-derived. transgene. mRNA. and. the. genomic.
RNA. of. an. infecting. virus. 2,11-13 . It. is. thought.that.this.type.of.recombination.could.
potentially.give.rise.to.a.new.virus.strain.or.new.virus.species.possessing.characteris-
tics.different.from.those.of.the.parent.viruses..Recombination.may.involve,.for.example,.
an. incoming. virus. containing. a. viral. coat. protein. gene. whose. product. is. defective in.
its.ability.to.be.vectored,.and.transcripts.of.a.homologous.viral.transgene.sequence.
possessing.a.functional.copy.of.the.coat.protein.gene..Recombination.between.these.
sequences.may.then.restore.vector.transmissibility..Unlike.the.risk.of.transencapsida-
tion,.recombination.cannot.be.as.easily.dismissed.as.an.evolutionary.dead.end,.since.
viral.progeny.identical.to.the.recombined.strain.could.be.produced.in.a.new.host. 2,12,13 .
Recombination. between. transcripts. of. a. viral. gene. construct. in. a. transgenic. plant.
and.an.incoming.virus.has.been.shown.by.several.groups. 32 .If.resistance.were.effec-
tive.through.RNA.silencing,.the.occurrence.of.recombination.is.less.likely.unless.the.
incoming.virus.were.a.related,.but.divergent,.isolate.of.the.virus.from.which.the.trans-
gene.is.derived..In.that.case,.it.is.conceivable.that.the.incoming.virus.would.replicate.
to.the.same.extent.as.in.a.susceptible.plant,.providing.opportunities.for.recombination..