Virus-Resistant Transgenic Horticultural Crops: Safety Issues and Lessons from Risk Assessment Studies - Transgenic Horticultural Crops: Challenges and Opportunities - page 265

ield. studies. are. available,. thereby. using. each. released. crop. as. a. case. study. in. our.
examination.of.the.safety.issues.associated.with.transgenic.horticultural.crops..Finally,.
we. summarize. lessons. from. risk. assessment. research. and. evaluate. whether. safety.
issues.account.for.the.limited.number.of.horticultural.transgenic.crops.released.to.date.
p athoGen -d erIved r eSIStance and e nGIneered
r eSIStance to v IruSeS In p lantS
The. irst. report. on. engineered. resistance. to. viruses. in. plants. was. published. in.
1986. 16 . These. researchers. noted. that. tobacco. plants. expressing. the. coat. pro-
tein. gene. of Tobacco mosaic virus . (TMV). exhibited. delayed. infection. following.
mechanical. inoculation. with. TMV.. This. observation. conformed. to. the. concept. of.
pathogen-derived.resistance.that.had.been.postulated.earlier. 10 .Following.the.initial.
breakthrough. by. Abel. et. al., 16 . viral. coat. protein. genes. from. various. viruses. were.
introduced.into.numerous.economically.important.crop.species.in.hopes.of.achieving.
resistance. 17 .It.was.initially.believed.that.resistance.was.provided.by.the.viral.protein.
itself.via.a.mechanism.involving.excess.plant-expressed.coat.protein.that.interfered.
with.the.uncoating.step.in.viral.replication. 18 .However,.it.soon.became.apparent.that.
resistance.could.be.achieved.in.transgenic.plants.producing.low.or.undetectable.lev-
els. of. coat. protein. 19 . Further. observations. indicated. that. the. mechanism. involved.
degradation.of.the.transgene-derived.messenger.ribonucleic.acid.(mRNA).into.small.
fragments.in.a.sequence-speciic.manner. 20-23
rna S IlencInG and e nGIneered r eSIStance to v IruSeS In p lantS
Extensive. research. has. shown. that. engineered. virus. resistance. in. transgenic. plants.
works. primarily. through. the. antiviral. pathways. of. the. mechanism. known. as. RNA.
silencing..This.mechanism.regulates.the.expression.of.genes.at.the.RNA.level.follow-
ing. RNA. transcription. by. the.host. plant.. RNA. silencing. is.triggered. by. the. replica-
tion. of. genomic. viral. RNA. within. the. host. cell. 18,21,24 . Following. entry. into. the. cell,.
most.viruses.form.double-stranded.RNA.(dsRNA).intermediates.during.their.replica-
tion..These.dsRNA.are.recognized.by.host.RNase.III.Dicer-like.enzymes.and.cleaved.
into.short.fragments.called.small.interfering.(si).RNA.(∼21.nts)..These.fragments.then.
associate.with.the.RNA-induced.silencing.complex.(RISC),.which.proceeds.to.target.
homologous. RNA. sequences. within. the. cell.. Once. identiied. by. the. RNA. silencing.
machinery,.these.target.RNA.sequences.are.similarly.cleaved. 18,21,23,24 .In.this.way,.the.
host.cell.severely.limits.or.entirely.prevents.viral.replication,.resulting. in. resistance..
Though.originally.identiied.in.plant-virus.interaction.studies,.RNA.silencing.has.been.
shown. to. be. present. in. a. wide. range. of. organisms. including. humans,. plants,. nema-
todes,.and.fruit.lies.and.is.believed.to.function.as.a.sort.of.primitive.immune.system. 25
The.activation.of.these.cellular.viral.defenses.within.host.plants.is.believed.to.
explain.the.long-recognized.phenomenon.of.recovery.from.viral.symptoms.seen.in.
the.upper.leaves.of.some.infected.plants,.as.it.has.been.shown.that.a.silencing.sig-
nal.can.move.systemically.within.the.plant,.activating.silencing.ahead.of.the.viral.
replication.advance. 26