Agriculture Reference
In-Depth Information
Lending.further.support.to.the.importance.of.the.RNA.silencing.system.in.plants.
is.the.discovery.that.most.virus.families.encode.one.or.more.proteins.that.suppress.
silencing.
21,24,27-29
. Viral. suppressors. have. been. shown. to. interact. in. multiple. ways.
with. the. RNA. silencing. machinery. to. prevent. effective. control. of. the. expression.
of.the.viral.genome.within.the.host..One.such.example.is.the.HC-Pro.protein.from.
PVY..This.protein.binds.the.siRNAs.produced.by.the.cell's.RNA.silencing.machin-
ery,. thereby. squelching. the. signal,. which. effectively. suppresses. RNA. silencing.
within.the.host.cell.
30
.This.not.only.can.allow.for.continued.PVY.replication,.but.
has.also.been.shown. to.be.primarily.responsible.for.the.observed.phenomenon. of.
viral.synergism.where.two.unrelated.viruses,.when.co-infecting.the.host.plant,.can.
exhibit. more. severe. symptoms. than. either. virus. on. their. own.. Experiments. with.
PVY.and.
Potato virus X
.(PVX).have.shown.that.HC-Pro.of.PVY.can.suppress.host.
defenses.and.allow.for.PVX,.which.very.weakly.suppresses.silencing.on.its.own,.to.
replicate.and.produce.much.more.severe.symptoms.than.either.PVX.or.PVY.cause.
on.their.own.
13
The. fact. that. RNA. silencing. is. a. nucleotide. sequence-based. resistance. mecha-
nism.has.unique.implications.for.risk.assessment.and.the.development.of.new.virus-
resistant.transgenic.plants..This.feature.is.discussed.in.the.next.section.
SAFETY ISSUES AND RISKS
Due. to. the. expression. of. viral. gene. constructs. to. provide. resistance. to. viruses. in.
transgenic.plants,.there.are.unique.safety.issues.associated.with.this.technology
2,11-13
.
versus.many.of.the.other.commercialized.transgenic.crop.plants..Among.these.are.
the. risks. of. viral. recombination. and. transencapsidation.. However,. not. all. of. the.
potential.risks.associated.with.this.technology.are.unique.to.virus-resistant.transgenic.
plants..Risks.also.associated.with.other.transgenic.plant.technology.including.gene.
low. to. free-living. relatives,. allergenicity. and. other. human. health. effects,. break-
down.of.resistance,.and.effects.on.nontarget.organisms.also.need.to.be.assessed.with.
regard.to.engineered.resistance.against.viruses..In.the.following.section,.we.examine.
each.of.these.areas.of.concern.and.assess.their.scientiic.merits.
t
ranSencapSIdatIon
One.of.the.concerns.associated.with.virus-resistant.transgenic.crops.is.the.potential.
that.viral.capsid.proteins,.when.produced.in.a.transgenic.host.plant,.may.transencap-
sidate.the.genome.of.a.challenge.virus.
2,11-13
.Since.viral.capsid.proteins.play.roles.
in.such.diverse.processes.as.movement.within.the.host,.replication,.suppression.of.
gene.silencing,.and.vector.transmission.speciicity,
31
.it.is.plausible.that.a.transencap-
sidated.virus.may.have.altered.properties..If.the.coat.protein.of.a.virus.vectored.by.
aphids,.for.example,.were.transgenically.produced.within.a.host.plant,.it.is.conceiv-
able.that.an.aphid.non-transmissible.virus.could.be.encapsidated.within.the.coat.pro-
tein.derived.from.the.transgene..This.transencapsidated.virus.(with.the.coat.protein.
from. the. aphid-borne. virus. and. the. genome. of. the. aphid. non-transmissible. virus).
might.then.acquire.the.characteristics.of.an.aphid-borne.virus,.potentially.allowing.
it.to.move.more.expeditiously.into.new.host.plants—possibly.even.to.plant.species.
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