Virus-Resistant Transgenic Horticultural Crops: Safety Issues and Lessons from Risk Assessment Studies - Transgenic Horticultural Crops: Challenges and Opportunities - page 277

Transgenic. plum. trees. containing.multiple.copies.of.the.PPV.coat.protein.gene.
were.developed 114 .and.line.C5.was.shown.to.be.highly.resistant.to.PPV.infection. 113 .
The.involvement.of.the.posttranscriptional.RNA.silencing.mechanism.in.the.resis-
tant.line.C5.was.conirmed. 115,116 .Clone.C5.was.tested.extensively.in.the.ield.in.the.
Czech. Republic,. Poland,. Romania,. and. Spain. to. demonstrate. the. effectiveness. of.
the. engineered. resistance.. Transgenic. C5. trees. all. remained. free. from.PPV.infec-
tion.even.6 117,118 .and.8.years.after.transfer.to.the.ield, 119 .while.100%.of.the.control,.
non-transgenic.trees.were.infected.with.PPV.after.these.periods..Though.these.data.
indicate. the. durability. of. the. engineered. resistance. to. PPV,. trees. inoculated. with.
PPV. via. chip. budding. exhibited. very. mild. symptoms. after. several. years,. though.
these.symptoms.did.not.progress.to.severe. 118,119
r ISk S tudIeS
Fruit. compositional. analyses. indicated. that. PPV-resistant. transgenic. C5. is. typical.
for. P. domestica . plums. in. terms. of. protein,. total. fat,. antioxidant. capacity,. pheno-
lics,.starch,.dietary.iber,.ash,.moisture,.acidity,.carbohydrates,.sugar.proiles.(glucose,.
sucrose,.lactose,.maltose,.and.fructose),.calcium,.magnesium,.sodium,.potassium,.iron,.
and.vitamins.A,.B1,.B2,.B3,.and.C. 120
The.potential.for.viral.recombination.between.transgene.transcripts.and.incoming.
PPV.RNA.was.examined.in.transgenic.plums.as.well.as.the.effects.of.PPV-resistant.
transgenic.plums.on.aphid.vector.populations. 121 .Utilizing.transgenic.European.plum.
lines.as.well.as.non-transgenic.plums.from.an.experimental.orchard.and.Japanese.plums.
from.an.external.control.plot,.85.PPV.isolates.were.collected.from.these.three.popu-
lations.of.trees.and.their.genetic.diversity.was.compared..Looking.at.variable.regions.
of. the. PPV. genome. including. the. coat. protein. gene,. no. signiicant. differences. in.
genetic.variability.were.found.among.isolates.from.the.three.populations,.indicating.
that.the.PPV.populations.were.not.being.selectively.altered.in.the.transgenic.trees. 121 .
Subsequent.analysis.of.12.PPV.isolates.showed.no.detectable.recombinant.virus. 121 .
Of. note. in. this. experiment,. C5. could. not.be.used. as. a. source. of. virus. isolates. for.
testing.since.it.remained.free.from.infection.8.years.after.natural.exposure.to.PPV.
populations..Recombination.in.C5.trees.therefore.would.have.been.impossible.due.
to.lack.of.virus.infection.
For.assessing.the.impact.of.transgenic.plums.on.nontarget.organisms,.the.diver-
sity.of.aphid.populations.visiting.transgenic.and.non-transgenic.plums.was.investi-
gated. 121 .Aphids.were.captured,.identiied,.counted,.and.their.viruliferous.potential.
was.subsequently.characterized..These.comparisons.found.no.signiicant.differences.
between.the.aphid.populations.from.either.transgenic.or.control.plums.in.terms.of.
total.aphid.numbers,.aphid.species.distributions,.and.viruliferous.potential.over.the.
2.year.period.of.the.study. 121 .To.test.the.effect.of.heterologous.viruses.on.the.stability.
of.RNA.silencing.in.transgenic.plum.line.C5,.trees.were.graft-inoculated.with.differ-
ent.combinations.of.PPV.and.either. Prunus necrotic ringspot virus .(PNRSV),. Prune
dwarf virus .(PDV),.or. Apple chlorotic leafspot virus .(ACLSV). 75 .PNRSV,.PDV,.and.
ACLSV.are.common.in. Prunus .sp..The.engineered.resistance.to.PPV.was.stable.
and.was.not.suppressed.by.the.presence.of.heterologous.viruses.during.a.3.year.ield.
trial.in.Romania.and.Spain. 75