Agriculture Reference
In-Depth Information
culture. methods.
69
.It.is.important.to.also.note.that.variation.in.composition.is.not.
limited.to.transgenic.crops.generated.by.recombinant.DNA.technologies.and.tissue.
culture.methods..Nutritional.variation.has.been.reported.for.a.number.of.plant.prod-
ucts.derived.from.conventionally.bred.varieties,.and.ranges.for.most.of.the.compo-
sitional.variables.are.available.in.the.literature.
70
.Variation.in.conventional.crops.is.
attributed.to.genetics.as.well.as.preharvest.conditions,.maturity.at.harvest,.harvest-
ing.methods,.postharvest.handling,.and.storage.conditions.
Although.numerous.animal.studies.have.been.conducted.with.transgenic.crops.
carrying.
cry
. insecticidal,. cowpea. trypsin. inhibitor,. phytase,. and. snowdrop. lec-
tin. genes,
58
. there. are. only. a. few. published. studies. on. the. safety. assessment. of.
whole.foods.derived.from.transgenic.crop.plants.transformed.with.viral.coat.pro-
tein.genes..Presumably.viral.coat.proteins.are.not.regarded.as.potential.allergens.
or.toxins.given.the.physicochemical.and.structural.properties.of.the.proteins.and.
the. low. exposure. levels. due. to. low. or. undetectable. transgene. protein. expression.
(because. of.RNA. silencing)..For.transgenic. viral.proteins.expressed. in. commer-
cialized.horticultural.crops,.sequence.relatedness.of.35%.(or.higher).or.a.continuous.
stretch.of.eight.amino.acids.is.not.shared.with.known.allergens.
71
.Moreover,.resis-
tance. to. digestion. under. acidic. conditions. has. not. been. demonstrated.
2,72,73
. It. is.
important.to.bear.in.mind.that.many.of.the.crop.plants.available.in.commerce.contain.
natural.toxins.and.allergens.
48
.Peanuts,.tomatoes,.soybeans,.kiwi,.and.potatoes.are.
a.few.examples.
d
urabIlIty
and
S
pecIfIcIty
of
e
nGIneered
r
eSIStance
to
v
IruSeS
In
p
lantS
The. issues. of. broad-spectrum,. durable. resistance. with. regard. to. virus-resistant.
transgenic. horticultural. crops. do. not. conceivably. have. any. impact. on. the. envi-
ronment.and.human.health..If.the.engineered.resistance.to.viruses.were.to.show.
limitations. in. terms. of. durability. and. speciicity,. it. would. likely. only. create. an.
agronomic.problem.and.affect.growers.
2
.However,.these.issues.should.be.considered.
in.light.of.an.effective.management.of.the.technology.
Breakdown of Engineered Virus Resistance
The.risk.that.virus.resistance.may.break.down.or.not.prove.durable.is.not.unique.to.
virus-resistant.transgenic.crops..It.is.a.risk.shared.by.conventional.crops.and.other.
transgenic.technologies.including.the.pest.resistance.of.
Bt
.crops..The.durability.of.
resistance.refers.to.the.ability.of.a.gene.conferring.resistance.to.hold.up.over.time.
after.being.widely.deployed..In.the.case.of.virus.resistance,.however,.the.potential.
mechanisms. for. resistance. breakdown. are. different.. As. alluded. to. previously,. one.
potential.breakdown.of.resistance.could.occur.if.virus.isolates.that.are.genetically.
divergent.enough,.at.the.nucleotide.sequence.level,.from.the.transgene.are.not.rec-
ognized. by. the. RNA. silencing. machinery. and. are. subsequently. capable. of. infect-
ing.the.genetically.modiied.host.plant..Another.potential.mechanism.of.resistance.
breakdown. centers. on. the. virus'. ability. to. mutate. in. such. a. way. as. to. overcome.
the. resistance. triggered. by. the. transgene.. A. third. potential. mechanism. for. resis-
tance. breakdown. involves. infection. of. the. resistant. host. plant. with. a. heterologous.
virus
74,75
. encoding. a. strong. suppressor. of. gene. silencing.. This. heterologous. virus.
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