Agriculture Reference
In-Depth Information
displayed. 80%-90%. reductions. in. S6PDH. enzyme. activity. and. 70%-80%. reduc-
tions. in. sorbitol. levels.. Dramatic. changes. in. fruit. composition. were. also. observed.
in.the.transgenic.lines,.including.signiicant.increases.in.glucose.levels,.lesh.irm-
ness,.and.soluble.solids.content,.and.corresponding.signiicant.decreases.in.fructose.
levels,.sorbitol.levels,.starch.levels,.malic.acid.levels,.and.titratable.acidity..Overall,.
these.results.demonstrate.that.fruit.quality.characteristics.(and.likely.lavor).are.real-
istic.targets.for.manipulation/improvement.via.biotechnology.
r
emoval
of
u
ndeSIrable
p
hytochemIcalS
Postharvest.enzymatic.browning,.caused.primarily.by.the.enzyme.polyphenol.oxi-
dase,. is. a. substantial. quality. problem. in. a. variety. of. fruit. crops,. including. apple,.
pear,. grape,. and. pineapple.. Polyphenol. oxidase. comes. in. contact. with. its. pheno-
lic.substrates.during.fruit.ripening.or.tissue.damage,.generating.reactive.quinones.
that.crosslink.to.form.dark-colored.phytomelanins.
196
.Following.the.lead.of.studies.
performed. in. annual. crops,
197
. several. groups. have. attempted. to. silence. the. poly-
phenol. oxidase. gene. in. order. to. reduce. postharvest. browning. in. tree. fruits.. Most.
notable. is. work. by. Murata. et. al.,. in. which. expression. of. an. antisense. polyphenol.
oxidase.gene.in.transgenic.apple.lines.reduced.polyphenol.oxidase.activity.and.tis-
sue.browning.by ~50%.compared.to.wild-type.controls.
198,199
.However,.only.callus.
and.shoot.tissues.were.examined;.it.remains.to.be.seen.whether.reductions.in.post-
harvest.browning.will.also.be.observed.in.apple.fruit.
198,199
.Similar.approaches.have.
been. pursued. to. reduce. blackheart. in. pineapple
200
. and. grape. berry. darkening,
196
.
though.no.characterization.of.the.transgenic.polyphenol.oxidase-silenced.plants.has.
yet.been.reported.
Posttranscriptional. gene. silencing. has. also. been. utilized. to. reduce. caffeine.
content. in. coffee. plants.. Caffeine. is. generated. from. xanthosine. by. two. sequential.
methylation.reactions.catalyzed.by.the.enzymes.theobromine.synthase.and.caffeine.
synthase.. Ogita. et. al.. transformed. coffee. plants. with. an. RNAi. vector. designed. to.
initiate.posttranscriptional.gene.silencing.of.the.theobromine.synthase.gene.
201
.The.
resultant. transgenic. lines. were. morphologically. normal,. but. displayed. 50%-70%.
less.caffeine.in.leaf.tissues.than.controls..Effects.on.caffeine.content.in.coffee.fruit.
were.not.described..Although.not.yet.reported.in.the.literature,.a.similar.gene.silenc-
ing.strategy.could.be.used.to.reduce.the.levels.of.allergenic.proteins.in.tree.nuts.(e.g.,.
walnuts.and.chestnuts)..This.approach.was.recently.applied.to.reduce.the.levels.of.the.
immunodominant.Ara.h.2.protein.in.transgenic.peanut.plants.
202
a
ntIGen
p
roductIon
Several. studies. have. shown. that. transgenic. plants. that. produce. viral. or. bacterial.
antigen.proteins.can.induce.a.protective.immune.response.(as.measured.by.the.pro-
duction. of. antigen-speciic. serum. antibodies). when. fed. to. mice. and. humans.
203,204
.
These.plants,.which.act.as.“edible.vaccines,”.have.several.signiicant.advantages.over.
traditionally.puriied.and.administered.vaccines,.including.ease.of.human. delivery,.
increased.stability,.and.decreased.production.and.transport.costs.
205
.While.most.stud-
ies. on. edible. vaccines. have. focused. on. annual. crops,. recently. the. “
s
”. gene. of. the.
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