Transgenic Fruit and Nut Tree Crops Review - Transgenic Horticultural Crops: Challenges and Opportunities - page 16

d elayed f ruIt r IpenInG
Many. tree. fruits. are. transported. long. distances. from. their. point. of. cultivation. to.
their.point.of.sale.to.the.consumer..As.demonstrated.in.several.annual.plant.systems,.
genetically.manipulating.the.biosynthesis.of.the.hormone.ethylene.can.substantially.
slow.fruit.ripening,.allowing.increased.lexibility.in.transport.time.and.fruit.maturity.
at.harvest. 184,185 .In.climacteric.fruits,.ethylene.triggers.a.rapid.increase.in.respiratory.
rate.and.initiates.a.cascade.of.biochemical.and.physiological.changes.associated.with.
ripening. 88 .Beginning.with.the.precursor. S -adenosyl.methionine.(SAM),.ethylene.is.
synthesized.in.two.enzymatic.steps.in.plants..First,.SAM.is.converted.to.1-amino-
cyclopropane-1-carboxylic. acid. (ACC),. a. reaction. catalyzed. by. the. ACC. synthase.
enzyme..Second,.the.ACC.oxidase.enzyme.converts.ACC.to.ethylene. 186 .The.strat-
egies. applied. to. minimize. ethylene. production. in. transgenic. fruit. tree. crops,. and.
thereby. slow. ethylene-associated. fruit. ripening. processes,. have. primarily. focused.
on.posttranscriptional.silencing.of.the.genes.encoding.the.ACC.synthase.and.ACC.
oxidase.enzymes.
Delayed.ripening.in.transgenic.fruit.trees.has.been.demonstrated.most.convinc-
ingly.in.apple.and.papaya. 88,187 .Silencing.of.ACC.oxidase.in.papaya.resulted.in.a.40%.
reduction.in.fruit.ethylene.production,.with.a.corresponding.delay.in.fruit.softening.
and.the.retention.of.green.peel.color..However,.the.fruit.appears.to.have.limited.com-
mercial.application,.as.the.unripe.fruit.retains.high.susceptibility.to.pathogen.infection/
decay,. leading. to. a. direct. transition. from. unripe. fruit. to. rotten. fruit. 88 . Dandekar.
et. al.. pursued. a. similar. approach. in. apple,. silencing. both. the. ACC. synthase. and.
ACC..oxidase.genes. in.separate.transgenic. lines..Apples.collected.from.both.ACC.
synthase-. and. ACC. oxidase-silenced. lines. displayed. >90%. reduction. in. ethylene.
production..The.transgenic.fruits.were.irmer.and.had.a.longer.shelf.life.than.compa-
rable.controls.(Figure.1.1),.but.their.sugar/acid.balance.(a.key.aspect.of.fruit.lavor).
was.unaffected..Fruit.aroma.was.likely.altered.in.these.lines,.however,.as.total.vola-
tile.ester.production.was.reduced.by.65%-70%.compared.to.controls. 187
Several.additional.studies.have.described.comparable.delayed.ripening.strategies.
in.pineapple, 188 .mango, 189 .and.avocado, 190 .though.no.phenotypic.characterization.of.
the.transgenic.plants.has.been.reported..Similarly,.the.silencing.of.ACC.oxidase.in.
transgenic.pear.reduced.ethylene.production.by.85%.in.in.vitro.shoots,.but.effects.on.
pear.fruit.have.not.yet.been.determined. 191
I mprovInG f ruIt Q ualIty and n utrItIve v alue
Several.recently.published.studies.have.focused.on.the.generation.of.transgenic.fruit.
and.nut.tree.crops.with.enhanced.nutritive.value.or.increased.levels.of.speciic.phyto-
chemicals.beneicial.to.human.health..For.example,.the.grape.stilbene.synthase.gene,.
which.is.responsible.for.the.synthesis.of.the.phytoalexin.resveratrol,.has.been.intro-
duced.into.apple.and.kiwifruit. 143,192 .Resveratrol.is.an.antifungal.compound.involved.
in.pathogen-induced.plant.defense.responses,.but.it.also.has.a.variety.of.beneicial.
effects.on.human.health,.including.anti-inlammatory.and.anticarcinogenic.proper-
ties. 143 .The.accumulation.of.glycosylated.resveratrol.derivatives.was.demonstrated.in.
the.leaves.of.both.transgenic.kiwifruit.and.apple.plants,.though.it.is.currently.unclear.