Transgenic Fruit and Nut Tree Crops Review - Transgenic Horticultural Crops: Challenges and Opportunities - page 7

time-consuming.. However,. genetic. engineering. offers. a. real. possibility. to. create.
fruit.and.nut.tree.crop.varieties.with.desirable.agronomic.traits.that.exhibit.increased.
resistance.to.pathogens.and.insects..Here,.we.discuss.recent.progress.in.increasing.
resistance.to.viruses,.bacteria,.fungi,.and.insects.through.the.genetic.transformation.
of.fruit.and.nut.tree.crops.
p athoGen r eSIStance
Viral Resistance
Viruses. are. the. main. cause. of. economic. losses. to. agricultural. crops. worldwide..
For.example,. Citrus tristeza virus .(CTV).causes.huge.economic.losses.for.the.cit-
rus. industry. and. Plum pox virus . (PPV). causes. one. of. the. most. devastating. viral.
diseases. of. stone. fruit. ( Prunus . spp.).. Several. transgenic. approaches. have. been.
successfully.applied.for.virus.resistance.in.fruit.crops.based.on.pathogen-derived.
resistance.(PDR).strategies..Virus.resistant.transgenic.crops.have.been.developed.
during.the.last.two.decades.through.both.the.expression.of.transgenic.virus.RNA.
in. host. plants. and. protein-mediated. resistance. 36-39 . Described. as. a. potent. genetic.
vaccination. against. viral. sequences,. RNA-mediated. virus. resistance. (RMVR). is.
less. susceptible. to. variation. in. virus. sequence. and. is. highly. sequence-speciic,.
often. providing. complete. immunity. to. the. inoculated. virus. or. RNA. 40,41 . Protein-
mediated.resistance.has.the.potential.for.broader.protection.but.generally.results.in.
mild.resistance,.which.only.delays.symptoms.and.decreases.viral.titers..However,.
the. combination. of. RNA-. and. protein-mediated. resistance. in. transgenic. plants.
could. prove. to. be. potent,. broad,. and. durable. 41 . Gene. silencing. is. another. power-
ful.biotechnological.tool.used.to.gain.virus.resistance.for.fruit.crops..This.method.
has.produced.transgenic.plants.able.to.switch.off.endogenous.genes.and.invading.
nucleic.acids. 42-47
Papaya Ringspot Virus
Papaya ringspot virus . (PRSV). causes. a. destructive. disease. in. papaya. ( Carica
papaya . L.),. an. economically. important. fruit. crop. in. tropical. and. subtropical. areas,.
resulting.in.drastic.reduction.of.fruit.quality..PRSV.is.naturally.transmitted.by.aphids.
in. a. nonpersistent. manner. and. induces. stunting. in. growth,. symptoms. of. mosaic.
and. distortion. on. leaves,. and. streaks. on. petiole. and. stem.. Resistance. against. PRSV.
in. papaya. has. not. been. obtained. by. conventional. breeding.. In. the. last. two. decades,.
effective.strategies.based.on.PDR.have.been.widely.used..The.coat.protein.(CP).gene.
of.PRSV,.HA.5-1,.was.transferred.into.papaya.via.micro-projectile.bombardment. 48-50 .
Transgenic.papaya.lines.highly.resistant.to.the.severe.Hawaiian.PRSV.strain.PRSPV.
HA.were.selected.under.greenhouse.and.ield.conditions. 49,50 .Rainbow.and.SunUp.cul-
tivars.have.been.successfully.commercialized.in.Hawaii.since.1998,.representing.the.
irst. practical. application. of. transgenic. fruit. crop. 51,52 . However,. the. CP-hemizygous.
line.Rainbow.is.susceptible.to.non-Hawaiian.PRSV.isolates.and.the.CP-homozygous.line.
SunUp,.while.resistant.to.a.wider.range.of.isolates.from.Jamaica.and.Brazil,.is.still.
susceptible.to.isolates.from.Thailand.and.Taiwan. 51,53,54
In.Taiwan.transgenic.papaya.lines.carrying.the.CP.gene.of.a.Taiwan,.severe.strain,.
PRSV.YK,.have.also.been.successfully.generated. 55 .Several.transgenic.lines.provide.