Transgenic Vegetables - Transgenic Horticultural Crops: Challenges and Opportunities - page 39

stresses.(salinity.and.drought),.production.of.edible.vaccines.and.therapeutic.prod-
ucts,.nutritional.enhancement,.and.metabolic.engineering..The.published.results.from.
these.studies.show.some.very.interesting.trends.toward.better.stress.tolerance,.pest.
and.disease.resistance,.and.more.nutritious.vegetable.crops.(Table.2.1)..For.the.most.
part,.these.transgenic.vegetable.crop.species.have.been.evaluated.under.laboratory.
and.greenhouse.conditions,.with.limited.ield.trials..Several.studies,.however,.have.
evaluated.these.materials.in.trials.with.animal.systems.for.the.assessment.of.vaccines.
and.therapeutics..Collectively,.these.studies.clearly.show.the.potential.of.transgenic.
vegetable.crops.with.beneicial.traits,.and.they.are.discussed.in.more.detail.below.

INSECT RESISTANCE
Resistance.of.plants.to.insects.is.a.much.desired.attribute,.potentially.reducing.the.
number. of. insecticide. sprays. required.. The. following. are. examples. of. transgenic.
strategies.used.to.reduce.insect.damage.on.vegetable.crops.

. 1.. Manipulation.of.polyphenol.oxidase.(PPO).activity.could.provide.resistance.

simultaneously.to.both.diseases.and.insect.pests. 2 .Transgenic.tomato.plants.
constitutively.expressing.potato. PPO .gene.were.tested.for.insect.resistance..
These.transgenic.plants.exhibited.up.to.nearly.sixfold.higher.PPO.activity.
levels.compared.to.controls. 2 .The.PPO.transgenic.plants.clearly.showed.an.
increase.in.resistance;.simple.growth.rates.of.common.cutworms.on.these.
plants.were.up.to.three.times.lower.than.on.controls.and.larvae.consumed.
less.foliage..In.addition,.increased.PPO.activity.led.to.higher.larval.mortality. 2 .
These.results.suggest.a.critical.role.for.PPO-mediated.phenolic.oxidation.in.
resistance.to.this.insect. 2

. 2.. Insect.cytoskeletons.and.digestive.systems.contain.large.amounts.of.chitin,.

which.can.be.broken.down.by.certain.plant.chitinases..Transgenic.tomato.
seedlings. expressing. a. poplar. chitinase. gene. were. tested. for. resistance.
toward.the.Colorado.potato.beetle,.which.can.cause.severe.losses.in.tomato.
and.other.crops. 3 .The.plants.expressing.the.poplar.chitinase.gene.reduced.
the. larval. development. allowing. approximately. 50%. to. reach. the. second.
larval.instar,.as.compared.to.over.90%.in.the.control.plants, 3 .suggesting.the.
possibility.of.using.a.plant.chitinase.gene.for.promoting.insect.resistance. 3

. 3.. Plant.proteinase.inhibitors.(PIs).have.been.well.established.to.play.a.potent.

defensive. role. against. predators. and. pathogens. (reviewed. by. Mosolov. and.
Valueva 4 )..The.defensive.capacities.of.plant.PIs.rely.on.inhibition.of.proteases.
present.in.insect.guts.or.secreted.by.microorganisms,.causing.a.reduction.in.
the.availability.of.amino.acids.necessary.for.their.growth.and.development..
Unlike. other. insecticidal. proteins,. for. PIs. to. be. active,. very. high. levels.
of.recombinant.PI.protein.are.required.to.achieve.pest.resistance. 4 .Tissue.
speciic. or. inducible. expression. would. yield. higher. levels. at. the. required.
site. and. time.. Leaf-speciic. overexpression. of. the. potato. protease. inhibi-
tor.and.carboxypeptidase.inhibitors.in.tomato. resulted.in.increased.resis-
tance.to. Heliothis obsoleta .and. Liriomyza trifolii .larvae.in.high-expressing.