Transgenic Vegetables - Transgenic Horticultural Crops: Challenges and Opportunities - page 42

in. transgenic. lettuce. 37 . Both. of. these. genes. exhibited. the. ability. to. degrade.oxalic.
acid,.an.important.pathogenicity.factor.for.this.fungus. 36,37
For.bacterial.resistance,.gene.products.such.as.peroxidase,.ferredoxin,.and.defen-
sins.reduced.disease.development.on.tomato.(Table.2.1).caused.by.several.pathogens..
In.tomato,.bacterial.wilt.development.was.reduced.through.combined.expression.of.
a.β-1,3-glucanase.and.defensin.gene 38 .while.expression.of.a.sweet.pepper.ferredoxin.
gene.also.reduced.the.development.of.bacterial.blight. 39
ABIOTIC STRESS TOLERANCE
Abiotic.stresses,.including.salinity,.temperature,.and.water.stresses,.are.among.the.
most. limiting. stresses,. lowering. crop. yield. and. quality. worldwide.. Growing. areas.
that.experience.extreme.salt.and.water.stress.typically.grow.limited.amounts.of.veg-
etable. crops,. which. can. lead. to. local. human. nutritional. deiciencies.. Transgenic.
vegetable. crops. capable. of. growing. on. saline. soil. or. with. minimum. irrigation. in.
these.areas.could.help.stabilize.food.and.nutritional.production.in.these.regions. 40
Osmotic.adjustment.at.the.cellular.level.is.a.vital.response.in.dealing.with.water,.
salinity,. and. freezing. stresses.. Natural. plant. defenses. to. these. stresses. include.
the. synthesis. and. accumulation. of. plant. compatible. solutes,. including. glycine.
betaine,. sugars,. proline,. and. compatible. proteins. (late-embryogenesis. abundant.
protein.[LEA],.dehydrins,.and.heat.shock.proteins).(reviewed.in.Cheong.and.Yun 41 )..
Compatible. osmolytes. protect. the. plant. through. osmotic. adjustment,. which. helps.
maintain. turgor,. scavenging. of. reactive. oxygen. species. (ROS),. and. the. stabiliza-
tion.of.protein.structure. 40 .The.ability.to.regulate.osmotic.potential.using.compatible.
solutes.has.increased.yield.under.drought.conditions.in.many.crop.plants. 42,43 .There.
are. reports. of. transgenic. vegetables. with. increased. production. of. compatible. sol-
utes,.resulting.in.increased.resistance.to.abiotic.stresses.(Table.2.1)..Two.of.the.more.
commonly. used. strategies. in. transgenic. vegetables.are.the. overexpression.of. LEA.
genes. and. overproduction. of. glycine. betaine.. Normally,. LEA. genes. are. expressed.
during. the. late. stages. of. seed. development;. however,. many. LEA. genes. are. also.
expressed.in.vegetative.tissue.when.the.plants.undergo.an.abiotic.stress. 44 .Many.of.
the.LEA.proteins.have.been.shown.to.confer.tolerance.to.drought,.salinity,.and.freez-
ing.stresses. 45 .Overexpression.of.heterologous.LEA.proteins.has.been.successful.in.
increasing. abiotic. stress. resistance. in.kidney.bean, 46 .Chinese.cabbage, 45,47 .and. let-
tuce. 48 .Glycine.betaine.is.naturally.produced.in.many.plant.species,.and.high.accu-
mulation.levels.are.associated.with.drought.and.salt.tolerance..Tomato.plants.do.not.
produce.endogenous.glycine.betaine;.therefore,.plants.were.transformed.to.express.
heterologous.betaine.aldehyde.dehydrogenase.(BADH) 49 .or.choline.oxidase. 50 .These.
tomato.plants.were.able.to.withstand.moderately.high.levels.of.salinity.and.produced.
viable. fruit. and. seeds.. Carrot. accumulates. low. levels. of.glycine. betaine. naturally,.
but.chloroplastic.overexpression.of.endogenous.BADH.resulted.in.enhanced.glycine.
betaine.levels. 51 .These.transgenic.carrots.were.able.to.grow.on.soil.containing.physi-
ologically.very.high.salt.levels.
Another. strategy. used. by. drought. and. salinity-resistant. plants. is. to. compart-
mentalize. the. abundant. but. toxic. Na + . ions. to. utilize. them. for. osmotic. adjustment.