Transgenic Vegetables - Transgenic Horticultural Crops: Challenges and Opportunities - page 43

by. transporting. the. ions. into. the. vacuole. using. tonoplastic. Na + /H + . antiporters. 52 .
Transgenic.tomato.plants,.overexpressing.the. Arabidopsis .Na + /H + .antiporter.( Atnhx ).
gene,.were.able.to.grow.and.produce.fruit.at.relatively.high.levels.of.Na + ,.without.
accumulating.excess.Na + .in.the.fruit. 52 .The. Atnhx .gene.has.also.been.overexpressed.
in.sugar.beet,.which.became.more.resistant.to.drought.stress,.in.addition.to.increas-
ing.the.natural.level.of.sucrose.in.the.taproot. 53,54
Abiotic. stresses. disrupt. normal. plant. metabolism. by. affecting. reaction. rate,.
protein. stability,. and. membrane. integrity.. This. alteration. in. metabolism. leads.
to. an. increase. in. accumulation. of. ROS. to. potentially. toxic. levels. (reviewed. in.
Ashraf 55 ).. Detoxiication. of. ROS. occurs. naturally. to. some. extent. in. all. plant.
species.through.the.production.of.detoxifying.enzymes.and.antioxidants. 56 .The.
recently.employed.strategies.for.detoxifying.ROS.in.vegetable.crops.include.the.
overexpression. of. the. detoxifying. enzymes. superoxide. dismutase. and. catalase.
in. tomato 57 . and. Chinese. cabbage, 58,59 . resulting. in. enhanced. tolerance. to. abi-
otic.stresses..Additionally,.more.complex.metabolic.engineering.to.increase.the.
production.of.antioxidants.has.been.made.to.enhance.resistance.toward.abiotic.
stresses.in.tomato. 60-63
MEDICAL APPLICATIONS
e dIble v accIneS
Transgenic.vegetables.offer.an.attractive.option.for.vaccine.production,.since.nearly.
any.antigenic.protein.can.be.expressed.in.plants.at.a.large.scale,.and.the.crops.can.
be.grown.locally. 64 .Vaccines.can.also.be.produced.in.edible.parts.of.plants,.which.
aids.in.delivery..Edible.plant-based.vaccines.can.be.made.against.an.entire.protein.or.
just.the.desired.antigenic.peptides.from.viruses,.parasites,.or.bacterial.pathogens. 64 .
Development. of. edible. vaccines. has. been. primarily. directed. to. pathogens,. which.
infect.the.host.via.the.mucosal.surfaces.lining.the.digestive,.respiratory,.and.urinor-
eproductive.tracts.of.the.body,.since.these.vaccines.need.to.be.absorbed.in.a.similar.
fashion..Much.of.the.research.on.edible.vaccines.has.focused.on.tomato.fruit,.car-
rot.taproots,.and.leafy.greens, 65-73 .since.they.can.be.consumed.raw,.eliminating.the.
possibility. of. degradation. of. the. vaccine. during. the. cooking. process.. Expression.
of. these. polypeptides. has. been. mainly. with. constitutive. promoters. while. some.
research.in.tomatoes.used.fruit-speciic.E8.promoter. 65,66 .In.recent.years,.successful.
.vegetable-based.edible.vaccines.have.been.produced.against.common.viruses.such.
as.measles, 69,70,74,75 .rabies, 73 .and.hepatitis.B, 66,68 .common.bacterial.diseases.such.as.
cholera, 71 .and.HIV, 65,72,76 .which.are.extremely.dificult.to.treat.
While.there.are.many.beneits.associated.with.the.production.of.vegetable-based.
edible. vaccines,. there. are. a. number. of. limitations. to. this. technology.. One. of. the.
major.limitations.is.obtaining.adequate.protein.concentrations.that.initiate.suficient.
immune.response.to.result.in.complete.immunity..Additionally.timing.of.doses.and.
application.of.multiple.doses.is.crucial.and.will.need.to.be.addressed.before.these.
edible.vaccines.are.used.for.larger.scale.applications..Despite.the.promise.in.allevi-
ating.important.global.diseases,.there.are.currently.no.commercial.transgenic.veg-
etable.plants.producing.edible.vaccines.