Transgenic Vegetables - Transgenic Horticultural Crops: Challenges and Opportunities - page 45

folate. content. 84 . When. transgenic. PABA-. and. pteridine-overproduction. traits. were.
combined.by.crossing,.vine-ripened.fruit.accumulated.up.to.25-fold.more.folate.than.
controls. 85 .Transferring.the.folate.biosynthetic.branch.developed.in.tomatoes.to.staple.
food.plants.such.as.sweet.potato,.could.potentially.increase.the.folate.intake.in.poor.
countries.with.limited.access.to.fresh.vegetables.
Modiication.of.starch.composition.of.tubers.enables.them.to.be.used.for.diverse.
industrial. applications.. Amylose. and. amylopectin. are. the. two. principal. polysac-
charide. components. of. most. natural. starches. 86 . In. sweet. potato. plants,. the. range.
of.amylose.content.(10%-20%).is.narrow.in.comparison.with.other.crops,.thereby.
making.them.less.amenable.for.industrial.uses..Since.the.ratio.of.amylase.to.amy-
lopectin. is. an. important. textural. property. of. starch,. development. of. a. plant. type.
that.contains.amylose-free.or.low.amylose.would.envisage.new.industrial.applica-
tions. 86,87 . Amylose-free. transgenic. sweet. potato. plants. were. produced. by. inhibit-
ing. sweet potato granule-bound starch synthase I (GBSSI) .gene.expression.through.
RNA.interference. 86,87 . GBSSI .is.one.of.the.key.enzymes.catalyzing.the.formation.of.
amylose,.a.linear.alpha.(1,4)d-glucan.polymer,.from.ADP-glucose..Due.to.silencing.
of. GBSSI .gene,.over.70%.of.the.regenerated.transgenic.plant.roots.contained.amylo-
pectin.but.not.amylose..These.reports.suggest.that.RNA.interference.is.an.effective.
method.for.manipulating.gene.expression.in.the.starch.metabolic.pathway. 86,87
e nhanced t aSte
Fruit.taste.is.a.complex.genetic.trait.governed.by. multiple.genes.and.gene. interac-
tions.. Although. taste. improvement. in. food. crops. has. been. classically. achieved. by.
careful.selection.and.hybridization.for.centuries,.it.is.one.of.the.traits.that.still.cannot.
be.directly.manipulated.by.conventional.plant-breeding.methods.but.may.be.possible.
through.genetic.engineering..Taste-modifying.proteins.are.alternative.sweeteners.and.
lavor.enhancers,.and.manipulation.of.their.expression.may.enhance.the.nutritional.
quality.and.consumer.preference..Thaumatin.is.a.sweet-tasting,.lavor-enhancing.pro-
tein.present.in.the.fruits.of. Thaumatococcus daniellii ,.an.African.shrub. 88 .Transgenic.
tomato.lines.expressing.recombinant-thaumatin.protein.in.fruits.were.sweeter.than.
the.controls,.which.was.conirmed.by.organoleptic.evaluation. 88 .Miraculin.is.a.taste-
modifying. protein. present. in. the. red. berries. of. Richadella dulciica .. Miraculin. by.
itself.is.not.sweet;.it.converts.the.sour.taste.of.ascorbic.acid,.acetic.acid,.and.citric.acid.
into.a.sweet.taste.after.being.held.in.the.mouth.for.some.time..This.unique.property.
has.led.to.increasing.interest.in.this.protein.for.expression.in.other.plants..Miraculin.
was.constitutively.expressed.in.lettuce,.and,.as.a.result,.the.transgenic.leaves.accumu-
lated.signiicant.levels.of.miraculin,.which.eventually.attributed.sweetness-inducing.
activity. 89,90 . Similarly,. transgenic. tomato. plants. expressing. recombinant. miraculin.
protein.accumulated.high.levels.in.both.leaves.and.fruits..The.puriied.recombinant.
miraculin.protein.from.transgenic.tomato.plants.showed.strong.sweetness-inducing.
activity,.which.was.similar.to.that.of.native.miraculin. 90
Similar.to.taste,.lavor.and.aroma.are.traits.that.are.often.rarely.available.within.
existing.germplasm.and.hence.have.to.be.imported.from.related.or.alien.gene.pool..
Ocimum basilicum . geraniol. synthase. gene. was. expressed. in. tomatoes. in. a. fruit-.
speciic.manner.using.the.tomato.ripening-speciic.polygalacturonase.(PG).promoter..