Transgenic Ornamental Crops - Transgenic Horticultural Crops: Challenges and Opportunities - page 60

pathway.for.orange.pelargonidin.anthocyanin.production..However,.a.commercially.
acceptable,.stable.color.was.not.obtained.in.the.initial.report. 20 .Commercially.accept-
able,.stable,.improved.orange.colors.were.achieved.through.introgression.of.the.maize.
DFR .into.elite.breeding.lines.accumulating.cyanidin,.peonidin,.delphinidin,.petuni-
din,.and.malvidin.anthocyanins.and.three.generations.of.seling.the.F1.crosses. 21
c arotenoIdS
Carotenoids.are.plastid.synthesized,.lipid.soluble,.isoprenoid-derived.C40.compounds.
essential.to.all.photosynthetic.organisms..Carotenoids.are.structurally.diverse.with.
over.700.structures.identiied..They.function.in.light.harvesting,.energy.transfer,.and.
photoprotection..They.also.provide.pink,.red,.orange,.and.yellow.color.to.lowers.and.
fruits,.are.precursors.of.plant.hormones.and.fragrance.compounds,.and.have.signii-
cant.nutritional.value.in.animal.diets.(reviewed.by.Lu.and.Li 22 ).
Carotenoid.synthesis.is.initiated.by.the.condensation.of.two.molecules.of.gera-
nylgeranyl. pyrophosphate. (GPP). by. phytoene. synthase. (PSY). to. form. colorless.
phytoene..Phytoene.is.converted.to.all.trans.lycopene.through.four.desaturation.reac-
tions.catalyzed.by.phytoene.desaturase.(PDS).and.ζ-carotene.desaturase.(ZDS).and.
an.isomerization.reaction.catalyzed.by.carotenoid.isomerase.(CRTISO)..The.desatu-
ration.reactions.introduce.conjugated.double.bonds,.leading.to.a.shift.in.the.absorp-
tion.spectra.that.causes.color.to.appear..Lycopene.represents.a.branch.point.in.the.
pathway,.where.either.(1).δ-carotene,.α-carotene,.and.ultimately.lutein.are.produced.
through. the. action. of. lycopene. ε-cyclase,. lycopene. β-cyclase,. and. carotenoid. ring.
hydroxylases. or. (2). γ-carotene,. β-carotene,. and. zeaxanthin. are. produced. through.
the. action. of. lycopene. β-cyclase. and. carotenoid. β-ring. hydroxylases. (Figure. 3.2).
(reviewed. by. Hirschberg 23 ).. Naturally. occurring. mutants. for. many. of. these. carot-
enoid. biosynthetic. genes. exist. in. tomato,. and. fruits. from. these. lines. show. visible.
differences.in.color. 24 .For.example,.the.tomato.R.mutant.has.yellowish-green.fruit.
due.to.a.mutation.in.the.phytoene.synthase.gene. 25 .Mutants.with.ectopic.expression.
of. a. lycopene. β-cyclase.have. orange. fruit,.accumulating. high. levels.of. β-carotene.
and.less.lycopene.than.control.plants. 26
Most.of.the.research.on.engineering.carotenoid.synthesis.in.transgenic.plants.has.
focused.on.the.alteration.of.nutritional.characteristics.in.food.plants..For.example,.
β-carotene.accumulating.“golden”.rice.varieties.were.developed.by.introducing.daf-
fodil. or. maize. phytoene synthase . ( Psy ). and. a. bacterial. phytoene desaturase ( crtl ).
gene. into. the. rice. genome. 27,28 . Although. these. plants. may. be. valuable. for. helping.
solve. worldwide. vitamin. A. deiciency,. they. also. demonstrate. the. ability. to. drive.
heterologous.expression.of.yellow.color.in.plants.
By.transferring.bacterial.genes.for.ketocarotenoid.synthesis,.deep.yellow,.orange,.
and. red. colors. have. been. introduced. into. plants. normally. displaying. pale. yellow.
colors. (Table. 3.1).. Bright. orange. nectaries. were. created. in. Nicotiana . by. transfor-
mation. with. the. alga. Haematococcus pluvialis . β-carotene. ketolase. gene,. causing.
accumulation.of.astaxanthin.in.nectary.tissue..This.application.has.more.of.a.neu-
traceutical. value. since. astaxanthin.has.many. uses. related. to. animal. health..While.
nectaries.generally.do.not.contribute.much.to.the.overall.aesthetic.appeal.of.a.plant,.
it. demonstrates. that. novel. carotenoids. can. be. expressed. in. plants. to. introduce.