Transgenic Ornamental Crops - Transgenic Horticultural Crops: Challenges and Opportunities - page 71

agronomic. and. ornamental. industries. since. creating. plants. with. more. desirable.
architecture.can.increase.yield,.decrease.the.amount.of.input.in.growing.a.crop,.and.
improve.the.overall.aesthetic.value.of.a.plant.
p lant h eIGht
Plant.height.is.routinely.regulated.during.greenhouse.crop.production.through.the.
use.of.chemical.growth.regulators.that.control.the.synthesis.of.gibberellic.acid.(GA),.
a.phytohormone.that.promotes.stem.elongation..Ideally,.plants.would.produce.inter-
nal.growth.regulators.to.decrease.labor.and.chemical.costs.and.create.a.product.that.
would. be. easier. to. ship. to. the. consumer. and. easier. to. maintain. in. the. landscape..
Transgenic.control.of.plant.height.through.manipulation.of.gibberellin.synthesis.has.
been.proven.as.a.successful.means.for.controlling.plant.height.
Plant.height.can.be.transgenically.controlled.by.altering.signaling.of.the.phyto-
hormone.GA.or.lowering.the.levels.of.endogenous.GA.in.the.plant.through.inhibit-
ing. GA. synthesis. or. increasing. GA. breakdown.. GAs. are. chemically. diverse. with.
more. than. 120. different. GAs. identiied;. many. are. precursors. or. breakdown. prod-
ucts.and.only.a.handful.are.biologically.active.in.plants.(reviewed.by.Yamaguchi 110 )..
GA-induced.cell.elongation.has.been.attenuated.in. Chrysanthemum morifolium .by.
using. the. Arabidopsis. gibberellic insensitive . ( gai ). gene.. Transgenic. gai . chrysan-
themum.plants.continually.repress.GA.responses,.exhibit.dwarism.with.transgenic.
plants. growing. to. about. 30%. as. tall. as. the. control. plants. 111 . Several. GA. oxidase.
enzymes. that. hydroxylate.GA. precursors.to. form.inactive. GA.products.have. been.
identiied.in.Arabidopsis. 112 .Two.GA2-oxidases.(AtGAox7.and.AtGAox8).hydroxyl-
ate. C20-GA. precursors,. but. not. active. C19-Gas. 112 . Based. on. work. by. Schomburg.
et. al., 112 . petunias. with. reduced. plant. height. were. produced. by. overexpressing. GA.
oxidase.(35S:: AtGAox7 .and. AtGAox8 ),.thereby.reducing.endogenous.levels.of.GA..
Transgenic.GA.oxidase.petunias.have.a.range.of.dwarf.phenotypes. 2 .The.dwarf.phe-
notype. can. be. rescued. by. application. of. exogenous. C19-Gas,. a. feature. useful. to.
commercial.growers.as.a.means.of.elongating.cuttings.for.vegetative.propagation. 2
f lower m orpholoGy
Over. the. past. two. decades. genes. responsible. for. loral. organ. identity. have. been.
identiied. and. models. for. lower. development. are. rapidly. progressing.. In. general,.
loral. organ. identity. is. controlled. by. the. sole. and. combinatorial. activity. of. mul-
tiple. classes. of. organ. identity. genes. encoding. A,. B,. C,. D,. and. E. class. transcrip-
tion.factors..The.A class.transcription.factors.are.responsible.for.sepal.formation..
Interaction.of.A and.B.class.factors.speciies.petals..B.and.C.class.together.specify.
stamens,.while.the.C class.gives.rise.to.carpels. 113 .Ovule.development.requires.D.
class. genes,. possibly. in. combination. with. C. class. genes. (reviewed. by. Angenent.
and. Immink 114 ).. The. E. class. functions. in. combination. with. A,. B,. and. C. factors.
to. determine. identity. of. petals,. stamens,. and. carpels. (reviewed. by. Angenent. and.
Immink 114 ).. This. mechanism. of. lower. development,. known. as. the. ABC. model,.
is.generally.conserved.among.lowering.plants..Mutants.with.ectopic.or.repressed.
expression.of.these.genes.have.altered.loral.form..Loss.of.function.of. PMADS3 ,.