Transgenic Ornamental Crops - Transgenic Horticultural Crops: Challenges and Opportunities - page 70

Petunia.has.been.used.as.a.model.for.engineering.delayed.leaf.senescence.using.
different.SAG.promoters.and.the. ipt .gene..The.SAG12:: ipt .construct.was.introduced.
into. 'V26'. petunia. and. over. 20. lines. were. screened. by. drought. stress. for. delayed.
leaf. senescence.. Drought. stress. is. a. common. problem. encountered. in. commercial.
and.home.garden.environments.and.induces.the.leaf.senescence.program,.but.does.
not. induce. SAG12. expression. 105 . Two. lines. out. of. 20. SAG12:: ipt . transgenic. lines.
displayed.a.normal.growth.phenotype.and.had.delayed.leaf.senescence. 2,106-108 .The.
numbers.of.chlorotic.basal.leaves.on.transgenic.SAG12:: ipt .plants.were.about.14.times.
less.that.of.control.plants. 2,106
To.engineer.plants.for.extended.cold.storage.tolerance,.Khodakovskaya.et.al. 109 .
utilized.cor15a,.a.cold-inducible.promoter.from.Arabidopsis,.to.drive. ipt .expression.
in. petunia. and. chrysanthemum.. During. horticultural. propagation,. plant. parts. are.
commonly. stored. in. cool,. dark. environments.. These. conditions. cause. leaf. senes-
cence.and.ultimately.increase.mortality.when.plants.are.in.prolonged.storage..Plants.
transformed.with.cor15a:: ipt .had.morphologically.normal.growth.at.a.normal.tem-
perature. (25°C),. similar. to. that. of. controls.. When. a. propagation. environment. was.
simulated. with. cold. storage,. the. transgenic. plants. resisted. senescence. exhibiting.
greener. leaves. with. elevated. cytokinin. levels. and. leaves. with. normal. chlorophyll.
levels.while.control.tissue.was.visibly.senescing.
Other.approaches.for.delaying.leaf.senescence.have.been.successful.and.have.
resulted. in. interesting. phenotypes. useful. for. manipulation. in. ornamental. plants..
The. SAG12. promoter. was. used. to. drive. expression. of. the. maize. Knotted1. gene..
KN1 .is.a.homeobox.protein.involved.in.meristem.formation,.cytokinin.synthesis,.
and.repression.of.senescence..Another.SAG.promoter,.SAG13,.was.used.to.drive.
ipt .expression.in.petunia..Plants.expressing.SAG12:: KN1 .or.SAG13:: ipt .exhibited.
delayed.leaf.senescence.after.drought.stress. 107 .Compared.to.controls,.SAG13:: ipt .
transgenic.plants.were.more.branched,.leaves.were.thicker.and.larger,.root.growth.
was. reduced,. and. lowering. was. delayed.. These. phenotypes. were. ascribed. to.
“leakier”. expression. of. the. ipt . transgene. 107 . Nutrient. stress. experiments. showed.
that. source-sink. relationships. were. altered. in. SAG13:: ipt . and. SAG12:: KN1 . lines.
as. new. growth. after. nutrient. stress. was. chlorotic. and. necrotic. 107 . Interestingly,.
SAG13:: ipt .and.SAG12:: KN1 .plants.also.showed.enhanced.tolerance.to.pathogens..
In.controlled.greenhouse.experiments.in.which. Cercospora .was.spray-inoculated.
onto. plants,. control. plants. had. sixfold. more. senescing. leaves. than. SAG13:: ipt . or.
SAG12::Knotted1. plants,. with. transgenic. lines. exhibiting. local. hypersensitive.
responses.to.infection. 107 .These.experiments.highlight.the.value.of.exhaustive.hor-
ticultural.evaluations.of.multiple.lines.of.transgenic.plants..Selection.of.elite.culti-
vars.will.be.required,.similar.to.conventional.breeding.
PLANT ARCHITECTURE
Plants. have. evolved. to. grow. a. fascinating. array. of. loral. and. vegetative. forms.
that. are. inluenced. by. genetic. and. environmental. factors.. Plant. height,. branching,.
lower.number,.and.loral.morphology.are.just.some.of.the.readily.apparent.forms.
giving. individual. plants. their. unique. architectural. characteristics.. Understanding.
how.a.plant.develops.its.architectural.attributes.is.of.economic.importance.to.both.