Agriculture Reference
In-Depth Information
expression.to.loral.tissues..Use.of.lower-speciic.promoters.to.drive.ethylene.insensi-
tivity.has.been.demonstrated,.but.exhaustive.physiological.characterization.and.horti-
cultural.examination.have.not.been.documented.for.these.transgenic.lines..For.example,.
lower-speciic.expression.of.the.Arabidopsis.
etr1-1
.in.carnation.with.the.petunia.
fbp1
.
promoter.extended.lower.longevity.to.twice.that.of.control.lowers.
89
.Flower-speciic.
expression. of. the. Arabidopsis.
etr1-1
. gene. with. the. petunia.
fbp1
. promoter. extended.
loral.display.in.
Campanula carpatica
.
92
.The.petunia.
fbp1
.promoter.was.also.used.to.
drive. Arabidopsis.
etr1-1
. expression. in.
Kalanchoe blossfeldiana
.
93
. While. transgenic.
Kalanchoe
. plants. had. a. longer. lower. display,. expression. of. the. transgene. did. not.
appear.to.be.entirely.lower.speciic.with.some.expression.in.root.tissue..There.was.no.
consistent,. obvious. difference. in.all. transgenic. ethylene-insensitive.
Kalanchoe
. lines.
for.shoot.dry.weight,.leaf.area,.or.root.length;.however,.exhaustive.physiological.and.
horticultural.characterization.was.not.performed..These.results.demonstrate.that.trans-
genic.ethylene.insensitivity.can.be.engineered.in.speciic.plant.tissues.
l
eaf
l
onGevIty
Leaf.senescence.is.a.highly.ordered,.deteriorative.process.where.cellular.components.
are.broken.down.and.nutrients.are.remobilized.from.dying.tissue.to.other.parts.of.the.
plant..Although.this.process.involves.macromolecular.breakdown,.synthesis.of.new.
proteins,. from. genes. known. as. senescence. associated. genes. (SAGs),. is. required.
94
.
The.initiation.of.the.senescence.process.is.known.to.involve.alteration.in.the.levels.
of.the.phytohormone.cytokinin.(reviewed.by.Sakakibara
95
)..In.addition.to.their.role.
in.senescence,.cytokinins.regulate.a.number.of.developmental.processes.in.plants..
These. include. promoting. cell. division,. transducing. nutritional. signals,. promoting.
shoot.initiation,.and.increasing.crop.productivity
96
.(reviewed.by.Sakakibara
95
)..The.
irst. cytokinin. biosynthetic. gene. identiied. and. characterized. was. the.
isopentenyl
transferase
.(
ipt
).gene.from.
Agrobacterium tumefaciens
.
97
.The.IPT.protein.catalyzes.
synthesis.of.the.cytokinin.Δ2-isopentenyladenosine.monophospate.through.a.trans-
ferase. reaction. between. dimethylallylpyrophosphate. (DMAPP). and. 5′AMP.. The.
gene.encoding.IPT.is.located.on.the.Ti-plasmid.of.
A. tumefaciens
.and.is.integrated.
into.the.host.genome.after.infection..The.enzyme.is.active.in.plants;.increased.cyto-
kinin.production.is.observed.in.plants.expressing.
ipt
.
98
.Since.the.
Agrobacterium ipt
.
was.cloned.and.characterized,.cytokinin.biosynthetic.genes.have.been.isolated.from.
plants.
99,100
.Work.to.date.has.centered.on.heterologous.use.of.the.
Agrobacterium ipt
.
gene.in.transgenic.plants.
Increasing. cytokinin. levels. in. plants. is. a. strategy. for. delaying. leaf. senescence,.
but.most.early.attempts.resulted.in.plants.with.irregular.growth.patterns.
101-103
.These.
reports.made.use.of.modiied.constitutive.promoters,.Cu
2+
.inducible,.or.heat.shock.
promoters. for. driving. expression. of.
ipt
.. Gan. and. Amasino
104
. designed. constructs.
for.auto-regulated,.senescence-speciic.expression.of.
ipt
.using.the.SAG12.promoter.
from.Arabidopsis..With.this.construct,.expression.of.
ipt
.is.restricted.until.the.plant.
initiates.senescence..Expression.of.
ipt
.is.self-attenuating.as.leaf.senescence.ceases.
thereby.preventing.overproduction.of.cytokinins.and.eliminating.abnormal.growth..
Tobacco.plants.expressing.SAG12::
ipt
.had.longer.lasting.leaves.and.plants.were.mor-
phologically.normal.
104
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