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
Search WWH ::




Custom Search