Agriculture Reference
In-Depth Information
ire. blight. resistance. are.
epo
,. a. gene. encoding. EPS-depolymerase,
97,98
. SB-37,
91,99
.
T4. lysozyme,
91,95,99,100
.
hrp
N. (harpin),
101
. cecropin. MB. 39. (modiied. SB-37),
102,103
.
and.
Shiva
-1.
91
. Transgenic. apples. resistant. to. ire. blight. have. also. been. obtained.
by.partially.silencing.certain.genes.encoding.pathogen-protein.receptors.
104
.There.
are.also.opportunities.for.breeding.ire.blight-resistant.apple.and.pear.cultivars.by.
exploiting.genetic.variation.in.germplasm.and.by.developing.quantitative.trait.loci.
(QTL).markers.
105,106
Agrobacterium tumefaciens
Crown.gall,.caused.by.the.soil.bacterium.
Agrobacterium tumefaciens
,.greatly.dimin-
ishes.tree.productivity.and.is.a.serious.disease.for.many.fruit,.nut,.and.ornamental.
crops..Walnuts.in.particular.are.very.susceptible.to.this.disease..Losses.are.incurred.
from.both.contaminated.nursery.stock.and.infected.orchard.trees..Current.prophy-
lactic.measures.and.the.mechanical.removal.of.galls.have.not.adequately.controlled.
the. problem.. Pathogenic.
A. tumefaciens
.transforms.infected.plants.with.the.genes.
iaaM
. (tryptophan. monooxygenase),.
iaaH
. (indole-3-acetamide. hydrolase),. and.
ipt
.
(isopentenyl. transferase).. The.
iaaM
. and.
iaaH
. gene. products. convert. tryptophan.
into. indole-3-acetic. acid,. an. auxin,. while. the.
ipt
. gene. product. catalyzes. the. pro-
duction. of. adenosine. monophosphate. (AMP),. which. is. converted. to. cytokinins. by.
endogenous. plant. enzymes.. The. resulting. overproduction. of. auxin. and. cytokinins.
induces.proliferation.of.callus.tissue.at.the.wound.site,.resulting.in.the.development.
of.large.galls.
17,107
To.engineer.resistance.to.this.pathogen,.a.binary.vector.plasmid.containing.inverted.
repeats.of.portions.of.the.
iaaM
.and.
ipt
.genes.was.constructed.and.transformed.into.
walnut.
108
.Constitutive.expression.of.this.construct.induces.RNAi-mediated.degrada-
tion.of.the.
iaaM
.and.
ipt
.transcripts,.demonstrating.the.use.of.RNAi.to.generate.resis-
tance.to.a.major.bacterial.disease.
109
.Because.the.construct.and.the.oncogenes.do.not.
need.to.have.perfect.homology.for.silencing.to.be.effective,.the.resulting.transgenic.
plants.are.resistant.to.a.very.wide.range.of.
A. tumefaciens
.strains,.displaying.a.broad.
spectrum.durable.resistance.
108
.Transgenic.walnut.plants.containing.the.
iaaM
-.and.
ipt
-silencing.constructs.are.currently.in.ield.trials..RNAi has.also.been.used.to.gen-
erate.crown.gall.resistance.for.tomato,
110
.apple,
111,112
.and.grape.
113
Xylella fastidiosa
Xylella fastidiosa
.(
Xf
).is.a.xylem-limited,.Gram-negative.bacterium,
114
.transmitted.
by. insect. vectors. (especially. the. glassy. winged. sharpshooter,.
Homalodisca
coagulate
).. Different.
Xf
. strains. cause. devastating. diseases. in. economically.
important.plants,.for.example.PD.in.grape,.citrus.variegated.chlorosis.(CVC).in.
citrus,
115
. and. almond. leaf. scorch. disease. (ALSD). in. almond.. Grape. PD. symp-
toms.include.yellowing.and.gradual.necrosis.of.the.petiole.attached.to.the.cane.
after. leaf. fall.. The. disease. progresses. rapidly,. resulting. in. occlusion. of. xylem.
vessels. and. consequent. water.stress.. Vine. death. often.occurs. within. 2. years.
116
.
Citrus. CVC. symptoms. include. leaf. variegated. chlorosis,. defoliation,. twig. die-
back,.size.reduction,.and.hardening.of.fruits.
115
.The.disease.management.strat-
egy.for.PD.in.grape.focuses.on.containing.vector.transmission.using.insecticides,.
with.limited.success.
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