Agriculture Reference
In-Depth Information
broad.spectrum.resistance.against.homologous.and.geographically.distinct.strains.
of.PRSV.under.greenhouse.conditions 56 .and.a.high.degree.of.resistance.under.ield.
trials. 57 . During.the. ield. trial,.some. transgenic. papaya. lines. showed. susceptibility.
to. the. new. emerging. papaya leaf distortion mosaic virus . (PLDMV). which. indi-
cates.that.the.virus.is.a.threat.for.PRSV-resistant.transgenic.papaya.in.Taiwan.and.
elsewhere. 58
Recently,.transgenic.papaya.lines.with.double.resistance.to.PRSV.and.PLDMV.
were.generated.using.a.chimeric.construct.strategy.containing.a.fused.cDNA.frag-
ment.composed.of.the.truncated.PLDMV.P-TW-WF.CP.and.PRSV.YK.CP.coding.
regions..Three.of.nine.resistant.transgenic.papaya.lines.carrying.the.chimeric.con-
struct.showed.high.levels.of.resistance.to.heterologous.PRSV.strains.from.Hawaii,.
Thailand,. and. Mexico.. Transgenic. papaya. lines. look. promising. for. resistance. to.
PRSV.strains.and.PLDMV.in.Taiwan.and.elsewhere. 59
Citrus Tristeza Virus
Citrus tristeza virus . (CTV). (Closterovirus). is. considered. the. most. economically.
important. virus. affecting. citrus. production. worldwide.. CTV. is. spread. by. several.
species. of. aphids. with. piercing-sucking. mouthparts,. Toxoptera citricidus . being.
the. most. eficient. vector.. This. viral. disease. is. now. endemic. in. several. economi-
cally.important.production.regions.around.the.world..Different.strains.of.CTV.can.
cause. diverse. disease. syndromes. that. vary. in. severity.. Symptoms. may. be. as. mild.
as.weak.and.scattered.vein.clearing,.whereas.quick.decline,.a.syndrome.in.which.a.
tree.with.normal.appearance.starts.showing.wilt.symptoms.and.completely.collapses.
in. a. few. weeks,. is. the. most. dramatic. manifestation. of. disease.. The. development.
and.identiication.of.CTV.tolerant.rootstocks.was.one.of.the.main.strategies.for.the.
control. of. quick-decline. disease. 60 . Recently,. biotechnology. tools. like. gene. silenc-
ing. (GS). have. been. successfully. applied.. Speciically,. targeting. CTV. using. RNA.
interference.(RNAi).strategies.is.the.major.strategy.in.plant.transformation.for.CTV.
resistance..Citrus.lines.expressing.p23.CTV,.a.silencing.suppressor,.were.reported.
as.resistant.to.CTV,.and.transgenic.viral.protein.accumulation.was.negatively.cor-
related. to. resistance.. In. this. study,. posttranscriptional. gene. silencing. (PTGS). was.
proposed.as.the.mechanism.for.resistance.to.CTV,.since.resistant.citrus.plants.had.
multiple.copies.of.the.transgene,.low.levels.of.the.corresponding.mRNA,.methyla-
tion.of.the.silenced.transgene,.and.accumulation.of.the.p23-speciic.small.interfer-
ing. RNAs. (siRNAs). 61 . RNA-mediated. resistance. for. CTV. has. also. been. obtained.
in. transgenic. grapefruit. 62,63 . To. successfully. obtain. CTV. resistant. plants. through.
gene.silencing,.it.is.important.to.understand.all.the.viral.counterdefense.strategies,.
like.the.virus's.RNAi.suppression.strategies.developed.during.their.coevolution.with.
plants. 43,45,64 . Protein-mediated. resistance. strategies. have. also. been. described. for.
CTV..CTV-resistant.citrus.plants.that.express.the.CP.of.the.virus.have.been.obtained.
and. in. these. plants. virus. resistance. was. related. to. the. accumulation. of. the. trans-
genic. viral. protein. in.plant.tissue,. with.PDR. as. the.proposed.mechanism. of.resis-
tance. 65-69 .Protoplast.fusion.is.another.promising.biotech.tool.because.it.can.produce.
somatic.hybrids,.which.would.be.very.valuable.for.the.development.of.CTV-resistant.
rootstocks..Citrus.somatic.hybrids.with.resistance.to.CTV.infection.have.been.suc-
cessfully.obtained.by.protoplast.fusion. 70
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