Transgenic Vegetables - Transgenic Horticultural Crops: Challenges and Opportunities - page 41

lesions,.reduced.rate.of.systemic.disease.development,.and.very.low.levels.of.virus.
accumulation.compared.to.control.plants..In.some.instances,.the.transgenic.plant.was.
protected.not.only.against.the.virus.from.which.the.coat.protein.gene.was.derived.but.
also.against.other.serologically.unrelated.viruses.(reviewed.in.Prins.et.al. 18 )..Despite.
the.widespread.use.of.CPMR,.it.is.believed.that.multiple.molecular.mechanisms.of.
resistance.exist,.which.are.not.fully.understood. 21 .Currently,.CPMR.is.registered.for.
use. in. transgenic. squash. and. zucchini,. with. the. potential. for. many. new. vegetable.
crops.in.the.future.
Expression.of.viral.RNA-dependant.RNA-polymerases.(replicase).using.antisense.
or.sense.constructs.has.also.been.effective.at.limiting.infection.and.spread.of.RNA.
viruses.. Replicase-mediated. resistance. appears. to. be. strain-speciic. and. functions.
through.lowering.the.overall.level.of.viral.replicases,.thus.reducing.the.viral.concen-
tration..Replicase-mediated.viral.resistance.has.been.successfully.used.in.tomato.to.
increase.resistance.to.tomato.leaf.curl.virus. 22,23
Plant. DNA. viruses. do. not. encode. polymerases;. instead,. they. require. an. inter-
action. between. viral. replication-associated. protein. (rep).and. the. host. plant's. DNA.
polymerase..Expression.of.mutated.viral.rep.gene.or.rep-gene.silencing.through.anti-
sense.technologies.can.reduce.viral.titer.and.has.been.used.to.improve.viral.resis-
tance.in.cabbage 24 .and.bean. 25,26
Other.mechanisms.of.viral.resistance.include.expression.of.viral.nucleoproteins,.
which.serve.to.disrupt.normal.movement.of.viral.proteins.and.nucleic.acid.packing.
leading. to. resistance,. reducing. viral. symptoms.. Expression. of. nucleoproteins. has.
been.used.in.tomato.to.reduce.symptoms.in.response.to.tomato-spotted.wilt.virus. 27,28
f unGal and b acterIal r eSIStance
Enhanced. resistance. to. fungi. and.bacteria.has. been. demonstrated. using. a. number.
of.different.strategies.for.several.vegetable.crop.species.(Table.2.1)..These.include.
carrot,. cucumber,. lettuce,. and. tomato.. The. genes. and. corresponding. enzymes.
.produced. include. hydrolytic. enzymes. (chitinases. and. glucanases),. peroxidase,.
.pathogenesis-related.(PR).proteins.(thaumatin),.and.peptides..Expression.of.chitin-
ases.or.β-1,3-glucanases,.which.are.PR-proteins.that.can.hydrolyze.the.major.struc-
tural.polysaccharides.in.the.fungal.cell.wall.(chitin.and.laminarin),.provided.varying.
levels. of. resistance. to. Botrytis cinerea . in. cucumber 29 . and. to. a. number. of. fungal.
pathogens.infecting.carrot. 30,31 .Chitinase.expression.has.been.achieved.in.combina-
tion. with. other. genes. including. osmotin. in. tomato 32 . to. reduce. Fusarium. wilt. and.
with. lipid-transfer. protein. in. carrot, 33 . which. resulted. in. reduced. fungal. infection..
Expression. of. another. PR-protein,. thaumatin,. also. enhanced. fungal. disease. resis-
tance.in.carrot. 34 .Peroxidase.expression.reduced. Botrytis .and. Sclerotinia .infection.of.
carrot.leaves 31 .and,.when.expressed.in.combination.with.PR-1,.reduced. Phytophthora .
development.in.tomato. 35
A.secondary.approach.has.been.to.engineer.expression.of.enzymes.that.degrade.
pathogen. virulence. products,. e.g.,. oxalic. acid. oxidase. or. decarboxylase. to. break.
down.oxalic.acid..For.example,.expression.of.a.wheat.oxalate.oxidase.gene.in.tomato.
reduced.the.development.of. Botrytis .and. Sclerotinia 36 .while.an.oxalate.decarboxyl-
ase.gene.from.the.fungus. Flammulina .also.led.to.reduced. Sclerotinia .development.