Transgenic Vegetables - Transgenic Horticultural Crops: Challenges and Opportunities - page 40

homozygous. transgenic. lines. compared. to. controls. 5 . Seeds. of. transgenic.
peas.expressing.a.α-amylase.inhibitor.reduced.pea.weevil.( Bruchus piso-
rum .L.).survival.by.nearly.100%,.with.the.larval.mortality.occurring.at.an.
early.instar..Conversely,.in.nontransgenic.cultivars,.essentially.100%.of.the.
pea.weevils.emerged.as.adults. 6

. 4.. Bacillus thuringiensis . (Bt). crystalline. toxin. genes. (cry). have. been. exten-

sively.used.to.develop.transgenic.insect.resistant.crop.plants.over.the.past.
two.decades..Several.reports.are.available.on.different.synthetic.Bt.genes.
expressed.both.individually.and.in.a.combinatorial.way.and.their.eficiency.
in. pest. control. (reviewed. in. Christou. et. al. 7 ).. A. cry1Ca . or. a. H04 . hybrid.
gene. was. expressed. in. leaf. tissues. of. beet. and. shallot. 8 . When. either. of.
these.genes.were.expressed.at.high.levels.in.the.leaves,.it.resulted.in.nearly.
complete.resistance.against.beet.armyworm.in.shallot.or.beet. 8 .Similarly,.
transgenic. garlic. plants. expressing. cry1Ca . were. completely. resistant. to.
beet.armyworm.in.a.number.of.in.vitro.bioassays. 8 .There.are.several.other.
successful. examples. of. increased. insect. resistance. in. a. variety. of. trans-
genic.vegetables.using.Bt.cry.genes.alone.or.in.combination..These.include.
cabbage, 9 . collards, 10 . broccoli, 11-13 . and. caulilower. 14 . The. success. of. Bt. in.
controlling.insects.has.been.very.promising,.with.the.only.major.concern.
being. the. development. of. pest. resistance. to. particular. Cry. proteins.. The.
use. of. trap. plants. and. pyramiding. different. Cry. proteins. at. high. levels.
through. chloroplast. expression. can. help. alleviate. the. potential. for. future.
pest.resistance. 13,15

NEMATODE RESISTANCE
Some.of.the.Bt.toxin.genes.also.have.known.nematicidal.activity..Expression.of.nema-
ticidal.Cry6A.in.transgenic.tomato.plants.provided.protection.against.plant-parasitic.
nematodes. 16 . When. Cry6A-expressing. roots. were. challenged. with. Meloidogyne
incognita ,.the.nematode.was.able.to.ingest.the.toxin,.which.resulted.in.a.signiicant.
decrease.in.gall. production. by.up.to. fourfold..This.report.suggests. that.Bt. protein.
can.confer.resistance.to.endoparasitic.nematodes,.and.therefore.Bt.toxin.proteins.are.
potential.candidates.for.developing.nematode-resistant.transgenic.plants. 17

PATHOGEN RESISTANCE

v IruS r eSIStance
The.earliest.successful.development.of.virus-resistant.transgenic.vegetable.crops.uti-
lized.the.viral.coat.protein-mediated.resistance.(CPMR).strategy.to.engineer.patho-
gen.resistance. 18 .The.use.of.CPMR.continues.to.be.one.of.the.most.useful.mechanisms.
for.viral.resistance,.having.been.used.for.tomato.leaf.curl.virus.resistance.in.tomato 19 .
and.big.vein-associated.virus.resistance.in.lettuce. 20 .CPMR.requires.expression.of.
the. viral. coat. (or. capsid). protein. gene. from. the. virus. in. transgenic. plants.. When.
exposed. to. the. viral. inoculum,. the. transgenic. plants. displayed. reduced. number. of.