Genetic Engineering of Grapevine and Progress toward Commercial Deployment - Transgenic Horticultural Crops: Challenges and Opportunities - page 326

against.fungal.infection. 77 .Stilbene.synthase.is.the.key.enzyme.that.catalyzes.the.
formation.of.resveratrol.via.phenolic.substrates.in.the.shikimic.acid.pathway..Stilbene.
biosynthesis.only.speciically.requires.the.presence.of.stilbene.synthase..The.family.
of.stilbene.synthase.genes.have.been.cloned.and.characterized.from.grape. 78 .Studies.
with.transgenic.apple, 79 .papaya, 80 .rice, 81 .tobacco, 77 .wheat,.and.barley 82 .overexpress-
ing.the.STS.1.gene.from.grape.reported.enhanced.resistance.to.various.fungal.dis-
eases.like.blast,.fruit,.root,.and.stem.rot.
Polyphenol Oxidase
Polyphenol. oxidases. (PPOs). are. nuclear-encoded. enzymes. of. almost. ubiquitous.
distribution. in. plants. and. are. known. to. catalyze. the. oxygen. dependent. oxidation.
of. phenols. to. quinones. 83 . Systemic. induction. of. PPO. in. response. to. wounding.
and. pathogen. attack. may. protect. plants. against. further. attack. by. pathogens. and.
insects. 84 .PPOs.have.been.cloned.from.a.number.of.plant.species.including.grape, 85 .
potato, 86 .tomato, 87 .and. Vicia .species. 88 .Transgenic.plants.overexpressing.PPO.were.
found.to.exhibit.enhanced.resistance.to.bacterial.diseases.caused.by. Pseudomonas
synringae . 89,90 .Additionally,.an.antisense.downregulation.of.polyphenol.oxidase.in.
transgenic.tomato.plants.resulted.in.increased.disease.susceptibility.as.compared.to.
non-transgenic.controls. 90 .Although.only.bacterial.resistance.has.been.speciically.
demonstrated,.PPOs.are.considered.to.have.broad.antimicrobial.activity.
Marker-Free Plants
Genetic.transformation.requires.a.method.to.separate.transformed.cells.from.non-
transformed.cells.and.typically.is.achieved.by.expression.of.a.marker.gene.that.pro-
vides.a.growth.advantage.to.transformed.cells.in.a.selection.medium.(reviewed.by.
Dutt.et.al. 91 )..In.the.absence.of.a.selectable.marker.gene,.transformed.cells.tend.to.
be.at.a.competitive.disadvantage.compared.to.nontransformed.cells.and.die.out..The.
marker.gene.is.needed.only.for.selection.of.transgenic.cells.and.typically.is.linked.
to. a. gene. of. interest.. Thus,. selection. for. ability. to. proliferate. in. the. presence. of. a.
selective. agent. results. in. isolation. of. transgenic. cells. containing. both. the. gene. of.
interest.and.the.marker.gene..However,.once.a.desired.plant.is.selected.from.trans-
genic. cells,. the. marker. gene. is. no. longer. needed.. The. presence. of. marker. genes.
may. complicate. future. commercialization. due. to. concerns. regarding. their. effects.
on. ecosystems. and/or. human. health. 91 . For. example,. one. concern. is. that. selectable.
marker.genes.could.become.transferred.to.other.organisms,.leading.to.the.creation.
of. antibiotic.resistant.bacterial.strains. (from.antibiotic.resistance.markers).or.new,.
aggressive.weedy.plant.species.(from.herbicide.resistance.markers)..Development.of.
transgenic.plants.without.marker.genes.alleviates.such.concerns..Dutt.et.al. 91 .used.a.
cotransformation.treatment.to.demonstrate.production.of.transgenic.grapevines.free.
of.the.selectable.marker.gene(s).that.are.typically.required.only.for.initial.identiica-
tion. and. selection. of.transgenic.cells..Transgenic. V. vinifera .'Thompson. Seedless'.
plants.containing.only.a.single.stably.integrated.gene.of.interest.and.not.a.selective.
marker.gene.were.produced..The.system.provided.the.possibility.of.producing.plants.
that.contain.only.a.gene.of.practical.use.while.preventing.the.formation.of.genetic.
chimeras.that.might.occur.through.meristem.transformation..The.technique.can.be.