Transgenic Fruit and Nut Tree Crops Review - Transgenic Horticultural Crops: Challenges and Opportunities - page 2

INTRODUCTION
Fruit.crop.plants.constitute.an.important.group.among.agricultural.crops,..adding.
tens. of. billions. of. dollars. per. year. to. the. global. economy.. Worldwide,. over.
45 million.hectares.of.land.have.been.devoted.to.their.production,.and.millions.
of. growers. depend. on. continued. global. trade. 1 . Among. fruit. crops,. citrus. pro-
duction.is.most.signiicant,.with.more.than.134.million. tons.produced.from.an.
area. of. 8.7. million. hectares. in. 2008.. This. is. followed. by. bananas. with. world-
wide. production. at. just. under. 100. million. tons,. apples. at. 76. million. tons,. and.
grapes. at. 74. million. tons. 2 . In. California,. the. production. of. fruit. and. nut. crops.
contributes. to. more. than. one-third. of. the. total. farm. gate. value. of. the. state's.
agricultural.commodities..The.fruit.and.nut.crops.are.cultivated.using.a.highly.
sophisticated.production.system.that.uses.vegetatively.propagated.clonal.cultivar.
materials. grafted. to. clonal. or. seedling. rootstocks.. The. plantings. are. perennial.
and.orchard.systems.represent.several.decades.of.investment.to.a.grower..These.
crop.plants.have.a.juvenile.phase,.which.can.last.from.2.to.8.years,.during.which.
they.grow.vegetatively.without.any.fruit.production..Thus,.traditional.fruit.tree.
breeding.takes.a.very.long.time—20.to.40.years.to.develop.new.varieties..This.is.
primarily.due.to.the.time.it.takes.to.backcross.promising.selections.with.current.
commercial.cultivars,.which.is.required.to.create.a.new.commercial.cultivar.with.
optimal. fruit.characteristics.. Transgenic.plants.offer. a. more.direct. and.quicker.
strategy.to.provide.genetic.solutions.
Fruit.and.nut.crops.face.many.challenges,.including.the.need.to.be.grown.on.
less.land.with.less.resources.and.the.need.to.yield.consistently.high.quality.fruit.
that. can. sustain. the. economics. of. production.. Disease. and. pests. pose. the. great-
est.challenge.to.fruit.and.nut.crop.production,.and.we.discuss.below.the.different.
diseases. and. pests. that. are. important. to. these. crops. and. the. speciic. solutions.
that.can.be.provided.using.transgenic.technologies..Many.of.these.are.production-
eliminating. diseases,. so. inding. genetic. solutions. is. critical.. In. some. cases,. the.
disease.resistance.traits.are.present.in.wild.germplasm,.so.introgression.of.disease.
resistance. by. traditional. breeding. takes. place. at. the. cost. of. fruit. quality,. which.
means. that. many. backcrosses. have. to. be. conducted. to. restore. fruit. quality.. In.
some. cases,.there. is. no. resistance.even. in. the. wild.germplasm,. so. novel. sources.
of. resistance. have. to. be. developed. (e.g.,. pathogen-derived. resistance).. For. these.
strategies,.transgenic.technologies.need.to.be.employed..These.technologies.have.
been.developed.in.many.of.the.tree.crops.(as.outlined.in.next.section);.however,.a.
major.challenge.is.the.recalcitrance.that.these.plants.display.in.tissue.culture..The.
percentage. of. tissue-cultured. explants. that. give. rise. to. a. transgenic. plant. is. low.
and.thus.it.is.dificult.to.transform.these.plants.using. Agrobacterium tumefaciens ..
Successful.plant.transformation.requires.high.frequencies.of.both.transformation.
and.regeneration,.such.that.a.successful.overlap.of.such.events.can.take.place.at.a.
tissue.level.to.obtain.transgenic.plants..While.there.are.many.examples.of.success,.
there.are.also.many.fruit.tree.species.that.have.proved.very.dificult.to.transform.
due.to.this.problem.
Two.general.classes.of.traits.that.are.of.commercial.signiicance.are.discussed.
in.this.review:.input.traits.and.output.traits..Input.traits.are.speciically.important.