Expression and Manufacture of Pharmaceutical Proteins in Genetically Engineered Horticultural Plants - Transgenic Horticultural Crops: Challenges and Opportunities - page 104

Other factors such as the. distance. between. sites. of. biomass. production. and. pro-
cessing.will.determine.the.required.period.for.biomass.storage,.which,.in.turn,.will.
determine.the.choice.between.leafy.and.seed.production.system..In.addition,.consid-
erations.have.to.be.in.place.in.choosing.an.expression.system/species.to.address.regu-
latory.compliance.and.public.concerns.on.issues.of.transgene.containment,.product.
biosafety,.and.others.related.to.GM.plants.in.general.
l eafy c ropS
Tobacco.and.its.related. Nicotiana .species.have.been.one.of.the.earliest.crop.systems.
for.successful.transgenic.plant.pharmaceutical.expression,.in.which.target.proteins.
can.be.extracted.directly.from.harvested.leaves..Tobacco.is.one.of.the.easiest.plants.to.
transform,.and,.therefore,.the.molecular.tools.for.transgene.expression.in.this.spe-
cies. are. well. established.. Biomass. yield. for. tobacco. is. higher. than. most. of. other.
leafy.crops..Tobacco.is.also.a.proliic.seed.producer.allowing.rapid.scalability..The.
existence. of. large-scale. processing. infrastructures. in. traditional. tobacco. industry.
provides.the.advantage.of.quick.adaptation.for.processing.pharmaceutical-containing.
biomass.. Since. tobacco. is. a. nonfood,. nonfeed. crop,. the. risk. of. contamination. of.
human.food.chain.or.animal.feed.chain.by.transgenic.tobacco.is.relatively.low..All.
these.advantages.of.tobacco.make.it.one.of.the.most.advanced.candidates.for.molec-
ular.farming.in.commercial.production.of.pharmaceutical.proteins.
Other. green. leaf. expression. systems. also. include. alfalfa,. soybean,. Arabidopsis
thaliana ,.lettuce,.and.spinach..They.share.many.common.advantages.with.tobacco.
including.easy.to.engineer.and.high.biomass.yield..One.of.the.most.advantageous.
properties. of. the. leaf. expression. systems. is. that. it. allows. the. lexibility. to. choose.
from.all.three.plant.expression.technology.platforms.including.stable.nuclear,.chlo-
roplast.transformation,.and.viral.vector-based.transient.expressions..This.lexibility.
increases.the.chance.of.success.for.many.target.proteins,.since.multiple.options.are.
available. to. identify. an. optimal. expression. platform. based. on. the. property. of. the.
target.protein.or.the.nature.of.the.particular.project.
Alfalfa. and. soybean. are. legumes. and,. therefore,. can. obtain. organic. nitrogen.
through. nitrogen. ixation.. This. may. provide. some. advantages. in. reducing. the. con-
sumption.of.nitrogen-containing.chemicals..Alfalfa.also.can.be.harvested.up.to.nine.
times.a.year,.adding.to.a.higher.biomass.yield.. Arabidopsis thaliana .has.been.a.model.
plant. for. plant. molecular. genetics. and. molecular. biology. research.. It.has. been. used.
to.produce.a.variety.of.pharmaceutical.proteins..However,.the.small.size.of.this.plant.
will.limit.its.scalability..Lettuce.and.spinach.are.palatable.without.cooking.and,.there-
fore,. have. been. particularly. targeted. to. produce. edible. vaccines. for. oral. delivery. 124 .
Because.they.are.food.crops,.the.biosafety.and.containment.of.the.transgene.are.major.
concerns.for.their.commercial.application..One.of.the.drawbacks.for.leaf.expression.
systems.is.the.potential.interference.of.expressed.transgenic.proteins.on.plant.growth.
and.development..For.this.type.of.proteins,.inducible.promoters.may.be.advantageous.
as. transgenic. proteins. are. not. expressed. during. plant. growth. under. the. control. of.
such.promoters.but.will.only.be.produced.upon.application.of.speciic.inducer.agent.
post-biomass. harvest. 229 . Leaves. also. tend. to. contain. more. pigments,. alkaloids,. and.
polyphenols,. which. sometimes. may. be. dificult. to. remove. from. the. target. protein..