Genetic modification of plants: significant issues and hurdles to

Genetic
hurdles
Peter
modification
to success3
Transformation
many
crop plants.
A genetically
shelf
life
at full
and
issues and
genes
the
Foreign
DNA
in the
resistance
for
United
products,
States
both
to pests,
foods
that can deliver
of controlling
fruit ripening.
inadvertent
can
uptake
particles
plant
ways
include
introduced
is routine for
with a longer
foods
production
be introduced
by protoplasts,
or other
materials.
of
into
toxicants
plants
Limitations
concerns
and
allergens.
little
on metal
or no control
of copy number
or site of integration
of the introduced
DNA,
dependence
on selectable
markers for recovery of traits, and madequate knowledge
of how to control key metabolic
steps to maximize desirable
traits. Directed
genetic change still requires
conventional
crop breeding
to deliver
benefits
to farmers
and
consumers.
KEY
Am
WORDS
neered
foods,
J C/in
Plant
allergens,
Nutr
l996;63:65lS-6S.
transformation,
toxicants,
genetically
plant
breeding,
engi-
fatty
acids,
INTRODUCTION
Ever since the discoveries
of Mendel,
geneticists
have been
interested
in the prospect
of directed
genetic
change.
Conventional plant breeding
always requires
progeny
large enough
to
recover
forms that recombine
the desired
features
from the
parents
of a cross. Recombination
results from random
segregation
due to chromosome
reassortment
and from crossingover during gamete
formation
at meiosis.
The gametes,
pollen
and egg cells, fuse randomly
to form fertilized
eggs, or zygotes,
that develop
into seeds. Hundreds
of different
crosses
are made
each year in large-scale
crop-breeding
programs,
and individual segregating
(F2) populations
of
1000 of each cross are
grown
in successive
years for selecting
recombinants
in later
generations.
In a major program
of breeding
winter
wheat in
the United
Kingdom,
several
miles of single-row
segregating
families
are grown
each year to recover
desirable
forms for
selection
and evaluation.
The number
of families
is progressively
reduced
by selection
of the plants
3 or 4 y yield
trials
succeeding
family
are performed,
in larger
plots
in different
Am
Nuir
l996;63:65lS-6S.
J Clin
in each
of each
first
generation
increases
in small
with
the
PLANT
TRANSFORMATION
original
parental
material.
plots
and
encoded
that
of DNA
in 1944
in the
it might
and
realization
nucleotide
be
DNA-coding
transformation
the
base
possible
in bacteria
that
genetic
sequence
of
to transform
sequences
to effect
DNA
higher
directed
by Avery
information
is
suggested
organisms
genetic
with
changes.
One
of the first successful
methods
harnessed
a circular
DNA
molecule,
or plasmid,
carried
by the pathogenic
plant bacterium
Agrobacterium
tumefaciens.
The
bacterium
invades
wounds
in plant tissue,
where
it stimulates
rapid host cell
growth,
resulting
in the production
of a plant tumor or gall.
This occurs
because
a tumor-inducing
(Ti) segment
of the
plasmid
DNA becomes
integrated
in a chromosome
of the host
nucleus.
Deleting
the
genes
that
control
the
production
of
tumor
cells from the plasmid
leaves
right and left border
fragments,
each 24 nucleotides
long, that mark the ends of the
integrating
Ti segment.
This segment
of bacterial
plasmid
DNA can be used as a vector to introduce
foreign
DNA. If a
bacteriophage
lambda
cos site is included
in the vector,
up to
‘40
kb of DNA
can
be packaged
in vitro
between
the
borders.
Transformation
vectors
include
marker
genes that can be detected in bacterial
and plant host cells and controlling
elements,
or promoters,
to drive the expression
of the desired
gene in
plant tissue at the appropriate
phase of development.
A drawback
of the Ti vector system
is that it is restricted
to
plants
that are hosts for Ag. tumefaciens.
Another
method
uses
protoplasts
that are prepared
by treating
plant cells with enzymes
that dissolve
such as a 4-mollL
not burst.
The
forming
DNA,
cell walls.
Held in a suitable
solution
of mannitol,
they are
protoplasts
some
of
are treated
which
with
is taken
osmoticum,
stable
and
do
a solution
of transup and may become
2
after
3
Address
then
Biology,
and
so that
discovery
et al (I)
From the Center for Agricultural
Molecular
Biology,
Rutgers,
The
University
of New Jersey, New Brunswick.
Supported
by the New Jersey Commission
on Science and Technology.
locations.
Printed
compared
cell
phytase
the number
respects
The
vectors,
introduction
include
After World
War II there was some interest
in producing
mutations
by exposing
seeds to ionizing
radiation.
This was
followed
by treatment
with chemical
mutagens.
Much time and
effort was spent in searching
for useful variants
but few were
found. Most of the mutants
were inferior
in vigor and in other
antigens,
safety
by bacterial
and mechanical
and
altered
grain and
are
immunizing
Food
in
and
diseases,
benign herbicides.
Other possibilities
oils, methionineand lysine-enhanced
proteins,
and other
was
soon-to-be-released
incorporate
legume
and regeneration
engineered
tomato
I
State
reprint
Rutgers,
requests
to PR Day,
The State University
Center
for Agricultural
of New Jersey,
Molecular
New Brunswick,
NJ
08902.
in USA.
© 1996
American
Society
for Clinical
Nutrition
651S
Downloaded from www.ajcn.org at TS/Medicina/SBA 2 on February 15, 2007
fibers,
ripeness
other
environmentally
plant fats and
direct
significant
R Day
ABSTRACT
1994,
of plants:
6525
DAY
integrated
increased
in the host cell genome.
The uptake of DNA can be
by treatment
with polyethylene
glycol or by electrical
disruption
of the
form
new
cell
with
culture
protoplast
walls
membrane.
as the osmoticum
medium.
Embryogenic
the resulting
unorganized
The biolistic,
or gene
coated
metal
erated
toward
penetrate
particles
plant
must
is gradually
cell
lines
replaced
selected
from
callus that is formed.
gun, method
uses very
small
DNA-
are rapidly
accel-
or tungsten)
of plant
cell
protoplasts
are
(gold
a target
the
The
walls
that
cells
and
or tissues.
some
enter
The
nuclei.
to recover
putative
development
increased
and diseases.
use
Attempts
In
transformants.
toxin
TRANSFORMATION
of food-crop
be achieved
costs
of raw
cessing
by strain
materials,
costs,
tolerance
quality
and
on the mechanisms
isolation
of
genes
that
as herbicide
may reduce
both
in raw materials.
Novel
materials
can be intro-
desirable
qualities
and very
on
such
a wide
target
as a short
St Louis)
land
an alternative
free
from
acid
acetohydroxyacid
in the
are
not
into
with
environment
organisms,
The
Roundup
the enzyme
5-enolwhich
is involved
in
acids in the chloroplast.
was available,
Roundup
growth
or to prepare
Until
was
soil
to chemical
tests
imidazolinone
biosynthesis
of leucine
screening
in plants
(AHAS),
and valine.
for
which
Mutations
only
a loss
larva
loses
fluids
been
identified
for
and
that
of insects.
are
The
larvae,
interfere
prokaryote,
the
expression
resistance
to
PPT
is responsible
for
at two different
(4).
An
forms
alternative
method
of creating
is to select herbicide-tolerant
(5).
are
in
Herbicide-tolerant
mutants
were used
Pursuit
(American
resistant
transgenic
herbicide-tolerant
crop
mutants
from plant cell
corn
to make
Cyanamid
(6).
(7). The
proteins
against
active
resting
intestinal
cavity
specifically
have
different
against
cells
enthe
insect,
of the
endotoxin
protein,
a delta
many
lepi-
insecticide
BT.
containing
the cryl
codon
usage
had
to be
adjusted
to maximize
in a eukaryote.
Transgenic
plants
tices
and
such
that
as using
reducing
express
numbers
of
remain
The
several
the
level
plants
Bt genes
may
different
have
carrying
resistant
control
of expression
a short
be
control
collection
to the
life
larvae
and
interfering
insect
press
was
with
larvae
this
hibitors
introduced
feeding
gene
and
on
show
other
food
and feed
Virus
resistance
the
that
attack
stored
plants
proteins
high
plants
resistance.
inhibitors
are
(9).
By
ingested
transgenic
moderately
protease
germplasm
that
the
is to
inhibitor.
The gene that
cloned
from the cowpea
plant
plants,
or
if BT
transgenic
of the
gene
necessary
of moths
into
the digestion
together
Bt
agent.
of cowpea
grain
were found
to contain
a trypsin
encodes
this inhibitor
was isolated
and
accession
methods
of the
it may
a useful biological
seeds of an exotic
by
that
ex-
Trypsin
commonly
in-
found
in
crops.
plant
sucking
viruses
plant
protein
sap.
gene
are transmitted
In
1986
for the RNA
resistant
(10).
other
plant
virus
resistance
the host
This
The
lines
recovered
from
F1 hybrids
tolerant
to
Co, Princeton,
NJ)
chini
yellow,
it was
virus
plant
their
is uncommon,
first
that causes
has
and
commercial
coat
and
watermelon
pares
favorably
with
commonly
consumed
fected with virus.
the
fruit
the discovery
much
discovery
mosaic,
in squash
is
zuc(12).
viral coat proteins
that account
of the squash
fruit. This com-
somewhat
from
in
occurring
showed
of this
viruses
the
the plant
observed
of cucumber
mosaic
The engineered
squash contains
for <0.1%
of the total protein
been
was
with
making
also
by
coat
mosaic
(I 1). Naturally
genes
the
interfered
application
protein
feed
when
tobacco
of the virus,
hosts
that
that
its expression
spread
and
by insects
reported
phenomenon
viruses
the use of viral
plants
tissue
cultures.
tissue culture
the herbicide
herbicides
year
in
break-
in agriculture
because
of the strong
selection
pressure
they
exert for insects that are insensitive
to the endotoxin
protein.
Resistance
to BT was reported
in diamondback
moths
after
extensive
use of this insecticide
(8). Special management
prac-
Many
the enzyme
that
already
and
protein
were shown
to be safe and effective
in large-scale
applications
over many years. Expressed
in transgenic
crops,
the Bt gene that encodes
the cryl protein
is effective
in controlling the larvae of hornworm,
cabbage
looper, diamondback
moth, and leaf miner.
Because
the toxin gene came from a
promise.
conferring
forms
active
of B. thuringiensis
thus
enzyme
Several
cryl
and systemic
create
past
was first used as the biological
replication
gene
the
into the intestinal
dies.
to both types of herbicides
(3). Phosphinothricin
(PPT) inhibits
glutamine
synthetase
in plants. The bar gene, from Streptomyces hygroscopicus,
encodes
an enzyme,
phosphinothricin
acetyltransferase,
that converts
PPT to a nontoxic
derivative,
AHAS
genes
difficult
semipermeability
leaks
into
in the
within
of the
introduced
sites
the resistance
has
identifying
herbicides
by inhibiting
occurred
induces
is unusually
especially
herbicide
inhibits
synthase,
plant
synthase
the synthesis
that
It is rapidly
decomposed
in the soil and thus
toxicity.
Genes for herbicide
tolerance
pro-
selective
herbicides
(2).
The sulfonyl
urea and
amino
led to the
specific
genes
herbicides
half-life
of crops.
the synthesis
of aromatic
amino
a gene for glyphosate
tolerance
to keep
enzymes
to nontarget
range
(glyphosate;
Monsanto,
pyruvyl-shikimate-3-phosphate
crop cultivation.
has little residual
toxicity
Incorporating
these
to use nonselective
low or no toxicity
mammals,
of herbicide
encode
inhibited
by the agent.
crops
makes
it possible
with
pro-
tolerance
Research
vide
the
reduce
such
and diseases
agrochemical
residues
that add value
to raw
reduce
can
characteristics
to pests
benefits
yields
by transformation.
Herbicide
used
several
High
improvements
agronomic
and resistance
farm costs
characteristics
duced
and
improvement
modification.
to pests
resistance
greater
plants
that
virus
content
are naturally
of
in-
Downloaded from www.ajcn.org at TS/Medicina/SBA 2 on February 15, 2007
In the context
may
tool
for
to be unexpectedly
wall so that hemolymph
Suspensions
PLANT
as a breeding
genes
to clone
proved
have
dopteran
OF
in identifying
by breeders
groups
USES
of transformation
interest
The soil bacterium
Bacillus
thuringiensis
forms
dotoxin
protein.
Ingested
by the larva of a sensitive
a small proportion
of these cells the introduced
DNA becomes
integrated
in a host chromosome.
A selectable
marker
is
needed
resistance
The
throughs
particles
the cell
Pest
GENETIC
MODIFICATION
Other forms of virus resistance
include viral genes that when
expressed
in plants interfere
with replication
of the viral genome or impede
virus movement
from cell to cell (13, 14). The
pokeweed
plant produces
several ribosornal
inhibitor
proteins.
One
of
against
these
expressed
a broad
range
Resistance
to pathogenic
Many
fungal
Novel
transgenic
escens
(16).
were
in
transgemc
of viruses
plants
protects
genes
have
been
reported
to show
regions
of DNA
with
repeat
chitin.
Phytase
pathogens
structural
lysine
encoding
on pesticides.
APPLICATIONS
ripening
The first food product
from this recombinant
DNA
technology, genetically
engineered
chymosin
(rennin),
is widely used
in Europe
and the United
States for cheese
making.
The first
genetically
engineered
food
plant
to be introduced
in the
United States, the Flavr Savr tomato (Calgene,
Davis, CA), was
released
early
that carries
in 1994.
These
an antisense
polygalacturonase
form
(PG).
tomatoes
incorporate
of the tomato
Produced
during
a Ti vector
gene
that encodes
fruit
ripening,
this
enzyme
breaks down pectin, the cement
that holds the cells of
the fruit tissue
together,
and is responsible
for fruit softening.
The PG antisense
gene reduces
the rnRNA
transcript
formed
and hence the amount of PG. The result is a fruit with a longer
shelf life at full ripeness.
Because
transformation
is inefficient
and very few treated
cells are transformed,
it is necessary
to
have a marker to select cells that carry integrated
and expressed
foreign
DNA.
neomycin
The
Ti vector
thus
phosphotransferase
the antibiotic
kanamycin.
mato cells. The kanamycin
stitutive
promoter,
is expressed
only
is of no
carries
for
consumers
that
confers
Kanamycin
resistance
Food
and Drug
of the neomycin
significance
a gene
H, which
encodes
resistance
to
kills untransformed
togene, driven
by a con-
is always
expressed.
after flowering
and
development.
The
that the expression
The PG
fertilization,
antisense
alternative
means
derstood.
Cosuppression
tomatoes,
a process
ethylene.
codes
those,
gene
Administration
concluded
phosphotransferase
of the
II gene
fruit.
of delaying
is also
that
being
fruit
used
is regulated
by
to delay
the
ripening
production
in
of
In this case, additional
copies
of the gene that enthe enzyme
1-aminocyclopropane1-carboxylate
synwhich is important
in ethylene
synthesis,
blocks ethylene
production
by
also
be used
to control
cosuppression.
The
produce
production
engineering
promises
improvements
in
the
nutri-
(19).
Phytase-expressing
plants
were
normal
in growth
and development
and their seeds were enriched
in phytase.
Seeds from transgemc
tobacco
were tested
in feeding
trials
with male broiler
chicks.
This supplemented
diet was cornpared
with diets supplemented
with and without
inorganic
phosphate
and As. niger
phytase.
Growth
in animals
fed diets
containing
milled
transgenic
phytase-expressing
seeds
was
sig-
nificantly
greater than that in animals
fed control seeds or diets
without
supplementation.
The production
of transgenic
oilseed
rape and soybean
that constitutively
overexpress
phytase
in
their
seeds
is now
under
way.
Other
work
phytase
added to soil increases
the availability
has
shown
that
of phosphorus
to
findings
suggest
that the expression
of
of transgenic
plants may have a similar
plant roots (20). These
the enzyme
in the roots
effect.
Reducing
phytate
in soybean
and other vegetable
products
may play a role in human
nutrition.
Phytate
is an effective
chelator
of divalent
cations,
such as iron and zinc. Consuming
protein
sources
that contain
relatively
high concentrations
of
phytate
results
suming
vegetable
promotes
in decreased
proteins
mineral
mineral
with
low
bioavailability,
concentrations
and
con-
of phytate
absorption.
late in plant
ripening
is by cosuppression.
The addition
of sense copies of a gene by transformarion
often results
in the failure
of the resident
gene to be
expressed.
The mechanism
of cosuppression
is not fully unAn
can
tional value of livestock
feed. Phytate
(myoinositol
hexaphosphate)
is the principal
storage
form of phosphorus
in plant
seeds.
Inorganic
phosphate
is released
from phytate
by an
endogenous
enzyme,
phytase,
at seed germination.
However,
when plant seeds are fed to monogastric
animals,
such as pigs
and poultry,
these animals
cannot use phytate
and thus cannot
access the phosphorus.
After the oil has been extracted
from
oilseed rape (canola)
in Europe
the meal is used as animal
feed
with inorganic
phosphate
added
as a supplement.
Phytase,
prepared
from
Aspergillus
niger,
can replace
the phosphate
supplement
(18). A gene from As. niger
that encodes
the
enzyme
was used to transform
tobacco
with an agrobacterium
vector
Controlled
or both
industry
triggers
the ripening
of fruit that are harvested
before
ripe by treating
them with ethylene
gas in storage.
routinely
they
Phytochelatins
Another
in human
class of plant compounds
with potential
importance
nutrition
is the phytochelatins,
a class of inducible
peptides
that bind metals
and may determine
the availability
from plant foods of trace metals such as zinc and copper
that
are important
in human nutrition.
However,
phytochelatins
are
also active in the uptake of toxic heavy metals. Phytochelatins
contain
four to six repeated
units of -y-glutamyl
cystine.
Recent
research
has shown
that cadmium
sensitivity
in Arabidopsis
thaliana
is caused
by mutations
that reduce
the synthesis
of
glutathione,
This
are
suggests
might
provide
in food plants,
the
substrate
for
phytochelatin
biosynthesis
(21).
that selection
for greater
cadmium
sensitivity
a method
for reducing
unwanted
phytochelatin
thereby
reducing
their heavy metal content.
Downloaded from www.ajcn.org at TS/Medicina/SBA 2 on February 15, 2007
FOOD
cosuppression,
Genetic
motifs-suggests
that they have a role in signaling
biochemical
defense
responses.
An earlier example
was of a corn gene that
appears
to encode the structure
of a membrane
receptor
protein
to which a fungal pathotoxin
binds (17). General
methods
for
recovering
and redeploying
such genes will help to reduce our
dependence
Antisense,
the amounts
of other materials
that plants produce.
The candidates include
raffinose
and stachyose,
oligosaccharides
in legume seeds (beans
and soybean)
that are responsible
for flatulence.
Naturally
occurring
variants
of the bean (Phaseolus
vulgaris)
with low concentrations
of stachyose
have been used
to produce
bean cultivars
that cause less flatulence.
marc-
resistance
Four genes for resistance
to different
plant
recenfly
cloned
from higher
plants (6). Their
had
653S
PLANTS
fungi
pathogens
have cell walls made of
plants that express
bean or Serratia
chitinase
similarity-all
them
(15).
OF
654S
DAY
Expression
of antigens
is excessive.
The ability of gut mucosal
cells to respond
to antigens
(22)
has prompted
attempts
to create inexpensive
oral vaccines
by
expressing
antigens
in transgenic
food plants.
Mason et al (23)
showed
that tobacco
that was transformed
with a gene encoding
hepatitis
B surface
antigen
expressed
this
protein
in
its
leaves in a form that was antigenically
similar to that in human
serum. Because
so much of our food is cooked
and because
heating
may degrade
the antigenic
proteins,
this approach
will
have limitations
for human
vaccination.
There
is also the
prospect
of engineering
forage
for livestock
so that
induced
to include antibodies
that are effective
of human infants, but, again, heating to sterilize
milk may render the antibodies
ineffective.
Edible
plant
Vegetable
cow
milk
consist
largely
of
is a target
for
further
triacylglycerols
reduction
acid
synthesis
in
which
either
the
and
their
by molecular
biology.
is palmitic
acid (16:0).
a branch point in fatty
precursor
palmitoyl-acyl
protein
is elongated
to form stearic
acid or the acyl
carrier protein is released
to form palmitic
acid. A reduction
in
the palmitic
acid content
was achieved
by overexpressing
the
enzyme
that is responsible
for elongation.
Another
successful
approach
was to use cosuppression
to reduce the enzyme
that
releases
palmitic
acid.
carrier
Hypoallergenic
rice
Rice grains contain a 16-kDa globulin
protein
that is heat-stable
and that resists proteolytic
enzymes
in the human gut. This protein
is the cause
The allergen
of an ectopic
can
dermatitis
be destroyed
in sensitive
by enzyme
in which
the amount
of
encodes
it sequenced
(27)
so it is theoretically
transgenic
rice with an antisense
that would be hypoallergenic.
possible
to produce
gene or a cosuppressing
construct
Seed
proteins
and canola seed storage
proteins,
although
imporin human and livestock
nutrition,
could be of more value
if their balance of amino acids were improved
by increasing
the
methionine
content.
Altenbach
et al (28) used a gene from
Soybean
Japanese
treatment,
children.
but the cost
Brazil
nut
that
encodes
a 25 storage
protein
with
a methionine
content of 18% and introduced
it by transformation
into canola.
It increased
the methionine
content of the seed meal by >30%.
The same gene expressed
in the seeds of transgenic
soybean
also increased
their nutritional
quality but, unfortunately,
made
them allergenic
to people who are sensitive
to Brazil nuts (29).
Evidently,
the 2S protein
is a major allergen
in Brazil nuts. In
view of this, it seems unlikely
that the transgenic
soybean
will
be commercialized
in its present
form.
Much attention
is now paid in wheat breeding
to the genes
that encode high-molecular
weight glutens
and gliadins
in the
grain endosperm.
Some of these proteins
are responsible
for the
viscoelastic
properties
of doughs
for making
bread.
Some
progress
has been made in cloning
and expressing
genes for
low-molecular
weight wheat storage
proteins
in tobacco
seed
(30). Engineering
high-molecular
weight proteins
in wheat has
yet to be achieved
but once done offers prospects
of improving
the flour of other cereals,
such as sorghum
and millet, that are
important
in some developing
countries.
A CRUCIFEROUS
WEED
AS A CROP
PLANT
ANALOGUE
Many of the spectacular
advances
made in molecular
biology
in the past 25 y have depended
on the availability
of a wide
variety
of model organisms,
such as Escherichia
coli, yeast,
mice, Caenorhabditis
elegans
(a nonparasitic
nematode),
and
corn.
During
this time there has been growing
interest
in the
small
cruciferous
weed
Ar.
thaliana.
number
of four chromosomes
l0 nucleotide
bases. Because
content
of crop plants, which
is a concerted
plant.
Ar.
cycle
from
effort
seed
several
hundred
grown
in growth
32).
It is now
to
has
thaliana
to
sequence
other
seed
seeds,
chambers
an important
This
plant
has
a haploid
and a DNA content
of “‘0. 1 X
this is much lower than the DNA
is up to 100 times greater,
there
the
entire
advantages:
in
and
6
wk,
large
genome
it completes
each
flower
of
produces
plant
populations
or greenhouses
in little
space
in plant
molecular
tool
for research
this
its life
can
be
(31,
biology.
Genes of interest
may be more readily
identified
and
cloned from Ar. thaliana
than from a crop plant. These genes
may also be altered or their expression
adjusted
before they are
used
in an economically
crop plant analogue
that
ogists to try out systems
important
plant.
Ar.
thaliana
is thus
makes it possible
for molecular
biolthat can later be applied
to economic
a
Downloaded from www.ajcn.org at TS/Medicina/SBA 2 on February 15, 2007
contain
mutants
tant
compositions
vary
considerably
among
genera
and species.
Plant breeding
has exploited
this variation
to produce
a variety
of improved
forms. The composition
of canola oil was changed
by breeding
and selection
to reduce the high amounts
of erucic
acid (22: 1) that characterized
early varieties.
At the same time,
glucosinolates
(goitrogenic
glycosides)
were removed
from
canola
seed
so that the meal left after oil extraction
in a
crushing
mill has greater
value as livestock
feed. In recent
reviews
Somerville
(24) and Ohlrogge
(25) described
increasing the value of plant edible oils in human nutrition
by using
cloned genes to alter the activities
of specific
enzymes
responsible for lipid and fatty acid biosynthesis.
The most common
plant fatty acids have chain lengths
of 16 or 18 carbon atoms
and from 1 to 3 double
bonds.
They are liquid at room ternperature,
and for making
margarine,
in which a higher melting
temperature
is required,
the double bonds must be reduced
by
hydrogenation.
However,
this not only increases
the saturated
fat content
of the oil but also converts
many of the double
bonds from the cis to the less desirable
trans
configuration.
Because
the enzyme
steraoyl-acyl
carrier
protein
desaturase
introduces
the first double
bond,
it has been a target
for
antisense
RNA. This approach
was successful
in two species of
Brassica
(B. napus
and B. rapa) in which the content of stearic
acid, which has no double
bonds (18:0),
in the seed oil was
increased
from 2% to 40%.
Although
vegetable
oils have less saturated
fatty acid than do
animal
fats, the 10-20%
(depending
on species)
that they do
The major saturated
fatty acid in plants
One approach depends on manipulating
recovered
protein was reduced after the rice seeds had been treated
with a chemical
mutagen
(26). In one mutant with good agronomic qualities
the allergenic
protein
content
was reduced
by
50%.
Two other mutants with trace amounts of the allergen
were largely sterile and were of little use in breeding
and seed
production.
The allergen
has been characterized
and the gene that
is
against diseases
or process such
oils
oils
Scientists
allergenic
GENETIC
plants (33). One of the first examples
isolation
of the acetolactate
synthase
followed
by the recovery
of mutant
MODIFICATION
of such a use was the
gene from Ar. thaliana
forms
that
are insensitive
to
the herbicide
imidazolinone.
The resultant
gene for tolerance
could then be used to confer tolerance
to unrelated
crops after
it had been introduced
by transformation.
CONCLUSIONS
Biotechnology
is opening
up many
new
opportunities
for crop-
New
products
mentally
include
crop
plants
with
tolerance
to environ-
benign
herbicides
and resistance
to insect pests and
viruses,
fruit
with delayed
ripening,
and oil seeds with improvements
in the balance
of fatty acids. Other possibilities
include
food plants
resistant
to fungal
pathogens,
foods that
express
antigens
that can immunize
consumers
through
the
gastrointestinal
press
phytase
tract,
before
hypoallergenic
germination
available
to monogastric
be
to ensure
taken
animals
that
no
rice, and seeds
that exto make
stored
phosphate
that are fed them.
allergenic
proteins
are
Care
must
introduced
inadvertently
by engineering.
Some of these developments
owe
much
to advances
made in studies
of the small
weed Ar.
thaliana,
which has promise
as a crop plant analogue.
New
crops
that
will encounter
try
make
claims
regulatory
is unwilling
barriers
to pay
for
proved
if there
are cheaper,
quality
of feed
stocks.
icals
and
engineered
to enhance
raw
costs
pharmaceuticals
crops to produce
and
nutrition
to commercialization.
materials
alternative
The
health
that
means
of high-value
will likely
them.
be
have
Indusbeen
im-
of adjusting
specialty
reduced
the
chem-
by
using
A
655S
PLANTS
7. Gill SS, Cowles
EA, Pietrantonio PV. The mode of action of Bacillus
endotoxins. Annu Rev Entomol l992;37:615-36.
thuringiensis
8. Hama H, Suzuki K, Tanaka H. Inheritance
and stability
of resistance
to Bacillus
thuringiensis
formulations
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Plutella
xylostella
(Linnaeus)
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Appl
Entomol Zool 1992;27:355-62.
9. Hilder VA, Gatehouse
AMR, Sheerman
SE, Barker RF, Boulter D. A
novel mechanism
of insect resistance
engineered
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Nature
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10. Abel PP, Nelson RS, Dc B, et al. Delay of disease development
in
transgenic
plants that express the tobacco mosaic virus coat protein
gene. Science 1986;232:738.-43.
1 1. Turner NE, O’Connell
KM. Nelson RS, et al. Expression
of alfalfa
mosaic virus coat protein gene confers cross-protection
in transgemc
tobacco and tomato plants. EMBO J 1987;6: I 181-8.
12.
Tricoli
DM,
Camey
K!,
resistance
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of American
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for their helpful discussions.
Wyeth-Ayerst,
and Jeanne
Hoskin,
of
abstracts
of the
Virology.
13th
WI:
Lodge
JK,
Kaniewski
tochemical
17.
Briggs
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Iowa
State
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3. Hattori J, Rutledge
R, Labb#{233}
H, Brown D, Sunohara
G, Miki B.
Multiple resistance
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the
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mosaic
program
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Society
Turner
of
chitin
Identification
Findenegg
GR,
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and
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Broad-spectrum
breakdown.
and
(N Y)
Nelernans
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1994:250
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1993;l 1:811-4.
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glutathione-deficient
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position
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pokeweed
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about the safety of
food products
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however,
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the products
of genetic engineering
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on the part of farmers
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