Genetic Engineering and Sustainable Agriculture Field of Dreams or...

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Focus: Endgame
16
Harvard Science Review


spring 2007
Genetic Engineering and
Sustainable Agriculture
By Megan Bartlett
Field of Dreams or...
E
ven in modern times, when most
of the population in the wealthi
-
est nations is uninvolved in agriculture,
and starvation is a very remote pos
-
sibility, the size of the world’s popula
-
tion ensures that our urgent need for
high agricultural productivity is still
present. This continued dependence
on agriculture requires us to examine
how successful our current agricultural
methods are, and how well they can be
sustained and expanded to meet the
demands of a growing population in
the indefinite future. While our agri
-
cultural system is currently capable of
producing enough food to support our
population, it consumes a great deal
of water, fertilizer, pesticide and land,
which has been harmful both for the
environment and for the workers who
are exposed to agricultural chemicals,
and will become exceedingly expensive
as the demand for crop yield increases.
One solution to these problems utilizes
the developing technology of genetic
engineering, which has so far allowed
researchers to alter crops so that they
require less of these resources while
producing more nutrients for their
to eat and cheaper to raise.
One of the most important aspects
of crop nutrition is protein content
because protein is a difficult nutrient
to obtain, especially for people who
cannot afford meat. Some crop variet
-
ies may be too susceptible to disease or
may grow too slowly to be economi
-
cally viable, but they do produce seeds
with large amounts of protein or fat.
The protein- or oil- producing genes
from these crops can be identified by
sequencing their DNA and profiling
their gene expression patterns to see
what distinguishes them from normal
varieties (2). Then, these genes can be
transferred into embryonic plants of
another variety, where they will increase
protein or oil formation but still have
the growth characteristics of the origi
-
nal variety. These methods have been
used to produce soybean and corn
crops with high levels of the amino
acids serine and lysine and the fatty oils
oleic acid, vernolic acid, and ricinoleic
acid, which have either industrial uses
or nutritional benefits (2). These genetic
manipulations have been so successful
that the DuPont Corporation has sev
-
Frankenfoods?
consumers. By modifying the natural
traits of crop plants and compensating
for their genetic weaknesses, genetic
engineering has the potential to make
conventional agriculture considerably
more sustainable in the future.
Making vegetables even better
for you
One very serious problem with con
-
ventional agriculture is that meeting
the demand for high plant growth
requires extremely heavy use of fertil
-
izer, amounting to 80 million tons per
year. However, only about a third of the
fertilizer applied is actually absorbed by
the crops (1). The rest is leached into
the environment, where it induces algal
blooms, contaminates drinking water
and depletes aquatic oxygen to cre
-
ate dead zones like those now found
in the Gulf of Mexico. Fertilizer is
also expensive to produce and often
too costly for farmers in developing
countries to buy. An ideal crop would
accumulate nutrients so efficiently that
it would require less fertilizer and, at the
same time, contain more nutrients than
normal crops, making it both healthier

By modifying the natural traits of
crop plants and compensating for their
genetic weaknesses, genetic engineering
has the potential to make conventional
agriculture considerably more sustainable
in the future.”
credit: Heselmans, M.
Nature Biotechnology
19 (2001): 700-1.
Focus: Endgame
spring 2007



Harvard Science Review
17
Parched plants
Water usage is currently one of the
greatest constraints on increasing ag
-
ricultural productivity. The demand
for high crop yield requires
that 70% of the world’s water
supply be used for irrigation,
which is an enormous amount
of a resource already made
scarce by urban and industrial
consumption and pollution
(4). This scarcity is expected to
worsen as the growing impact
of climate change significantly
alters precipitation patterns, produc
-
ing drought in some currently thriving
agricultural areas. However, even if
water scarcity were not a concern, the
process of irrigation would still pose
a problem for agriculture because the
water used for irrigation contains salt,
metals, and minerals that are left behind
in the soil when the water is consumed
or evaporated. As a result, irrigated soil
becomes too salty over time to sup
-
port agriculture, eventually destroying
arable land.
To address these problems, it is neces
-
sary to make plants less dependent on
irrigation and more tolerant of salty
soil. Changing the tolerance thresholds
of crop plants is achieved through
genetic engineering, which alters gene
expression or introduces new genes to
make plants more tolerant of drought
or salinity. To counter drought, the
Frankenfoods?
Figure 1. Several ideas have been
proposed to combat anthrax on a
cellular level. These include: A)
Competing molecules occupy the
heptamer pore, preventing ede
-
ma factor (EF) and lethal factor
(LF) from entering the host cell.
B) Competing receptor molecules
bind protective antigen (PA), be
-
fore PA can bind to a membrane
receptor and form the heptamer.
C) Replace parts of the heptamer
with a close analog of PA. This
complex will be unable to release
EF and LF into the cytosol.
credit: Zhang, J., et. al. “From Laboratory to Field. Using Information from Arabidopsis to Engineer Salt, Cold, and Drought Tolerance in Crops.”
Plant Physiology
135 (2004): 615-621.
Figure 1. Drought tolerant
Arabidopsis
plants overcome a week-long water deprivation, while
wild-type plants die.
eral strains in various stages of testing
or commercialization.
To improve their mineral nutritional
value, plants can be engineered to in
-
crease the efficiency with which they
absorb and use such nutrients. Re
-
search in this area has not progressed
as quickly as in drought and salt toler
-
ance because nutrient uptake pathways
are complicated and poorly under
-
stood. However, studying mutant plant
strains that have nutrient deficiencies
has enabled researchers to identify
many nutrient transporter genes. Some
of these genes, which transport ions
into the plant, can be overexpressed to
increase uptake of such ions as nitrate,
potassium, organic phosphate and iron
(1). Others, which control the efflux of
ions from the cell, can be engineered to
accumulate nutrients instead of leach
-
ing them, and have been identified for
nitrogen and sulfur pathways. A less
well-understood mechanism involves
changing the extent and shape of root
growth. Engineering plants to have the
ability to increase root growth in nutri
-
ent rich soil would increase nutrient
uptake into the rest of the plant, even
though the exact proteins involved in
nutrient-specific transport and uptake
mechanisms may be unknown (3).
Although these experiments are still
hypothetical, they provide a starting
point for further research.
most successful studies have engineered
plants to produce excess CBF (C-repeat
binding factor) protein, which binds
to the C-repeat region in the promoter
DNA of drought-tolerance genes and
increases their expression (5) (Figure
1). Arabidopsis plants showing this
overexpression pattern both survived
and grew during a 1-week period of
drought, while unmodified plants died.
Analogs of this protein are found in
most crop plants, and they have been
successfully modified to induce drought
resistance in numerous species, includ
-
ing strawberries, tomatoes, maize,
wheat, and rice (6). Other studies have
targeted the overexpression of genes
whose protein products control detoxi
-
fication of free radicals (such as man
-
nitol, which eliminates free hydroxyl
radicals), osmosis (such as trehalose,
which stabilizes membranes), signal
-
ing (such as NtC7, which signals genes
that regulate osmosis), transport (such
as aquaporins, which transport water),
and plant energy status (such as PEPC,
or phosphoenolpyruvate carboxylase,
which aids in sugar production) (4).
Salinity tolerance can be conferred
by altering the expression of salt trans
-
porter genes in plant cells. One pos
-
sible methods is overexpressing Na+
antiporters, which transport sodium
ions out of the cells, or tonoplast
Na+ importers, which take sodium
ions out of the cell and store them in
the vacuole; another method involves
decreasing the expression of plasma
membrane Na + importers, which
bring sodium ions into the cell. The
most successful strategy in several spe
-
cies has been the overexpression of the
antiporter genes AtNHX1 and SOS1,
The demand for high crop yield
requires that 70% of the world’s
water supply be used for irrigation,
which is an enormous amount of a
resource already made scarce by
urban and industrial consumption
and pollution
Focus: Endgame
18
Harvard Science Review


spring 2007
which are responsible for removing

excess sodium from the cells and al
-
lowing the plants to grow in highly
saline conditions (6). Although these
engineered plants are not yet ready
for large-scale commercial produc
-
tion, their success and the insight they
provide into how plants tolerate water
stress indicate that genetic engineering
is a potent strategy for improving the
efficiency of agricultural water use.

Poisoning the pests
A third problem with conventional agri
-
culture is the huge amount of pesticide
it consumes – up to 2.5 million tons per
year worldwide. These toxins can leach
into the environment, killing fish and
amphibians, and inflicting cancer and
birth defects on agricultural workers
who cannot protect themselves from
overexposure. Furthermore, excessive
exposure to these toxins can spawn pes
-
ticide-resistant insect strains, and create
a biotechnological arms race resembling
that between antibiotics and resistant
bacteria (7). However, avoiding the use
of pesticides can have equally devastat
-
ing consequences – 37% of all crops
produced each year are lost to pests or
disease, with insects devouring as much
as 13% of our food supply (8).
A promising solu
-
tion to the problem of
protecting plants from
insects without the use
of harsh chemicals is
to genetically engineer
the plants to express
their own pesticides, which kill insects
without affecting animals or people.
Although this has sparked an enormous
amount of controversy, especially from
consumers who question the harmless
-
ness of these plant-produced insecti
-
cides, research in this area is sufficiently
advanced that several insect-resistant
crop varieties have reached commercial
production.
The first successful gene transfer to
produce insect resistance in plants used
bacteria to carry the gene for the pro
-
tease inhibitor CpTI from resistant to
wild-type cow
-
pea plants. This
inhibitor binds
to insect diges
-
tive proteases
and prevents
t h e m f r o m
breaking down
plant material,
killing them but
leaving mam
-
mals unaffect
-
ed. Such genes
encoding pro
-
tease inhibitors
have been intro
-
duced into valu
-
able crop plants
such as tobacco, potato, rice and straw
-
berries. These methods have been ex
-
panded to engineer plants that express
chitinases, which dissolve the cell walls
of fungi, as well as a-amylase inhibitors,
which interfere with lectins, another in
-
sect digestive enzyme, thereby blocking
insect protein translation (8).
While these methods transfer exist
-
ing genes between species, a technology
known as DNA shuffling can actually
direct and speed up the evolution of
an existing gene in order to change its
virulence, protein chemical proper
-
ties or target spe
-
cies range. In DNA
shuffling, a parental
gene sequence is
copied using DNA
polymerase for a
very short reaction
time, which creates many small frag
-
ments of the copied gene. These
small fragments are allowed to join in
a random order, and their sequence is
expressed in an organism vector. Those
that grow best have their DNA chosen
for further rounds of shuffling, and the
process is repeated until a sequence with
the desired properties is produced. This
method has been used to increase BT
toxin expression in corn to 3.8 times its
normal potency and widen its range of
target insects to include an entirely new
genus of pests (9). A second possible
approach ignores the plants entirely,
and instead engineers natural insect
pathogens to be more virulent. The
bacterium Bacillus thuringiensis, which
actually produces the BT insect toxin, is
an extremely efficient insecticide when
modified to overproduce BT and ap
-
plied to plants (10). These experiments
have been successful and have been
used with other toxins and bacteria, but
require more health and safety testing
to ensure that these increased quantities
will be safe for humans before they can
be commercialized.
Research on herbicide resistance has
been concerned primarily with resis
-
tance to glyphosate, which is the active
ingredient in the herbicide Round-Up.
Glyphosate functions by inhibiting the
activity of the enzyme EPSPS, which
is necessary to make certain kinds of
amino acids. To overcome this inhibi
-
tory effect, plants have been modified
to express a bacterial version of EPSPS,
known as CP4, which contains a muta
-
tion that prevents it from binding with
glyphosate but does not interfere with
its metabolic function. This modifica
-
tion allows fields to be sprayed with
glyphosate without causing crop dam
-
age, thereby eliminating invasive weeds
without a drop in productivity (1l). Al
-
though this feat of genetic engineering
has been extremely successful, there is
considerable debate over how safe this
Meet the Marburg virus . . .
37% of all crops produced
each year are lost to pests
or disease, with insects
devouring as much as 13%
of our food supply
Figure 2. The caterpillar and mature moth of the maize pest Cotesia mar
-
giniventris. Genetic engineering has elucidated the plant compounds that
are involved in preventing damage by these pests.
credit: Lincoln, T. “Chemical ecology: in defense of maize.”
Nature
439 (2006): 278.
Focus: Endgame
spring 2007



Harvard Science Review
19
of people who can afford to eat meat
(which requires 10 times more energy
to produce than vegetables and grains),
and, potentially, biofuel production.
Although biofuel is an infant industry,
fulfilling its potential to replace oil in
the US would require a doubling of
current US agricultural productivity, an
expansion on a scale that we currently
do not know how to achieve.
Genetic engineering strategies have
been elucidated that enhance natural
plant resistance and stress tolerance
mechanisms to make agriculture more
efficient in its use of natural resources.
Our upcoming challenges are to con
-
tinue developing this technology to
minimize the environmental impact
of agriculture, and to ensure that the
system that controls such a vital com
-
modity is environmentally sustainable
and socially fair.
—Megan Bartlett ‘09 is an Organismic and
Evolutionary Biology concentrator in Lowell
House.
References
1. Hirsch, R. E. and Sussman, M. R. “Improving
nutrient capture from soil by the genetic manipu
-
lation of crop plants.”
Trends in Biotechnology
17
(1999): 356-1.
2. Mazur, B. et al. “Gene Discovery and Product
Development for Grain Quality Traits.”
Science
285
(1999): 372-6.
3. Hell, R. and Hillebrand, H. “Plant concepts for
mineral acquisition and allocation.”
Current Opinion
in Biotechnology
. 12 (2001): 161-8.
4. Chaves, M. and Oliveira, M. “Mechanisms underly
-
ing plant resilience to water deficits: prospects for
water-saving agriculture.”
Journal of Experimental
Botany
55 (2004): 2365-2384.
5. Shinozaki, K. and Yamaguchi-Shinozaki, K. “Gene
networks involved in drought stress response and
tolerance.”
Journal of Experimental Botany
58
(2007): 221-7.
6. Zhang, J., et. al. “From Laboratory to Field. Using
Information from Arabidopsis to Engineer Salt, Cold,
and Drought Tolerance in Crops.”
Plant Physiology
135 (2004): 615-621.
7. Pimentel, D., et. al. “Environmental and eco
-
nomic costs of pesticide use.”
BioScience
. 42 (1992):
750-60.
8.Gatehouse, A. M. and Gatehouse, J. “Identifying
proteins with insecticidal activity: use of encoding
genes to produce insect-resistant transgenic crops.”
Pesticide Science
52 (1998): 165-75.
9.Lassner, M. and Bedbrook, J. “Directed molecular
evolution in plant improvement.”
Current Opinion in
Plant Biology
. 4 (2001): 152-6.
10.Gerhardson, B. “Biological substitutes for pesti
-
cides.”
Trends in Biotechnology
. 20 (2002): 338-43.
11.Dill, G. “Glyphosate-resistance crops: history,
status, and future.”
Pest Management Science
. 61
(2005): 219-24.
modification really is – it is still not
known whether these genes can be
introduced into weed species through
the notably flexible hybridization abili
-
ties of plants, and how safe the altered
protein will be for human and animal
consumption. However, the speed with
which this GM strain has been incor
-
porated into the market suggests that
experience will answer these questions
before experimentation will; modified
soybean, for instance, now accounts for
85% of all soybean grown today (11).
Conclusions
Agricultural productivity is threatened
by a variety of problems. To reduce the
economic and environmental cost of
agriculture, it is vital to reduce the ag
-
ricultural consumption of water, fertil
-
izer, pesticide and herbicide. Although
we may not currently feel any sense of
urgency, agricultural productivity will
be taxed even further by the growing
population, the increasing number