GM CROPS AND THE ENVIRONMENT

The debate over the environmental impact of genetically modified (GM) crops is growing increasingly
complex, intense, and extremely emotional. It is further complicated as new research is published. Are GM
crops safe for the environment?
This Pocket K attempts to shed light on this issue by addressing basic questions regarding GM crops and the
environment.
Assessing the environmental impact of GM crops is often difficult as many factors
are considered. Some scientists focus on the potential risks of GM crops, while
others emphasize their potential benefits. Just what are the issues and how can we
address them?
What is the current environmental situation?
A growing population, global warming, and loss of biodiversity have a tremendous
impact on our environment.
By year 2050, there will be 9.3 billion people living on this planet. This means that
in less than 50 years, world population is expected to increase by 3 billion. Feeding
these people will mean massive changes in the production, distribution, and
stability of food products.
Unfortunately, cropland and population are not uniformly distributed. For example, China has only 1.4% of
the world’s productive land1 but 20-25% of the world’s population. This situation is further aggravated by
diminishing cropland due to erosion, fewer renewable resources, less water, and a reduced population
working the land.
The destruction of wilderness and forests and continued use of coal and oil have led to a steady increase in
carbon dioxide levels, resulting in global warming. It is predicted that the average global temperature will
rise by 1.4-5.8ºC by 2100, with increasing fluctuations in weather conditions. Climate change can radically
alter rainfall patterns and therefore require the migration of people and shifts in agricultural practices.

Further, an increasing human population is responsible for wilderness destruction, water quality problems,
and diversion of water. The loss of habitat has resulted in many species being displaced.
Thus, to conserve forests, habitats, and biodiversity, it is necessary to ensure that future food requirements
come only from cropland currently in use.
What are the environmental benefits of GM crops?
One of the significant environmental benefits of GM crops is the dramatic reduction in pesticide use, with
the size of the reduction varying between crops and introduced trait.
 A study assessing the global economic and environmental impacts of biotech crops for the first
fifteen years (1996-2010) of adoption showed that the technology has reduced pesticide spraying by
443 million kg and has reduced environmental footprint associated with pesticide use by 17.9%. The
technology has also significantly reduced the release of greenhouse gas emissions from agriculture
equivalent to removing nine million cars from the roads.2
 In the USA, adoption of GM crops resulted in pesticide use reduction of 46.4 million pounds in
2003.3
 The use of Bt cotton in China resulted in pesticide use reduction of 78,000 tons of formulated
pesticides in 2001. This corresponds to about a quarter of all the pesticides sprayed in China in the
 mid-1990s.4 Additionally, the use of Bt cotton can substantially reduce the risk and incidence of
pesticide poisonings to farmers.5
 Herbicide tolerant crops have facilitated the continued expansion of conservation tillage, especially
no-till cultivation system, in the USA. The adoption of conservation and no-till cultivation practices
saved nearly 1 billion tons of soil per year.6
 Biotech cotton has been documented to have a positive effect on the number and diversity of
beneficial insects in the US and Australian cotton fields.7
 Adoption of Bt corn in the Philippines did not show an indication that Bt corn had negative effect on
insect abundance and diversity.15
How are GM crops assessed for environmental safety?
GM crops are thoroughly evaluated for environmental effects before entering the marketplace. They are
assessed by many stakeholders in accordance with principles developed by environmental experts around
the world. 8,9,10 Among those who conduct risk assessment procedures are the developers of GM crops,
regulatory bodies, and academic scientists.
Most countries use similar risk assessment procedures in considering the interactions between a GM crop
and its environment. These include information about the role of the introduced gene, and the effect that it
brings into the recipient plant. Also addressed are specific questions about unintentional effects such as:
 impact on non-target organisms in the environment
 whether the modified crop might persist in the environment longer than usual or invade new habitats
 likelihood and consequences of a gene being transferred unintentionally from the modified crop to
other species

In addition to performing pre-commercialization tests for environmental safety, every GM crop should also
be subjected to post approval monitoring by the product developer, independent researchers, and
government scientists. This helps ensure that biotech crops continue to be safe for consumers and the
environment.17
What are the potential risks?
Potential of the introduced genes to outcross to weedy relatives as well as the potential to create weedy
species
Outcrossing is the unintentional breeding of a domestic crop with a related plant. A major environmental
concern associated with GM crops is their potential to create new weeds through outcrossing with wild
relatives, or simply by persisting in the wild themselves.
The potential for the above to happen can and is assessed prior to introduction and is monitored after the
crop is planted as well. A ten-year study initiated in 1990 demonstrated that there is no increased risk of
invasiveness or persistence in wild habitats for GM crops (oilseed rape, potatoes, corn, and sugar beet) and
traits (herbicide tolerance, insect protection) tested when compared to their unmodified counterparts. 11 The
researchers stated, however, that these results “do not mean that genetic modifications could not increase
weediness or invasiveness of crop plants, but they do indicate that productive crops are unlikely to survive
for long outside cultivation.” A recent study by De Nijs, et al (2004, in press) shows that only very limited
effects on the environment have been detected in relation to outcrossing. It is therefore important, however,
as regulations require, to evaluate individual GM crops on a case-by-case basis, both prior to release and
after commercialization.
Direct effects on non-target organisms
In May 1999, it was reported that pollen from Bacillus thuringiensis (Bt)-insect resistant corn had a negative
impact on Monarch butterfly larvae. This report raised concerns and questions about potential risks to
Monarchs and perhaps other non-target organisms. Some scientists, however, urged caution over the
interpretation of the study because it reflects a different situation than that in the environment. The author
indicated “Our study was conducted in the lab and, while it raises an important issue, it would be
inappropriate to draw any conclusions about the risk to Monarch populations in the field solely on these
initial results.” In 2001, a study published in PNAS concluded that the impact of Bt corn pollen on Monarch
butterfly populations is neglible. 14
A report from the US Environmental Protection Agency (EPA) indicated that the “data provide a weight of
evidence indicating no unreasonable adverse effects of Bt proteins expressed in plants to non-target
wildlife”. Furthermore, a collaborative research effort by North American scientists has concluded that in
most commercial hybrids, Bt expression in pollen is low, and laboratory and field studies show no acute
toxic effects at any pollen density that would be encountered in the field.12 A Nature publication of Losey,
1999; and lab experiments on force-fed predators (Hilbeck, et al, 1998; Hilbeck, et al, 1999) and extensive
field work demonstrated no significant impact on Monarch Butterfly populations (Fitt and Wilson, 2003;
Gatehouse, et al, 2002; Hansen and Obrycki, 2000; Hellmick, et al, 2001).16
Development of insect resistance
Another concern over the use of Bt crops is that it will lead to the development of insect resistance to Bt.
Insect resistance management plans have been developed by government, industry, and scientists to address
this issue. These plans include a requirement that every field of insect-resistant crops must have an
associated refuge of non-GM crops in order for the insects to develop without selection to the insect resistant
varieties.
Additional resistance management practices are also being developed by scientists all over the world. These
must be performed in line with post-approval monitoring, where GM crops, as well as their immediate
environment, will be constantly evaluated for changes even after the crop has been released.

Conclusion
The environmental and ecological concerns potentially associated with GM crops are evaluated prior to their
release. In addition, post-approval monitoring and good agricultural systems need to be in place to detect
and minimize potential risks, as well as to ensure that GM crops continue to be safe after their release.
Comparisons among GM, conventional, and other agricultural practices, such as organic farming, will bring
to light the relative risks and benefits of adopting GM crops.
BT INSECT RESISTANT TECHNOLOGY
Have you ever seen a leaf eaten off by plant pests? What about an entire harvest destroyed by insects? Plant
pests cause a lot of problems to farmers and home gardeners alike. Because of this, they have had very little
recourse other than to continually spray their plants with pesticides. Unfortunately, some of these pesticides
pose health risks to people who are exposed to them.

It is for this reason that scientists are constantly looking for alternative ways of dealing with plant pests.

The Bt Organism
Bt stands for Bacillus thuringiensis (Bt) a common soil bacterium so called because it was first isolated in
the Thuringia region of Germany.

Bt produces a protein that paralyzes the larvae of some harmful insects, including the cotton bollworm and
the Asian and European corn borers, all of which are common plant pests whose
infestations produce devastating effects on important crops.

Mode of Action

When ingested by the larva of the target insect, the Bt protein is activated in the gut’s
alkaline condition and punctures the mid-gut leaving the insect unable to eat. The insect
dies within a few days.

It is because of its ability to produce the insecticidal protein that much research is being USDA Photo
done to exploit the organism’s agronomic value. To date, there are more than 200 types of Corn borer
Bt proteins identified with varying degrees of toxicity to some insects.

Earlier Bt Technology

Bt is easily cultured by fermentation. Thus, over the last 40 years, Bthas been used as an insecticide by
farmers worldwide. Organic farming in particular has benefited from Bt insecticide, as it is one of the very
few pesticides permitted by organic standards. The insecticide is applied either as a spray, or as ground
applications. It comes in both granules and liquefied form.

The efficiency of both applications is quite limited, as target organisms often do not come in contact with
the insecticide as they are found on the underside of leaves or have already penetrated into the plant.
Scientists are working to overcome this problem through the use of modern biotechnology.

Modern Bt Technology

Scientists have taken the Bt gene responsible for the production of the insecticidal protein from the
bacterium and incorporated it into the genome of plants. Thus, these plants have a built-in mechanism of
protection against targeted pests. The protein produced by the plants does not get washed away, nor is it
destroyed by sunlight. The plant is thus protected from the bollworm or the corn borer round the clock
regardless of the situation.

Safety Aspects of Bt Technology

Effects on Human Health
So how safe is the Bt protein to non-target organisms? The specificity of Bt for its target insects is one of
the characteristics that make it an ideal method of biological pest control. In fact, different strains of Bt
have specific toxicity to certain target insects. The specificity rests on the fact that the toxicity of the Bt
protein is receptor-mediated. This means that for an insect to be affected by the Bt protein, it must have
specific receptor sites in its gut where the proteins can bind. Fortunately, humans and majority of beneficial
insects do not have these receptors.

Before Bt crops are placed on the market, they must pass very stringent regulatory tests, including those for
toxicity and allergenicity.

The U.S. Environmental Protection Agency (US-EPA) has already administered toxicology assessments,
and Bt proteins have already been tested even at relatively higher dosages. According to the Extension
Toxicology Network (Extoxnet), a pesticide information project of several universities in the US, “no
complaints were made after 18 humans ate one gram of commercial Btpreparation daily for five days, on
alternate days...Humans who ate one gram per day for three consecutive days were not poisoned or
infected.” Furthermore, the protein was shown to be degraded rapidly by human gastric fluid in vitro
(Extoxnet, 1996).

Effects on the Environment

Soil ecosystems and groundwater

The Bt protein is moderately persistent in soil and is classified as immobile, as it does not
move, or leach, with groundwater. It does not particularly persist in acidic soil conditions
and, when exposed to sunlight, is rapidly destroyed due to UV radiation.

Independent experts have conducted studies to investigate the impact of Btcrops on soil
organisms and other insect species that are considered beneficial in agriculture. No adverse USDA Photo
effects have been found on non-target soil organisms, even when these organisms were Monarch
exposed to quantities of Bt far higher than what would actually occur under natural crop- Caterpillar
growing conditions. Likewise, research done by the US-EPA revealed no changes in the
soil microbiota in fields with Bt plant material or conventional plant material (Donegan, et al., 1995), or
between fields of Bt and non-Bt crops (Donegan, et al., 1996).

Animals and insects

On tests conducted on dogs, guinea pigs, rats, fish, frogs, salamanders, and even birds, the Btprotein was
found not to have any harmful effects. It is also noteworthy that no toxic effects were found on beneficial or
predator insects, such as honeybees and lady beetles. (Extoxnet, 1996).

In 1999, it was reported that pollen from Bt corn had a negative impact on Monarch butterfly larvae. This
report raised concerns and questions about the risks of Bt crops on non-target organisms. Recent studies,
however, show that Bt corn poses “negligible” threat to Monarch butterflies in the field. A collaborative
research effort by scientists in the US and in Canada has produced information to develop a formal risk
assessment of the impact of Bt corn on Monarch butterfly populations. They concluded that
in most commercial hybrids, Bt expression in pollen is low, and laboratory and field studies
show no acute toxic effects at any pollen density that would be encountered in the field.

Advantages of Bt Crops
Improved pest management.Insect-protected Bt crops provide the farmer with season-long
protection against several damaging insect pests, and reduce or eliminate the need for
insecticide sprays. This eliminates the yield loss that results from less than optimal pest
control by applied farm insecticides, and it allows the farmer more time for other farm
management duties.
USDA Photo
Rotten corn
Reduction in insecticide use.A study by the US Department of Agriculture reported that 8.2 million pounds
of pesticide active ingredients were eliminated by farmers who planted Bt crops in 1998. Significant
reductions have also been reported in China and Argentina, where the use of Bt cotton resulted in a 60-70%
reduction in pesticide use.)

Greater net return. Lower input costs often contribute to a higher net return compared to conventional
crops. Btcotton farmers in the US earned an incremental $99 million as a result of decreased pesticide costs
and/or increased yields. Similarly, Btcotton farmers in Argentina reported that Btcotton generated an
average incremental benefit of $65.05/ha

Improved conditions for non-target organisms.Since Bt crops are able to defend themselves against pests,
the use of chemical insecticides is significantly reduced, thereby encouraging the proliferation of beneficial
organisms. These beneficial organisms can help control other secondary pests, which can often become a
problem when predator and parasite populations are reduced by conventional broad-spectrum insecticides.

Less mycotoxin in corn.Aside from being effective against insect pests, Btcrops have lower incidences of
opportunistic microbial pathogens, such as the fungus Fusarium. This fungus produces mycotoxins that can
be deadly to livestock and also cause cancer in humans.

Insect Resistance Management (IRM)

Since Bt crops are capable of season long expression of the Bt protein, precautionary steps have to be taken
in order to avoid the development of insect resistance. In the US, for example, the EPA usually requires a
“buffer zone,” or a structured refuge of non-Bt crops that is planted in close proximity to the Bt crops.

IRM is said to be the key to sustainable use of the insecticide in both genetically modified crops and
Btmicrobial spray formulations.

Current Status of Bt Technology

At the end of 2011, an estimated 23.9 million hectares of land were planted with crops containing the Bt
gene. The following table shows the countries that have commercialized Bt crops, from 1996 to 2011.

Table 1. Countries that have commercialized Bt cotton and/or Bt corn, 1996-2004

Crop Country
Cotton Argentina, Australia, Brazil, Burkina Faso, Canada, China, Colombia, Costa Rica, European
Union (EU), India, Japan, Korea, Mexico, Myanmar, New Zealand, Pakistan, Paraguay,
Philippines, South Africa, United States of America (USA)
Maize Argentina, Australia, Brazil, Canada, Chile, China, Colombia, Czech Republic, Egypt, EU,
Honduras, Japan, Korea, Malaysia, Mexico, Netherlands, New Zealand, Philippines, Poland,
Portugal, Romania, Russian Federation, Singapore, Slovak Republic, South Africa, Spain,
Switzerland, Taiwan, United Kingdom, USA, Uruguay
Poplar China
Potato Australia, Canada, Japan, Korea, Mexico, New Zealand, Philippines, Russian Federation, USA
Rice China, Iran
Soybean Australia, Canada, New Zealand, Taiwan, USA
Tomato Canada, USA
Source: Source: ISAAA's GM Approval Database. http://www.isaaa.org/gmapprovaldatabase/.
For the first fifteen years of commercialization (1996-2010), benefits from insect resistant crops are
valued at US$ 43.4 billion, 55% of the global value of biotech crops of US$78.4 billion; and for 2010
alone at 9.5 billion, 69% of the global value of biotech crops.

Conclusion
Bt crops are an addition to our arsenal against plant pests. With an increasing population and decreasing
arable land, it is necessary to exploit all options with as little compromise to produce more crops. When
used side by side with proper agricultural practices, Bt insect resistance technology can bring many benefits
to crops, farmers, and consumers alike.

HERBICIDE TOLERANCE TECHNOLOGY

Ask any farmer and he will surely tell you that weeds are a constant problem. Weeds not only compete with
crops for water, nutrients, sunlight, and space but also harbor insect and disease pests; clog irrigation and
drainage systems; undermine crop quality; and deposit weed seeds into crop harvests. If left uncontrolled,
weeds can reduce crop yields significantly.
Farmers can fight weeds with tillage, hand weeding, herbicides, or typically a combination of all techniques.
Unfortunately, tillage leaves valuable topsoil exposed to wind and water erosion, a serious long-term
consequence for the environment. For this reason, more and more farmers prefer reduced or no-till methods
of farming.
Similarly, many have argued that the heavy use of herbicides has led to groundwater contaminations, the
death of several wildlife species and has also been attributed to various human and animal illnesses.
Weed Control Practices
The tandem technique of soil-tilling and herbicide application is an example of how farmers control weeds
in their farms.
Generally, they till their soil before planting to reduce the number of weeds present in the field. Then they
apply broad-spectrum or non-selective herbicides (one that can kill all plants) to further reduce weed growth
just before their crop germinates. This is to prevent their crops from being killed together with the weeds.
Weeds that emerge during the growing season are controlled using narrow-spectrum or selective herbicides.
Unfortunately, weeds of different types emerge in the field, and therefore, farmers have to use several types
of narrow-spectrum herbicides to control them. This weed control method can be very costly and can harm
the environment.
Researchers postulated that weed management could be simplified by spraying a single broad-spectrum
herbicide over the field anytime during the growing season.
Development of Glyphosate and Glufosinate Herbicide Tolerant Plants
Herbicide-tolerant (HT) crops offer farmers a vital tool in fighting weeds and are
compatible with no-till methods, which help preserve topsoil. They give farmers the
flexibility to apply herbicides only when needed, to control total input of herbicides
and to use herbicides with preferred environmental characteristics.
Technology Background
How do these herbicides work?
These herbicides target key enzymes in the plant metabolic pathway, which disrupt
plant food production and eventually kill it. So how do plants elicit tolerance to
herbicides? Some may have acquired the trait through selection or mutation; or more
recently, plants may be modified through genetic engineering. USDA Photo
Why develop HT crops?
What is new is the ability to create a degree of tolerance to broad-spectrum herbicides - in particular
glyphosate and glufosinate - which will control most other green plants. These two herbicides are useful for
weed control and have minimal direct impact on animal life, and are not persistent. They are highly effective
and among the safest of agrochemicals to use. Unfortunately, they are equally effective against crop plants.
How do Glyphosate and Glufosinate HT crops work?
1. Glyphosate-tolerant crops
Glyphosate herbicide kills plants by blocking the EPSPS enzyme, an enzyme involved in the biosynthesis of
aromatic amino acids, vitamins and many secondary plant metabolites. There are several ways by which
crops can be modified to be glyphosate-tolerant. One strategy is to incorporate a soil bacterium gene that
produces a glyphosate-tolerant form of EPSPS. Another way is to incorporate a different soil bacterium gene
that produces a glyphosate degrading enzyme.
2. Glufosinate-tolerant crops
Glufosinate herbicides contain the active ingredient phosphinothricin, which kills plants by blocking the
enzyme responsible for nitrogen metabolism and for detoxifying ammonia, a by-product of plant
metabolism. Crops modified to tolerate glufosinate contain a bacterial gene that produces an enzyme that
detoxifies phosphonothricin and prevents it from doing damage.
Other methods by which crops are genetically modified to survive exposure to herbicides including: 1)
producing a new protein that detoxifies the herbicide; 2) modifying the herbicide’s target protein so that it
will not be affected by the herbicide; or 3) producing physical or physiological barriers preventing the entry
of the herbicide into the plant. The first two approaches are the most common ways scientists develop
herbicide tolerant crops.
Safety Aspects of Herbicide Tolerance Technology
Toxicity and Allergenicity
Government regulatory agencies in several countries have ruled that crops possessing herbicide-tolerant
conferring proteins do not pose any other environmental and health risks as compared to their non-GM
counterparts.
Introduced proteins are assessed for potential toxic and allergenic activity in accordance with guidelines
developed by relevant international organizations. They are from sources with no history of allergenicity or
toxicity; they do not resemble known toxins or allergens; and they have functions, which are well
understood.
Effects on the Plants
The expression of these proteins does not damage the plant’s growth nor
result in poorer agronomic performance compared to parental crops. Except
for expression of an additional enzyme for herbicide tolerance or the
alteration of an already existing enzyme, no other metabolic changes occur
in the plant.
Persistence or invasiveness of crops
A major environmental concern associated with herbicide-tolerant crops is their potential to create new
weeds through outcrossing with wild relatives or simply by persisting in the wild themselves. This potential,
however, is assessed prior to introduction and is also monitored after the crop is planted. The current
scientific evidence indicates that, in the absence of herbicide applications, GM herbicide-tolerant crops are
no more likely to be invasive in agricultural fields or in natural habitats than their non-GM counterparts
(Dale et al., 2002).
The herbicide-tolerant crops currently in the market show little evidence of
enhanced persistence or invasiveness.
Advantage of Herbicide Tolerant Crops
 Excellent weed control and hence higher crop yields;
 Flexibility – possible to control weeds later in the plant’s growth;
 Reduced numbers of sprays in a season;
 Reduced fuel use (because of less spraying);
 Reduced soil compaction (because of less need to go on the land to spray);
  Use of low toxicity compounds which do not remain active in the soil; and
 The ability to use no-till or conservation-till systems, with consequent benefits to soil structure and
organisms (Felsot, 2000).
A study conducted by the American Soybean Association (ASA) on tillage frequency on soybean farms
showed that significant numbers of farmers adopted the “no-tillage” or “reduced tillage” practice after
planting herbicide-tolerant soybean varieties. This simple weed management approach saved over 234
million gallons of fuel and left 247 million tons of irreplaceable topsoil undisturbed.
Current Status of Herbicide Tolerance
From 1996 to 2011, herbicide- tolerant crops consistently occupied the largest planting area of biotech
crops. In 2011 alone, herbicide tolerant crops occupied 93.3 million hectares or 59% of the 160 million
hectares of biotech crops planted globally. The most common are the glyphosate and glufosinate tolerant
varieties. The following table shows countries that have approved major HT crops for food use.

Crop Countries

Alfalfa Australia, Canada, Japan, Mexico, New Zealand,

Philippines, United States of America (USA)

Argentine Australia; Canada; Chile, China; European Union

Canola (EU); Japan; Korea, Rep.; Mexico; New Zealand;
Philippines; South Africa; USA

Cotton Argentina; Australia; Brazil; Canada; China;

Colombia; Costa Rica; EU; Japan; Korea, Rep.;
Mexico; New Zealand; Philippines; Singapore;
South Africa; USA

Flax, Linseed Canada; USA

Maize Argentina; Australia; Brazil; Canada; China;

Colombia; El Salvador; EU; Honduras; Japan;
Korea, Rep.; Malaysia; Mexico; New Zealand;
Philippines; Russian Federation; Singapore; South
Africa; Spain; Taiwan; Thailand; USA; Uruguay

Rice Australia; Canada; Colombia; Mexico; New

Zealand; Russian Federation; USA

Soybean Argentina; Australia; Bolivia; Brazil; Canada;

Chile; China; Colombia; Costa Rica; Czech
Republic; EU; Japan; Korea, Rep.; Malaysia;
Mexico; New Zealand; Paraguay; Philippines;
Russian Federation; South Africa; Switzerland;
Taiwan; Thailand; Turkey; United Kingdom; USA;
Uruguay

Sugarbeet Australia; Canada; Colombia; EU; Japan; Korea,

Rep.; Mexico; New Zealand; Philippines; Russian
Federation; Singapore; USA

Wheat Colombia; USA

 Source: ISAAA's GM Approval Database.
http://www.isaaa.org/gmapprovaldatabase/.

A literature review conducted by the Council for Agricultral Science and Technology concluded that the
environment benefits from the use of HT crops. In the US, for example, no-till soybean acreage has
increased by 35% since the introduction of HT soybean. A similar trend is observed in Argentina where
soybean fields are 98% planted with HT varieties. The CAST paper entitled “Comparative Environmental
Impacts of Biotechnology-derived and Traditional Soybean, Corn and Cotton Crops” is available at
http://www.cast-science.org.

BIOTECHNOLOGY FOR THE DEVELOPMENT OF

DROUGHT TOLERANT CROPS
Water and Agriculture
Adverse environmental factors, of which water scarcity represents the most severe constraint to agriculture,
account for about 70 percent of potential yield loses worldwide1. Agriculture is the largest consumer of
water in the world, and in the drier areas of the world, which include many developing countries, the use of
water for agriculture can exceed 90 percent of consumption.

Global warming is also predicted to affect most severely developing countries, where agricultural systems
are most vulnerable to climatic conditions and where small increases in temperature are very detrimental to
productivity. The Food and Agricultural Organization of the United Nations2 estimates that by 2025
approximately 480 million people in Africa could be living in areas with very scarce water, and that as
climatic conditions deteriorate, 600,000 square km currently classed as moderately constrained will become
severely limited.

Water becomes an increasingly scarce and precious commodity. It is thus

essential to improve water use efficiency in agriculture. This will require
an integrated approach to water resources management to encourage an
efficient and equitable use of the resource, and to ensure sustainability.
The development of crop varieties with increased tolerance to drought,
both by conventional breeding methods and by genetic engineering, is
also an important strategy to meet global food demands with less water.

Developing Drought Tolerant Crops

Conventional breeding requires the identification of genetic variability to drought among crop varieties, or
among sexually compatible species, and introducing this tolerance into lines with suitable agronomic
characteristics. Although conventional breeding for drought tolerance has and continues to have some
success, it is a slow process that is limited by the availability of suitable genes for breeding. Some examples
of conventional breeding programs for drought tolerance are the development of rice, wheat and Indian
mustard varieties tolerant to salt and to alkali soils by the Central Soil Salinity Research Institute in Karnal,
India3; the development of maize hybrids with increased drought tolerance4; efforts to incorporate salt
tolerance to wheat from wild related species5; and the incorporation of drought tolerance as a selection trait
in the generation of new maize and wheat germplasm by the International Maize and Wheat Improvement
Center6.

The development of tolerant crops by genetic engineering, on the other

hand, requires the identification of key genetic determinants underlying
stress tolerance in plants, and introducing these genes into crops. Drought
triggers a wide array of physiological responses in plants, and affects the
activity of a large number of genes: gene expression experiments have
identified several hundred genes which are either induced or repressed
during drought7.

Plant Drought Tolerance Mechanisms

Plants respond to their changing environment in a complex, integrated way
that allows them to react to the specific set of conditions and constraints
present at a given time. Therefore, the genetic control of tolerance to abiotic
stresses is not only very complex, but is also highly influenced by other
environmental factors and by the developmental stage of the plant.

The physiological responses of plants to a deficit of water include leaf

wilting, a reduction in leaf area, leaf abscission, and the stimulation of root
growth by directing nutrients to the underground parts of the plants. Plants are more susceptible to drought
during flowering and seed development (the reproductive stages), as plant’s resources are deviated to
support root growth. In addition, abscisic acid (ABA), a plant stress hormone, induces the closure of leaf
stomata (microscopic pores involved in gas exchange), thereby reducing water loss through transpiration,
and decreasing the rate of photosynthesis. These responses improve the water-use efficiency of the plant on
the short term.

Plant cells are required to maintain water balance. To mantain this water
balance, plants absorb water when water potential is negative Cells can
decrease their water potential through the accumulation of solutes, such as
sugars, amino acids, organic acids and ions – especially potassium (K+). As
cellular enzymes are severely inhibited by the presence of ions, these must be
removed from the cytosol (the ground fluid substance of the cell) and stored
in special storage cell organelles, the vacuoles. Compatible solutes that
accumulate in the cytosol and do not interfere with enzymatic reactions
comprise sugar alcohols (mannitol and sorbitol), the amino acid proline, and glycine betaine. The synthesis
of these compounds by the plant enhances tolerance to drought8.

The plant’s response to drought is accompanied by the activation of genes involved in the perception of
drought stress and in the transmission of the stress signal. One group are genes that encode proteins that
protect the cells from the effects of desiccation9. These genes include those that govern the: accumulation of
compatible solutes; passive transport across membranes; energy-requiring water transport systems; and
protection and stabilization of cell structures from desiccation and damage by reactive oxygen species 9.

A second group of genes activated by drought is comprised by regulatory

proteins that further regulate the transduction of the stress signal and
modulate gene expression. At least four independent stress-responsive genetic
regulatory pathways are known to exist in plants, forming a highly complex
and redundant gene network8, 9. Two of the pathways are dependent on the
hormone ABA, and two are ABA-independent. These pathways are also
implicated in the perception and response to additional stress factors,
including cold, high temperature and salinity.

Genetic Engineering Drought Tolerant Plants

Although not a crop plant, Arabidopsis has played a vital role in the elucidation of the basic processes
underlying stress tolerance, and the knowledge obtained has been transferred to a certain degree to important
food plants10. Many of the genes known to be involved in stress tolerance have been isolated initially in
Arabidopsis. The introduction of several stress-inducible genes into plants by genetic engineering has
resulted to increased tolerance of transgenics to drought, cold and salinity stresses8, 9. Some examples are
reviewed in the following section.

Genetic manipulation of the stress response to abscisic acid (ABA)

ABA levels in the plant greatly increase in response to water stress, resulting in the closure of stomata
thereby reducing the level of water loss through transpiration from leaves and activate stress response
genes. The reaction is reversible: once water becomes available again, the level of ABA drops, and stomata
re-opens. Increasing the plant’s sensitivity to ABA has therefore been a very important target for improving
drought tolerance.

ERA1, a gene identified in Arabidopsis, encodes the ß-subunit of a farnesyl-

transferase, and is involved in ABA signaling. Plants lacking ERA1 activity have
increased tolerance to drought, however are also severely compromised in yield. In
order to have a conditional, reversible down-regulation of ABA, a group of
Canadian researchers used a drought-inducible promoter to drive the antisense
expression of ERA1, in both Arabidopsis and canola plants11. Transgenic plants
performed significantly better under water stress, with consistently higher yields over conventional varieties.
Importantly, there was no difference in performance between transgenic and controls in conditions of
sufficient water, demonstrating that the technology has no yield-drag11. Multi-location trials have confirmed
yield increases due to enhanced protection to drought to be 15-25 percent compared to non-transgenic
controls (http://www.performanceplants.com).

Performance Plants Inc, a Canadian plant biotechnology company, is developing the technology for
commercialization, under the name Yield Protection Technology™ (YPT™). YPT™ is also being
developed for maize, soybean, cotton, ornamentals and turf grass to be available to farmers in early 2011.

ABA-independent gene regulation to drought stress

The transcription factors DREB1 and DREB2, are important in the ABA-
independent drought tolerant pathways, that induce the expression of stress
response genes. Over-expression of the native form of DREB1, and of a
constitutively active form of DREB2, increases the tolerance of transgenic
Arabidopsis plants to drought, high salinity and cold. Although these genes were
initially identified in Arabidopsis plants, their presence and role in stress tolerance
have been reported in many other important crops, such as rice, tomato, barley, canola, maize, soybean, rye,
wheat and maize, indicating that this is a conserved, universal stress defense mechanism in plants 9. This
functional conservation makes the DREB genes important targets for crop improvement for drought
tolerance through genetic engineering.

Conclusion
Although significant progress has been made in elucidating the genetic mechanisms underlying drought
tolerance, considerable challenges remain. In field conditions, crops are subjected to variable levels of
multiple stresses, thus one area of studies that deserves much more attention is the response of plants to a
combination of stresses. There, plant’s response to multiple stresses cannot be inferred from the response to
individual stress12. It is thus essential to test newly developed varieties to multiple stresses, and to carry out
extensive field studies in a large range of conditions that assess tolerance as absolute yield increases.

Another major challenge is the increasing difficulty and expense in obtaining approvals for field trials of
GM plants. As a number of measures are in place to ensure the safe and responsible design of field tests,
excessive precaution should not become a barrier to making sure we use all the tools available to us for a
more sustainable agriculture.