Bt corn, a genetically modified organism (GMO), has been both the
poster-child and thorn-in-the-side of the plant biotechnology industry from the
late 1990’s to present. There are several versions of this transgenic crop that
each have a gene from an insect pathogen, Bacillus thuringiensis (Bt),
which encodes a protein toxic to the European corn borer (ECB), an insect pest
that eats and destroys corn stems (see Figure 1). Bt corn has proven effective
in reducing crop damage due to ECB, yet public opposition to Bt corn has
escalated amid fears of human health and environmental risks associated with the
production and consumption of Bt corn.
Figure 1. Engineering resistant corn. Following the insertion of a
gene from the bacteria Bacillus thuringiensis, corn becomes resistant to corn
borer infection. This allows farmers to use fewer insecticides
History of Bt
Bt corn draws its humble origins from France, where in 1938
B.
thuringiensis bacteria was grown in large quantities and sprayed on corn
crops to prevent ECB damage[1]. Artificial selection of Bt strains has led to
the successful targeting of many insect pests. Because no toxic effects of Bt on
humans have been detected in its seventy years of use, it is now considered an
acceptable pest control measure for the organic food industry[2]. To this day,
Bt is an important part of many integrated pest management strategies. The
success of the Bt spray has been limited because the bacteria cannot survive for
very long on the plant’s surface. Bt is particularly ineffective at controlling
ECB because these insect live most of their larval life inside the corn stem,
not on the surface: sprays are only effective when the insects are starting its
journey into the stem. Thus, a means of penetrating corn tissue with Bt is
required to offer long-term anti-feeding measures against tunneling insects such
as ECB.
Mechanism of Bt toxicity
Researchers investigated how this bacteria kills particular insects and
discovered that Bt has two classes of toxins; cytolysins (Cyt) and crystal
delta-endotoxins (Cry)[3]. While Cyt proteins are toxic towards the insect
orders Coleoptera (beetles) and Diptera (flies), Cry proteins selectively target
Lepidopterans (moths and butterflies). As a toxic mechanism, Cry proteins bind
to specific receptors on the membranes of mid-gut (epithelial) cells resulting
in rupture of those cells[4]. If a Cry protein cannot find a specific receptor
on the epithelial cell to which it can bind, then the Cry protein is not toxic.
Bt strains will have different complements of Cyt and Cry proteins, thus
defining their host ranges[5]. The genes encoding many Cry proteins have been
identified providing biotechnologists with the genetic building blocks to create
GM crops that express a particular Cry protein in corn that is toxic to a
particular pest such as ECB yet potential safe for human consumption.
Making Bt corn
As it turns out, nature has its own biotechnologist called
Agrobacterium
tumefaciens which induces the growth of tumours on woody plants. These
tumours are engineered by
A.tumefaciens to produce a special food for the
bacteria (opines) that plants normally cannot make. These tumours arise from a
unique bacterial transformation mechanism involving the Ti-plasmid which
coordinates the random insertion of a subset of its DNA (t-DNA) containing opine
synthase genes into a plant chromosome[6] (see Figure 2). By replacing portions
of the t-DNA sequence with genes of interest (such as Cry), researchers have
been able to harness this transformational mechanism and confer new traits to
many flowering plants including grasses such as corn7 and rice[8].
Cry-transformed corn varieties, called ‘Bt corn’, produce sufficient levels of
Cry proteins to provide an effective measure of resistance against ECB and are
now widely grown in North America.
Figure 2. General schematic of GM crop production
Human health and environmental risks
The promise of this technology has been largely overshadowed by concerns
about the unintended effects of Bt corn on human health and the environment. Cry
protein toxicity, allergenicity, and lateral transfer of antibiotic-resistance
marker genes to the microflora of our digestive system threaten to compromise
human health. Despite these alarming possibilities, the risks to human health
appear small based upon what is known about the bacterial endotoxin, its
specificity, and confidence in the processes of plant transformation and
screening[9]. The task of determining the levels of such risks, however, are
immense. Human diets are complex and variable. How can we trace the acute or
chronic effects of eating GM ingredients when they are mixed in with many other
foods that may also present their own health hazards? It is even more
complicated to determine the indirect risk of eating meat from animals raised on
transgenic crops. These tests take time, and the results of clinical trials are
not always clear-cut. It will likely take decades before we can know with any
certainty if Bt corn is as safe for human consumption as its non-GM
alternatives[10].
We currently know very little about the actual ecological risks posed by Bt
corn. Bt corn may be toxic to non-target organisms, transgenic genes may escape
to related corn species, and ECB and other pests may become resistant to Cry
proteins[11]. The alleged effect of Bt corn pollen on Monarch butterfly larvae
has rocketed to the front pages of major newspapers around the world (ex. CNN).
Some research has shown that Monarch butterfly larvae fed their normal diet of
milkweed leaves suffer a significant decline in fitness when those leaves are
dusted with Bt corn pollen (Losey et al. 1999). The methodology of this
experiment, however, has been harshly criticized by members of the scientific
community.
Most recently, the threat of Cry gene escape into wild populations has been
substantiated by the discovery that artificial DNA from transgenic corn has been
detected in traditional corn varieties in remote areas of Mexico (Quist and
Chapela, 2001). However, this study was pulled from NATURE magazine in an
unprecedented fashion following a heated scientific and political debate[12].
While few contest that such transgenes are present in the local corn races of
Mexico, there is still no evidence to suggest that these genetic constructs are
“escaping” to become established in local corn races. We are limited to an
educated guess as to the likelihood and speed of such genetic pollution[13].
Balancing risk and benefit
Despite the lack of conclusive evidence that GM foods present considerable
risk to human health and environment, widespread use of this new technology is
being compared to past mistakes such as broadcast spraying of populated towns
with DDT to control mosquitoes during the 1950s. Notions of “frankenfoods”[14]
and “agroterrorism”[15] corrupting our planet present theoretical possibilities
that cannot be discounted given the remarkable ability of the unlikely to become
an actuality. In truth, we must plead ignorance of the long-term impacts of GM
crops[16].
Arguably, every food in our current diet carries with it associated risks,
determined through “trial-and-error” extending back before to our
hunter-gatherer origins. Often, we will accept a certain degree of exposure to
known hazards to receive known benefits. Bt corn has obvious benefits for
agricultural production, increasing profit margins through more efficient and
consistent corn production and improving the working environment for farmers
through reduced exposure to pesticides. In a surplus market, these benefits may
be passed on to the consumer as a grocery bill reduction. On a global scale,
decreased crop losses due to herbivory may translate into improved world food
supply since corn remains a major staple in the global diet. Ecosystems are not
likely to benefit from ECB-resistant Bt corn propagation since this technology
replaces a largely mechanical (non-chemical) control for ECB.
These benefits, real or imagined, have been used as leverage by Bt corn
proponents in the argument to accept what they argue are minimal levels of
health and environmental risk. Yet many consumer, civil rights, and
environmental advocacy groups characterize such arguments as industry
propaganda, asserting that corporate benefits should not out-weigh the
undetermined human health, socioeconomic and environmental risks.
The relative ease in engineering Bt biopesticides into crops such as corn,
cotton and rice, combined with the cost effectiveness of Bt crops for growers
under threat of ECB, makes banning this technology in North America seem
unlikely. This reality highlights the necessity for the research community to
improve methods for assessing risks posed by GM crops. While some industry
proponents may resist, it is ultimately the public’s responsibility to ensure
that this new technology is properly managed in the context of other pest
management methods that have their own set of risks and benefits.
Notes
Glossary
Artificial Selection – the encouragement of certain traits in an
animal through selective breeding by humans, both intentional or
unintentional
Ti plasmid – “tumour-inducing” plasmid: originally found in the
bacterium Agrobacterium tumefaciens, this plasmid integrates into a host cell
genome and causes galls on plants. Biotechnologists can take advantage of this
integration to insert genes of their choice into plant cells.
Lateral transfer – also called horizontal gene transfer, the movement
of genetic material from one organism to another other than from parent to
offspring, and often across species, genus, or even domain.
Antibiotic resistance marker genes – genes that allow biotechnologists
to distinguish between plants that have been modified properly and those that
have not depending on their suceptibility to antibiotics.
Screening – the process of selection of desirable plants from a large
population of transformants (different insertional events) with variation in
trait depending on location and number of t-DNA insertions.
Herbivory – the consumption of plants by animals, in this case to the
detriment of the plant (predation).
References
1. Van Frankenhuyzen, K. in Bacillus thuringiensis, An environmental
biopesticide: Theory and practice (John Wiley & Sons, 1993).
2. Whalon, M.E. & Wingerd, B.A. Bt: mode of action and use. Arch Insect
Biochem Physiol 54, 200-211 (2003).
3. Crickmore, N. et al. Revision of the nomenclature for the Bacillus
thuringiensis pesticidal crystal proteins. Microbiol Mol Biol Rev 62, 807-813
(1998).
4. Dorsch, J.A. et al. Cry1a Toxins of Bacillus Thuringiensis Bind
Specifically to a Region Adjacent to the Membrane-Proximal Extracellular Domain
of Bt-R-1 in Manduca Sexta: Involvement of a Cadherin in the Entomopathogenicity
of Bacillus Thuringiensis. Insect Biochemistry and Molecular Biology 32,
1025-1036 (2002).
5. De Maagd, R.A., Bravo, A. & Crickmore, N. How Bacillus Thuringiensis
Has Evolved Specific Toxins to Colonize the Insect World. Trends in Genetics 17,
193-199 (2001).
6. Bevan, M.W. & Chilton, M.D. T-DNA of the Agrobacterium Ti and Ri
plasmids. Annu Rev Genet 16, 357-384 (1982).
7. Ishida, Y. et al. High efficiency transformation of maize (Zea mays L.)
mediated by Agrobacterium tumefaciens. Nat Biotechnol 14, 745-750 (1996).
8. High, S.M., Cohen, M.B., Shu, Q.Y. & Altosaar, I. Achieving successful
deployment of Bt rice. Trends Plant Sci 9, 286-292 (2004).
9. Kuiper, H.A., Kleter, G.A., Noteborn, H.P. & Kok, E.J. Assessment of
the food safety issues related to genetically modified foods. Plant J 27,
503-528 (2001).
10. Sudakin, D.L. Biopesticides. Toxicol Rev 22, 83-90 (2003).
11. Sharma, H.C. & Ortiz, R. Transgenics, Pest Management, and the
Environment. Current Science 79, 421-437 (2000).
12. Ochert, A. Caught in the maize at Berkeley. California Monthly
(2002).
13. Letourneau, D.K., Robinson, G.S. & Hagen, J.A. Bt crops: predicting
effects of escaped transgenes on the fitness of wild plants and their
herbivores. Environ Biosafety Res 2, 219-246 (2003).
14. Golden, F. Who’s afraid of Frankenfood? Time 154, 49-50 (1999).
15. van Bredow, J. et al. Agroterrorism. Agricultural infrastructure
vulnerability. Ann N Y Acad Sci 894, 168-180 (1999).
16. Hoffmann-Riem, H. & Wynne, B. In risk assessment, one has to admit
ignorance. Nature 416, 123 (2002).
(Art by Jiang Long and Jen Philpot)
Original source:
http://www.scq.ubc.ca/bt-corn-is-it-worth-the-risk/
