Utilization of Bacillus thuringiensis in Genetically Modified Crops

Introduction

Figure 1. Scanning electron micrograph of a colony of Bacillus thuringiensis cells prior to sporulation. http://www.visualphotos.com/photo/1x3740021/bacillus-thuringiensis-bacteria-scanning-electron-micrograph-sem-of-bacillus-thuringiensis-colony-before-sporulation-microscope-magn-3000x-image-width-w-375-um-b-thuringiensis-is-used-as-biological-control-for.jpg

By Zoë Frazier

The bacterium Bacillus thuringiensis is a gram-positive and rod-shaped microbe that is 2–5 µm in length and 1.0 µm wide [9]. It is found in soil ecosystems, moist environments, and within caterpillar guts [5]. Bacillus thuringiensis is non-pathogenic for most organisms expect for a variety of insect species. Bacillus thuringiensis contains a gene that is known to code for Cry proteins. These proteins are produced during the sporulation phase of the Bacillus thuringiensis lifecycle. The Cry proteins are crystalline in structure and incredibly toxin to insects if consumed. The insect species that are killed by a variety of different Bacillus thuringiensis strains include vegetable insects (tomato and tobacco hornworms), field crop insects (European corn borers, alfalfa caterpillars and webworms), fruit crop insects (Leaf roller and Achemon sphinx), and tree insects (tent caterpillars, fall webworm, pine butterfly, Western spruce budworm). Additionally, mosquitoes, black flies, fungus gnats, cottonwood leaf beetles, elm leaf beetles, and Colorado potato beetles can also be affected if they consume the toxic Cry proteins [4].

Historically the Cry proteins produced by Bacillus thuringiensis have been used as a topical pesticide. However, due to advances in biotechnology, scientists have developed a mechanism to extract the Cry gene and insert it into another organism’s genome. The Cry gene is inserted into common crops that are in high demand, such as corn, wheat, cotton, canola, soy, and potato crops. Once the Cry gene is inserted into the genome, the plant can express its own insecticidal properties. This genetic alteration results in plants that are continuously producing pesticides, which helps to cut down or even eliminate the usage of spray-on pesticides. This new technological advancement is so popular in agriculture that by 2014, a total of 93% of all corn crops in the United States contained the Cry gene [3]. There are many different strains of Bacillus thuringiensis, all of which can be used to genetically modify plants to be resistant to specific target organisms. Despite the many seeming advantages to using genetically modified crops, there are several ecological effects that could drastically change the success rate of Bt crops. Both target organisms and secondary pests have been observed to adapt or evolve to resist the toxic effects of Bt crops. The following page will further describe the integration of Bacillus thuringiensis into our modern day agricultural system along with the social and ecological implications associated with the usage of this microbe.

History

In 1901, a Japanese scientist by the name of Shigetane Ishiwata was the first to discover Bacillus thuringiensis. In the early twentieth century, Bacillus thuringiensis was the cause of a widespread disease that targeted silkworms. Ishiwata was able to isolate the microbe from samples of dead silkworm larvae. In 1927, a German scientist named Ernst Berliner isolated another strain of Bacillus thuringiensis and observed that the microbe developed endospores and parasporal crystals simultaneously. It was not until 1955 that Christopher Hannay finally determined that the parasporal crystals directly caused insect death. This discovery sparked research involving the potential uses for this naturally occurring insecticide. Finally in 1961, the first Bt-based insecticide was registered by the US Environmental Protection Agency to be used in the United States. However, the market for the Bacillus thuringiensis pesticide properties changed drastically in 1982 when the technology for the genetic modification of plants was developed. Scientists discovered that if they extracted the Cry gene from Bacillus thuringiensis, inserted it into a plasmid, and integrated that plasmid into a plants genome, the plant would express a resistance to insect activity. After several years of working with the plasmid, the first crop of transgenic plants was grown in 1996 [5].

Structure and Phylogeny

Include some current research, with at least one figure showing data.

Life Cycle

Figure 2. Transition electron micrograph of Bacillus thuringiensis at the final stage of sporulation. The parasporal crystals are indicated using white arrows. These crystals are produced in the fourth through sixth stages of sporulation and are toxic to insects if consumed [9].

Figure 2.

Bt Toxins

Figure 3. Electron micrograph of the crystalline protein toxin produced by Bacillus thuringiensis. The insecticidal properties of these proteins were first discovered by Christopher Hannay in 1955. Micrography by Jim Buckman (2006) http://en.wikipedia.org/wiki/Bacillus_thuringiensis#/media/File:Bt-toxin-crystals.jpg.

Figure 3.

Bt Crops

Figure 4. The overall process of the production and binding of the Cry proteins. The Cry proteins are directly produced by Bt crops that have the Cry gene integrated into its genome. The toxins produced by the Cry proteins bind to the gut receptors of the target organism, resulting in the organism's death [10].

Figure 4.

Ethical Issues Surrounding Bt Crops

Include some current research, with at least one figure showing data.

Evolved Resistance and Secondary Pests

Figure 5. The survival of western corn rootworm larvae when exposed to (A) Cry3Bb1 corn, (B) mCry3A corn, and Cry34/35Ab1 corn. The black bars indicate populations of western rootworm larvae that had been collected from cornfields in 2011. The white bars indicate the control group of western rootworm larvae that had been isolated in the NCARL since before 2003 [7].

Figure 5.

Conclusion

Include conclusion

References

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[2] Catarino, R., G. Ceddia, F.J. Areal, and J. Park. (2015). The impact of secondary pests on Bacillus thuringiensis (Bt) crops. Plant Biotechnology Journal, 1-12.

[3] Cheeke, T.E., U.M. Schütte, C.M. Hemmerich, M.B. Cruzan, T.N. Rosenstiel, and J.D. Bever. (2015). Spatial soil heterogeneity has a greater effect on symbiotic aburscular mycorrhizal fungal communities and plant growth than genetic modification with Bacillus thuringiensis toxin genes. Molecular Ecology, 1-33.

[4] Cranshaw, W.S. Bacillus thuringiensis: Fact Sheet No. 5.556. Insect Series, Home and Garden. Colorado State Univserity.

[5] de Maagd, R.A. 2015. Chapter 20: Bacillus thuringiensis- Based Products for Insect Pest Control. Principles of Plant-Microbe Interactions. Springer International Switzerland, 2015. 185-192.

[6] Finucane, M.L. & J.L. Holup. (2005). Psychosocial and cultural factors affecting the perceived risk of genetically modified food: an overview of the literature. Social Science & Medicine, 60, 1603 – 1612.

[7] Gassmann, A.J. J.L., Petzold-Maxwell, E.H. Clifton, M.W. Dunbar, A.M. Hoffmann, D.A. Ingber, and R.S. Keweshan. (2014). Field-evolved resistance by western corn rootworm to multiple Bacillus thuringiensis toxins in transgenic maize. PNAS, 111(14), 5141-5146.

[8]Hansen Jesse, L.C. & J.J. Obrlycki. (2000). Field deposition of Bt transgenic corn pollen: lethal effects on the monarch butterfly. Oecologia, 125, 241 – 248.

[9] Ibrahim, M.A., N. Griko, M. Junker, and L.A. Bulla. 2010. Bacillus thuringiensis: A genomic and proteomics perspective. Bioengineered Bugs 1:1, 31-50.

[10] Jurat-Fuentes, J.L. "What Is Bacillus Thuringiensis (Bt)?" Bt Mode of Action. The University of Tennessee Institute of Agriculture. Web. <http://web.utk.edu/~jurat/Btresearchtable.html>.

[11] Petras, S.F. and L.E. Casida. (1985). Survival of Bacillus thuringiensis Spores in Soil. Applied and Environmental Microbiology, 59, 1496 – 1501.

[12] Uzogara, S.G. (2000). The impact of genetic modification of human foods in the 21st century: A review. Biotechnology Advances, 18, 179 – 206.