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Bacillus thuringiensis

Bacillus thuringiensis (or Bt) is a Gram-positive, soil-dwelling bacterium, commonly used as a biological pesticide; alternatively, the Cry toxin may be extracted and used as a pesticide. B. thuringiensis also occurs naturally in the gut of caterpillars of various types of moths and butterflies, as well as on the dark surfaces of plants.[1]

During sporulation, many Bt strains produce crystal proteins (proteinaceous inclusions), called -endotoxins, that have insecticidal action. This has led to their use as insecticides, and more recently to genetically modified crops using Bt genes. Many crystal-producing Bt strains, though, do not have insecticidal properties.[2]

Contents


Discovery and study

B. thuringiensis was first discovered in 1901 by Japanese biologist Shigetane Ishiwatari. In 1911, B. thuringiensis was rediscovered in Germany by Ernst Berliner, who isolated it as the cause of a disease called Schlaffsucht in flour moth caterpillars. In 1976, Robert A. Zakharyan reported the presence of a plasmid in a strain of B. thuringiensis and suggested the plasmid's involvement in endospore and crystal formation.[3][4] B. thuringiensis is closely related to B.cereus, a soil bacterium, and B.anthracis, the cause of anthrax: the three organisms differ mainly in their plasmids. Like other members of the genus, all three are aerobes capable of producing endospores.[1] Upon sporulation, B. thuringiensis forms crystals of proteinaceous insecticidal -endotoxins (called crystal proteins or Cry proteins), which are encoded by cry genes.[5] In most strains of B. thuringiensis, the cry genes are located on a plasmid (in other words, cry is not a chromosomal gene in most strains).[6][7][8]

Cry toxins have specific activities against insect species of the orders Lepidoptera (moths and butterflies), Diptera (flies and mosquitoes), Coleoptera (beetles), Hymenoptera (wasps, bees, ants and sawflies) and nematodes. Thus, B. thuringiensis serves as an important reservoir of Cry toxins for production of biological insecticides and insect-resistant genetically modified crops. When insects ingest toxin crystals, the alkaline pH of their digestive tract denatures the insoluble crystals, making them soluble and thus amenable to being cut with proteases found in the insect gut, which liberate the cry toxin from the crystal.[6] The Cry toxin is then inserted into the insect gut cell membrane, forming a pore. The pore results in cell lysis and eventual death of the insect.[9][10]

Use in pest control

Spores and crystalline insecticidal proteins produced by B. thuringiensis have been used to control insect pests since the 1920s.[11] They are now used as specific insecticides under trade names such as Dipel and Thuricide. Because of their specificity, these pesticides are regarded as environmentally friendly, with little or no effect on humans, wildlife, pollinators, and most other beneficial insects and are used in Organic farming.[12] The Belgian company Plant Genetic Systems was the first company (in 1985) to develop genetically engineered (tobacco) plants with insect tolerance by expressing cry genes from B. thuringiensis.[13][14]

B. thuringiensis-based insecticides are often applied as liquid sprays on crop plants, where the insecticide must be ingested to be effective. The solubilized toxins are thought to form pores in the midgut epithelium of susceptible larvae. Recent research has suggested the midgut bacteria of susceptible larvae are required for B. thuringiensis insecticidal activity.[15]

Bacillus thuringiensis serovar israelensis, a strain of B. thuringiensis is widely used as a larvicide against mosquito larvae, where it is also considered an environmentally friendly method of mosquito control.

Genetic engineering for pest control

Bt toxins present in peanut leaves (bottom image) protect it from extensive damage caused by Lesser Cornstalk Borer larvae (top image).<!-- cite web -->
Bt toxins present in peanut leaves (bottom image) protect it from extensive damage caused by Lesser Cornstalk Borer larvae (top image).[16]

Usage

In 1995, potato plants producing Bt toxin were approved safe by the Environmental Protection Agency, making it the first pesticide-producing crop to be approved in the USA.[17] By 1996, Bt maize, Bt potato and Bt cotton were being grown by farmers in the USA.[18]

Bt crops (in corn and cotton) were planted on 281,500 km in 2006 (165,600 km of Bt corn and 115900 km of Bt cotton). This was equivalent to 11.1% and 33.6%, respectively, of global plantings of corn and cotton in 2006.[19] Claims of major benefits to farmers, including poor farmers in developing countries, have been made by advocates of the technology, and have been challenged by opponents. The task of isolating impacts of the technology is complicated by the prevalence of biased observers, and by the rarity of controlled comparisons (such as identical seeds, differing only in the presence or absence of the Bt trait, being grown in identical situations). The main Bt crop being grown by small farmers in developing countries is cotton, and a recent exhaustive review of findings on Bt cotton by respected and unbiased agricultural economists concluded, "the overall balance sheet, though promising, is mixed. Economic returns are highly variable over years, farm type, and geographical location".[20]

Environmental impacts appear to be positive during the first ten years of Bt crop use (1996 2005). One study concluded insecticide use on cotton and corn during this period fell by 35.6 million kg of insecticide active ingredient, which is roughly equal to the amount of pesticide applied to arable crops in the EU in one year. Using the environmental impact quotient (EIQ) measure of the impact of pesticide use on the environment,[21] the adoption of Bt technology over this ten-year period resulted in 24.3% and 4.6% reduction, respectively, in the environmental impact associated with insecticide use on the cotton and corn area using the technology.[19]

Advantages

There are several advantages in expressing Bt toxins in transgenic Bt crops:

  • The level of toxin expression can be very high, thus delivering sufficient dosage to the pest.
  • The toxin expression is contained within the plant system, hence only those insects that feed on the crop perish.
  • The toxin expression can be modulated by using tissue-specific promoters, and replaces the use of synthetic pesticides in the environment. The latter observation has been well documented worldwide.[19]

Health and safety

Overall, Bt-modified crops appear to be environmentally safe.[22] The proteins produced by Bt have been used in sprays for insect control in France since 1938 and the USA since 1958 with no ill effects on the environment reported.[23]

Bt toxins are a potential alternative to broad-spectrum insecticides. The toxicity of each Bt type is limited to one or two insect orders; it is nontoxic to vertebrates and many beneficial arthropods, because Bt works by binding to the appropriate receptor on the surface of midgut epithelial cells. Any organism that lacks the appropriate receptors in its gut cannot be affected by Bt.[24][25]

There is evidence from laboratory settings that Bt toxins can affect nontarget organisms, usually organisms closely related to the intended targets.[26] Typically, exposure occurs through the consumption of plant parts, such as pollen or plant debris, or through Bt ingestion by their predatory food choices. The methodology used by these researchers has been called into question.[27]

A 2007 study funded by the European arm of Greenpeace suggested the possibility of a slight but statistically meaningful risk of liver damage in rats.[28] The observed changes have been found to be of no biological significance by the European Food Safety Authority.[29]

Limitations of Bt crops

Kenyans examining insect-resistant transgenic Bt corn
Kenyans examining insect-resistant transgenic Bt corn
Constant exposure to a toxin creates evolutionary pressure for pests resistant to that toxin. Already, a diamondback moth population is known to have acquired resistance to Bt in spray form (i.e., not engineered) when used in organic agriculture.[30] The same researcher has now reported the first documented case of pest resistance to biotech cotton.[31][32]

One method of reducing resistance is the creation of non-Bt crop refuges to allow some nonresistant insects to survive and maintain a susceptible population. To reduce the chance an insect would become resistant to a Bt crop, the commercialization of transgenic cotton and maize in 1996 was accompanied with a management strategy to prevent insects from becoming resistant to Bt crops, and insect resistance management plans are mandatory for Bt crops planted in the USA and other countries. The aim is to encourage a large population of pests so any genes for resistance are greatly diluted. This technique is based on the assumption that resistance genes will be recessive.

This means that with sufficiently high levels of transgene expression, nearly all of the heterozygotes (S/s), i.e., the largest segment of the pest population carrying a resistance allele, will be killed before they reach maturity, thus preventing transmission of the resistance gene to their progeny.[33] The planting of refuges (i. e., fields of nontransgenic plants) adjacent to fields of transgenic plants increases the likelihood that homozygous resistant (s/s) individuals and any surviving heterozygotes will mate with susceptible (S/S) individuals from the refuge, instead of with other individuals carrying the resistance allele. As a result, the resistance gene frequency in the population would remain low.

Nevertheless, limitations can affect the success of the high-dose/refuge strategy. For example, expression of the Bt gene can vary. For instance, if the temperature is not ideal, this stress can lower the toxin production and make the plant more susceptible. More importantly, reduced late-season expression of toxin has been documented, possibly resulting from DNA methylation of the promoter.[34] So, while the high-dose/refuge strategy has been successful at prolonging the durability of Bt crops, this success has also had much to do with key factors independent of management strategy, including low initial resistance allele frequencies, fitness costs associated with resistance, and the abundance of non-Bt host plants that have supplemented the refuges planted as part of the resistance management strategy.[35]

Insect resistance

In November 2009, Monsanto scientists found the pink bollworm had become resistant to Bt cotton in parts of Gujarat, India. In four regions, Amreli, Bhavnagar, Junagarh and Rajkot, the crop is no longer effective at killing the pests. This was the first instance of Bt resistance confirmed by Monsanto anywhere in the world.[36] Monsanto confirmed field resistance of the worm to the Cry1Ac first generation Bollgard cotton, which expresses a single Bt gene.[37]

Secondary pests

Several studies have documented surges in "sucking pests" (which are not affected by Bt toxins) within a few years of adoption of Bt cotton. In China, the main problem has been with mirids,[38][39] which have in some cases "completely eroded all benefits from Bt cotton cultivation .[40]

Similar problems have been reported in India, with both mealy bugs [41][42] and aphids.[43]

Possible problems

Lepidopteran toxicity

The most publicised problem associated with Bt crops is the claim that pollen from Bt maize could kill the monarch butterfly.[44] This report was puzzling because the pollen from most maize hybrids contains much lower levels of Bt than the rest of the plant[45] and led to multiple follow-up studies.

The initial study apparently was flawed by faulty pollen-collection procedure; researchers fed nontoxic pollen mixed with anther walls containing Bt toxin.[46] The weight of the evidence is that Bt crops do not pose a risk to the monarch butterfly.[47] Monarch butterflies have no innate relationship to maize crops in the wild, and are not believed to consume maize pollen (or pollen of related plants) in either life stage.

Wild maize genetic mixing

A study in Nature reported that Bt-containing maize genes were found in maize in its center of origin, Oaxaca, Mexico.[48] Nature later "concluded that the evidence available is not sufficient to justify the publication of the original paper."[49] A significant controversy happened over the paper and Natures unprecedented notice.[50][51] In 1998, Chapela, one of the original paper's authors, spoke out against Berkeley accepting a multimillion-dollar research grant from the Swiss pharmaceutical company, Novartis.[50]

A subsequent large-scale study, in 2005, failed to find any evidence of genetic mixing in Oaxaca.[52] A further study has disagreed, finding evidence of transgenic DNA in traditional Mexican maize.[53] Another, apparently unpublished study has likewise shown some small-scale (about 1%) introduction of the 35S promoter in sampled fields in Mexico; it did not find evidence for or against this introduced genetic material being inherited by the next generation of plants.[54] One meta-study has found evidence both for and against Bt introduction into traditional Mexican maize, concluding that the preponderance of evidence points to the introduction of Bt genes into maize in Mexico.[55]

Disproven link to colony collapse disorder

As of 2007, a new phenomenon called colony collapse disorder (CCD) began affecting bee hives all over North America. Initial speculation on possible causes ranged from new parasites to pesticide use[56] to the use of Bt transgenic crops.[57] The Mid-Atlantic Apiculture Research and Extension Consortium published a report in March 2007 that found no evidence that pollen from Bt crops is adversely affecting bees.[58] The actual cause of CCD remains unknown, and scientists believe that it may have multiple causes.[59]

See also

  • Biological insecticides
  • Western corn rootworm
  • Genetically modified food

References

Further reading

External links

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