Relevance of Bt-Crops
Insect pest management in agriculture is important to safeguard crop yields and productivity. Chemical insecticides that effectively control insect pests have been proven to be harmful to human health and environment. There is a need to reduce the dependence on pesticides by using safer alternatives to manage insect pests. Many insecticidal proteins and molecules are available in nature, which are effective against agriculturally important pests but innocuous to mammals, beneficial insects and other organisms. Insecticidal proteins present in the soil borne bacterium, Bacillus thuringiensis (Bt), which has demonstrated its efficacy as a spray formulation in agriculture over the past five decades, have been expressed in many crop species with positive results. Three such transgenic crop species (cotton, corn and potato) have been commercialized with substantial benefits to the farmers. Bt crops occupy an area of 42 million hectares out of the global transgenic area of >150 million hectares in 2011. In India, Bt cotton was cultivated in more than 10.0 million hectares.
Bt is a gram-positive, aerobic, endospore-forming bacterium belonging to morphological group I along with Bacillus cereus, Bacillus anthracis and Bacillus laterosporus. All these bacteria have endospores. Bt, however, is recognized by its parasporal body (known as the crystal) that is proteinaceous in nature and which possesses insecticidal properties. The parasporal body comprises of crystals varying in size, shape and morphology. The crystals are tightly packed with proteins called protoxins or d-endotoxins. The first record on Bt goes back to 1901, when a Japanese microbiologist Ishiwata discovered a bacterium from diseased silkworm larvae, which he named Bacillus sotto. Between 1909 and 1912, Berliner, working at a research station for grain processing in Germany, investigated an infectious disease of the Mediterranean flour moth and described a spore-forming bacterium as the causative agent and designated it as B. thuringiensis.
There are many subspecies and serotypes of Bt with a range of well-characterized insecticidal proteins or Bt toxins (d-endotoxins). At present it has been estimated that over 60,000 isolates of Bt are being maintained in culture collections worldwide. Known Bt toxins kill subsets of insects among the Lepidoptera, Coleoptera, Diptera and nematodes. The host range of Bt has expanded considerably in recent years due to extensive screening programs. Currently more than 160 different genes encoding Bt toxins have been cloned. Recent information about Bt toxins/genes can be obtained from http://www.biols.susx.ac.uk/Home/Neil_Crickmore/Bt/.
Mode of action
The Bt toxins exert their toxicity by forming pores in the larval midgut epithelial membranes. Initially the protoxins are activated in the midgut by trypsin-like proteases to toxins. The active toxins bind to specific receptors located on the apical brush border membrane of the columnar cells. Binding is followed by the insertion of the toxin into the apical membrane leading to pore formation. The formation of toxin-induced pores in the columnar cell apical membrane allows rapid fluxes of ions. Different studies revealed that the pores are K+ selective, permeable to cations, anions or permeable to small solutes like sucrose, irrespective of the charge. It appears that the toxin forms or activates a relatively large aqueous channel in the membrane. The disruption of gut integrity results in the death of the insect from starvation or septicemia.
Applications of Bt
The first practical application of Bt dates back to 1938 when it was sold as ‘Sporeine’ in France for the control of European corn borer. The growing realization that organic insecticides are deleterious to the environment and human health spurred a renewed interest in Bt in the 1960s, which led to the introduction of viable Bt biopesticides like Thuricide and Dipel. Bt is the most popular biological control agent with worldwide sales of about $100 million. Bt spray formulations comprise 5% of total global pesticide market. The use of conventional Bt biopesticides, however, was found to have limitations like narrow specificity, short shelf life, low potency, lack of systemic activity, and the presence of viable spores.
An elegant and the most effective delivery system for Bt toxins is the transgenic plant. The major benefits of this system are economic, environmental, and qualitative. In addition to the reduced input costs to the farmer, the transgenic plants provide season-long protection independent of weather conditions, effective control of burrowing insects difficult to reach with sprays and control at all of the stages of insect development. The important feature of such a system is that only insects eating the crop are exposed to the toxin. Genetic transformation of almost all the major crop species is now feasible with the development of an array of techniques ranging from the Agrobacterium approach to electric discharge-mediated particle acceleration procedure (Pattanayak et al., 2000).
The initial attempts to introduce and express native Bt genes encoding protoxins or truncated toxins in plants were not very successful because the levels of toxin expression were very low. In 1990, researchers at Monsanto made a significant advancement in the expression of Bt genes in plants. They noticed that Bt genes were excessively AT rich in comparison with normal plant genes. This bias in nucleotide composition of the DNA could have a number of deleterious consequences to gene expression because AT-rich regions in plants are often found in introns or have a regulatory role in determining polyadenylation. Introduction and expression of codon-modified genes in crop plants conferred significant protection against target pests. The first transgenic Bt-crops viz., cotton, corn and potato were commercialized in USA in 1995 and 1996. Currently more than a dozen countries cultivate Bt-crops. In 2009, China’s Ministry of Agriculture released biosafety certificates for Bt rice Huahui No. 1 and Bt Shanyou 63 with possible planting in 2012. Bt-cotton was permitted for commercial cultivation in India in 2002, which has brought about a revolution in cotton production.
Benefits of Bt-crops
In the past fifteen years, all the countries that have introduced Bt cotton and maize have derived significant and multiple benefits. These include increases in yield, decreased production costs, a reduction of at least 50% in insecticide applications, resulting in substantial environmental and health benefits to small producers, and significant economic and social benefits.
Bt maize: Globally, the farm level benefit of using Bt maize cumulatively since 1996 has been $6.34 billions. In terms of the total value of maize production from the countries growing Bt maize in 2008, the additional farm income generated by the technology is equal to a value added equivalent of 2.2%.
Bt cotton: Globally, the farm level benefit of using Bt cotton cumulatively since 1996 has been $15.61 billions. In terms of the total value of cotton production from the countries growing Bt in 2008, the additional farm income generated by the technology is equal to a value added equivalent of 19.3%. The economic benefits of Bt cotton in India have been enormous and well documented (http://www.apcoab.org/publications.html). Millions of farmers benefited from Bt cotton in developing countries such as China, India and South Africa where Bt cotton contributed to the reduction in poverty by increasing incomes of small farmers.
The environmental benefits of cultivating Bt crops are:
- Reduction in use of pesticides: The cumulative reduction of pesticide applications due to Bt crops from 1996-2008 is estimated to be 356 million kilograms of active ingredient.
- Less insecticides in aquifers and the environment: The substantial decrease in insecticides associated with the cultivation of Bt cotton has lead to significant decrease in insecticide run off into watersheds, aquifers, soils and generally into the environment. More widespread global cultivation of Bt-cotton will further improve the water quality.
- Reduced farmer exposure to insecticides and improvement of human health: Chemical insecticides used in cotton have high toxicity to humans and animals. Substitution of the chemical insecticides with Bt cotton has clearly reduced the risks to farm workers and to others in the farm community who may be exposed. These effects are particularly important in developing countries where modern application techniques are neither always adopted nor available for use.
- Increased populations of beneficial insects: The global use of broad spectrum insecticides on cotton has adversely affected and decreased the populations of non-target species including the arthropod natural enemies that can provide effective control of non-lepidopteran pests. Various studies confirmed that the arthropod natural enemy populations in Bt cotton are greater than in non-Bt cotton. In addition to reducing the number of sprays for the bollworm/budworm complex, Bt cotton has also reduced the number of sprays for other insects such as thrips and aphids. This effect has been attributed to higher populations of beneficial predators and parasitic insects that are eliminated by insecticide sprays.
- Reduced risk for wildlife: Reduction in the use of insecticides, many of which are highly toxic to wildlife will reduce the risks to mammals, birds, bees, fish and other organisms. Many birds are dependent on insects for food and their elimination through the use of insecticides deprives birds of their food source.
- Reduced fuel and raw material consumption and decreased pollution: Lowering the demand for insecticides, through the use of Bt cotton reduces tractor fuel usage as a result of reduction in number of sprays, which in turn reduces air pollution.
Safety of Bt toxins in terms of toxicity and allergenicity towards mammals and other non-target organisms is well documented. Lack of receptors that bind to Bt toxins and instant degradation of Bt toxins in human digestive system makes them innocuous to human beings. Community exposure to Bt toxins/spray formulations over a period of six decades has not resulted in any adverse effects. Human volunteers consumed Bt toxins at very high concentration without any undesirable effects. Lack of homology to any allergenic protein/epitope sequences makes Bt toxins non-allergenic. Extensive testing of Bt cotton and Bt maize has proven that the crops and the products derived thereof are totally safe.
Insect resistance management
One of the important considerations of introduction of insect resistant transgenic plants into environment is to prevent the development of resistance in insects towards Bt toxins. Instances where resistance has developed in laboratory and field populations of Diamondback moth and Indian meal moth point towards great caution to be applied in the implementation of this technology. Various resistance management strategies have been suggested to prevent or delay the development of resistance to Bt. Proposed strategies include the use of multiple toxins (stacking or pyramiding), crop rotation, high or ultra-high dosages, and spatial or temporal refugia. Gene stacked Bt cotton is currently available which will ensure the longevity of the technology. The most promising and currently practical strategy is that of using refugia. This strategy calls for reducing the possibility of long-term impact by preventing the creation of a resistant population. This is achieved by ensuring that there are always plenty of susceptible insects nearby for the few resistant ones to mate with.
Introduction of various Bt crops is extremely important towards achieving the goals of eco-friendly and sustainable pest management in agriculture. Deployment of Bt vegetables will go a long way to protect human health by reducing the pesticide use in vegetable crops, which are often consumed raw. There is an urgent need to develop pod borer-resistant pulses, which would usher in a revolution in pulse production in India. Caution is needed, however, while introducing a particular Bt gene in multiple crops. Large-scale bioprospecting of Bt strains will provide us novel Bt genes, which can widen the spectrum of insecticidal activity as well as ensure resistance management.