An ingenious method of organic farming relies on a toxin derived from a bacterium to protect plants from insect attack. Bacillus thuringiensis is a gram-positive, rod shaped bacterium that occurs naturally in the soil and on plants. This bacterium is most noteworthy because of its use to kill butterfly and moth caterpillars (Lepidoptera), the larvae of mosquitoes and some species of black fly that are a damage to economically important plants or a health threat.
Originally identified in 1901, by Japanese biologist Shigetane Ishiwatari when it decimated Japan's silkworm population. Bacillus thuringiensis was not so named until 1911, during an outbreak among flour moths in the German province of Thuringia.
A protein endotoxin, actually a so called protoxin, is responsible for the bacterium's insecticidal power. Inside the bacterium the protoxin molecules collect together to form a crystal. The crystal is a hardy structure and does not readily dissolve. However, in the gut of the insects, where the pH is very basic the toxin is degraded. One of the toxins fragments, the "delta endotoxin " confers lethal effects to the insects by binding to the epithelial cells lining of the gut wall of the insect by creating holes in the cells, the toxin destroys the functioning of the gut and causes massive cell death. Destruction is also due to the modification of the pH to a more neutral level that is hospitable for the germination of the endospores. The reviving and growth of Bacillus thuringiensis within the insect gut kills the larva.
In the age of Recombinant DNA technology, the gene for the Bt toxin were engineered into the genome of crop plants and today we have a whole range of Bt designer crops including Bt corn, Bt cotton, Bt potato, Bt soybean and so on which resulted in reduced use of chemical pesticides. Bt strains account for nearly 90% of the world microbial pest control agent (MCPA) market. Most Bt products contain insecticidal crystal proteins and viable spores (spores that produce live bacteria). Studies indicate that Bt products degrade rapidly in the environment and is practically non-toxic to birds, fish and humans.
The use of Bacillus thuringiensis as an insecticide has been practiced since the 1930's. With the advent of techniques of molecular rearrangement, the specificity of the bacterium for target insect pests has been refined. The disadvantages of the use of Bt spray as an insecticide include, the spray cannot be applied uniformly to all parts of the plant, it cannot be delivered to the pests that are found inside the plant tissues and is rapidly degraded by UV light and also removed by water run off. So, multiple applications can be required for extended pest protection. Transgenic crops with the Bt "cry" genes might overcome these kinds of the disadvantages. The use of transgenic plants also eliminates the difficulties in targeting pests that burrow into plants as well as the labour and expenses associated with applying sprays.
Evolution of insect resistance threatens the continued success of transgenic crops producing Bt toxins that kill pests. The approach used most widely to delay insect resistance to Bt crops is the "refuge strategy" in fields. The refuge area, supply a source of wild type (non- mutant) insects to mate with possible resistant insects to produce non resistant insects. In this, Bt crops are planted with alternating rows of regular non Bt crops. The insects that have developed the resistance to Bt have more chances of mating with an insect that has not developed resistance to Bt and the progenies produced will be insects that are not resistant to Bt.
Environmentally safe insect control methods based on the Bt's and their insecticidal crystal proteins are going to increase with the wide adoption of transgenic crops. Development of recombinant microorganisms and proteomic technology could be adapted to the study of Bt crystal proteins, along with interaction studies between target insects and Bt involving modes of action of Bt cry proteins. Studies of resistance mechanisms should be carried out all of which are fundamental that will allow the improvement of existing Bt application methods and ability to design alternative ways. For sustainable use of Bt it is absolutely necessary that investigations should be carried out on long term effects of Bt on non-target organisms and the environment.
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