Thuringiensis toxin is produced by Bacillus thuringiensis. The toxins are called cry proteins. These are endotoxins. The bacterium is present in the soil. Thuringiensis toxin (cry protein) has been used for protecting crops against insect pests.
Tissues sprayed with this toxin kill the insect pests. This toxin is an effective biocontrol against insect pests. Through genetic engineering, the Cry gene has been inserted into plants. The transformed plants produce their own thuringiensis toxin.
Using the chemical Pesticides
The pesticides have always been erratic in their performance as;
Their concentration is not equally distributed on the plant tissues that are sprayed with the toxin
They may not be eaten by insects.
Sprayed the toxin may be washed away by the rain, so the endotoxin performs irregularly.
The use of toxic pesticides pollutes the soil.
It may be dissolved in water that pollutes the water as well.
MECHANISM OF ACTION OF CRY PROTEIN
When sprayed on the crops it has been found that Bacillus thuringiensis produce crystalline protein. This is known as cry protein this endotoxin when entered into an insect is metabolized by insect Gut enzymes which convert the inactive cry protein into active cry protein and this protein breakdown the cells of the insect gut and causes paralysis of insect gut muscles in this way this toxin kills the insect pest.
MOLECULAR MECHANISM OF ACTION
The crystalline protein is insoluble and inactive.
It is converted into soluble protein by proteases of the insect gut.
The soluble cry AC protein bind to cell membrane protein cadherin.
This can kill the gut cells by two processes.
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| MOLECULAR MECHANISM OF cry1AC PROTEIN |
They form pores in the cell membrane. As binding of cry oligomer to the cadherin protein activates binding it to GPI anchor protein, this causes pore formation in the cell.
The cell dies due to leakage.
Another method is a cascade of events. This initiates programmed cell death.
PLASMID FOR Bt GENE
Gene for cry protein has been isolated from the bacterium this gene is under the constitutive promoter of cauliflower mosaic virus CaMV and G7 Terminator. The gene has been inserted into the plasmid and an expression vector has been created. These genes are also associated with an antibiotic-resistant marker gene.
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| cry1AC plasmid (simplified) |
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| Production of Bt plant |
Transformed plants are selected through marker-assisted methods. These Transformed plants are utilized to reduce modified plants which are known as BT cotton.
CRY PROTEINS
Cry proteins are classified basically into four classes. Each of these classes is effective against the special class of insects. Cry I is effective against Lepidoptera try to be effective against Lepidoptera and Diptera Cry 3 is effective against Colepetra and Cry 4 is effective against specific Diptera.
Other endotoxins that are produced by Bacillus thuringiensis are- cry which are crystal proteins there is 126 type of Crystal protein that is produced by bacteria as endotoxin cyt proteins that are cytolytic there are 22 classes of this endotoxin, and the toxin is VIP which is vegetative insecticidal protein.
Bt cotton was first produced in 1987 it got permission to be grown in India only in 2003 Monsanto is a seed company that markets BT cotton seeds. Other crops that have been improved by utilizing the cry gene are Bt brinjal and Bt tobacco.
Drawbacks of using pest-resistant crop
The endotoxin produced in young tissue is efficient against the larva of insects. In older tissue effective production of endotoxin has not been achieved yet this endotoxin is effective against certain types of insects only. It has not found its universal application. Using terminator Technology has also made it controversial.
Cotton happens to be the first crop to receive environmental clearance as a genetically modified crop in Indian agriculture. Cotton bollworm belongs to Lepidoptera and these are sensitive to cry 1 endotoxin. The cotton bollworm reduces the yield of the cotton crop by up to 50% in India and this endemic effect Rajasthan Haryana and Punjab.
Benefits of using BT cotton
It helps in managing the bollworm infection without any adverse effect on the environment.
It reduced the use of pesticides the reduced the cost of production of the crop. It reduced the risk of cotton cropping. It reduced the adverse effects of utilizing pesticides on soil, it improved the yield of cotton. Bt gene has no adverse effect on human and cattle health. Seeds produced by cotton are used as cattle feed.
Seeds of Bt cotton can also be used as cattle feed, and they are not harmful to cattle these are easily digested by the cattle. The cry endotoxin is not harmful to cattle. This toxin was not found in cattle after eating reset escaping the gene into the environment is negligible for BT Cotton as the BT gene is inserted into tetraploid cotton with chromosome number 52 and this cotton does not breed with a wild variety of cotton as well cotton does not outbreed with other members of Malvaceae so, there is least of this gene escaping into the environment.
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