CBSE · Class 12 · Biology · Biotechnology and its ApplicationsExplain the process of creating genetically modified pest-resistant plants with special reference to Bt cotton. How do Bt toxins work in the body of the insect? Describe the mechanism in detail.

Introduction to Bt Cotton\nBt cotton is a genetically modified pest-resistant plant variety created to protect crops from the attacks of lepidopteran insects, particularly the cotton bollworm (Helicoverpa armigera). The name 'Bt' comes from a bacterium called Bacillus thuringiensis.

Origin of the Bt Toxin Gene

Bacillus thuringiensis produces certain proteins during a particular phase of its growth that kill certain insects such as lepidopterans (tobacco budworm, armyworm), coleopterans (beetles), and dipterans (flies, mosquitoes). The bacterium produces crystals containing a toxic insecticidal protein. Interestingly, this toxin does not kill the bacterium itself because it exists as an inactive protoxin.

Mechanism of Action in the Insect

1. Ingestion: When an insect ingests the inactive protoxin along with the plant material, it enters the insect's gut.
2. Activation: The alkaline pH of the insect's gut solubilizes the crystals and converts the inactive protoxin into its active form.
3. Binding: The activated toxin binds to the surface of midgut epithelial cells and creates pores.
4. Cell Lysis and Death: The creation of pores causes cell swelling and lysis, eventually leading to the death of the insect due to starvation and septicemia.

Genetic Engineering Aspect\nScientists have isolated the specific toxin genes (e.g., cryIAb, cryIIAb) from the bacterium and incorporated them into several crop plants like cotton. Since the resistance genes are specific, the choice of gene depends upon the crop and the targeted pest. For example, the protein encoded by the gene cryIAb controls corn borer, while cryIAc and cryIIAb control cotton bollworms.