Question
What is Bt cotton? Explain how the Bt toxin gene is transferred to cotton plants and how it provides resistance against bollworm.
(NCERT Class 12, commonly asked in NEET)
Solution — Step by Step
Bt cotton is a genetically modified (transgenic) cotton plant that produces its own insecticide. The gene comes from a soil bacterium called Bacillus thuringiensis (Bt).
This bacterium naturally produces crystal proteins (Cry proteins) that are toxic to certain insect larvae. The specific genes used for cotton bollworm resistance are cry1Ac and cry2Ab (effective against lepidopteran insects like the bollworm).
The cry gene from Bacillus thuringiensis is isolated using restriction enzymes and inserted into a suitable vector (like the Ti plasmid of Agrobacterium tumefaciens).
The recombinant plasmid is introduced into cotton plant cells using Agrobacterium-mediated transformation. The cry gene integrates into the cotton plant’s genome and is expressed in the plant tissues.
The resulting transgenic cotton plant produces Cry protein in its leaves, stems, and bolls.
The mechanism of action is specific and elegant:
- The Bt cotton plant produces inactive protoxin (Cry protein) — it is a crystalline protein that is harmless in its inactive form.
- When the bollworm larva eats the cotton leaves, the protoxin enters its alkaline gut (pH > 9.5).
- The alkaline pH solubilises the crystal and gut proteases cleave the protoxin into the active toxin.
- The active toxin binds to specific receptors on the midgut epithelial cells of the insect.
- It creates pores in the cell membrane, causing cell lysis.
- The gut lining is destroyed, leading to septicemia (bacterial infection of blood) and death of the larva.
The Cry protein is highly specific — it requires:
- An alkaline gut environment (human stomach is acidic, pH ~2)
- Specific receptor proteins on gut cells (absent in human gut cells)
Without both conditions, the protein is simply digested like any other protein. This is why Bt crops are considered safe for human consumption.
Why This Works
Bt cotton is a classic example of biotechnology solving an agricultural problem. The cotton bollworm (Helicoverpa armigera) was devastating cotton crops in India, requiring heavy pesticide use. Bt cotton reduced pesticide use by 50-70% and significantly increased yields for Indian farmers.
The specificity of the Cry toxin is remarkable — it kills target insects but is harmless to beneficial insects (like bees), birds, mammals, and humans. This specificity comes from the receptor-binding step — only insects with matching gut receptors are affected.
NEET commonly tests: “What is the active form of Bt toxin?” Answer: The Cry protein is produced as an inactive protoxin that becomes active only in the insect’s alkaline gut. Also frequently asked: “Name the specific Cry genes for different pests” — cry1Ac/cry2Ab for bollworm (lepidopterans), cry3Bb for corn rootworm (coleopterans).
Common Mistake
The most common error: writing that Bt toxin is active when produced by the plant. The plant produces the inactive protoxin (crystal form). It becomes active ONLY in the insect’s alkaline gut. If the toxin were active in the plant, it would damage the plant’s own cells.
Another mistake: confusing Bt cotton with a pesticide-spraying plant. Bt cotton does not spray or release any toxin externally. The Cry protein is present inside the plant tissues. The insect must eat the plant tissue to be affected — this is why Bt crops are effective against chewing pests but not against sucking pests (like aphids and whiteflies, which feed on phloem sap, not leaf tissue).