Bt cotton plants are genetically engineered to resist insect pests by expressing endotoxin genes derived from the soil bacterium Bacillus thuringiensis. How does the Bt toxin kill susceptible insect larvae?
Answer and explanation
Correct answer: A. Protoxin solubilizes in alkaline insect gut, cleaved to active toxin creating lytic pores
- Step 1: Bacillus thuringiensis produces crystalline protein inclusions containing inactive protoxins (Cry proteins) during sporulation.
- Step 2: When an insect larva ingests the crystal, the strongly alkaline pH of the midgut () dissolves the crystal, and midgut proteases cleave it into the active toxin.
- Step 3: The activated toxin binds specific cadherin receptors on the surface of midgut epithelial cells, oligomerizing to form lytic membrane pores; this leads to osmotic cell swelling, gut paralysis, starvation, and death.
Answer
The ingested inactive protoxin is solubilized in the alkaline pH of the insect midgut, cleaved into an active toxin that binds to epithelial cells, creating pores that cause cell swelling and lysis.
Why not the others: Bt toxin is not an acetylcholinesterase inhibitor (that describes organophosphate insecticides); It does not hydrolyze chitin; It does not act as a mitochondrial electron transport blocker.
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