Question

What triggers activation of protoxins to active Bt toxin of Bacillus thuringiensis in bollworm?
A. Body temperature
B. Moist surface of midgut
C. Alkaline pH of gut
D. Acidic pH of stomach

Answer 1

Hint: Bacillus thuringiensis (Bt) spores are available in sachets as dried spores which are mixed with water and sprayed on to vulnerable plants (brassicas, fruit trees etc), to protect them from crop damage.

Complete Answer:
Some strains of Bacillus thuringiensis have proteins that kill insects like coleopterans (beetles), lepidopterans (tobacco budworm, armyworm) & dipterans (flies, mosquitoes).

Option A, B and D: There is no effect of body temperature, moist surface of midgut and acidic pH of stomach on activation of protoxins to active Bt toxin. Therefore, these are the incorrect options.

Option C: B. thuringiensis forms a toxic insecticidal protein (Bt toxin) crystal during a particular phase of their growth. It does not kill the Bacillus as it exists as inactive proteins. When an insect ingest the inactive toxin, it is converted into active toxin due to the alkaline pH of the gut which solubilise the crystals. Therefore, this is the correct option.

The toxin binds to the surface of midgut epithelial cells and creates pores. It causes cell swelling and lysis and death of the insect.

Thus, the correct option is C) Alkaline pH of gut.

Note: Bt toxin genes were isolated from B. thuringiensis and incorporated into crop plants such as cotton. Most Bt toxins are insect-group specific. The toxin is coded by a gene named cry. E.g. proteins encoded by genes cryIAc & cryIIAb control the cotton bollworms that of cryIAb controls corn borer.