Lesson 5 of 12 · 9 min
Bt cotton
NCERT §10.1
At a cotton farm in Telangana, the farmer splits open a boll to show Kavya: no bollworm inside. 'Before Bt, every boll had a worm,' he says.
The lesson in notes
In short
Bt toxin is made by the bacterium Bacillus thuringiensis (Bt). Its gene has been cloned and expressed in plants, giving insect resistance without insecticides: in effect, a bio-pesticide.
Crops given Bt genes include cotton and corn, and also rice, tomato, potato and soyabean.
Some strains of B. thuringiensis make proteins lethal to particular insects: lepidopterans such as the tobacco budworm and armyworm, coleopterans (beetles), and dipterans (flies and mosquitoes).
During one phase of growth the bacterium forms protein crystals that contain a toxic insecticidal protein.
The toxin does not kill the Bacillus because it is stored as an inactive protoxin.
In an insect that eats it, the alkaline pH of the gut solubilises the crystals and converts the protoxin into the active toxin.
The active toxin binds to the surface of the midgut epithelial cells and creates pores, so the cells swell and lyse and the insect dies.
Most Bt toxins act on one insect group, so the gene chosen depends on the crop and its pest. The toxin genes are named cry.
Proteins coded by cryIAc and cryIIAb control cotton bollworms; the protein of cryIAb controls the corn borer.