Bt crops: pest resistance with Bt toxin
Bt crops carry cry genes from Bacillus thuringiensis. The inactive protoxin turns active in the insect's alkaline gut, makes pores in gut cells and kills the pest.
The Bt bacterium
Some strains of the bacterium Bacillus thuringiensis make proteins that kill certain insects: lepidopterans (tobacco budworm, armyworm), coleopterans (beetles) and dipterans (flies, mosquitoes). During a particular phase of growth, the bacterium forms protein crystals. These crystals contain a toxic insecticidal protein. The gene that codes for the toxin is called cry.
How Bt toxin kills an insect
- 1The bacterium stores the toxin as an inactive protoxin in crystals; the Bt plant makes the same protoxin from the cry gene.
- 2A caterpillar, such as a bollworm, eats the plant tissue.
- 3★The alkaline pH of the insect's gut solubilises the protoxin (dissolving the crystals, if it came from the bacterium) and turns it into the active toxin.
- 4The active toxin binds to the surface of the midgut epithelial cells (cells lining the middle gut).
- 5It makes pores in these cells, so they swell and burst (lysis).
- 6★The insect dies.
★ key step
Making a Bt cotton plant
cry genes and their targets
| Gene | Protein controls | Crop |
|---|---|---|
| cryIAc | Cotton bollworms | Cotton |
| cryIIAb | Cotton bollworms | Cotton |
| cryIAb | Corn borer | Corn (maize) |
Protoxin vs active toxin
Most Bt toxins are insect-group specific, so the cry gene is chosen to suit the crop and its pest.
Bt crops grown
Bt genes have been put into cotton, corn, rice, tomato, potato and soybean. Bt cotton is the main GM crop grown in India. Farmers spray far less insecticide against bollworms, which lowers costs and protects useful insects.
Bt cotton is grown widely in India and has cut insecticide spraying against bollworms. The pink bollworm has since become resistant in many cotton areas, so farmers are advised to grow a few refuge rows of non-Bt cotton to slow this down.
3 marks: “Explain how Bt toxin kills an insect but does not kill the bacterium that produces it.” In the bacterium the toxin is an inactive protoxin in crystals. In the insect's alkaline gut the crystals dissolve and the toxin becomes active. It binds midgut epithelial cells, makes pores, the cells swell and lyse, and the insect dies.
2 marks: “Name the genes that control cotton bollworms and the gene that controls corn borer.” cryIAc and cryIIAb control cotton bollworms; cryIAb controls corn borer.
2 marks: “Why are different cry genes chosen for different crops?” Most Bt toxins are specific to an insect group, so the gene must match the pest that attacks that crop.
Alkali activates: Bt protoxin wakes up only in the alkaline insect gut. Bollworm: cryIAc and cryIIAb; corn borer: cryIAb.