how bt cotton plant which is an insect resistant plant is produced????
Bacillus thuringiensis is a bacterium that produces proteins to kill certain insects such as lepidopterans (armyworm), coleopterans (beetles), and dipterans (flies/ mosquitoes). B. thuringiensis produces a protein crystal containing a toxic protein (inactivated state).
Inactivated toxinActivated toxin (gut of insect)
Activated toxin binds to the epithelial cells in the midgut of insect and creates pores that cause lyses and swelling and eventually death of insect.
This toxin is encoded by a gene called Cry in the bacterium. Genes encoded by Cry IAc and Cry II Ab control cotton bollworms and those encoded by Cry IAb control corn borer.
Cry genes are introduced into the cotton plants to produce Bt cotton, which is an insect resistant variety of cotton. The steps involved in the production of the Bt cotton includes
1. Isolation of the Cry Genes encoding toxin from the Bacillus thuringiensis.
2. Introduction of these genes into the plant cells. This is achieved by using the natural vehicle called Agrobacterium tumefaciens. This bacteria is a natural pathogen of plants and causes tumour called crown gall disease in plants. The Cry genes are introduced into plasmid of this bacteria by genetic engineering and it is allowed to infect the plant. This way it introduce the toxin producing genes of Bt along with its own plasmid into plant cells.
3. The genes can be introduced into single cells or tissue also and new insect resistant plant can be produced by tissue culture.
After the introduction of genes into plant cells. These genes are expressed and produced the toxin crystals in plant and when insect feeds on this plant , it dies.
Insect-resistant crops have been one of the major successes of applying plant genetic engineering technology to agriculture; cotton (Gossypium hirsutum) resistant to lepidopteran larvae (caterpillars) and maize (Zea mays) resistant to both lepidopteran and coleopteran larvae (rootworms) have become widely used in global agriculture and have led to reductions in pesticide usage and lower production costs.
The source of the insecticidal toxins produced in commercial transgenic plants is the soil bacterium Bacillus thuringiensis (Bt). Bt strains show differing specificities of insecticidal activity toward pests, and constitute a large reservoir of genes encoding insecticidal proteins, which are accumulated in the crystalline inclusion bodies produced by the bacterium on sporulation (Cry proteins, Cyt proteins) or expressed during bacterial growth (Vip proteins). The three-domain Cry proteins have been extensively studied; their mechanism of action involves a proteolytic activation step, which occurs in the insect gut after ingestion, followed by interaction of one or both of domains II and III with “receptors” on the surface of cells of the insect gut epithelium. This interaction leads to oligomerization of the protein, and domain I is then responsible for the formation of an open channel through the cell membrane....