One of the applications of biotechnology is the production of insect resistant crop plants." Justify the statement with reference to Bt-cotton.

Explanation

Bt cotton :
• Bacillus thuringiensis produces crystal proteins called Cry proteins which are toxic to larvae of insects like tobacco bud worm, army worm, beetles and mosquitoes.
• Cry proteins exist as inactive protoxins and get converted into active toxin when ingested by the insect, as the alkaline pH of the gut solubilises the crystals.
• The activated toxin binds to the surface of epithelial cells of midgut and creates pores.
• This causes swelling and lysis of cells leading to the death of the insect (larva).
• The genes (cry genes) encoding this protein are isolated from the bacterium and incorporated into several DE Lcrop plants like cotton, tomato, corn, rice, soyabean, etc.
• The proteins encoded by the following cry genes control the corresponding pests. = Cry I Ab controls corn borer.
= Cry III Ab controls colarado potato beetle.
Cry III Bb controls corn rootworm.
(b) Protection Against Nematodes :
• A nematode called Meloidegyne incognitia infects plants and reduces their yield.
• The specific genes in the form of cDNA) from the parasite are introduced into the plant using Agrobacterium as the vector.
• The genes are introduced in such a way that both sense/coding RNA and antisense RNA (complementary to the sense/coding RNA) are produced.
• Since these two RNAs are complementary, they form a double stranded RNA (ds RNA).
• This silences the specific RNA of the nematode, by a process called RNA-interference. It prevents the translation of a specific mRNA (silencing).
• As a result, the parasite cannot live in the transgenic host and the transgenic plant is protected from the pest.