Question

The toxic protein produced by B. thuringiensis
a. It acts in neutral medium and binds to epithelial cells of hindgut
b. It acts in acidic medium and binds to epithelial cells of foregut
c. It acts in alkaline medium and binds to epithelial cells of foregut
d. It acts in alkaline medium and binds to epithelial cells of midgut

Answer 1

Hint: The key to answer this question is to understand the mechanism of action of the Cry toxin in the gut of the insects and to remember the pH condition of the insect gut. The Cry toxin binds to the insect gut cell membrane and paralyzes the digestive tract forming a pore.

Complete answer:
Bacillus thuringiensis is the bacteria which is used to make crops insect resistant. The bacteria produce the crystal toxin or the Cry toxin which kills the insects by forming or inserting pores into the membrane. Previously insecticides such as DDT were used but knowing the effects of DDT on the environment now it is banned which makes using bacterium to control the insects a better option.

The Cry toxin is not toxic to all cells but only certain cells of certain insect species. The mode of action of the Cry toxin is simple yet complicated. It is widely accepted that the Cry toxins lyse the cells of the midgut epithelium by forming pores in the apical microvilli membrane of the cells. The crystal proteins dissolve in the alkaline environment of the gut.

The mechanism involves a chain of reactions of the protein being cleaved and binding to specific receptors in the midgut epithelium that’s why it's mentioned that it’s simple yet complicated. Further the lysis of the cells happens and the cell contents provide a medium for the spores of the bacterium to germinate and it leads to severe infection in the gut of the insect eventually leading to the death of the insect as it stops eating food.

Hence, the correct answer is option (D).

Additional information:
Transgenic plants are the plants that contain genes other than the normal genes present in them. In BT crops, the Cry gene from the bacterium Bacillus thuringiensis which produces the Cry toxin is introduced into the plants which then becomes resistant to the insects as when the insects feed on them they are unable to survive. The other mode of using this bacterium as a mode of insecticide is to spray the bacterial suspension on the plants as they don’t harm the plants.

Note: Due to the widespread use of BT crops, the insects have evolved a resistance to the toxin and continue to damage the crops and cause reduction in production of the crops. Also, the BT crops like BT cotton, BT brinjal etc., are more expensive than the normal crops. The long-term effects of consumption of BT crops are not yet understood. The introduction of transgenic crops like BT cotton etc., can disrupt the gene flow and cause the wild variety to disappear from nature.