Q.An application of biotechnology in agriculture involves production of pest resistant plants using ‘cry’ genes from a bacterium. Name the bacterium. Explain why the ‘cry’ gene kills the pest but not the bacterium itself.
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Start your 14-day free trial to unlock the full solution →The bacterium is Bacillus thuringiensis (Bt). The cry gene encodes a protein that exists as an inactive crystal in the bacterium; it becomes toxic only when activated by the alkaline pH and proteases in the insect gut, leaving the bacterium unharmed.
The bacterium in question is Bacillus thuringiensis, commonly abbreviated as Bt. This soil-dwelling microbe has been a cornerstone of biological pest control for decades, and its cry genes have revolutionized agricultural biotechnology by enabling the creation of pest-resistant crop varieties.
Bt produces a family of proteins encoded by different cry genes—cry I, cry II, cry III, and so on—each targeting specific groups of insect pests. When scientists isolate these genes and insert them into crop plants like cotton, corn, or brinjal, the transgenic plants manufacture the Bt toxin in their tissues. Insects that feed on these plants ingest the toxin and die, reducing the need for chemical pesticides.
Why the cry gene kills the pest but spares the bacterium
The elegance of this system lies in the form in which the toxin exists inside the bacterium versus inside the insect. In Bacillus thuringiensis itself, the cry gene produces a protein that remains as an inactive protoxin or crystal. These crystalline inclusions, called Bt toxin crystals, are completely harmless in their dormant state. The bacterium carries them without suffering any damage because the protein has not yet been activated.
The transformation happens only when an insect larva—say, a caterpillar feeding on a cotton plant—ingests the Bt crystals. The insect's gut environment is markedly different from the bacterial cell:
- The insect midgut is highly alkaline (pH around 9–10), whereas the bacterial cytoplasm is neutral or slightly acidic.
- The alkaline pH solubilizes the protein crystals, breaking them down from their inert crystalline form into a soluble protoxin.
- Proteases (protein-digesting enzymes) present in the insect gut then cleave the protoxin, converting it into the active toxin.
Once activated, the toxin binds to specific receptor sites on the epithelial cells lining the insect's midgut. This binding creates pores in the cell membranes, disrupting the gut lining. The insect stops feeding, its gut is paralyzed, and it eventually dies from starvation and septicemia as gut bacteria invade the body cavity. …
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