Question

Arrange the following steps which are involved in action of Bt toxin proteins on insects in correct sequence.
(A) Conversion of inactive toxin into active one by alkaline pH of the gut of insect
(B) Creation of pores that cause cell swelling and lysis which leads to death of insect
(C) Inactive protoxins is ingested by insect
(D) Activated toxin binds to the surface of midgut epithelial cells of insect
Choose the correct answer from the options given below:

• A. (A), (D), (C), (B)
• B. (D), (B), (C), (A)
• C. (C), (A), (D), (B)
• D. (C), (B), (A), (D)

Hint:

Bt Toxin Action Cycle:
$\text{Protoxin Ingestion (C)} \xrightarrow{\text{Alkaline pH}} \text{Active Toxin (A)} \xrightarrow{\text{Midgut Epithelium}} \text{Binding (D)} \xrightarrow{\text{Pores}} \text{Lysis \& Death (B)}$.

Answer 1

The Correct Option is C

Concept:
Bacillus thuringiensis produces crystal endotoxin proteins (Bt toxins) that act as insecticidal agents following ingestion and activation within susceptible insect larvae.

Step 1: Mechanism of Action of Bt Toxin:
1. Ingestion (C): The insect feeds on the transgenic plant parts and ingests the insecticidal crystalline protein in its inactive protoxin form.
2. Solubilization and Activation (A): Inside the insect's alimentary canal, the alkaline pH of the midgut solubilizes the protein crystals. Proteolytic cleavage converts the inactive protoxin into an active form of the toxin.
3. Receptor Binding (D): The activated toxin binds specifically to receptors present on the apical brush border membrane of the insect midgut epithelial cells.
4. Pore Formation and Lysis (B): The bound toxin oligomerizes and inserts into the membrane, creating trans-membrane lytic pores. This causes osmotic swelling, cell lysis, gut perforation, and ultimately results in the death of the insect.

Step 2: Determining the Correct Sequence:
The correct order of events is:
Ingestion of inactive protoxins (C) $\rightarrow$ Activation by alkaline pH (A) $\rightarrow$ Binding to midgut epithelium (D) $\rightarrow$ Pore formation and cell lysis (B).
This forms the sequence (C), (A), (D), (B).

Step 3: Final Answer:
Hence, Option (C) is the correct answer.