Subtopic: Fundamentals of Biomolecules, enzyme kinetics
Question Statement:
Which one of the following plant secondary metabolites is a natural insecticide?
Options:
(A) Digitoxin
(B) Pyrethrin
(C) Salicylic acid
(D) Avenacin A-1
GATE 2018 Biotechnology
Official Answer Key:
B
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2018 Q.30)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (1 Mark)
- **Topic**: Plant Biotechnology / Secondary Metabolites (Natural Pesticides)
**2. Detailed Step-by-Step Analysis**:
- Plant secondary metabolites provide evolutionary chemical defenses against herbivores, pests, and pathogens:
- **Pyrethrins**: Natural monoterpene esters extracted from the flower heads of *Chrysanthemum cinerariifolium* (Dalmatian pyrethrum). Pyrethrins act as potent neurotoxins in insects by binding to voltage-gated sodium channels along nerve axons, preventing channel inactivation and leading to repetitive discharges, paralysis ("knockdown effect"), and death. They are widely used commercial botanical insecticides.
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: *Digitoxin* is a cardiac glycoside extracted from *Digitalis purpurea* (foxglove) used clinically as a cardiotonic to treat congestive heart failure.
- **Option (B) [CORRECT]**: *Pyrethrin* is a well-characterized natural botanical insecticide.
- **Option (C) [INCORRECT]**: *Salicylic acid* is a phenolic plant hormone mediating systemic acquired resistance (SAR) against plant pathogens, not an insecticide.
- **Option (D) [INCORRECT]**: *Avenacin A-1* is a triterpenoid saponin found in oat roots that provides antifungal resistance against the "take-all" fungus (*Gaeumannomyces graminis*).
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **B**
- **Scoring Criteria**: +1.0 Mark (Negative marking: -0.33 marks).
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