Subtopic: Fundamentals of Biomolecules, enzyme kinetics
Question Statement:
The plant hormone indole-3-acetic acid is derived from
Options:
(A) histidine
(B) tyrosine
(C) tryptophan
(D) proline
Official Answer Key:
C
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2017 Q.11)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (1 Mark)
- **Topic**: Plant Physiology & Biochemistry (Auxin Biosynthesis Pathways)
**2. Detailed Step-by-Step Analysis**:
- **Indole-3-acetic acid (IAA)** is the principal, naturally occurring auxin in higher plants, regulating cell elongation, apical dominance, vascular differentiation, and tropisms.
- The chemical structure of IAA consists of an indole ring attached to a carboxymethyl group.
- In plants, the indole core is derived directly from the aromatic amino acid **L-Tryptophan** via several well-characterized tryptophan-dependent biosynthetic pathways:
1. The **IPA (Indole-3-pyruvic acid) pathway**: Tryptophan is transaminated to IPA by TAA1, followed by oxidative decarboxylation to IAA by YUCCA flavin monooxygenases.
2. The **TAM (Tryptamine) pathway**.
3. The **IAN (Indole-3-acetonitrile) pathway**.
- In all these classic pathways, **tryptophan** is the biochemical precursor.
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: Histidine contains an imidazole ring, not an indole ring, and is a precursor for histamine.
- **Option (B) [INCORRECT]**: Tyrosine has a phenol side chain and serves as a precursor for catecholamines, dopamine, and thyroid hormones.
- **Option (C) [CORRECT]**: Tryptophan is the specific aromatic amino acid precursor for indole-3-acetic acid (IAA) biosynthesis in plants.
- **Option (D) [INCORRECT]**: Proline is an imino acid involved in collagen structure and osmoprotection.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **C**
- **Scoring Criteria**: +1.0 Mark (Negative marking: -0.33 marks).
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