11. The plant hormone indole-3-acetic acid is derived from
(a) Histidine (b) Tyrosine
(c) Tryptophan (d) Proline
Indole-3-acetic acid (IAA), a key auxin hormone, drives plant growth processes like cell elongation and root development. The correct answer to the query is (c) Tryptophan, as plants primarily synthesize IAA from this amino acid through well-established biochemical pathways.
Correct Answer
The plant hormone indole-3-acetic acid is derived from (c) Tryptophan. IAA biosynthesis mainly occurs via tryptophan-dependent pathways, such as conversion through indole-3-pyruvic acid (IPyA) or indole-3-acetaldoxime in species like Arabidopsis thaliana.
Tryptophan, an essential amino acid with an indole ring, serves as the primary precursor in shoot apices and young leaves, where enzymes transform it into IAA to regulate tropisms and apical dominance.
Option Explanations
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(a) Histidine: Incorrect. Histidine, rich in imidazole rings, acts as a precursor for histamine in animals but plays no direct role in IAA synthesis in plants.
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(b) Tyrosine: Incorrect. Tyrosine contributes to catecholamine synthesis and phenylpropanoid pathways for lignin, not auxin production like IAA.
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(c) Tryptophan: Correct. Its indole structure directly yields IAA via pathways including IPyA, indole-3-acetamide, or indole-3-acetonitrile routes.
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(d) Proline: Incorrect. Proline supports osmotic stress responses and collagen-like proteins but lacks connection to auxin biosynthesis.
Biosynthesis Pathways
Plants produce IAA predominantly from tryptophan in multiple steps, influenced by pH and temperature. Key routes include:
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Tryptophan → Indole-3-pyruvic acid → Indole-3-acetaldehyde → IAA (most common).
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Tryptophan → Indole-3-acetaldoxime → Indole-3-acetonitrile → IAA (in Brassicaceae).
Minor tryptophan-independent paths exist but are less significant.
This knowledge aids biotechnology students in understanding microbial engineering for auxins and plant growth regulation.
