4. The plant hormone indole-3-acetic acid (I.) is ores,: in most plants. The structure of this hormone is related to which one of the following amino acids?

(1) Glutamic acid            (2) Aspartic acid

(3) lysine      (4) Tryptophan

Correct answer: (4) Tryptophan.

Indole-3-acetic acid (IAA) is the main natural auxin and is structurally an indole ring attached to an acetic acid side chain, directly derived from the amino acid tryptophan, which also contains an indole ring. In plants, the major IAA biosynthetic pathways are tryptophan-dependent, where tryptophan is converted through intermediates such as indole-3-pyruvic acid to IAA. Glutamic acid, aspartic acid, and lysine lack the indole ring and are not structural precursors of IAA, so options (1), (2), and (3) are incorrect.