This is the precursor of Indole-3-acetic acid

The correct option is Tryptophan.

Indole-3-acetic acid (IAA) is the most common naturally occurring plant hormone of the auxin class. Auxins play a crucial role in regulating plant growth and development, including processes like cell elongation, division, and differentiation.

To understand why tryptophan is the precursor, we look at the biosynthesis of IAA in plants:
1. Plants primarily synthesize indole-3-acetic acid (IAA) using the amino acid L-tryptophan as the starting material.
2. Tryptophan contains an indole ring structure in its side chain, which is chemically inherited by indole-3-acetic acid.
3. There are several biochemical pathways (such as the indole-3-pyruvic acid pathway and the indole-3-acetaldoxime pathway) through which plants convert tryptophan into IAA by removing the amino group and decarboxylating the side chain.

Let us briefly evaluate the other options to confirm why they are incorrect:
• Methionine: This amino acid serves as the precursor for the gaseous plant hormone ethylene, not auxin (IAA).
• Glycine: This is the simplest amino acid and is involved in various metabolic pathways (like photorespiration and purine synthesis), but it does not serve as a precursor for auxin.
• Isopentenyl pyrophosphate: This is a key intermediate in the family of isoprenoid compounds and serves as a precursor for other plant hormones like cytokinins, gibberellins, and abscisic acid, but not for IAA.