IAA is a hormone responsible for plant growth. The amino acids tryptophan is the precursors of IAA.It is mainly produced in the buds and young leaves of plants.

Auxin Precursor for IAA is Tryptophan

Explanation:
Auxins are a group of plant hormones that play a crucial role in the growth and development of plants. One of the most important auxins is Indole-3-acetic acid (IAA), which is synthesized from the amino acid tryptophan. Tryptophan serves as the precursor for IAA biosynthesis in plants.

IAA Biosynthesis:
The biosynthesis of IAA involves several enzymatic steps, with tryptophan as the starting point. Here is a brief overview of the biosynthesis pathway:

1. Tryptophan is converted into indole-3-pyruvic acid (IPA) by the enzyme tryptophan aminotransferase (TAA). This reaction involves the transfer of an amino group from tryptophan to a keto acid, resulting in the formation of IPA.

2. IPA is then converted into indole-3-acetaldehyde (IAld) by the enzyme indole-3-pyruvate decarboxylase (IPDC). This step involves the decarboxylation of IPA, leading to the formation of IAld.

3. IAld is further converted into IAA by the enzyme indole-3-acetaldehyde oxidase (IAAO). This reaction involves the oxidation of IAld, resulting in the formation of IAA.

Importance of Tryptophan:
Tryptophan is an essential amino acid that is not only a building block for proteins but also serves as a precursor for the synthesis of various plant metabolites, including IAA. It is obtained by plants through their diet or by de novo synthesis.

Regulation of IAA Biosynthesis:
The biosynthesis of IAA is tightly regulated by various factors, including light, hormones, and environmental cues. The availability of tryptophan is one of the key factors that influence the synthesis of IAA. Changes in tryptophan levels can affect the overall levels of IAA in plants.

Conclusion:
In conclusion, the correct answer is option 'D' - Tryptophan. Tryptophan serves as the precursor for IAA biosynthesis in plants. It undergoes a series of enzymatic reactions to form IAA, which is an essential auxin hormone involved in plant growth and development.