The IAA is a natural plant growth regulator. It is synthesized in large amounts by apices of stem and in small amounts by apices of roots. It is synthesized by an amino acid tryptophan through multiple step metabolic reactions. Other metabolic precursors are indole acetonitrile, indoleacetaldehyde and indole pyurvic acid. It promotes cell enlargement, cell differentiation, growth, flowering and fruit formation. IAA was first isolated from human urine. During the oxidation of protein and amino acid tryptophan in cells (hepatocytes) of liver IAA or precursors of IAA are synthesized. IAA does not have metabolic role so excreted out of body through urine.