Biosynthesis of indolic compound in plants and animals. The essential amino acid tryptophan arises from the shikimate pathway. chorismate/anthranilate, whereas the other aromatic amino acids, phenylalanine and tyrosine, come from prephenate/arogenate branching and lead to phenylpropanoids. Tryptophan is converted to 5-hydroxy tryptophan and, subsequently, to serotonin, from which melatonin is synthesized by HIOMT, in both animals and plants. Indole-3-acetic acid synthesis, by tryptophan decarboxylase, is exclusive to plants and leads to tryptamine precursors.