Kogl and Haagen-smit introduces the term auxin. They isolated three chemical auxin-a (from human urine), auxin-b (from corn germ oil) and heteroauxin (from human urine), out of these heteroauxin proved to be real auxin. It is indole-3-acetic acid or IAA. It is synthesised mainly from tryptophan in shoot apex and developing seeds. IAA is a natural auxin.
So, tryptophan is precursor of IAA. Tryosine is precursor of thyroxine and adrenalin. Phenylalanine is precursor of pigment, melanin.