On the question of β-indolyl-acetic acid synthesis from indole without a tryptophan intermediate.

Experiments with sterile grown maize coleoptiles were carried out to decide whether or not a biosynthetic path for β-indolyl-acetic acid (IAA) from indole exists without tryptophan occurring as an intermediate. β-Indolyl-acrylic acid as a tryptophan synthetase inhibitor significantly reduces the yield of [(3)H]tryptophan obtained from [(3)H]indole while the reduction in the [(3)H]IAA yield is considerably less pronounced. This, however, indicates only a non-linear relationship between the tryptophan concentration and the IAA yield and not the sought path. Moreover, double labelling combined with isotope competition methods in experiments with [(3)H]indole and L-[(14)C]serin show that all IAA synthesized from [(3)H]indole is produced on a path involving the synthesis of tryptophan as an intermediate.