Indole-3-acetic acid (IAA), the main natural auxin in plants, is essential for plant growth and development.The amino acid tryptophan (Trp) is a precursor for synthesizing IAA in plants.It has long been hypothesized that plants use multiple routes to synthesize auxin from Trp.However, recent findings demonstrate that Arabidopsis mainly uses a two-step pathway to convert Trp to IAA.