Figure 1.
Overview of auxin biosynthetic pathways for (A) IAA and (B) PAA. (A) It is widely accepted that Trp is converted to indole-3-pyruvic acid by the TAA1 family of amino-transferases.2,10 Indole-3-pyruvic acid is then converted to IAA by the YUCCA family of flavin-containing mono-oxygenases.4 (B) The available evidence indicates that PAA is biosynthesised from phenylalanine via phenylpyruvate.5,7 However, while Sugawara5 suggested that TAA1 enzymes catalyze the first step, and YUCCAs the second, we suggest that these enzymes play at most only minor roles.