The auxins, IAA and PAA, are synthesized by similar steps catalyzed by different enzymes

One of the fundamental plant growth substances, indole-3-acetic acid (IAA), belongs to a class of phytohormones known as auxins. The main IAA biosynthesis pathway involves the conversion of tryptophan to indole-3-pyruvic acid, which is in turn converted to IAA. The two enzymes responsible for these conversions, members of the TAA1 and YUCCA gene families, respectively, have recently been implicated in the synthesis of another auxin, phenylacetic acid (PAA). While there is some evidence to support this theory, there are also some concerns. Here we address the question: to what extent does the TAA1/YUCCA system contribute to the biosynthesis of PAA? In addition, we highlight the importance of measuring auxin metabolites and conjugates in addressing such questions.