The biosynthesis of auxin: how many paths truly lead to IAA?

The plant auxin indole-3-acetic acid (IAA) plays critical roles in plant growth and development. There are two main strategies proposed for plant synthesis of IAA: the Trp-dependent (TD) and the Trp-independent (TI) pathways. Four TD pathways, namely the indole-3-acetamide pathway, the indole-3-pyruvic acid pathway, the tryptamine pathway and the indole-3-acetaldoxime pathway, have been postulated, identified and extensively studied. On the other hand, neither genes nor mutants involved in the TI pathway have been identified to date. Interestingly, some bacteria have auxin synthesis pathways that are similar to those in plants, indicating conserved biosynthetic mechanisms. Over the past few years, genetic, biochemical and molecular studies have greatly advanced our understanding of auxin biosynthesis. This review both summarizes recent advances in genetic and molecular knowledge and addresses the unsolved questions regarding auxin biosynthesis pathways in plants.