The IAOx-Dependent IAA Biosynthesis Pathway: Acquired Insights, Paradigm Shifts, and Unresolved Questions

The auxin indole-3-acetic acid (IAA) is essential for plant growth and stress adaptation. Its biosynthesis via the indole-3-acetaldoxime (IAOx) pathway has recently undergone a paradigm shift. Recent genetic and metabolomic studies have fundamentally revised the indole-3-acetaldoxime (IAOx) pathway from a linear route (IAOx→IAN→IAM→IAA) to a dynamic network. This review synthesizes this paradigm shift by integrating evidence from key Arabidopsis studies. Crucially, mutants disrupting multiple downstream enzyme families fail to block IAA overproduction in the IAOx-accumulating superroot 2 (sur2) background. Functioning as a central branching point between auxin and defense metabolism, the tryptophan-derived metabolite IAOx, along with indole-3-acetonitrile (IAN) and indole-3-acetamide (IAM), elicits auxin responses via independent, tissue-specific pathways, with no metabolic requirement for IAM as a universal intermediate. Furthermore, IAN and IAM levels do not increase with massive IAOx accumulation, indicating a bypass route from IAOx to IAA. We conclude that IAOx acts as a central metabolic hub, partitioning flux competitively between growth and defense. Resolving the unknown IAOx-converting enzyme, the signaling roles of IAN/IAM, and the logic of metabolic channeling is vital to understanding how plants integrate hormonal and stress responses.