The shifting paradigms of auxin biosynthesis

•We summarize the long and complex history of auxin biosynthesis in plants.•At least five pathways have been proposed, including four from tryptophan.•Tryptophan can be converted to auxin via a single intermediate, indole-3-pyruvic acid.

Auxins are an important group of hormones found in all land plants and several soil-dwelling microbes. Although auxin was the first phytohormone identified, its biosynthesis remained unclear until recently. In the past few years, our understanding of auxin biosynthesis has improved dramatically, to the stage where many believe there is a single predominant pathway in Arabidopsis (Arabidopsis thaliana L.). However, there is still uncertainty over the applicability of these findings to other plant species. Indeed, it appears that in certain organs of some species, other pathways can operate. Here we review the key advances that have led to our current understanding of auxin biosynthesis and its many proposed pathways.