Reassessing the role of YUCCAs in auxin biosynthesis.

It is remarkable that although auxin was the first growth-promoting plant hormone to be discovered, and although more researchers work on this hormone than on any other, we cannot be definitive about the pathways of auxin synthesis in plants. In 2001, there appeared to be a dramatic development in this field, with the announcement of a new gene, and a new intermediate, purportedly from the tryptamine pathway for converting tryptophan to the main endogenous auxin, indole-3-acetic acid (IAA). Recently, however, we presented evidence challenging the original and subsequent identifications of the intermediate concerned.