Indole-3-acetic acid (IAA), a plant hormone belonging to the auxin family, activates a variety of signaling cascades involved with critical functions that include shoot growth, root extension and flowering. How auxin is synthesized in plants is still unclear.

Findings from Hiroyuki Kasahara, RIKEN Plant Science Center, Yokohama, Japan, and colleagues, have now overturned earlier proposed mechanisms, revealing that the YUCCA (YUC) protein family and TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS 1 (TAA1)  work together within a single, unified pathway instead of separated ones.

The TAA family produces indole-3-pyruvic acid (IPA) and the YUC family functions in the conversion of IPA to IAA in Thalis cress (Arabidopsis thaliana). This was demonstrated by a quantification method of IPA using liquid chromatography–electrospray ionization–tandem MS. YUC proteins catalyze a rate-limiting step of the IPA pathway, which is the main IAA biosynthesis pathway in Arabidopsis.

The study represents a major step towards understanding the biochemical processes that ultimately govern plant growth. By learning the core components of the auxin biosynthesis machinery, geneticists and botanists stand to gain valuable tools for manipulating growth behavior and thereby maximizing the utility of plants with essential roles in textile and food production.

• The main auxin biosynthesis pathway in Arabidopsis,
  K. Mashiguchi, K. Tanaka, T. Sakai, S. Sugawara, H. Kawaide, M. Natsume, A. Hanada, T. Yaeno, K. Shirasu, H. Yao et al.,
  Proceedings of the National Academy of Sciences USA 2011, 108, 18512–18517.