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October 25, 2011

Researchers clarify main auxin biosynthesis pathway

New findings by scientists at the RIKEN Plant Science Center (PSC) have shed first-ever light on the biosynthesis pathways of a family of phytohormones known as auxins, which play a central role in plant growth and development. The findings open the door to new techniques for boosting crop yields, helping to tackle the world's pressing agricultural challenges.

Proposed IAA biosynthesis pathway

Proposed IAA biosynthesis pathway

By promoting cell expansion, division and differentiation, and regulating developmental events and environmental responses, the family of plant hormones known as auxins is central to a wide variety of growth and behavioral processes in the plant's life cycle. As the most abundant and potent auxin, indole-3-acetic acid (IAA) in particular, discovered more than 70 years ago, plays a fundamental role in plant growth and development, yet surprisingly little is known about how it is synthesized.

With their latest research, published in the Proceedings of the National Academy of Sciences (PNAS), the PSC research team and the collaborators, including Yunde Zhao of University of California San Diego, sought to clarify the pathways via which this synthesis occurs. Earlier research showed that plants synthesize IAA from tryptophan, an amino acid, via a set of four such pathways, and the team's own research in 2009 indicated that IAA biosynthesis may vary among plant species. The overall picture of these pathways, however, remained unclear.

One point of contention with existing hypotheses concerns two families of enzymes named TAA (TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS) and YUC (YUCCA), each of which is required for IAA biosynthesis. While earlier research had placed these two families on distinct biosynthesis pathways, mutants deficient in TAA and YUC have very similar phenotypes, suggesting a linear relationship between the two.

Through a series of experiments on Arabidopsis using advanced mass spectroscopy techniques and enzyme assays, the research team confirmed for the first time this linear relationship. Their findings show that the TAA and YUC families are not on separate pathways, as earlier believed, but on the same pathway, with YUC catalyzing a rate-limiting step in the synthesis of IAA from indole-3-pyruvic acid (IPA). By clarifying this key step, the discovery unravels the 70-year old mystery of how auxins are synthesized and sets the groundwork for new agricultural techniques aimed at solving the world's pressing agricultural challenges.

Reference

  • Kiyoshi Mashiguchi, Keita Tanaka, Tatsuya Sakai, Satoko Sugawara, Hiroshi Kawaide, Masahiro Natsume, Atsushi Hanada, Takashi Yaeno, Ken Shirasu, Hong Yao, Paula McSteen, Yunde Zhao, Ken-ichiro Hayashi, Yuji Kamiya and Hiroyuki Kasahara “The main auxin biosynthesis pathway in Arabidopsis”. Proceedings of the National Academy of Sciences of the United States of America, 2011, doi:10.1073/pnas.1108434108

Contact

Hiroyuki Kasahara
Growth Regulation Research Group
RIKEN Plant Science Center
Tel: +81-(0)45-503-9660 / Fax: +81-(0)45-503-9662

Jens Wilkinson
RIKEN Global Relations and Research Coordination Office
Tel: +81-(0)48-462-1225 / Fax: +81-(0)48-463-3687
Email: pr@riken.jp

Figure 1

IAA biosynthesis pathway proposed in the present study. The bold arrows indicate proposed functions of TAA1 and YUC, respectively.

Figure 2

Plants cannot grow normally if IAA biosynthesis by TAA and YUC gene families is disrupted (Left: normal, upper right: TAA-deficient mutant, lower right: YUC-deficient mutant).