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DEVELOPMENT AND HORMONE ACTION

The TRANSPORT INHIBITOR RESPONSE2 Gene Is Required for Auxin Synthesis and Diverse Aspects of Plant Development^{1,[C],[W],[OA]}

Department of Biology, Indiana University, Bloomington, Indiana 47405

The plant hormone auxin plays an essential role in plant development. However, only a few auxin biosynthetic genes have been isolated and characterized. Here, we show that the TRANSPORT INHIBITOR RESPONSE2 (TIR2) gene is required for many growth processes. Our studies indicate that the tir2 mutant is hypersensitive to 5-methyl-tryptophan, an inhibitor of tryptophan synthesis. Further, treatment with the proposed auxin biosynthetic intermediate indole-3-pyruvic acid (IPA) and indole-3-acetic acid rescues the tir2 short hypocotyl phenotype, suggesting that tir2 may be affected in the IPA auxin biosynthetic pathway. Molecular characterization revealed that TIR2 is identical to the TAA1 gene encoding a tryptophan aminotransferase. We show that TIR2 is regulated by temperature and is required for temperature-dependent hypocotyl elongation. Further, we find that expression of TIR2 is induced on the lower side of a gravitropically responding root. We propose that TIR2 contributes to a positive regulatory loop required for root gravitropism.

² Present address: Department of Biological Sciences, Graduate School of Science, University of Tokyo, Tokyo 113–0033, Japan.

³ Present address: Cell and Developmental Biology, University of California San Diego, La Jolla, CA 92093.

⁴ Present address: Department of Plant Pathology, Pennsylvania State University, University Park, PA 16802.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Mark Estelle (mestelle{at}ucsd.edu).

^[C] Some figures in this article are displayed in color online but in black and white in the print edition.

^[W] The online version of this article contains Web-only data.

^[OA] Open access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.109.138859

^* Corresponding author; e-mail mestelle{at}ucsd.edu.

Received March 19, 2009; accepted June 27, 2009; published July 22, 2009.