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Plant and Cell Physiology Advance Access originally published online on October 27, 2006
Plant and Cell Physiology 2006 47(12):1603-1611; doi:10.1093/pcp/pcl027
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© The Author 2006. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

Involvement of HLS1 in Sugar and Auxin Signaling in Arabidopsis Leaves

Masa-aki Ohto1,4,*, Shingo Hayashi2,5, Shinichiro Sawa3, Akiko Hashimoto-Ohta1 and Kenzo Nakamura1,2

1 Division of Developmental Biology, National Institute for Basic Biology, Myodaiji-cho, Okazaki, 444-8585 Japan
2 Laboratory of Biochemistry, Graduate School of Bioagricultural Sciences, Nagoya University, Chikusa-ku, Nagoya, 464-8601 Japan
3 Department of Biological Sciences, Graduate School of Science, University of Tokyo, Hongo, Tokyo, 113-0033 Japani

* Corresponding author: E-mail, maohto{at}ucdavis.edu; Fax, +1-530-752-2278.


  Abstract

Sugar regulates a variety of genes and controls plant growth and development similarly to phytohormones. As part of a screen for Arabidopsis mutants with defects in sugar-responsive gene expression, we identified a loss-of-function mutation in the HOOKLESS1 (HLS1) gene. HLS1 was originally identified to regulate apical hook formation of dark-grown seedlings (Lehman et al., 1996, Cell 85: 183–194). In hls1, sugar-induced gene expression in excised leaf petioles was more sensitive to exogenous sucrose than that in the wild type. Exogenous IAA partially repressed sugar-induced gene expression and concomitantly activated some auxin response genes such as AUR3 encoding GH3-like protein. The repression and the induction of gene expression by auxin were attenuated and enhanced, respectively, by the hls1 mutation. These results suggest that HLS1 plays a negative role in sugar and auxin signaling. Because AUR3 GH3-like protein conjugates free IAA to amino acids (Staswick et al., 2002, Plant Cell 14: 1405–1415; Staswick et al., 2005, Plant Cell 17: 616–627), enhanced expression of GH3-like genes would result in a decrease in the free IAA level. Indeed, hls1 leaves accumulated a reduced level of free IAA, suggesting that HLS1 may be involved in negative feedback regulation of IAA homeostasis through the control of GH3-like genes. We discuss the possible mechanisms by which HLS1 is involved in auxin signaling for sugar- and auxin-responsive gene expression and in IAA homeostasis.

Keywords: Arabidopsis - Auxin - HOOKLESS1 - Sugar signaling

Abbreviations: ARF, auxin response transcription factors; AuxRE, auxin response element; HXK, hexokinase; Suc, sucrose.

4 Present address: Department of Plant Sciences, Plant Reproductive Biology Building, Mail Stop 5, University of California, One Shields Avenue, Davis, CA 95616, USA.

5 Present address: Medical and Biological Laboratories, Ina, Nagano, 396-0002 Japan.

(Received September 3, 2006; Accepted October 16, 2006)
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