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Published on January 11, 2006; 10.1104/pp.105.067918

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Received July 1, 2005
Returned for revision August 25, 2005
Accepted December 19, 2005

The regulation of DWARF4 expression is likely a critical mechanism in maintaining the homeostasis of bioactive brassinosteroids in Arabidopsis thaliana

Ho Bang Kim , Mi Kwon , Hojin Ryu , Shozo Fujioka , Suguru Takatsuto , Shigeo Yoshida , Chung Sun An , Ilha Lee , Ildoo Hwang , and Sunghwa Choe *

Department of Biological Sciences, College of Natural Sciences, Seoul National University, Seoul 151-747, Korea
Division of Molecular and Life Science, Pohang University of Science & Technology, Pohang 790-784, Korea
RIKEN (The Institute of Physical and Chemical Research), Wako-shi, Saitama 351-0198, Japan
Department of Chemistry, Joetsu University of Education, Joetsu-shi, Niigata 943-8512, Japan

* Corresponding author; email: shchoe{at}snu.ac.kr.

Mutants that are defective in brassinosteroid (BR) biosynthesis or signaling display severely retarded growth patterns due to absence of growth promoting effects by BRs. Arabidopsis (Arabidopsis thaliana) DWARF4 catalyzes a flux-determining step in the BR biosynthetic pathways. Thus, it is hypothesized that the tissues of DWF4 expression may represent the sites of BR biosynthesis in Arabidopsis. Here we show that DWF4 transcripts accumulate in the actively growing tissues, such as root, shoot apices with floral clusters, joint tissues of root and shoot, and dark-grown seedlings. Conforming to the RNA gel blot analysis, DWF4:GUS histochemical analyses more precisely define the tissues that express the DWF4 gene. Examination of the endogenous levels of BRs in 7 and 6 different tissues of wild type and brassinosteroid insensitive1-5 (bri1-5) mutant, respectively, revealed that BRs are significantly enriched in roots, shoot tips, and joint tissues of roots and shoots. In addition, DWF4:GUS expression was negatively regulated by BRs. DWF4:GUS activity was increased by treatment with brassinazole, a BR biosynthetic inhibitor, and decreased by exogenous application of bioactive BRs. When DWF4:GUS was expressed in a different genetic background, its level was down-regulated in brassinazole resistant1 (bzr1-D), confirming that the BZR1 act as negative regulator of DWF4. Interestingly, in the bin2/dwf12-1D background, DWF4:GUS expression was intensified and delocalized to elongating zones of root, suggesting that BIN2 is an important factor that limits DWF4 expression. Thus, it is likely that the DWF4 promoter serves as a focal point in maintaining homeostasis of endogenous bioactive BR pools in specific tissues of Arabidopsis.




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