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Plant Cell Advance Online Publication
Published on November 9, 2007; 10.1105/tpc.106.048900

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Received November 14, 2006
Returned for revision October 11, 2007
Accepted October 21, 2007

SAD2, an Importin {beta}-Like Protein, Is Required for UV-B Response in Arabidopsis by Mediating MYB4 Nuclear Trafficking

Jinfeng Zhao 1, Wenhui Zhang 2, Yang Zhao 1, Ximing Gong 1, Lei Guo 2, Guoli Zhu 3, Xuechen Wang 3, Zhizhong Gong 3, Karen S. Schumaker 4, and Yan Guo 5*

1 State Key Laboratory of Plant Physiology and Biochemistry, College of Biological Sciences, China Agricultural University, Beijing 100094, China; National Institute of Biological Sciences, Beijing, Zhongguancun Life Science Park, Beijing 102206, China
2 Department of Biology, Liaocheng University, Shandong Province, 252059, China
3 State Key Laboratory of Plant Physiology and Biochemistry, College of Biological Sciences, China Agricultural University, Beijing 100094, China
4 Department of Plant Sciences, University of Arizona, Tucson, Arizona 85721
5 National Institute of Biological Sciences, Beijing, Zhongguancun Life Science Park, Beijing 102206, China

* To whom correspondence should be addressed. E-mail: guoyan{at}nibs.ac.cn.

We report that the Arabidopsis thaliana mutant sensitive to ABA and drought2 (sad2), which harbors a T-DNA insertion in an importin {beta}-like gene, is more tolerant to UV-B radiation than the wild type. Analysis of cyclobutane pyrimidine dimer accumulation revealed that less DNA damage occurred in sad2 than in the wild type during UV-B treatment. No significant growth difference was observed between sad2 and the wild type when treated with the genotoxic drug methyl methanesulfonate, suggesting that SAD2 functions in UV-B protection rather than in DNA damage repair. Whereas the R2R3-type transcription repressor MYB4 has previously been shown to negatively regulate the transcription of cinnamate 4-hydroxylase (C4H) and thus to regulate the synthesis of sinapate esters, expression of both MYB4 and C4H and accumulation of UV-absorbing compounds were significantly higher in sad2 than in the wild type. MYB4 did not localize to the nucleus in the sad2 mutant, suggesting that SAD2 is required for MYB4 nuclear trafficking. SAD2 and MYB4 coimmunoprecipitated, indicating that these proteins localize in the same complex in vivo. MYB4 protein specifically bound to its own promoter in gel shift assays and repressed its own expression, demonstrating that MYB4 protein and mRNA are part of a negative autoregulatory loop. This feedback loop is altered in the sad2 mutant due to the absence of MYB4 protein in the nucleus, leading to the constitutive expression of MYB4 and C4H and resulting in accumulation of UV-absorbing pigments that shield the plant from UV-B radiation.




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