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Plant and Cell Physiology Advance Access originally published online on December 2, 2008
Plant and Cell Physiology 2009 50(2):254-264; doi:10.1093/pcp/pcn187
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© The Author 2008. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

Arabidopsis MBF1s Control Leaf Cell Cycle and its Expansion

Takuto Tojo1, Kenichi Tsuda2, Takeshi Yoshizumi3, Akira Ikeda4, Junji Yamaguchi4, Minami Matsui3 and Ken-ichi Yamazaki1,*

1Laboratory of Environmental Molecular Biology, Graduate School of Environmental Science, Hokkaido University, Kita 10, Nishi 5, Kita-ku, Sapporo, 060-0810 Japan
2Department of Plant Biology, Microbial and Plant Genomics Institute, University of Minnesota, 1500 Gortner Avenue, St. Paul, MN 55108, USA
3Plant Functional Genomics Research Team, Plant Functional Genomics Research Group, Plant Science Center RIKEN Yokohama Institute, Tsurumi-ku, Yokohama, Kanagawa, 230-0045 Japan
4Faculty of Advanced Life Science, Hokkaido University, Kita 10, Nishi 8, Kita-ku, Sapporo, 060-0810 Japan

*Corresponding author: E-mail, ymzk{at}ees.hokudai.ac.jp; Fax, +81-11-706-4522.


  Abstract

Multiprotein bridging factor 1 (MBF1) is known as a transcriptional co-activator that enhances transcription of its target genes by bridging between transcription factors and TATA-box-binding protein in eukaryotes. Arabidopsis thaliana has three MBF1 genes: AtMBF1a–AtMBF1c. However, details of the functions of AtMBF1 remain unclear. For this study, transgenic Arabidopsis overexpressing AtMBF1 fused to an active transcriptional repression domain (SRDX) was constructed. The chimeric protein putatively functions as a transcriptional co-repressor and as a suppressor of functions of endogenous AtMBF1 in transgenic plants. Transgenic Arabidopsis overexpressing AtMBF1–SRDX (AtMBF1-SRDXOE) showed an extremely small leaf phenotype under a continuous white light condition. Its leaf cells—especially those around vascular tissues, where strong expression of endogenous AtMBF1s is observed—were much smaller than those from the wild type (WT). In addition, a lower cell number was observed in leaves from AtMBF1-SRDXOE plants. Time course analysis of cell size revealed that cell expansion of leaves of AtMBF1-SRDXOE plants was dramatically suppressed during the late leaf developmental stage (cell expansion stage), when endogenous AtMBF1b is strongly expressed in the WT. The results show that ploidy levels of leaves from AtMBF1-SRDXOE plants were dramatically lower than those from the WT; moreover, expression levels of several negative regulators of endoreduplication were more elevated in AtMBF1s-SRDXOE plants than those in the WT. These observations suggest that AtMBF1–SRDX interacts with regulators of endoreduplication. Therefore, AtMBF1s are considered to affect not only leaf cell expansion but also regulation of the ploidy level in leaf cells during the leaf expansion stage.

Keywords: Arabidopsis thaliana - Cell cycle - Endore-duplication - Leaf expansion - SRDX - Transcriptional co-activator

Abbreviations: AtMBF1OE, transgenic Arabidopsis overexpressing AtMBF1; GUS, β-glucuronidase; MBF1, multiprotein bridging factor 1; RT–PCR, reverse transcription–PCR; TBP, TATA box-binding protein; TF, transcription factor; WT, wild type

(Received November 3, 2008; Accepted November 28, 2008)
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