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Plant Cell Advance Online Publication
Published on July 6, 2007; 10.1105/tpc.106.042697

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Received March 21, 2006
Returned for revision May 29, 2007
Accepted June 12, 2007

The TORMOZ Gene Encodes a Nucleolar Protein Required for Regulated Division Planes and Embryo Development in Arabidopsis

Megan E. Griffith 1, Ulrike Mayer 2, Arnaud Capron 3, Quy A. Ngo 3, Anandkumar Surendrarao 3, Regina McClinton 4, Gerd Jürgens 2, and Venkatesan Sundaresan 3*

1 Institute of Molecular and Cell Biology, Singapore 138673, Republic of Singapore; Victorian AgriBiosciences Centre, Primary Industries Research Victoria, Department of Primary Industries, Victoria 3083, Australia
2 Centre for Molecular Biology of Plants, University of Tübingen, D-72076 Tübingen, Germany
3 Plant Biology and Plant Sciences, University of California, Davis, California 95616
4 Biology Department, Grand Valley State University, Allendale, Michigan 49401

* To whom correspondence should be addressed. E-mail: sundar{at}ucdavis.edu.

Embryogenesis in Arabidopsis thaliana is marked by a predictable sequence of oriented cell divisions, which precede cell fate determination. We show that mutation of the TORMOZ (TOZ) gene yields embryos with aberrant cell division planes and arrested embryos that appear not to have established normal patterning. The defects in toz mutants differ from previously described mutations that affect embryonic cell division patterns. Longitudinal division planes of the proembryo are frequently replaced by transverse divisions and less frequently by oblique divisions, while divisions of the suspensor cells, which divide only transversely, appear generally unaffected. Expression patterns of selected embryo patterning genes are altered in the mutant embryos, implying that the positional cues required for their proper expression are perturbed by the misoriented divisions. The TOZ gene encodes a nucleolar protein containing WD repeats. Putative TOZ orthologs exist in other eukaryotes including Saccharomyces cerevisiae, where the protein is predicted to function in 18S rRNA biogenesis. We find that disruption of the Sp TOZ gene results in cell division defects in Schizosaccharomyces pombe. Previous studies in yeast and animal cells have identified nucleolar proteins that regulate the exit from M phase and cytokinesis, including factors involved in pre-rRNA processing. Our study suggests that in plant cells, nucleolar functions might interact with the processes of regulated cell divisions and influence the selection of longitudinal division planes during embryogenesis.






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