First published online May 20, 2005; 10.1104/pp.105.063289
Plant Physiology 138:715-733 (2005)
© 2005 American Society of Plant Biologists
CELL BIOLOGY AND SIGNAL TRANSDUCTION
A Molecular-Genetic Study of the Arabidopsis Toc75 Gene Family1
Amy Baldwin3,
Anthony Wardle2,
Ramesh Patel2,
Penny Dudley,
Soon Ki Park4,
David Twell,
Kentaro Inoue and
Paul Jarvis*
Department of Biology, University of Leicester, Leicester LE1 7RH, United Kingdom (A.B., A.W., R.P., P.D., S.K.P., D.T., P.J.); and Department of Plant Sciences, University of California, Davis, California 95616 (K.I.)
Toc75 (translocon at the outer envelope membrane of chloroplasts, 75 kD) is the protein translocation channel at the outer envelope membrane of plastids and was first identified in pea (Pisum sativum) using biochemical approaches. The Arabidopsis (Arabidopsis thaliana) genome contains three Toc75-related sequences, termed atTOC75-I, atTOC75-III, and atTOC75-IV, which we studied using a range of molecular, genetic, and biochemical techniques. Expression of atTOC75-III is strongly regulated and at its highest level in young, rapidly expanding tissues. By contrast, atTOC75-IV is expressed uniformly throughout development and at a much lower level than atTOC75-III. The third sequence, atTOC75-I, is a pseudogene that is not expressed due to a gypsy/Ty3 transposon insertion in exon 1, and numerous nonsense, frame-shift, and splice-junction mutations. The expressed genes, atTOC75-III and atTOC75-IV, both encode integral envelope membrane proteins. Unlike atToc75-III, the smaller atToc75-IV protein is not processed upon targeting to the envelope, and its insertion does not require ATP at high concentrations. The atTOC75-III gene is essential for viability, since homozygous atToc75-III knockout mutants (termed toc75-III) could not be identified, and aborted seeds were observed at a frequency of approximately 25% in the siliques of self-pollinated toc75-III heterozygotes. Homozygous toc75-III embryos were found to abort at the two-cell stage. Homozygous atToc75-IV knockout plants (termed toc75-IV) displayed no obvious visible phenotypes. However, structural abnormalities were observed in the etioplasts of toc75-IV seedlings and atTOC75-IV overexpressing lines, and toc75-IV plants were less efficient at deetiolation than wild type. These results suggest some role for atToc75-IV during growth in the dark.
1 This work was supported by the University of Leicester (Ph.D. studentship to A.B.), by the United States Department of Agriculture Cooperative State Research, Education, and Extension Service (grant no. 2003–02860 to K.I.), by the Royal Society (Rosenheim Research Fellowship to P.J.), and by the Biotechnology and Biological Sciences Research Council (grant nos. 91/C12976, 91/P12928, and 91/C18638 to P.J.).
2 These authors contributed equally to the paper.
3 Present address: Department of Plant Sciences, University of California, One Shields Avenue, Davis, CA 95616.
4 Present address: Division of Plant Bioscience, Kyungpook National University, Daegu 702–701, Korea.
Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.063289.
* Corresponding author; e-mail rpj3{at}le.ac.uk; fax 44–116–252–3330.
Received March 23, 2005;
returned for revision April 26, 2005;
accepted April 27, 2005.
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