Plant Cell Advance Online Publication
Published on January 19, 2005; 10.1105/tpc.104.028332
Received October 6, 2004
Accepted November 24, 2004
The Arabidopsis HOMOLOGY-DEPENDENT GENE SILENCING1 Gene Codes for an S-Adenosyl-L-Homocysteine Hydrolase Required for DNA Methylation-Dependent Gene Silencing
Pedro S.C.F. Rocha 1, Mazhar Sheikh 1, Rosalba Melchiorre 1, Mathilde Fagard 2, Stéphanie Boutet 2, Rebecca Loach 3, Barbara Moffatt 3, Conrad Wagner 4, Hervé Vaucheret 2, and Ian Furner 1*
1 Department of Genetics, University of Cambridge, Cambridge, CB2 3EH, United Kingdom
2 Laboratoire de Biologie Cellulaire, Institut Jean-Pierre Bourgin, Institut National de la Recherche Agronomique, 78026, Versailles, Cedex, France
3 Department of Biology, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada
4 Department of Biochemistry, Vanderbilt University Medical Centre, Nashville Tennesse, 37232-0146
* To whom correspondence should be addressed. E-mail: ijf{at}mole.bio.cam.ac.uk.
Genes introduced into higher plant genomes can become silent (gene silencing) and/or cause silencing of homologous genes at unlinked sites (homology-dependent gene silencing or HDG silencing). Mutations of the HOMOLOGY-DEPENDENT GENE SILENCING1 (HOG1) locus relieve transcriptional gene silencing and methylation-dependent HDG silencing and result in genome-wide demethylation. The hog1 mutant plants also grow slowly and have low fertility and reduced seed germination. Three independent mutants of HOG1 were each found to have point mutations at the 3' end of a gene coding for S-adenosyl-L-homocysteine (SAH) hydrolase, and hog1-1 plants show reduced SAH hydrolase activity. A transposon (hog1-4) and a T-DNA tag (hog1-5) in the HOG1 gene each behaved as zygotic embryo lethal mutants and could not be made homozygous. The results suggest that the homozygous hog1 point mutants are leaky and result in genome demethylation and poor growth and that homozygous insertion mutations result in zygotic lethality. Complementation of the hog1-1 point mutation with a T-DNA containing the gene coding for SAH hydrolase restored gene silencing, HDG silencing, DNA methylation, fast growth, and normal seed viability. The same T-DNA also complemented the zygotic embryo lethal phenotype of the hog1-4 tagged mutant. A model relating the HOG1 gene, DNA methylation, and methylation-dependent HDG silencing is presented.
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