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GENETICS, GENOMICS, AND MOLECULAR EVOLUTION

Invasion of the Arabidopsis Genome by the Tobacco Retrotransposon Tnt1 Is Controlled by Reversible Transcriptional Gene Silencing^1,[W]

Station de Génétique et d'Amélioration des Plantes, UR254, INRA, F–78026 Versailles, France

Long terminal repeat (LTR) retrotransposons are generally silent in plant genomes. However, they often constitute a large proportion of repeated sequences in plants. This suggests that their silencing is set up after a certain copy number is reached and/or that it can be released in some circumstances. We introduced the tobacco (Nicotiana tabacum) LTR retrotransposon Tnt1 into Arabidopsis (Arabidopsis thaliana), thus mimicking the horizontal transfer of a retrotransposon into a new host species and allowing us to study the regulatory mechanisms controlling its amplification. Tnt1 is transcriptionally silenced in Arabidopsis in a copy number-dependent manner. This silencing is associated with 24-nucleotide short-interfering RNAs targeting the promoter localized in the LTR region and with the non-CG site methylation of these sequences. Consequently, the silencing of Tnt1 is not released in methyltransferase1 mutants, in contrast to decrease in DNA methylation1 or polymerase IVa mutants. Stable reversion of Tnt1 silencing is obtained when the number of Tnt1 elements is reduced to two by genetic segregation. Our results support a model in which Tnt1 silencing in Arabidopsis occurs via an RNA-directed DNA methylation process. We further show that silencing can be partially overcome by some stresses.

² These authors contributed equally to the article.

³ Present address: Instituto de Recursos Naturales y Agrobiología de Sevilla, Consejo Superior de Investigaciones Científicas, Avda. Reina Mercedes 10, 41012 Sevilla, Spain.

⁴ Present address: Laboratoire National de la Protection des Végétaux, Unité de Détection des OGM, 93 rue de Curembourg, 45404 Fleury-les-Aubrais cedex, France.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Nicolas Bouché (bouche{at}versailles.inra.fr).

^[W] The online version of this article contains Web-only data.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.117846

^* Corresponding author; e-mail bouche{at}versailles.inra.fr.

Received February 15, 2008; accepted May 6, 2008; published May 8, 2008.

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