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Published on May 8, 2008; 10.1104/pp.108.117846

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Received February 15, 2008
Accepted May 6, 2008

Invasion of the Arabidopsis genome by the tobacco retrotransposon Tnt1 is controlled by reversible transcriptional gene silencing

Javier Perez-Hormaeche , Frederique Potet , Linda Beauclair , Ivan Le Masson , Beatrice Courtial , Nicolas Bouche *, and Helene Lucas

Station de Genetique et d'Amelioration des Plantes, UR254, INRA, F-78026 Versailles, France

* Corresponding author; email: bouche{at}versailles.inra.fr.

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 LTR-retrotransposon Tnt1 into 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 localised in the LTR region and with the non-CG site methylation of these sequences. Consequently, the silencing of Tnt1 is not released in methyltransferase1 (met1) mutants in contrast to decrease in DNA methylation1 (ddm1) or PolIVa (nrpd1a) mutants. Stable reversion of Tnt1 silencing is obtained when the number of Tnt1 elements is reduced down 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.






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