Silencing of retrotransposons in arabidopsis and reactivation by the ddm1 mutation.

Gene silencing associated with repeated DNA sequences has been reported for many eukaryotes, including plants. However, its biological significance remains to be determined. One important function that has been proposed is the suppression of transposons. Here, we address transposon suppression by examining the behavior of the tobacco retrotransposon Tto1 and endogenous retrotransposons in Arabidopsis. After an initial increase in copy number because of active transposition in the Arabidopsis genome, Tto1 became silent. The amount of transcript was reduced, and the inactivated Tto1 became methylated. This silencing correlated with an increase in copy number. These phenomena mimic repeat-induced gene silencing. The homozygous ddm1 (for decrease in DNA methylation) mutation of Arabidopsis results in genomic DNA hypomethylation and the release of silencing in repeated genes. To investigate the role of DNA methylation and the gene-silencing machinery in the suppression of Tto1, we introduced the ddm1 mutation into an Arabidopsis line carrying inactivated Tto1 copies. In the homozygous ddm1 background, Tto1 became hypomethylated and transcriptionally and transpositionally active. In addition, one of the newly isolated endogenous Arabidopsis retrotransposon families, named Tar17, also became hypomethylated and transcriptionally active in the ddm1 mutant background. Our results suggest that the inactivation of retrotransposons and the silencing of repeated genes have mechanisms in common.