Repeat elements and the Arabidopsis DNA methylation landscape.

DNA methylation is an epigenetic mark that has key roles in the control of genome activity in plants and mammals. It is critical for the stable silencing of repeat elements and is also involved in the epigenetic regulation of some genes. Despite similarities in the controlling functions of DNA methylation, its dynamics and deposition patterns differ in several respects between plants and mammals. One of the most striking differences is that plants tend to propagate pre-existing DNA methylation states across generations, whereas mammals re-establish them genome wide at every generation. Here, we review our current understanding of DNA methylation in the flowering plant Arabidopsis. We discuss in particular the role of RNAi in the incremental methylation and silencing of repeat elements over successive generations. We argue that paramutation, an epigenetic phenomenon first described in maize, is an extreme manifestation of this RNAi-dependent pathway.