Active DNA Demethylation in Plant Companion Cells Reinforces Transposon Methylation in Gametes

Abstract

Intergenerational Transposable Shutdown Transposable elements (TEs) are a potential threat, especially to the germline genome. In many eukaryotes, TEs are shut down by DNA methylation and/or small-RNA–mediated silencing. Therefore, it seems counterintuitive that results obtained by Ibarra et al. (p. 1360 ) on Arabidopsis showed that in the cells of this plant's sexual apparatus, many small TEs are demethylated by DEMETER (DME) DNA glycosylase and become activated. But it turns out that activation of the TEs triggers the formation of small-interfering RNAs, which in these experiments were seen to travel from the surrounding cells to the egg. Thus, activation of TEs in the companion cells “immunizes” the gametes via the interfering RNAs that shutdown the TEs in the gametes permanently.