Mobile small RNAs and their role in regulating cytosine methylation of DNA.

Small (s)RNAs of 21 to 24 nucleotides are associated with RNA silencing and methylation of DNA cytosine residues. All sizes can move from cell-to-cell and long distance in plants, directing RNA silencing in destination cells. Twenty-four nucleotide sRNAs are the predominant long-distance mobile species. Thousands move from shoot to root, where they target methylation of transposable elements both directly and indirectly. We derive several classes of interaction between small RNAs and methylation and use these to explore the mechanisms of methylation and gene expression that associate with mobile sRNA signaling.