Small Silencing RNAs in Plants Are Mobile and Direct Epigenetic Modification in Recipient Cells

SiRNA Movement in Plant Tissues Long-distance movement of RNA interference (RNAi)–derived signals in plants plays an important role in development and in defense against viral attack. The nature of the signals that spread from cell to cell is not known, although evidence suggests that they are nucleic acids of some sort (see the Perspective by Martienssen ). Molnar et al. (p. 872 , published online 22 April) and Dunoyer et al. (p. 912 , published online 22 April) now show that in Arabidopsis , both exogenous and endogenous small interfering RNAs (siRNAs), rather than their long double-stranded precursor RNAs, are the molecules that transfer information between plant cells. A viral protein that counters RNAi…