PHLOEM MECHANISM. “The authors integrate recent work on a phenomenon called “cosupression,” in which introduced genes can suppress expression of the plant’s own genes, with other research on how plants resist viral infections and suggest that information-carrying molecules (likely RNAs) move through the plant in long tubes (phloem) and regulate cell physiology at a distance.”
http://science.sciencemag.org/content/279/5356/1486