Systemic, posttranscriptional silencing of transgenes in Nicotiana benthamiana was initiated in localized regions of the plant by introduction of transgene-homologous DNA fragments, including those without a promoter. Following this initiation step, a sequence-specific signal of gene silencing spread from cells that had received the ectopic DNA via a relay mechanism that employs plasmodesmatal and phloem channels. The nature of this mechanism, the mode of its initiation, and the ability of the signal to move long distances indicate the existence of a sequence-specific signaling mechanism in plants that may have roles in developmental control as well as in protection against transposons and viruses.