Fig. 1. Evidence for a systemic silencing signal and for a maintenance step in PTGS. (A) Wild-type plants (wt) do not undergo PTGS after grafting onto silenced transgenic plants. (B) Transgenic plants that do not spontaneously undergo PTGS (class I) undergo PTGS after grafting onto silenced transgenic plants but do not maintain silencing after elimination of the silenced rootstock. (C) Non-silenced transgenic plants derived from lines that can spontaneously undergo PTGS (class II) undergo PTGS after grafting onto silenced transgenic plants and maintain silencing after elimination of the silenced rootstock. (D) Non-transgenic plants that express an endogenous gene at high level owing to metabolic derepression (class III) undergo PTGS after grafting onto silenced transgenic plants but do not maintain silencing after elimination of the silenced rootstock.