Background:In plants, post-transcriptional gene silencing results in RNAdegradation after transcription. Among tobacco transformants carrying a nitratereductase (Nia) construct under the control of the cauliflower mosaic virus 35Spromoter (35S–Nia2), one class of transformants spontaneously triggers Niapost-transcriptional gene silencing (class II) whereas another class does not(class I). Non-silenced plants of both classes become silenced when graftedonto silenced stocks, indicating the existence of a systemic silencing signal.Graft-transmitted silencing is maintained in class II but not in class I plantswhen removed from silenced stocks, indicating similar requirements forspontaneous triggering and maintenance.Results:Introduction of 35S–Nia2DNA by the gene transfer method calledbiolistics led to localised acquired silencing (LAS) in bombarded leaves of wild-type, class I and class II plants, and to systemic acquired silencing (SAS) inclass II plants. SAS occurred even if the targeted leaf was removed 2 days afterbombardment, indicating that the systemic signal is produced, transmitted andamplified rapidly. SAS was activated by sense, antisense and promoterlessNia2DNA constructs, indicating that transcription is not required although itdoes stimulate SAS. Conclusions:SAS was activated by biolistic introduction of promoterlessconstructs, indicating that the DNA itself is a potent activator of post-transcriptional gene silencing. The systemic silencing signal invaded thewhole plant by cell-to-cell and long-distance propagation, and reamplificationof the signal