Activation of systemic acquired silencing by localised introduction of DNA

Background: In plants, post-transcriptional gene silencing results in RNA d egradation after transcription. Among tobacco transformants carrying a nitr ate reductase (Nia) construct under the control of the cauliflower mosaic v irus 35S promoter (35S-Nia2), one class of transformants spontaneously trig gers Nia post-transcriptional gene silencing (class II) whereas another cla ss does not (class I). Non-silenced plants of both classes become silenced when grafted onto silenced stocks, indicating the existence of a systemic s ilencing signal. Graft-transmitted silencing is maintained in class II but not in class I plants when removed from silenced stocks, indicating similar requirements for spontaneous triggering and maintenance. Results: Introduction of 35S-Nia2 DNA by the gene transfer method called bi olistics led to localised acquired silencing (LAS) in bombarded leaves of w ildtype, class I and class II plants, and to systemic acquired silencing (S AS) in class II plants. SAS occurred even if the targeted leaf was removed 2 days after bombardment, indicating that the systemic signal is produced, transmitted and amplified rapidly. SAS was activated by sense, antisense an d promoterless Nia2 DNA constructs, indicating that transcription is not re quired although it does stimulate SAS. Conclusions: SAS was activated by biolistic introduction of promoterless co nstructs, indicating that the DNA itself is a potent activator of posttrans criptional gene silencing. The systemic silencing signal invaded the whole plant by cell-to-cell and long-distance propagation, and reamplification of the signal.