Systemic silencing signal(s)

Grafting experiments have revealed that transgenic plants that undergo co-s uppression of homologous transgenes and endogenous genes or PTGS of exogeno us transgenes produce a sequence-specific systemic silencing signal that is able to propagate from cell to cell and at long distance. Similarly, infec tion of transgenic plants by viruses that carry (part of) a transgene seque nce results in global silencing (VIGS) of the integrated transgenes althoug h viral infection is localized. Systemic PTGS and VIGS strongly resemble re covery from virus infection in non-transgenic plants, leading to protection against secondary infection in newly emerging leaves and PTGS of transient ly expressed homologous transgenes. The sequence-specific PTGS signal is pr obably a transgene product (for example, aberrant RNA) or a secondary produ ct (for example, RNA molecules produced by an RNA-dependent RNA polymerase with transgene RNA as a matrix) that mimics the type of viral RNA that is t argeted for degradation by cellular defence. Whether some particular cases of transgene TGS could also rely on the production of such a mobile molecul e is discussed.