Systemic silencing signal(s)

Abstract

Grafting experiments have revealed that transgenic plants that undergo co-suppression of homologous transgenes and endogenous genes or PTGS of exogenous transgenes produce a sequence-specific systemic silencing signal that is able to propagate from cell to cell and at long distance. Similarly, infection of transgenic plants by viruses that carry (part of) a transgene sequence results in global silencing (VIGS) of the integrated transgenes although viral infection is localized. Systemic PTGS and VIGS strongly resemble recovery from virus infection in non-transgenic plants, leading to protection against secondary infection in newly emerging leaves and PTGS of transiently expressed homologous transgenes. The sequence-specific PTGS signal is probably a transgene product (for example, aberrantRNA) or a secondary product (for example,RNAmolecules produced by anRNA-dependent RNApolymerasewith transgeneRNAas amatrix) that mimics the type of viralRNAthat is targeted for degradation by cellular defence.Whether some particular cases of transgene TGS could also rely on the production of such a mobilemolecule is discussed.