ABSTRACT
The reliable expression of transgenes that have been introduced into plants, is a key requirement for the application of recombinant DNA technology in modern agriculture. Although single copy transgenes can become inactivated, most gene silencing events described so far are linked to the presence of multiple copies of the transgene or homologous endogenous genes. Silencing either occurs at the transcriptional level, frequently associated with DNA methylation, or post-transcriptionally, which involves RNA degradation. Transgene silencing does not necessarily occur in primary transformants, but can develop during the propagation of transgenic material or in future generations. The dependence of certain silencing events on developmental and endogenous effects often complicates the selection of stable transgene activity. Furthermore, any long-term use of transgenic material in breeding programmes will be hampered when silencing is triggered in different genetic backgrounds or when genetic crosses combine multiple transgenes that share common, homologous sequences. Our understanding of the various molecular mechanisms and physiological effects that regulate gene inactivation is still very limited. However, the current picture that emerges from the molecular analysis of different silencing phenomena, allows us to define some principles that should reduce the probability of transgenes to become inactivated. This includes criteria for the design of recombinant constructs, the DNA transfer technique, the structure of the integrated transgene and the selection conditions to test transgene stability.
