THE SILENCE OF GENES IN TRANSGENIC PLANTS

In genetically modified plants, the introduced transgenes are sometime s not expressed. They can be silenced. Transgenes can also cause the s ilencing of endogenous plant genes if they are sufficiently homologous , a phenomenon known as co-suppression. Silencing occurs transcription ally and post-transcriptionally but silencing of endogenous genes seem s predominantly post-transcriptional. If viral transgenes are introduc ed and silenced, the posttranscriptional process also prevents homolog ous RNA Viruses from accumulating; this is a means of generating virus -resistant plants. A major goal of current research is to dissect the mechanism(s) of these sequence-homology-dependent gene silencing pheno mena. Various factors seem to play a role, including DNA methylation, transgene copy number and the repetitiveness of the transgene insert, transgene expression lever, possible production of aberrant RNAs, and ectopic DNA-DNA interactions. The causal relationship between these fa ctors and the link between transcriptional and post-transcriptional si lencing is not always clear. In this review we discuss various observa tions associated with gene silencing and attempt to relate them. (C) 1 997 Annals of Botany Company