Abstract
The trigger of cosuppression has been ascribed to the RNA threshold theory. In this theory, the RNA silencing commences if intracellular concentration of transgene's transcripts surpasses a gene-specific threshold at the RNA level. Here we analyzed the cosuppression of the tobacco ER-localized a-linolenate synthase gene (NtFAD3). When the NtFAD3-cosuppressed plants were re-transformed with the NtFAD3-harboring transgene, several lines showed increased 18:3 content in comparison with the wild type plants. Similar results can be obtained by introduction of the RNAi construct against the CaMV 35S promoter sequence into the cosuppressed plants. Since the transgene's expression was regulated by the CaMV 35S promoter, reduction of transcriptional activity by siRNA-mediated methylation should reduce the level of the transgene's transcripts. Agreeing with the RNA threshold theory, cosuppression may be evaded by reduction of the transcripts level, and a-linolenate content may elevate as a result of expression of the remaining transgene's transcripts.
