Abstract
Silencing of a Nicotiana plumbaginifolia beta-1,3-glucanase (gn1) transgene in tobacco line T17 occurs in homozygous and in haploid plants with one transgene locus dosage per chromosome set. We have previously shown that the silent state is manifested by a reduced gn1 steady state mRNA level and results from a post-transcriptional process that is under developmental control in homozygous T17 plants. In this study, we show that specific endogenous beta-1,3-glucanase genes are cosuppressed with gn1 in homozygous T17 plants. We also demonstrate that the developmental timing of cosuppression depends on environmental conditions and that once silencing is established it is stably maintained during plant development. Analyses of additional transgenic tobacco lines revealed that gn1 silencing is not restricted to the T17 line and showed that silencing can also take place in R0 plants containing independent loci in hemizygous states. Furthermore, silencing can also be obtained in progeny plants in which expressing loci have been combined. Importantly, cosuppression occurs via a post-transcriptional mechanism that does not interfere with the accumulation of transgene nuclear mRNA. These results strongly suggest that the silencing mechanism operates at RNA transport and/or RNA stability levels.