Post-transcriptional silencing of chalcone synthase in Petunia by inverted transgene repeats

To induce post-transcriptional silencing of flower pigmentation genes by homologous sense transgenes in transgenic petunias, it is not necessary for the transgenes to be highly transcribed. Even promoterless transgenes can induce silencing. Here it is shown that in these cases silencing is mediated by multimeric transgene/T-DNA loci in which the T-DNAs are arranged as inverted repeats (IRs). With the transgene constructs used, monomeric T-DNA loci are unable to confer silencing even though they modulate IR-induced silencing. IRs with the silencing sequences proximal to the centre (IRc) induce a more severe silencing than IRs with these sequences distal to the centre (IRn). Somatic reversion of silencing, as observed in a side branch of one of the chalcone synthase (Chs) transformants, was associated with a deletion of the IR locus from L1 cells, the meristematic cell layer that expresses the endogenous Chs genes in the flower corolla. Taken together, these data indicate that the post-transcriptional silencing mechanism can be activated by inverted transgene repeats. It is also shown that a silent IR UidA-ChsA locus silences the expression of a monomeric 35S promoter-driven UidA-ChsA transgene only in corollas where the endogenous Chs genes are highly transcribed. These results are consistent with a model in which an IR, by virtue of its palindromic sequence organization, is able to promote the production of aberrant RNAs from the endogenous homologs as a result of ectopic pairing.