The present invention relates to methods to regulate gene expression in plants. In particular, manipulation of the expression in a plant cell of a nucleotide sequence encoding a polypeptide comprising a 3'-5' exonuclease domain is disclosed. More stable and predictable expression is thus obtained. The present invention also relates to method of increasing or decreasing post-transcriptional silencing. The invention further relates to novel nucleic acid molecules comprising nucleotide sequences encoding polypeptides comprising a 3'-5' exonuclease domain.