(Trans)gene silencing in plants: How many mechanisms?

Epigenetic silencing of transgenes and endogenous genes can occur at the tr anscriptional level (TGS) or at the posttranscriptional level (PTGS). Becau se they can be induced by transgenes and viruses, TGS and PTGS probably ref lect alternative (although not exclusive) responses to two important stress factors that the plant's genome has to face: the stable integration of add itional DNA into chromosomes and the extrachromosomal replication of a vira l genome. TGS, which results from the impairment of transcription initiatio n through methylation and/or chromatin condensation, could derive from the mechanisms by which transposed copies of mobile elements and T-DNA insertio ns are tamed. PTGS, which results from the degradation of mRNA when aberran t sense, antisense, or double-stranded forms of RNA are produced, could der ive from the process of recovery by which cells eliminate pathogens (RNA vi ruses) or their undesirable products (RNA encoded by DNA viruses). Mechanis ms involving DNA-DNA, DNA-RNA, or RNA-RNA interactions are discussed to exp lain the various pathways for triggering (trans)gene silencing in plants.