EPIGENETIC SILENCING OF PLANT TRANSGENES AS A CONSEQUENCE OF DIVERSE CELLULAR DEFENSE RESPONSES

Linked and unlinked copies of transgenes and related endogenous genes in plants can be epigenetically silenced by homology-based mechanisms that operate al either the transcriptional or post-transcriptional lev el. Transcriptional inactivation is associated with promoter homology and meiotically heritable methylation. Post-transcriptional silencing requires homology in protein-coding regions and is fully reversed duri ng meiosis. Recently, the notion that both of these processes reflect the action of different host defence systems has been strengthened: (i ) Obvious parallels have emerged between promoter homology-dependent s ilencing/methylation of transgenes and paramutation of endogenous gene s that contain transposable elements in their promoters; (ii) remarkab le similarities have been observed between post-transcriptional silenc ing involving transgenes and natural forms of virus resistance in nont ransgenic plants. These results and others implicate two distinct cell ular defence responses in transgene silencing. One is active in the nu cleus and is manifested by transgene methylation, a reaction that migh t have originated as a means to oppose the spread of transposable elem ents. A second line of defence resides in the cytoplasm and operates t hrough enhanced RNA turnover, a process that might help plants overcom e viral infection.