Noncoding RNAs: Molecular Biology and Molecular Medicine
, edited by Jan Barciszewski
and Volker A. Erdmann. ©2003 Eurekah.com and Kluwer Academic / Plenum Publishers.
Post-Transcriptional Gene Silencing in Plants
Matthew A. Escobar and Abhaya M. Dandekar
ccumulating genetic and biochemical evidence suggests that antisense-mediated gene
silencing, cosuppression, RNA interference and virus-induced gene silencing are all
unique inputs into a common RNA silencing pathway triggered by double stranded
RNA. This pathway, termed post-transcriptional gene silencing (PTGS) is characterized by
accumulation of 21-25 nt small-interfering RNAs, sequence-specific degradation of target
mRNAs, and methylation of target gene sequences. PTGS appears to be ancient and highly
conserved, as several groups of homologous genes required for silencing in plants, animals, and
fungi have been identified. Though biochemical dissection of PTGS is still in its infancy, sev-
eral key activities have been identified, such as Dicer, the endonuclease responsible for synthe-
sis of short-interfering RNAs, and RISC, the nucleoprotein complex which mediates mRNA
degradation. Several lines of evidence suggest that PTGS plays a key role in viral defense in
plants, but further study is required to investigate the intriguing possibility that PTGS can act
as an endogenous gene control mechanism.
The characterization of antisense RNA-mediated controls of plasmid replication and
maintenance in prokaryotes prompted several studies of mRNA silencing in eukaryotic genes
in the mid-1980s.
Rothstein et al (1987) first demonstrated an “antisense effect” in intact
plants by silencing an integrated nopaline synthase (
) transgene through expression of antisense
RNA in tobacco.
Interestingly, this eukaryotic antisense silencing appeared to result from
increased turnover of the targeted
mRNA, in contrast to the inhibitory effects on transla-
or DNA synthesis
commonly associated with antisense RNA in bacteria. A similar
homology-dependent RNA degradation phenomenon, resulting from the over expression of
an endogenous gene in transgenic plants, was discovered soon thereafter.
termed antisense-mediated gene silencing and cosuppression, became powerful techniques for
plant improvement and functional gene analysis, but relatively little was understood about
their underlying mechanisms.
The seminal discoveries that double stranded RNA (dsRNA) and plant viruses are potent
initiators of gene silencing has recently invigorated basic research on silencing mechanisms.
It has become apparent that the diverse threads of antisense-mediated gene silencing,
cosuppression, dsRNA-mediated gene silencing (RNA interference), and virus-induced gene
silencing converge with the synthesis of dsRNA.