Plant Molecular Biology 43: 295–306, 2000.
M.A. Matzke and A.J.M. Matzke (Eds.), Plant Gene Silencing.
© 2000 Kluwer Academic Publishers. Printed in the Netherlands.
RNA viruses as inducers, suppressors and targets of post-transcriptional
Rajendra Marathe, Radhamani Anandalakshmi, Trent H. Smith, Gail J. Pruss and Vicki B.
Department of Biological Sciences, University of South Carolina, Columbia, SC 29208, USA (
correspondence; e-mail: email@example.com)
Key words: plant virus, post-transcriptional gene silencing, suppressor of gene silencing
Post-transcriptional gene silencing (PTGS) is a fundamental regulatory mechanism operating in diverse types of
organisms, but the cellular components of the gene silencing machinery and the regulation of the process are not
understood. Recent ﬁndings that cytoplasmically replicating RNA viruses act as both targets and inducers of PTGS
has led to the idea that PTGS may have evolved as an anti-viral defense mechanism in plants. Consistent with
this hypothesis, it has been found that certain plant viruses encode proteins that suppress PTGS. From a practical
standpoint, an understanding of the mechanisms by which viruses regulate PTGS may well lead to better ways
to control gene expression in plants. It is often desirable to overexpress selected beneﬁcial genes or to silence
detrimental ones in order to confer a particular phenotype. Induction of PTGS using RNA viruses as vectors or as
transgenes provides a reliable and efﬁcient way to interfere with the expression of a speciﬁc gene or with a family
of genes. Conversely, expression of viral suppressors has signiﬁcant potential to improve yields in technologies
that use plants to express beneﬁcial gene products. Given the antiviral nature of gene silencing in plants and the
indications that PTGS is an ancient mechanism in eukaryotic organisms, understanding the phenomenon in plants
could well lead to the development of anti-viral strategies in both plants and animals.
Abbreviations: CMV, cucumber mosaic virus; GFP, green ﬂuorescent protein; GUS, β-glucuronidase; HC-Pro,
helper component proteinase; PTGS, post-transcriptional gene silencing; PVX, potato virus X; RYMV, rice yel-
low mottle virus; TAV, tomato aspermy virus; TBSV, tomato bushy stunt virus; TEV, tobacco etch virus; TGS,
transcriptional gene silencing; TBRV, tomato black ringspot virus; VIGS, virus-induced gene silencing
Homology-basedgene silencing was ﬁrst observed by
plant biotechnologists attempting to construct trans-
genic plants that overexpress either endogenous or
foreign genes. Instead of enhanced expression, in-
creased gene dosage in many instances resulted in
silencing of both the transgene and the endogenous
gene (for recent reviews see Vaucheret et al., 1998;
Kooter et al., 1999; Wassenegger and Pélissier, 1999).
Subsequent research showed that there are two dif-
ferent types of gene inactivation. Transcriptional gene
silencing (TGS) is characterized by reduced transcrip-
tion of the silenced gene in the nucleus and is associ-
ated with de novo methylation of the promoter region
of the affected locus. In post-transcriptional gene si-
lencing (PTGS), transcription of the silenced gene is
unaffected, but the transcripts fail to accumulate, indi-
cating that there is a sequence-speciﬁc targeting and
degradation of the RNA. PTGS occurs in the cyto-
plasm and is associated with the appearanceof speciﬁc
low-molecular-weight RNA fragments (Hamilton and
AlthoughPTGSwas ﬁrst discoveredin plants, sim-
ilar processes have been described for ciliates (Ruiz
et al., 1998), for ﬁlamentous fungi, where it is called