Characteristics of RNA silencing in plants: similarities and diﬀerences
*, M. Hohkuri
, T. Wahlroos
and N.J. Kilby
Joint Biotechnology Laboratory, Department of Biochemistry and Food Chemistry, University of Turku,
katu 6 A, 20520 Turku, Finland (*author for correspondence; e-mail: pesusi@utu.ﬁ);
address: Protein Production Laboratory, Lemminka
isenkatu 30, 20520 Turku, Finland;
Breeding Ltd., 31600 Jokioinen, Finland.
Received 28 November 2003; accepted in revised form 8 March 2004
Key words: DNA methylation, post-transcriptional gene silencing, quelling, RNA interference, RNA
silencing, suppressor protein
RNA silencing is a collective term that encompasses the sequence of events that leads to the targeted
degradation of cellular mRNA and thus to the silencing of corresponding gene expression. RNA silencing is
initiated after introduction into the host genome of a gene that is homologous to an endogenous gene.
Transcription of the introduced gene results in the formation of double-stranded RNA (dsRNA) that is cut
into smaller dsRNA species termed small interfering RNAs (siRNAs) by an RNaseIII-like enzyme called
‘Dicer’. siRNAs associate with a protein complex termed the ‘RNA-induced silencing complex’ (RISC),
which mediates the binding of one strand of siRNAs with mRNAs transcribed from the native ‘target’ gene.
The binding of siRNAs with native gene mRNAs earmarks native gene mRNAs for destruction, resulting
in gene silencing. In plants, RNA silencing appears to serve as a defence mechanism against viral pathogens
and also to suppress the activity of virus-like mobile genetic elements. In an apparent response to RNA
silencing, some plant viruses express suppressors of RNA silencing. RNA silencing also is directly impli-
cated in the regulation of the function(s) of microRNAs, which are the key determinants in an additional
cellular mechanism related to the translational repression of genes, the eﬀect of which ultimately impinges
on development. The high degree of sequence similarity that exists between genes involved in RNA
silencing in widely diﬀerent organisms underscores the conserved nature of many aspects of the RNA
silencing mechanism. However, depending (for example) on the precise nature of the target gene involved,
there also are signiﬁcant diﬀerences in the silencing pathways that are engaged by various organisms.
RNA silencing involves a coordinated series of
sub-cellular events that ultimately lead to the post-
transcriptional termination of gene expression.
The RNA silencing phenomenon has been termed
co-suppression or post-transcriptional gene
silencing (predominantly used to describe plant-
and plant virus-related silencing events), quelling
(in fungi) and RNA interference (in vertebrates
and invertebrates). Recently, the mechanisms that
underlie these phenomena have been recognized as
being based on broadly similar principles, and that
comparable signalling molecules are involved.
There are, however, signiﬁcant diﬀerences in the
mechanisms of RNA silencing that are engaged by
diﬀerent organisms as inﬂuenced, for example, by
the nature of the target gene. In this regard, it
would be an oversimpliﬁcation to suggest that
observations made in one species are necessarily of
direct equivalence in a related species. However,
given the overall conserved nature of the core
Plant Molecular Biology 54: 157–174, 2004.
Ó 2004 Kluwer Academic Publishers. Printed in the Netherlands.