RNA as a signalling molecule - Intercellular Communication in Plants - page 61

powerful system for the amplification of molecules that are actively involved in
RNA silencing, namely, the dsRNA and the siRNAs that derive from it.

3.3.1.6 Biological functions of RNA silencing in plants
Most plant viruses have an RNA genome whose replication involves synthesis of
cytoplasmic, double-stranded intermediates that are potent inducers of RNA silenc-
ing, as shown by the high accumulation of virus-derived siRNAs in infected tissues
(Hamilton & Baulcombe, 1999; Szittya et al. , 2002; Lakatos et al. , 2004). Thus,
silencing has evolved as an antiviral mechanism in plants, a proposal consistent with
the findings that most, if not all, plant viruses encode proteins that inhibit various
steps of the silencing mechanism (Voinnet, 2001). The existence of these 'silencing
suppressors', extremely diverse in sequence and structure, provides compelling evi-
dence for an ongoing silencing-based arms race between plants and viruses (Voinnet,
2001). A second defensive role of RNA silencing is in the control of transposable
elements and maintenance of genome integrity. For instance, 24 nt siRNAs derived
from the atSIN1 elements are associated with methylation of the corresponding
DNA that impact on RNA levels of this element (Hamilton et al. , 2002; Zilberman
et al. , 2003; Xie et al. , 2004).
Recent findings in both animals and plants have highlighted an important role of
RNA silencing in patterning endogenous gene expression. For instance, reduction
of Dicer activity has dramatic developmental consequences in mice and Arabidop-
sis (Jacobsen et al. , 1999; Bernstein et al. , 2003). In many organisms, abundant
endogenous, single-stranded 21-24 nt long RNAs are processed by Dicer-like en-
zymes from short stem-loop precursor transcripts that are encoded in intergenic
regions. These microRNAs (miRNAs) are perfectly or partially complementary to
the coding region or the 3 UTR of cellular transcripts that they regulate at the level
of stability or translation (for review: Carrington & Ambros, 2003; Bartel, 2004).
Analyses in plants suggest that endonucleolytic cleavage is the prevalent mode of
action of miRNAs (Llave et al. , 2002; Bartel & Bartel, 2003). Like siRNAs, they
appear to be incorporated into an RISC (Tang et al. , 2003) upon their processing
by a Dicer-like enzyme called DCL-1 (Jacobsen et al. , 1999; Papp et al. , 2003; Xie
et al. , 2003, 2004). Many transcription factors that have been crucially implicated in
plant development have been predicted or experimentally characterized as miRNA
targets in plants and it is currently thought that at least some miRNAs play a crucial
role in clearing the cellular content of specific transcription factors to orchestrate the
emergence of cell lineages that are crucial for organ formation in plants (Aukerman
& Sakai, 2003; Bartel & Bartel, 2003; Palatnik et al. , 2003; Chen, 2004).

3.3.2 The discovery of systemic RNA silencing

The first hint of the non-cell-autonomous nature of PTGS in plants was provided by
studies of transgenic tobacco lines exhibiting spontaneous silencing of a highly
expressed nitrate reductase transgene ( Nia ). Co-suppression was manifested by
the occurrence of cholorosis due to perturbation in nitrogen availablility, which