Agriculture Reference
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short-distance signal (Himber
et al.
, 2003). In principle, limited and extensive cell-
to-cell movement of silencing could be mediated by distinct mechanisms involving
separate molecules. However, the difference could be more simply explained in
terms of a single movement process with varying intensities. In their model, Himber
et al.
(2003) proposed that local initiation of silencing would produce 21 nt and
24 nt primary siRNAs. The primary 21 nt siRNA would move to 10-15 adjacent
cells, independently of the presence of homologous transcripts in those cells. This
initial wave of movement could then have two possible outcomes. First, primary
21 nt siRNAs could initiate synthesis of secondary 21 nt siRNAs through the ac-
tion of SDE1 and SDE3 using homologous transcripts as templates. As proposed
for primary 21 nt siRNAs, the newly synthesized 21 nt siRNAs could then move
overafurther distance of 10
15 cells in which the same SDE1/SDE3-mediated
process would be initiated. Such reiterated short-distance signalling events would
then eventually translate into extensive movement. The second possible outcome
would be that silencing does not move any further because of a lack or inability of
homologous transcripts to act as templates for SDE1 and SDE3. This would pre-
clude further production of 21 nt siRNAs and movement would stop (Himber
et al.
,
2003).
±
3.3.6.3 No specific RNA species has been correlated with long-distance
transport of silencing in plants
The nature of the nucleic acid component of the signal involved in systemic si-
lencing remains highly controversial. On the one hand, the use of several silencing
suppressors indicated a tight correlation between the occurrence of the 24 nt long
GFP siRNA in infiltrated tissues and the onset of systemic silencing in GFP trans-
genic
N. benthamiana
(Hamilton
et al.
, 2002). However, experiments involving
stable expression of the HcPro suppressor of silencing in GUS-silenced tobacco
lines showed that this protein eliminated the production of both 21 nt and 24 nt
siRNAs, yet it did not prevent graft transmission of the silenced state from HcPro
rootstocks to non-HcPro scions (Mallory
et al.
, 2001). The presence of HcPro in
the scions did, however, prevent perception of the silencing signal in those tissues.
Additional use of HcPro in conjunction with other silencing systems involving an
inverted repeat construct and a transgene encoding a replicating virus, respectively,
further suggested that long-distance silencing in those systems was not correlated
with accumulation of any small RNA species, larger RNA or long dsRNA molecules
(Mallory
et al.
, 2003). The picture is even more complicated by the fact that different
classes of transgenes are likely to produce different patterns of systemic silencing,
if any, as exemplified here by the very distinct response of the Class-I, Class-II
and Class-III plants in systemic silencing of
Nia
. Therefore, it may well be that
there is not one single nucleic acid species that serves as the long-distance signal.
Rather, potentially any RNA intermediate in the silencing pathway that is able to
move systemically could act as the systemic RNA silencing signal (Mallory
et al.
,
2003).
