Agriculture Reference
In-Depth Information
3.3.7 Plant factors required for movement of RNA silencing
To date, no plant factors that are required for silencing movement have been iden-
tified. However, the demonstrated sensitivity of long-distance silencing transport to
non-toxic concentrations of cadmium (Ueki & Citovsky, 2001) could pave the way
to the identification of such factors. Indeed, it has recently been shown that a tobacco
glycine-rich protein, cdiGRP, is specifically induced by low cadmium concentration
and accumulates in the cell walls of vascular tissues (Ueki & Citovsky, 2002). Con-
stitutive expression of cdiGRP was shown to inhibit long-distance spread of turnip
vein-clearing tobamovirus (TVCV) by enhancing callose deposits in the vascula-
ture. It remains to be addressed whether enhanced cdiGRP expression also inhibits
systemic movement of RNA silencing. If that is the case, cdiGRP could provide a
powerful handle towards identification of plant proteins that regulate transport of
silencing.
Factors required for silencing cell-to-cell movement in
Arabidopsis
are currently
being investigated through a genetic approach developed in our laboratory. The
principle of this approach is the same as described in Plate 3.2, except that the
target of silencing in this system is an endogenous mRNA (
SULPHUR
) rather than
the GFP mRNA. Phloem-triggered silencing of
SULPHUR
causes the appearance
of yellow chlorosis that expands 10-15 cells away from the vein network (see
Plate 3.7, following page 146). These plants have been mutagenized in order to
retrieve individuals in which the vein-centred silencing phenotype is compromised.
Some of these individuals should carry mutations that impair or enhance the short-
distance movement of
SULPHUR
silencing. We have indeed retrieved several classes
of such mutants and they are currently being characterized.
3.3.8 Biological functions of non-cell autonomous RNA silencing in plants
3.3.8.1 Antiviral defence
Obviously, plants did not elaborate such sophisticated signalling systems for the
purpose of long-distance silencing of transgenes. The link with antiviral defence
first became apparent from the striking similarities between the timing and path-
ways of systemic silencing and virus movement in plants (Santa Cruz
et al.
, 1996,
1998; Voinnet
et al.
, 1998). Based on the discovery that viruses were potent trig-
gers of a PTGS-like response within infected cells (see Section 3.3.1.6), it was
speculated that non-cell autonomous silencing could represent the systemic arm
of this response, whereby transmission of a virus-induced-silencing signal ahead
of the infection front would prime silencing in cells that are yet to be infected.
Consequently, movement of the pathogen into those cells would be delayed or pre-
cluded (Voinnet
et al.
, 1998). This hypothesis received support from experiments
in which silencing was activated in upper leaves of wt tobacco plants by inoculat-
ing lower leaves with movement defective mutants of PVX (Voinnet
et al.
, 2000).
Systemic spread of silencing could be monitored
in planta
, because the modified
PVX contained fragments of endogenous genes. Thus, the silencing signal generated
