RNA as a signalling molecule - Intercellular Communication in Plants - page 54

conformation on the oligomeric viroid RNA, dictating cleavage specificity (Baum-
stark et al. , 1997; Liu & Symons, 1998). The last step of the replication cycle
involves circularization of linear viroid monomers through RNA ligation. Whether
this reaction is mediated by a host-encoded RNA ligase (Tabler et al. , 1992; Baum-
stark et al. , 1997) or by self-ligation (Lafontaine et al. , 1995; Cote et al. , 2001) is
as yet undetermined (Flores et al. , 2000; Diener, 2001; and references therein).

3.2.2 Intercellular movement of viroids

In spite of the small size and non-coding nature of their genome, viroids are able
to replicate, move from cell to cell, spread systemically, and cause symptoms on
their hosts (Davies et al. , 1974; Hall et al. , 1974; Flores, 2001). Thus, these RNA
molecules must rely entirely on cellular factors to carry out all the steps that are
required for successful infection. They are therefore extremely valuable probes of
the mechanisms and roles of endogenous RNA trafficking processes in plants.

3.2.2.1 Cell-to-cell movement of viroids
Cell-to-cell transport of PSTVd was analysed by microinjections of in vitro PSTVd
RNA transcripts labelled with TOTO-1 iodide, a nucleotide-specific fluorescent dye
(Glazer & Rye, 1992). When introduced into a symplastically connected mesophyll
cell, rapid cell-to-cell movement of PSTVd occurred (Ding et al. , 1997). In contrast
when similarly injected into symplastically isolated guard cells (Galatis, 1980; Wille
& Lucas, 1984; Palevitz & Hepler, 1985), TOTO-labelled PSTVd remained in the
injected cells, indicating that cell-to-cell movement of this viroid occurs through
plasmodesmata. Interestingly, a 1400 nt long, heterologous RNA containing only
vector sequences did not move unless it was fused to the PSTVd transcript (Ding
et al. , 1997). This finding strongly suggested that PSTVd possesses a sequence
or structural motif for plasmodesmata transport. Whether PSTVd 'piggy-backs'
on cellular proteins that are transported between cells or interacts directly with
plasmodesmata components remains to be determined.

3.2.2.2 Long-distance movement of viroids
At the whole plant level, PSTVd infection in tomato spreads in a pattern similar to
photoassimilate transport, from source to sink tissues (Palukaitis, 1987; Zhu et al. ,
2001), suggesting that PSTVd moves long distance through the phloem. Consistent
with this hypothesis, in situ hybridization experiments have shown that PSTVd traf-
fics through and replicates actively in the phloem during long-distance movement
(Zhu et al. , 2001). Presumably, replication along the transport pathway provides
more infectious PSTVd molecules either for further long-distance trafficking or for
invasion of neighbouring, non-vascular cells. Analysis of mature floral organs indi-
cated that PSTVd was detected in sepals but not in petals, stamens or ovaries. This
observation was unexpected, since phloem connections exist in all floral parts and
since ovaries, stamens and petals are strong sink tissues in which movement of plant
mRNAs (Ruiz-Medrano et al. , 1999) and of the green fluorescent protein (GFP)
had been reported (Imlau et al. , 1999). Absence of PSTVd in specific floral organs