RNA as a signalling molecule - Intercellular Communication in Plants - page 50

The second mode of transport, known as targeted movement, relies on specific
interactions between plasmodesmata components and transported macromolecules,
which leads to a dynamic increase in SEL. Our knowledge of the processes govern-
ing targeted movement through plasmodesmata has been greatly enhanced by the
study of plant viruses (Lazarowitz, 1999). Successful infection by these pathogens
implies an ability to move through the cell primary physical defence: the wall. To
bypass this barrier, most plant viruses encode movement proteins (MPs) that actively
target the plasmodesmata (Deom et al. , 1990) to increase their SEL (Wolf et al. ,
1989; Oparka et al. , 1997), allowing movement of the viral ribonucleoprotein com-
plex to the neighbouring cells. The possible mechanisms of viral-targeted transport
in plants have been reviewed extensively in the past few years and will not be cov-
ered here. However, the existence of such mechanisms provided an important clue
that, perhaps, endogenous ribonucleic complexes could also be transported through
plasmodesmata.
3.1.1.2 Cell-to-cell movement of a transcription factor with its mRNA
Most plant organs originate post-embryonically from meristems, which correspond
to groups of undifferentiated stem cells that are set aside during embryogenesis. The
aerial parts of the plants are generated by the shoot apical meristem (SAM), which
consists of three consecutive layers of tissue (respectively L1, L2 and L3, from the
outer most layer to the inner most layer) that produce the epidermis, subepider-
mis and vasculature (see also Chapter 6). Extensive intercellular communication
is required between these three layers to coordinate cell proliferation and organ
differentiation. The demonstration that transcription factors could move together
with their mRNA through different SAM layers was provided by studies of the
KNOTTED1 (KN1) protein, a homeobox transcriptional regulator found in maize
(Vollbrecht et al. , 1991; Reiser et al. , 2000). In situ hybridization and immunolocal-
ization experiments revealed that the KN1 protein could be detected in the L1 of the
maize SAM even though the kn1 mRNA was apparently limited to L2 and L3 (Lucas
et al. , 1995), therefore suggesting intercellular movement of KN1. This hypothesis
was confirmed by microinjection of fluorescently labelled KN1 in maize and to-
bacco mesophyll cells. It was found that movement of KN1 into the surrounding
cells was associated with an increase in plasmodesma SEL (Lucas et al. , 1995).
The interaction between KN1 and plasmodesmata, the resulting increase in SEL,
and the capacity of the protein to mediate its own cell-to-cell transport were highly
reminiscent of the standard properties of viral MPs. This suggested that KN1 could
also promote the transport of nucleic acids, an idea that was subsequently confirmed
experimentally. However, in contrast to the situation observed with viral MPs, move-
ment of nucleic acids through KN1 was found to be sequence-specific, as it occurred
only if sense KN1 transcripts were involved. A phage display procedure involving
plasmodesmata-enriched cell wall fractions was used to identify a KN1-derived
peptide that strongly antagonized the increase in SEL normally potentiated by the
full-length protein, presumably through competition for plasmodesmata-binding
domains. Interestingly, this peptide completely abolished movement of the KN1