RNA as a signalling molecule - Intercellular Communication in Plants - page 51

sense mRNA while it only reduced movement of the protein (Kragler et al. , 2000),
suggesting that increased plasmodesmata SEL was not essential for KN1 transport
(which probably occurred through passive diffusion, at least partly) but it was a
prerequisite for the transport of the ribonucleoprotein KN1/ KN1 complexes.
A series of alanine scanning mutants was generated to investigate protein domains
essential for KN1 transport through plasmodesmata. This study identified a potential
nuclear localization signal (NLS) in the N-terminal part of the homeodomain, which,
when mutated, significantly reduced the KN1 cell-to-cell movement ability. Since
the antagonist peptide evoked did not span the region corresponding to the putative
NLS, it may be that different domains in the KN1 protein have separate functions
for its transport. For instance, the N-terminal part of KN1 could be involved in
specific interactions of the protein with receptors located near the plasmodesmata
pore, whereas the C-terminal part, containing the homeodomain, could be involved
in targeting the protein to plasmodesmata and/or in modification of the SEL.
It is of note that although the ability of KN1 to traffick its own mRNA has been
unambiguously established, the biological relevance of this process is not entirely
clear, since, in vivo , KN1 - but not its mRNA - is detected in the L1 layer of the
SAM. Perhaps, the level of sensitivity of the in situ hybridization protocol used
in this study was insufficient to detect the kn1 mRNA. Quantitative RT-PCR on
laser-microdissected tissue sample could help to resolve this issue.
The Antirrhinum MADS domain protein DEFICIENS was shown to move from
the L2 into the L1 layer of floral meristems (Perbal et al. , 1996). In Arabidopsis , the
SHORTROOT (SHR) (Nakajima et al. , 2001) and LEAFY (LFY) (Sessions et al. ,
2000) transcription factors have also been shown to function non-cell autonomously.
For instance, the SHR mRNA is expressed in the stele of the root (Helariutta et al. ,
2000), yet the SHR protein is found both in the stele and in the surrounding en-
dodermis, which is absent in shr mutants. Likewise, expression of LFY from the
epidermis-specific ML1 promoter was correlated with a gradient of LFY protein
that extended over several internal cell layers beyond the epidermis (Sessions et al. ,
2000). So far, however, no evidence has been obtained to suggest that these tran-
scription factors are able to mediate transport of nucleic acids, as seen in the case
of KN1.

3.1.2 Long-distance transport of plant mRNAs

Long-distance transport of water, nutrients, hormones and other signals in plants
relies on the plant vascular system, which comprises the xylem and the phloem.
The xylem consists of rows of dead cells, whereas the phloem contains living, albeit
enucleate cells called the sieve elements (SE) that are surrounded by nucleated
companion cells (CCs). The thermodynamic driving force of the phloem sap relies
on a pressure gradient created by the loading of sugars produced in source leaves and
their subsequent unloading in sink tissue (Turgeon, 1996). The SE and surrounding
CCs are connected by a considerable number of plasmodesmata that typically have
a larger SEL compared to the ones found in non-vascular tissues of mature leaves