RNA as a signalling molecule - Intercellular Communication in Plants - page 52

(Imlau et al. , 1999). This peculiarity is probably explained by the fact that extensive
mRNA and protein communication between the SE and CCs is required to maintain
the integrity of SE, which are not competent for protein synthesis because they lack
a nucleus and Golgi apparatus.
Evidence for this kind of transport through SE-CCs plasmodesmata comes from
studies of the spatial distribution of the sucrose transporter SUT1 mRNA (Kuhn
et al. , 1997). In situ hybridization and immunogold labelling revealed that both
SUT1 mRNA and protein are detected in SE although the SUT1 mRNA is produced
exclusively in CCs (Kuhn et al. , 1997). The SUT1 mRNA has also been directly
detected in the phloem sap (Ruiz-Medrano et al. , 1999) of cucurbits, providing the
first experimental clue that movement of endogenous mRNAs through the CC-SE
plamodesmata could allow their translocation into the phloem stream where they
could potentially serve as long-distance signalling molecules.
This initial observation prompted the thorough analysis of the mRNA content
of the cucurbit phloem sap (Sasaki et al. , 1998; Ruiz-Medrano et al. , 1999). These
studies revealed a large population of at least 500 polyadenylated RNA molecules,
several of which were subsequently confirmed to traffic over long distances, as
discussed later on. One of these mRNAs, isolated from Cucurbit maxima (pumpkin),
was found to encode an RNA-binding protein named CmPP16 (Ruiz-Medrano et al. ,
1999), which shows functional similarity and a limited degree of sequence identity
to the MP of red clover necrotic mosaic virus (RCNMV) (Xoconostle-Cazares et al. ,
1999). CmPP16 was indeed detected with an antibody directed against the RCNMV-
MP and exhibited a similar capacity to increase plasmodesmata SEL, to mediate its
own cell-to-cell movement and to potentiate the intercellular trafficking of RNA in
a non-sequence-specific manner (Xoconostle-Cazares et al. , 1999).
Heterografting experiments between pumpkin (rootstocks) and Cucumis sativus
(cucumber scions) clearly demonstrated that both CmPP16 and its mRNA are indeed
translocated over long distances through the phloem. The experiments exploited the
polymorphism found between cucumber and pumpkin CmPP16 (both at the protein
and mRNA level). Therefore, its detection in the scions could have only resulted
from graft transmission. Altogether, these findings supported the idea that systemic
transport of ribonucleoprotein complexes could participate in the integration of
biological functions in plants.
The demonstration that phloem-mobile mRNA can play a role in developmental
events was recently provided by grafting experiments conducted with tomato plants
carrying the dominant gain-of-function mutation Mouse ears ( Me ), which affects
leaf morphology (Kim et al. , 2001). The Me phenotype is caused by accumula-
tion of fusion transcripts between the PHOSPHATE-DEPENDENT PHOSPHO-
FRUCTOKINASE ( PFP ) gene, and a KN1 -like homeobox gene, LeT6 (Chen et al. ,
1997). The resulting chimaeric PFP-LeT6 mRNA was shown to be translocated in
the phloem from Me stocks to wild-type (wt) tomato scions. Moreover, the graft-
transmitted transcript accumulated in the SAM, resulting in phenotypic changes in
the wt scions that were similar to those of the Me stocks (Kim et al. , 2001).
Implicit to the idea that phloem long-distance movement of mRNA could orches-
trate crucial biological functions in plants is the notion that there must be control