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'sphincters' in cells of the SAM of birch buds entering endodormancy (Rinne
et al.
,
2001). The proteinaceous sphincters were found to contain pronounced deposits
of b -1,3-D-glucan and thought to prevent any form of symplastic communication
between cells in endodormant tissue (Rinne & van der Schoot, 1998). These authors
found that removal of callose from plasmodesmata during chilling coincided with
the production of b -1,3-glucanase (Rinne
et al.
, 2001). Blockage of plasmodesmata
by callose in the SAM of poplar buds entering into endodormancy, together with
elevated levels of cytosolic Ca
2
+
, has also been reported by Jian
et al.
(1997). Inter-
estingly, Karlson
et al.
(2003) have recently reported the immunolocalisation of a
24-kDa dehydrin protein to plasmodesmata in cold acclimated xylem parenchyma
cells of
Cornus sericea
. The pattern of labelling was identical to that observed with
monoclonal antibodies specific for callose. The phloem of perennials has also been
shown to become inactivated in the autumn by the deposition of b -1,3-glucan at the
sieve-plate pores, which become unplugged in the spring (Esau, 1977) by the activa-
tion of b -1,3-glucanase (Krabel
et al.
, 1993; Rinne
et al.
, 2001). In the sucrose export
deficient maize mutant (
sxd1
), phloem loading is also prevented by callose deposits
that specifically block plasmodesmata at the interface between bundle sheath and
vascular parenchyma cells (Botha
et al.
, 2000). Hofius and Sonnewald (2003) have
noted that the
sxd1
mutation in maize corresponds to a
VTE1
-tocopherol cyclase
(vitamin E)
Arabidopsis
knockout mutant (Porfirova
et al.
, 2002), and have therefore
suggested a putative link between tocopherol cyclase and plasmodesmata function.
Caution may be required in interpreting some studies on callose deposition be-
cause the preparation of tissue for EM studies can itself induce callose by stimulating
awounding response (Radford
et al.
, 1998). It is interesting to note that, although the
effects of callose are generally thought to be coarser than other control mechanisms,
the extent of callose synthesis and degradation is clearly highly regulated (Roberts
& Oparks, 2003).
5.3.6 Phosphorylation, protein unfolding and chaperones
Plasmodesmal trafficking of protein and/or RNA is unique to plants (Ding
et al.
,
2003). Haywood
et al.
(2002) remark that all viral MPs and NCAPs studied so
farhave been found to expose a motif(s) that can induce dilation of the SEL. This
has led various authors to comment that selective movement of protein through
plasmodesmata involves the interaction between this motif and cognate cellular
factors (Haywood
et al.
, 2002; Ding
et al.
, 2003; Oparka, 2004). In a recent review
of protein movement through plasmodesmata, Haywood
et al.
(2002) suggest that,
in the simplest scenario, plasmodesmal dilation is necessary and sufficient to allow
protein movement by diffusion into neighbouring cells (i.e. non-selective gating;
see Schulz, 1999). However, Wu
et al.
(2003) recently provided evidence that the
transcription factor LEAFY (Sessions
et al.
, 2000) moves from the L1 layer of
the
Arabidopsis
SAM into the L2 and L3 layers by diffusion, and therefore defined
the movement of LEAFY as non-targeted (Crawford & Zambryski, 2001). Wu
et al.
(2003) concluded that the LEAFY protein sequence does not contain a specific
