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in 1968 (Robards, 1968a,b) various structural models have been proposed, together
with considerable debate as to the effect of different fixation and staining procedures
on their appearance (Gunning & Robards, 1976; Robards & Lucas, 1990; Beebe &
Turgeon, 1991; Tilney
et al.
, 1991; Ding
et al.
, 1992b; Botha
et al.
, 1993; Turner
et al.
, 1994; White
et al.
, 1994; Overall & Blackman, 1996; Ding, 1997, 1999;
Waigmann
et al.
, 1997; Radford
et al.
, 1998; Overall, 1999). On the basis of ultra-
structure alone, plasmodesmata are generally classified into two basic types: simple
and branched. Simple plasmodesmata consist of a single channel traversing the cell
wall, whereas branched plasmodesmata have a more complex structure with two or
more channels on either side of the middle lamella, often joined by a central cavity
(Roberts & Oparka, 2003). Many structural models depict simple plasmodesmata
only, usually assuming that simple and branched plasmodesmata have a common
basic architecture (Blackman & Overall, 2001; Ehlers & Kollmann, 2001). As van
Bel
et al.
(1999) comment, this may give the impression that higher plants possess
only one type of plasmodesma, forcing interpretations of function into a single struc-
tural model. The basic structure of simple and branched plasmodesmata is shown in
Fig. 5.1. When discussing plasmodesmata ultrastructure it is important to note that
a wide range of plasmodesmal morphologies and substructural variations have been
found both between plant species and within the tissues of the same plant (Robinson-
Beers & Evert, 1991; Waigmann
et al.
, 1997). Furthermore, electron micrographs
provide a two-dimensional image of an essentially dynamic structure (Robards &
Lucus, 1990; Botha & Cross, 1999; van Bel
et al.
, 1999). However, since the first
observation of plasmodesmata in the electron microscope (Buvat, 1957), images of
plasmodesmata have revealed a remarkable structural consistency (Overall, 1999).
A longitudinal section through a plasmodesma reveals a plasma-membrane-lined
cylindrical channel that transverses the cell wall (Fig. 5.2; Blackman & Overall,
2001); the plasma membrane defines the symplastic boundary of the plasmodesma
and is continuous between adjacent cells (Overall & Blackman, 1996). Grabski
et al
.
(1993) suggested that the plasma membrane in plasmodesmata may be modified as
it fails to allow diffusion of lipids between neighbouring cells. In the centre of the
channel lies a strand of modified cortical endoplasmic reticulum, the desmotubule
(Robards & Lucas, 1990; Ding
et al.
, 1992b; Lucas & Wolf, 1993; Epel, 1994).
The modified endoplasmic reticulum of the desmotubule is continuous with the en-
doplasmic reticulum in the adjoining cells, forming an endomembrane continuum
throughout the plant (Robards & Lucus, 1990; Denecke, 2001). This continuity has
been shown by 3,3
-dihexyl-oxacarocyanine iodide (DiOC
6
) staining after plasmol-
ysis (Oparka
et al.
, 1994), and lipids may apparently diffuse along the endoplasmic
reticulum membranes between adjacent cells (Grabski
et al.
, 1993). Although the
endoplasmic reticulum is continuous between cells, it is the cytoplasmic sleeve (the
space between the desmotubule and plasma membrane) through which the bulk of
cell-to-cell communication is thought to occur (Ding, 1997).
At both ends of the plasmodesma channel, just inside the cell wall, is the neck
region (see Fig. 5.2). This area is often constricted at either or both ends, where the
plasma membrane comes into tight association with the entrance of the cytoplasmic
