Structure

5.2.1 The general ultrastructure of plasmodesmata

Structural models of plasmodesmata have generally been based on data from transmission electron microscopy. Since the first published model of plasmodesmata 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 ultrastructure 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 structural 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 (DiOC6) 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

Figure 5.1 (A) Longitudinal section through simple plasmodesmata between adjacent parenchyma cells of potato tuber. ER, endoplasmic reticulum; DT, desmotubule; CW, cell wall. Bar: 25 nm. (B) Transverse section through simple plasmodesmata between parenchyma cells of potato tuber, showing the desmotubule and the plasma membrane. The central electron dense dot is thought to be formed by the appressed hydrophilic phospholipid head groups of the inner leaflet of the desmotubule. The cytoplasmic sleeve appears to be partially occluded; this is thought to be due to gobular subunits surrounding the desmotubule. PM, plasma membrane; CS, cytoplasmic sleeve. Bar: 20 nm. (C) Longitudinal section through branched plasmodesmata between parenchyma cells of potato tuber. Multiple branches can be seen extending from median central cavities. CC, central cavity; CW, cell wall; ER, endoplasmic reticulum. Bar: 25 nm.

Figure 5.1 (A) Longitudinal section through simple plasmodesmata between adjacent parenchyma cells of potato tuber. ER, endoplasmic reticulum; DT, desmotubule; CW, cell wall. Bar: 25 nm. (B) Transverse section through simple plasmodesmata between parenchyma cells of potato tuber, showing the desmotubule and the plasma membrane. The central electron dense dot is thought to be formed by the appressed hydrophilic phospholipid head groups of the inner leaflet of the desmotubule. The cytoplasmic sleeve appears to be partially occluded; this is thought to be due to gobular subunits surrounding the desmotubule. PM, plasma membrane; CS, cytoplasmic sleeve. Bar: 20 nm. (C) Longitudinal section through branched plasmodesmata between parenchyma cells of potato tuber. Multiple branches can be seen extending from median central cavities. CC, central cavity; CW, cell wall; ER, endoplasmic reticulum. Bar: 25 nm.

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