The Cytoplasmic Sleeve

The plasma membrane within a plasmodesma is lined on its cytoplasmic face with electron-dense structures (Botha et al. 1993; Ding et al. 1992b; Tilney et al. 1991). These globular particles are connected by elongated spokes to the particles embedded in the wall of the desmotubule (Fig. 1) and may regulate transport by expanding or contracting the cytoplasmic sleeve (Ding et al. 1992b; Overall and Blackman 1996). Although the diameter of a plasmo-desma channel is 20-50 nm and the desmotubule is about 15 nm in diameter (Ehlers and Kollmann 2001), the helical arrangement of the embedded particles produces spiralling channels that may reduce the functional diameter of the cytoplasmic sleeve (Zee 1969; Robards 1976; Olesen 1979; Overall et al. 1982; Wolf et al. 1989; Olesen and Robards 1990; Robards and Lucas 1990; Ding et al. 1991; Robinson-Beers and Evert 1991; Lucas and Wolf 1993; Lucas et al. 1993; Overall and Blackman 1996; Waigmann et al. 1997). Various estimates give the diameter of the resulting microchannels between 2.5 and 6 nm (Overall et al. 1982; Ding et al. 1992b; Botha et al. 1993).

Following the localisation of actin to plasmodesmata (White et al. 1994), Overall and Blackman (1996) proposed a model, in which a cable of actin runs through the centre of the pore, closely associated with the desmotubule. It has been suggested that the spokes may be composed of myosin and con-

Desmotubule

Fig. 1 Diagrammatic representation of the structure of a simple plasmodesma including all sub-structural components that have been confirmed to date. A longitudinal section through the pore is shown to the left, while a transverse section through the central cavity region is shown to the right. Based primarily on Ding (1992b) and reproduced with permission from Roberts (2005)

Fig. 1 Diagrammatic representation of the structure of a simple plasmodesma including all sub-structural components that have been confirmed to date. A longitudinal section through the pore is shown to the left, while a transverse section through the central cavity region is shown to the right. Based primarily on Ding (1992b) and reproduced with permission from Roberts (2005)

nect proteins embedded in the plasma membrane to the actin cable at the centre of the pore, forming an important regulatory structure that can alter the size exclusion limit of plasmodesmata (Oparka 2004). Additional support for this model is provided by the identification of myosin within higher-plant plasmodesmata (Radford and White 1998) and the green alga Chara corallina (Blackman and Overall 1998).

The neck region at each end of a plasmodesma may appear constricted, bringing the plasma membrane into close proximity with the desmotubule. This constriction may be the result of callose deposition since treatment with inhibitors of callose synthesis result in funnel-shaped rather than constricted plasmodesmata (Radford et al. 1998). Since callose deposition can be triggered as a response to wounding or chemical fixation, it remains to be determined whether the constricted state is an artefact of fixation or whether callose has a regulatory role in the control of plasmod-

esmal aperture (for discussions, see Overall 1999; Schulz 1999; Botha and Cross 2000).

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