Open plasmodesmata allow the diffusive exchange of metabolites and small molecules (Tyree 1970; Tucker et al. 1989). The size of molecules able to pass through plasmodesmata determines the size exclusion limit (SEL) of the pore but should more accurately be referred to as the molecular exclusion limit (MEL) when it is measured in terms of the mass, rather than the physical dimensions, of the molecules involved. On the basis of investigations using predominantly fluorescently labelled dextrans and other conjugates, the MEL for such movement through many plasmodesmata has been determined to be in the order of 850-900 Da. However, it is not the mass of the molecule that determines its ability to move through plasmodesmata but rather its physical size, which is usually measured in terms of its hydrodynamic radius or Stokes radius (Rs) (Terry and Robards 1987). Many open plasmodesmata allow the diffusion of molecules with an Rs of 0.75 nm. Predictions of the diameter of pores within plasmodesmata, determined using fluorescent dextrans or peptides, fall in the region of 3-4 nm (Terry and Robards 1987; Fisher 1999), which corresponds well with the measurements obtained from electron micrographs (2.5-6 nm) (Ding et al. 1992b; Overall and Blackman 1996; Overall et al. 1982). It should be noted that the relationship between molecular mass and size for dextrans differs greatly from that for proteins. For example, a 27-kDa dextran has the same Rs value (3.5 nm) as a 67-kDa protein (Jorgensen and Moller 1979; Le Maire et al. 1986; Fisher 2000). This becomes of particular relevance in considering more recent studies investigating the movement of the fluorescent protein GFP, which has a molecular mass of 27 kDa and an Rs of 2.82 nm (Terry et al. 1995).
As described previously, a plant is divided into a number of functional sym-plasmic domains (Erwee and Goodwin 1985; Robards and Lucas 1990; Lucas et al. 1993; McLean et al. 1997; Ding et al. 1999; Ehlers and van Bel 1999; Lucas 1999). In this context, it has been demonstrated that plasmodesmata do not all have the same SEL. Whilst leaf mesophyll cells are frequently quoted to have an MEL for dextrans in the region of 1 kDa, some cell types including epidermal cells have a lower MEL of370 Da, as assessed by the movement of car-boxyfluorescein (Erwee and Goodwin 1985; Duckett et al. 1994), while others have a higher MEL(dextran), including tobacco trichomes at 7 kDa (Rs 1.6 nm, estimated from Jorgensen and Moller 1979; Waigmann and Zambryski 1995).
Simple plasmodesmata have a much larger SEL than branched plasmo-desmata (Oparka et al. 1999). In sink tobacco leaf epidermis, simple plasmodesmata predominate and allow the passage of GFP fusion proteins up to 47 kDa. However, after the sink-source transition, during which the simple plasmodesmata are replaced or modified to the branched morphotype, the SEL decreases so that GFP is able to move only from the bombarded cell to its immediate neighbours (Oparka et al. 1999).
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