It is now generally accepted that the majority of molecular transport through plasmodesmata takes place via the cytoplasmic sleeve, through the helical channels between the plasma membrane and the desmotubule. This of course includes molecules moving in association with the ER. In many cases it is assumed that molecules are moving through the cytoplasmic sleeve without requiring any specific interaction with components of the pore, i.e. non-selectively (Schulz 1999; Zambryski and Crawford 2000; Oparka 2004).
The conductive properties of plasmodesmata have been established using a combination of techniques including monitoring of cell-to-cell transfer of radiolabelled substances (Arisz 1969), electrical coupling experiments (Overall and Gunning 1982) and dye coupling (Tucker 1982; Erwee and Goodwin 1983, 1984, 1985; Goodwin 1983; Palevitz and Hepler 1985; Tucker and Span-swick 1985; Madore et al. 1986; van der Schoot and van Bel 1989). Fluorescent probes in the form of fluorescein were first used to investigate short-distance transport through plasmodesmata in trichomes by Schumacher (1936), but it was not until 40 years later that fluorescent probes started to make a significant impact on the study of cell-to-cell communication.
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