In many cases plasmodesmata have been observed to be occluded by the proteins embedded in the desmotubule membrane, so it is envisaged that only a single water molecule could pass (Overall et al. 1982). However, other plasmodesmata have been observed to have an open desmotubule lumen. It appears that the open lumen in plasmodesmata of some trichomes may be a particular adaptation to their secretory role. In chickpea (Cicer arietinum) trichomes there appears to be a specialised system of vacuoles and tubules belonging to the ER system. Following the endocytic uptake of Lucifer Yellow carbohydrazide (LYCH) into this system the fluorescent probe was able to move, apparently within the ER network, through plasmodesmata (Laz-zaro and Thomson 1996). Although not conclusively demonstrated, it has been proposed that the transport of pre-nectar in cotton papillae takes place through the open desmotubule lumen, followed by budding of pre-nectar filled vesicles from the ER and secretion of their contents via reverse pinocy-tosis (Eleftheriou and Hall 1983; Waigmann and Zambryski 2000). Movement of fluorescent probes through the desmotubule lumen has also been demonstrated. Fluorescently labelled dextrans with a molecular weight of 10 000 were able to move from cell to cell in Torenia fournieri stem explants following microinjection into the ER but not after injection into the cytoplasm, although it was not demonstrated whether the plasmodesmata involved had an open or closed lumen (Cantrill et al. 1999).
Although some fluorescent probes have been observed to move from cell to cell presumably via the ER lumen, ER-targeted green fluorescent protein (GFP) does not. When expressed under the AtSUC2 promoter, GFP targeted to the ER
lumen of companion cells was not able to move through the desmotubules into the neighbouring sieve elements (Martens et al. 2001; Wright et al. 2003).
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