Endoplasmic reticulum in the formation of the cell plate and plasmodesmata

Summary The association of endoplasmic reticulum (ER) with the developing cell plate has been analyzed in lettuce roots fixed in glutaraldehyde and post-fixed in a mixture of osmium tetroxide-potassium ferricyanide (OsFeCN). Electron microscopic observations show that elements of ER, which are selectively stained by the OsFeCN reagent, become loosely associated with aggregating dictyosome vesicles at the onset of plate formation. Subsequently the ER, in a tubular reticulate network, surrounds the vesicular aggregates creating a three dimensional membrane matrix. It is suggested that the ER (1) provides a structural framework that holds the vesicles in position and directs their fusion within the plane of the plate and/or (2) regulates the local release of calcium ions required for vesicle fusion.OsFeCN post-fixation also provides new information about the cell plate vesicles themselves. The results demonstrate that vesicles derived from dictyosomes undergo an abrupt increase in staining as they fuse at the plate.Finally the ER associated with developing and mature plasmodesmata has been examined. Electron micrographs reveal that the OsFeCN staining, seen traversing the cell plate in early stages, later becomes restricted from that portion of the ER extending through the plasmodesmatal canal. These structural observations support the idea that during formation of the plasmodesma a tubular element of ER is tightly furled upon itself and that its inner leaflet is compressed into a rod. The ER cisternal space appears occluded and thus it is argued that intercellular transport occurs through the cytoplasmic annulus of the plasmodesmata.