Behaviour of plasma membrane, cortical ER and plasmodesmata during plasmolysis of onion epidermal cells

During plasmolysis of onion epidermal cells, the contracting protoplast remains connected to the cell wall by an intricate, branched system of plasma membrane (PM) ‘Hechtian strands’ which stain strongly with the fluorescent probe DiOC₆. In addition, extensive regions of the cortical endoplasmic reticulum (ER) network remain anchored to the cell wall during plasmolysis and do not become incorporated into the contracting protoplast with the other cell organelles. These ER profiles become tightly encased by the PM as the latter contracts towards the centre of the cell. Thus, although the cortical ER is left outside the main protoplast body, it is nonetheless still bound by the PM of the cell. As well as being anchored to the wall, the cortical ER remains intimately linked with plasmodesmata and retains continuity between cells via the central desmotubules which become distended during plasmolysis. The PM also remains in close contact with the plasmodesmatal pore following plasmolysis. It is suggested that plasmodesmata, although sealed, may not be broken during plasmolysis, their substructure being preserved by continuity of both ER and PM through the plasmodesmatal pore. A structural model is presented which links the behaviour of PM, ER and plasmodesmata during plasmolysis.