Plasmodesmata of maize root tips: structure and composition.

A procedure is described for obtaining clean maize cell wall preparations that contain embedded plasmodesmata. Negative staining and rotary shadowing have been used with transmission electron microscopy to visualise the plasmodesmata in these isolated walls, and to assess the effects of simple biochemical treatments on plasmodesmal components. Light protease treatment removes material from the exposed ends of plasmodesmata but does not extract the plasmodesmal core, which lies within the cell wall. However, heavy proteolysis occasionally removes the complete plasmodesma, including its enclosing collar structure, from the wall. Extraction with urea has a similar effect. The collar itself appears not to be proteinaceous in composition, although protein may bind it into the wall. Callose is localised in the wall around plasmodesmata, but does not appear to be a constituent of the collar. The membrane components of the plasmodesma (plasma membrane and desmotubule) can be extracted with membrane-solubilising detergents. This treatment releases from the wall a small number of proteins that are regarded as being potentially of plasmodesmal origin. These results show that plasmodesmata from maize can be dissected biochemically and suggest a strategy for the characterisation of individual molecular components.