Plasmodesmata. A not so open-and-shut case.

In 1897, Eduard Tangl observed striations between cells in the cotyledons of Strychnos nuxvomica and hypothesized that the “the protoplasmic bodies. . . are united by thin strands passing through connecting ducts in the walls, which put the cells into connection with each other and so unite them to an entity of higher order” (for review, see 2). This was a prophetic view of the symplasm, a term coined much later by Münch to describe the cytoplasmic continuum that occurs between higher plant cells. Tangl’s work challenged the then-held view that plant cells functioned as autonomous units and stimulated several studies on other species. However, it was Strasburger in 1901 who finally named these delicate structures plasmodesmata (2). Space constraints do not permit us to do justice to the several pioneers of plamodesmal research. However, we have attempted to select some notable milestones in the development of the field. Although the concept of the symplasm became accepted by many plant physiologists, it was not until the advent of electron microscopy that the fine structure of plasmodesmata was resolved. Following the introduction of glutaraldehyde as a fixative, several studies showed plasmodesmata to be plasma membrane-lined channels containing a central endoplasmic reticulum (ER)-derived structure termed the desmotubule (16). The demonstration of ER continuity between adjacent cells clearly distinguished the plasmodesma from its functional counterpart in animal cells, the gap junction. Although some studies suggested that there may be homologies between plasmodesmata and gap junctions, the consensus today is that these structures differ considerably in both form and function. With the introduction of immunological techniques, plasmodesmata have been shown to be extremely complex structures containing several unique proteins, including cytoskeletal elements (13), and recent studies suggest that there may be important differences between simple and branched plasmodesmal architectures with respect to molecular trafficking (11).