Current perspectives on plasmodesmata: structure and function

Recent studies on plasmodesmata have shown that these important intercellular passages for communication and transport are much more sophisticated in both structure and regulatory abilities than previously imagined. A complex, but not well understood, substructure has been revealed by a variety of increasingly reliable ultrastructural techniques. Proteinaceous particles are seen within the cytoplasmic sleeve surrounding the desmotubule. Dye-coupling studies have provided experimental evidence for the physical pathway of solute movement, supporting conclusions about substructural dimensions within plasmodesmata drawn from the ultrastructural studies. Calcium has been identified as a major factor in the regulation of intercellular communication via plasmodesmata. Evidence from studies on virus movement through plasmodesmata suggests a direct interaction between virallycoded movement proteins and plasmodesmata in the systemic spread of many viruses. There is increasing evidence, albeit indirect, that in some plant species phloem loading may involve transport of photoassimilate entirely within the symplast from mesophyll cells to the sieve element-companion cell complexes of minor veins.     

Fine structure of plasmodesmata in mature leaves of sugarcane

The fine structure of plasmodesmata in vascular bundles and contiguous tissues of mature leaf blades of sugarcane (Saccharum interspecific hybrid L62–96) was studied with the transmission electron microscope. Tissues were fixed in glutaraldehyde, with and without the addition of tannic acid, and postfixed in OsO₄. The results indicate that the fine structure of plasmodesmata in sugarcane differs among various cell combinations in a cell-specific manner, but that three basic structural variations can be recognized among plasmodesmata in the mature leaf: 1) Plasmodesmata between mesophyll cells. These plasmodesmata possess amorphous, electron-opaque structures, termed sphincters, that extend from plasma membrane to desmotubule near the orifices of the plasmodesmata. The cytoplasmic sleeve is filled by the sphincters where they occur; elsewhere it is open and entirely free of particulate or spokelike components. The desmotubule is tightly constricted and has no lumen within the sphincters, but between the sphincters it is a convoluted tubule with an open lumen. 2) Plasmodesmata that traverse the walls of chlorenchymatous bundle-sheath cells and mestome-sheath cells. In addition to the presence of sphincters, these plasmodesmata are modified by the presence of suberin lamellae in the walls. Although the plasmodesmata are quite narrow and the lumens of the desmotubules are constricted where they traverse the suberin lamellae, the cytoplasmic sleeves are still discernible and appear to contain substructural components there. 3) Plasmodesmata between parenchymatous cells of the vascular bundles. These plasmodesmata strongly resemble those found in the roots of Azolla, in that their desmotubules are closed for their entire length and their cytoplasmic sleeves appear to contain substructural components for their entire length. The structural variations exhibited by the plasmodesmata of the sugarcane leaf are compared with those proposed for a widely-adopted model of plasmodesmatal structure.Abbreviation ER endoplasmic reticulum This study was supported by National Science Foundation grants DCB 87-01116 and DCB 90-01759 to R.F.E. and a University of Wisconsin-Madison Dean's Fellowship to K. R.-B. We also thank Claudia Lipke and Kandis Elliot for photographic and artistic assistance, respectively.     

Primary and secondary plasmodesmata: structure, origin, and functioning

Summary In the multicellular organisms of higher plants, plasmodesmata provide pathways for intimate symplasmic communication between neighboring cells. The arguments summarized in the present review demonstrate that plasmodesmata are diverse and highly dynamic structures. Differences in the plasmodesmal origin and modifications of the plasmodesmal structure and functioning at the various cell interfaces are the basic means which give rise to a complicated and flexibile symplasmic network. This complex communication system is discussed to serve a significant role in the coordinated development and in the concerted physiological functioning of the cells within the plant tissues, organs, and organisms.     

Observations on the structure of pullulan

An extracellular α-glucan, pullulan, elaborated by a strain of Pullularia pullulans, contains 0.6% of a maltotetraose subunit, as well as the already known major component, maltotriose. The majority, at least, of the maltotetraose is contained within the polymeric chains and is linked, through its terminal glucose units, by α-1→6— bonds.     

The neck constriction in plasmodesmata

Simple plasmodesmata between mesophyll and bundle sheath cells in actively expanding leaves of Salsola kali L. and roots of Epilobium hirsutum L. are shown to possess specialized structures, called sphincters, around their neck regions. The sphineters are made visible by the combined effects of tannic acid and heavy metal staining; they are localized just outside that area of the plasmalemma, which forms the collar around the entrance to each plasmodesmos. This localization corresponds to a very active area of the plasmodesmos/olasmalemma complex (i.e. enzyme activity and/or presence of strongly reducing substances).Evidence is presented that these ring structures are structural equivalents to hypothetical sphincters performing some valve function; i.e. participating in the control of rates and directions of symplastic transport of solutes through plasmodesmata. The middle layer of the plasmalemma in the neck region is composed of closely-packed, globular subunits appearing in negative contrast. Apparently, these subunits correspond to particle clusters observed at the plasmodesmatal entrance in freeze-fracture preparations. They appear similar to particle clusters in animal tight junctions, and their possible function in providing electrical coupling via low resistance junctions between plant cells is discussed.     

The structure of plasmodesmata as revealed by plasmolysis, detergent extraction, and protease digestion

Plasmodesmata or intercellular bridges that connect plant cells are cylindrical channels approximately 40 nm in diameter. Running through the center of each is a dense rod, the desmotubule, that is connected to the endoplasmic reticulum of adjacent cells. Fern, Onoclea sensibilis, gametophytes were cut in half and the cut surfaces exposed to the detergent, Triton X 100, then fixed. Although the plasma membrane limiting the plasmodesma is solubilized partially or completely, the desmotubule remains intact. Alternatively, if the cut surface is exposed to papain, then fixed, the desmotubule disappears, but the plasma membrane limiting the plasmodesmata remains intact albeit swollen and irregular in profile. Gametophytes were plasmolyzed, and then fixed. As the cells retract from their cell walls they leave behind the plasmodesmata still inserted in the cell wall. They can break cleanly when the cell proper retracts or can pull away portions of the plasma membrane of the cell with them. Where the desmotubule remains intact, the plasmodesma retains its shape. These images and the results with detergents and proteases indicate that the desmotubule provides a cytoskeletal element for each plasmodesma, an element that not only stabilizes the whole structure, but also limits its size and porosity. It is likely to be composed in large part of protein. Suggestions are made as to why this structure has been selected for in evolution.