Summary
It is shown that simple, unbranched, plasmodesmata between young xylem ray cells of willow have no direct intercellular continuity apart from the plasmalemma which limits the cytoplasm and lines the plasmodesmatal canal. Each plasmodesma is traversed by a 200 Å diameter tubule (the desmotubule) which has a wall with probably 11 subunits arranged around a central cavity through which runs a 40 Å diameter rod. This rod is connected to the inside of the tubule wall, by fine filaments. At the ends of each plasmodesma the plasmalemma and cell wall are closely appressed to the tubule, thus precluding direct continuity between the cytoplasm of adjacent cells. Through the central part of the plasmodesmata the tubule is separated from the plasmalemma by a 90–100 Å wide gap. Cytoplasmic microtubules in the same tissue have a diameter of approximately 250 Å and a wall probably composed of 13 subunits: both desmotubules and cytoplasmic microtubules therefore have a centre-to-centre subunit spacing of about 47 Å. It is suggested that the desmotubules are not microtubules but may be nuclear spindle fibres which become trapped in the wall during cell plate formation. The endoplasmic reticulum, while closely approaching the plasmodesmata, is not continuous across them. It is thought most unlikely that the endoplasmic reticulum traverses plasmodesmata, as the dimensions of the central tubule — found here as well as by other workers — are smaller than those which would be expected to allow a stable molecular configuration in a unit membrane. The plasmalemma, where it lines the plasmodesmatal canal, appears to have particulate subunits in the outer opaque layers and the presence of these subunits may be attributable to the need for stability in membranes arranged about so small a radius.
Article PDF
Similar content being viewed by others
Avoid common mistakes on your manuscript.
References
Agrawal, H. O., J. W. Kent, and D. M. Mackay: Rotation technique in electron microscopy of viruses. Science 148, 638–640 (1965).
Branton, D.: Fracture faces of frozen membranes. Proc. nat. Acad. Sci. (Wash.) 55, 1048–1056 (1966).
Buvat, R.: L'infrastructure des plasmodesmes chez les cellules parenchymateuses des cordons conducteurs jeunes de “Cucurbita pepo” L.. C. R. Acad. Sci. (Paris) 250, 170–172 (1960).
Cronshaw, J.: Cytoplasmic fine structure and cell wall development in differentiating xylem elements. In: Cellular ultrastructure of woody plants, ed. W. A. Côté, p. 99–124. Syracuse: Syracuse University Press 1965
Frey-Wyssling, A., and K. Mühlethaler: Ultrastructural plant cytology. Amsterdam: Elsevier 1965.
Glauert, A. M.: Electron microscopy of lipids and membranes.. J. roy. mier. Soc. 88 49–70 (1968).
Jensen, W. A. and R. B. Park: Cell ultrastructure. Wadsworth: Belmont 1967.
Jungers, V.: Recherches sur les plasmodesmes chez les vegetaux. II. Les synapses des Algues rouges. Cellule, 42, 1–28 (1933).
Kollmann, R., u. W. Schumacher: Über die Feinstruktur des Phloems von Metasequoia glyptostroboides und seine jahreszeitlichen Veränderungen II. Mitt. Vergleichende Untersuchungen der plasmatischen Verbindungsbrücken in Phloemparenchymzellen und Siebzellen. Planta (Berl.) 58, 366–386 (1962).
——: Über die Feinstruktur, des Phloems von Metasequoia glyptostroboides und seine jahreszeitlichen Veränderungen. IV. Mitt. Weitere Beobactungen zum Feinbau der Plasmabrücken in den Siebzellen. Planta (Berl.) 60, 360–389 (1963).
Ledbetter, M. C., and K. R. Porter: Morphology of microtubules of plant cells. Science 144, 872–874 (1964).
López-Sáez, J. F., G. Giménez-Martín, and M. C. Risueño: Fine structure of the plasmodesm. Protoplasma (Wien) 61, 81–84 (1966).
Markham, R., S. Frey, and G. J. Hilis: Methods for the enhancement of image detail and accentuation of structure in electron microscopy. Virology 20, 88–102 (1963).
O'Brien, T. P., and K. V. Thimann: Observations on the fine structure of the oat coleoptile. II. The parenchyma cells of the apex. Protoplasma (Wien) 63, 417–442 (1967).
Porter, K. R., and R. D. Machado: Studies on the endoplasmic reticulum. IV. Its form and distribution during mitosis in cells of onion root tip. J. biophys. biochem. Cytol. 7, 167–180 (1960).
Ringo, D. L.: The arrangements of subunits in flagellar fibres. J. Ultrastruct. Res. 17, 266–277 (1967).
Robards, A. W.: On the ultrastructure of differentiating secondary xylem in willow. Protoplasma (Wien) 65, 449–464 (1968).
Robertson, J. D.: Unit membranes.: a review with recent new studies of experimental alterations and a new subunit structure in synaptic membranes. In: Cellular membranes in development, ed. M. Locke, p. 1–81. London and New York: Academic Press 1964.
Wardrop, A. B.: Cellular differentiation in xylem. In: Cellular ultrastructure of woody plants, ed. W. A. Côté, p. 61–97. Syracuse: Syracuse University Press 1965.
Whaley, W. G., H. H., Mollenhauer, and J. H. Leech: The ultrastructure of the meristematic cell. Amer. J. Bot. 47, 401–449 (1960).
Wooding, F. B. P., and D. H. Northcote: The fine structure and development of the companion cell of the phloem of Acer pseudoplatanus. J. Cell Biol. 24, 117–128 (1965).
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Robards, A.W. A new interpretation of plasmodesmatal ultrastructure. Planta 82, 200–210 (1968). https://doi.org/10.1007/BF00398199
Received:
Issue Date:
DOI: https://doi.org/10.1007/BF00398199