- Plasmodesma
Plasmodesmata (singular: plasmodesma) are microscopic channels which traverse the cell walls of
plant cells Oparka, K. J. (2005) "Plasmodesmata." Blackwell Pub Professional. ISBN 10: 1405125543ISBN 13: 9781405125543] [ [http://dictionary.reference.com/browse/Plasmodesmata Plasmodesmata (www.dictionary.com)] ] and enable transport and communication between them. Plants having plasmodesmata include the highest derived charophyceans, Charales and Coleochaetales, as well as all embryophytes, better known as land plants. [Graham, LE; Cook, ME; Busse, JS (2000), Proceedings of the National Academy of Sciences 97, 4535-4540.] Unlike animal cells, everyplant cell is surrounded by apolysaccharide cell wall . Neighbouring plant cells are therefore separated by a pair of cell walls and the intervening middle lamella, forming an extracellular domain known as theapoplast . Although cell walls are permeable to small soluble proteins and othersolutes , plasmodesmata enable direct, regulated, symplastic intercellular transport of substances between cells. Plant cells can use both passive andactive transport to move molecules and ions through the passage Fact|date=August 2007.tructure
Plasmodesmatal plasma membrane
A typical plant cell may have between 103 and 105 plasmodesmata connecting it with adjacent cells. Plasmodesmata are approximately 50-60nm in diameter at the mid-point and are constructed of three main layers, the
plasma membrane , the "cytoplasmic sleeve", and the "desmotubule" AW Robards (1975) Plasmodesmata. Annual Review of Plant Physiology 26, 13-29]The plasma membrane portion of the plasmodesma is a continuous extension of the cell membrane or
plasmalemma AW Robards (1976) Plasmodesmata in higher plants. In: "Intercellular communications in plants: studies on plasmodesmata." Edited by BES Gunning and AW Robards Springer-Verlag Berlin pps 15-57.] It is similar in structure to the cellular phospholipid bilayers.Cytoplasmic sleeve
The cytoplasmic sleeve is a fluid-filled space enclosed by the plasmalemma and a continuous extension of the
cytosol . Trafficking ofmolecule s andion s through plasmodesmata is assumed to occur through this passage. Smaller molecules (e.g.sugar s andamino acid s) and ions can easily pass through plasmodesmata bydiffusion without the need for additional chemical energy. It is not yet known how the selective transport of larger molecules, such as proteins, occurs. Onehypothesis is that thepolysaccharide callose accumulates around the neck region of plasmodesmata to form a collar, reducing their diameter and thereby controlling permeability to substances in the cytoplasm .Desmotubule
The desmotubule is a tube of appressed endoplasmic reticulum that runs between two adjacent cells RL Overall, J Wolfe, BES Gunning (1982) Intercellular communication inAzolla roots: I. Ultrastructure of plasmodesmata. Protoplasma 111: 134-150] Some molecules are known to be transported through this channel [LC Cantrill, RL Overall and PB Goodwin (1999) Cell-to-cell communication via plant endomembranes. Cell Biology International 23: 653–661] , but it is not thought to be the main route for plasmodesmatal transport.
Around the desmotubule and the plasma membrane areas of an electron dense material have been seen, often joined together by spoke-like structures that seem to split the plasmodesma into smaller channels These structures may be composed of
myosin JE Radford and RG White (1998) Localization of a myosin‐like protein to plasmodesmata. Plant Journal 14: 743-750] LM Blackman and RL Overall (1998) Immunolocalisation of the cytoskeleton to plasmodesmata of "Chara corallina". Plant Journal 14: 733-741] [S Reichelt, AE Knight, TP Hodge, F Baluska, J Samaj, D Volkmann and J Kendrick-Jones (1999) Characterization of the unconventional myosin VIII in plant cells and its localization at the post-cytokinetic cell wall. Plant Journal 19: 555–569] andactin [RG White, K Badelt, RL Overall and M Vesk (1994) Actin associated with plasmodesmata. Protoplasma 180: 169-184] , which are part of the cell'scytoskeleton . If this is the case these proteins could be used in the selective transport of large molecules between the two cells.Transport
Plasmodesmata have been shown to transport
protein s,messenger RNA and viralgenome s from cell to cell Fact|date=August 2007. The best studied of these are viralmovement proteins such as those of thetobacco mosaic virus MP-30 Fact|date=August 2007. MP-30 is thought to bind to the virus's own genome and shuttle it from infected cells to uninfected cells through plasmodesmataFact|date=September 2007. Flowering Locus T protein moves from leaves to the shoot apicalmeristem through plasmodesmata to initiateflowering Fact|date=August 2007.References
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