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Plasmodesma

Plasmodesma allow molecules to travel between plant cells through the symplasticpathway

The structure of a primary plasmodesma. CW=Cell wall CA=CallosePM=Plasma membrane ER=Endoplasmic reticulum DM=Desmotubule

Red circles=Actin Purple circles and spokes=Other unidentifiedproteins[1]

Plasmodesmata (singular: plasmodesma)are microscopic channels which traverse thecell walls of plant cells[2] [3] and some algalcells, enabling transport and communicationbetween them. Species that haveplasmodesmata include members of theCharophyceae, Charales and Coleochaetales(which are all algae), as well as allembryophytes, better known as landplants.[4] Unlike animal cells, every plantcell is surrounded by a polysaccharide cellwall. Neighbouring plant cells are thereforeseparated by a pair of cell walls and theintervening lamella, forming an extracellulardomain known as the apoplast. Althoughcell walls are permeable to small solubleproteins and other solutes, plasmodesmataenable direct, regulated, symplasticintercellular transport of substances betweencells. There are two forms ofplasmodesmata: primary ones are formedduring cell division and secondary ones canform between mature cells.[5]

Similar structures, called gap junctions[6]

and membrane nanotubes, interconnectanimal cells[7] and stromules form betweenplastids in plant cells.[8]

Formation

Plasmodesmata are formed when portions ofthe endoplasmic reticulum are trappedacross the middle lamella as new cell wall islaid down between two newly divided plantcells and these eventually become thecytoplasmic connections between cells(primary plasmodesmata). Here the wall isnot thickened further, and depressions orthin areas known as pits are formed in thewalls. Pits normally pair up betweenadjacent cells. Alternatively, plasmodesmatacan be inserted into existing cell walls

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between non-dividing cells (secondary plasmodesmata)[9]

Structure

Plasmodesmatal plasma membraneA typical plant cell may have between 103 and 105 plasmodesmata connecting it with adjacent cells[10] equating tobetween 1 and 10 per µm2.[11] Plasmodesmata are approximately 50-60 nm in diameter at the mid-point and areconstructed of three main layers, the plasma membrane, the cytoplasmic sleeve, and the desmotubule.[10] They cantransverse cell walls that are up to 90 nm thick.[11]

The plasma membrane portion of the plasmodesma is a continuous extension of the cell membrane or plasmalemma[12] It is similar in structure to the cellular phospholipid bilayers.

Cytoplasmic sleeveThe cytoplasmic sleeve is a fluid-filled space enclosed by the plasmalemma and a continuous extension of thecytosol. Trafficking of molecules and ions through plasmodesmata occurs through this passage. Smaller molecules(e.g. sugars and amino acids) and ions can easily pass through plasmodesmata by diffusion without the need foradditional chemical energy. Proteins can also pass through the cytoplasmic sleeve (for example Green fluorescentprotein).[13] It is not yet known how the selective transport of larger molecules, such as proteins, occurs. Onehypothesis is that the polysaccharide callose accumulates around the neck region of plasmodesmata to form a collar,reducing their diameter and thereby controlling permeability to substances in the cytoplasm.[12]

DesmotubuleThe desmotubule is a tube of appressed endoplasmic reticulum that runs between two adjacent cells [14] Somemolecules are known to be transported through this channel,[15] but it is not thought to be the main route forplasmodesmatal transport.Around the desmotubule and the plasma membrane areas of an electron dense material have been seen, often joinedtogether by spoke-like structures that seem to split the plasmodesma into smaller channels [14] These structures maybe composed of myosin[16] [17] [18] and actin,[17] [19] which are part of the cell's cytoskeleton. If this is the case theseproteins could be used in the selective transport of large molecules between the two cells.

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Transport

Tobacco mosaic virus movement protein 30 localizesto plasmodesmata.

Plasmodesmata have been shown to transport proteins (includingtranscription factors), short interfering RNA, messenger RNA andviral genomes from cell to cell. One example of a viral movementproteins is the tobacco mosaic virus MP-30. MP-30 is thought tobind to the virus's own genome and shuttle it from infected cells touninfected cells through plasmodesmata.[13] Flowering Locus Tprotein moves from leaves to the shoot apical meristem throughplasmodesmata to initiate flowering.

The size of molecules that can pass through plasmodesmata isdetermined by the size exclusion limit. This limit is highly variableand can is subject to active modification.[5] MP-30 is able toincrease the size exclusion limit from 700 Daltons to 9400 Daltonsthereby aiding its movement through a plant.[20]

Several models for possible active transport throughplasmodesmata exist. It has been suggested that such transport ismediated by interactions with proteins localized on the desmotubule, and/or by chaperones partially unfoldingproteins, allowing them to fit through the narrow passage. A similar mechanism may be involved in transportingviral nucleic acids through the plasmodesmata.[21]

References[1] Maule, Andrew (December 2008). "Plasmodesmata: structure, function and biogenesis". Current Opinion in Plant Biology 11 (6): 680–686.

doi:10.1016/j.pbi.2008.08.002. PMID 18824402.[2] Oparka, K. J. (2005) Plasmodesmata. Blackwell Pub Professional. ISBN 10: 1405125543 ISBN 13: 9781405125543[3] Plasmodesmata (www.dictionary.com) (http:/ / dictionary. reference. com/ browse/ Plasmodesmata)[4] Graham, LE; Cook, ME; Busse, JS (2000), Proceedings of the National Academy of Sciences 97, 4535-4540.[5] http:/ / www. pubmedcentral. nih. gov/ articlerender. fcgi?artid=1692983 The shoot apical meristem: the dynamics of a stable structure. Jan

Traas and Teva Vernoux : Philos Trans R Soc Lond B Biol Sci. 2002 June 29; 357(1422): 737–747. (page 744)[6] Bruce Alberts (2002). Molecular biology of the cell (4th ed.). New York: Garland Science. ISBN 0-8153-3218-1.[7] Gallagher KL, Benfey PN (January 2005). "Not just another hole in the wall: understanding intercellular protein trafficking" (http:/ / www.

genesdev. org/ cgi/ pmidlookup?view=long& pmid=15655108). Genes Dev. 19 (2): 189–95. doi:10.1101/gad.1271005. PMID 15655108. .[8] Gray JC, Sullivan JA, Hibberd JM, Hansen MR (2001). "Stromules: mobile protrusions and interconnections between plastids". Plant Biology

3: 223–33. doi:10.1055/s-2001-15204.[9] Lucas W., Ding, B. and Van der Schoot, C. (1993) Tansley Review No.58 "Plasmodesmata and the supracellular Nature of Plants" New

Phytologist, Vol. 125, No. 3, pp. 435-476, Stable URL: http:/ / www. jstor. org/ stable/ 2558257[10] AW Robards (1975) Plasmodesmata. Annual Review of Plant Physiology 26, 13-29[11] "22". Molecular Cell Biology (4 ed.). 2000. pp. 998. ISBN 0-7167-3706-X.[12] 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.[13] http:/ / www. ingentaconnect. com/ content/ bsc/ pce/ 2003/ 00000026/ 00000001/ art00007 Plasmodesmata and the control of symplastic

transport A. G. ROBERTS & K. J. OPARKA[14] RL Overall, J Wolfe, BES Gunning (1982) Intercellular communication in Azolla roots: I. Ultrastructure of plasmodesmata. Protoplasma

111: 134-150[15] LC Cantrill, RL Overall and PB Goodwin (1999) Cell-to-cell communication via plant endomembranes. Cell Biology International 23:

653–661[16] JE Radford and RG White (1998) Localization of a myosin‐like protein to plasmodesmata. Plant Journal 14: 743-750[17] LM Blackman and RL Overall (1998) Immunolocalisation of the cytoskeleton to plasmodesmata of Chara corallina. Plant Journal 14:

733-741[18] 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[19] RG White, K Badelt, RL Overall and M Vesk (1994) Actin associated with plasmodesmata. Protoplasma 180: 169-184

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[20] http:/ / www. sciencemag. org/ cgi/ content/ abstract/ 246/ 4928/ 377 Science 20 October 1989: Vol. 246. no. 4928, pp. 377 - 379 MovementProtein of Tobacco Mosaic Virus Modifies Plasmodesmatal Size Exclusion Limit SHMUEL WOLF, WILLIAM J. LUCAS, CARL M.DEOM,and ROGER N. BEACHY

[21] http:/ / jpkc. zju. edu. cn/ k/ 437/ content/ 05. pdf Plant Physiology lectures, chapter 5

Article Sources and Contributors 5

Article Sources and ContributorsPlasmodesma  Source: http://en.wikipedia.org/w/index.php?oldid=426648361  Contributors: 24Adrianus, 2overCosC, Alexchris, Athaenara, DARTH SIDIOUS 2, DabMachine, Dan Gluck,EncycloPetey, Heron, JFreeman, Jonnabuz, Kingdon, Kurgus, Liveste, Loupeter, Mccready, Mgiganteus1, Nick Number, Nina Gerlach, Nonagonal Spider, Onorem, Plantsurfer, R. S. Shaw,Redhookesb, Serephine, Signalhead, Simpsonce, Smartse, Synapomorphy, Taka, Temporaluser, WriterHound, 57 anonymous edits

Image Sources, Licenses and ContributorsImage:Apoplast and symplast pathways.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Apoplast_and_symplast_pathways.svg  License: Public Domain  Contributors:User:Jackacon vectorised by User:SmartseFile:Plasmodesmata structure.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Plasmodesmata_structure.svg  License: Creative Commons Attribution-Sharealike 3.0  Contributors:SmartseImage:MP-30-GFP.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:MP-30-GFP.jpg  License: Creative Commons Attribution-ShareAlike 3.0 Unported  Contributors: Synapomorphy

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