{"id":242,"date":"2015-11-04T14:52:51","date_gmt":"2015-11-04T22:52:51","guid":{"rendered":"http:\/\/labs.wsu.edu\/knoblauch\/?page_id=242"},"modified":"2026-03-20T10:14:55","modified_gmt":"2026-03-20T17:14:55","slug":"p-proteins","status":"publish","type":"page","link":"https:\/\/labs.wsu.edu\/knoblauch\/p-proteins\/","title":{"rendered":"Phloem-Proteins (P-Proteins)"},"content":{"rendered":"
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Historically, P-proteins have been defined as \u201cproteinaceous material in the phloem which is sufficiently characteristic when observed with the electron microscope to warrant a special term\u201d (Cronshaw, (1975) P-Proteins. In: Phloem Transport (eds S. Aronoff, J. Dainty, P.R. Gorham, L.M. Srivastava, & C.M. Swanson), pp. 79-115. Plenum, New York, London. Thus, this term mostly refers to structural proteins. A variety of other proteins withing the phloem sap have been described. Since they are soluble and not visible in the transmission electron microscope, they do not strictly qualify as P-proteins.<\/p>\n\n\n\n

P-proteins have been devided into two major groups: <\/h2>\n\n\n\n
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  1. A) Non dispersive P-protein bodies: <\/strong>Non dispersive P-protein bodies are formed early during ontogeny (ontogeny = the development of individual cells or organisms) in young sieve tubes. Once the sieve element matures, the sieve pores open and they become active in translocation, non-dispersive P-protein bodies remain as visible, large bodies within the sieve tube. A typical example are forisomes (see link on the left to learn more about forisomes). About 10% of the Angiosperm species contain non-dispersive P-proteins.<\/li>\n\n\n\n
  2. B) Dispersive P-protein bodies:<\/strong> In the rest of the angiosperms (about 90%), the initial development of P-protein bodies follows the same principle. However, once the sieve element matures, the bodies do not remain in the lumen of the tube, but disperse into fine filaments.<\/li>\n<\/ol>\n\n\n\n

    With the exception of the forisomes, the function of P-proteins is still obscure.<\/p>\n\n\n

    \n
    \"forisome<\/figure><\/div>\n\n\n

    Non dispersive P-protein bodies (asterisk) as seen in the confocal microscope (left) and the transmission electron microscope (right). SE= sieve element, CC = companion cell. From Knoblauch et al. (2001) Calcium regulated stopcocks in legume sieve tubes. Plant Cell 13, 1221 – 1230. Copyright American Society of Plant Biologists<\/p>\n\n\n

    \n
    \"dispersive<\/figure><\/div>\n\n\n

    A bundle of filaments (left) and individual filaments (right) in mature sieve elements, derived from dispersive P-protein bodies. From Froelich et al. (2011) Phloem ultrastructure and pressure flow: SEOR protein agglomerations do not affect translocation. Plant Cell doi\/10.1105\/tpc.111.093179 Copyright American Society of Plant Biologists<\/p>\n\n\n\n

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