Translocation in Legumes: Assimilates, Nutrients, and Signaling Molecules

Craig Atkins, P.M. Smith

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    44 Citations (Scopus)


    Translocation or long distance transport in plants isachieved by a vascular network that connects and is anintegral part of all organs. The vasculature comprisestwo distinctly different and separate cellular translocationpathways: xylem and phloem. The principalxylem pathway is the transpiration stream that movesnutrients and water taken up by roots to the shoot.This stream also bears products of root metabolismandsolutes that reflect features of the internal and externalroot environment. Phloem provides the means for redistributingxylem-delivered solutes to weakly transpiringorgans, but most significantly phloem distributesthe carbon assimilated by photosynthesis (principallyas Suc) to heterotrophic organs like roots, vegetativeand reproductive apices, flowers, fruits, and developingseeds. Together these two translocation streamsprovide all the nutrients and assimilates, in appropriateforms and proportions, to enable growth and developmentin an ordered and regulated fashion. Becausetranslocation connects distant components of the plantbody, xylem and phloem have long been considered tofulfill a role in communicating between organs, throughthe movement of plant hormones and other signalingmolecules. Such signals are envisaged to move withassimilates by mass flow. However, phloem also transmitspressure/concentration (turgor) information atrates greatly in excess of mass flow of solutes (Thompsonand Holbrook, 2004) and long distance electrical signalingis also thought to be directionally propagatedvia vascular bundles (Brenner et al., 2006). These actionpotential or osmotic signals may prove to have asignificant regulatory role in terms of phloem functionbut are outside the scope of this article. Most recentlyour understanding of the functional significance ofphloem has been extended with the realization that italso provides a conduit for trafficking macromolecules(nucleic acids and proteins), some of which may regulategene expression as a consequence of their translocation(Banerjee et al., 2006; Lough and Lucas, 2006;Jones-Rhoades et al., 2006). Similarly root-derived signalsthat are postulated to regulate shoot processes arebelieved to move in xylem (Beveridge, 2006; Kinkemaet al., 2006) together with a suite of secreted proteins(Buhtz et al., 2004).While the supporting evidence for these diverse rolesof translocation has been gathered from many species,this article will highlight information that is specific tolegumes where it is available, drawing particularly ondata from the author’s laboratory for members of thegenus Lupinus.
    Original languageEnglish
    Pages (from-to)550-561
    JournalPlant Physiology
    Issue number2
    Publication statusPublished - 2007


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