Symplasmic Transport in Phloem Loading and Unloading

  • Johannes Liesche
  • Alexander Schulz


The coordinated distribution of carbohydrates between different plant organs takes place in the phloem. Understanding how carbohydrates are loaded into and unloaded from this long-distance transport system is essential for our understanding of carbon allocation in plants and the mechanism of phloem transport.

In this chapter, we present the current knowledge on how sugars move from the site of production in the leaf parenchyma towards the phloem and how they exit the phloem in sink organs and move to the sites of consumption or storage. The main focus lies on symplasmic transport through plasmodesmata, which is central to all questions of intercellular carbohydrate transport. Recent investigations in non-model plant species, especially the gymnosperms, provide a more comprehensive picture of phloem loading and unloading processes than ever before, but many questions regarding underlying mechanisms, evolution, pathway capacity, and regulation remain.


Sucrose Photoassimilate transport Carbon allocation Plasmodesmata Sucrose transporter 



Bundle sheath cell




Companion cell


Intermediary cell




Sieve element


Companion cell/sieve element complex


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© Springer Science+Business Media New York 2013

Authors and Affiliations

  1. 1.Department of Plant and Environmental SciencesUniversity of CopenhagenCopenhagenDenmark

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