Phloem – University of Copenhagen

Phloem

Sieve elements of the phloem constitute the long-distance pathway for assimilates in the plant. They form a symplasmic, low-resistance domain due to their reduced cytoplasm and wide sieve pores. With their reduced cytoplasm, intimate association to neighboring cells and specific proteins that only occur in this cell type, they are the most highly specialized cells in a plant.

Phloem transport is driven by the loading of assimilates in the source leaves. Prerequisites for the function of phloem elements are a "sugar-tight" plasma membrane, tightly controlled plasmodesmata, linking to surrounding tissues and presence and activity membrane transporters. Then a Münch-type pressure flow can develop that carries sugars and amino acids from the sources to all organs consuming them.

The biophysics of phloem transport is challenging in large trees.  Is the accumulating in the phloem of leaves sufficient to drive assimilates all the way from the leaves to the roots including the friction and losses occurring on the way? In cross-discipline collaboration with physicists we are trying to understand the mechanisms of sugar accumulation and osmotic water movements in the phloem network, considering both angiosperms and gymnosperms.

Recent papers

  • Liesche J, Schulz A (2012) In Vivo Quantification of Cell Coupling in Plants with Different Phloem-Loading Strategies. Plant Physiology 159: 355-365
  • Schulz A (2015) Diffusion or bulk flow: how plasmodesmata facilitate. J Plant Res 128 49-61   
  • Ronellenfitsch H, Liesche J, Jensen KH, Holbrook NM, Schulz A, Katifori E (2015) Scaling of phloem structure and optimality of photoassimilate transport in conifer needles. Proc Biol Sci 282: 20141863
  • Liesche J, Windt C, Bohr T, Schulz A, Jensen KH (2015) Slower phloem transport in gymnosperm trees can be attributes to higher sieve element resistance. Tree Physiology 35: 376-386
  • Dölger J, Rademarker H, Liesche J, Schulz A, Bohr T (2014) Diffusion and bulk flow in phloem loading: a theoretical analysis of the polymer trap mechanism for sugar transport in plants. Phys Rev E Stat Nonlin Soft Matter Phys 90: 042704

Collaboration

Tomas Bohr, DTU