Diffusion and bulk flow in phloem loading: A theoretical study of the polymer trap mechanism for sugar transport in plants

Our analytical calculations on sugar loading in leaves show that the polymer trap mechanism can in principle function, and give new insights on the pathway of water inside a leaf.
J. Dölger1, H. Rademaker1, J. Liesche2, A. Schulz2, T. Bohr1,

Physical Review E 90, no. 4, 042704, (2014)

Plants produce sugars they need for survival and growth in photosynthesis. In vascular plants the transport of these sugars, from production sites in mature leaves to regions of storage and growth, takes place in the phloem tubes. These tubes consist of living cells, sieve elements (SEs), well connected to each other. The process, in which sugars are taken up into the transport system, is called phloem loading and takes place in the minor veins of the leaf.

In one special loading mechanism, the polymer trap, sucrose enters the intermediary cell (IC) coming from the bundle sheath cell (BSC) through small plasmodesmata (PDs), channels which connect the cytosols of the two cells (see figure). The sucrose in the IC is then enzymatically converted to sugar oligomers like raffinose and stachyose. These molecules are too large to diffuse back into the BSC, and are now “trapped” in the phloem system, which consists of ICs and SEs. According to the Münch hypothesis (1930), water is then osmotically taken up into the phloem and the resulting pressure drives the flow through the sieve elements.

Polymer trap

Figure: In the polymer trap mechanism, sucrose diffuses from the bundle sheath cell into the intermediary cell and is enzymatically transformed into oligomers, which are too large to diffuse back. The figure shows the relevant cells inside a leaf with the assumed sugar and water flows. The flows are driven by differences in concentration and pressure between the cells, and influenced by the properties of the interfaces (pore size and density).

One major concern about the polymer trap mechanism, postulated by Robert Turgeon in 1990, was, if there could still diffuse enough sucrose through the PDs, which need to be narrow enough to hold back oligosaccharides just about 20% larger than the sucrose molecules. Our recent theoretical study shows, that the mechanism can indeed function. It further suggests that water is entering the phloem together with the sugars through PDs. This water would then actually be enough to drive the flow, so there would be no further osmotic uptake of water necessary, which could make the system more efficient.

1Department of Physics and Center for Fluid Dynamics, Technical University of Denmark, 2Department of Plant and Environmental Sciences, University of Copenhagen