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Plants create sugar in the mesophyll cells of their leaves by photosynthesis.This sugar, mostly sucrose, has to be loaded via the bundle sheath into thephloem vascular system (the sieve elements), where it is distributed to growingparts of the plant. We analyze the feasibility of a particular loadingmechanism, active symplasmic loading, also called the polymer trap mechanism,where sucrose is transformed into heavier sugars, such as raffinose andstachyose, in the intermediary-type companion cells bordering the sieveelements in the minor veins of the phloem. Keeping the heavier sugars fromdiffusing back requires that the plasmodesmata connecting the bundle sheathwith the intermediary cell act as extremely precise filters, which are able todistinguish between molecules that differ by less than 20% in size. In ourmodeling, we take into account the coupled water and sugar movement across therelevant interfaces, without explicitly considering the chemical reactionstransforming the sucrose into the heavier sugars. Based on the available datafor plasmodesmata geometry, sugar concentrations and flux rates, we concludethat this mechanism can in principle function. We find that the water flowthrough the plasmodesmata, which has not been quantified before, contributesonly 10-20% to the sucrose flux into the intermediary cells, while the mainpart is transported by diffusion. On the other hand, the subsequent sugartranslocation into the sieve elements would very likely be carriedpredominantly by bulk water flow through the plasmodesmata. Thus, in contrastto apoplasmic loaders, all the necessary water for phloem translocation wouldbe supplied in this way with no need for additional water uptake across theplasma membranes of the phloem.

Diffusion and bulk flow in phloem loading - a theoretical analysis of the polymer trap mechanism in plants