Dölger Julia, Rademaker Hanna, Liesche Johannes, Schulz Alexander, Bohr Tomas
Department of Physics and Center for Fluid Dynamics, Technical University of Denmark, Kgs. Lyngby, Denmark and Institute for Condensed Matter Physics, Darmstadt University of Technology, Darmstadt, Germany.
Department of Physics and Center for Fluid Dynamics, Technical University of Denmark, Kgs. Lyngby, Denmark.
Phys Rev E Stat Nonlin Soft Matter Phys. 2014 Oct;90(4):042704. doi: 10.1103/PhysRevE.90.042704. Epub 2014 Oct 8.
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 the phloem vascular system (the sieve elements), where it is distributed to growing parts of the plant. We analyze the feasibility of a particular loading mechanism, active symplasmic loading, also called the polymer trap mechanism, where sucrose is transformed into heavier sugars, such as raffinose and stachyose, in the intermediary-type companion cells bordering the sieve elements in the minor veins of the phloem. Keeping the heavier sugars from diffusing back requires that the plasmodesmata connecting the bundle sheath with the intermediary cell act as extremely precise filters, which are able to distinguish between molecules that differ by less than 20% in size. In our modeling, we take into account the coupled water and sugar movement across the relevant interfaces, without explicitly considering the chemical reactions transforming the sucrose into the heavier sugars. Based on the available data for plasmodesmata geometry, sugar concentrations, and flux rates, we conclude that this mechanism can in principle function, but that it requires pores of molecular sizes. Comparing with the somewhat uncertain experimental values for sugar export rates, we expect the pores to be only 5%-10% larger than the hydraulic radius of the sucrose molecules. We find that the water flow through the plasmodesmata, which has not been quantified before, contributes only 10%-20% to the sucrose flux into the intermediary cells, while the main part is transported by diffusion. On the other hand, the subsequent sugar translocation into the sieve elements would very likely be carried predominantly by bulk water flow through the plasmodesmata. Thus, in contrast to apoplasmic loaders, all the necessary water for phloem translocation would be supplied in this way with no need for additional water uptake across the plasma membranes of the phloem.
植物通过光合作用在叶片的叶肉细胞中产生糖分。这种糖，主要是蔗糖，必须通过维管束鞘装载到韧皮部维管系统（筛管分子）中，然后在那里被输送到植物的生长部位。我们分析了一种特定装载机制——活跃共质体装载（也称为聚合物陷阱机制）的可行性，在这种机制中，蔗糖在韧皮部小叶脉中与筛管分子相邻的中间型伴胞中转化为较重的糖类，如棉子糖和水苏糖。要防止较重的糖类扩散回去，就要求连接维管束鞘和中间细胞的胞间连丝起到极其精确的过滤器作用，能够区分大小差异小于20%的分子。在我们的建模中，我们考虑了水和糖在相关界面上的耦合运动，而没有明确考虑将蔗糖转化为较重糖类的化学反应。根据胞间连丝几何形状、糖浓度和通量率的现有数据，我们得出结论，这种机制原则上可以发挥作用，但它需要分子尺寸的孔隙。与糖输出率的一些不确定实验值相比，我们预计这些孔隙仅比蔗糖分子的水力半径大5%-10%。我们发现，之前未被量化的通过胞间连丝的水流对进入中间细胞的蔗糖通量的贡献仅为10%-20%，而主要部分是通过扩散运输的。另一方面，随后糖向筛管分子的转运很可能主要是由通过胞间连丝的大量水流携带的。因此，与质外体装载者不同，韧皮部转运所需的所有水分都将以这种方式供应，无需通过韧皮部质膜额外吸收水分。
