韧皮部装载中的扩散和整体流动--植物中聚合物捕获机制的理论分析
Diffusion and bulk flow in phloem loading - a theoretical analysis of the polymer trap mechanism in plants
摘要：
植物通过光合作用在叶子的叶肉细胞中产生糖。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（筛元件）中，在那里它被分配到植物的生长部位。我们分析了一个特定的加载机制，主动共质加载，也被称为聚合物陷阱机制，其中蔗糖转化为较重的糖，如棉子糖和水苏糖，在中间型同伴细胞接壤的筛元素在韧皮部的小静脉的可行性。为了防止较重的糖扩散回来，需要连接维管束鞘和中间细胞的胞间连丝作为非常精确的过滤器，能够区分大小相差不到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的化学反应。根据现有的数据胞间连丝的几何形状，糖浓度和流量率，我们得出结论，这种机制可以在原则上的功能。我们发现，通过胞间连丝的水流，这还没有被量化之前，只贡献了10-20%的蔗糖通量进入中间细胞，而主要部分是通过扩散运输。另一方面，随后的糖转移到筛元件中很可能主要由通过胞间连丝的大量水流携带。因此，与质外体装载器相反，韧皮部移位所需的所有水将以这种方式供应，而不需要额外的水穿过韧皮部的质膜吸收。
英文摘要：
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. 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.
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