人造树片中韧皮部的被动装载和长距离运输
Passive phloem loading and long-distance transport in a synthetic tree-on-a-chip
摘要：
维管植物依靠渗透压的差异将糖从合成区域（成熟叶）输出到糖库（根、果实）。在这个过程中，被称为Münch压力流，糖从光合细胞到输出管道（韧皮部）的装载是至关重要的，因为它设置了为长距离运输提供动力所必需的压头。而大多数草本植物使用主动机制，以增加韧皮部浓度以上的光合细胞，在大多数树种，运输距离是最大的，负载似乎发生通过被动共质体扩散从叶肉到韧皮部。在这里，我们使用一个合成的被动加载器的微流体模型来探索出口过程中出现的非线性动力学，并确定被动加载驱动长距离运输的能力。我们首先证明，在我们的设备中，韧皮部浓度是由从源和对流出口通过韧皮部的扩散负载的电阻之间的平衡。对流限制出口对应于经典模型的明希运输，其中韧皮部浓度接近的源;相反，扩散限制出口导致小韧皮部浓度和弱缩放的流速与水力阻力。然后，我们表明，对流限制出口的有效制度是占主导地位的大运输阻力和低木质部压力的植物。此外，在我们的合成被动加载器中开发的静水压力可以达到高达10巴的植物学相关值。我们的结论是，被动加载足以驱动长距离运输的大型植物，树木非常适合充分利用被动韧皮部加载策略。
英文摘要：
Vascular plants rely on differences of osmotic pressure to export sugars from regions of synthesis (mature leaves) to sugar sinks (roots, fruits). In this process, known as Münch pressure flow, the loading of sugars from photosynthetic cells to the export conduit (the phloem) is crucial, as it sets the pressure head necessary to power long-distance transport. Whereas most herbaceous plants use active mechanisms to increase phloem concentration above that of the photosynthetic cells, in most tree species, for which transport distances are largest, loading seems to occur via passive symplastic diffusion from the mesophyll to the phloem. Here, we use a synthetic microfluidic model of a passive loader to explore the nonlinear dynamics that arise during export and determine the ability of passive loading to drive long-distance transport. We first demonstrate that in our device, phloem concentration is set by the balance between the resistances to diffusive loading from the source and convective export through the phloem. Convection-limited export corresponds to classical models of Münch transport, where phloem concentration is close to that of the source; in contrast, diffusion-limited export leads to small phloem concentrations and weak scaling of flow rates with the hydraulic resistance. We then show that the effective regime of convection-limited export is predominant in plants with large transport resistances and low xylem pressures. Moreover, hydrostatic pressures developed in our synthetic passive loader can reach botanically relevant values as high as 10 bars. We conclude that passive loading is sufficient to drive long-distance transport in large plants, and that trees are well suited to take full advantage of passive phloem loading strategies.
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