Department of Plant and Environmental Sciences, University of Copenhagen Copenhagen, Denmark.
Front Plant Sci. 2013 Jun 19;4:207. doi: 10.3389/fpls.2013.00207. eCollection 2013.
Plasmodesmata (PD) play a key role in loading of sugars into the phloem. In plant species that employ the so-called active symplasmic loading strategy, sucrose that diffuses into their unique intermediary cells (ICs) is converted into sugar oligomers. According to the prevalent hypothesis, the oligomers are too large to pass back through PD on the bundle sheath side, but can pass on into the sieve element to be transported in the phloem. Here, we investigate if the PD at the bundle sheath-IC interface can indeed fulfill the function of blocking transport of sugar oligomers while still enabling efficient diffusion of sucrose. Hindrance factors are derived via theoretical modeling for different PD substructure configurations: sub-nano channels, slit, and hydrogel. The results suggest that a strong discrimination could only be realized when the PD opening is almost as small as the sugar oligomers. In order to find model parameters that match the in vivo situation, we measured the effective diffusion coefficient across the interface in question in Cucurbita pepo with 3D-photoactivation microscopy. Calculations indicate that a PD substructure of several sub-nano channels with a radius around 7 Å, a 10.4 Å-wide slit or a hydrogel with 49% polymer fraction would be compatible with the effective diffusion coefficient. If these configurations can accommodate sufficient flux of sucrose into the IC, while blocking raffinose and stachyose movement was assessed using literature data. While the slit-configuration would efficiently prevent the sugar oligomers from "leaking" from the IC, none of the configurations could enable a diffusion-driven sucrose flux that matches the reported rates at a physiologically relevant concentration potential. The presented data provides a first insight on how the substructure of PD could enable selective transport, but indicates that additional factors are involved in efficient phloem loading in active symplasmic loading species.
胞间连丝（PD）在将糖装载到韧皮部中起着关键作用。在采用所谓的主动共质体装载策略的植物物种中，扩散到其独特的中间细胞（IC）的蔗糖会转化为糖寡聚物。根据流行的假设，这些寡聚物太大而无法通过束鞘侧的 PD 反向扩散，但可以进入筛分子中，在韧皮部中运输。在这里，我们研究 PD 在束鞘-IC 界面处是否确实可以阻止糖寡聚物的运输，同时仍然能够有效地扩散蔗糖。通过理论建模为不同的 PD 亚结构配置（亚纳米通道、狭缝和水凝胶）推导出阻碍因素。结果表明，只有当 PD 开口几乎与糖寡聚物一样小时，才能实现强烈的歧视。为了找到与体内情况匹配的模型参数，我们使用 3D 光激活显微镜测量了南瓜中该界面的有效扩散系数。计算表明，具有 7Å 左右半径的几个亚纳米通道、10.4Å 宽的狭缝或 49%聚合物分数的水凝胶的 PD 亚结构将与有效扩散系数兼容。如果这些配置可以容纳足够的蔗糖通量进入 IC，同时阻止 raffinose 和 stachyose 运动，则使用文献数据进行评估。虽然狭缝配置可以有效地防止糖寡聚物从 IC“泄漏”，但没有一种配置可以使扩散驱动的蔗糖通量与在生理相关浓度势下报道的速率相匹配。所提出的数据提供了对 PD 亚结构如何能够实现选择性运输的初步了解，但表明在主动共质体装载物种中，有效的韧皮部装载还涉及其他因素。
