Department of Plant and Environmental Sciences, University of Copenhagen, Thorvaldsensvej 40, 1871, Frederiksberg C, Denmark,
J Plant Res. 2015 Jan;128(1):49-61. doi: 10.1007/s10265-014-0676-5. Epub 2014 Dec 17.
Assimilates synthesized in the mesophyll of mature leaves move along the pre-phloem transport pathway to the bundle sheath of the minor veins from which they are loaded into the phloem. The present review discusses the most probable driving force(s) for the pre-phloem pathway, diffusion down the concentration gradient or bulk flow along a pressure gradient. The driving force seems to depend on the mode of phloem loading. In a majority of plant species phloem loading is a thermodynamically active process, involving the activity of membrane transporters in the sieve-element companion cell complex. Since assimilate movement includes an apoplasmic step, this mode is called apoplasmic loading. Well established is also the polymer-trap loading mode, where the phloem-transport sugars are raffinose-family oligomers in herbaceous plants. Also this mode depends on the investment of energy, here for sugar oligomerization, and leads to a high sugar accumulation in the phloem, even though the phloem is not symplasmically isolated, but well coupled by plasmodesmata (PD). Hence the mode polymer-trap mode is also designated active symplasmic loading. For woody angiosperms and gymnosperms an alternate loading mode is currently matter of discussion, called passive symplasmic loading. Based on the limited material available, this review compares the different loading modes and suggests that diffusion is the driving force in apoplasmic loaders, while bulk flow plays an increasing role in plants having a continuous symplasmic pathway from mesophyll to sieve elements. Crucial for the driving force is the question where water enters the pre-phloem pathway. Surprisingly, the role of PD in water movement has not been addressed so far appropriately. Modeling of assimilate and water fluxes indicates that in symplasmic loaders a considerable part of water flux happens through the PD between bundle sheath and phloem.
成熟叶片的叶肉合成的同化产物沿韧皮前质体运输途径移动到小维管束的韧皮部鞘，从中它们被装载到韧皮部。本综述讨论了韧皮前质体途径最可能的驱动力，是沿着浓度梯度扩散还是沿着压力梯度的整体流动。驱动力似乎取决于韧皮部装载的模式。在大多数植物物种中，韧皮部装载是一个热力学活性过程，涉及筛分子伴胞复合体中膜转运体的活性。由于同化产物的移动包括质外体步骤，因此这种模式称为质外体装载。聚合物陷阱装载模式也得到了很好的建立，其中韧皮部运输糖是草本植物中的棉子糖家族寡糖。这种模式也依赖于能量的投入，这里是糖寡聚化的能量投入，导致韧皮部中糖的积累很高，即使韧皮部不是共质体隔离的，而是通过胞间连丝（PD）很好地偶联的。因此，聚合物陷阱装载模式也被指定为主动共质体装载。对于木本被子植物和裸子植物，目前正在讨论一种替代的装载模式，称为被动共质体装载。基于有限的可用材料，本综述比较了不同的装载模式，并提出在质外体装载器中扩散是驱动力，而在具有从叶肉到筛分子的连续共质体途径的植物中，整体流动发挥着越来越重要的作用。对于驱动力至关重要的问题是水进入韧皮前质体途径的位置。令人惊讶的是，PD 在水运动中的作用迄今为止尚未得到适当的解决。对同化产物和水流的建模表明，在共质体装载器中，相当一部分水流通过 PD 在韧皮部鞘和韧皮部之间发生。
