Department of Agronomy and Purdue Center for Plant Biology, Purdue University, West Lafayette, IN 49707, USA.
Plant Biology Section, School of Integrative Plant Science, Cornell University, Ithaca, NY 14853, USA.
Curr Opin Plant Biol. 2018 Jun;43:71-75. doi: 10.1016/j.pbi.2018.01.009. Epub 2018 Feb 12.
The complex form of higher plants requires continuous, balanced transport of nutrients in the phloem. The initial step of transferring sugars, amino acids, and other materials from photosynthetic cells to the conducting sieve tubes is known as phloem loading. Three phloem loading mechanisms have been described. The first involves release of sucrose into the apoplast and subsequent retrieval by the phloem. The initial release step in this process is now known to be mediated by a new class of transporters, the SWEET proteins. In the other two loading mechanisms, polymer trapping and diffusion, sucrose passes into the phloem through cytoplasmic channels, the plasmodesmata. Recent models have shed additional light on these mechanisms and their ability to sustain the growth of even the tallest trees.
高等植物的复杂形式需要韧皮部中养分的持续、平衡运输。将糖、氨基酸和其他物质从光合细胞转移到导筛管的初始步骤称为韧皮部装载。已经描述了三种韧皮部装载机制。第一种机制涉及蔗糖释放到质外体中，然后被韧皮部回收。这个过程中的初始释放步骤现在已知是由一类新的转运蛋白，即 SWEET 蛋白介导的。在另外两种装载机制中，聚合物捕获和扩散，蔗糖通过细胞质通道（胞间连丝）进入韧皮部。最近的模型进一步阐明了这些机制及其维持即使是最高的树木生长的能力。
