Sieve Tube Unloading: Control Point for Assimilate Partitioning?

尽管库在决定植物生长过程中光合作用同化物分配方面起着核心作用，但对同化物输入到库组织的控制知之甚少。这项研究的总体目标是了解水槽是如何控制进口的。由于开发小麦谷物提供了显著的实验优势，费舍尔博士的工作重点放在这个系统上。然而，这项研究的发现对所有筛分元件/伴胞复合体的卸载遵循共生体途径的汇都有重要的意义。所有的运输步骤，从筛管内到胚乳腔，都是被动的、可逆的和相对非特异性的。同化物从籽粒韧皮部向周围薄壁组织细胞的移动伴随着源-库途径中任何一步的最大膨胀率和浓度差异。由于溶质流出筛管是通过压力驱动流进行的，而颗粒生长速率是恒定的，因此流速必须与这部分通道上的压力和浓度梯度成反比。这一点，以及所涉及的高抗性，意味着这一步骤是同化物输入谷物的重要控制点。这项研究将更紧密地集中在同化进口这一步骤上。小麦发展中的重大问题仍有待解决，并将继续成为工作的主要对象。然而，为了测试在其他汇中是否可以在此步骤中控制导入，还将启动一些与生长的根尖相关的工作。在根的生长中需要解决的主要问题是：筛管脱胶是否可逆，所涉及的压力和浓度梯度，以及它们与根生长速率的关系。

Despite the central role of sinks in determining photosynthetic assimilate partitioning in plant growth, very little is known about the control of assimilate import into a sink tissue. The overal goal of this research is to understand how import by a sink is controlled. Because the developing wheat grain offers significant experimental advantages, Dr. Fisher's work has focused on that system. However, findings from this research have important implications for all sinks in which unloading of the sieve element/companion cell complex follows a symplastic pathway. All of the transport steps, from within the sieve tubes to the endosperm cavity, are passive, reversible and relatively nonspecific. Assimilate movement out of the grain phloem into surrounding parenchyma cells is accompanied by the largest turgor and concentration differences over any step of the source-to-sink pathway. Because solute movement out of the sieve tubes occurs via pressure-driven flow, while grain growth rate is constant, flow rate must be inversely related to the pressure and concentration gradients across this part of the pathway. This, and the high resistance involved, implicate this step as an important control point for assimilate import into the grain. This research will focus more closely on this step of assimilate import. Important questions remain to be addressed in developing wheat grains, and they will continue to be the main object of the work. However, to test whether import might be controlled at this step in other sinks, some work with growing root tips will also be initiated. The main questions to be addressed in growing roots are whether sieve tube unloadiong is reversible, the pressure and concentraiton gradients involved, and their relationship to root growth rate.