The Efficiency of Long-Distance Translocation: Retention Properties of Sugars in the Transport Phloem

植物沿着流体静压梯度运输光合作用的产物，这种压力梯度建立在一个被称为韧皮部的特殊维管系统中。韧皮部网络通常被细分为三个功能域。在叶片中产生的光同化物被装载到收集韧皮部中，并从释放的韧皮部中卸载到生长和储存区域。中间的区域是运输韧皮部，构成植物中最长的连续韧皮部。为了使压力驱动的运输有效，必须保持在收集韧皮部中产生的渗透势，直到它到达释放韧皮部。因此，运输韧皮部经常被降级为不透水管道的状态，因此它在碳分配和整个植物生理中的作用被低估。然而，韧皮部的运输是动态的，营养物质既被卸载又被重新装载，以滋养侧翼组织，并从短期和长期储存储备中调动糖分。因此，运输韧皮部中发生的事件缓冲了营养供需的波动，并最终影响了整个植物的资源分配，但在很大程度上仍未被开发。这一建议的具体目标是：1)通过蔗糖转运蛋白的组织特异性表达，确定蔗糖转运蛋白在蔗糖转运和沿韧皮部转运过程中的作用。蔗糖转运蛋白与组织特异性启动子的限制表达将分离它们对韧皮部装载和韧皮部运输的相对贡献。据预测，蔗糖的运输效率相对较低，能量消耗较大，如果没有主动回收，通过运输韧皮部的运输效率将严重降低。2)通过代谢工程产生收集韧皮部中的四糖水苏糖，并测定沿运输韧皮部的转运效率。有人提出，较大的低聚糖将表现出比蔗糖更少的外流，因此更有效地转移到需要营养的组织。3)通过组织特异性过表达参与水苏糖分解代谢的基因来调节植物的库强度，目的是控制生物量的分配。

Plants transport the products of photosynthesis along hydrostatic pressure gradients established in a specialized vascular system called the phloem. The phloem network is commonly subdivided into three functional domains. Photoassimilate produced in leaves is loaded into the collection phloem and unloaded in areas of growth and storage from the release phloem. The region in-between is the transport phloem, and constitutes the longest contiguous stretch of phloem in the plant. For pressure driven transport to be effective, the osmotic potential generated in the collection phloem must be maintained until it reaches the release phloem. The transport phloem is thus frequently relegated to the status of an impermeable pipe, and its role in carbon partitioning and whole-plant physiology is consequently underrepresented. The transport phloem is however dynamic, and nutrients are both unloaded and reloaded to nourish flanking tissues and to mobilize sugars to and from short- and long-term storage reserves. Events occurring in the transport phloem therefore buffer against fluctuations in nutrient supply and demand, and ultimately affect resource allocation throughout the plant, but remain largely unexplored. The specific objectives of this proposal are to 1) determine the role of sucrose transporters in sugar efflux and retrieval along the transport phloem by tissue-specific expression of sucrose transporters in null-mutant plants. Confined expression of sucrose transporters with tissue-specific promoters will separate their relative contribution to phloem loading and phloem transport. It is predicted that sucrose transport is relatively inefficient and energetically expensive, and that in the absence of active retrieval, transport efficiency through the transport phloem will be severely reduced. 2) Generate the tetrasaccharide stachyose in collection phloem through metabolic engineering, and determine transport efficiency along the transport phloem. It is proposed that larger oligosaccharides will demonstrate less efflux than sucrose, and therefore move to nutrient-requiring tissues more efficiently. 3) Modulate sink strength in plants engineered to translocate stachyose by tissue-specific overexpression of genes involved in stachyose catabolism, with the goal of manipulating biomass distribution.