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

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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.