Collaborative Research: Physiology of Long Distance Assimilate Transport

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The basis for life on earth is the conversion of solar energy into chemical energy by a process called photosynthesis that takes place in plants. The high energy containing end product of photosynthesis is sugar that has to be translocated, via a tissue called phloem, from the site of generation (in most cases leaves) to the sites of consumption and storage (stems, roots, fruits). Humans either consume energy rich plant tissues directly in the form of salads, cereals, vegetables etc., or indirectly via the consumption of meat, which was produced by animals consuming plants. The translocation of sugars within the phloem is a key mechanism in the production of high quality, high yield, and healthy food. For example, photosynthesis is actively reduced by the plant if export of sugars through the phloem is insufficient. Pests like aphids attack the phloem, insert their mouthparts into the tissue and feed on the sugar rich solution, which results in loss of production. Plant viruses travel through the phloem and cause severe damage to the plant. Despite the central role the phloem plays in plant performance and food production, the knowledge of the underlying processes of phloem loading and transport are poorly understood. This project will investigate the flow path of sugars through leaves, the entry or "loading" of sugars into the phloem and the distribution of sugars within the plant. Enhanced understanding of phloem loading and transport could lead to new strategies to protect plants from pests, to increase crop yield and to produce healthier food by reducing or eliminating the necessity for application of pesticides.The goal of this award is to investigate physiological parameters of phloem loading and the physics of transport in plants with long phloem networks (vines and trees). Methods developed during previous awards will be used to generate cell and tissue-type based maps of leaves outlining turgor pressure and phloem flow patterns in different vein orders. In addition, the distribution of plasmodesmata between cells and tissues, and cell-to-cell conductivity from source cells to sieve tubes will be mapped. New systems to measure sieve tube turgor and flow velocity, as well as straightforward SEM based methods will be used to gather data on plasmodesmal frequencies and sieve plate structure. The relationship between plant size and source turgor pressure and phloem architecture in trees and vines with different loading types will also be investigated. This will allow current models of phloem transport to be re-parameterized so as to reflect better the hydraulic architecture of the pathway for photoassimilate transport through the plant. This award will also bring together researchers specializing in plant cell biology and vascular transport, and support education and training for a postdoctoral researcher and graduate students, as well as engage undergraduates in research and provide opportunities for high-school students and teachers to learn about the transport of photoassimilates in plants.