Investigation of the structural, physiological, and biophysical premises for assimilate allocation in plant sinks

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Plants are the primary producers on our planet. The mechanism of photosynthesis allows them to turn a gas (CO2) and a liquid (H2O) into energy-rich solid sugars. The sugar can then be used to fuel cellular functions and also as a building block for structural plant components such as wood. It is also the basis for all human food sources. The process of photosynthesis is well studied and understood. However, the vast majority of sugars are not utilized at the place of their photosynthetic production (the leaves), but are translocated to distant tissues. For example, with a few exceptions such as lettuce, humans do not eat leaves, but roots, fruits, seeds etc. Another example is wood, which has been used for centuries and is one of the fundamental materials in our civilization. The tissue that transports the sugars from the site of photosynthesis to the site of storage and utilization is called phloem. Phloem transport is much less well studied and understood than photosynthesis. This project will investigate the mechanisms that define the location and amount of sugar delivery that happens through the phloem. The ultimate goal of understanding these mechanisms is to direct phloem transport to specific locations to improve food or biomass production.Vascular systems enable organisms to distribute resources internally by bulk flow and thus to overcome size limitations set by diffusion. In plants, the evolution of vascular tissues enabled the development of trees and was accompanied by a major increase in the productivity of terrestrial ecosystems. The phloem distributes the products of photosynthesis throughout the plant, allowing non-photosynthetic structures to be formed. The process of phloem unloading plays a critical role in allocating photoassimilates to sinks which in the form of seeds, tubers, roots etc. represent the major food sources for humans. The cellular processes of allocation control at the sites of phloem unloading, however, are understudied and poorly understood. This project aims at elucidating the structural, physiological, and biophysical basis for phloem unloading with the future goal of controlling allocation of assimilates to sinks for the improvement of food- and bioenergy crops. The project will utilize newly developed techniques such as in situ flow and unloading observations and high resolution serial block face imaging to create a detailed understanding of the mechanisms of phloem unloading in various plant species.