RESEARCH-PGR: Sieve Tube Proteomics - Unraveling the Physiology and Cell Biology of an Arcane Cell Type

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The food consumed by humans is produced in plants by photosynthesis, either directly (think of potatoes) or indirectly (meat from plant-eating animals). Photosynthesis occurs in the green parts of plants, but, with exceptions such as lettuce, hungry humans are more interested in non-photosynthesizing storage organs of plants including grain, tubers, and fruits. Plants transfer the products of photosynthesis (sugars, chemically speaking) from their green parts to storage organs via a network of microscopic pipes, the so-called sieve tubes, in the phloem tissue. Due to their extreme sensitivity, the function of live sieve tubes is notoriously hard to study, and our understanding of sieve tube transport and its regulation has remained lamentably poor. This is unfortunate, especially since numerous phloem-sucking pests, for instance aphids, exploit the continuous stream of energy-rich substances in the sieve tubes. What often is worse are the many pathogenic viruses and bacteria that hitch rides in this stream to infest the entire plant, starting from a single point of entry. As these factors cause massive crop losses every year, the critical function of phloem in plant performance translates into an economic significance that can hardly be overstated. This project will make important contributions to comprehension of fundamental mechanisms of sieve tube function, which will facilitate the design of treatments and preventive measures targeting pests and pathogens. Proteins potentially involved in sieve tube physiology will be identified by a large-scale biochemical approach (‘proteomics’), and then further analyzed one by one through directed modification of the genes that encode them. With regard to Broader Impacts, the project will engage public interest with an artistic exhibition that incorporates large format photographs of phloem cells displayed in the terminal of the Pullman/Moscow regional airport and will also continue updating a “Phloem” webpage. In addition the PI will work with the Office of Multicultural Student Services at Washington State University to recruit minority undergraduate students to work in the lab.Vascular systems allow organisms to distribute resources internally by bulk flow, thus overcoming size limitations set by diffusion. The development of vascular tissues including sieve tubes drove the evolution of large land plants (tracheophytes) which caused a major increase in the productivity of terrestrial ecosystems. This productivity supports life on earth in its various forms. The largest photoautotroph marine organisms, kelps, have convergently evolved transporting sieve tubes, highlighting the significance of this transport and communication system for large photosynthetic and sessile life forms. This project aims at elucidating open questions in the cell biology and physiology of sieve tubes. State-of-the-art proteomics approaches will be applied to identify proteins potentially involved in transport regulation, by comparing the proteomes of vascular tissue and isolated sieve elements. Candidate proteins will be fluorescently tagged by modifications of their encoding genes, and the subcellular localization of the proteins in transporting tubes will be determined. Advanced bioimaging tools will be applied to assign sieve tube proteins to specific organelles or membrane systems, to generate 3D models of sieve tube organelle interactions, and to characterize the responses of protein distribution to stress. The insights gained are expected to provide essential information for the generation of more robust and pest-resistant crops.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.