RUI: Apoplastic lipids in xylem of vascular plants: Composition, locations, origins, and possible functions

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Vascular plants are the only organisms known to transport water under negative pressure, using solar energy to create the force for moving water from soil to roots to leaves and out into the atmosphere. How plants can do this without constantly creating bubbles in their hydraulic systems, which act to block the water transport is still not completely understood. Engineers have not been able to replicate water transport under negative pressure, except in very small amounts over very short distances under idealized laboratory conditions. So how can trees move vast quantities of water through the xylem (wood) in their trunks under negative pressure? The goal of this research is to determine the role of natural oils (lipids) in this process. Lipids have been found covering the surfaces of xylem vessels in plants. The project will determine the composition, locations, origins, and possible functions of lipids in xylem of vascular plants and test the hypothesis that lipids are required for negative pressure transport. The research will increase the understanding of plant adaptation to water stress, and could potentially lead to engineered systems that replicate plant water transport under negative pressure. The research will involve undergraduate students, including from groups currently underrepresented in the sciences, and its broader impacts will include education about plant water transport both in academic courses and public outreach events.It has been an axiom of the cohesion-tension theory that it is the high surface tension of water that protects xylem conduits against bubble entry. This axiom was recently challenged by findings that xylem sap of flowering plants contains significant amounts of surface-active lipids and that surfactants isolated from xylem sap strongly reduced the sap's surface tension. Moreover, surfactant-coated bubbles were found in sap, suggesting that plants can transport water in the presence of bubbles, rather than needing to prevent their formation completely. This research addresses the questions: 1. Are apoplastic lipids found in xylem of all major groups of vascular plants? 2. What is the composition of apoplastic lipids in different plants? 3. Where in the xylem are apoplastic lipids located? 4. Where do apoplastic xylem lipids originate? The research will use 15 representative species of all major vascular plant lineages using imaging via TEM, confocal super-resolution microscopy, atomic force microscopy, and energy-dispersive X-ray spectroscopy, as well as by lipidomic analyses of xylem sap and xylem tissue using mass spectrometry. The research will test the paradigm-reversing hypothesis that lipid-coatings of gas-water surfaces provide the hydraulic system with a degree of protection against bubbles expanding into embolism and causing hydraulic failure. The proposed research on lipids in water under negative pressure will contribute to scientific knowledge not only in plant biology, but also in physics and chemical engineering.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.