EAGER: Biology, Chemistry, and Physics of Xylem Surfactants

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Scientists first proposed in 1895 that water transport in plants often occurs under negative pressure, which is generated though surface tension in the cell walls of leaves and which causes water to flow from the soil into the roots and as sap up towards the leaves. Much evidence has accumulated since then to support this hypothesis, known as the cohesion-tension theory, but it is still unknown how plants can move water under negative pressure without constantly creating bubbles in their hydraulic system, the xylem. The question how negative pressure transport works in plants has achieved new urgency with the recent discovery of surfactants in the sap. This finding contradicted the assumption that sap is essentially pure water, and that the high surface tension of water prevents bubbles from forming or from entering the hydraulic system through small pores in xylem walls. This research will determine the chemical composition of xylem surfactants, characterize their physical properties, including surface tension, locate surfactant micelles and their cellular origin in the xylem, and survey a number of plant species from different evolutionary backgrounds to determine if xylem surfactants are ubiquitous in vascular plants. Broader impacts of the project include involvement of several undergraduate and graduate students in the research, including many from groups underrepresented in science. The research has the potential to result in biomimetic applications, such as solar-powered microfluidic devices that transport liquids under negative pressure.Previous findings have shown that xylem sap of woody angiosperms contains insoluble surfactants, including numerous proteins, glycoproteins, and phospholipids. The proposed research is motivated by a new hypothesis that insoluble surfactants enable water transport under negative pressure by controlling bubble sizes to remain smaller than a critical threshold size, below which bubbles do not expand to form embolisms. Such a mechanism could explain how it is possible to transport large amounts of gas-saturated or super-saturated water under normal, non-stressed conditions, down to several MPa of negative pressure, a feat that human engineers have been unable to replicate. The aim of the research is to characterize xylem surfactants and determine how common they are in vascular plants, including angiosperms and gymnosperms, because this information is needed before any hypotheses about their functions in plants can be tested. Methods will include lipidomic and proteomic studies of xylem sap, constrained drop surfactometry of xylem surfactants, and electron microscopy of xylem sap and xylem to locate surfactant micelles and their origin in the xylem.