EAGER: Biology, Chemistry, and Physics of Xylem Surfactants

科学家们在1895年首次提出，植物中的水分运输通常发生在负压下，这是通过叶片细胞壁中的表面张力产生的，导致水分从土壤流入根部，并作为汁液流向叶片。从那时起，已经积累了许多证据来支持这一假设，即所谓的凝聚力-张力理论，但仍然不知道植物如何在负压下移动水，而不会在其液压系统（木质部）中不断产生气泡。随着最近在树液中发现表面活性剂，植物中负压运输如何起作用的问题变得更加紧迫。这一发现与树液基本上是纯水的假设相矛盾，并且水的高表面张力防止气泡形成或通过木质部壁中的小孔进入液压系统。本研究将确定木质部表面活性剂的化学组成，表征其物理性质，包括表面张力，定位表面活性剂胶束及其在木质部的细胞起源，并调查了一些植物物种从不同的进化背景，以确定木质部表面活性剂是否普遍存在于维管植物。该项目更广泛的影响包括参与研究的几名本科生和研究生，包括许多来自科学界代表性不足的群体。这项研究有可能导致仿生应用，如太阳能驱动的微流体装置，在负压下传输液体。以前的研究表明，木本被子植物的木质部汁液含有不溶性表面活性剂，包括许多蛋白质，糖蛋白和磷脂。这项研究的动机是一个新的假设，即不溶性表面活性剂通过控制气泡尺寸保持小于临界阈值尺寸来实现负压下的水运输，低于临界阈值尺寸的气泡不会膨胀形成栓塞。这种机制可以解释如何在正常、无压力的条件下运输大量气体饱和或过饱和水，低至几MPa的负压，这是人类工程师无法复制的壮举。这项研究的目的是表征木质部表面活性剂，并确定它们在维管植物（包括被子植物和裸子植物）中的常见程度，因为在测试它们在植物中的功能之前需要这些信息。方法将包括木质部汁液的脂质组学和蛋白质组学研究，木质部表面活性剂的约束滴表面测定法，木质部汁液和木质部的电子显微镜定位表面活性剂胶束和它们在木质部的起源。

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.