EAGER: Biology, Chemistry, and Physics of Xylem Surfactants

科学家在1895年首次提出，植物体内的水分运输通常是在负压下发生的，负压是通过叶片细胞壁的表面张力产生的，导致水分从土壤流入根部，并以汁液的形式向上流向树叶。从那时起，已经积累了很多证据来支持这个被称为内聚力-张力理论的假说，但植物如何在负压下移动水分，而不不断地在它们的液压系统--木质部中产生气泡--仍然是未知的。随着最近在汁液中发现表面活性剂，负压运输在植物中是如何发挥作用的问题变得更加紧迫。这一发现与以下假设相矛盾，即树液本质上是纯水，水的高表面张力阻止气泡形成或通过木质部壁上的小孔进入液压系统。这项研究将确定木质部表面活性剂的化学组成，表征它们的物理性质，包括表面张力，确定表面活性物质胶束及其在木质部中的细胞来源，并调查来自不同进化背景的一些植物物种，以确定木质部表面活性物质是否普遍存在于维管束植物中。该项目的更广泛影响包括几名本科生和研究生参与研究，其中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控装置，在负压下输送液体。先前的研究结果表明，木本被子植物的木质部汁液中含有不溶性表面活性剂，包括大量的蛋白质、糖蛋白和磷脂。这项拟议的研究是由一种新的假设推动的，即不溶性表面活性剂通过控制气泡大小保持小于临界阈值大小，使水在负压下得以传输，低于临界阈值大小的气泡不会膨胀形成血栓。这样的机制可以解释如何在正常、无压力的条件下输送大量气体饱和或过饱和的水，直到几兆帕的负压，这是人类工程师无法复制的壮举。这项研究的目的是描述木质部表面活性物质的特征，并确定它们在包括被子植物和裸子植物在内的维管束植物中的普遍程度，因为在检验关于它们在植物中的功能的任何假说之前，需要这些信息。方法将包括木质部汁液的脂组学和蛋白质组学研究，木质部表面活性剂的限制性液滴表面测定法，以及木质部汁液和木质部的电子显微镜，以确定表面活性物质胶束及其在木质部中的来源。

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.