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

维管植物是已知的唯一能在负压下输送水分的生物，它们利用太阳能产生动力，将水分从土壤输送到根部，再输送到叶片，最后进入大气。植物是如何做到这一点的，而不会在液压系统中不断产生阻碍水输送的气泡，目前还不完全清楚。工程师们无法在负压下复制水的输送，除非在理想的实验室条件下，在极短的距离内进行极少量的水输送。那么树木是如何在负压下通过树干的木质部（木材）运输大量水分的呢？本研究的目的是确定天然油（脂质）在这一过程中的作用。在植物的木质部导管表面发现了脂质。该项目将确定维管植物木质部中脂质的组成、位置、起源和可能的功能，并验证脂质是负压运输所必需的假设。这项研究将增加对植物适应水分胁迫的理解，并可能导致在负压下复制植物水分运输的工程系统。这项研究将涉及本科生，包括目前在科学领域代表性不足的群体，其更广泛的影响将包括在学术课程和公共宣传活动中开展关于植物水运的教育。内聚张力理论的一个公理是，水的高表面张力保护木质部导管防止气泡进入。这一公理最近被一项发现所挑战，即开花植物的木质部汁液中含有大量的表面活性脂质，并且从木质部汁液中分离出的表面活性剂强烈地降低了汁液的表面张力。此外，在树液中发现了表面活性剂包裹的气泡，这表明植物可以在气泡存在的情况下运输水分，而不需要完全阻止它们的形成。本研究解决了以下问题：1。在所有主要维管植物类群的木质部中是否都发现了外体脂质？2. 不同植物的脂质组成是什么？3. 木质部中脂质位于哪里？4. 外体木质部脂质起源于哪里？该研究将使用所有主要维管植物谱系的15个代表性物种，通过透射电镜、共聚焦超分辨率显微镜、原子力显微镜和能量色散x射线光谱进行成像，并使用质谱对木质部汁液和木质部组织进行脂质组学分析。该研究将验证一种范式逆转假说，即气-水表面的脂质涂层为液压系统提供了一定程度的保护，防止气泡膨胀成栓塞并导致液压故障。在负压条件下对水中脂质的研究不仅对植物生物学，而且对物理和化学工程的科学知识都有贡献。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.