Testing the High-Pressure Manifold Model of Phloem Transport and Unloading

植物通过一种叫做光合作用的光能过程为地球提供食物。产生的糖构成了植物和动物生命的一般能量基础。光合作用主要发生在叶子（被称为“源”）中，而叶子并不是消耗大部分糖的器官。相反，糖被运送到发育中的汇器官，如根、水果、谷物和块茎，我们利用这些器官来获取营养和喂养动物。糖从源到汇的转运发生在一种叫做韧皮部的维管组织中。鉴于韧皮部在食物和饲料生产中的核心作用，人们对韧皮部的分配过程知之甚少。目前还不清楚是什么控制着韧皮部中糖的分配。光合作用过程中是否产生了足够的糖，而下游的运输过程是限制因素？还是植物汇有足够的容量，而光合作用不足以满足汇的存储容量？这个项目将调查源或汇是否限制了植物的生产力，以及植物进化出了什么策略来填充它们的汇组织。为了解决这些基本问题并研究细胞水平上的糖分配，我们将采用最近开发的韧皮部原位观察工具，高分辨率组织三维重建的显微镜协议，以及一种新的机器学习算法软件来量化结构变化。总之，我们的各种方法将为评估未来作物改良最有希望的战略提供坚实的基础。此外，该项目还通过细胞生物学网页和为教育目的而生成的软件工具吸引了世界各地不同的学生团体和教师。植物生产力的一个核心问题是植物是受汇还是源限制。这个问题与韧皮部生理学直接相关，因为韧皮部将光同化物分配到水槽。韧皮部在整个植物中形成低水力阻力的管状网络。远距离转运的物理驱动力是由远端器官的同化物装卸产生的。从汇或源的分离会导致运输功能和结构伪影的立即停止。因此，有意义的实验必须在现场进行。这在技术上要求很高，也是一些基本问题仍未得到解答的主要原因。在这里，我们通过对汇源关系的理解来解决一个基本问题。根据目前教科书上关于韧皮部卸载和运输的物理学假设——高压流形模型——植物通常应该是汇有限的，这意味着增加作物光合能力的价值有限。这个问题涉及到植物中筛管导电性是否得到主动控制的核心问题。原位观察、电子显微镜协议和机器学习算法的新方法已经开发出来，可以精确和定量地表征韧皮部成分的结构。我们将利用控制源库距离的植物来激发韧皮部的形态和功能反应。如果存在对管材电导率的主动调节，则控制因素的识别将开辟一个新的研究领域，并为作物改良提供潜在的靶点。如果没有这样的调整，高压歧管模型将被拒绝，为寻找替代汇填充模型奠定了基础。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。

Plants feed our planet by a light-powered process called photosynthesis. The sugars produced form the general energy basis for plant and animal life. Photosynthesis mostly takes place in leaves (referred to as sources), which are not the organs where most of the sugars are consumed. Instead, sugars are transported to developing sink organs such as roots, fruits, grains, and tubers that we utilize for our nutrition and for animal feeding. Sugar translocation from sources to sinks takes place in a vascular tissue called phloem. The partitioning process in the phloem is surprisingly poorly understood, given its central role in food and feed production. Today it remains unclear what controls the allocation of sugars in the phloem. Are sufficient sugars produced during photosynthesis, while the downstream transport process is the limiting factor? Or do plant sinks have sufficient capacity while photosynthesis is insufficient to meet the sink storage capacity? This project will investigate if sources or sinks are limiting plant productivity, and what strategies plants have evolved to fill their sink tissues. To tackle these fundamental questions and to investigate sugar allocation on the cellular level, we will employ recently developed tools for in situ observation of the phloem, microscopy protocols for 3D reconstruction of tissues at high resolution, and a novel machine learning algorithm software to quantify structural changes. Together, our diverse approaches will provide a solid basis for the evaluation of the most promising strategies for future crop improvement. Additionally, this project also engages diverse student groups and teachers worldwide through webpages on cell biology and software tools generated for educational purposes.A core question in plant productivity is whether plants are sink- or source-limited. This question is directly linked to phloem physiology because the phloem distributes photoassimilates to sinks. The phloem forms a network of tubes of low hydraulic resistance throughout the plant. The physical driving forces for long-distance translocation are generated by assimilate loading and unloading in distant organs. Detachment from either sinks or sources causes an instant cessation of transport function and structural artifacts. Meaningful experiments therefore have to be performed in situ. This is technically demanding and the main reason why fundamental questions have remained unanswered. Here we address a basic problem with our understanding of sink-source relations. Based on the physics of the current textbook hypothesis of phloem unloading and transport – the High-Pressure Manifold Model – plants generally should be sink-limited, which would imply that increased crop photosynthetic capacities would be of limited value. This problem relates to the central question if sieve tube conductivity is actively controlled in plants. New methods for in situ observation, electron microscopy protocols, and machine learning algorithms have been developed to precisely and quantitatively characterize the structure of phloem components. We will utilize plants with controlled source-to-sink distances to provoke morphological and functional responses in the phloem. If there is an active adjustment of tube conductivity, the identification of the controlling factors would create a new research field and provide potential targets for crop improvement. If there is no such adjustment, the High-Pressure Manifold Model will be rejected, setting the stage for the search for alternative sink filling models.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.