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

Washington State University, Pullman WA

Investigators

Abstract

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.