IOS: The Link between Chloroplast Metabolism and Plasmodesmata-mediated Carbon Partitioning

University Of Tennessee Knoxville, Knoxville TN

Investigators

Abstract

The end product of photosynthesis, sucrose, is exported from leaves to the rest of the plant where it is used as an energy source for metabolism. This distribution of sucrose is essential for plant growth and development. A major route for passage of sucrose between cells is via cytoplasmic channels called plasmodesmata (PD). Although PD have long been known to exist and to be important for sucrose transport, we still do not know how PD development and function are regulated. There is accumulating evidence that chloroplasts, the plant organelles that perform photosynthesis to generate sucrose, produce signals that can regulate PD. This project will determine how chloroplast function impacts PD to regulate sucrose transport. The results of this research will be of broad interest to plant biologists, and in the long term should also provide novel targets for genetic engineering in efforts to develop plants with fine-tuned control of carbon partitioning. This may produce plants with better carbon allocation patterns to meet increased human demands for crop production. Several high school and undergraduate students will be involved in this project, and will gain hands-on experience in conducting research. This will help the NSF meet its goal of integrating research and education. Carbon partitioning in plants depends on the coordination of carbon fixation with transport. Plasmodesmata (PD) allow the passage of sugars to and from the plant vasculature for redistribution to tissues that require photoassimilates. It has long been known that plant physiology significantly impacts PD and intercellular trafficking, but how this is accomplished remains unclear. On the basis of previous work with the ise2 mutant, it is proposed that chloroplasts regulate the balance between photosynthesis and PD. ISE2 is an evolutionarily conserved nuclear gene that encodes a chloroplast RNA helicase. The objective of this study is to characterize the function of this chloroplast RNA helicase in chloroplasts and to elucidate its impacts on PD. Specific goals of this project are to: 1) elucidate the molecular function of ISE2 in the chloroplast, 2) understand how chloroplast photosynthetic output alters PD formation and intercellular trafficking, and 3) characterize other chloroplast mutants with altered PD structure and trafficking properties. The questions addressed in this proposal impact two important areas of plant cell biology. The first is the longstanding question of how intercellular flux of carbon via PD is regulated. Second is RNA processing in the chloroplast, an intriguing process that represents a crucial step in the coordination of gene expression between nuclear and organellar genomes, providing insight into the evolution of the organelle from its prokaryotic origins as an endosymbiont. A combination of biochemical and molecular biological approaches will be used to investigate ISE2's molecular function; intercellular trafficking will be monitored through the use of fluorescent molecules; and PD will be visualized by transmission electron microscopy and confocal fluorescence microscopy. Insight into the relationship between chloroplasts and PD will increase our understanding of how plants integrate local physiological and environmental cues into decisions that have systemic implications. The research in this proposal takes a holistic view of intercellular trafficking, and the results generated could potentially shift the paradigm from PD as passive mediators of carbon flux to PD as part of a larger strategy for integrating local, cellular physiology with whole organism responses.