Integrated Studies on Cellular and Physiological Roles of Higher Plant Plasmodesmata

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In plants, cell-to-cell contact is established by special intercellular organelles, termed plasmodesmata. These unique cytoplasmic bridges interconnect the protoplasm of the cells within an organ, such as a leaf, forming a symplasmic state, which potentiates the exchange of both metabolites and information molecules. In recent years, genetic, molecular, cellular and physiological studies provided direct evidence that plasmodesmata have the capacity to mediate the transport of macromolecules, such as proteins and nucleic acids (RNA and DNA). Importantly, it has been shown that nucleic acids move cell to cell in the form of nucleoprotein complexes. Based on these studies, the emerging concept is that plasmodesmata have evolved to (a) enhance the efficacy of biochemical processes occurring within specific organs, and (b) facilitate the coordination of both physiological and developmental events, via the selective trafficking of information molecules. To better understand the mechanisms by which plants exert control over these plasmodesmal functions, experiments are described that will allow identification and cloning of the component parts involved in the selective trafficking of macromolecules between plant cells. An integrated approach will be taken to achieve this goal, and will incorporate the use of biochemical, cellular, molecular and ultrastructural techniques. Methods developed for the identification of the components involved in protein trafficking between the cytoplasm and the nucleoplasm will be integrated into these experiments. The specific involvement of chaperones in the overall process of protein and nucleoprotein transport to and through plasmodesmata will also be explored. Attention will be focused on a class of cytoplasmic chaperones termed the heat shock proteins. These studies will be integrated with an analysis of the symplasmic pathway that exists within the vascular system of higher plants. This vascular system is comprised of the xylem, which conducts water and mineral nutrients from the root to the aerial regions of the plant, and the phloem, which translocates fixed carbon (sugars) and nitrogen (amino acids) to developing organs. Two cell types, the companion cells and the sieve elements, form the conducting tissue of the phloem. The plasmodesmata that interconnect these cell types fulfill an important function in the operation of this long-distance translocation system. Experiments will be conducted to identify, at the genetic level, ways by which the plant can control this pivotal symplasmic route, from the photosynthetic tissues of the leaf, all the way to the phloem conducting tissue. These studies will provide a framework for understanding how the plant employs the phloem long-distance translocation system to orchestrate processes at the whole-plant level.