Mechanisms of phloem-mediated RNA traffic

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In a multicellular organism, cells signal each other to coordinate gene expression patterns that dictate growth and development. In plants, cytoplasmic channels called plasmodesmata form one pathway for cell-cell signaling. Furthermore, the vascular tissue phloem forms a network of channels to allow signaling between distant cells or organs. Increasing evidence indicates that selected RNAs and proteins traffic from cell to cell through plasmodesmata and from organ to organ through the phloem. RNA traffic appears to have important roles in plant development, physiology, and defense. This project addresses the question of how RNA traffic is regulated in a plant, using viroid traffic as a highly tractable model system. Viroids are small pathogenic RNAs that infect plants. They do not encode proteins and can replicate and move systemically within an infected plant. It is likely that viroid RNAs have evolved the capacity to utilize pre-existing cellular mechanisms to traffic through plasmodesmata and the phloem. This work has three objectives: 1) testing the hypothesis that an RNA uses different mechanisms to enter and exit the phloem during systemic traffic, 2) identifying the cellular boundary that controls phloem exit of an RNA, and 3) identifying the RNA motif(s) that mediates phloem exit. Results from this project are expected to yield novel insights about the RNA structures and cellular factors that are important for intercellular traffic. Such insights may impact studies on how endogenous plant RNAs traffic to regulate cellular processes underlying growth and development and how viral RNAs traffic systemically to spread infection.