Uncovering Molecular Mechanisms Underlying Cell-to-Cell Movement and Long-Distance Trafficking of Viruses in Plants

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Plant viruses infect all major crops and cause economic losses of multibillion dollars worldwide each year. As obligate intracellular parasites, plant viruses can only multiply in their host plants. In order to establish a systemic infection, upon successful reproduction of progeny viruses in primarily infected cells, the nascent viral genome must spread locally to neighboring cells, termed cell-to-cell movement, and further move systemically to other parts of the plant, named long-distance movement. Cell-to-cell movement of viruses occurs through plasmodesmata (PD), a specialized intercellular organelle, unique to the plant kingdom. PD are structurally complex microchannels that cross the cell wall and establish cytoplasmic and endomembrane continuity between neighbouring cells. PD allow small molecules to diffuse between cells and regulate the intercellular trafficking of macromolecules or macromolecular complexes such as viral particles and ribonucleic protein complexes. Viral cell-to-cell movement through PD is an active process mediated by virus-encoded protein(s). Long-distance movement is the process by which the virus moves from the mesophyll via bundle sheath cells, phloem parenchyma, and companion cells into phloem sieve elements (SE) where they are translocated, and then unloaded at a remote site from which further infection will occur. Due to the complexity of PD and various types of cells involved in viral spread, the molecular mechanism of viral intercellular movement is poorly understood. Our recent studies have shown that plant potyviruses, the largest and most agricultural important group of known viruses, form a special structure at PD via the coordinated action of two viral proteins, e.g., CI and P3N-PIPO, to facilitate viral cell-to-cell movement. Our most recent genetic work has also demonstrated that (1) the coat protein (CP) is required for systemic spread of potyviruses; and (2) in addition to viral genome replication and cell-to-cell movement, the potyviral CI protein also plays a distinct, essential role in viral long-distance movement. In this proposal, we propose to further investigate the pathways recruited for the formation of conical structures at PD and how CI and CP are involved in viral systemic movement. We will also identify host gene products required in potyviral cell-to-cell and long-distance movement, and further characterize their functional roles. Therefore, this proposal will advance our knowledge of viral local and systemic spread and directly contribute to the development of novel antiviral strategies. Moreover, the results obtained from the proposed work will also help better understanding of essential cellular processes such as cell-to-cell and long-distance trafficking of macromolecular complexes in plants, and thus contribute to life science by advancing knowledge in fundamental cell and molecular biology.