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

植物病毒感染所有主要作物，每年在全球造成数十亿美元的经济损失。植物病毒作为专性细胞内寄生物，只能在寄主植物中繁殖。为了建立全身性感染，在主要感染细胞中成功繁殖后代病毒后，新生病毒基因组必须在局部传播到邻近细胞，称为细胞间运动，并进一步系统地传播到植物的其他部分，称为长距离运动。病毒通过胞间连丝（plasmodesmata， PD）进行细胞间运动，胞间连丝是一种特殊的细胞器，是植物界所特有的。PD是结构复杂的微通道，它们穿过细胞壁，在邻近细胞之间建立细胞质和内膜的连续性。PD允许小分子在细胞间扩散，并调节大分子或大分子复合物（如病毒颗粒和核糖核蛋白复合物）的细胞间运输。病毒通过PD细胞间运动是一个由病毒编码蛋白介导的活跃过程。长距离运动是指病毒从叶肉通过束鞘细胞、韧皮部薄壁细胞和伴生细胞进入韧皮部筛元（SE），在那里它们被转运，然后在一个遥远的地方卸下病毒，从而发生进一步的感染。由于帕金森病的复杂性和参与病毒传播的细胞类型的多样性，人们对病毒细胞间运动的分子机制知之甚少。我们最近的研究表明，植物多型病毒是已知病毒中最大和最重要的一类，通过两种病毒蛋白（如CI和P3N-PIPO）的协同作用，在PD上形成一个特殊的结构，以促进病毒细胞间的运动。我们最近的遗传工作也证明了：(1)外壳蛋白（CP）是多病毒系统传播所必需的；(2)除了病毒基因组复制和细胞间运动外，多病毒CI蛋白在病毒长距离运动中也起着独特的重要作用。在这篇论文中，我们建议进一步研究PD形成锥形结构的途径，以及CI和CP如何参与病毒的全身运动。我们还将鉴定多病毒细胞间和长距离运动所需的宿主基因产物，并进一步表征它们的功能作用。因此，这一建议将提高我们对病毒局部和全身传播的认识，并直接有助于开发新的抗病毒策略。此外，从所提出的工作中获得的结果也将有助于更好地理解植物中基本的细胞过程，如细胞间和大分子复合物的远距离运输，从而通过推进基础细胞和分子生物学的知识为生命科学做出贡献。

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.