Mechanisms of disease symptom expression by potexviruses

植物病毒感染植物寄主后，会引起花叶病、黄化、黄萎病、环斑病、斑驳病等多种疾病症状。然而，这些症状表达的分子机制几乎没有被发现。在这个项目中，我使用了属于Potexvirus属的两种植物病毒来研究其机制。马铃薯X病毒（PVX）和车前草花叶病毒（PIAMV）。PVX菌株OS和BH侵染烟草植株后分别引起环斑和花叶病。两株病毒嵌合基因组的构建和病毒基因组的位点定向突变表明，病毒基因组5 '非翻译区的第58个核苷酸是决定不同症状的原因。利用原位杂交技术监测了PVX各菌株的病毒RNA分布及宿主防御基因的转录。在环斑区，表达了一些与过敏反应（hypersensitive response， HR）相关的植物基因，如致病相关蛋白1a （patho致病相关蛋白1a， PR-1a）。在环状斑点区域内外均发现了病毒RNA。这些数据表明，菌株OS引起的环斑症状是诱导hr样植物细胞死亡的结果，但未能限制病毒的传播。PlAMV菌株Li1和Li6侵染烟叶后分别引起全身坏死和花叶病。分子分析表明，PlAMV基因组中编码的病毒复制酶的1154个氨基酸负责症状的决定。在感染菌株Li1的植株中也观察到hr样反应。这些数据支持这样一种观点，即一些症状，如环斑和全身性坏死，是由寄主植物试图限制病毒传播的反应引起的。

Upon infection to plant hosts, plant viruses cause vast varieties of disease symptoms including mosaic, yellows, chlorosis, ring spot, mottle, and so on. Molecular mechanisms underlying expression of these symptoms, however, have scarcely been uncovered. In this project, I studied the mechanism using two species of plant viruses belonging to Genus Potexvirus. Potato virus X (PVX) and Plantago asiatica mosaic virus (PIAMV). The strains OS and BH of PVX cause ring spot and mosaic, respectively, upon infection to tobacco plant (Nicotiana tabacum). Construction of chimeric genomes of the two strains and site-directied mutagenesis of the viral genomes revealed that the 58th nucleotide of the 5 prime untranslated region of the viral genome was responsible for determination of distinct symptoms. Distribution of viral RNA of each PVX strain, as well as transcription of host defense genes, was monitored in the infected leaves by in situ hybridization technique. In the area of ring spot, some plant genes related to the hypersensitive response (HR) such as pathogenesis-related protein 1a (PR-1a) were expressed. Viral RNA was found both inside and outside of the area of ring spot. These data suggest that the ring spot symptom caused by strain OS is a result of induced HR-like plant cell death yet unsuccessful in restricting the spread of the virus. The strains Li1 and Li6 of PlAMV cause systemic necrosis and mosaic, respectively, upon infection to Nicotiana benthamiana plants. Molecular analysis revealed that the 1154th amino acid of the viral replicase encoded in the PlAMV genome was responsible for the symptom determination. HR-like response was also observed in plants infected with the strain Li1. These data support the idea that some symptoms such as ring spot and systemic necrosis are caused by host plant responses which tried to restrict viral spread.