Mechanisms of disease symptom expression by potexviruses

植物病毒感染植物后会引起花叶、黄化、褪绿、环斑、斑驳等多种病害，但其分子机制尚不清楚。在本项目中，我研究了两种植物病毒属于马铃薯X病毒属的机制。马铃薯X病毒（PVX）和亚洲车前花叶病毒（PIAMV）。PVX的OS和BH菌株在侵染烟草（Nicotiana tabacum）后分别引起环斑病和花叶。两株病毒嵌合基因组的构建和病毒基因组的定点突变显示，病毒基因组的5 ′非翻译区的第58个核苷酸负责确定不同的症状。用原位杂交技术检测了病毒RNA在感染叶片中的分布以及寄主防御基因的转录。在轮斑病区，一些与过敏反应相关的基因如病程相关蛋白1a（PR-1a）等得到表达。病毒RNA在环斑区内外均有分布。这些数据表明，OS株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的菌株Li 1和Li 6在感染本氏烟草（Nicotiana benthamiana）植物后分别引起系统性坏死和花叶。分子生物学分析表明，病毒复制酶的第1154位氨基酸编码的基因组中负责症状的决定。在感染菌株Li 1的植物中也观察到HR样反应。这些数据支持这样的观点，即一些症状，如环斑病和系统性坏死是由宿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.