Mechanisms of disease symptom expression by potexviruses

植物病毒侵染植物寄主后，会引起花叶、黄化、黄化、环斑、斑驳等多种病害症状。然而，这些症状表达的潜在分子机制几乎还没有被发现。本课题以马铃薯Y病毒属的两种植物病毒为材料，对其作用机理进行了研究。马铃薯X病毒(PVX)和车前花叶病毒(PIAMV)。PVX的OS和BH菌株在侵染烟草时分别引起环斑和花叶病。构建了两株病毒的嵌合基因组，并对病毒基因组进行了定点突变，结果表明病毒基因组5个非翻译区的第58位核苷酸负责决定不同的症状。用原位杂交技术检测了各PVX株系病毒RNA的分布，以及寄主防御基因的转录。在环斑区，一些与过敏性反应(HR)相关的植物基因如病程相关蛋白1a(PR-1a)被表达。环斑内外均可检测到病毒RNA。这些数据表明，OS菌株引起的环斑症状是诱导类HR植物细胞死亡但未能成功限制病毒传播的结果。PlAMV的Li1和Li6株系在侵染烟草植株时分别引起系统坏死和花叶病。分子分析表明，在PlAMV基因组中编码的病毒复制酶的第1154个氨基酸负责决定症状。在感染菌株Li1的植物中也观察到类似HR的反应。这些数据支持一些症状，如环斑和全身性坏死是由寄主植物的反应引起的，这些反应试图限制病毒的传播。

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.