Plant-virus interactions: Suppression activity of the viral proteins, coat protein trans effects on recombination, and viral host-RNA encapsidation in horizontal gene transfer

Virus-host interactions are complex and have been a driving force of the everlasting evolutionary progress of both virus and host. Ever since the first virus infection took place, it has been an arms race of defense and counter defense between virus and host. To further uncover the underpinnings of this arms race, plant RNA viruses were utilized to uncover the mechanisms of virus-host interactions. Three projects were chosen that focus on different aspects of the viral interaction. Specifically, three separate projects investigated viral-host relationships in suppression activities of the viral proteins, coat protein's trans effect on recombination, and co-encapsidation of host-RNA. The first project focused on the host effect on the virus through the RNA silencing system that can degrade viral RNA. Additionally, the possibility that virus-host interactions might provide an evolutionary advantage in favor of the host was investigated.;This type of interaction where the host influences the virus was explored by testing the proteins encoded by the Brome Mosaic Virus (BMV) for their silencing suppression activity. Surprisingly, none of the proteins demonstrated local or systemic RNA silencing suppression activity. The proteins were not able to recover silenced green fluorescence protein (GFP), nor did they show the ability to suppress the two types of siRNAs, 21-22 nucleotides (nt) and 25nt. The lack of suppression activity of the proteins gives the host an advantage in host-virus interactions that has to be compensated by the virus. BMV might evade silencing in membrane bound vesicles, which are shaped through invaginations at the endoplasmic reticulum.;The second project studied the virus's effect on itself through recombination. This level of interaction was analyzed via the trans effect of the coat protein (CP) on recombination. The coat protein promotes recombination in a trans-acting manner. The in vivo experiments with five different mutated coat protein genes revealed that CP's B-box binding domain promotes recombination by slowing RNA polymerase. Whereas the stem-loop C (SLC) binding domain induces recombination by binding CP at the same time to sgRNA3a and bringing the two RNAs in close proximity. The C-terminus of CP also plays a role in recombination and virion stability. RNA4 encapsidation was shown to be dependent on the N-terminus of CP. These results corroborate similar studies, further confirming and strengthening the theories on the molecular mechanisms of BMV RNA recombination driven by CP as a trans -acting factor.;The third project examined the relationship between virus and host through horizontal gene transfer on an evolutionary time scale. Two Bromoviridae model viruses were extracted from different hosts and their RNA virion content was sequenced using Next Generation Sequencing (NGS). Both viruses co-encapsidated different patterns and quantities of host-RNAs. The co-encapsidation rate was host dependent, whereas the type of RNA co-encapsidated was virus specific. Transposable elements were found among the genetic material in all cases. These findings support the theory that RNA viruses have acted as vectors in horizontal gene transfer and have benefited their hosts through an evolutionary time scale.;Together these three projects examined multiple facets of virus-host interactions and have provided a greater understanding of the complex relationship. Virus and host influence each other on an immediate level by RNA silencing but also on an evolutionary time scale through horizontal gene transfer. The results suggest that host-viral evolution involves selection for behavioral adaptations, such as the specific encapsidation host RNA. Intrinsic molecular mechanisms are also used in host-viral evolution in form of CP trans -activated recombination, to keep pace in the host-virus arms race. Further work in this area my likewise identify unique RNA virus adaptations that can be targeted for therapies of viral disease.