Host-pathogen interactions in bromoviruses: Molecular and genetic studies of movement and coat proteins

The work presented in this thesis is divided into the following individual but related topics. In Chapter 1, the specificity of brome mosaic virus (BMV) and cucumber mosaic virus (CMV) coat protein (CP) was studied by exchanging the CP genes between the two viruses and testing the encapsidation competence in vivo. Eventhough each RNA3 chimera replicated to near wild type (wt) levels and synthesized CPs of expected parental origin, inoculum containing each chimera was non-infectious to common permissive hosts. Additional encapsidation assays in protoplasts revealed that the CMV CP expressed from chimeric BMV RNA3 was capable of packaging heterologous BMV RNA, but at a lower efficiency than parental BMV CP. On the other hand, BMV CP expressed from chimeric CMV RNA3 was unable to package heterologous CMV RNA. Chapter 2, describes experiments involving CP exchanges between two members of bromoviruses, BMV and CCMV. The results of the infectivity and host range assays performed using BMV and CCMV RNA3 having heterologous CPs, indicate that the CPs can be freely exchanged between the two viruses without having a detectable effect on long distance movement or symptom phenotype in hosts selective for each parental virus. These observations suggest that neither BMV CP nor CCMV CP has host specific determinants for long distance movement. Additional experiments involving two sets of reassortants between BMV and CCMV RNA3 hybrids and their genomic RNAs 1 and 2 revealed that the viral replicase genes encoded by RNA1 and 2 and/or the MP genes are involved in long distance spread. Chapter 3 deals in delineating the minimal sequences of the CCMV MP required to support movement function. Seven viable CCMV RNA3 variants with deletions in the C-terminal region of the MP (...