Ultrastructure of carnation etched ring virus-infected saponaria vaccaria and dianthus caryophyllus

Abstract

Carnation etched ring virus (CERV), a member of the cauliflower mosaic virus (CaMV) group, forms cytoplasmic inclusions similar to those produced by other members of the group. CERV virions in cytoplasmic inclusions are reportedly synthesized in the nucleus and transported as fully assembled particles to the cytoplasm. In contrast, all other members of the CaMV group are reported to form virions in association with cytoplasmic inclusions, and virions have not been observed in nuclei of cells infected with CaMV, dahlia mosaic virus, and mirabilis mosaic. We made an ultrastructural study of CERV-infected leaf cells of Saponaria vaccaria, Pink Beauty and Dianthus caryophyllus, florists' carnation, to further study the unique nuclear origin of CERV. We found CERV particles in nuclei of some infected cells of inoculated and systemically infected leaves of S. vaccaria, but not in nuclei of systemically infected D. caryophyllus. Cells of S. vaccaria that contain virions in the nuclei may also contain intranuclear membranes. Since nuclear virions are absent in some cells that contain cytoplasmic inclusions, we postulate that CERV is assembled in the nucleus and in the cytoplasm in separate processes in S. vaccaria. The absence of nuclear virions in the two cultivars of D. caryophyllus we examined suggests that nuclear assembly of CERV is host specific. CERV is the first DNA plant virus proposed to have two separate processes of virus assembly.