Comparative electron microscopic observation of viroplasms induced by plant viruses.

Abstract

Viroplasms (Vps) are considered the assembly sites of virus particles associated with virus multiplication in host cells. The intracellular Vps of 37 viruses containing classified and unclassified plant viruses were observed with electron microscopy and compared. Vps were generally amorphous or spherical, electron-dense or -lucent, granular/fibrillar/striated structures. These profiles differed among viruses or virus groups. In Caulimoviridae, viruses of Caulimovirus (4 viruses), Petuvirus (PVCV) and Soymovirus (SbCMV) induced similar Vps in the cytoplasm, but their profiles differed. Phytoreovirus (RDV) and Fijivirus (RBSDV) in Reoviridae induced different cytoplasmic Vps. Viruses of Alphacryptovirus (RYEV) in Partitiviridae and Waikavirus (RTSV=RWV) in Sequiviridae were localized in the phloem cells, and induced cytoplasmic Vps. RTSV also induced specific vesicles containing nucleic acid-like strands associated with the Vps, suggesting a relationship with genome replication of the virus. Tospovirus (TSWV, MYSV) of Bunyaviridae induced cytoplasmic Vps, and seemed to occur where the ER membrane was budding around the Vps. Very few particles of MYSV were found. In Rhabdoviridae, Nucleorhabovirus (8 viruses) induced nuclear Vps and seemed to bud at the nuclear inner membrane, whereas Cytorhabovirus (6 viruses) induced cytoplasmic Vps, and seemed to bud at the ER membrane. In unclassified viruses, small, non-enveloped bacilliform viruses were grouped into the nuclear and cytoplasmic types based on multiplication profiles. Virus particles seemed to mature at the nuclear inner membrane in the nuclear types (5 viruses), and at the ER membrane in the cytoplasmic types (5 viruses). Rod-shaped BuMoV from Japan induced electron dense-cytoplasmic Vps in many cells.