Ultrastructure of maize necrotic streak virus infections.

ABSTRACT: Maize necrotic streak virus (MNeSV) has 32 nm isometric particles that encapsidate a single stranded RNA genome ca. 4.3 kb in size, and is a tentative member of the genus Tombusvirus. Since tombusviruses elicit the formation of cytopathological structures known as "multivesicular bodies" that assist in their classification at the genus level, the cytology of thin-sectioned maize leaf tissues 7, 15, and 45 days after inoculation was investigated. The prominent cytopathological features were: (i) an extensive accumulation of virus particles scattered throughout the cytoplasm or in crystalline arrays in the cytoplasm or, occasionally, in intercellular spaces; (ii) extensive plasmolysis and the appearance of small patches of amorphous, dark-staining material in the cytoplasm; (iii) the proliferation of membranes and the presence of empty double membrane-bound vesicular structures of various sizes. No apparent connection between these membranous elements and the nuclear envelope was detected, even when large clusters of vesic es were adpressed to the nuclei. Some of the largest double-membrane bound structures had rows of small vesicles located inside peripheral dilations of the bounding membrane. Although most of these structures appeared to originate from disintegrating mitochondria, they bore little resemblance to genuine multivesicular bodies like those arising from the modified mitochondria that follow infection by certain members of the family Tombusviridae, i.e. Carnation Italian ringspot virus, Pelargonium necrotic spot virus (PNSV), Galinsoga mosaic virus (GaMV), and Turnip crinkle virus (TCV). Immunogold labelling using antibodies to virus particles was detected over cytoplasm associated with virus particles and patches of electrondense amorphous material, suggesting that this material consists of virus coat protein.