Pangola stunt virus in thin sections and in negative stain

Abstract

Pangola grass (Digitaria decumbens) infected with pangola stunt virus (PSV) was thin-sectioned and examined in the electron microscope. Particles assumed to be those of the virus were found in the cytoplasm of phloem and phloem-associated cells. The virions had a reovirus-like double shell with an outer capsid 65–68 nm in diameter, an inner capsid 50–55 nm in diameter and a densely staining core about 50 nm in diameter. The inner capsid was in favorable cases seen to have spikes. Virions, occasionally enclosed in tubes, occurred only outside the viroplasms, which occupied most of the cytoplasm of the cells. Inner capsids with or without dense cores were confined to the viroplasms. Where viroplasms were disintegrating, inner capsids were released prematurely and the viroplasms were seen to be composed of kinked filaments. Negatively stained extracts of the plants were also examined. In uranyl acetate, the virus had a diameter of about 66 nm and the inner capsid had a diameter of about 52 nm. The virion possessed A spikes and the inner capsid possessed B spikes similar to those of maize rough dwarf virus (MRDV) and Fiji disease virus (FDV). Apparently helical filaments like those observed with FDV were seen in fixed negatively stained preparations and may be the same as the viroplasm filaments seen in sections. Morphologically, PSV was indistinguishable from MRDV, FDV, and other similar viruses, but it was distinct from wound tumor virus and rice dwarf virus.