Purification and electron microscopy of three soil-borne plant viruses

Highly infective, purified preparations of tomato black ring, raspberry ringspot and arabis mosaic viruses were obtained by macerating infected Petunia hybrida or tobacco leaves in a butanol-chloroform mixture, followed by clarification, differential centrifugation, and rate zonal centrifugation in sucrose density gradients. The sedimentation rates of the three viruses relative to that of tobacco mosaic virus were estimated. Electron micrographs of shadowcast mounts of formalin-treated preparations showed that all three viruses have icosahedral particles of average diameters 29–30 mμ. Particles embedded in phosphotungstate often appeared polygonal in outline but were smaller in diameter than those in shadowed mounts. Some particles of each virus were deeply penetrated by phosphotungstate, but others were not. Infectivity was always associated with the presence of unpenetrated particles but there was no evidence that the penetrable ones were infective. Particle shape, mode of transmission, properties in vitro, and behavior in plants all suggest that these viruses should be placed in the same general group in any classification of plant viruses.