The Interaction between Raspberry Ringspot and Tobacco Rattle Viruses in Doubly Infected Protoplasts

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Abstract
Tobacco mesophyll protoplasts infected with raspberry ringspot virus (RRV) show faint generalized staining when treated with fluorescent antibody to RRV, but many brightly staining granules develop after inoculation with a mixture of RRV and the CAM strain of tobacco rattle virus (TRV-CAM). The phenomenon occurred with 3 strains of RRV but not when tobacco mosaic virus, or 2 other strains of tobacco rattle virus, were substituted for the CAM strain. The RRV antigen aggregates were produced only in protoplasts that synthesized TRV-CAM nucleoprotein, and they did not seem to result from enhanced accumulation of RRV. They were also produced in intact leaves of doubly infected Nicotiana benthamiana plants, and in protoplasts from singly infected plants that were superinfected with the 2nd virus of the pair. On mixing in vitro in a wide range of conditions, purified preparations of RRV and TRV-CAM formed aggregates containing particles of both viruses, and this reaction had the same virus and strain specificity as the production of RRV antigen aggregates in vivo. Aggregation seems to result from a reaction that increases hydrophobicity and not from electrostatic attraction of particles of opposite charge. The production of these aggregates in vivo results from a redistribution of RRV particles or coat protein within the protoplast or cell; this occurs because of the affinity between RRV particle antigen and TRV-CAM nucleoprotein, the longer particles of which accumulate predominantly on the surface of mitochondria.