Plant Viruses with a Divided Genome

Abstract

A considerable number of plant viruses produce in infected plants more than one nucleoprotein component. Typical examples are tobacco rattle virus (TRV) , alfalfa mosaic virus (AMV), viruses of the cowpea mosaic virus (CPMV) group, tobacco streak virus (TSV) and brome mosaic virus (BMV). It has been shown in recent years for some of these muItiparticulate viruses that the genetic information necessary for virus multiplication is di­ vided among two or more nucleoprotein components. Discovery of plant viruses with a divided genome has changed the con­ cept of the nature of viruses. The classical concept implied that the mature virus, the ultimate phase of viral development, was a single particle, the vir­ ion, containing the genome of the virus. This is of course still true for many plant viruses, but no longer for all. Several important groups of plant viruses have been recognized to have a genome divided among two or more physi­ cally separable particles. A certain heterogeneity occurs in practically all purified preparations of plant viruses. In preparations of tobacco mosaic virus (TMV) for example, rodshaped particles with a modal length of 300 m,u predominate, but smaller and longer rods occur also. The presence of such particles has been explained by possible fragmentation of 300 mft particles during purification, faulty syn­ thesis of particles, or, in case of longer particles, by aggregation. Such hetero­ geneity is quite normal in preparations of rodshaped and threadlike viruses. Similarly, preparations of spherical viruses may contain some particles with fragmented RNA. This type of particle we shall regard as defective; it con­ sists of viral structural proteins and fragments of the viral genome. It is possi­ ble that these defective particles have some biological significance, and that they can interfere with virus replication. The infectivity of TMV, for exam­ ple, is enhanced by the presence of short particles derived from the virus in the inoculum (23, 37). In these cases it is a question of occurrence of defec­ tive particles, besides nondefective homOlogous virus particles which contain a complete genome necessary for multiplication (36). Heterogeneity in such virus preparations does not reflect functional heterogeneity. This is different from the multiparticulate viruses we will discuss here. Plant viruses with a divided genome are characterized by physical hetero-