Biological Significance of Multicomponent Viruses

Abstract

One of the significant advances in plant virology in the past decade has been the development of knowledge of multicomponent plant viruses. The sub­ ject has been reviewed periodically and in some detail (7, 21, 38, 40, 54, 60, 63). The purpose of this paper is not to present another review, but to speculate on why different plant viruses exist as multicomponent systems. The term divided genome (60) may be more proper than multicomponent when applied to these viruses, because their significant characteristic seems to be that the genetic material is distributed among two or more nucleo­ protein particles. In the case of the rod-shaped tobraviruses (tobacco rattle and pea early browning viruses) particles are rods of two different lengths. This is also the case with the smaller, round-ended rods of alfalfa mosaic virus and some other ilarviruses. Most of the multicomponent viruses, however, are isometric. The particles of some, such as tobacco streak virus, differ in diameter; others, such as the comoviruses and nepoviruses, have all particles the same size. The fact that some particles contain more nucleic acid than others results in differences in sedimentation rates upon ultracentrifugation. When these viruses, as well as those with particles differing in size or shape, are layered on the top of sucrose density gradients and centrifuged for several hours, the different particle types (components) form discrete bands some distance down the tube. For certain other multicomponent viruses, bromoviruses and cucumoviruses for example, quantitative differences in nucleic acid content of different particle types are not great enough to provide significant differences in sedimentation rates. The particle types do differ, however, in