Abstract
Electron microscopic analyses have revealed that the bottom (B) component of cowpea mosaic virus alone induces cytopathic structures in cowpea mesophyll protoplasts, similar to those induced by the complete virus [i.e. B plus middle (M) components]. This indicates that the development of such structures is not linked to accumulation of virus particles but to virus RNA replication and expression. When purified B component was inoculated to primary cowpea leaves, symptoms were not produced and B component RNA was incapable of spreading to surrounding cells. The results are discussed in terms of limits to the independence of B component RNA and of possible functions for M component RNA-encoded proteins.