ADAPTATION OF POSITIVE-STRAND RNA VIRUSES TO PLANTS

The vast majority of positive-strand RNA viruses (more than 500 specie s) are adapted to infection of plant hosts. Genome sequence comparison s of these plant RNA viruses have revealed that most of them are genet ically related to animal cell-infecting counterparts; this led to the concept of ''superfamilies''. Comparison of genetic maps of representa tive plant and animal viruses belonging to the same superfamily (e.g. cowpea mosaic virus [CPMV] versus picornaviruses and tobacco mosaic vi rus versus alphaviruses) have revealed genes in the plant viral genome s that appear to be essential adaptations needed for successful invasi on and spread through their plant hosts. The best studied example repr esents the ''movement protein'' gene that is actively involved in cell -to-cell spread of plant viruses, thereby playing a key role in virule nce and pathogenesis. In this paper the host adaptations of a number o f plant viruses will be discussed, with special emphasis on the cell-t o-cell movement mechanism of comovirus CPMV.