UC Riverside

For many years, it has been known that changes in plant RNA virus populations can result from replication in different host plants or in different environments, but the genetics underlying such changes could not be explored. Recent developments in the molecular biology of tobacco mosaic virus (TMV) make such studies possible.

A typical population of "wild type" TMV (U1) contains a small subpopulation of genetic variants (necrotic lesion-NL) that evoke the hypersensitive reaction on Nicotiana sylvestris, a systemic host for wild type. This variant is strongly selected for during replication in plants that are undergoing development of shoot axes during bolting. This selection changes the composition of the virus population dramatically. No accumulation of NL-type occurred when N. sylvestris plants were inoculated with TMV204, a clonal population derived from RNA transcripts of a plasmid carrying a cDNA copy of the TMV genome or when U1 infected other systemic hosts.

Tissue cultures initiated from virus infected explants are stressed. The extent of stress varies with the age of infected explants. NL-type virus is also strongly selected for during the regeneration of plantlets from leaf disks of plants infected with TMV U1. When whole plantlets were transferred into a greenhouse, those containing NL variants were severely stunted, showed mosaic symptoms, and most of them died slowly. Some that had originally contained lower titers of NL-type virus survived to flower but produced only a few seeds. Another set of these plants contained only wild type and grew well. Repeated assays of virus in these plants before or after bolting revealed no detectable NL-type virus. Apparently, during callus development and organogenesis in culture, partial segregation of the mixed U1 population occurred, and NL variants preferentially infected the developing tissues. This represents another situation in which dramatic change in the genetic structure of an RNA virus population occurs during development of the host plant.