Plant viruses are among the simplest of plant pathogens. Most contain fewer than a dozen genes. A growing number of plant virus genomes have been molecularly defined and cloned as cDNA. The goal of our research is to determine the roles of virus genes in virus replication and in disease development with the expectation that the results will lead to novel approaches for virus control.
The virus causing mosaic in alfalfa, alfalfa mosaic virus (AMV), has been the focus of our research. This virus is similar to the majority of plant viruses in that it has a single-stranded RNA genome. Synthesis of cDNAs to the entire AMV genome and determination of their sequences enabled us to study the individual genes in vitro and in plant protoplasts. We found that there are four proteins synthesized during infection that correspond to the expected products of the four genes encoded by AMV RNAs and to the proteins synthesized in vitro.
To study the role of virus genes in disease development and to devise and test strategies for the control of plant viruses, we transferred individual AMV genes into plant genomes. Plants which expressed the coat protein gene are similar to normal plants in morphology; however, they are resistant to AMV infection. The resistance is similar to natural cross protection, whereby infection with a virus protects a plant from subsequent infection by other strains of the same virus. Our studies, like studies of natural cross protection, suggest that coat protein interferes with an early event in virus replication.
An AMV gene involved in the movement of infection from one cell to the next was also transferred to tobacco plants. Our results suggest that this protein, like that in virus infections, is associated with the cell wall. Accumulation of this protein does not confer virus resistance, as does virus coat protein. Our goals are to determine more precisely where the protein accumulates in the plant cell and how it facilitates the cell-to-cell movement of infection.