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.