| Summary: | Four bean cultivars (“Bountiful”, “Pinto”, “Top Crop”, and
“Tendergreen”) allowed only subliminal replication of the cowpea strain of
southern bean mosaic virus (SBMV-C). Bean protoplasts, on the other hand,
sustained replication of SBMV-C upon in vitro inoculation. Antigen accumulation
of the bean strain of southern bean mosaic virus (SBMV-B) and SBMV-C in bean
protoplasts was similar, indicating that the replicating capacity of both viruses
does not differ in bean cells.
When the four bean cultivars were inoculated with a mixture of sunn
hemp mosaic virus (SH1VIV), a tobamovirus, and SBMV-C, the latter was readily
detected in the inoculated primary leaves. The rate of spread of SBMV-C in the
presence of SHMV was compared to the rate of spread of SBMV-B in bean co
inoculated with SHMV. Virus accumulation in leaf blades, lateral veins, mid-ribs,
petioles, stems and roots was similar for both strains in the non-vascular tissue of
the inoculated leaf; a sharp decline in SBMV-C accumulation was observed
starting from the lateral veins towards the mid and distal parts of the petiole,
where virtually no virus could be found. These results contrasted with the
uniform presence of SBMV-B throughout infected bean plants.
Leaf strips blotted on nitrocellulose paper and developed as for
Western blotting confirmed these results, with SBMV-C antigen being detected in
mesophyll tissue and in epidermal cells of the lateral veins of the inoculated primary leaves. Electron micrographs of immunogold-labelled sections revealed
the absence of uniform SBMV-C particles in the mesophyll cells; instead, heavily
labelled, amorphous protein clumps in the vacuole were found. SBMV-C coat
protein from infected cowpea and bean plants showed no difference in its mobility
during electrophoresis in denaturing polyacrylamide gels.
These results indicate that SHMV facilitates cell to cell spread of
SBMV-C in inoculated bean leaves but does not allow for the movement of the
latter through the vascular system. Lack of efficient assembly of SBMV-C does
not impede cell-to-cell movement of the virus in the doubly-infected leaves, yet it
is probably an important factor involved in determining the inability of SBMV-C
to move into and/or through the vascular system. === Land and Food Systems, Faculty of === Graduate
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