Sunn-hemp mosiac virus as a helper in the intercellular spread of southern bean mosaic in a resistant host

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 ofGraduat