Interactions between Alfalfa mosaic virus and Soybean mosaic virus in soybean

Viral synergism occurs when two or more unrelated viruses simultaneously infect the same plant and the multiplication of one of the viruses is enhanced. This is generally associated with no change(s) in multiplication of the other viruses involved. Synergism also results in intensification of symptoms. In mixed-infection, viruses may also interact in an antagonistic manner, where one virus suppresses the replication or accumulation of another virus. This phenomenon is uncommon, and only two cases have been reported where the coat protein (CP) accumulation of one of the viruses has decreased. A number of synergistic interactions studied involve viruses belonging to the Potyviridae family. The increase in CP accumulation of the non-potyviruses in such an interaction has been attributed to the effect of the helper-component proteinase (HC-Pro) of potyviruses. Plant antiviral defense mechanism called “gene silencing”. HC-Pro is known as a strong suppressor of gene silencing and represents the first identified and characterized plant viral suppressor of gene silencing. The ability of Soybean mosaic virus (SMV), a member of the Potyviridae family, to interact synergistically with Bean pod mottle virus (BPMV) and Cowpea mosaic virus (CPMV) in mixed-infection in soybean has been demonstrated, but no change in the level of accumulation of CP of SMV was reported. In addition to SMV, soybean is infected by many other potyviruses or non-potyviruses, including Alfalfa mosaic virus (AMV). This research was aimed at studying the interaction of SMV with AMV in mixed-infection in soybean. Two biologically distinct SMV strains and three AMV isolates were used in this study and their interactions in mixed-infection in two different cultivars of soybeans (Williams 82 and Lee 68) were investigated. It was demonstrated that (a) mixed-infection between AMV and SMV can be easily established, irrespective of sequential or simultaneous inoculation of the two viruses; (b) based on CP accumulation and disease phenotype, AMV interaction with SMV is synergistic resulting in enhancement in symptom severity and AMV CP accumulation; (c) synergistic interaction of AMV with SMV is strain and cultivar independent; (d) interaction of SMV with AMV is antagonistic, which is also strain and soybean cultivar-independent.