Genomic reassortment of barley mild mosaic virus: evidence for the involvement of RNA1 in pathogenicity.

A reverse transcription-polymerase chain reaction (RT-PCR) was developed for specific detection of RNA1 and RNA2 of two barely mild mosaic virus strains (BaMMV-Ka1 and BaMMV-Na1) and a barley yellow mosaic virus strain (BaYMV-II-1). Mechanical inoculation of barley cultivars with a mixture of BaMMV-Ka1 and BaMMV-Na1, followed by RT-PCR to detect RNA components in infected plants, revealed that the RNAs of the two strains were exchangeable in vivo to generate all nine possible combinations containing at least one RNA1 and one RNA2. Infected plants with mixed or reassorted RNAs were selected and used as inocula for further analysis of cultivar reactions. The results demonstrate that the pathogenicity and symptomatology are determined solely by RNA1. In contrast, following inoculation with mixtures of BaYMV-II-1 and either BaMMV-Ka1 or BaMMV-Na1, no heterologous combinations of their RNAs were observed.