Molecular characterisation and infectivity of a “Legumovirus” (genus Begomovirus: family Geminiviridae) infecting the leguminous weed Rhynchosia minima in Pakistan

The legume yellow mosaic viruses (LYMVs) that cause extensive losses to grain legumes across southern Asia are an evolutionarily unusual group of begomoviruses (genus Begomovirus; family Geminiviridae) with bipartite genomes. All previously identified LYMVs were isolated from leguminous crop species. Here we have identified a virus related to the LYMVs in a common weed, the legume Rhynchosia minima originating from Pakistan. Analysis of the sequence of the virus shows it to be a typical bipartite begomovirus. Sequence comparisons to all other begomovirus sequences available in the databases show the virus from R. minima to be distinct, with the highest level of sequence identity (69.5%) to an isolate of Mungbean yellow mosaic virus. This indicates that the virus identified here is a new species in the genus Begomovirus for which we propose the name Rhynchosia yellow mosaic virus (RhYMV). By Agrobacterium-mediated inoculation we show that, in common with the other LYMVs, the clones of RhYMV are not infectious to the experimental host Nicotiana benthamiana. In soybean, the results of inoculation depended upon the variety. In soybean var. Ig6 the symptoms were mild and plants recovered from infection. However, in var. FS-85, symptoms were severe and progressed to necrosis, indicative of a hypersensitive response. These results indicate that there is resistance to RhYMV in the soybean germplasm. The significance of these results is discussed.