Research Accomplishment Reports 2007

Epidemiology, Genetic Diversity and Strategies to Control Bean Pod Mottle Virus

S.A. Ghabrial
Department of Plant Pathology

 

Project Description

Bean pod mottle virus (BPMV) is widespread in the major soybean growing regions of the United States. BPMV, like other comoviruses, has a bipartite genome consisting of RNA1 and RNA2.  We previously reported the occurrence in nature of two distinct subgroups of BPMV strains (subgroups I and II), as well as reassortants between the two subgroups. Furthermore, we recently documented the presence of partial diploid strains in natural populations of BPMV.

We now report the isolation and molecular characterization of RNA1 recombinants from soybean plants infected with the partial diploid reassortant strain IL-CB1 (diploid for RNA1 and monoploid for RNA2). Three types of local lesion isolates, mild, moderate and severe, were derived from strain IL-CB1, which induces very severe symptoms on soybean, with the majority of the local lesion isolates being severe. The parental strain, IL-CB1, and all local lesion isolates examined contained only one type of RNA2 (subgroup i).

Whereas the mild and moderate local lesion isolates lost their diploid status as their RNA1s belonged to either subgroup i or ii, the severe local lesion isolates retained their diploid genotypes. Moreover, some of these severe isolates contained intramolecular and intermolecular recombinant RNA1s. The presence of recombinant RNA1 in the original IL-CB1 strain was verified using northern hybridization analysis, RT-PCR cloning and sequencing. The recombinant RNA1, however, accumulated at low levels in the presence of wild-type RNA1. 

Structurally similar recombinant RNA1 molecules were also generated in soybean after four passages using mixed infection with infectious RNA1 transcripts from two distinct strains, suggesting that recombination events are frequent and that a recombination hot spot exists.  In addition, the different types of recombinant RNA1s that we isolated and characterized (mosaic, chimeric and deletion) shared similar recombination sites thus supporting the presence of a recombination hot spot. Furthermore, sequence analysis showed that the recombination region has au rich sequences characteristic of recombination hot spots. The identification of natural recombination events between virus strains is important in considerations of virus evolution. Our study provided the first report on recombination in a virus belonging to the comovirus genus.

Impact

Our studies on the genetic diversity of BPMV documented the occurrence in nature of two distinct subgroups of BPMV strains (subgroups I and II) as well as reassortants between these two subgroups.  Furthermore, evidence was recently presented that some of the naturally occurring strains that induce very severe symptoms in soybean are partial diploid reassortants, i.e., they are diploid for RNA1 and haploid for RNA2.  The occurrence of such partial diploids at relatively high frequency suggests that mixed infections with two distinct strains are also common.  The isolation and characterization of intermolecular BPMV RNA1 recombinants provided undisputed evidence that two distinct BPMV strains may replicate in the same cells.  The occurrence of recombinants and reassortants in the natural populations of BPMV and their roles in the emergence of new strains that induce different symptoms are of considerable significance to studies on BPMV evolution and epidemiology.  Furthermore, such extensive diversity in BPMV genotypes should be a matter of great concern to plant pathologists and plant breeders who seek to screen soybean germplasm for BPMV resistance.

Publications

Zhang, C., Gu, H., and Ghabrial, S. A. 2007. Molecular characterization of naturally occurring RNA1 recombinants of the comovirus Bean pod mottle virus. Phytopathology 97:1245-1254.

Gu, H., Zhang, C. and Ghabrial, S. A. 2007. Novel naturally occurring Bean pod mottle virus reassortants with mixed heterologous RNA1 genomes. Phytopathology 97:79-86.