Constant ratio between the genomic components of bipartite begomoviruses during infection and transmission.

The genomic components of multipartite viruses are encapsidated in separate virus particles, and the frequencies of genomic components represent one of the key genetic features. Many begomoviruses of economic significance are bipartite, and the details of the association between their genomic components remain largely unexplored. We first analyzed the temporal dynamics of the quantities of DNA-A and DNA-B and the B/A ratio of the squash leaf curl China virus (SLCCNV) in plants and found that while the quantities of DNA-A and DNA-B varied significantly during infection, the B/A ratio remained constant. We then found that changes in the B/A ratio in agrobacteria inoculum may significantly alter the B/A ratio in plants at 6 days post inoculation, but the differences disappeared shortly thereafter. We next showed that while the quantities of DNA-A and DNA-B among plants infected by agrobacteria, sap transmission and whitefly-mediated transmission differed significantly, the B/A ratios were similar. Further analysis of gene expression revealed that the ratio of the expression of genes encoded by DNA-A and DNA-B varied significantly during infection. Finally, we monitored the temporal dynamics of the quantities of DNA-A and DNA-B and the B/A ratio of another bipartite begomovirus, and a constant B/A ratio was similarly observed. Our findings highlight the maintenance of a constant ratio between the two genomic components of bipartite begomoviruses during infection and transmission, and provide new insights into the biology of begomoviruses.