Temporal Dynamic of the Ratio between Monopartite Begomoviruses and Their Associated Betasatellites in Plants, and Its Modulation by the Viral Gene βC1

Abstract

The begomovirus–betasatellite complex constantly threatens crops in Asia. However, the quantitative relationship between begomoviruses and betasatellites remains largely unknown. The quantities of tobacco curly shoot virus (TbCSV) and its betasatellite (TbCSB) and their ratio varied significantly in initial infection, and thereafter, the ratio tended to become constant. The TbCSB/TbCSV ratio in agrobacteria inoculum significantly affected that in plants in the initial infection but not thereafter. Null-mutation of βC1 that encodes a multifunctional protein important for pathogenesis in TbCSB significantly reduced the TbCSB/TbCSV ratio in plants. Viral inoculum plants with higher TbCSB/TbCSV ratios promoted whitefly transmission of the virus. The expression of AV1 encoded by TbCSV, βC1 encoded by TbCSB and the βC1/AV1 ratio varied significantly in the initial infection and thereafter the ratio tended to become constant. Additionally, the temporal dynamics of the ratio between another begomovirus and its betasatellite was similar to that of TbCSV and was positively regulated by βC1. These results indicate that the ratio between monopartite begomoviruses and betasatellites tend to become constant as infection progresses, and is modulated by βC1, but a higher betasatellite/begomovirus ratio in virally inoculated plants promotes virus transmission by whiteflies. Our findings provide novel insights into the association between begomoviruses and betasatellites.

The Role of Satellites in the Evolution of Begomoviruses

“…In plants infected with tobacco curly shoot virus (TbCSV) and its betasatellite (TbCSB), the amount of TbCSB is higher at the initial stages of infection, and becomes constant at a later stage of infection. It was shown that a higher TbCSB/TbCSV ratio promotes transmission of the virus by the vector whitefly [ 52 ]. The efficiency of transmission of five different begomoviruses by the vector whitely was found to vary from virus to virus; similarly, in the vector transmission of beta- and alphasatellites, transmission efficiency by four different cryptic species of whiteflies have also been shown to vary [ 35 ].…”