Submitted to: Viruses
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/16/2017
Publication Date: 6/21/2017
Citation: Tian, B., Gildow, F.E., Stone, A.L., Sherman, D.J., Damsteegt, V.D., Schneider, W.L. 2017. Host adaptation of soybean dwarf virus following serial passages on pea (Pisum sativum) and soybean (Glycine max). Viruses. 9:155. doi: 10.3390/v9060155.. Interpretive Summary: Soybean dwarf virus (SbDV) causes two types of diseases in soybeans. One form of the disease, a near complete loss of color in the leaves, is caused by the yellowing strain of the virus (SbDV-Y). The other form of the disease, a severe stunting of the plant with little or no growth, is caused by the dwarfing strain of the virus (SbDV-D). Both forms of the disease have caused significant damage to soybean crops in Japan in the past. The only way SbDV can move from plant to plant is by insects. Recently, there have been reports of the dwarfing strain causing damage to soybean crops in the Midwest. A survey of several eastern US states determined that SbDV was also present there. However, unlike Japan, the SbDV outbreaks in the Midwest are very limited in scope, and it appears as though SbDV in the US is nearly always found in clover, not soybean. A series of experiments was conducted to determine if the change in host from clover to soybean was so difficult for SbDV that it may explain the relative lack of outbreaks. SbDV from clover was used to infect soybeans and clover, and the changes in the virus were monitored. SbDV was able to adapt to these new hosts, and several changes that improved the virus’s ability to infect soybeans. However, in the process of jumping from clover to soybean the virus lost the ability to be carried from plant to plant by aphids. The soybean adapted virus would be trapped in the originally infected plant, and this could explain why outbreaks of SbDV have been limited in the United States.
Technical Abstract: Soybean dwarf luteovirus (SbDV) has been identified in both Japan and the United States. Like all luteoviruses, SbDV must be transmitted from host to host by aphid vectors. Previous efforts determined that isolates representing both D-like and Y-like isolates exist in the eastern United States. In Japan, SbDV causes significant losses in soybeans. However, the SbDV isolates found in the United States are mostly limited to clover, with occasional sporadic outbreaks occurring in the Midwest. To determine if the ability of SbDV to adapt to a new host was the limiting factor preventing the emergence of outbreaks in US soybeans, an endemic clover strain of SbDV, called MD-6, was subjected to a passage experiment. In this experiment, MD-6 was passaged on peas and soybeans repeatedly, and the adaptation to the new host was measured in terms of viral titer, viral symptoms and transmission efficiency. MD-6 did adapt to both soybean and peas, but in the process of adapting to soybeans lost its ability to be aphid transmitted. The adapted populations were sequenced, and the sequences were compared to the original SbDV sequence found in clover. A number of mutations were identified, including non-synonymous mutations in the replication, movement and read-through proteins. All of the non-synonymous mutations reverted when the MD-6 population was returned to clover, suggesting that many of these mutations were host adaptive. It appears as though the mutations made in response to the new host have a deleterious effect on aphid transmission for MD-6.