Abstract
Soybean vein necrosis virus (SVNV) is a newly discovered species of tospovirus infecting soybean plants that is transmitted by the primary vector, soybean thrips (Neohydatothrips variabilis), and two additional secondary vectors, tobacco thrips (Frankliniella fusca) and eastern flower thrips (F. tritici). This study was undertaken to elucidate the association between virus acquisition [6, 12, 24, and 48 h acquisition access period (AAP)] and transmission efficiency [12, 24, and 48 h inoculation access period (IAP)] in the primary vector, N. variabilis, and to examine the mechanisms of vector competence by analyzing the effect of AAP (6, 12, and 24 h) on virus infection in various tissues. In addition, we examined virus infection in tissues of the two secondary vectors. We found a significant effect of virus acquisition on transmission efficiency, transmission rate post 6 and 48 h AAP was significantly lower than 12 and 24 h AAP. Our analysis did not reveal a correlation between virus transmission rate and virus RNA in corresponding N. variabilis adults. On the contrary, N. variabilis adults harboring higher accumulation of the virus (>104) resulted in lower transmission rates. Analysis of SVNV infection in the tissues revealed the presence of the virus in the foregut, midgut (region 1, 2, and 3), tubular salivary glands and principal salivary glands (PSG) of adults of all three vector species, however, the frequency of infected tissues was highest in N. variabilis followed by F. fusca and F. tritici. The frequency of SVNV infection in individual tissues specifically the salivary glands was lowest after 6 h AAP compared to 12 and 24 h AAP. This finding is in agreement with the transmission assays, where significantly lower virus transmission rate was observed post 6 h AAP. In addition, N. variabilis adults with high PSG infection (12 and 24 h AAP) were likely to have high percentage of foregut and midgut region 2 infection. Overall, results from the transmission assays and immunolabeling experiments suggest that shorter AAP results in reduced virus infection in the various tissues especially PSG, which are important determinants of vector competence in SVNV-thrips interaction.
Highlights
Soybean [Glycine max (L.) Merr.] is an important crop throughout the world due to its versatility as a food source along with applications in various industries such as textiles and biofuels
The dark and pale forms of F. schultzei differ in their ability to transmit tospoviruses, with the dark form considered to be a more efficient vector (Wijkamp et al, 1995). These findings suggest that the transmission efficiency of a newly identified tospovirus in a specific area should be studied in the different populations of thrips species that live there
Nagata et al (2002) found that adults of F. occidentalis that transmitted tomato spotted wilt virus (TSWV) efficiently harbored higher titers of the virus compared to T. tabaci that failed to transmit the virus
Summary
Soybean [Glycine max (L.) Merr.] is an important crop throughout the world due to its versatility as a food source along with applications in various industries such as textiles and biofuels. The United States is the global leader in soybean production with approximately 83 million acres planted and an estimated 117 million metric tons produced in 2017 (Soystats, 2017). There are several viral diseases that affect soybean crops in the United States. Emergence of new viral diseases such as soybean dwarf virus, tobacco streak virus, and more recently soybean vein necrosis virus (SVNV) can further reduce yield. Soybean vein necrosis virus is a species in the genus Orthotospovirus (Bunyavirales: Tospoviridae) and was first identified in Tennessee in 2008 (Tzanetakis et al, 2009). SVNV has been documented in all the major soybean-growing in the United States and Ontario, Canada, including Arkansas, Illinois, Indiana, Iowa, Kentucky, Michigan, Minnesota, Ohio, Tennessee, Wisconsin (Bloomingdale et al, 2014). The symptoms begin with vein clearing followed by chlorosis or appearance of lightgreen to yellow blotchy patches near the main vein, followed by necrosis or dying of the leaf tissue at late stage of infection; the name, SVNV
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