Resistance Screening Of Rice Varieties For Rice Black Streaked Dwarf Virus And Virus Quantification In The Vector Insects

Rice black streaked dwarf virus(RBSDV), belongs to genus Fijivirus, family Reoviridae, is causing Rice black streaked dwarf disease(RBSDD) one of the most destructive disease of rice in East and Southeast Asia. During recent decades, the disease is causing serious threats to rice, wheat and maize production. Small brown planthopper(SBPH, Laodelphax striatellus Fallen) and two other planthoppers(Unkanodes sapporona Matsumura and U. albifascia Matsumura) transmit RBSDV in a persistent propagative manner. In addition, SBPH was also known to acquire southern rice black streaked dwarf virus(SRBSDV), a tentative member of genus Fijivirus which is potentially transmitted by white-backed planthopper(WBPH, Sogatella furcifera Horvath). SRBSDV was first recorded in 2001 in Yangxi County, China and has spread to Southern China, Vietnam, and Japan, becoming major threats to rice production. Present study was conducted to screen different rice varieties against RBSDD and to explore the spatiotemporal dynamics of SRBSDV and RBSDV over time in their vector and nonvector insects using absolute quantitative real time-PCR.Here we screened different rice varieties for resistance to RBSDV under greenhouse and field conditions and developed a Taq Man q RT-PCR assay utilizing primers from the S4 genomic segment to detect and quantify RBSDV in different rice varieties. Among 66 tested varieties, seven varieties including Liangyou 3399, Da Liang 207, Lian dao 9805, X 008, Ningjing 4, Shengdao 301 and Liangyou 1129 showed a resistant response to the disease, 17 other varieties were moderately resistant, 30 moderately susceptible and 12 susceptible. Disease assessment in the greenhouse was significantly correlated with disease incidence in the field and with RBSDV titers in the plant. Incorporation of these findings in a disease management program will aid in avoiding disease epidemics in the future.On the other hand, we showed the spatial distribution of SRBSDV and RBSDV over time in their vector and nonvector insects using absolute quantitative real time-PCR. Viral copy number was assessed every 2 days from 0 to 14 days after a 3-day acquisition access period(AAP) on infected plants. Results revealed 2932.4 ± 215.9 to 4041.5 ± 464.9 copies of SRBSDV in WBPH, and 5135.0 ± 884.7 to 8168.0 ± 1107.3 copies of RBSDV in SBPH. Highest copy numbers of both viruses were found in the gut of their respective vectors. Although SRBSDV was detected in the gut of SBPH, it did not spread into the hemolymph or other organs as shown by the significantly lower copies number compared with WPBH(P < 0.0001). After an 8-day latent period, the transmission efficiency of SRBSDV and RBSDV by their respective vectors was significantly positive correlated with the number of copies in the salivary gland(r2 = 0.7808, P = 0.0036 and r2 = 0.9351, P < 0.0001, respectively, at α = 0.05). Together, these results confirm that viral copies need to reach a threshold in the insect gut for further spread. For each virus, copy numbers in the salivary gland were significantly correlated with transmission efficiency by their respective vectors.Collectively, these results provided knowledge about different resistant level of widely grown varieties in different provinces of China, practically application of these varieties may provide better insights to control severe disease epidemic. Quantitative studies on transmission mechanism of SBPH confirm that viral copies need to reach a threshold in the insect gut for further spread. Due to lower copy numbers of SRBSDV in the gut of SBPH could not disseminate in the hemolymph or other organs to complete circulation process. Copy numbers in the salivary gland of SBPH or WBPH was significantly correlated with transmission efficiency.