BIOLOGICAL, EPIDEMIOLOGICAL AND MOLECULAR INSIGHTS INTO THRIPS-IRIS YELLOW SPOT TOSPOVIRUS PEST COMPLEX
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Iris yellow spot tospovirus (IYSV) (genus Tospovirus, family Bunyaviridae), transmitted by Thrips tabaci L. causes an economically important disease in both onion bulb and seed crop in the USA and other onion-growing regions of the world. Onion thrips as a pest alone can cause up to >60 % crop loss. Besides Allium spp, several weeds were found to be hosts of IYSV. IYSV isolates collected from different states in the USA were evaluated to determine the existence of biologically distinct strains. On the basis of the ability to cause systemic infection, disease severity, senescence and death of the inoculated plants, isolates were delineated as mild or severe isolates. Since the genome structure of only the small (S) RNA of IYSV was known, the large (L) and medium (M) RNAs of the virus were sequenced. The L RNA was 8,880 nucleotides in length, coding the 331.17 kDa RNA-dependent RNA polymerase in the viral complementary (vc) strand. The M RNA was 4,817 nucleotides long coding the movement protein (34.7kDa) in the viral sense and the glycoprotein precursor (128.4 kDa) in the vc strand. An ELISA protocol was developed for detecting IYSV in single adult thrips using a polyclonal antiserum produced against the nonstructural protein (NSs) coded by the small (S) RNA. The approach enabled estimating the proportion of viruliferous thrips among the field-collected thrips. This will help better understand the epidemiology of IYSV. To understand the molecular basis of the emergence of new tospoviruses, a system was developed to study virus-virus interactions. It was found that two distinct and economically important tospoviruses, IYSV and Tomato spotted wilt virus (TSWV) complement each other to overcome host defense. The small RNA expression profiles of IYSV and TSWV in single-and dually-infected datura plants showed that systemic leaves of dually-infected plants had reduced levels of TSWV N gene-specific small interfering RNAs (siRNAs). This identifies a new role for the viral gene silencing suppressor in potentially modulating the biology and host range of viruses and underscores the important role of virally-coded suppressors of gene silencing in virus infection of plants.