TOSPOVIRUSES: EVOLUTIONARY RELATIONSHIPS AND RNAi-MEDIATED RESISTANCE

Afsha Tabassum

Tospoviruses continue to be a major constraint to a wide range of agronomic and horticultural crops worldwide. There are no practical control measures available for Orthotospoviruses. Expansion of thrips distribution in the world along with resurgence of the virus species, breakdown of resistance in cultivars, emergence of resistant breaking strains etc. requires the development of new strategies to manage the tospoviruses. Comprehensive genetic architecture and evolutionary genomic analysis of viral population and development of resistant cultivars using artificial microRNA mediated mechanism is necessary to manage tospoviruses. Considering the importance of the tospoviruses and knowledge gap in understanding and managing tospoviruses, this study was taken up. Allium fistulosum (green onion) showing atypical symptoms of Iris yellow spot orthotospovirus (IYSV) where tested for presence of the virus by ELISA and sequencing confirmed the presence the IYSV showing the first report of IYSV infecting green onions in the US. The complete N gene sequence of 142 IYSV isolates available in GenBank was used to determine the evolutionary pattern. IYSV isolates showed two major genotypes viz., IYSV Netherlands (IYSVNL) IYSV Brazil (IYSVBR). A little over 5 percent of the isolates fell in neither group (IYSV other). IYSVNL and IYSVBR genotypes are under purifying selection and population expansion, whereas IYSVother showed decreasing population size and hence appear to be under balancing selection. The substitution rates identified are similar to other plant RNA viruses. The evolution studies will help us design appropriate disease control measures. Plants have innate defense mechanism against plant viruses and plant viruses encode silencing suppressor gene to counteract this defense. microRNAs engineered [artificial microRNA(amiRNA)] against silencing suppressor (NSs) against Tomato spotted wilt orthotospovirus. There was no difference in appearance, development of localized and systemic symptoms, severity of symptoms, and relative virus levels among any of the constructs compared to controls suggesting that the constructs did not offer protection to TSWV infection under the conditions used. Further studies need to be done to ascertain this no resistance provided by amiRNA targeting the NSs. New insights into host expansion, virus evolution and efficacy of amiRNA were revealed through this study.

TOSPOVIRUSES: EVOLUTIONARY RELATIONSHIPS AND RNAi-MEDIATED RESISTANCE

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