Exogenous Application of IR-Specific dsRNA Inhibits Infection of Cucumber Green Mottle Mosaic Virus in Watermelon

Cucumber green mottle mosaic virus (CGMMV) represents a serious threat in the production of watermelon. Small RNAs facilitate a mechanism known as RNA interference (RNAi), which regulates gene expression. RNAi technology employs foreign double-stranded RNAs (dsRNAs) to target and reduce the expression levels of specific genes in plants by interfering with their mRNAs. In this study, watermelon plants were treated with dsRNAs of CGMMV MET, IR, and HEL fragments that had been generated in E. coli HT115. We investigated variations in several factors, including viral accumulation, virus-derived small interfering RNAs (vsiRNAs), and symptom severity. MET-dsRNA, IR-dsRNA and HEL-dsRNA dramatically decreased the symptoms of CGMMV in plants in the growth chamber test. Plants treated with viral-derived dsRNA showed a considerable decrease in both virus titers and vsiRNA levels. We also explored the mobility of spray-on dsRNA-derived long dsRNA and discovered that it could be identified in both inoculated leaves and the systemic leaves. IR-dsRNA outperformed MET-dsRNA and HEL-dsRNA in dsRNA therapy. Illumina sequencing of small RNAs from watermelon plants treated with IR-dsRNA and those that were not treated showed that the decreased accumulation of vsiRNAs was consistent with interference with CGMMV infection in systemic leaves. dsRNA-treated plants showed a higher level of 24-nt viral siRNA and lower level of 22-nt viral siRNA accumulation, while 22-nt viral siRNA predominated in untreated plants, indicating that dsRNA treatment improved DCL3 activity. In conclusion, our research provides deeper insights into the mechanism of antiviral RNA interference and confirms the effectiveness of applying dsRNA locally to enhance plant antiviral activity.