Turnip mosaic virus NIb weakens the function of eukaryotic translation initiation factor 6 facilitating viral infection in Nicotiana benthamiana

Construction of watermelon mosaic virus-Beijing isolate infectious clone and study of the dynamic localization and accumulation of virus-encoded proteins

Potyvirus is the largest genus of plant RNA viruses, and the members in the genus are known to cause significant damage to a wide variety of crops. In this study, we performed small RNA (sRNA) deep sequencing for identification of potential virus (es) in collected cucumber leaves with mosaic symptoms from fields in Beijing. Through this high-throughput sequencing and subsequent PCR amplification, we obtained a complete viral genome sequence of 10,044 bp, which showed the highest similarity to the watermelon mosaic virus (WMV) isolate FBR04-37 and exhibited the typical characteristics of potyviruses in its genome organization. The obtained WMV isolate was designated as WMV-BJ. We then constructed the infectious clone of WMV-BJ, which can successfully infect six kinds of cucurbitaceous plants by agroinfiltration. Among these, Cucumis melo showed obvious symptoms such as shrinkage and mosaic compared to other cucurbitaceous plants infected by this virus infectious clone. To understand the biological function of WMV-BJ, we further analyzed the dynamic subcellular localization and protein accumulation of WMV-BJ encoded 11 viral proteins. The results showed the WMV-BJ-encoded proteins displayed diverse subcellular localizations, and most viral proteins were easily degraded after expression in plant cells. Together, the construction of the WMV-BJ infectious clone in this study provides a valuable tool for further exploring the biology of this virus and the interaction between WMV-BJ and host plant, and useful information for investigating the functions of WMV-encoded proteins.

Turnip mosaic virus NIb weakens the function of eukaryotic translation initiation factor 6 facilitating viral infection in Nicotiana benthamiana

Viruses rely completely on host translational machinery to produce the proteins encoded by their genes. Controlling translation initiation is important for gaining translational advantage in conflicts between the host and virus. The eukaryotic translation initiation factor 4E (eIF4E) has been reported to be hijacked by potyviruses for virus multiplication. The role of translation regulation in defence and anti-defence between plants and viruses is not well understood. We report that the transcript level of eIF6 was markedly increased in turnip mosaic virus (TuMV)-infected Nicotiana benthamiana. TuMV infection was impaired by overexpression of N. benthamiana eIF6 (NbeIF6) either transiently expressed in leaves or stably expressed in transgenic plants. Polysome profile assays showed that overexpression of NbeIF6 caused the accumulation of 40S and 60S ribosomal subunits, the reduction of polysomes, and also compromised TuMV UTR-mediated translation, indicating a defence role for upregulated NbeIF6 during TuMV infection. However, the polysome profile in TuMV-infected leaves was not identical to that in leaves overexpressing NbeIF6. Further analysis showed that TuMV NIb protein, the RNA-dependent RNA polymerase, interacted with NbeIF6 and interfered with its effect on the ribosomal subunits, suggesting that NIb might have a counterdefence role. The results propose a possible regulatory mechanism at the translation level during plant-virus interaction.