A number of plant and animal viruses involve the nucleolus and specific nucleolar proteins in their life cycles. The functions of these proteins and nucleolar trafficking in virus infections are largely unknown. Taliansky and colleagues have shown recently that the ORF3 protein of the plant virus, groundnut rosette virus (GRV), has been shown to cycle through the nucleus, passing through Cajal bodies to the nucleolus, and then exiting back into the cytoplasm. This journey is absolutely required for the formation of viral ribonucleoprotein particles (RNPs) that themselves are essential for the spread of the virus from leaf to leaf. All of these processes rely on the interaction of the ORF3 protein with fibrillarin, a major nucleolar protein (see model).

Silencing of the fibrillarin gene prevents long-distance movement of GRV but does not affect replication and cell-to-cell movement of the virus. Silencing of fibrillarin also localises the ORF3 protein to multiple Cajal body-like aggregates which are unable to fuse with the nucleolus. The ORF3 protein and fibrillarin interact in vitro, forming an RNP complex when mixed with umbravirus RNA. This complex has a filamentous structure with some regular helical features, similar to the RNP complex formed in vivo during umbravirus infection. Significantly, the viral RNA is protected from RNase digestion by the ORF3 protein-fibrillarin complex and is infectious when inoculated to plants.

Figure 1
Fig. 1. Fibrillarin knock-down suppresses systemic infection of GRV. Non-silenced (Non-s) plants show yellow blotch symptoms characteristic of the GRV-YB isolate, whereas fibrillarin-silenced (Fib-s) plants are symptomless.

We have shown that the coat protein of another plant virus potato leafroll virus (PLRV, a polerovirus) also targets the nucleolus. Our preliminary results also indicate that like GRV, PLRV is unable to cause systemic infection in the fibrillarin-silenced plants, although accumulation of PLRV in the inoculated leaves is not affected. These results suggest that fibrillarin involvement in virus infection could be a more general mechanism among viruses with a nucleolar phase of life cycle and may have implications for other plant and animal viruses which interact with the nucleolus. This opens up new opportunities for development of novel approaches for resistance against such viruses based on the demolishing interaction of virus-encoded proteins with fibrillarin or other nuclear factors. Figure 1 provides an example demonstrating how such approaches may provide resistance against viruses: fibrillarin silencing in Nicotiana benthamiana plants leading to fibrillarin deficiency strongly protects these plants from GRV systemic infection.

References

Taliansky, M., Roberts, I.M., Kalinina, N., Ryabov, E.V., Raj, S.K., Robinson, D.J. and Oparka, K.J. 2003. An umbraviral protein, involved in long-distance RNA movement, binds viral RNA and forms unique, protective ribonucleoprotein complexes. Journal of Virology 77, 3031-3040.

Kim, S.H., MacFarlane, S., Kalinina, N.O., Rakitina, D., Ryabov, E., Gillespie, T., Haupt, S., Brown, J.W.S. and Taliansky, M. 2007. Cajal bodies and the nucleolus are required for a plant virus systemic infection. The Embo Journal 26, 2169-2179.

Kim, S.H., Ryabov, E., Kalinina, N.O., Rakitina, D., Gillespie, T., MacFarlane, S., Haupt, S., Brown, J.W.S. and Taliansky, M. 2007. Interaction of a plant virus-encoded protein with the major nucleolar protein, fibrillarin, is required for virus systemic infection. Proceedings of National Academy of Sciences USA 104, 11115-11120.