Factors involved in the systemic transport of plant RNA viruses: the emerging role of the nucleus

Compatible virus-host interactions depend on a suitable milieu in the host cells permitting viral gene expression, replication, and spread. During pathogenesis, viruses hijack the plant cellular machinery to access molecules, subcellular structures, and host transport pathways needed for infection. Vascular trafficking of virus transport forms (VTF) within the phloem is a crucial step in setting-up virus infection within the entire plant. Moreover, vascular trafficking is an essential step for the further transmission of the viruses by their natural vectors as movement of the viruses to the distant parts of the plant from the initial site of infection guarantees accessibility of the virus particle for vector transmission. With the recent advances in the field of plant virology several emerging themes of viral systemic movement occur linking the role of virus-mediated transcriptional reprogramming and nuclear factors in vascular trafficking. Recent studies have uncovered host factors involved in virus vascular trafficking. Surprisingly, it appears that the role of the nucleus and nuclear factors in virus movement is still under-appreciated. This review describes how these new themes started to emerge by using two contrasting modes of virus vascular trafficking. It is argued that the translocation of viral movement proteins into the nuclei is, in many cases, an essential step in promoting virus systemic infection.