Host factors involved in viral movement through plants

Tomato bushy stunt virus (TBSV) is a positive-sense single-stranded RNA virus. It encodes five open reading frames (ORFs), including two nested genes, expressing movement-associated proteins. One of these proteins, P22, interacts with a host transcription factor containing a homeodomain leucine-zipper motif, known as HFi22. Similar proteins of this type traffic their RNA from cell-to-cell, suggesting the possiblity that HFi22 is involved in the cell-to-cell movement of TBSV RNA. To further characterize the nature of the interaction between P22 and HFi22 on the cellular level, cellular fractionation experiments were conducted. To investigate the functional role of HFi22 in viral movement I attempted to inactivate its expression using a virus induced gene silencing system with a Tobacco rattle virus (TRV) vector. A final objective was based on the notion that different hosts can impact the stability of viruses used to express foreign genes of biotechnological interest. To compare virus stability in different hosts, TBSV expressing the green fluorescent protein (GFP) was inoculated onto various TBSV hosts, and infected leaf tissue was then used as inoculum to be rubbed onto a local lesion host. This technique made it possible to quantify the number of fluorescent foci versus total lesions. Results obtained for the first objective indicate that P22 and HFi22 co-fractionate in nucleus and membrane-enriched samples. In addition, it was found that HFi22 is largely conserved through a wide variety of plant species but not in lettuce, which was found to be compromised for effective virus spread. Control experiments for the second objective showed that plants were successfully silenced with TRV carrying the phytoene desaturase (PDS) gene resulting in photobleaching, however attempts to silence HFi22 have not yielded conclusive results. The results obtained for the third objective indicate there is a difference in how efficiently a foreign gene insert is maintained by TBSV in different host plants. In summary, the overall results of this research showed that host factors influence the host-virus interaction but their exact contributions remain elusive.