| In this study, I demonstrated that a non-coding RNA sequence from potato virus X as small as 127 nucleotides (located near the 3´end of 8 kDa and the start of CP genes as well as the non-coding intergenic region) is capable of translocating not only its own sequence but also a reporter gene, fluorescent green protein mRNA into chloroplasts of the transgenic tobacco plants. This is the first evidence showing that a small viral RNA sequence (designated "RNA tractor") is capable of translocating RNA sequences to the chloroplast. The chloroplast translocation efficiency of the PVX RNA tractor was determined to be 120 X lower than that of Eggplant latent viroid, a member of the Avsunviroidae family that replicates and accumulates in the chloroplast. Furthermore, I investigated two begomoviruses on various Nicotiana species to assess the effects of their ploidy level on infectivity and symptomatology. For this purpose, infectious clones of Ageratum enation virus (AEV), a monopartite (DNA-A with Beta-satellite DNA particle) and Tomato leaf curl New Delhi virus (ToLCNDV), a bipartite (DNA-A and DNA-B), begomoviruses were used. All plants inoculated with ToLCNDV were systemically infected and showed characteristic symptoms. However, in the case of AEV, all plants except N. tabacum cv. Xanthi were infected by the virus but remained symptomless. Taken together, these results indicate that there is no clear relationship between infectivity and ploidy levels; furthermore, symptomatology depends on the type of virus and/or plant species. Another key question to answer was whether or not the genomes of the begomoviruses could be isolated from chloroplasts of the infected tobacco and tomato plants. PCR results confirmed the presence of only DNA-A of the AEV in the chloroplasts. Preliminary studies clearly show that the "RNA tractor" sequence and AEV genome are incapable of targeting the mitochondria. These findings suggest that members from different viral families may be associated with the same organelle, but that members do not necessarily target the different organelles. Thus, the present study could be important for understanding the evolutionary importance of the interactions of viral genomes with different organelles of plant cells and their consequential pathological effects. |
