Cell-to-cell movement of potato spindle tuber viroid

Viroids are non-translatable, autonomously replicating circular RNAs that infect only plants. An important component of the viroid infection process is cell-to-cell movement; however, there is virtually no information available about the pathways and mechanisms of this process. In this study, potato spindle tuber viroid (PSTVd) has been used as a model system to investigate the mechanism of viroid cell-to-cell transport. Infectious RNA transcripts were produced from PSTVd cDNA clones in vitro, labeled with the nucleotide-specific fluorescent dye TOTO-1 iodide, and used for micro-injection. When injected into symplasmically isolated guard cells of mature tomato and tobacco leaves, PSTVd remained in the injected cells; in contrast, PSTVd injected into symplasmically connected mesophyll cells moved rapidly from cell to cell. A 1400 nt RNA containing only vector sequences was unable to move out of the injected mesophyll cells, but when PSTVd was fused to this transcript, the fusion RNA moved from cell to cell. At the DNA level, PSTVd cDNA also appears able to mediate cell-to-cell movement of plasmid DNA. These data indicate that (i) PSTVd moves from cell to cell via plasmodesmata, and (ii) this movement may be mediated by a specific sequence or structural motif.