Viroids: uniquely simple and tractable models to elucidate regulation of cell-to-cell trafficking of RNA.

Cell-to-cell trafficking of RNA is an emerging biological principle that integrates systemic gene regulation, viral infection, cellular antiviral response, and cell-to-cell communication. The mechanisms regulating this trafficking remain to be understood. A testable hypothesis is that many RNAs have distinct structural motifs to direct trafficking across various cellular boundaries. The identification of structural motifs in a noncoding viroid RNA required for trafficking between specific cell types in a plant provides compelling genetic evidence to support this hypothesis. Here we discuss the unique features of viroids as well as experimental data to demonstrate these RNAs as simple and highly tractable models to elucidate the general mechanisms of RNA trafficking.