Viroids/virusoids

Viroids and virusoids are the smallest subviral replicons. Virusoids refer to a specific group of satellite RNAs, associated with sobemoviruses, that share some common structural and functional properties with viroids. Both replicons accumulate in infected plant cells as single-stranded circular RNAs not encoding (with the possible exception of a virusoid) any protein, and replicate via rolling-circle mechanisms with only RNA intermediates. Despite these similarities, they differ in their biological properties. Viroids replicate by utilizing the nuclear or chloroplastic transcription and processing machinery encoded by their host plants, whereas virusoids replicate in the cytoplasm by utilizing the transcription and processing machinery encoded in part by their helper viruses and in part by their hosts. Furthermore, while viroids are not encapsidated, virusoids are encapsidated by their helper virus coat proteins. There is ample evidence that viroid RNAs contain all the information necessary to direct intracellular trafficking, replication, systemic trafficking, and pathogenicity. The biological functions of virusoids remain elusive.