What has been happening with viroids?

Viroids are naked nucleic acids that do not code for any proteins and yet are able to be replicated, processed, moved cell-to-cell and systemically through their host plants, as well as resist plant defense response and be transmitted from plant to plant, without a protective coat. All of the information specifying these functions lies within their nucleotide sequence and the RNA structures they form. This review examines what information about these processes has been acquired since 2008. Sequences involved in viroid replication and movement within the plant have been identified, in particular for the nuclear-associated (Pospiviroidae) viroids, as have sequences of one chloroplast-associated viroid (Avsunviroidae) involved in chloroplast uptake. The enzymes involved in ligation of viroids of either of the above two types also have been identified. Viroid sequences that are involved in pathogenicity through the RNA silencing system and the target of their viroid-specific small RNAs also have been identified. Effects of viroid infection on plant gene expression have been assessed for several viroids, and further specific interactions between viroids and host proteins have been identified. The variation in sequence of natural or passaged populations of viroids in various host species has been examined, and the effects of the host have been evaluated. New approaches to obtaining resistance to viroid infection have been examined or implemented, as have combinations of approaches to control viroid infection, and to better understand how viroids are transmitted. Finally, new viroids have also been discovered and characterized.