pairing with target messenger RNA. Secondly, siRNAs corresponding to sequences from viroid genomes have been isolated from infected plants. Finally, transgenic expression of the noninfectious hpRNA of potato spindle tuber viroid develops all the corresponding viroid-like symptoms. This indicates that when viroids replicate via a double stranded intermediate RNA, they are targeted by a dicer enzyme and cleaved into siRNAs that are then loaded onto the RNA-induced silencing complex. The viroid siRNAs contain sequences capable of complementary base pairing with the plant's own messenger RNAs, and induction of degradation or inhibition of translation causes the classic viroid symptoms. Retroviroids Retroviroids and retroviroid-like elements are viroids, which are RNA that have a DNA homologue. These entities are thought to be largely exclusive to the carnation, Dianthus caryophyllus, that are closely related to the family of viruses termed 'carnation small viroid-like RNA' (CarSV RNA). These elements may act as a homologous substrate upon which recombination may occur and are linked to double-stranded break repair. These elements are dubbed retroviroids as the homologous DNA is generated by reverse transcriptase that is encoded by retroviruses. RNA world hypothesis Diener's 1989 hypothesis had proposed that the unique properties of viroids make them more plausible macromolecules than introns, or other RNAs considered in the past as possible "living relics" of a hypothetical, pre-cellular RNA world. If so, viroids have assumed significance beyond plant virology for evolutionary theory, because their properties make them more plausible candidates than other RNAs to perform crucial steps in the evolution of life from inanimate matter (abiogenesis). Diener's hypothesis was mostly forgotten until 2014, when it was resurrected in a review article by Flores et al., in which the authors summarized Diener's evidence supporting his hypothesis as: Viroids' small size, imposed by error-prone replication. Their high guanine and cytosine content, which increases stability and replication fidelity. Their circular structure, which assures complete replication without genomic tags. Existence of structural periodicity, which permits modular assembly into enlarged genomes. Their lack of protein-coding ability, consistent with a ribosome-free habitat. Replication mediated in some by ribozymes—the fingerprint of the RNA world. The presence, in extant cells, of RNAs with molecular properties predicted for RNAs of the RNA world constitutes another powerful argument supporting the RNA world hypothesis. However, the origins of viroids themselves from this RNA world has been cast into doubt by several factors, including the discovery of retrozymes (a family of retrotransposon likely