Because RNA can be a carrier of genetic information and a biocatalyst, there is a consensus that it emerged before DNA and proteins, which eventually assumed these roles and relegated RNA to intermediate functions. If such a scenario—the so-called RNA world—existed, we might hope to find its relics in our present world. The properties of viroids that make them candidates for being survivors of the RNA world include those expected for primitive RNA replicons: (a) small size imposed by error-prone replication, (b) high G + C content to increase replication fidelity, (c) circular structure for assuring complete replication without genomic tags, (d) structural periodicity for modular assembly into enlarged genomes, (e) lack of protein-coding ability consistent with a ribosome-free habitat, and (f) replication mediated in some by ribozymes, the fingerprint of the RNA world. With the advent of DNA and proteins, those protoviroids lost some abilities and became the plant parasites we now know.

As a consequence of two unique physical properties, small size and circularity, viroid RNAs do not code for proteins and thus depend on RNA sequence/structural motifs for interacting with host proteins that mediate their invasion, replication, spread, and circumvention of defensive barriers. Viroid genomes fold up on themselves adopting collapsed secondary structures wherein stretches of nucleotides stabilized by Watson–Crick pairs are flanked by apparently unstructured loops. However, compelling data show that they are instead stabilized by alternative non-canonical pairs and that specific loops in the rod-like secondary structure, characteristic of Potato spindle tuber viroid and most other members of the family Pospiviroidae, are critical for replication and systemic trafficking. In contrast, rather than folding into a rod-like secondary structure, most members of the family Avsunviroidae adopt multibranched conformations occasionally stabilized by kissing-loop interactions critical for viroid viability in vivo. Besides these most stable secondary structures, viroid RNAs alternatively adopt during replication transient metastable conformations containing elements of local higher-order structure, prominent among which are the hammerhead ribozymes catalyzing a key replicative step in the family Avsunviroidae, and certain conserved hairpins that also mediate replication steps in the family Pospiviroidae. Therefore, different RNA structures – either global or local – determine different functions, thus highlighting the need for in-depth structural studies on viroid RNAs.

There is a subviral world, whose most prominent representatives are viroids. Despite being solely composed by a circular, highly structured RNA of ~250 to 400 nucleotides without protein-coding ability (all viruses code for one or more proteins), viroids can infect and incite specific diseases in higher plants. The RNA of human hepatitis delta virus (HDV), the smallest genome of an animal virus, displays striking similarities with viroids: It is circular, folds into a rodlike secondary structure, and replicates through a rolling-circle mechanism catalyzed by host enzymes and cis-acting ribozymes. However, HDV RNA is larger (~1700 nucleotides), encodes a protein in its antigenomic polarity (the ∂ antigen), and depends for transmission on hepatitis B virus. The presence of ribozymes in some viroids and in HDV RNA, along with their structural simplicity, makes them candidates for being molecular fossils of the RNA world that presumably preceded our extant world based on DNA and proteins.

Preliminary transmission assays conducted under greenhouse conditions demonstrated that Citrus exocortis viroid (CEVd), Citrus bent leaf viroid (CBLVd), Hop stunt viroid (HSVd), Citrus viroid III (CVd-III), and Citrus viroid IV (CVd-IV) can be mechanically transmitted from citron to citron (Citrus medica) by a single slash with a knife blade. The impact of mechanical transmission of viroids by pruning and harvesting operations was also demonstrated in experimental and commercial field plots. Transmission efficiency under field conditions ranged from 4% in ‘Nules’ clementine to 10% in ‘Navelina’ sweet orange and 21% in ‘Verna’ lemon. Transmission efficiency varied only slightly with viroid and donor hosts. The impact of viroid transmission on tree height, canopy volume, and crop harvest was minimal. When the donor host was coinfected with several viroids, the viroids were not necessarily cotransmitted. Considerations regarding viroid transmission in other climates are discussed. Measures to control viroid spread in nurseries should be mandatory in certification programs.

Citrus psorosis virus (CPsV), genus Ophiovirus , causes a bark scaling disease of citrus. CPsV virions are kinked filaments with three negative‐stranded RNA molecules (vRNA) and a 48 kDa coat protein. The effect of temperature on symptom expression, virus accumulation and RNA silencing was examined in sweet orange seedlings ( Citrus sinensis ) graft‐inoculated with three different CPsV isolates and grown in a glasshouse at 26/18°C or 32/26°C (day/night). Most plants kept in the cooler glasshouse showed a shock reaction in the first flush with shoot necrosis, and then moderate to intense chlorotic flecking and spotting in young leaves, whereas plants incubated at 32/26°C did not exhibit shoot necrosis, and young leaf symptoms were milder. Virus titre estimated by ELISA and by northern and dot blot hybridization paralleled symptom intensity, with significantly higher virus accumulation in plants incubated at 26/18°C. The amount of CPsV‐derived small RNAs (CPsV‐sRNAs) slightly increased at 32/26°C, with the ratio of CPsV‐sRNA/vRNA being higher at 32/26°C than at 26/18°C. These results suggest that (i) CPsV infection induces RNA silencing in citrus plants, (ii) symptom intensity is associated with virus accumulation, and (iii) temperature increase enhances the RNA silencing response of citrus plants and decreases virus accumulation.