Genomes of non-defective viruses range in size from 2,400,000 bp of dsDNA (Pandoravirus salinus) to 1,759 bp of ssDNA (porcine circovirus). Are even smaller viral genomes possible? The subviral agents called viroids provide an answer to this question.
Viroids, the smallest known pathogens, are naked, circular, single-stranded RNA molecules that do not encode protein yet replicate autonomously when introduced into host plants. Potato spindle tuber viroid, discovered in 1971, is the prototype; 29 other viroids have since been discovered ranging in length from 120 to 475 nucleotides. Viroids only infect plants; some cause economically important diseases of crop plants, while others appear to be benign. Two examples of economically important viroids are coconut cadang-cadang viroid (which causes a lethal infection of coconut palms) and apple scar skin viroid (which causes an infection that results in visually unappealing apples).
The 30 known viroids have been classified in two families. Members of the Pospiviroidae, named for potato spindle tuber viroid, have a rod-like secondary structure with small single stranded regions, a central conserved region, and replicate in the nucleus (illustrated; click to enlarge; figure credit). The Avsunviroidae, named for avocado sunblotch viroid, have both rod-like and branched regions, but lack a central conserved region and replicate in chloroplasts. In contrast to the Pospiviroidae, the latter RNA molecules are functional ribozymes, and this activity is essential for replication.
There is no evidence that viroids encode proteins or mRNA. Unlike viruses, which are parasites of host translation machinery, viroids are parasites of cellular transcription proteins: they depend on cellular RNA polymerase for replication. Such polymerases normally recognize DNA templates, but can copy viroid RNAs.
In plants infected with members of the Pospiviroidae, viroid RNA is imported into the nucleus, and copied by plant DNA-dependent RNA polymerase II. The viroid is copied by a rolling circle mechanism that produces complementary linear, concatameric, RNAs. These are copied again to produce concatameric, linear molecules, which are cleaved by the host enzyme RNAse III. Their ends are joined by a host enzyme to form circles.
In plants infected with members of the Avsunviroidae, viroid RNA is imported into the chloroplast, and complementary concatameric RNAs are produced by chloroplast DNA-dependent RNA polymerase. Cleavage of these molecules is carried out by a ribozyme, an enzyme encoded in the viroid RNA.
After replication, viroid progeny exit the nucleus or chloroplast and move to adjacent cells through plasmodesmata, and can travel systemically via the phloem to infect other cells. Viroids enter the pollen and ovule, from where they are transmitted to the seed. When the seed germinates, the new plant becomes infected. Viroids can also be transmitted among plants by contaminated farm machinery and insects.
Symptoms of viroid infection in plants include stunting of growth, deformation of leaves and fruit, stem necrosis, and death. Because viroids do not produce mRNAs, it was first proposed that disease must be a consequence of viroid RNA binding to host proteins or nucleic acids. The discovery of RNA silencing in plants lead to the hypothesis that small interfering RNAs derived from viroid RNAs guide silencing of host genes, leading to induction of disease. In support of this hypothesis, peach latent mosaic viroid small RNAs have been identified that silence chloroplast heat shock protein 90, which correlates with disease symptoms. The different disease patterns caused by viroids in their hosts might all have in common an origin in RNA silencing.
Our current understanding is that the disease-causing viroids were transferred from wild plants used for breeding modern crops. The widespread prevalence of these agents can be traced to the use of genetically identical plants (monoculture), worldwide distribution of breeding lines, and mechanical transmission by contaminated farm machinery. As a consequence, these unusual pathogens now occupy niches around the planet that never before were available to them.
The origin of viroids remains an enigma, but it has been proposed that they are relics from the RNA world, which is thought to have been populated only by non-coding RNA molecules that catalysed their own synthesis. Viroids have properties that make them candidates for survivors of the RNA world: small genome size (to avoid error catastrophe caused by error-prone replication), high G+C content (for greater thermodynamic stability), circular genomes (to avoid the need for mechanisms to prevent loss of information at the ends of linear genomes), no protein content, and the presence of a ribozyme, a fingerprint of the RNA world. Today’s viroids can no longer self-replicate, possibly having lost that function when they became parasites of plants. What began as a search for virus-like agents that cause disease in plants has lead to new insights into the evolution of life.