Viroids

Plant viroids are infectious agents that have no capsid and have an RNA genome that encodes no gene product; they do not require a helper for infectivity. Potato spindle tuber viroid is the prototype of this class of agents. The viroids are covalently closed, circular, single-stranded RNAs, 246-375 nucleotides long, whose sequence is such that base pairing occurs across the circle, as shown in Fig. 15.13. As a result, these agents have the form of a dsRNA rod with regions of unpaired loops. Their replication is carried out by plant RNA polymerase, and likely proceeds through an antigenome. Large multimeric structures can be observed in infected plant nuclei, and self-cleavage of such multimers into unit-length RNA molecules is involved in "maturation" of the infectious form.

Viroids spread from plant to plant through mechanical damage caused by insects or by cultivation. They are also spread by propagation of cuttings from infected plants. Viroids may also be present in seeds. Very often, viroids are transmitted during the manipulation of crop plants for harvest, as is the case with the coconut Cadang-Cadang viroid, transmitted from tree to tree on the metal spikes harvesters wear on their shoes to climb the trunk.

Left-hand terminal domain (T1)

Pathogenic domain

Conserved central domain (C)

Variable domain (V)

Right-hand terminal domain (T2)

Left-hand terminal domain (T1)

Pathogenic domain

Conserved central domain (C)

Variable domain (V)

Right-hand terminal domain (T2)

Fig. 15.13 The potato spindle tuber viroid genome. Various pathogenic strains range from 250 to 360 nucleotides in length. This circular RNA does not encode a protein, but the sequences indicated as pathogenic are required to cause the disease. Modification of these sequences leads to a viroid that is nonpathogenic and can protect the plant from pathogenesis by the original viroid. Viroid RNA is replicated with cellular RNA polymerase, forming large multimeric structures of both positive and negative sense. Individual viroid RNA is released by RNA self-cleavage.

More than 20 viroids have been described infecting a wide variety of plant species. Many of these have great agricultural significance and are known to destroy fields of economically important crops. The actual mechanism of their pathogenesis is obscure but it clearly involves specific sequences within the viroid RNA, as there are examples where a viroid RNA with sequence very similar to a pathogenic one is not pathogenic and can provide some protection to the host plant. It has been postulated that pathogenic regions of the viroid RNA interact with one or more host factors, but this has not been demonstrated.

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