Viroids and Satellites and Their Vector Interactions—This Special Issue Is Dedicated to the Memory of Theodor O. Diener Who Discovered Viroids

Dear Colleagues,

Many diseases of unknown etiology with symptoms like those caused by plant viruses but for which no virions could be found were described during the early and mid-20th century. In 1971, T.O. Diener demonstrated that the causal agent of potato spindle disease, first described in the early 1920s, is a free RNA of 25,000–110,000 Daltons, which is much smaller than a viral genome, and that no viral coat proteins were synthesized in infected plants. He concluded that the RNA is too small to contain the genetic information necessary for self-replication and that it must rely on host enzymes for its replication. In 1972, Joseph S. Semancik and L. G. Weathers reported similar findings for the causal agent of citrus exocortis disease. For such an unconventional agent, the term viroid, suggested by Diener, was adopted in 1972. Diener remarked that the term viroid had earlier been put forward by Altenburg in 1946 to describe possible “...symbionts akin to the viruses...” in animal cells. Viroids are small, covalently closed circular or linear highly structured RNA molecules with a high degree of base paring, replicate autonomously, do not encode proteins, and use pre-existing host-cell RNA polymerase and processing enzymes for replication and pathogenesis. Viroids belong to the order of subviral agents, which currently includes two families, eight genera, and 33 species that have been biologically and molecularly characterized. 

The natural host range of viroids includes vegetables, field and ornamental crops, fruit and palm trees, and grapevines. Recently, it has been shown that apple scar skin viroid naturally infects several species of filamentous fungi in viroid-infected apple trees.  Moreover, under laboratory conditions, few viroids replicated in filamentous fungi. The term mycoviroids was coined for viroids that infect plant pathogenic fungi.

Viroids have had an impact on the science of virology, plant pathology, botany, microbiology, genetics, genomics, molecular biology, and molecular evolution as they represent the leaders of life as they are infectious agents consisting of only 234‒401 ribonucleotides.

In 1961, Basil Kassanis coined the term satellite for the very small virus particles that he found in some cultures of tobacco necrosis virus. He demonstrated that these spherical 17-nm viral satellite particles were unable to replicate in the absence of the larger 30-nm TNV particles. In 1972, Irving R. Schneider described that an RNA dependent on a helper virus for replication and movement was found in association with tobacco ringspot virus and was named “satellite RNA”. In 1972, a devastating epidemic disease swept through the tomato fields of the French Alsace province destroying the crop. The causal agent was not revealed until 1977, when Jacobus M. Kaper clearly demonstrated that RNA 5 of cucumber mosaic virus (CMV) was required to cause the Alsace disease. Because RNA 5 alone cannot cause the disease but needed other CMV RNAs for replication, this CMV RNA was recognized as the first RNA satellite to cause a plant disease. The DNA satellites of geminiviruses were described about two decades later. The first one identified in 1997 was found to be a defective form of a larger satellite designated DNA beta in 2000, one of many since described and now referred to as the Betasatellites of Begomoviruses.

In general, with a few exceptions, most satellites are considered unique and different from either sub-genomic RNAs or viral defective interfering RNAs/DNAs with sequences identical to and derived from part(s) of the viral genome. Historically, satellites were not necessary for the replication of their helper viruses and were thus considered completely redundant. This view, however, has changed to some extent after more than two decades, when satellite-like RNAs were found to be required for their helper viruses’ specific movement within the infected host plant, plant-to-plant transmission, or plant-to-vector transmission.

Alphasatellites of both geminiviruses and nanoviruses are small circular DNAs that depend on their associated viruses for encapsidation, movement, and transmission; however, they are capable of autonomous replication. Hence, they are not true satellites and will not be covered.

Dr. Ahmed Hadidi
Prof. Dr. Henryk Hanokh Czosnek
Prof. Dr. Kriton Kalantidis
Prof. Dr. Peter Palukaitis
Guest Editors

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