Abstract
Viroids are small, single-stranded, circular RNAs infecting plants. Composed of only a few hundred nucleotides and being unable to code for proteins, viroids represent the lowest level of complexity for an infectious agent, even below that of the smallest known viruses. Despite the relatively small size, viroids contain RNA structural elements embracing all the information needed to interact with host factors involved in their infectious cycle, thus providing models for studying structure-function relationships of RNA. Viroids are specifically targeted to nuclei (family Pospiviroidae) or chloroplasts (family Avsunviroidae), where replication based on rolling-circle mechanisms takes place. They move locally and systemically through plasmodesmata and phloem, respectively, and may elicit symptoms in the infected host, with pathogenic pathways linked to RNA silencing and other plant defense responses. In this review, recent advances in the dissection of the complex interplay between viroids and plants are presented, highlighting knowledge gaps and perspectives for future research.
Highlights
Viroids are the simplest known infectious agents
Members of the family Pospiviroidae [representative member potato spindle tuber viroid (PSTVd)] replicate and accumulate in the nucleus and contain a central conserved region (CCR) [4], while those of the family Avsunviroidae [representative member avocado sunblotch viroid (ASBVd)] replicate and accumulate in chloroplasts and are endowed with self-cleaving activity mediated by ribozymes [5]
Being exclusively composed of RNA unable to code for proteins, viroids provide a unique model to address structural-functional relationships of noncoding RNAs
Summary
Viroids are the simplest known infectious agents. Composed exclusively of a small (246–434 nucleotides), circular, single-stranded RNA molecule, viroids replicate and move systemically in host plants, in which they may induce symptoms. Viroids are pathogens affecting several agricultural crops [1]. Since their discovery about 50 years ago [2, 3], viroids with different biological, structural, and biochemical features have been identified and classified in two families (Figure 1). Recent reviews have addressed in detail different aspects related to the structure [6], cytopathology [7], pathogenesis [8], and host interactions [9, 10] of these infectious RNAs. Here, we present a general overview of these intracellular molecular parasites at frontiers of life, highlighting recent advances and knowledge gaps, which represent promising topics for future investigations
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