VIROIDS AND VIROID DISEASES OF PLANTS

Viroids, in spite of their name that hints at a relationship with viruses, differ from them in fundamental aspects that include structure, function, and evolutionary origin. Viroids are the smallest replicons described so far, being exclusively composed of small (in the range of 250-430 nt) circular RNA (Diener 2003; Flores et al. 2005, 2015; Tsagris et al. 2008; Ding 2009; Kovalskaya and Hammond 2014; Hadidi et al. 2017; Steger and Riesner 2018), whereas the genome of a typical plant virus like tobacco mosaic virus (TMV) is a linear RNA of about 6000nt (Goelet et al. 1982). Moreover, virus genomes (either DNA or RNA) encode at least one (and most frequently several) proteins that mediate their replication, movement, and suppression of the host antiviral response, while viroids are non-protein-coding RNAs. Consequently, RNA viruses and viroids need to parasitize primarily the translation and transcription apparatus of their hosts, respec­ tively. Finally, RNA viruses and viroids have independent evolutionary lineages, with the lat­ ter being considered remnants of the "RNA world" that presumably preceded our present world based on DNA and proteins (Diener 1989; Flores et al. 2014), although other alternative and more recent scenarios have been discussed (Diener 2016; De la Pen?a and Cervera 2017). From an ecological standpoint, viruses have been found infecting all cell types, from myco­ plasmas and prokaryotes (bacteria and archaea) to eukaryotic cells, in contrast to viroids that have been described so far only in vascular plants (in monocots and dicots), where they may incite symptoms similar to those character­ istic of virus infections. Actually, potato spindle tuber viroid (PSTVd), the first viroid reported, was identified when searching for the virus pre­ sumed to cause a potato disease (Diener and Raymer 1967; Diener 1971, 1972), and soon after that, a second one, citrus exocortis viroid (CEVd) (Semancik and Weathers 1972) was discovered. The similar phenotypic effects induced by viruses and viroids may just reflect a lack of specificity in the ultimate macroscopic response of their hosts, although the possibility that they may affect the same host defensive response, particularly RNA silencing (see below), cannot be dismissed. In this chapter, we will first present the molecular properties of viroids and their diversity, and then move on to describe the interactions of these unique biological entities with their environment and, principally, with their hosts.