Family: Pospiviroidae

Genus: Pospiviroid

Distinguishing features

Circular ssRNA genomes between 356 and 375 nt (excluding duplications) depending on species and sequence variants. The most stable secondary structure is a rod-like or quasi-rod-like conformation (Figure 1. Pospiviroidae) with five domains, a characteristic central conserved region (CCR) identical to that of the potato spindle tuber viroid (PSTVd), the reference variant of the type species, and the terminal conserved region (TCR) (Figure 3. Pospiviroidae). Columnea latent viroid (CLVd) is classified in the genus Pospiviroid but with a genome containing the TCR and a CCR identical to that of members of the genus Hostuviroid, and so only partially fulfills these criteria.  

Biology

Members of the genus Pospiviroid are found world-wide and infect a broad range of plants, mostly solanaceous species but also citrus, avocado and some ornamentals. They can be experimentally transmitted to many other hosts in which symptom expression ranges from symptomless to almost lethal. In field or greenhouse, members of the genus Pospiviroid are readily transmitted by vegetative propagation of infected plants, by cutting tools and by mechanical inoculation. Seed transmission has been shown for several members of the genus, including PSTVd (Matsushita et al., 2011, Singh 1970), citrus exocortis viroid (CEVd) (Wan Chow Wah and Symons 1999, Singh and Dilworth 2009), pepper chat fruit viroid (PCFVd) (Verhoeven et al., 2009) and tomato apical stunt viroid (TASVd) (Antignus et al., 2007). However, failure of seed transmission has also been reported (Faggioli et al., 2015, Verhoeven et al., 2020). Horizontal transmission through infected pollen has been documented for PSTVd, tomato planta macho viroid (TPMVd) and chrysanthemum stunt viroid (CSVd) (Kryczyński et al., 1988, Singh et al., 1992, Yanagisawa and Matsushita 2018). For TPMVd horizontal transmission has also been observed in the absence of fertilization (Yanagisawa and Matsushita 2018). Some members of the genus, particularly TPMVd, can be persistently transmitted by aphids under specific ecological conditions (Galindo et al., 1989). PSTVd is also aphid-transmissible when trans-encapsidated in particles of potato leafroll virus (Querci et al., 1997), with the virion acting only as a carrier of the viroid RNA (Syller et al., 1997). TASVd and tomato chlorotic stunt viroid (TCDVd) have been reported to be transmitted in greenhouses by bumblebees (Bombus ignites), possibly through the transfer of viroid-contaminated pollen during insect-mediated pollination (Antignus et al., 2007, Matsuura et al., 2010). In contrast, no transmission of PSTVd by bumblebees, honeybees or thrips has been observed (Nielsen et al., 2012).  

Species demarcation criteria

Viroids with molecular features (rod-like conformations, CCR, TCH) identical to that of members of the type species of the genus, less than 90% sequence identity and distinct biological properties (i.e. differential host range, movement and distribution within the host, differential fitness in competition assays, seed transmission) with respect to the other members of the genus, are classified in different species (Di Serio et al., 2014). 

Related, unclassified viroid-like RNAs

Virus names and virus abbreviations are not official ICTV designations.