Family: Avsunviroidae

Genus: Pelamoviroid

Distinguishing features

Three species, Pelamoviroid latenspruni, Pelamoviroid maculachrysanthemi, and Pelamoviroid malleusmali are included in this genus, whose members have circular genomic RNAs between 337 and 434 nt depending on the isolates and sequence variants. The most stable secondary structure is a branched conformation (Ambrós et al., 1998), stabilized by a kissing-loop interaction in the (+) strand (Bussière et al., 1998, Gago et al., 1998, Serra et al., 1998) (Figure 1 Avsunviroidae). Similar structures have been proposed to occur both in vitro and in vivo (Gago et al., 1998, Giguère et al., 1998, Serra et al., 1998). An additional element of tertiary structure has been reported in the (+) strand of peach latent mosaic viroid (PLMVd) (Hernández et al., 1998). RNAs are insoluble in 2 M LiCl. Thermodynamically stable single-hammerhead structures can be formed in both strands, which self-cleave in vitro and in vivo as predicted by these structures (Figure 2 Avsunviroidae). Replication occurs by a symmetric rolling-circle mechanism since the monomeric circular (−) RNA has been identified in infected tissue. PLMVd and, most likely, chrysanthemum chlorotic mottle viroid (CChMVd) and apple hammerhead viroid (AHVd) replicate in plastids, mostly chloroplasts. 

Biology

PLMVd, AHVd and CChMVd appear to be restricted to their natural hosts, but some reports indicate that PLMVd naturally infects additional species within and outside the genus Prunus (Kyriakopoulou et al., 2017). PLMVd and CChMVd have been recorded in many peach- and chrysanthemum-growing areas, respectively. Most isolates of PLMVd are non-symptomatic or incite symptoms of leaf chlorosis in peach trees (Flores et al., 2017), but others containing a specific pathogenic determinant incite an extreme albinism (peach calico) (Malfitano et al., 2003, Rodio et al., 2003) by silencing a host mRNA via two 21-nt viroid-derived small RNAs resembling functionally host microRNAs (Navarro et al., 2003, Delgado et al., 2003). Symptomatic and non-symptomatic isolates of CChMVd differ in the sequence of a hairpin tetraloop, wherein the pathogenic determinant has been mapped (de la Peña et al., 1999). Association with symptoms of AHVd is unclear (Serra et al., 1999). 

Species demarcation criteria

Viroids with similar molecular features (multi-branched conformations stabilized by a kissing-loop interaction in the (+) strands, and thermodynamically stable hammerhead ribozymes) but with less than 73% sequence identity should be classified as different species. Evidence of infectivity in the absence of a helper virus should be provided (Chiumenti et al., 1999).