Cucumoviruses are transmitted in a non-persistent manner by over 80 species of aphids belonging to more than 30 genera. RNA2 is bicistronic producing a 2b protein associated with suppression of post-transcriptional gene silencing.
Virions are icosahedral, of uniform size and sedimentation properties (Figure 1.Bromoviridae, panels C and F). In electron micrographs they appear to have electron dense centers (Figure 1C. Bromoviridae).
Physicochemical and physical properties
Purified virions are labile and especially susceptible to anionic detergents and high ionic strength buffers that disrupt the RNA-protein interactions required for particle integrity. Most strains are unstable in the presence of Mg2+, but at least one strain of cucumber mosaic virus (CMV) requires Mg2+ for stability. Extinction coefficient at 260 nm: 5.0.
Within each species, all members have RNA 3′-termini that are highly similar and can form tRNA-like structures that are aminoacylatable with tyrosine; the 5′-untranslated regions of RNA1 and RNA2 are also very similar to each other. At least two strains of CMV (CMV-Fny and CMV-M) can form defective RNAs that arise by deletions in the 3a ORF of RNA3. Subgroup II strains of CMV encapsidate the subgenomic RNA for the 2b ORF, called RNA4A, and an additional small RNA of about 300 nt, called RNA5, that is co-terminal with the 3′-ends of RNA3 and RNA4. In addition, CMV may harbor satellite RNAs of about 330 to 400 nt. The satellite RNAs are more common under experimental conditions than in field conditions and may dramatically alter the symptoms of infection by the helper virus in certain hosts like tomato.
Virions contain a coat protein of 24.5 kDa.
Genome organization and replication
The genome is organized as depicted in Figure 2C.Bromoviridae. Coat protein is not required for activation of the genome. An additional ORF, the 2b ORF, has been shown to be active in CMV, tomato aspermy virus (TAV) and peanut stunt virus (PSV). There is evidence for recombination among natural CMV isolates at low frequency (Bonnet et al., 2005, Nouri et al., 2014) as well as inter-strain recombination in CMV reassortants (Pita and Roossinck 2013). The control elements of recombination frequency reside predominantly within the 2a gene (Pita et al., 2015).
CMV has an extremely broad host range, infecting plants in 85 families and more than 1000 species under experimental conditions (Yoon et al., 2019). Other cucumoviruses have narrower host ranges; PSV is largely limited to legumes and solanaceous hosts, while TAV predominantly infects composites and solanaceous plants. All species are transmitted by aphids in a non-persistent manner. Recent evidence expands the CMV host range to other kingdoms to now include oomycetes (kingdom Stramenopila), ascomycetes and basidiomycetes (kingdom Fungi) (Andika et al., 2017, Mascia et al., 2018). CMV is prone to recombination and shows high genetic diversity among isolates so far sequenced. However, the presence of reassortants and recombinants is not frequent in CMV populations isolated within plants, in a field or in nearby wild plants suggesting that at least some combinations are not favored under natural infection. Local low genetic diversity correlates positively with severe bottlenecks occurring during both virus movement within a plant and transmission between plants (Escriu et al., 2007, Jacquemond 2012).
CMV has been divided into two subgroups (I and II) based on serology. PSV also has more than one serological group. Sequence analysis has upheld the divisions, although subgroup I can be further divided into two groups (IA and IB) by phylogenetic analyses.
Species demarcation criteria
Criteria used for demarcation of species within the genus are serological relatedness and complete functional interaction of the 1a and 2a replicase proteins (but these distinctions may break down in the case of naturally occurring reassortants), and sequence similarity.
Serology and nucleotide sequence similarity is used to distinguish subgroups within a species; subgroups generally have at least 65% whole genome sequence identity.