Family: Bromoviridae

Genus: Ilarvirus

Distinguishing features

Ilarviruses are transmitted mechanically by thrips feeding on pollen grains containing the virus or by pollen grains carrying the virus on their surface. RNA2 of members of subgroups 1 and 2 is bicistronic producing a 2b protein, which for asparagus virus 2 is proven to be a suppressor of post-transcriptional gene silencing (Shimura et al., 2013).

Virion

Morphology

Virions of most members of the genus have quasi-isometric (26–36 nm in diameter) or occasionally bacilliform particles of four different size classes (Figure 1B. Bromoviridae). Bacilliform particles occur in plants infected by prune dwarf virus (PDV), Tulare apple mosaic virus (TAMV), Prunus necrotic ringspot virus (PNRSV), apple mosaic virus (ApMV) and spinach latent virus (SpLV). Cryoelectron microscopy of preparations of tobacco streak virus (TSV), a member of the species Tobacco streak virus, reveals isometric particles permeated by numerous pores, which may explain the lability of these viruses (Figure 1E.Bromoviridae) (Pallas et al., 2013).

Nucleic acid

There is a short region of sequence similarity between the different RNA segments at their 3′-ends.

Proteins

Virions contain a coat protein (CP) of 24.5 kDa.

Genome organization and replication

The genome is organized as depicted in Figure 2A.Bromoviridae for ilarviruses of subgroups 3 and 4 and Figure 2C.Bromoviridae for ilarviruses of subgroups 1 and 2. The CP is required for activation of replication, but may be substituted with CP from alfalfa mosaic virus (genus Alfamovirus).

Biology

Ilarviruses mainly infect woody plants. Viruses are present in/on pollen and may be transmitted when wind-blown pollen and populations of vector thrips are coincident on a susceptible host.

Antigenicity

Virions are “unpromising subjects for the raising of good antisera” (Bujarski et al., 2012). Subgroups within the genus are defined based on the presence of serological relationships among some, but not all, members of each subgroup. Sequence data support many of the serological relationships but also show potential relationships among viruses not previously demonstrated as related. Serological relationships between viruses in different subgroups may be associated with conserved amino acids and secondary structures. Monoclonal antibodies and antibodies produced against recombinant proteins (Abou-Jawdah et al., 2004, Menzel et al., 2012) have addressed some of the problems associated with the serological detection of ilarviruses (Pallas et al., 2013).

Species demarcation criteria

Criteria used for demarcation of species within the genus are serology, host range and sequence similarity, although specific levels of sequence similarity have not been defined.
Ilarviruses were initially grouped on the basis of serological relationships. Sequence data have confirmed some of these groupings but have also shown that some species were grouped inappropriately (Pallas et al., 2013).

Related, unclassified viruses

Virus names and virus abbreviations are not official ICTV designations.