Bromoviridae

Joseph Bujarski, Donato Gallitelli, Fernando García-Arenal, Vicente Pallás, Peter Palukaitis, M. Krishna Reddy and Aiming Wang

Edited by F. Murilo Zerbini and Sead Sabanadzovic

Chapter contents

Citation

A summary of this ICTV Report chapter has been published as an ICTV Virus Taxonomy Profile article in the Journal of General Virology, and should be cited when referencing this online chapter as follows:

Joseph Bujarski, Donato Gallitelli, Fernando García-Arenal, Vicente B. Pallás, Peter Palukaitis, M Krishna Reddy, Aiming Wang and ICTV Report Consortium. 2019, ICTV Virus Taxonomy Profile: Bromoviridae, Journal of General Virology, 100, 1206–1207.

Summary

The Bromoviridae family of plant viruses contains tri-segmented, single-stranded, positive-sense RNA viruses with a total genome size of about 8 kb (Table 1.Bromoviridae). Genomic RNAs are packaged in separate virions that may also contain subgenomic, defective or satellite RNAs. Virions are variable in morphology (spherical or bacilliform) and are transmitted mechanically, in/on the pollen and non-persistently by insect vectors. Members of the family cause major disease epidemics in fruit, vegetable and fodder crops such as tomato, cucurbits, bananas, and alfalfa.

Table 1.Bromoviridae. Characteristics of members of the family Bromoviridae

Virion

Morphology

Virions are either spherical or quasi-spherical (Figure 1.Bromoviridae, panels A–D), having T=3 icosahedral symmetry, and a diameter of 26–35 nm (genera Anulavirus, Bromovirus, Cucumovirus and Ilarvirus) or bacilliform (genera Alfamovirus, Ilarvirus and Oleavirus) having (within a species) diameters of 18–26 nm and lengths of 30–85 nm, depending on the virus. Genomic RNAs are packaged in separate virions that may also contain subgenomic RNAs (sgRNAs), defective RNAs or satellite RNAs (Table 2.Bromoviridae).

Physicochemical and physical properties

The Mr of virions varies from 3.5 to 6.9×10⁶, depending on the nucleic acid content. The buoyant density of formaldehyde-fixed virions ranges from 1.35 to 1.37 g cm⁻³ in CsCl. The S_20,w varies from 63S to 99S. Virion integrity is dependent on RNA–protein interactions and virion RNA is susceptible to RNase degradation in situ at neutral pH. Heat inactivation occurs at 60 °C for members of some genera; this has not been tested for all genera. In some cases, virions are unstable in the presence of divalent cations. Virions are generally stable in the presence of chloroform, but not in the presence of phenol. Virions are unstable in the presence of strong anionic detergents such as SDS, but can tolerate low concentrations of mild detergents such as Triton X-100.

Nucleic acid

Total genome length is approximately 8 kb. Genomes consist of three linear, positive-sense ssRNAs with 5′-terminal cap structures. The 3′-termini are not polyadenylated, but generally are highly conserved within members of the same species or isolate, and form strong secondary structures. They are either tRNA-like and can be aminoacylated (genera Bromovirus and Cucumovirus) or form other structures that are not aminoacylated (genera Alfamovirus, Anulavirus, Ilarvirus and Oleavirus) (Table 2.Bromoviridae).

 DI / sat: defective interfering / satellite RNAs

* Complex secondary structure.

† Aminoacylatable, with pseudoknot folding.

Proteins

A single 20–24 kDa coat protein (CP) is expressed from a sgRNA. The CP is generally required for systemic movement and may be required for cell-to-cell spread in some cases. CP may be also required for virus expression and replication (genus Alfamovirus) or activation of replication (genus Ilarvirus). The phenomenon of genome activation has been demonstrated to cross group boundaries since the CP of an alfamovirus can activate the genome of an ilarvirus and vice versa. An intriguing feature of the alfamovirus CP is its accumulation in nucleus and nucleolus (Herranz et al., 2012)

Lipids

Lipids are not associated with virions.

Carbohydrates

Carbohydrates are not associated with virions.

Genome organization and replication

RNA1, RNA2, and RNA3 can act as mRNAs. The genomic RNA1 and RNA2 each encodes a single large ORF. Proteins encoded by these ORFs (1a and 2a) act with host factors as the viral replicase. In members of the genus Cucumovirus and in some members of the genus Ilarvirus (subgroups 1 and 2 only) a small 2b protein is expressed from an additional sgRNA that may or may not be encapsidated. These 2b proteins are involved in cell-to-cell movement, suppression of post-transcriptional gene silencing and symptom induction. The 3a protein is a movement protein and ORF3b encodes the CP, expressed from a sgRNA (Figure 2.Bromoviridae; Table 3.Bromoviridae).

Table 3.Bromoviridae. Features of the proteins encoded by members of the family Bromoviridae

* Functions of 1a and 2a are putative in most cases, by analogy to related viruses.

† The sgRNA for the CP derived from RNA3 is encapsidated in all but the genus Oleavirus.

There is no clear evidence of proteolytic or other post-translational processing. Virus replication occurs on cytoplasmic membranes via full-length negative-sense strand synthesis and subsequent plus (+) strand synthesis. The sgRNAs are synthesized from the (−) template, and may or may not be found in the virions. The CP accumulates to high levels in infected cells, whereas the nonstructural proteins accumulate to much lower levels. Virions accumulate in the cytoplasm. The replication cycle of the virus takes place predominantly in the cytoplasm, although in the case of Alfamovirus it has been demonstrated that a cytoplasmic/nuclear balance of CP accumulation accurately modulates viral expression (Pallas et al., 2013).

Biology

The natural host range of the viruses spans from very narrow (genera Bromovirus, Oleavirus) to extremely broad (genus Cucumovirus). Members of the family cause major disease epidemics in fruit, vegetable and fodder crops such as tomato (Figure 3.Bromoviridae), cucurbits, bananas, and alfalfa. Bromoviruses are transmitted mechanically, in/on the pollen, through seeds and non-persistently by insect vectors.

Antigenicity 

Native virions are generally poor immunogens and require stabilization with formaldehyde prior to use as antigens. There are few or no serological relationships between members of different genera, and weak relationships between members of the same genus.

Derivation of names

Alfamo: from alfalfa mosaic virus.

Anula: from Latin “anularis” (ring-shaped) for the concentric symptom associated with infection by this virus.

Bromo: from Brome mosaic, also, from Bromus (genus of the host of brome mosaic virus).

Cucumo: from cucumber mosaic virus.

Ilar: from isometric labile ringspot.

Olea: from the genus name of the host, olive (Olea).

Genus demarcation criteria

Genera in the family have been designated on the basis of natural host range, mode of transmission, particle morphology and  physicochemical properties, genome structure and replication strategies and abilty to support replication of defective RNAs and satellite RNAs.

Relationships within the family

Relationships within this family have been examined using the complete genomic sequence, genes and gene products. The three genera with multiple members (Bromovirus, Cucumovirus and Ilarvirus) form clades that are distinct. The genera Anulavirus and Oleavirus are unique and distinct from other genera within the family. A longstanding debate as to whether alfalfa mosaic virus (AMV, genus Alfamovirus) should be regarded as a member of the genus Ilarvirus has yet to be resolved. Ilarviruses share many features at the molecular level with AMV, including the activation of ilarvirus replication by AMV CP and vice versa. A major difference between AMV and ilarviruses is the mode of transmission. AMV is transmitted non-persistently by aphids whereas ilarviruses are transmitted through pollen and feeding of thrips. Studies defining recombination events in the genomes of members of the Bromoviridae suggest that AMV should be grouped with the ilarviruses (Pallas et al., 2013).

Relationships with other taxa

Bromovirids are members of the “alpha-like” supergroup, sharing sequence similarity in the 1a protein domains for methyltransferase and helicase activities, and in the 2a protein polymerase domain with plant viruses belonging to the family Virgaviridae and the order Tymovirales, and with animal viruses in the family Togaviridae. The 3a proteins of bromoviruses and the 35 kDa protein of members of the genus Dianthovirus (red clover necrotic mosaic virus, RCNMV) form a distinct “family” of movement-associated proteins. Raspberry bushy dwarf virus (RBDV, genus Idaeovirus) and the newly recognized idaeovirus privet leaf blot associated virus (PrLBaV) are similar to bromovirids in genome organization and in the sequence of certain genes. The CP of RBDV and PrLBaV have RNA-binding domains like those found in the CP of AMV and some members of the genus Ilarvirus and involved in genome activation (Navarro et al., 2017). Like the members of the genus Ilarvirus, RBDV is transmitted in association with pollen whereas no natural routes of transmission are known for PrLBaV.

Related, unclassified viruses

Virus name	Accession number	Virus abbreviation
grapevine angular mosaic virus	AY590305	GAMV

Virus names and virus abbreviations are not official ICTV designations.