Family: Bromoviridae

Genus: Alfamovirus

Distinguishing features

Alfalfa mosaic virus (AMV) is transmitted in a non-persistent manner by at least 20 aphid species. AMV shares many properties at the molecular level with members of the genus Ilarvirus, which are not transmitted by aphids.

Virion

Morphology

Virions are generally bacilliform, the structure of which is based on T=1 icosahedral symmetry having a constant diameter of 18 nm, and varying from 30 to 57 nm in length, depending on the size of the nucleic acid encapsidated (Figure 1A.Bromoviridae).

Physicochemical and physical properties

The Mr values of virions are from 3.5–6.9×10⁶. Virions can be separated into six components on sucrose density gradients. The three larger components, named bottom (B), middle (M), and top b (Tb), in order of decreasing size, are essential for infection whereas the two or three additional top components (Ta, To, and Tz) are made up of spherical particles and are not necessary for infection.

The sedimentation coefficients (S_w20) of the three larger components are: 94 S (B), 82 S (M) and 73 S (Tb).

The virus is unstable in CsCl but all components sediment in Cs₂SO₄ density gradients at a mean density of 1.278 g/cm³  (Hull 1976).

Extinction coefficient E^0.1% =5.1–4.7  

Nucleic acid

The genome of alfalfa mosaic virus (AMV) is divided into three species of linear, positive-sense ssRNA of 3.2 kb (RNA1, encapsidated in the B component), 2.6 kb (RNA2, encapsidated in the M component), and 2.1 kb (RNA3, encapsidated in the Tb component). Subgenomic RNA4 (0.8 kb) containing CP coding ORF is packaged separately in the Ta component. RNA1 and RNA2 encode the replicase subunits P1 and P2, respectively, whereas RNA3 encodes the movement protein (MP) and serves as a template for the synthesis of the non-replicating subgenomic RNA4 (sgRNA4), which encodes the CP (Bol 2005).

Proteins

Virions contain a single CP of 24.3 kDa, which in addition to virion formation is involved in the regulation of replication and translation, and in systemic movement. 

Genome organization and replication

The genome is organized as depicted in Figure 2A.Bromoviridae. Replication is activated by CP binding to the complex 3′-end structure. CP (or RNA4) from members of the genus Ilarvirus can also activate AMV replication (van Vloten-Doting 1975, Gonsalves and Fulton 1977).  

Biology

The host range is very broad. Virus is transmitted by aphids in a non-persistent manner and is also reported to be transmitted through seeds (Frosheiser 1974, Hemmati and McLean 1977) and pollen.

Antigenicity

Native virions are generally poor immunogens and require stabilization with formaldehyde prior to use as antigens. 

Species demarcation criteria

Not applicable as there is only a single species in the genus.