Type Species

Pothos latent virus

(PoLV)

Distinguishing Features

The virion is a 30 nm icosahedron that packages the 4.4 kb genomic RNA. The RNA contains four ORFs. The CP ORF is located internally in the genomic RNA and is expressed in vivo from a 2 kb sgRNA. ORFs 3 and 4 are 3-proximal and ORF4 is contained within ORF3, in a different reading frame. Both ORFs are expressed from a second 0.8 kb sgRNA. The genome organization and expression strategy are identical to those of viruses in genus Tombusvirus. While conserved, the polymerases of the two genera are no more closely related than with even the least conserved genus in Tombusviridae. The aureusvirus ORF3 is significantly larger and ORF4 significantly smaller than those in the genus Tombusvirus. The virus is soil-transmitted without the aid of a biological vector.

Virion Properties

Morphology

Virions are isometric with a rounded outline, a knobby surface and a diameter of approximately 30 nm. Based on comparative amino acid sequence alignments, the CP subunits of Pothos latent virus (PoLV) appear to be made up of three structural domains, i.e., the N-terminal internal domain, the shell domain and the C-terminal protruding domain.

Physicochemical and Physical properties

Preparations of purified virus sediment as a single component in sucrose density gradients, and to equilibrium in solutions of CsCl and Cs₂SO₄. Buoyant density in CsCl is 1.36 g/cm³. The thermal inactivation point is above 80°C. Virus particles resist organic solvents but are readily disrupted by sodium dodecyl sulphate.

Nucleic Acid

Virions contain a single molecule of linear, uncapped, non-polyadenylated, positive sense ssRNA of 4415 nts, constituting 17% of the particle weight. Virions can contain two sgRNAs 2.0 and 0.8 kb in size. Three dsRNA species corresponding to the full-size genomic RNA and the two sgRNAs can be recovered from infected plants. Satellite or defective interfering (DI) RNAs do not occur naturally, nor does PoLV genomic RNA support the replication of tombusvirus satellite or DI RNAs.

Proteins

Virions possess a single CP species of Mr 40 10³. It is predicted that the virion is formed from 180 copies of the CP.

Lipids

None reported.

Carbohydrates

None reported.

Genome Organization and Replication

The viral genome contains four ORFs (Fig. 3). ORF1 encodes a protein of Mr 25 10³. The readthrough of its amber stop codon results in translation into ORF1-RT yielding a Mr 84 10³ protein possessing the conserved motifs of RNA-dependent RNA polymerases. ORF2 encodes the Mr 40 10³ CP. ORF3 and 4 are nested in different reading frames. The Mr 27 10³ product of ORF3 is the movement protein and the Mr 14 10³ product of ORF4 is responsible for symptom severity. CP is important in regulating the synthesis of the Mr 14 10³ protein, the excess production of which is lethal to infected plants. In vitro translation of genome-length RNA transcribed from an infectious full-length cDNA clone, yields only one protein Mr 25 10³ in size. Translation of the 2.0 kb and 0.8 kb sgRNAs gives rise to the Mr 40 10³ CP and the Mr 27 10³ and Mr 14 10³ proteins, respectively. Replication may occur in the cytoplasm, possibly in association with nucleus-derived vesicles and vesiculated bodies, i.e., globose aggregates of vesicular elements surrounded by a unit membrane. The strategy of replication includes readthrough and sgRNA production. Virus particles assemble and accumulate in the cytoplasm.

Antigenic Properties

Virions are efficient immunogens. Polyclonal antisera yield a single precipitin line in immunodiffusion tests and uniformly decorate virus particles. Distant relationships were found with members of the genera Tombusvirus and Carmovirus.

Biological Properties

Host Range

Pothos (Scindapsus aureus), the only known natural host, is infected symptomlessly. The experimental host range is moderately wide. Localized infections are induced in most hosts, except for Nicotiana benthamiana and N. clevelandii, which are systemically invaded.

Transmission

PoLV is readily transmitted by mechanical inoculation. Natural transmission occurs through the soil or the circulating solution in hydroponics, apparently without the intervention of a vector.

Geographical Distribution

Reported only from Southern Italy.

Cytopathic Effects

PoLV is very invasive in systemically infected plants and is found in parenchyma and conducting tissues. Virus particles often form intracellular crystalline aggregates. Distinctive cytopathological features are the extensive vesiculation of the nuclear envelope and the single-membrane vesiculated bodies in the cytoplasm.

List of Species Demarcation Criteria in the Genus

Not applicable.

List of Species in the Genus

Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names, genome sequence accession numbers [ ], and assigned abbreviations ( ) are:

Species in the Genus

Pothos latent virus

[X87115]

(PoLV)

Tentative Species in the Genus

None reported.