Potyvirus


Potyvirus is a genus of viruses in the family Potyviridae. Plants serve as natural hosts. There are currently 183 species in this genus including the type species Potato virus Y. The genus is named after the type virus. Potyviruses account for ~30% of the currently known plant viruses. Like begomoviruses, members of this genus may cause significant losses in agricultural, pastoral, horticultural and ornamental crops. More than 200 species of aphids spread potyviruses and most are from the subfamily Aphidinae.

Virology

The virion is non-enveloped with a flexuous and filamentous nucleocapsid, 680 to 900 nanometers long and is 11–20 nm in diameter. The nucleocapsid contains ~2000 copies of the capsid protein. The symmetry of the nucleocapsid is helical with a pitch of 3.4 nm.
The genome is a linear positive sense ssRNA ranging in size from 9000–12000 bases/nucleotides. Most potyviruses have non-segmented genomes, though a number of species are bipartite. The base composition is: 21–23.51–26% G; 23–30.15–44% A; 14.9–22.41–28% C; 15.6–24.41–30.9% U.
GenusStructureSymmetryCapsidGenomic arrangementGenomic segmentation
PotyvirusFilamentousNon-envelopedLinearMonopartite

In the species with a single genome, at the 5' end a protein is covalently linked. It encodes a single open reading frame expressed as a 350kDa polyprotein precursor. This is processed into seven smaller proteins: P1, helper component, P3, cylindrical inclusion, nuclear inclusion A, nuclear inclusion B, capsid protein and two small putative proteins known as 6K1 and 6K2. The P3 cistron also encodes a second protein—P3N-PIPO—which is generated by a +2 frameshift.

Molecular biology

Protein P1 is a serine protease.
HC is a protease that is also involved in aphid transmission. As a protease it cleaves a glycine-glycine dipeptide at its own C terminus. It also interacts with eukaryotic initiation factor 4. It acts as a viral RNA silencing suppressor.
The function of P3 is not known. It interacts with large subunit of the ribulose-1,5-bisphosphate carboxylase/oxygenase.
CI is an RNA helicase with ATPase activity. It is also involved in membrane attachment.
NIa is cleaved into a protease and the VPg protein.
NIb is an RNA-dependent RNA polymerase.
The functions of the 6K1 is not known. 6K2 protein, having a single trans membrane domain, is accumulating in the host cellular membranes and is thought to play a role in forming the replication vesicles of the virus.
The function of the P3N-PIPO is not known but it appears to be essential. It interacts with both the large and small subunits of the ribulose-1,5-bisphosphate carboxylase/oxygenase.
The capsid protein ranges between 30 and 35 kDa in weight.
The VPg protein interacts with eukaryotic initiation factor 4E. This interaction appears to be essential to viral infectivity.
Two proteinases, P1 and the helper component proteinase catalyse only autoproteolytic reactions at their respective C termini. The remaining cleavage reactions are catalysed by either trans-proteolytic or autoproteolytic mechanisms by the small nuclear inclusion protein. This latter protein is an evolutionary homology of the picornavirus 3C proteinase.

Life cycle

Replication may occur in the cytoplasm, nuclei, chloroplasts, Golgi apparatus, cell vacuoles or more rarely in unusual sites.
Potyviruses make proteinacous inclusions in infected plant cells. These may be crystals in either the cytoplasm or in the nucleus, as amorphous X-bodies, membranous bodies, viroplasms or pinwheels. The inclusions may or may not contain virions. These inclusions can be seen in the light microscope in leaf strips of infected plant tissue stained with Orange-Green but not Azure A. There are four different kinds of Potyvirus inclusions.
Replication follows the positive stranded RNA virus replication model. Positive-stranded RNA virus transcription is the method of transcription. Translation takes place by -1 ribosomal frameshifting. The virus exits the host cell by tubule-guided viral movement. Plants serve as the natural host. The virus is transmitted via a vector. Transmission routes are vector and mechanical.
GenusHost detailsTissue tropismEntry detailsRelease detailsReplication siteAssembly siteTransmission
PotyvirusPlants-Viral movement; mechanical inoculationViral movementCytoplasmCytoplasmMechanical inoculation: aphids

Evolution

The potyviruses evolved between 6,600 and 7,250 years ago. They appear to have evolved in southwest Eurasia or north Africa. The estimated mutation rate is about 1.15 nucleotide substitutions/site/year.

Geographical spread

Agriculture was introduced into Australia in the 18th century. This introduction also included plant pathogens. Thirty eight potyvirus species have been isolated in Australia. Eighteen potyviruses have been found only in Australia and are presumed to be endemic there. The remaining twenty appear to have been introduced with agriculture.

Species

Potyvirus contains the following species: