Tobamovirus

Tobamovirus is a genus of positive-strand RNA viruses in the family Virgaviridae.[1] Many plants, including tobacco, potato, tomato, and squash, serve as natural hosts. Diseases associated with this genus include: necrotic lesions on leaves.[1][2] The name Tobamovirus comes from the host and symptoms of the first virus discovered (Tobacco mosaic virus).[3]

Tobamovirus
Tobacco mosaic virus (TMV) diagram and electron micrograph of virions
Virus classification
(unranked): Virus
Realm: Riboviria
Kingdom: Orthornavirae
Phylum: Kitrinoviricota
Class: Alsuviricetes
Order: Martellivirales
Family: Virgaviridae
Genus: Tobamovirus
Type species
Tobacco mosaic virus

There are four informal subgroups within this genus: these are the tobamoviruses that infect the brassicas, cucurbits,malvaceous, and solanaceous plants. The main differences between these groups are genome sequences, and respective range of host plants. There are currently 37 species in this genus including the type species Tobacco mosaic virus.[4]

Structure

Tobamoviruses are non-enveloped, with helical rod geometries, and helical symmetry. The diameter is around 18 nm, with a length of 300–310 nm. Genomes are linear and non-segmented, around 6.3–6.5kb in length.[1][2]

Genome

The RNA genome encodes at least four polypeptides:[1] these are the non-structural protein and the read-through product which are involved in virus replication (RNA-dependent RNA polymerase, RdRp); the movement protein (MP) which is necessary for the virus to move between cells and the coat protein (CP). The read-through portion of the RdRp may be expressed as a separate protein in TMV.[5] The virus is able to replicate without the movement or coat proteins, but the other two are essential. The non-structural protein has domains suggesting it is involved in RNA capping and the read-through product has a motif for an RNA polymerase. The movement proteins are made very early in the infection cycle and localized to the plasmodesmata, they are probably involved in host specificity as they are believed to interact with some host cell factors.

Life cycle

Viral replication is cytoplasmic. Entry into the host cell is achieved by penetration into the host cell. Replication follows the positive stranded RNA virus replication model. Positive stranded RNA virus transcription is the method of transcription. Translation takes place by suppression of termination. The virus exits the host cell by monopartite non-tubule guided viral movement. Plants serve as the natural host. Transmission routes are mechanical.[1][2]

Routes of infection

The infection is localized to begin with but if the virus remains unchallenged it will spread via the vascular system into a systemic infection. The exact mechanism the virus uses to move throughout the plant is unknown but the interaction of pectin methylesterase, a cellular enzyme important for cell wall metabolism and plant development, with the movement protein has been implicated.[6]

Evolution

These viruses are thought to have codiverged with their hosts from a common ancestor.[7] There are at least 3 distinct clades of tobamoviruses, which to some extent follow their host ranges: that is, there is one infecting solanaceous species; a second infecting cucurbits and legumes and a third infecting the crucifers.[8]

Classification

Species
Phylogenetic tree of Tobamovirus

Proposed members

Proposed, but currently unrecognised members of the genus include:[10]

  • Chara corallina virus (CCV)
  • Nicotiana velutina mosaic virus (NVMV)

References
  1. "ICTV Online Report Virgaviridae".
  2. "Viral Zone". ExPASy. Retrieved 15 June 2015.
  3. https://talk.ictvonline.org/ictv/proposals/Ratification_1975.pdf
  4. ICTV. "Virus Taxonomy: 2014 Release". Retrieved 15 June 2015.
  5. Creager AN, Scholthof KB, Citovsky V, Scholthof HB (March 1999). "Tobacco mosaic virus. Pioneering research for a century". The Plant Cell. 11 (3): 301–8. doi:10.1105/tpc.11.3.301. PMC 1464663. PMID 10072391.
  6. Chen MH, Citovsky V (August 2003). "Systemic movement of a tobamovirus requires host cell pectin methylesterase". The Plant Journal. 35 (3): 386–92. doi:10.1046/j.1365-313X.2003.01818.x. PMID 12887589.
  7. Stobbe AH, Melcher U, Palmer MW, Roossinck MJ, Shen G (February 2012). "Co-divergence and host-switching in the evolution of tobamoviruses". The Journal of General Virology. 93 (Pt 2): 408–18. doi:10.1099/vir.0.034280-0. PMID 22049092.
  8. Lartey RT, Voss TC, Melcher U (December 1996). "Tobamovirus evolution: gene overlaps, recombination, and taxonomic implications" (PDF). Molecular Biology and Evolution. 13 (10): 1327–38. doi:10.1093/oxfordjournals.molbev.a025579. PMID 8952077.
  9. Min BE, Chung BN, Kim MJ, Ha JH, Lee BY, Ryu KH (January 2006). "Cactus mild mottle virus is a new cactus-infecting tobamovirus". Archives of Virology. 151 (1): 13–21. doi:10.1007/s00705-005-0617-7. PMID 16132178. S2CID 40760030.
  10. "Descriptions of Plant Viruses: Tobamovirus Group". The Association of Applied Biologists (AAB).

Further reading
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