Tobamovirus


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

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.

Genome

The RNA genome encodes at least four polypeptides: these are the non-structural protein and the read-through product which are involved in virus replication ; the movement protein which is necessary for the virus to move between cells and the coat protein. The read-through portion of the RdRp may be expressed as a separate protein in TMV. 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.

Enzymes

Tobamovirus RNA-dependent RNA polymerases are homologous with bromoviruses, ilarviruses, tobraviruses, and the carnation mottle virus.

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.

Evolution

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.

Classification

Species

The genus contains the following species, listed by scientific name and followed by their common names:Tobamovirus anthocercis, Yellow tailflower mild mottle virusTobamovirus brugmansiae, Brugmansia mild mottle virusTobamovirus cacti, Cactus mild mottle virus Tobamovirus capsici, Pepper mild mottle virus Tobamovirus clitoriae, Clitoria yellow mottle virusTobamovirus crotalariae, Sunn-hemp mosaic virus Tobamovirus cucumeris, Cucumber mottle virusTobamovirus cucurbitae, Zucchini green mottle mosaic virusTobamovirus fortpiercense, Hibiscus latent Fort Pierce virus Tobamovirus frangipani, Frangipani mosaic virus Tobamovirus fructirugosum, Tomato brown rugose fruit virus Tobamovirus kyuri, Kyuri green mottle mosaic virusTobamovirus latentabaci, Tobacco latent virusTobamovirus maculacapsici, Bell pepper mottle virus Tobamovirus maculafructi, Cucumber fruit mottle mosaic virusTobamovirus maculatessellati, Tomato mottle mosaic virusTobamovirus maracujae, Maracuja mosaic virus Tobamovirus mititessellati, Tobacco mild green mosaic virusTobamovirus muricaudae, Rattail cactus necrosis-associated virus Tobamovirus obudae, Obuda pepper virus Tobamovirus odontoglossi, Odontoglossum ringspot virus Tobamovirus opuntiae, Opuntia chlorotic ringspot virusTobamovirus paprikae, Paprika mild mottle virusTobamovirus passiflorae, Passion fruit mosaic virusTobamovirus plantagonis, Ribgrass mosaic virus Tobamovirus plumeriae, Plumeria mosaic virusTobamovirus rapae, Turnip vein-clearing virus Tobamovirus rehmanniae, Rehmannia mosaic virusTobamovirus singaporense, Hibiscus latent Singapore virus Tobamovirus streptocarpi, Streptocarpus flower break virusTobamovirus tabaci, Tobacco mosaic virus Tobamovirus tomatotessellati, Tomato mosaic virus Tobamovirus tropici, Tropical soda apple mosaic virusTobamovirus ulluci, Ullucus mild mottle virusTobamovirus viridimaculae, Cucumber green mottle mosaic virus Tobamovirus wasabi, Wasabi mottle virus Tobamovirus youcai, Youcai mosaic virus aka oilseed rape mosaic virus

Proposed members

Proposed, but currently unrecognised members of the genus include:
  • Chara corallina virus
  • Nicotiana velutina mosaic virus