Tospovirus

A Viral Biorealm page on the family Tospovirus

Baltimore Classification

Group V (negative sense ssRNA)

Higher order categories

Bunyaviridae

Species

Tomato spotted wilt virus (TSWV); Impatiens necrotic spot virus (INSV); Watermelon silver mottle virus (WSMoV)

Description and Significance

Tospoviruses are enveloped viruses that infect plants, leading to tissue necrosis. An extremely wide variety of plants are susceptible to tospoviruses, including tomatoes, watermelon, lettuce, groundnuts, and flowers such as irises, impatiens, lilies, and orchids (Jones, 2005). Tospoviruses rank among the ten most detrimental plant viruses worldwide, and the recent resurgence of the virus and spread into novel hosts has sparked concern among agriculturalists and horticulturists (Prins & Goldbach, 1998).

Genome Structure

The tospovirus genome consists of three ssRNA molecules. The molecules are simply known as L (Large), M (Medium), and S (Small). The entire genome codes for six proteins via five different open reading frames (ORFs). The L RNA encodes the virus' RNA-dependent RNA polymerase in its complement. The M RNA encodes two envelope precursor glycoproteins (G1 & G2) and a viral movement protein (NSm). The S RNA encodes the nucleoprotein (N) and a non-structural protein (NSs). The M and S RNAs are special, in that both are ambisense, meaning that one gene is encoded in the negative sense and another gene is encoded in the positive sense (Cortez et.al., 2001). The RNA molecules are capable of forming pseudo-circular panhandle structures, as well as stable hairpins, due to the presence of complementary terminal sequences (Dong et.al., 2008).

Virion Structure of a Tospovirus

The tospovirus virion consists of a quasi-spherical capsid particle 80-120 nm in diameter containing the ssRNA genome. The paritcle is enclosed in an envelope formed by budding from the host Golgi (Cortez et.al., 2001; Prins & Goldbach, 1998).

Reproductive Cycle of a Tospovirus in a Host Cell

Tospovirus enters the cell via an insect vector. The virus sheds its capsid, and the viral RNA polymerase is produced. Subsequent RNA polymerase activity depends on a chemical cue similar to quorum sensing. At low concentrations of the virus nucleocapsid protein (eg. at initial entry), the RNA polymerase makes mRNAs that encode the other viral proteins necessary for packaging and transport. Host cell machinery is used to translate the viral proteins. At high concentrations of nucleocapsid protein (eg. after protein translation has occurred), the RNA polymerase can begin replicating the viral genome. After packaging occurs, viral transmission can proceed in two ways. The virus can be transmitted directly to another host cell via the viral movement protein (NSm) or indirectly via uptake by an insect vector (Prins & Goldbach, 1998).

Viral Ecology & Pathology

The tospovirus insect vector is the thrip, a small, winged herbivore. Thrips can infect a plant by transferring virus particles during feeding. In other instances, thrips can ingest the virus and shed particles as waste (Jones, 2005). Common insect vectors include the Thrips and Frankliniella species.

Upon infection, the virus induces tissue necrosis resulting in spots and streaks on leaves and discolored rings and spots on fruits (EPPO, 2004). Symptoms may vary, depending on factors such as cultivation method, age of the plant, nutritional and environmental conditions, and the virus species and/or serotype (EPPO, 2004). Current research is focused on methods for virus diagnosis and prevention, as the tospovirus host range is wide and includes several economically important crops and ornamental plants (EPPO, 2004).

References

Cortez, I., J. Saaijer, K.S. Wongjkaew, A.M. Pereira, R. Goldbach, D. Peters, & R. Kormelink. "Identification and characterization of a novel tospovirus species using a new RT-PCR approach." Archives of Virology 146 (2001): 265-278.

Dong, J.H., X.F. Cheng, Y.Y. Yin, Q. Fang, M. Ding, T.T. Li, L.Z. Zhang, X.X. Su, & Z.K. Zhang. "Characterization of tomato zonate spot virus, a new tospovirus in China." Archives of Virology" 153 (2008): 855-864.

Jones, D.R. "Plant viruses transmitted by thrips." European Journal of Plant Pathology 113 (2005): 119-157.

Page authored for BIOL 375 Virology, September 2008