Definitions

Tobacco mosaic virus

Tobacco mosaic virus

Tobacco mosaic virus (TMV) is an RNA virus that infects plants, especially tobacco and other members of the family Solanaceae. The infection causes characteristic patterns (mottling and discoloration) on the leaves (thus the name). TMV was the first virus to be discovered. Although it was known from the late 19th century that an infectious disease was damaging tobacco crops, it was not until 1930 that the infectious agent was determined to be a virus.

History

In 1883, Adolf Mayer first described the disease that could be transferred between plants, similar to bacterial infections. Dimitri Ivanovski gave the first concrete evidence for the existence of a non-bacterial infectious agent, showing that infected sap remained infectious even after filtering through Chamberland filter candles, in 1892. However, he remained convinced despite repeated failures to produce evidence, that bacteria were the infectious agents. In 1898, Martinus Beijerinck showed that a filtered, bacteria-free culture medium still contained the infectious agent. Wendell Meredith Stanley crystallized the virus in 1935 and showed that it remains active even after crystallization. For his work, he was awarded 1/4 of the Nobel Prize in Chemistry in 1946, even though it was later shown some of his conclusions (in particular, that the crystals were pure protein, and assembled by autocatalysis) were incorrect. The first electron microscopical images of TMV were made in 1939 by Gustav Kausche, Edgar Pfankuch and Helmut Ruska - the brother of Nobel Prize winner Ernst Ruska. In 1955, Heinz Fraenkel-Conrat and Robley Williams showed that purified TMV RNA and its capsid (coat) protein assemble by themselves to functional viruses, indicating that this is the most stable structure (the one with the lowest free energy), and likely the natural assembly mechanism within the host cell.

The crystallographer Rosalind Franklin worked for Stanley for about a month at Berkeley, and later designed and built a model of TMV for the 1958 World's Fair at Brussels. In 1958, she speculated that the virus was hollow, not solid, and hypothesized that the RNA of TMV is single-stranded. This conjecture was proven to be correct after her death and is now know to be the + strand.

Structure

Tobacco mosaic virus has a rod-like appearance. Its capsid is made from 2130 molecules of coat protein (see image above) and one molecule of genomic RNA 6390 bases long. The coat protein self-assembles into the rod like helical structure (16.3 proteins per helix turn) around the RNA which forms a hairpin loop structure (see the Electron Micrograph below). The protein monomer consists of 158 amino acids which are assembled into four main alpha-helices, which are joined by a prominent loop proximal to the axis of the virion. Virions are ~300 nm in length and ~18 nm in diameter. Negatively stained electron microphotographs show a distinct inner channel of ~4 nm. The RNA is located at a radius of ~6 nm and is protected from the action of cellular enzymes by the coat protein. There are three RNA nucleotides per protein monomer. TMV is a thermostable virus. On a dried leaf, it can withstand upto 120 degrees for 30 minutes

Replication

When it enters the host, it enters as single stranded RNA and has a temporary double stranded intermediate. Its RNA directs the synthesis of viral proteins by the host machinery. After the RNA and the protein is produced, they undergo self assembly. After the assembly of capsomeres and RNA, they are released outside the cell after the lysis and death of the host. These newly synthesized viruses continue on to infect other cells.

Infection

When TMV infects a tobacco plant, the virus enters the plant cell and the virus replicates. After its multiplication, it enters the neighboring cells through plasmodesmata. For its smooth entry, TMV produces a 30,000 Dalton protein called P30 which tends to enlarge the plasmodesmata. TMV most likely moves from cell-to-cell as a complex of the RNA, P30, and replicase proteins. The first symptom of this virus disease is a light green coloration between the veins of young leaves. This is followed quickly by the development of a “mosaic” or mottled pattern of light and dark green areas in the leaves. These symptoms develop quickly and are more pronounced on younger leaves. Mosaic does not result in plant death, but if infection occurs early in the season, plants are stunted. Lower leaves are subjected to “mosaic burn” especially during periods of hot and dry weather. In these cases, large dead areas develop in the leaves. This constitutes one of the most destructive phases of tobacco mosaic virus infection. Infected leaves may be crinkled, puckered, or enlongated.

Scientific and Environmental Impact

In plants, tobacco mosaic virus leads to severe crop losses. It is known to infect members of nine plant families, and at least 125 individual species, including tobacco, tomato, pepper, cucumbers, and a number of ornamental flowers. There are many different strains.

The large amount of literature about TMV and its choice for many pioneering investigations in structural molecular biology, X-ray diffraction, virus assembly and disassembly, and so on, are fundamentally due to the large quantities that can be obtained, plus the fact that it does not infect animals. After growing a few infected tobacco plants in a greenhouse and a few simple laboratory procedures, a scientist can easily produce several grams of virus. As result of this, TMV can be treated almost as an organic chemical, rather than an infective agent.

References

flue-cured tobacco field manual published R.J.Reynolds Tobacco Company Winston-Salem,North Carolina, 1995

Further reading

  • Creager, Angela N. The Life of a Virus: Tobacco Mosaic Virus as an Experimental Model, 1930-1965. Chicago, IL: University of Chicago Press, 2002.

External links

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