Tobacco mosaic virus (TMV)
Tobacco mosaic virus (TMV) is a positive-sense single stranded 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 (hence the name). TMV was the first virus to be discovered. Although it was known from the late 19th century thatan infectious disease was damaging tobacco crops, it was not until 1930 that the infectious agent was determined to be a virus.
Tobacco mosaic virus has a rod-like appearance. Its capsid is made from 2130 molecules of coat protein (see image to the left) and one molecule of genomic single strand RNA 6400 bases long. The coat protein self-assembles into the rod like helical structure (16.3proteins per helix turn) around the RNA which forms a hairpin loop structure (see the electron micrograph above). 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 distinctinner 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. X-ray fiber diffraction structure of the intact virus was studied based on an electron density map at 3.6 Å resolution.
TMV does not have a distinct over-wintering structure. Rather, itwill over-winter in infected tobacco stalks and leaves in the soil, on the surface of contaminated seed (TMV can even survive in contaminated tobacco products for many years). With the direct contact with host plants through its vectors (normally insects such as aphids and leaf hoppers), TMV will go through the infection process and then the replication process.
After itsmultiplication, it enters the neighboring cells through plasmodesmata. For its smooth entry, TMV produces a 30 kDa movement protein called P30 which enlarge the plasmodesmata. TMV most likely moves from cell-to-cell as a complex of the RNA, P30, and replicase proteins.
It can also spread through phloem for longer distance movement within the plant. Moreover, TMV can be transmitted from one plant toanother by direct contact. Although TMV does not have defined transmission vectors, the virus can be easily transmitted from the infected hosts to the healthy plants, by human handling.
Following entry into its host via mechanical inoculation, TMV uncoats itself to release its viral RNA strand. Then the viral genome will further replicate to produce multiple mRNAs. The resultingmRNAs encode several proteins, including the coat protein and an RNA-dependent RNA polymerase (RdRp), as well as the movement protein. Thus TMV can replicate its own genome. After the coat protein and RNA genome of TMV have been synthesized, they spontaneously assemble into complete TMV virions in a highly organized process. The protomers come together to form disks composed of two layers ofprotomers arranged in a helical spiral. The helical capsid grows by the addition of protomers to the end of the rod. As the rod lengthens, the RNA passes through a channel in its center and forms a loop at the growing end. In this way the RNA can easily fit as a spiral into the interior of the helical capsid.
Host and symptoms
Tobacco mosaic virus symptoms tobacco
Tobacco mosaic virus symptomsorchid
Like other plant pathogenic viruses, TMV has a very wide host range and has different effects depending on the host being infected. The tobacco mosaic virus has been known to cause a production loss for flue cured tobacco of up to two percent in North Carolina. It is known to infect members of nine plant families, and at least 125 individual species, including tobacco, tomato,...
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