Tobacco Mosaic Virus(TMV)
More than 100 types of plant viruses are known which cause various diseases in plants of these, tobacco mosaic virus has been studied most extensively, both in the field as well as in the laboratory. This virus was discovered by D. Iwanowski in 1892, but it's isolation from infected plants and crystallization was done by W. M Stanley in 1935.
Structure:
TMV particles appear as a bundle of rods under electron microscope. Each rod is approximately 3000√\ in length and 170√\ in diameter and has molecular weight approximately 400,000. The electron microscopic and x-ray crystallographic investigations have revealed that these particles have two constituents, a protein coat and nucleic acid. The protein coat(capsid) is made up of approximately 2130 identical protein subunits called capsomeres. Each capsomere consists of a long chain of 158 Ami no acids and it's molecular weight is 18000. The capsomeres are helically arranged around a central single stranded RNA molecule. The latter consists of some 6,000 nucleotide pairs. The protein and nucleic acid ratio in these particle is 94.4:5.6.
The total length of each rod has about 130 helicals and in each helix there are approximately 16.5 protein subunits. In each helix of RNA there are 49 nucleotides and the helix has a pitch of 23√\. The genetic information necessary for the formation of a complete TMV particle is contained in its RNA. This information determines the replication of RNA and sequence of Amino acids in the protein subunits of the capsid. As all subunits are identical only one coded gene is required for the formation of all capsomeres. It is possible to remove the protein coat from RNA. The naked RNA is capable of infecting tobacco plant; once inside the host cell, the virus RNA directs the protein synthesizing apparatus of the host cell to synthesize it's own proteins. Thus RNA has two functions I) self replication, II) synthesis of virus specific proteins for which it takes the raw material from the host cell.
Although the naked RNA is capable of infecting the host cell, the efficiency of infection is usually one thousandth to one millionth that of the intact virion. At the time of infection, usually the entire virion(I, e protein coat and nucleic acid) enters into the host cell. Immediately after entering the host cell, the protein coat of virion degenerates, whereas RNA starts the synthesis of mRNA which eventually synthesizes protein subunits.
Sources of TMV infection:
Debris of infected plants lying in the soil is the chief source of infection of tobacco mosaic virus. The infection also posses from one host to another by mechanical means. Such as agricultural implements and rubbing of ibfected leaves over the surface of healthy leaves. The virus is highly contagious and stable and can retain infectivity even after the leaves have been processed into smoking tobacco. According to Johnson(1937), 67% ciger, 11% cigerettes and 62% pipe tobacco is virus infected. A person who has handled a cigarette containing infected tobacco leaves may transmit the virus to healthy plants. Besides, many aphids are vectors of the virus.
Symptoms of TMV on the host:
The first visible symptoms of TMV infection in tobacco plant appear in the form of downward curling and distortion of young apical leaves. Large spots of dark green colour appear on the upper surface of the leaf which eventually develop into irregular blistered areas. Other areas of the leaf show typical yellow and green mottling or mosaic symptoms. The plant becomes stunted.
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| Structure of TMV |
