Give symptoms, etiology and control of Tobacco mosaic virus.

Ans. Etiology:Tobacco is originally a plant of America which reached in India in 17th century. The leaves and stems of the plant are used in chewing and smoking in different forms throughout the world. The tobacco plant is infected by a virus giving mosaic appearance on leaves and therefore the disease is known as Tobacco mosaic virus (TMV). The disease is very dangerous because it is supposed to be the main source of cancer in human being. As we all know the cancer is an incurable disease up till now and results in more than 90% death.

Symptoms of the disease: The following symptoms appear on the tobacco plants when they are infected :

1. When the virus infects, the colour disappears near veins.

2. Late on the leaves give a mosaic appearance.

3. The growth of the plants is retarded and they become dwarf.

4. The leaves become thinner and smaller.

5. The shape of leaves becomes irregular and curled in different forms.

The spread of disease: The TMV disease is spread by (i) Rubbing (ii) Grass hoppers and (iii) Wounds.

Structure: TMV is an elongated rod shaped virus. It measures 300 ml x 15 ml. in diameter. It exhibits helical symmetry. It consists of about 95% protein coat and about 5% RNA. The protein coat called capsid. Capsid consists of 2130 sub units (capsomeres). These capsomeres are closely packed and arranged in a helical manner around RNA forming a hollow cylinder.

Franklin et al., (1957) have described the structure of TMV particle. The particles appear as bundle of rods or needles under electron microscope. Each rod is approximately 3000 Å (300 nm) in length and 170 Å (17 nm) in diameter and has a molecular weight of 39 x 10° daltons. These particles have a protein coat (capsid) and a nucleic acid core. The protein coat is made up of approximately 2130 identical protein subunits called capsomeres. Each capsomere consists of a single polypeptide chain of 158 amino acids and its molecular weight is 18,000 daltons. The capsomeres are helically arranged around a central hole of 40 Å (4 nm). Inside the protein coat, there is a single stranded RNA (ss RNA) molecule, which is also spirally coiled to form helix, with a pitch of 23 Å and at a radius of about 40 Å. The RNA does occupy the hole, but is deeply embedded in the protein subunits, thus having its own helix. The viral RNA consists of 6,500 nucleotides and has a molecular weight of 2.4 millions, daltons.

The total length of each rod has about 130 helicals (turns) and in each helix there are approximately 16 1/3 capsomeres. In each helix of RNA there 3 are 49 nucleotides and the helix has a pitch of 23 Å.

Multiplication (Reproduction): TMV multiplies into the cells of the host plant. According to Fraenkel Conrat (1956), when the TMV infects the host plant cell the protein remained in the cytoplasm of the cell and RNA moves from cell to cell through plasmodesmata. The viral RNA when enters into the host cell, takes control over the whole metabolic activities and taking nutrition from the cytoplasm of the host cell initiates its own replication.

Control of the disease: The following measures may be taken to control the disease:

1. Sanitation should be maintained as far as practicable.

2. The diseased plants should be burnt at a long distance.

3. The disease is contagious and therefore the diseased plant should be kept away from the healthy plants.

4. Only the healthy and resistant variety should be grown.

5. The seeds should be treated chemically before sowing and the chemical spray should be kept at regular intervals.

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