TOBACO MOSAIC VIRUS

Viruses are obligate, intercellular parasites which can be viewed with the aid of an electron microscope. In Latin word ‘virus’ means poison or venom. A simple virus particle behaves as living with host cell and as non living outside the cell. They use both animals and plants as host. Till today more than 100 types of plant viruses are discovered. Among the plant viruses Tobacco Mosaic Virus (TMV) has been studied most extensively both in the field as well as in the laboratory.

Structure:

TMV was discovered be D. Iwanoswski in 1892. But its isolation from infected plants and crystallization was done by W. M. Stanley in 1935. It is the most serious pathogen causing mosaic on tobacco leaves. Under electron microscope, it appears as a non enveloped rigid rod with a helical symmetry. Each rod is approximately 3000 A^◦ in length and 170 A^° in diameter and has a molecular weight of approximately 400000.

       The electron microscopic and X ray crystallographic investigations have revealed that these particles have 2 constituents a protein coat and a nucleic acid. The protein coat or capsid is made up of approximately 2130 identical protein subunits called capsomers. Each capsomer consists of a long chain of 158 amino acids and its molecular weight is 18000. The capsomers are helically arranged around a central single stranded RNA molecule which consists of 6500 nucleotides. The protein and nucleic acid ratio of these particles is 944:5-6.

       The total length of each rod has about 130 helices and in each helix these approximately 16.5 protein subunits. In each helix of RNA there are 49 nucleotides and the helix has a pitch of 23 A^◦.

       The genetic information, necessary for the formation of complete TMV particle is contained in its RNA. This information determined the replication of RNA and the sequence of amino acid in the protein subunit of the capsid. As all subunits are identical only one coded gene is required for the formation of all capsomers. It is possible to remove the protein coat from RNA. The naked RNA is capable of infecting tobacco plant. Although the naked RNA is capable of infecting the host cell the efficiency of infection is usually 1/1000 – 1/1 million-th of the intact virus.

Reproduction:-

       The genome of TMV is a monopartite linear positive sense mRNA of 6.3-6.5 kb that produces 5 protein during infection in host. After entry and uncoating the genetic RNA can function as mRNA and direct synthesis of viral proteins. The overall reproduction i.e. multiplication method is as follows-

(i)  A virus enters into the host plant either through the mechanical wounds or by the vectors or through infected pollen grains. It needs to enter the vascular system of the plant for successful colonization of entire plant. The process of cell to cell movement of TMV is relatively slow.

(ii) After entry and uncoating the single stranded RNA of TMV serves as both the genome and viral messenger RNA. The genome encodes at least four proteins. Among them methyl transferase/helicase and RNA dependant RNA polymerase involve in virus replication. The other two are a movement protein and the capsid protein.

(iii) After replication and protein synthesis results the formation of enough genetic RNA and capsid protein when a large number of capsomer proteins and single stranded RNA have accumulated in the injected cell, the virions are assembled spontaneously in a highly organized manner. The capsomers are assembled to form discs made up of two layers of helically arranged protomers. The TMV RNA is associated with it near the 3' end of the molecule. Additions of capsomers discs gradually lengthen the size of virion.

(iv) After successful assembly, TMV is transmitted through the cell sap of host and enters a new host through wound insition.

Pathogenic effect:-  

       TMV is the most serious pathogen causing mosaic on tobacco leaves. It is transmitted very rapidly. It is the most resistant virus known so far of which the thermal death point is 90°c for 10 mins. Recently TMV has classified into following strain-

(i) Typical tobacco mosaic virus,

(ii) Tobacco virus I,

(iii) Nicotiana virus 1 K.M. Smidth,

(iv) Marmor tabaci Holmes.

Symptoms:-

(i) The 1^st visible symptoms of TMV infection in tobacco plant in the form of downward curling and distortion of young apical leaves.

(ii) As the leaves enlarged, abnormally dark green spots appear which develop into irregular crumpled blisters areas while the remainder of the tissue becomes more and more chlorotic(green).

(iii) Plants are stunted in various degrees.

(iv) At the mature condition large spots of dark green colour appear on the upper surface of the leaf.

       To resists TMV one should avoid infested soil and the use of tobacco refuse on land on which tobacco is to be grown. Seed beds should be steam sterilized and well removed from tobacco wire house. Except that contamination of hands with virus from tobacco product should be avoided.