PLANT VIRUS - TOBACCO MOSAIC VIRUS

   In 1886, Adolf Mayer first described the tobacco plant disease that could be transferred between plants. 

   In 1898, Martinus Beijerinck showed that the infectious agent was able to reproduce and multiply in the host cells of the tobacco plant & coined the term of "virus" to indicate that the causal agent of tobacco mosaic disease was of non-bacterial nature.

  
  In 1935, Wendell Meredith Stanley showed that TMV remains active even after crystallization.

  In 1956, Rosalind Franklin builds a TMV model hypothesized that the TMV particle was hollow and that its RNA genome was single strand.

  TMV was the first virus to be discovered.

  The infection causes characteristic patterns (mottling and discoloration) on the leaves & hence the name mosaic.

         

STRUCTURE 

 

       Tobacco mosaic virus particles are typically helical shaped with dimensions of 300nm in length by 18nm in diameter. TMV is one of the most stable viruses. It is resistant to chemical and physical agents that inactivate many other viruses. 

              

         The coat protein (CP) of TMV is extraordinarily resistant to most of the proteases. Its stability is likely why the virus was the first one to be identified and purified to homogeneity. The exact structure of TMV has been worked out.               

               

        The capsid contains 2130 identical coat protein subunit. Capsid is a proteinic structure covering the genetic material. TMV contain a single single positive sense RNA ( +ss RNA) that is approximately 6400 to 6600 nucleotides in length. The protein : RNA ratio is 95% to 5%.

            

       The TMV virions can self-assemble. In others words, purified TMV RNA and coat protein will self-assemble into stable rodlike helical virus particles. This self-assembly process takes little free energy to occur.

HOST RANGE AND TRANSMISSION

                

          TMV has broad host range, over 550 species of flowering plants can be infected by TMV. Natural hosts that are important commercially include several tobacco species, peppers, tomatoes and potatoes. There are high concentrations of TMV in cells of tobacco and solanaceous plants.

         

          TMV is easily transmissible from crop to crop primarily by mechanical transmission or in the roots/soil from infected plants but is not known to spread by insect vector transmission. TMV is so stable that its infectivity can be maintained for about 2 years in the soil that is not exposed to freezing and drying. The virions are stable in plants residues in the soil and in the composts of tomato plants. 

     

       Tobacco mosaic is one of the most persistent diseases of tomatoes because the virus can remain infectious without a host for many years’ and is able to withstand heat. The hands and clothing of smokers who come in contact with TMV infected tobacco easily transmit TMV to susceptible plants while working in the fields.

SYMPTOMS AND DIAGNOSIS      

          

            Compared to other plant viruses, the symptoms of TMV infected plants are not that distinct. They vary depending upon virus strains, plant age and growing conditions. A common characteristics is yellow or light green coloration between the veins of young leaves followed by “mosaic” or mottled pattern of light and dark green areas in the leaves.  Lower leaves are subjected to “mosaic burns” especially during hot and dry weather. The virus almost never kills plants, but infection does lower the quality and quantity of the crop. TMV infected plants can easily be confused with plants affected by herbicides, mineral deficiencies and other plant disease. A plant pathologist and the use of electron microscope are required for positive identification of tobacco mosaic virus in infected plants.

CONTROL

                    

                It is nearly impossible to prevent TMV infection in nature because of the stability of the virus. TMV occurs wherever tobacco is grown. On average TMV reduces crop yield by 30% to 35%, reducing the market value. The following control measure are recommended to prevent the spread of TMV when maintaining and transplanting seedlings in the greenhouse or field.

·       Use only uncontaminated soils for seedling.

·       Prohibit smoking during working.

·       Remove & destroy all infected plants from nurseries.

·       Do not plant tomato or pepper seedlings in same soil.

·       Crop rotation should be carried out.

·       Spray plants with skim milk or buttermilk to reduce mechanical spread of TMV.

·       Plant TMV resistant varieties can introduce.

·       Workers hygiene should be maintain.

·       Destroy all weed hosts of TMV in tobacco and nearby fields.