tobacco mosaic virus

M.Sc 3rd semester

1. UNIVERSITY OF ALLAHABAD PRESENTED BY BHIMSEN M.Sc 3 rd SEMESTER ROLL 11

2. CONTANTS INTRODUCTION SYMPTOMS STRUCTURE PROTEIN SYNTHESIS TRANSMISSION CONTROL OF TMV

3. INTRODUCTION TMV (Tobacco mosaic virus) is the most serious pathogen causing mosaic on tobacco leaves . It is transmitted by artificial inoculation but not by insect vectors. TMV is the most resistant virus known so far of which the thermal death point is 90  °C for 10 minutes. This is the first virus that was crystallized in 1935 by W.M. Stanley in the U.S.A.

4. SYMPTOMS TMV damages the solanaceous plants. However , it can infect the other plants too. Several strains of TMV has also reported. After infection , it developes system of lightening of leaf colour among the veins in early stages.

5. There after,it turns in to light and dark green mosaic symptoms. Among the veins green colour turns in to dark green and terminal region turns in to chlorotic . Some time dark green blisters appear in leaf blade. If the plants are infected early in season they become stunted. However, symptoms vary with varieties of tobacco. The virus reduces the yield as the quality of the products i.e. the nicotine content is decreased by 20 to 30 %.

6. STRUCTURE Franklin et al. (1957) , have described the structure of TMV. It is rod shaped helical virus measuring about 280 multiple 150 micron with a molecular weight of 39 multiple 10 to power 6 daltons . The virion is made up of 2,130 protein subunits of identical size. The protein subunits are arranged around a central hole of 4nm (40 A). Each protein subunit is made up of a single polypeptide chain which possesses 158 amino acid , the molecular weight of which is 17,500daltons.

7. Inside the protein capsid there is single stranded RNA molecule which is also spirally coind to form helix. Virus RNA consist of 6,500 nucleotides. In one turn the RNA contains 49 nucleotides. The number of protein subunits counting in three turns is 49 i.e. 49/3 subunits per turn. Therefore a single protein subunit is linked with 3 nucleotides of RNA. Arrangement of capsomere of RNA is shown in fig.

8. PROTEIN SYNTHESIS Takeba (1975) demonstrated the direct entry of TMV in to the isolated protoplast from mesophyll cells of tobacco. After making entry , RNA rapidly starts uncoating by removing the subunits from the capsid by using host enzymes. The parental RNA is localized in nucleus but not in cytoplasm. It performs two important functions: 1)it acts as mRNA and directs the synthesis of protein, and 2) function and template for synthesis of complemetary strand.

9. The virus RNA utilizes the amino acids , ribosomes and tRNA of the host and synthesizes the complementary strand and proteins i.e. coat protein of 17,500 daltons and other polypeptides ( of molecular weight 160,000 daltons ). The ratio of nucleic acid and protein differs with each virus. Nucleic acid about 5-40% of the virus and protein 60-70%. Each protein subunits of TMV consists of 158 amino acid making a total number to about 17,531.

10. TRANSMISSION TMV is transmitted through the cell sap of host and enters a new host through wound incision. Wound is caused in plant due to various cultural operations such as clipping or topping the shoot. It is not seed transmitted but acts as seed contaminant. It is also transmitted by wind and water. Various control methods of the disease are regular roguing diseased plants and weeds, sanitation and use of resistant varieties.

11. CONTROL OF TMV TMV occurs in tobacco- growing areas, and is considered to be one of the most important tobacco viruses economically. Insects are not important in its spread. Control is by crop rotation, effective sanitation and use of resistant cultivars of tobacco. For the first time, TMV has been used to demonstrate for development of coat protein-mediated resistance, replicase -mediated resistance, and movement protein-mediated resistance.

12. Refence Dubey ,R.C. and Maheshwari ,D.K. (A textbook of Microbiology)……. Google images…