M.Sc 3rd semester

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…