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INTRODUCTION
More than 100 types of plant viruses have been discovered till now and out of this, tobacco mosaic virus is one the best studied virus.
Russian biologist D. Ivanovsky in 1892 discovered Tobacco Mosaic Virus (TMV).
W. M. Stanley in 1935 showed that TMV remains active even after crystallization. He was awarded Nobel prize in chemistry in 1946 for reporting that TMV virus consists of nucleic acid and protein only.
TMV is a single-stranded RNA virus species in the genus Tobamovirus that infects a wide range of plants, especially tobacco and other members of the family Solanaceae.
TMV infects about 350 different species of plants.
STRUCTURE
The complete TMV is a sub-microscopic, rigid and appears bundle of rod or needles (Fig 1A).
Each particle is approximately 300 nm in length and 18 nm in diameter (Fig. 1B).
It is made up of a piece of nucleic acid and a protein coat or capsid (Fig. 1C).
The nucleic acid is ssRNA molecule and consists of 6000 nucleotide pairs. It is self-replicating and synthesize protein subunits inside the host cell.
Its capsid is made from 2130 identical protein subunits molecules called Capsomeres.
The capsomeres are helically arranged around the ssRNA molecule and there are about 130 turns and each turn has approximately 16 capsomeres and 49 nucleotides (Fig. 1D).
Each capsomere is made of 158 amino acids of 16 different kinds which are assembled into four main alpha-helices.
The proteins amounts to approximately 95% and nucleic acid (RNA) approximately 5% (exact ratio; 94.4 : 5.6).
SOURCE AND TRANSMISSION
TMV can multiply only inside a living cell but it can survive in a dormant state in dead tissue, retaining its ability to infect growing plants for years after the infected plant part died.
Debris of infected plants lying in the soil is the chief source of infection and infects the plants of next crop or of same season.
Infected agriculture implements and rubbing infected leaves over the healthy leaf also transmit the diseases.
Many aphids Muzus pericae, Microsiphon are vector of the virus. These insects transmit the diseases from infected organism to other healthy plants.
A person who handled a cigarette containing infected tobacco leaves may transmit the virus.
The virus can also contaminate seed coats and the plants germinating from these seeds can become infected.
It is also transmitted artificially by grafting infected buds on to healthy plant.
TMV is extraordinarily stable. Purified TMV has been reported to be infectious after 50 years storage in the laboratory at 4°C.
INFECTION AND DISEASES CYCLE
TMV enters the plant cell through minor wounds or by other means (Fig 2A). Usually the entire virion (nucleic acid and protein) enters into the host cell but the protein coat falls away (Fig. 2 B)
Immediately nucleic acid (ssRNA) directs the plant cell to produce more virus nucleic acid and virus protein, disrupting the normal activity of the cell (Fig 2C-G).
The coat protein then interacts with the newly synthesized TMV ssRNA for assembly of progeny virions (Fig 2 H-I).
These virus particles are very stable and at some point, when the cells are broken or the leaf dries up, they are released to infect new plants (Fig 2 J-K). For more detail visit our YouTube channel “Biology by Dr. Imtiyaz”.
TMV uses its movement protein to spread from cell to cell through plasmodesmata. As the virus moves from cell to cell, it eventually reaches the plant's vascular system for rapid systemic spread through the phloem to the roots and tips of the growing plant.
SYMPTOMS
The symptoms are external or internal expression of a plant that indicates its sufferings and it may be primary or systematic
Primary Symptoms: These appear at the site of inoculation as result of local reaction and disappear as the diseases spreads. There are two main kinds of primary symptoms
1. Vein clearing: The yellow network of veins called vein clearing appears and soon vanishes when the mosaic disease appears in a systematic manner.
2. Local lesions: In this small necrotic or chlorotic areas (deaths cells) appear at the point of inoculation.
Systematic Symptoms: It appear as a result of spreading of virus throughout the infected plant and are as follows
Downward curling and distortion of young apical leaves is the first visible symptoms in this disease.
Mosaic mottling: Mosaic symptoms are variable and commonly include irregular leaf mottling i.e., light and dark green or yellow patches or streaks (Fig 3B).
When large spot of dark green colour appears on the upper surface of leaf which eventually develops into irregular blistered areas and it called interveinal mottling
Sometimes the tissues close to vein remain green but the remaining tissue undergoes mottling and it is called vein banding.
Necrosis: It starts as local lesion or ring spots on leaves and then spreads in the form of long streaks and finally the whole tissue dies. Necrosis is associated with distortion and dwarfing of flowers.
Sometime the entire plant stunted and in some cases premature defoliation takes place.
CONTROL OR DISEASE MANAGEMENT
To reduce infection of plants with TMV all tools should be washed with soap or a 10% solution of household bleach to inactivate the virus.
TMV contaminated soil should be discarded and care should be taken to dispose of dead leaves and old plants.
During the growing season, infected plants should be dug up, bagged, and removed from the field.
Rotation practices that include resistant plants or non-host crops also should be employed to reduce the amount of inoculum in the field.
It is important to treat TMV contaminated tobacco seed with a 10% solution of trisodium phosphate for 15 minutes.
Tomato seed contaminated with TMV can be incubated at 70 °C for 2-4 days prior to planting.
Inoculation of a mild strain of the virus on young plants can protect them from subsequent infection by more severe strains of TMV. This is a well-documented control strategy, called "cross protection,"
Several tobacco and tomato cultivars have been bred to be genetically resistant to TMV.