Tobacco mosaic virus or TMV belongs to the Tobamovirus group, which cause mosaic-like symptoms on tobacco leaves. TMV is a plant virus that is generally disseminated through artificial inoculation rather than insect vectors. It was the first virus identified as a virus and the first virus ever purified. Since from the late 19th century, it was believed that a non-bacterial infectious disease showed detrimental affects on tobacco crops. In the year 1930, the infectious agent was proved to be a virus.
TMV mainly infects tobacco leaves, but can also infect well over 350 different species belonging the family Solanaceae. The virus particle consists of a single piece of nucleic acid (RNA) and a surrounding capsid protein.
Content: Tobacco Mosaic Virus
Structure of Tobacco Mosaic Virus
Shape: TMV possesses a rod-like shape, where the RNA genome shows a helical symmetry and the capsid protein subunits show radial symmetry.
Size: TMV comprises a length of 300 nm and a diameter of 18 nm with a molecular weight of 39X106 Daltons.
Capsid: The virion contains 2130 identical protein subunits, which also refers to as “Capsomers“. Each protein subunit comprises a single polypeptide chain of 158 amino acid residues with a molecular weight of 17,500 Daltons. The number of protein subunits in three turns of RNA is 49, or we can say a single protein subunit is linked with three nucleotides of the RNA genome.
RNA: TMV possesses ssRNA genome with a hollow central cavity of diameter (4 nm). The RNA molecule is spirally twisted to form a helix, where one turn of helix contains 49 nucleotides. Generally, the virus RNA possesses 6500 nucleotides. It is protected from the cellular enzymatic action by the surrounded capsid.
The genome of tobacco mosaic virus is monopartite and linear. It contains (+) sense mRNA with a genome size of 6.3-6.5Kb. The virus genome produces five proteins during virus infection. Virus RNA possesses a cap (m7G5’pppG) at 5′-end and histidine group at 3′-end.
After the direct translation of viral RNA, two RNA polymerase proteins are formed. The genome encodes four open reading frames (ORFs). ORF1 encodes a protein that participates in the genome replication like methyltransferase and helicase. ORF2 produces a single protein due to ribosomal readthrough of a leaky UAG stop codon. ORF3 produces movement protein, and ORF4 produces capsid proteins.
There are three subgenomic RNAs have been identified during the time of host infection, namely I1 sgRNA, I2 sgRNA and LMC-RNA. Sulzinski has characterized another subgenomic TMV RNA (11-RNA) that is associated with polyribosomes.
Symptoms of TMV
Symptoms induced by TMV are somewhat depending on the age, genetic background of the infected plant, the environmental conditions etc. TMV can include mosaic disease, which can be characterized by the following signs and symptoms:
- TMV cause leaf necrosis and leaf curling.
- It also causes discolouration or yellowing of plant tissues.
- Sometimes blisters appear in the leaf blade.
- In severe case, the TMV results in a stunted growth of a plant.
- Rugosity can also be visualized by the formation of small localized random wrinkles on the leaves.
- Strains of TMV also infect tomato, sometimes causing poor yield or distorted fruits, delayed fruit ripening, and nonuniform fruit colour.
- Stability: TMV is one of the most stable viruses, which can withstand a temperature up to 50 °C for 30 minutes.
- Refractive index: TMV has a refractive index of about 1.57.
- Disease: Tobacco mosaic
- Pathogen: Tobacco mosaic virus
- Hosts: Tobacco, tomato, and other solanaceous plants
Transmission of TMV
TMV can be very easily transmitted from plant to plant by the following means:
- Contact of an infected leaf with a healthy leaf.
- By infected vegetative tools.
- TMV can also enter to the cell sap via wound incision and can also infect the seed coats and the plants germinating from it.
- The TMV infection can also disseminate via wind, water and other environmental factors.
Disease Cycle of Tobacco Mosaic Virus
The life cycle of TMV goes through the given infectious pathway:
Movement in the infected plant
To cause infection, TMV first needs to enter the plant’s vascular system. It uses its movement protein to spread from cell-to-cell through plasmodesmata, where the movement is quite slow due to the smaller diameter of plasmodesmata. The movement protein of virion helps the TMV RNA to move into an adjacent cell by widening the opening of plasmodesmata.
When the virion releases its genome into the host cell, the movement protein dissociates to initiate a new round of infection. After cell to cell transport, systemic transport takes place via phloem sieve elements, where the TMV particles reproduce and colonize through the phloem to the roots and tips of the growing plant.
Once, the TMV enters the plant cell, and the virus particles disassemble to expose its RNA. The virus RNA is “+ sense”, and serves directly as a messenger RNA. The translation of the viral genome occurs by the machinery of host cell ribosomes. The replicase-associated proteins translate within a few minutes after infection. The replicase enzymes bind to the 3′ end of the + sense RNA for the production of “- sense” TMV RNA.
The – sense RNA functions as a template for the formation of the genomic positive sense RNA and positive sense subgenomic RNAs (sgRNAs). The ribosomes of the host plant cell carry out the translation of positive sense sgRNAs and produce the movement protein (30 kDa) and the coat protein (17.5 kDa).
The coat protein synthesized within the host cell helps in the assembly of virions by interacting with the genomic + sense TMV RNA. These virus particles can remain stable for a long period of time and may infect other cells in case of wound incision, or the leaf dries up. Alternatively, the movement protein wraps the + sense TMV RNA, which can infect the neighbouring cells.
Control of TMV
There are few control measures through which we can manage the dissemination of TMV.
- One should purchase disease-resistant varieties or virus-free plants.
- Weeds growing along with the host plants should be removed as these may harbour TMV.
- The crop debris should be removed from benches and the greenhouse structure.
- If a plant shows, any of the signs and symptoms mentioned above should be instantly discarded.
- Disinfection is a crucial measure to control this disease, where the vegetative tools must be disinfectant for at least 10 min.
- Crop rotation is one of the best ways to control the disease.
Therefore, tobacco mosaic virus is the most resistant and stable virus which can remain infectious even after 50 years of storage. From the name itself, it is clear that it can induce mosaic-like symptoms on the tobacco leaves and also in the members of the Solanaceous family. It is one of the serious plant pathogen, which reduces not only the yield but also the quality of the crop.