Tobacco mosaic virus or TMV belongs to the Tobamovirus group, which causes mosaic-like symptoms on tobacco leaves. TMV is a plant virus that disseminates via artificial inoculation rather than insect vectors.
It was the first virus identified and the first virus ever purified. Since the late 19th century, it was believed that a non-bacterial infectious disease showed detrimental effects on tobacco crops. In the year 1930, the infectious agent was proved to be a virus.
TMV mainly infects tobacco leaves, but it can also infect well over 350 different species belonging to the family Solanaceae. The virus particles consist of a single piece of nucleic acid (RNA) and a surrounding capsid protein.
This post describes the structure, viral genome properties, signs or symptoms, properties, transmission, disease cycle and control measures of the tobacco mosaic virus.
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 or 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 an 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 the 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 the 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. Viral 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 proteins that participate in genome replication like methyltransferases and helicases.
ORF2 produces a single protein due to ribosomal readthrough of a leaky UAG stop codon. ORF3 produces movement protein, and ORF4 produces capsid proteins.
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 dependent on the age, the genetic background of the infected plant, and environmental conditions etc. TMV induces mosaic disease, which is characterized by the following signs and symptoms:
- TMV causes 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 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 crops and cause 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 disseminates 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 the cell sap via wound incision, thereby infecting the seed coats and the plants germinating.
- The TMV infection also disseminates 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 widens the opening of plasmodesmata and helps the TMV RNA to move into the adjacent cell. 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. The TMV particles reproduce and colonize through the phloem to the roots and tips of the growing plant.
Virus particles disassemble to expose their RNA once the TMV enters the plant cell. The viral RNA is “+ sense”, and it 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 to make a – sense TMV RNA.
The – sense RNA functions as a template in forming the genomic positive sense RNA and positive sense subgenomic RNAs (sgRNAs). The ribosomes of the host plant cell translate positive sense sgRNAs and produce movement proteins (30 kDa) as well as coat proteins (17.5 kDa).
The coat proteins form within the host cell, and it helps in the assembly of virions by interacting with the genomic + sense TMV RNA. The virus particles remain stable for a long period of time, and they may infect other cells in case of wound incision or the leaf dries up. Alternatively, the movement protein wraps the + sense TMV RNA, which eventually infects the neighbouring cells.
Control of TMV
There are few control measures through which we could manage the dissemination of TMV.
- One should purchase disease-resistant varieties or virus-free plants.
- You should also remove weeds growing along with the host plants, as they may harbour TMV.
- Remove crop debris from benches and the greenhouse structure.
- Instantly discard if any of the signs and symptoms of the mosaic disease appear.
- You need to disinfect the vegetative tools for at least 10 minutes, as disinfection is a crucial measure to control this disease.
- Crop rotation is one of the best ways to control the mosaic disease.
Therefore, the tobacco mosaic virus is the most resistant and stable virus that remain infectious even after 50 years of storage. From the name itself, it is clear that the virus can induce mosaic-like symptoms on the tobacco leaves and in the Solanaceous family members. It is a serious plant pathogen that reduces not only the yield but also the crop’s quality.