What is the Tobacco Mosaic Virus (TMV)?
Tobacco mosaic virus (TMV) is the first virus that has been discovered by two scientists, Dimitri Losifovich Ivanovski and Martinus Beijerinck. In 1892, Ivanski reported that the extracts of infected leaves still showed infectious actions even after passing it through a Chamberland filter candle. It was then depicted as a filterable pathogen that can pass through a filter that retains bacteria. Now that we know the full form of TMV, let us proceed to its structural features.
TMV Virus, its Structure, and Symptoms of Infection
Viruses invade and affect the normal functioning of the plants. One such virus is the tobacco mosaic virus (TMV). It is a rod-shaped virus belonging to the genus Tobamovirus. It generally infects the plants falling in the family Solanaceae. In this article, we will study the different structural features of this virus, the symptoms of its infection, and how to tackle it.
Structure of TMV
This virus is a rod-shaped simple helical virus that consists of centrally-located RNA enveloped by a structural coat of protein. The RNA is single-stranded and contributes to 5.6% of the mass of a virus. The rest 94.4% is the structural protein coat protecting this genetic material.
A virus is a single rod with a diameter of 180 Å and a length of 3,000 Å. The scientific name given to the protein coat is capsid. Nearly 2130 subunits of capsomeres are present in this coat. Every three helical turns has approximately 49 capsomeres forming a helical rod. As per the observation of R Franklin, each rod virus comprises 10 turns of capsomere protein units.
The diametric dimension of the single-stranded RNA present in each TMV virus is 80 Å. It means that this genetic material remains 50 Å underneath the protein sheath protecting it. A unit of capsomere is made of grape-like 158 amino acids and has a cumulative molecular weight of 17,000 Dalton.
The RNA strand, on the other hand, has 7300 nucleotides in sequence and the cumulative weight of this strand is 25,000 Dalton. The ssRNA is somewhat bigger in length with a dimension of 3300 Å.
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Lifecycle of Tobacco Mosaic Virus
Like all the other parasitic viruses, the tobacco mosaic virus also enters the host cells by penetrating the cell wall. The infected or invaded cells become the replication site for the virus. This virus dissociates its protein protective coat to release the RNA strand inside the cytoplasm.
The specific location of the replication is not known but the biologists suggest that multiplication occurs in the nucleus as the RNA strand enters the nucleolus. This RNA then induces the synthesis of RNA polymerase enzymes to produce a replicative RNA or a template. This template RNA is then used for the multiplication of copies of the parental RNA inside the host cell. The viral genome is then replicated and released into the cytoplasm to serve as mRNA for protein synthesis.
In the next step of the TMV virus life cycle, once the protein synthesis is done in collaboration with the t-RNA and ribosomes, capsomeres are produced to form TMV strands. A complete virus particle is produced called a virion inside the host cells. One of the prime biological phenomena is that even if the host cell is producing thousands of TMVs, it will not get destroyed rather the viruses will be transferred to the adjacent cells for repeating the same process. Hence, TMV causes a systemic infection.
Symptoms of TMV Virus
The mosaic disease of tobacco causes various symptoms to appear that help the botanists to identify this disease in plants. These symptoms are:
Stunted growth of the plant as a major part of the nutrition is engaged in the protein synthesis of the TMV viruses
Mosaic pattern of dark and light green patches on the leaves of an infected plant
Growing portions of the plant or the meristem will show malformation
Yellow distinct streaking occurs on leaves (seen mostly in the monocot plants)
Yellow spots appear on the leaves
Yellowing of the veins in the leaves
Non-uniform coloring of the fruits
Deformation of fruits
These are the symptoms of TMV that depict the presence of this virus in the infected cells. The propagation level of the infection can be determined from the degree of these symptoms.
Disease Management of Tobacco Diseases
There are no such chemicals that can cure a plant infected by the mosaic disease of tobacco. This is why biotechnological techniques are used to create disease-resistant species of plants that get affected by this virus. The varieties are chosen according to the potential of resisting this infection.
From the very beginning, virus-free saplings are used for agriculture. Properly-cleaned trays for saplings and other guidelines for strict hygiene are maintained to protect the crops from further infection.
This is all about the TMV virus and its structural features. As you can see, it is a very primitive virus that infects plants of a particular family. The name is given as it was discovered in tobacco leaves. It is also one of the prime viruses that infect tobacco plants. Focus on its structural features to correlate how its life cycle proceeds. Certain measures are taken to protect the crops from the beginning but once a plant gets infected, there is no coming back.
FAQs on Tobacco Mosaic Virus
1. What is Tobacco Mosaic Virus (TMV) and what disease does it cause?
Tobacco Mosaic Virus, or TMV, is a well-studied virus that primarily infects plants, especially tobacco. It causes Tobacco Mosaic Disease, which is easily recognised by the unique mosaic-like pattern of light green and yellow patches that appear on the leaves of infected plants. This discolouration stunts the plant's growth.
2. What is the basic structure of the Tobacco Mosaic Virus?
The Tobacco Mosaic Virus has a very simple yet effective structure. It is shaped like a rod and is composed of two main parts:
- A central core of genetic material, which is single-stranded RNA (ssRNA).
- A protective protein coat called a capsid, which encloses the RNA. This coat is made of thousands of identical protein molecules arranged in a spiral.
3. Why is a virus like TMV often described as being on the border between living and non-living?
TMV exhibits characteristics of both living and non-living things. Outside of a plant cell, it is an inert, non-living chemical particle that cannot perform any life processes and can even be crystallised. However, once it enters a living host cell, it takes over the cell's machinery to replicate itself, a key characteristic of life. This dual nature places it on the borderline.
4. How does Tobacco Mosaic Virus typically spread from one plant to another?
TMV spreads mainly through mechanical transmission. This means it is transferred via direct contact with the sap of an infected plant. Common ways this happens include contaminated hands or tools (like pruning shears), or simply when an infected leaf rubs against a healthy one. The virus is extremely stable and can survive in a dried state for a long time.
5. What actually causes the 'mosaic' pattern on the leaves of an infected plant?
The mosaic pattern is a direct result of the virus's activity inside the plant's leaf cells. The virus disrupts the development and function of chloroplasts, which contain the green pigment chlorophyll. This damage leads to patches of yellow or light green tissue where the chloroplasts are not working properly, creating a mottled or mosaic-like appearance against the healthy green parts of the leaf.
6. Can the Tobacco Mosaic Virus infect humans if they use products from an infected plant?
No, TMV cannot infect humans or any other animals. It is a plant-specific virus and does not have the necessary biological tools to attach to or replicate inside animal cells. Our immune system can easily neutralise the virus without causing any illness. Therefore, using tobacco products from an infected plant poses no risk of viral infection to people.
7. What does it mean for a TMV infection to become 'systemic'?
A systemic infection means the virus has spread throughout the entire plant, not just the initial point of entry. After TMV infects a cell, it multiplies and moves to neighbouring cells. Eventually, it enters the plant's vascular system (specifically the phloem), which acts like a highway, allowing the virus to travel to all parts of the plant, including the roots, stem, and new leaves.
