Viruses aren't actually alive! They don't have any cytoplasms, ribosomes, or a cell membrane – just genetic information surrounded by protein molecules. Viruses cannot reproduce independently. Instead, they invade cells of living organisms and use the host cells to replicate themselves. The tobacco mosaic virus has been groundbreaking in many ways. It was the first virus to be discovered – as well as the first to be purified, characterised, and sequenced. Genetic engineering was the first to combat the virus, which is now the leading tool for developing pharmaceuticals and plant nanomaterials.
What is the Tobacco Mosaic Virus?
Tobacco Mosaic Virus (TMV for short) is a viral plant pathogen. It's a member of the large genus Tobamovirus. TMV affects a wide range of plants including, tobacco, tomato and other members of the solanaceous plants.
TMV is named as such after the first plant it was discovered in.
Solanaceous plants are part of the Solanaceae family. This important plant family contains principal food plants (such as potatoes, tomatoes, and aubergines) and tropane alkaloids. These chemicals can be used for medicine and pesticides, or misused as addictive drugs. Nicotine is a tropane alkaloid.
Bacteria were first acknowledged in the 17th century, but it took much longer to detect viruses. The discovery of the tobacco mosaic virus is credited to two individuals - Dmitry Invanovsky and Martinus Beijerinck.
- In 1892, Ivanovsky studied tobacco mosaic disease. He discovered that leaves remained infectious even after filtration. This implied the existence of much smaller pathogens than bacteria.
- Six years later, Beijerinck studied the virus that causes tobacco mosaic disease and determined that it was infectious yet not living. He also coined the term 'virus'.
The tobacco mosaic virus was the first virus ever to be discovered!
Tobacco Mosaic Virus Structure
TMV is a rod-like virus, tall and narrow. It's 300 nanometres long and 18 nanometres across.
A nanometre is one billionth of a metre.
The virus has a protective protein shell or capsid that is made up of over 2000 protein units which assemble around the central nucleic acid genome to make a helical (spiral) structure.
As you may have noticed, TMV has a very simple structure. Since it can't reproduce itself and requires a host cell, the compact shape helps it to fit inside a cell:
The spiral shape of the central nucleic acid gives the virus an easy way for genetic modifications once it is inside the host cell (see the section below)
The outer protein shell is made up of one type of protein repeated several times; this means that the virus only requires one gene, helping to reduce its length
The capsid plays a vital role in helping the TMV carry out its function as a plant pathogen. Its primary role is to protect the virus' genetic information from environmental conditions like Digestive Enzymes. The protein units on the capsid shell also help the virus to initiate infection by allowing it to attach to a specific molecule on the host and penetrating the host's membrane.
Tobacco Mosaic Virus Genetic Material
Us humans store our genetic material in DNA molecules. These are found inside the nuclei of our cells. Viruses have a very different structure from humans. They don't have nuclei or DNA. Instead, the genetic material of the tobacco mosaic virus is contained within a molecule called RNA.
RNA (ribonucleic acid) has a similar structure to DNA. Both molecules comprise nucleotides containing a pentose sugar, a phosphate group and one of four different bases. The four bases of DNA are adenine (A), cytosine (C), guanine (G), and thymine (T). But there are a few key differences between the two molecules:
| RNA | DNA |
Sugars | RNA contains ribose sugar | DNA contains deoxyribose sugar |
Bases | A, C, G and U (uracil) | A, C, G and T |
Nucleotide Chain | RNA molecules are made from a shorter, single nucleotide chain | DNA is made of two long nucleotide chains in the shape of a double-stranded helix |
DNA is more stable and supports a larger genome than RNA. So, why do some viruses use RNA instead of DNA? It's the reproduction rate that gives RNA an edge over DNA. RNA viruses can replicate much faster because RNA bypasses transcription during protein synthesis. Furthermore, RNA viruses have a high mutation rate, enabling rapid evolution.
What Causes Tobacco Mosaic Virus to Spread?
Unlike most plant viruses, TMV is not spread by insects. Instead, it is spread by mechanical transmission; without infecting its vectors.
Mechanical transmission is the transfer of pathogens from something contaminated to a susceptible host.
A vector is an organism that carries and transmits a disease.
Humans often act as vectors for TMV (since we cannot be infected). Smokers can spread TMV through their hands or clothes. This is because cigarettes can contain viruses from an infected tobacco plant.
TMV can also be spread via contaminated tools and other gardening equipment.
The tobacco mosaic virus enters new host plants via wounds. Physical damage or scratches to a plant's cell membranes allows pathogens to enter the cytoplasm. Once inside, the virus hijacks the cell's ribosomes and uses them to make many copies of its own RNA.
Ribosomes are the tiny sites of protein synthesis within cells.
Once TMV has hijacked the ribosomes, it replicates itself and spreads into neighbouring cells, infecting the whole plant.
Tobacco Mosaic Virus Symptoms
Outside of a living cell, a virus is just an inactive particle composed of a protein shell and some sort of genetic information. A virus becomes active once it enters a host cell and takes over that cell's machinery to make more virus particles - known as an infection. Each type of viral infection causes a set of symptoms in the host. TMV is no different.
The namesake symptom of TMV is the mosaic patterns of discolouration on the leaves. Mottling, yellowing tissue and leaf curling are also common. Symptoms can vary depending on the species and background of the host plant, the environmental conditions and the strain of the virus.
Discolouration and symptoms that affect leaves limit a plant's ability to photosynthesise:
Plants require large, green leaves to absorb sunlight for photosynthesis
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Reduced photosynthesis impacts the growth, health, and reproduction of the plant
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As a result, tobacco mosaic disease leads to secondary symptoms like stunted growth and necrosis
Necrosis is the death of body tissue.
Many crops that the tobacco mosaic virus can infect and cause these devastating symptoms are of crucial economic significance. Major food crops, most notably tomatoes, are susceptible to infection. An outbreak can decimate a harvest, impacting farmers and driving up food prices. Farmers who grow tobacco, an important cash crop, are also vulnerable to attack. Many rely on this source of income worldwide.
Tobacco Mosaic Virus Treatment
TMV is transmitted easily between plants, making it difficult to control. Even rubbing against an infected leaf can spread the virus.
There is no cure for TMV, so once a plant is infected, it is infected for life! This usually results in the crop/infected plant being burned (where it is allowed) or bagged and disposed of safely. But fear not, TMV cannot infect humans, and there are several control methods to prevent it from spreading in plants.
Plant viruses lack the proteins needed for recognition and entry into animal cells, like humans' cells.
Sanitation and Greenhouse Management
Keeping a greenhouse clean is an effective method to control the spread of the virus. Dead plants and contaminated soil are carefully removed and destroyed to prevent the virus from spreading to healthy plants. Furthermore, any tools that contact infected plants are washed using soap or a 10% bleach solution.
Farmers are careful to avoid wounding healthy plants. They also don't allow watering cans or hoses to make direct contact with plants in case they become contaminated and spread the virus to the rest of the greenhouse.
Crop Rotation
If the soil has been contaminated with TMV, crop rotation is employed to avoid the infected area for two years.
Cross-Protection
Cross-protection is like the plant version of vaccination. Young plants are deliberately infected with a mild strain of TMV. This protects them against infection by more severe strains later in life.
Resistance
Breeders have selected genes for resistance using genetic engineering or selective breeding in solanaceous crops typically affected by TMV. Nowadays, the majority of tomato varieties are resistant to TMV.
Tobacco Mosaic Virus - Key takeaways
- The tobacco mosaic virus is a viral plant pathogen affecting solanaceous plants.
- It is made up of over 2000 protein units that surround a central single-helix of RNA.
- The virus spreads to new host plants through wounds. Once inside the cell, the viruses hijack the plant's ribosomes, producing viral RNA to reproduce itself.
- The main symptom of the virus is a mosaic pattern of discolouration of the leaves. This means the plant's ability to photosynthesise is affectec.
- Tobacco mosaic virus can be prevented through sanitation, greenhouse management, crop rotation, cross-protection and breeding resistance.
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