Tobacco Mosaic Virus

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.

Tobacco Mosaic Virus Structure

TMV is a rod-like virus, tall and narrow. It's 300 nanometres long and 18 nanometres across.

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

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

↓

Reduced photosynthesis impacts the growth, health, and reproduction of the plant

↓

As a result, tobacco mosaic disease leads to secondary symptoms like stunted growth and necrosis

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.

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.

^{1. Aria Nouri, The discovery of bacteria, American Association for the Advancement of Science, 2011}

^{2. British Society for Plant Pathology, Tobacco Mosaic Virus (TMV), 2019}

^{3. CGP, AQA A Level Biology, 2015}

^{4. Dean O. Cliver, Capsid and Infectivity in Virus Detection, Food and Environmental Virology, 2009}

^{5. H. Lecoq, Discovery of the first virus, the tobacco mosaic virus: 1892 or 1898?, Comptes Rendus de L'academie des sciences, 2001}

^{6. K-B. G. Scholthof, Tobacco Mosaic Virus, The Plant Health Instructor, 2000}

^{7. M. Hemia, Emerging Trends in the Development of Plant Virus-Based Nanoparticles and Their Biomedical Applications, Recent Developments in Applied Microbiology and Biochemistry, 2019}

^{8. Obayedul Hoque Reza, Determination of Economic Threshold Level (ETL) of Tobacco Mosaic and Leaf Curl Viruses, International Journal of Sciences: Basic and Applied Research, 2019}

^{9. PEDIAA, Difference Between DNA and RNA Viruses, 2017}

^{10. Ruolan Liu, Humans Have Antibodies against a Plant Virus: Evidence from Tobacco Mosaic Virus, PLoS One, 2013}

^{11. Ulrich Melcher, Tobamorviruses (Virgoaviridae), Encyclopedia of Virology (Fourth Edition), 2021}

^{12. United States Department of Agriculture, The Powerful Solanaceae, 2022}