Tobacco Mosaic Virus

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

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:

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.

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.

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

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.

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