TOBACCO MOSAIC VIRUS: CHARACTERISTICS, STRUCTURE, REPLICATION - BIOLOGY - 2024

The virus of the mosaic of the snuff (TMV, of the English Tobacco Mosaic Virus) is a plant RNA virus that causes the appearance of brown spots on the leaves of plants snuff and other crops of economic interest such as tomatoes and other Solanaceae.

Its name derives from the pattern of spots that it causes in infected plants, which is described as a "mosaic". It is the first virus that was identified and described in nature, events that took place between the late 1800s and early 1900s, that is, more than a century ago.

Electron micrograph of tobacco mosaic virus (Source: No machine-readable author provided. Chb assumed (based on copyright claims). / Public domain, via Wikimedia Commons)

It is estimated that the losses of tobacco caused by the tobacco mosaic virus are around 1%, as more and more resistant plants are cultivated. However, other crops such as tomatoes, for example, suffer losses of more than 20% due to the disease caused by TMV.

One of the main agronomic problems related to this virus has to do with the fact that it can live even when the plant that hosts it dies and, in addition, that it supports high temperatures, so that its elimination from a crop or from the facilities of a greenhouse is quite challenging.

Nevertheless, the tobacco mosaic virus has proven to be very useful as:

- Symbolic and didactic model to expose the essential characteristics that define viruses

- Prototype to investigate the biology of parasite host plants, especially tobacco

- Tool for the study of pathogen-host interactions and cell trafficking

- Biotechnological tool for the expression of proteins of pharmaceutical interest in tobacco.

Discovery

Since its identification, the tobacco mosaic virus had a momentous role in establishing the field of virology, as it was the first virus to be identified and described in history.

It all started in 1879, when the German agricultural chemist Adolf Meyer devoted himself to the study of some diseases that affected tobacco.

This scientist demonstrated that a disease that caused the appearance of spots on tobacco leaves could be transmitted from a diseased plant to a healthy one, just by rubbing the leaves of the latter with an extract of the former.

Meyer called this the "mosaic disease of tobacco" and initially suggested that the etiological agent (the one that produced it) was of bacterial origin, although he could not isolate it or cultivate it experimentally in vitro.

Structure of the tobacco mosaic virus. 1) Single-stranded RNA, 2) Capsomer or protomer, subunit of the capsid CP protein, and 3) Capsid structure (Source: Y_tambe / CC BY-SA (https://creativecommons.org/licenses/by-sa/ 3.0) via Wikimedia Commons)

Discovery of the first virus

Credit for the discovery of the viral particles goes to Dmitry Ivanovsky, a Russian microbiologist who investigated tobacco mosaic disease between 1887 and 1890, finding that the causative agent of the disease was so small that it could pass through the tiny pores of a porcelain filter, where bacteria could not pass.

With this event, Ivanovsky determined that the disease was due to a "filterable virus", taking the term "virus" from the Latin word for "poison."

Ivanovsky's works were later confirmed, in 1895, by the Dutchman Willem Beijerinck, who demonstrated the serial transmission of the virus (from plant to plant) using the filtered sap of diseased plants.

Beijerinck's work also served to prove that it was not simply a chemical toxin, but rather a living agent capable of self-replication.

Between 1927 and 1931 two scientists from the Boyce Thompson Institute in Philadelphia, Vinson and Petri, concentrated the virus by precipitation using the methods used for protein purification.

Later, in 1935, Stanley purified the virus and managed to crystallize highly active and infectious needle-shaped particles, marking an unprecedented event in which a "living" entity could occur in a crystalline state.

Years later, with the collaboration and work of numerous scientists, it was determined that the tobacco mosaic virus was a single band RNA virus, with a filamentous appearance or morphology.

characteristics

- It is a single band RNA virus whose virions or viral particles are rod-shaped

- Its genome, as well as most viruses, is protected by a protein coat

- It belongs to the Virgaviridae family and to the Tobamovirus genus

- It infects tobacco plants and also some related plants, especially nightshades (potato, tomato, eggplant, etc.), adding more than 200 possible hosts

- It is extremely stable and can stay on different surfaces for a long time

- In infected plants, this virus accumulates in considerably high titers

- The symptoms it causes in diseased plants are noticeable and easy to identify

Structure

The tobacco mosaic virus, as discussed, is a single-stranded (single-stranded) RNA virus whose viral particles are rod-shaped.

Generalized scheme of the structure of the tobacco mosaic virus, TMV (Source: TMV_Structure.png: Graham Colm TalkThe original uploader was GrahamColm at English Wikipedia.derivative work: Arnaugir / CC BY-SA (https://creativecommons.org/licenses /by-sa/3.0) via Wikimedia Commons)

Protein cover

The characteristic structure of each viral particle is given by a protein coat formed by a right-handed helix of subunits of a protein known as "coat protein".

This envelope has approximately 2,130 protein subunits, which translates into a viral particle with an average size of 300 nm in length, a diameter of 18 nm, and a hollow center of 2 nm in radius, where the genome occupies a radius close to the 4 nm.

Top view of the TMV protein coat (Source: Deposition authors: Stubbs, G., Pattanayek, R., Namba, K.; visualization author: User: Astrojan / CC BY (https://creativecommons.org/licenses/by /4.0) via Wikimedia Commons)

Genome

Genomic RNA is sandwiched between the successive turns of the helix that make up the envelope, joining three of its nucleotides to each protein subunit and thus remaining completely covered in protein.

This genome is 6,395 nucleotides long and has an inverted 7-methyl-guanosine "hood" attached to its 5 'end via a triphosphate bond.

The information encoded in the TMV genome corresponds to 4 genes that encode 4 different products:

- Two proteins associated with replication, one of 126 kDa and the other of 183 kDa, directly translated from the virus RNA

- A movement protein (MP, from the English Movement Protein) and a structural or coat protein (CP, from the English Coat Protein), which are translated from “subgenomic” RNAs

A successful TMV infection involves the cooperation of these four multifunctional products with many of the cellular components of the host plant, especially the cell membrane and the cytoskeleton.

Replication

To understand the replication mechanism of TMV it is necessary to understand some aspects of infection with this virus.

Initial infection

TMV only enters a plant through mechanical wounds that temporarily "open" the plasma membrane or cause pinocytosis events.

Infection can occur from wounds caused by handling with infected hands and through infected pruning instruments, etc., but is rarely transmitted by insects.

Once in the cytosol, the viral particles disassemble and release their genomic RNA, which is recognized by the cell as its own RNA and is translated by specialized cytosolic enzymes for this purpose.

The methyl guanosine “hood” of the TMV genomic RNA is of utmost importance for this process, since it manages to “bypass” the cell's “surveillance” system and promote its interaction with other cellular components.

The number of assembled viral particles increases rapidly and these can leave the infected cell and infect other neighboring cells through plasmodesmata, which are "channels" that connect the cytosol of a cell with that of the cells that surround it.

Eventually, the viral particles reach the plant's translocation system, that is, the xylem and phloem, thus dispersing throughout the plant.

How is the replication process?

The tobacco mosaic virus uses its genome as a template to synthesize negative complementary strands that serve as a template for the synthesis of a large number of positive strands.

These templates are also used for the synthesis of "subgenomic" messenger RNAs that contain the open reading frames for the MP and CP proteins.

The two replication-associated proteins that are encoded in TMV genomic RNA have methyl transferase, helicase, and RNA-dependent RNA polymerase domains.

Replication appears to occur in a complex associated with the endoplasmic reticulum membrane that contains these proteins, the movement protein (MP), viral RNA, and other plant host proteins.

Symptoms

Symptoms of the tobacco mosaic virus vary greatly from one plant species to another. In other words, they depend considerably on the type of host plant and, furthermore, on the virus strain, on the genetic "background" of the plant and on the environmental conditions where it is found.

Photograph of the leaf of a tobacco plant infected with TMV (Source: RJ Reynolds Tobacco Company Slide Set / Public domain, via Wikimedia Commons)

Symptoms usually appear around 10 days after the initial infection and these are:

- Appearance of brown or yellowish spots with a mosaic-like matron on the leaf blades

- Necrosis

- Stunted growth

- Leaf curl

- Yellowing of tissues

- Low yield in fruit production and even appearance of damaged and deformed fruits

- Delay in the ripening of the fruits

- Color of the fruits not uniform (especially in tomato)

References

1. Butler, PJG (1999). Self – assembly of tobacco mosaic virus: the role of an intermediate aggregate in generating both specificity and speed. Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, 354 (1383), 537-550.
2. Liu, C., & Nelson, RS (2013). The cell biology of Tobacco mosaic virus replication and movement. Frontiers in plant science, 4, 12.
3. Mphuthi, P. (2017). Tobacco Mosaic Virus symptoms, transmission and management. Farmer's Weekly, 2017 (17014), 60-61.
4. Rifkind, D., & Freeman, G. (2005). The Nobel Prize winning discoveries in infectious diseases. Elsevier.
5. Scholthof, KBG (2000). Lessons in plant pathology: tobacco mosaic virus. Plant Health Instr.
6. Scholthof, KBG (2004). Tobacco mosaic virus: a model system for plant biology. Annu. Rev. Phytopathol., 42, 13-34.