The first viral virus to be discovered, purified, crystallised, and sequenced – the tobacco mosaic virus is indeed worth knowing about.
Scientific viral classification
Kingdom – orthornavirae
Phylum – Kitrinoviricota
Species – tobacco mosaic virus
A helical structure – open to genetic modifications
diagrammatic representation of TMV
The virus is a rodlike, submicroscopic particle with a rigid piece of nucleic acid, the ribose nucleic acid encased in a protein coat.[viral] It has a length of around 300 nm and a diameter of 18 nm.[viral] The capsids contain 2130 identical subunits of protein which facilitate the formation of a helical structure by assembling around the nucleic acid.[viral]
Dates back to the colonial era
Tobacco was grown as a cash crop in America. Due to intensive cultivation, tobacco depleted the soil and left it vulnerable to diseases. One such disease was found in the early 19th century.[viral] It was then named “Amulatamiento” (the tobacco leaf appeared as mulatto or a mixture)
close up of a healthy leaf and an infected one
Green areas with a mosaic pattern.[viral] This caused the plants to take a laden-grey colour and the tobacco turned extremely bitter. In 1879, the disease was named ‘ mosaic disease of tobacco’ by Adolph Mayer. Later a Russian biologist named Dmitry Ivanovsky investigated the mosaic disease and found that it was caused by a virus. This was confirmed by the Dutch botanist William Beijerick.
Subsequently, the virus was studied in detail and proved to be composed of a single-stranded RNA virus of filamentous morphology.
The TMV disease cycle and its epidemiology are intimately related because the virus is completely dependent on the host for replication and spread. There is wide variation in disease incidence, depending on the time of disease onset in the field and on cropping practices.
For example, a few plants could become infected early in the season, either from TMV on the seed coat or by workers contaminating plants. The disease could then spread rapidly throughout the field or greenhouse by TMV-infected plants contacting healthy plants, or by equipment or workers. TMV can also survive or overwinter in infected plant debris or perennial (weedy) hosts and, perhaps, in the soil. Agricultural practices, such as continuous cropping, have the potential to be a particular problem, especially in greenhouse facilities, where TMV inoculum may increase in more than one plant species.
Mechanical transmission – spreads by abrasion
It has the potential to affect 350 different species of plants. Sucking insects do not spread the virus while chewing insects do. The virus can multiply in a living cell but can survive in a dormant state in dead tissues. Thus, the virus can infect growing plants for years even after the infected plant part is dead. This is rare in other viruses.
Vegetative propagation perpetuates the virus. It can survive on surfaces, and in the dried plant sap. Tobacco products may carry TMV. In cigarette making, repeated heating inactivates most of it. However, it is not transmitted through the smoke of burning tobacco.
Accumulation – disassembly- replication-cellular motion
Mechanical wounds that open up the plasma membrane are a doorway to the virus. Once the virus enters the cell, it accumulates itself by disassembling and reaches the cytoplasm through intra-cellular motion and then it replicates to undergo inter-cellular motion.
Mosaic patches and the other symptoms
The plants infected with TMV show different features based on the species type. In a few plants, the symptoms are masked while a few of them clearly show them. Stunting, mosaic patterns of light and dark patches on the leaf, malformation of leaves, and distinct yellowing are a few of them.
Applications in the modern technology
Apart from being first discovered, crystallised and sequenced, the virus is also the first one to be genetically modified and finds application in bioengineering.
Pooja kumari sha