by Microbiology Doctor dr (doctor-dr)(doctor_dr)
Plant Diversity - Nomenclature and Classification of Viruses 
The International Committee on Taxonomy of Viruses, has worked constructively towards a meaningful and practical scheme of viral classification.
Nomenclature
The criteria and procedures for naming different taxa have been agreed upon by animal and bacterial virologists. The family names finish in -viridae, the subfamily names in -virinae, and the genera and species in -virus, according to these. Plant virologists, on the other hand, do not divide viruses into families or genera. They employ viral families with comparable characteristics. The names of these groups are taken from the name of the group's prototype; for example, the tobamo group or tobamovirus is the name of a group of viruses related to tobacco mosaic virus.
Classification
Many attempts have been made to categorise viruses. One of the oldest categorization systems, which is still in use, classified viruses according to the type of host they typically infect, such as fowl plague virus, cucumber mosaic virus, tobacco mosaic virus, and so on. Another early categorization method, for example, was based on virus tissue affinities, such as neurotropic (nerve tissue) viruses. However, as methods for measuring the physical, chemical, and biological features of viruses have been discovered, data has gathered that allows for the creation of a categorization scheme based on these qualities for all viruses. These are some of the properties:
i. The chemical make-up of the nucleic acid, whether it's DNA or RNA, and whether it's single-stranded or double-stranded.
ii. The structure of virion: helical, icoshedral or complex; naked or enveloped.
iii. The replication site, whether in the nucleus or in the cytoplasm.
The chemical nature of nucleic acid plays an important role in virus classification. David Baltimore (1971) proposed a classification of viruses based on how the viral genome is replicated and expressed. All viruses are divided into six classes on the basis of their method of mRNA synthesis or transcription. However, this scheme group very different virions together in the same class, for example bacteriophages and animal viruses.
Viruses that infect animals and plants are typically categorised individually. Animal viruses are classified into families, genera, and species, each of which is given a Latin name, though English names are also used. Plant viruses are classified according to the following criteria:
(i) the structure of the virion,
(ii) whether it contains DNA or RNA, and
(iii) its mode of transmission. A classification scheme on the basis of Latin family names has been proposed for bacteriophages, but this system is rarely used.
In 1966, Lwoff et al presented a new scheme of categorization, which was adopted by the Provisional Committee on Virus Nomenclature (PCNV). The system is based on the nucleic acid type, particle symmetry, the existence or absence of an envelope around the nucleocapsid, the diameter of the helicoid nucleocapsid, and the number of capsomeres in cuboidal kinds.
AN OUTLINE OF CLASSIFICATION BY LWOFF et al.
PHYLUM: VIRA
The phylum includes all the viruses. It is further divided into two sub-phyla.
1. Sub-Phylum: Deoxyvira
The sub-phylum includes viruses with DNA core. It is classified into three classes.
i. Class: Deoxyhelica: The class includes DNA viruses exhibiting helical symmetry. The class comprises of only one order:
Chaetovirales, and a single family: Poxviridae.
ii. Class: Deoxycubica: The class contains DNA viruses with cubical symmetry. It is divided into two orders: Haplovirales (viruses without envelope) comprising of six families; and Peplovirles (viruses with envelope) containing only one family: Herpesvirdae.
iii. Class : Deoxybinala: The class possesses DNA containing binal viruses. It includes a single order: Urovirales comprising of only family: Phagoviridae.
2. Sub-Phylum: Ribovira
The sub-phylum includes viruses with RNA core. It is further divided into two classes.
i. Class: Ribohelica: The class includes RNA viruses exhibiting helical symmetry. The class includes two orders: Rhabdovirales (viruses without envelopes) comprising of six families; and Sagorvirales (viruses with envelope) consisting of three families.
ii. Class: Ribocubica: The class contains RNA viruses exhibiting cubical symmetry. It comprises of two orders: Gymnovirales (viruses without envelope) comprising of two families; and Togovirales (viruses with envelopes) consisting of single family.
VIROIDS
Viroids are a kind of infectious agent made up entirely of the nucleic acid RNA. In certain viroids, the RNA molecule is circular, single-stranded, and comprises less than 400 nucleotides. Others include single-stranded linear RNA molecules. They are responsible for a number of significant cultivated plant diseases, including potato spindle tuber, cucumber pale fruit, and chrysanthemum stunt, among others. The mechanism by which viroids cause illness is unknown. Because of their nucleoplasmic position and apparent inability to operate as mRNA, it is hypothesised that they induce host symptoms by interfering with gene regulation in infected host cells. Viroidlike nucleic acids may possibly exist in species other than higher plants, according to current data. They might be to blame for a variety of human and animal illnesses. However, how they get around nucleases in animal cells is still unknown.
PRIONS
Two human illnesses, kuru and Jakob-Creutzfeldt disease, as well as one sheep disease, scrapie, are caused by macromolecules (proteins) smaller than any known virus. A gradual, deadly deterioration of brain tissue characterises these illnesses. Attempts to purify the causative agents were unsuccessful, although a highly enriched preparation for the kuru and scrapie causative agents was developed. It's mostly made up of proteins with molecular weights of around 30,000. Prions are infectious macromolecules that cause these illnesses, and if they are shown to be fully made of protein, they will represent a fundamentally new class of infectious agents. It's unclear how such an entity might replicate.
TOBACCO MOSAIC VIRUS (TMV) DISEASE
The tobacco mosaic virus is a rod-shaped virus that causes tobacco mosaic disease (TMV). It consists of a single strand of RNA shielded by a capsid made up of protein subunits and measures around 300 nm in length and 15 nm in diameter. Each subunit is made up of 158 amino acids that are organised in a helix shape. TMV is one of the most thermostable viruses that has been discovered so far. It can endure temperatures of up to 93 degrees Celsius.
Symptoms
The development of mottled dark green and light-green patches on tobacco leaves is the most prevalent sign. These look as blister-like regions that are dense and raised. The plant's development is slowed, and the leaves curl downward and deform. The leaves may grow elongated and thin. Petioles are impacted as well, becoming puckered (enlarged). TMV mostly affects young leaves.
Infection and Distribution
TMV can be found in the soil in infected leaves and stalks, as well as on the surfaces of contaminated seeds. The virus first infects a plant by infecting the damaged tissues of seedlings or transplants in the field. It then spreads over the field. The infection spreads throughout the body, infecting parenchyma cells and spreading via the phloem.
TMV is easily transferred by mechanical sap, grafting, and dodder. The most frequent way for TMV to spread in a field or greenhouse is through the hands of employees who handle both infected and healthy plants.
Control of Disease
The following steps help eliminating the disease:
i. The sanitation is the main method of disease control. The crop should not be grown at least for two years in the fields where diseased crop was grown.
ii. Removal of diseased plants and weeds that harbour virus early in the season also helps in reduction and elimination of subsequent spread of virus.
iii. Workers in the field must wash their hands with 3% trisodium phosphate or soap.
iv. TMV resistant varieties of tobacco may be grown.
SOME IMPORTANT QUESTIONS
1. What is the difference between Viruses and Bacteria. Give in detail their importance as disease causing organisms in plants. Give an example of ay viral disease of plants in detail. (2000 S)
2. Discuss in detail various steps during replication of a virus.
3. Describe the nature of a bacterial virus (bacteriophage) with respect to its size, growth, metabolism and genome.
4. Give an account of ultrastructure and life cycle of a bacteriophage.
5. What are viroids? How do they differ from viruses? Give a list of diseases caused by viroids.
6. Write notes on the followings:
(i) Symmetry groups of viruses
(ii) TMV
(iii) Lysogeny
(iv) Retroviruses
(v) Prions
7. What major characteristic are used in classification of viruses? Discuss briefly nomenclature and classification of viruses. Enlist some common families and groups of animal, plant and bacterial viruses.
8.Write a comprehensive note on tobacco mosaic disease.
9. How viruses are future threat to humans? Explain with examples.
~1.webp)
