In this article we will discuss about:- 1. Shape of Viruses 2. Microstructure of Viruses 3. Economic Importance 4. Affinities 5. Control of Diseases.
Shape of Viruses:
The shapes of virus particles or viruses are known chiefly from electron micrographs. The viruses may be spherical, rod like, brick-shaped B or shaped like a tadpole with a head and slender tail (D).
On the basis of the shapes of the viruses, the plant viruses chiefly fall under the following three categories:
(i) Straight tubular, long, rigid rods with a helical architecture (A):
To this category belong the viruses of Tobacco mosaic. Barley stripe mosaic and Tobacco rattle.
(ii) Long flexuous thread-like rods (D):
The common examples of this type are potato latent mosaic, common Bean mosaic. Lettuce mosaic and wheat streak mosaic virus.
(iii) Polyhedral viruses (C):
Their gross shape has often been described as spherical. The common examples of this category are Turnip yellow mosaic. Cucumber mosaic. Squash mosaic. Tobacco ring spot.
The polyhedral virus particles have numerous facets. Some of the polyhedron viruses of Tomato spotted wilt have been reported to have a tail.
The Alfalfa mosaic virus particles which were formerly described as spherical have been shown to be short, rigid rods. The viruses of Lettuce necrotic yellow virus, Gomphrena virus and potato yellow dwarf virus are bullet-shaped. The viruses in gross appearance are thus as heterogeneous as bacteria.
Microstructure of Viruses:
The mature virus particle consists of an outer, inert, non-genetic protein coat or shell which surrounds and protects the genetic material or nucleic acid forming the core.
The protein coat is called the capsid and the nucleic acid molecules in its protein coat is termed nuclcocapsid. The capsid is made up of numerous small subunits called the capsomeres. In turnip yellow mosaic, the protein shell is composed of 32 capsomeres.
The capsomeres constituting the capsid are similar but those of different strains vary in kind and aminoacids. The capsomeres are closely packed and are arranged in a geometrical pattern around the nucleic acid core resulting in helical, cubical, or complex symmetry.
Helical symmetry is characteristic of Tobacco mosaic virus. Cubical symmetry is illustrated by many viruses. The capsomeres in this case are closely packed in triangular pentagonal, hexagonal or combination of arrangement.
Adenovirus is an example of cubic symmetry. Examples of complex symmetry are Vaccinia virus and some large bacterial viruses.
1. Phytophagineae:
The viruses of this suborder attacking plants are the most important plant pathogens. They are included in the suborder Phytophagineae. Some plant viruses have been separated from the host cells and crystallized.
Prof. W.M.Stanley shared a Nobel Prize in 1946 in isolating from the diseased plants of tobacco the needle or rod-shaped crystals of Tobacco mosaic virus. On analysis, the virus particles consist of a core of ribonucleic acid (RNA) with a heath of virus protein around it.
The isolated virus panicles, which apparently are non-living, will invade fresh host cells and cause the typical disease. RNA is the infective part. Within the host, it will produce many new virus particles. The virus can exist in the crystallized form indefinitely.
The plant viruses are vastly destructive. In the past few years several hundred new plant viruses have been discovered. The number of diseases they cause is not yet definitely known. Majority of these have been recognised in the flowering plants.
Many of the plant viruses attack a large number of our annual crops beans, potatoes, cucumbers, sugar beets, tobacco and tomatoes and cause considerable damage. Fortunately none of our major cereal crop plants is a victim to them.
Losses are far greater in horicultural plants, plantation crops, vegetables and tuber crops. The most important plant diseases caused by the viruses are a group of mosaics. The common virus mosaic diseases are on tobacco, sugarcane, cucumber, tomato, peach, potato and sugar beets.
The mosaic diseases (Fig. 19.2 A) cause mottling or variegation of leaves. They are not fatal. They cause lowering of vigour accompanied by stunted growth. There is, however, enormous loss in the crop both in quality as well as quantity of the yield.
Much work in viruses has not been done in India so far. The important virus diseases reported in India are Rugose masaic and Leaf roll of potato, yellow-vein mosaic of Bhindi, leaf curl of tomato, mosaic and streak of sugarcane, leaf curl of tobacco, mosaic of cucurbita and groundnut rosette.
Blossom malformation disease of mango, which is the well-known national fruit of India, is also a virus disease. It is a great menace to our mango plantations. The bunchy top disease of banana is also equally devastating.
Economic Importance of Viruses:
The viruses are generally considered of great negative economic importance as they cause serious human and animal diseases and vastly destructive plant diseases. However, not all viruses cause disease resulting in death or malformations of the host.
In some of them, for instance, those of fig trees and many ornamental plants, bring about only mottling of leaves and have no harmful effect. In some cases the viruses may destroy some chlorophyll but what usually remains is more than enough for the leaves to carry on photosynthesis. The petals of flowers of tulips develop beautiful striations of various shades due to viral infection.
The viruses also interfere in the working of many industrial processes dependent on bacterial action. They attack the bacteria involved in the industrial process and cause loss in economy.
The curative properties of waters of certain springs and Ganga at Haridwar are attributed to the presence of viruses in the water which attack and lyse the pathogenic bacteria (bacteriophages) and the bacteria of decay and decomposition.
From the evolutionary point of view, the viruses are important as they stand at the very threshold of life representing the starting point in the evolution of other organisms.
Affinities of Viruses:
Another group of organisms with possible affinities with viruses are the bacteria. The former are entirely heterotrophic in their mode of nutrition and the latter are largely so. There is a group of organisms closely related to bacteria known as the Rickettsiae.
The latter resemble bacteria in certain respects and viruses in others. In their spherical or rod-like form, they are bacteria but are in the range of large viruses. In fact it is doubted whether rickettsia are organisms at all.
Furthermore, like viruses they are obligate parasites. Outside the specific host cells they are quite inert like viruses and thus unable to carry out having functions.
Control of Viral Diseases of Plants:
Various practices and control measures are employed to check the spread of virus diseases.
These are:
1. Destruction of Weed Hosts of Viruses:
Many weeds harbour soil born viruses. They serve as the source of virus infection for cultivated crops grown in the same soil. Destruction of these will be of value to check the spread of a number of virus diseases.
2. Insect Control:
To reduce the number of insect vectors by sprays of insecticides and antibiotic techniques may prove to be of value in the control of some virus diseases.
3. Growing Resistant varieties of Crops:
The best method of control is the use of resistance varieties. Some success in the production of virus resistant varieties of plants has been achieved.
4. Careful rogueing of diseased or suspected plants and control of the vectors with effective insecticides are also helpful to a considerable extent. The plants showing suspicious symptoms should be removed from the field and burnt.
5. Inactivation of Virus by Chemicals:
Fumigation by insecticides, sprays or dusts under glass house conditions are quite effective. 8-a Zaquanine, Catechol, Quinol, Phenol, Thiouracil, Nicotine are some of the chemicals with which success has been obtained.
6. Thermal Inactivation:
Many viruses can be destroyed simply by heating. The thermal inactivation point varies with different species.
7. Desiccation:
A few viruses can be destroyed by drying.