What are viruses?

Why are viruses important?

How are viruses transmitted?

How are viruses classified?

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What are viruses?

Viruses are very small (submicroscopic) infectious particles (virions) composed of a protein coat and a nucleic acid core. They carry genetic information encoded in their nucleic acid, which typically specifies two or more proteins. Translation of the genome (to produce proteins) or transcription and replication (to produce more nucleic acid) takes place within the host cell and uses some of the host's biochemical "machinery". Viruses do not capture or store free energy and are not functionally active outside their host. They are therefore parasites (and usually pathogens) but are not usually regarded as genuine microorganisms.

Most viruses are restricted to a particular type of host. Some infect bacteria, and are known as bacteriophages, whereas others are known that infect algae, protozoa, fungi (mycoviruses), invertebrates, vertebrates or vascular plants. However, some viruses that are transmitted between vertebrate or plant hosts by feeding insects (vectors) can replicate within both their host and their vector. This web site is mostly concerned with those viruses that infect plants but we also provide some taxonomic and genome information about viruses of fungi, protozoa, vertebrates and invertebrates where these are related to plant viruses.

We also provide information about viroids, which are infectious RNA molecules that cause diseases in various plants. Their genomes are much smaller than those of viruses (up to 400 nucleotides of circular single-stranded RNA) and do not code for any proteins.


Why are viruses important?

Viruses cause many diseases of international importance. Amongst the human viruses, smallpox, polio, influenza, hepatitis, human immunodeficiency virus (HIV-AIDS), measles and the SARS coronavirus are particularly well known. While antibiotics can be very effective against diseases caused by bacteria, these treatments are ineffective against viruses and most control measures rely on vaccines (antibodies raised against some component of the virus) or relief of the symptoms to encourage the body's own defense system.

Viruses also cause many important plant diseases and are responsible for huge losses in crop production and quality in all parts of the world. Infected plants may show a range of symptoms depending on the disease but often there is leaf yellowing (either of the whole leaf or in a pattern of stripes or blotches), leaf distortion (e.g. curling) and/or other growth distortions (e.g. stunting of the whole plant, abnormalities in flower or fruit formation).

Yellow mosaic symptoms on lettuce caused by Lettuce mosaic virus. Figure from Description 399 (photo: INRA Avignon, France).
Yellow vein-banding symptoms on grapevine caused by Grapevine fanleaf virus. Figure from Description 385
Fruit distortion on eggplant fruit caused by Tomato bushy stunt virus. A healthy fruit is shown on the left. Figure courtesy of F. Garcia Arenal, from Description 382.
Bark scaling caused by Citrus psorosis virus. Figure from Description 401.
These symptoms and many others are further illustrated in the various virus Descriptions on this site. Sometimes the virus is restricted to certain parts of the plant (e.g. the vascular system; discrete spots on the leaf) but in others it spreads throughout the plant causing a systemic infection. Infection does not always result in visible symptoms (as witnessed by names such as Carnation latent virus and Lily symptomless virus, both members of the genus Carlavirus). Occasionally, virus infection can result in symptoms of ornamental value, such as 'breaking' of tulips or variegation of Abutilon.

Plant viruses cannot be directly controlled by chemical application. The major means of control (depending on the disease) include:


How are viruses transmitted?

Some important animal and human viruses can be spread through aerosols. The viruses have the "machinery" to enter the animal cells directly by fusing with the cell membrane (e.g. in the nasal lining or gut).

By contrast, plant cells have a robust cell wall and viruses cannot penetrate them unaided. Most plant viruses are therefore transmitted by a vector organism that feeds on the plant or (in some diseases) are introduced through wounds made, for example, during cultural operations (e.g. pruning). A small number of viruses can be transmitted through pollen to the seed (e.g. Barley stripe mosaic virus, genus Hordeivirus) while many that cause systemic infections accumulate in vegetatively-propagated crops. The major vectors of plant viruses are:


How are viruses classified?

The highest level of virus classification recognises six major groups, based on the nature of the genome:

Within each of these groups, many different characteristics are used to classify the viruses into families, genera and species. Typically, a combination of characters are used and some of the most important are:

Particle morphology: Amongst plant viruses, the most frequently encountered shapes are:
Isometric: apparently spherical and (depending on the species) from about 18nm in diameter upwards. The example here shows Tobacco necrosis virus, genus Necrovirus with particles 26 nm in diameter.
Rod-shaped: about 20-25 nm in diameter and from about 100 to 300 nm long. These appear rigid and often have a clear central canal (depending on the staining method used). Some viruses have two or more different lengths of particle and these contain different genome components. The example here shows Tobacco mosaic virus, genus Tobamovirus with particles 300 nm long.
Filamentous: usually about 12 nm in diameter and more flexuous than the rod-shaped particles. They can be up to 1000 nm long, or even longer in some instances. Some viruses have two or more different lengths of particle and these contain different genome components. The example here shows Potato virus Y, genus Potyvirus with particles 740 nm long.
Geminate: twinned isometric particles about 30 x 18 nm. These particles are diagnostic for viruses in the family Geminiviridae which are widespread in many crops especially in tropical regions. The example here shows Maize streak virus, genus Mastrevirus.
Bacilliform: Short round-ended rods. These come in various forms up to about 30 nm wide and 300 nm long. The example here shows Cocoa swollen shoot virus, genus Badnavirus with particles 28 x 130 nm.
Further details can be found in the genus description pages and on the Rothamsted Electron Micrographs of Plant Viruses page.

Genome properties: Important features include:

Biological properties:

Serological properties: Many viruses are good antigens (elicit strong antibody production when purified preparations are injected into a mammal) and this property has been widely exploited to produce specific antibodies that can be used for virus detection and for examining relationships between viruses. Earlier studies used agar diffusion plates but in the last 20 years these have been largely superseded by ELISA (enzyme-linked immunosorbent assay) procedures. Although serological properties are still important, their significance in taxonomy has declined to some extent now that nucleotide sequence data are available.