Plant viruses are viruses that multiply by infecting plant cells and utilizing the plant cell's genetic replication machinery to manufacture new virus particles.
Plant viruses do not infect just a single species of plant. Rather, they will infect a group of closely related plant species. For example, the tobacco mosaic virus can infect plants of the genus Nicotiana. As the tobacco plant is one of the plants that can be infected, the virus has taken its name from that host. This name likely reflects the economic importance of the virus to the tobacco industry. Two other related viruses that were named for similar economic reasons are the potato-X and potato-Y viruses. The economic losses caused by these latter two viruses can be considerable. Some estimates have put the total worldwide damage as high as $60 billion a year.
The tobacco mosaic virus is also noteworthy as it was the first virus that was obtained in a pure form and in large quantity. This work was done by Wendall Meredith Stanley in 1935. For this and other work he received the 1946 Nobel Prize in Chemistry.
Plants infected with a virus can display lighter areas on leaves, which is called chlorosis. Chlorosis is caused by the degradation of the chlorophyll in the leaf. This reduces the degree of photosynthesis the plant can accomplish, which can have an adverse effect on the health of the entire plant. Infected plants may also display withered leaves, which is known as necrosis.
Sometimes plant viruses do not produce symptoms of infection. This occurs when the virus become latent. The viral nucleic acid becomes incorporated into the host material, just as happens with latent viruses that infect humans such as herpes viruses and retroviruses .
Most of the known plant viruses contain ribonucleic acid (RNA ). In a virus known as the wound tumor virus, the RNA is present as a double strand. The majority of the RNA plant viruses, however, possess a single strand of the nucleic acid. A group of viruses known as gemini viruses contain single stranded deoxyribonucleic acid (DNA ) as their genetic material, and the cauliflower mosaic virus contains double stranded DNA.
As with viruses of other hosts, plant viruses display different shapes. Also as with other viruses, the shape of any particular virus is characteristic of that species. For example, a tobacco mosaic virus is rod-shaped and does not display variation in this shape. Other plant viruses are icosahedral in shape (an icosahedron is a 20-sided figure constructed of 20 faces, each of which is an equilateral triangle).
There are no plant viruses known that recognize specific receptors on the plant. Rather, plant viruses tend to enter plant cells either through a surface injury to a leaf or the woody stem or branch structures, or during the feeding of an insect or the microscopic worms known as nematodes. These methods of transmission allow the virus to overcome the barrier imposed by the plant cell wall and cuticle layer. Those viruses that are transmitted by insects or animals must be capable of multiplication in the hosts as well as in the plant.
Plant viruses may also be transmitted to a new plant host via infected seeds from another plant. In the laboratory, viral DNA can be introduced into the bacterium Agrobacterium tumefaciens. When the bacterium infects a plant, the viral DNA can be incorporated into the plant genome. Experimental infection of plants can be done by rubbing virus preparation into the leaves of the plant. The virus can enter the plant through the physical abrasion that is introduced.
As humans can mount an immune response against viral infection, so plants have defense strategies. One strategy is the presence of a tough cell wall on many plants that restricts the entry of viruses unless the surface barrier of the plant is compromised, as by injury. Many plants also display a response that is termed hypersensitivity. In this response the plant cells in the vicinity of the infected cell die. This acts to limit the spread of the virus, since the virus require living cells in which to replicate.
Some plants have been shown to be capable of warning each other of the presence of a viral infection. This communication is achieved by the airborne release of a specific compound. This behavior is similar to the cell to cell signaling found in bacterial populations, which is known as quorum sensing .
See also Viral genetics; Virology