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We know now that smallpox was caused by something considerably smaller in size than bacteria. During Pasteur's time, virtually all infectious agents, including bacteria, protozoans, and yeast, were called viruses. One of Pasteur's associates, Charles Chamberland, discovered that porcelain filters would block out bacteria but would not keep an unseen agent from passing through. The agent caused rabies, another serious disease of both animals and humans. Agents of disease that could pass through filters became known as filterable viruses, although the word filterable is no longer used. Today, we know that not only smallpox and rabies are caused by these viruses but also measles, mumps, chicken pox, polio, yellow fever, influenza, fever blisters, warts, and the common cold.

Only those organisms that have certain unique features are now called viruses. These features, which include a complete lack of cellular structure, make viruses quite different from anything else in the six kingdoms of living organisms we now recognize. In fact, some question whether or not viruses are even living organisms. In 1946, Wendell Stanley, an American chemist, received a Nobel Prize for demonstrating that a virus causing tobacco mosaic, a common plant disease, could be isolated, purified, and crystalized and that the crystals could be stored indefinitely but would always produce the disease in healthy plants at any time they were placed in contact with them. We also know that viruses do not grow by increasing in size or dividing, nor do they respond to external stimuli. They cannot move on their own, and they cannot carry on independent metabolism.

Viruses are incredibly numerous. In 1989, for example, marine biologists at the University of Bergen in Norway discovered that a teaspoon of sea water typically contains more than 1 billion viruses. They are about the size of large molecules, varying in diameter from about 15 to 300 nanometers (Fig. 17.16). Thousands of the smallest ones could fit inside a single bacterium of average size.

Viruses consist of a nucleic acid core surrounded by a protein coat. The architecture of the protein coats varies considerably, but many have 20 sides and resemble tiny geodesic domes, while others have distinguishable head and tail regions. The nucleic acid core consists of either DNA or RNA—never both. Viruses have been classified in several ways. Originally, they were grouped according to their hosts and the types of tissues or organs they affected. Now they

Hampton Park Zoo Charleston
Figure 17.16 Papavoviruses in a human wart, x52,000. (Electron micrograph courtesy Richard S. Demaree Jr.)
Ebola Protein Coat

The book Hot Zone and the movie Outbreak have created an awareness of emerging viruses and their dangers to the human population. The Ebola, Hanta, and HIV viruses are now everyday words that have become synonymous with death. There is another group of viruses that also has a significant impact—plant viruses—which cause an estimated $15 billion worth of crop loss per year worldwide. They infect plants and cause hundreds of diseases, such as tomato spotted wilt disease, tobacco mosaic disease, maize stripe disease, and apple chlorotic leaf spot disease.

In total, nearly 400 plant viruses have been identified and classified by the International Committee on Taxonomy of Viruses (ICTV). Another 320 have been identified but are awaiting final classification.

Surprisingly, the first viruses ever identified were in plants. In 1898, a Dutch professor of microbiology, Dr. Martinus Beijerinck, was working to identify the disease that caused tobacco leaves to become mottled with light green and yellow spots. He demonstrated that the condition was not caused by a bacterium as was commonly thought at the time, but rather by some other unknown pathogen in the sap of the tobacco plant. He proved this by collecting sap from a diseased plant that was then passed through a filter capable of straining out any bacteria. When the filtered solution was reinjected into the leaf veins of healthy plants and the disease was transmitted, he had made his point. He called this filtered sap a contagium vivum fluidium (a contagious living fluid) and introduced the term virus to describe its property of being able to reproduce itself within living plants. Dr. Beijerinck's virus was later named tobacco mosaic virus (TMV), consistent with the now-established practice of naming plant viruses both by the plant it infects and by describing the major disease symptom (e.g., mosaic, wilting, spotted, etc.).

Not only were the first viruses discovered in plants, but the understanding of their biochemical nature was first recognized through research on tobacco mosaic virus. Today, we know that viruses are submicroscopic, infectious particles that are composed of a protein coat and a nucleic acid center. They can be seen only with an electron microscope. As obligate parasites, they can reproduce themselves only with a living cell. The biochemical nature of viruses remained unknown until 1935 when Dr. Wendell Stanley, an organic chemist in the United States, succeeded in crystallizing the protein coat of tobacco mosaic virus (TMV). Stanley, however, did not recognize the nucleic acid content of the virus that was later shown to be RNA. The fact that RNA could exist separately from DNA was a discovery that has had great influence on the development of molecular biology thought.

Today, we know that tobacco mosaic virus is a rigid rod, 300 nanometers by 15 nanometers, composed of a protein coat of approximately 2,100 helically arranged protein subunits surrounding an axial canal that contains a single-stranded RNA molecule consisting of 6,400 nucleotides. It, like all plant viruses, is classified according to the type of nucleic acid that it contains, either DNA or RNA but never both; whether the nucleic acid is single- or double-stranded; and the shape of the virus particle (spheres, stiff rods, flexible rods).

TMV is highly contagious, so much so that it can be transmitted to healthy plants merely from the fingers of smokers of cigarettes that were made from infected tobacco. This is unlike most other plant viruses that can survive no more than a few hours outside their living host. Plant viruses can gain entry into a plant only through an open wound or puncture and are typically transmitted by insect vectors such as aphids, leafhoppers, white flies, and mites. Aphids are the most important vectors, infecting healthy plants when they insert their mouth parts, called stylets, into phloem tubes for feeding. During feeding, they inject salivary secretions containing the virus particles into the plant's sieve tubes.

Once injected, viruses are transmitted within the phloem and move throughout the plant. However, viruses cannot move directly through cell walls. Rather, cell-to-cell movement of viral particles occurs via the plasmodes-mata (singular: plasmodesma), which are membrane-lined cylindrical pores through cell walls. Plasmodesmata create cytoplasmic bridges that cross cell walls to connect adjacent cells, and this transport route explains why many viral infections are systemic, affecting the entire organism.

Kingd om Bacteria, Kingdom Archaea, and Viruses

Awareness Box Continued

Viruses seldom kill the plant outright, but rather weaken it by causing abnormalities in leaves (such as mottling or changes in leaf color, shape, or vein patterns); changes in flower color; or irregularities in fruit size, shape, or color. Viruses can also cause fruits to ripen prematurely and to have an unpleasant taste or reduced sugar content. Crop yields of fruits and vegetables, as well as quality, can be reduced.

Few options exist for controlling plant viral diseases. The most effective control is achieved by sanitation— removing and burning diseased plants and thus killing the virus-carrying insects. Additionally, naturally resistant varieties of some plants have been developed. Chemicals remain an ineffective treatment for plant viruses because of the cost and environmental concerns.

Viral diseases affect many important agricultural crops in addition to tobacco. Crop losses worldwide are enormous each year. With the world's human population increasing at about 1.6% yearly, any disease that threatens agricultural productivity and the ability of the human population to feed itself must be taken seriously. Although not as spectacular or newsworthy as Ebola or HIV, plant viruses are silent killers because they rob humanity by directly affecting the food supply.

D.C. Scheirer are separated first according to the DNA or RNA in their cores. Then they are grouped according to size and shape, the nature of their protein coats, and the number of identical structural units in their cores.

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