Explain how viruses reproduce in plant cells. Include in your discussion what or

Question

Explain how viruses reproduce in plant cells. Include in your discussion what organelles plants have that viruses lack (You can read more about viruses in Chapter 17 of your textbook, pages 315-320). Read about how viruses are used to treat plant diseases:

Website- Helping wheat defend itself against damaging viruses

Website-My enemies' enemy is my friend

Next, read about the roles of viruses in gene therapy(website)

Use to following prompts to shape your discussion about the use of viruses to treat diseases:

Explain the difference between these two techniques of using viruses to treat diseases. What is your opinion about both/either of these techniques?

Do you think that either is more promising?

What negative consequences could come from either treatment?

If you do think that there are possible negative consequences, does the promise of these techniques outweigh the risks?

(pages 315-320)

Some strains of certain Nostoc species produce antibiotics that kill related strains of the same species. Scytonema hofmannii and other cyanobacteria produce antibiotics called cyanobacterins that kill many different forms of both cyanobacteria and eukaryotic algae. These cyanobacteria undoubtedly play a role in their own survival by inhibiting the growth of competing organisms.

Swimmers in Hawaii occasionally suffer from “swimmers’ itch,” a severe skin inflammation that is caused by a species of Lyngbya that sometimes becomes abundant. Ironically, the toxin produced by these organisms has been demonstrated to suppress leukemia and several other types of cancer.

In human water supplies, cyanobacteria frequently clog filters, corrode steel and concrete, cause natural softening of water, and produce undesirable odors or coloration in the water. Many communities control cyanobacteria in reservoirs through the addition of very dilute amounts of copper sulfate.

More than 40 species of cyanobacteria are known to fix nitrogen from the air at roughly the same rates as the nitrogen-fixing bacteria of the leguminous plants discussed in Chapter 25. They may be more important than originally thought in this regard. In Southeast Asia, so much usable nitrogen is produced in the rice fields by naturally occurring cyanobacteria that rice is often grown for many years on the same land without the addition of fertilizer.

Class Prochlorobacteriae—the Prochlorobacteria

In 1976, Ralph A. Lewin of the Scripps Institute of Oceanography announced the discovery of unicellular, prokaryotic organisms with bright green cells that were living on marine animals called sea squirts found in shallow marine waters of Baja California. These organisms, which were given the name Prochloron, have the chlorophylls a and b of higher plants but no trace of the phycobilin accessory pigments associated with cyanobacteria. Instead, their accessory pigments were confined to the carotenoid pigments found in higher plants. Also, unlike the single membranes of cyanobacteria, those of the new organisms are double.

Lewin considered the pigment differences between cyanobacteria and the bright green cells of Prochloron to be basic enough to warrant recognition at the division level, and he proposed a new division to be known as the Prochlorophyta. Many microbiologists are reluctant to recognize these organisms as belonging to a separate bacterial division because the prokaryotic cell structure and chemistry are similar to those of cyanobacteria and other true bacteria (Fig. 17.13), but others agree with Lewin’s assessment of the significance of the pigment system. While the pigment system is indeed significant, they are treated here as a class of true bacteria because their remaining structure and features are essentially indistinguishable from those of other true bacteria.

In 1984, Dutch biologists discovered a similar organism, which they named Prochlorothrix, in shallow lakes in the Netherlands. It differs from Prochloron in being free-living and filamentous. In the late 1980s, Sally Chisholm of the Massachusetts Institute of Technology identified yet another marine prochlorobacterium that flourishes in dim light at a depth of about 100 meters (328 feet). This organism now appears to be one of the two most numerous bacteria living in ocean waters.

The discovery of prochlorobacteria adds weight to the theory that chloroplasts may have originated from such cells living within the cells of other organisms, especially because the pigments involved are identical with those of higher plants.

Domain (Kingdom) Archaea—the Archaebacteria

Figure 17.13    Colorized transmission electron micrograph of Prochloron. Note the absence of a nucleus and other organelles, and the concentric layers of photosynthetic membranes shown in green. ca. × 12,500. Courtesy of T.D. Pugh & E.H. Newcomb, University of Wisconsin-Madison

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In the early 1980s, these basic differences led University of Illinois microbiologist Carl Woese and his colleagues, who have conducted considerable research on the archaebacteria, to suggest that the organisms should be separated from the true bacteria in a kingdom of their own, a suggestion that was widely adopted. Subsequently, microbiologists suggested the differences between true bacteria and archaebacteria are so fundamental that they should be placed in two superkingdoms called domains. Three distinct groups of bacteria are included in the archaebacteria.

The Methane Bacteria

The methane bacteria, which are the largest group of the archaebacteria, are killed by oxygen and are active only under anaerobic conditions found in swamps, ocean and lake sediments, hot springs, animal intestinal tracts, sewage treatment plants, and other areas not exposed to air. Their energy is derived from the generation of methane gas from carbon dioxide and hydrogen.

Methane, or “marsh gas,” is a principal component of natural gas and may have been a major part of the earth’s atmosphere in early geologic times. It still is present in the atmospheres of the planets Jupiter, Saturn, Uranus, and Neptune and is the main ingredient of firedamp, which causes serious explosions in mines. Methane will burn when it constitutes only 5% to 6% of the air, and a flitting, dancing light called ignis fatuus, or “will-o’-the-wisp,” which is occasionally seen at night over swamps and marshy places, is said to be due to the spontaneous combustion of the gas.

The Salt Bacteria

Commercial salt evaporation ponds and other shallow areas in bodies of water with high salt content often have a unique appearance from above. They can be strikingly red due to the presence of a distinctive group of archaebacteria. These are the salt bacteria, whose metabolism enables them to thrive under conditions of extreme salinity that instantly kills other living cells. The bacteria carry out a simple form of photosynthesis with the aid of a membrane-bound red pigment called bacterial rhodopsin. The concentration of salt inside the cells is much lower than in their surroundings, but their metabolism is so closely tied to their environment that the bacteria die if placed in waters with lower salt concentrations (Fig. 17.14).

The Sulfolobus Bacteria

The sulfolobus bacteria constitute a third group of archaebacteria whose members occur in sulfur hot springs. The extraordinary metabolism of these bacteria allows them to thrive at very high temperatures—mostly in the vicinity of 80°C (176°F), with some doing very well at only 10°C below the boiling point of water. One genus (Pyrodictium), discovered in superheated ocean-floor areas, has a minimum temperature requirement of 82°C (179°F) and an optimum growth temperature of 105°C (220°F), and it can tolerate 110°C (230°F). The environment of species in one order of these thermophilic bacteria is also exceptionally acidic, their hot-springs habitats often having a pH of less than 2 (the neutral point on the 14-point pH scale is 7). One genus (Thermoplasma) in this group is bounded by only a plasma membrane and has no cell wall. It is found only in the embers of coal tailings. Another genus (Thermoproteus) appears to be confined to the geothermal areas of Iceland.

Figure 17.14    The north end of Utah’s Lake Bonneville, as seen from the air. The water has a very high salt content. The pinkish areas are due to a red pigment produced by salt bacteria. The pigment, called bacterial rhodopsin, is involved in a form of photosynthesis. © Kingsley Stern

James Lake and his colleagues discovered that the shape of the ribosomes of sulfolobus bacteria is significantly different from those of other archaebacteria, true bacteria, and eukaryotes and that the chemistry of sulfur-dependent bacteria also distinguishes them from other archaebacteria. They have proposed a third kingdom of page 317prokaryotes named Eocytes (dawn cells). Other microbiologists are hesitant to base kingdom status on ribosome shape, but the controversy over the significance of these discoveries will undoubtedly continue as further research into the matter is pursued.

Human Relevance of the Archaebacteria

Archaebacteria are significant to humans in several ways. In the future, methane bacteria may be used on a large scale to furnish energy for engine fuels and for heating, cooking, and light, because the methane gas they produce can be used to replace the methane in natural gas. Methane has an octane number of 130 and has been used as a motor fuel in Italy for over 40 years. It is clean, nonpolluting, safe, and nontoxic, and it prolongs the life of automobile engines, also making them easier to start. The methane is given off by the bacteria as they “digest” organic wastes in the absence of oxygen. Nine kilograms (20 pounds) of horse manure or 4.5 kilograms (10 pounds) of pig manure fed daily into a methane digester will produce all the gas needed for the average American adult’s cooking needs. Considerably less green plant material is required to produce the same amount of methane.

The digester basically consists of an airtight drum connected by a pipe to a storage tank with a means of drawing off the sludge left after the gas has been produced. The sludge itself makes an excellent fertilizer, although many sludges that originate from municipal and large-scale agricultural plants carry with them toxic levels of metals.

In the United States, anaerobic digesters are used on farms to generate methane from animal manure. Wastewater treatment plants also produce methane as a byproduct. Although some plants use the biogas, most simply burn it. Methane is the primary source of hydrogen in the commercial production of ammonia.

Viruses

Introduction

Smallpox is a communicable disease that apparently was widespread for thousands of years, periodically killing countless numbers of individuals. As it developed in its victims, it appeared as blisterlike lesions on the skin, which later often became permanent pits or depressions. In 1901, an outbreak of smallpox in New York caused 720 deaths. Since that time, however, it has been eliminated in the United States through vaccination. Vaccination involves the introduction of a weakened form of the disease agent into the body. The body’s natural defenses, in fighting the agent, build up an immunity to the disease. The first known vaccinations were performed in England by Benjamin Jesty, a farmer, and Edward Jenner, a country physician. They both had noticed that farmhands working with cows having cowpox, a comparatively mild disease related to smallpox, did not contract smallpox itself during the devastating epidemics of the disease that occurred in Europe from time to time.

In 1796, Jenner scratched the skin of a boy with fluid he had obtained from a cowpox blister on the hand of a milkmaid. Six weeks later, he deliberately inoculated the boy with fluid from a blister of a smallpox victim, but the boy developed no symptoms of the disease. Jenner thus had performed a successful vaccination against a dread disease more than 50 years before Louis Pasteur developed the germ theory of disease.

The World Health Organization believes that as a result of vaccinations and vigilance, smallpox has been eradicated throughout the world, but there has been speculation that it may have been cultured by rogue nations and could reappear as an agent of bioterrorism.

Size and Structure

We know now that smallpox was caused by something considerably smaller 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 organisms with certain unique features are now called viruses. These features, which include a complete lack of cytoplasm or cellular structure, make viruses quite different from anything else in the six kingdoms of living organisms we now recognize. In fact, viruses seem to represent the interface between biochemistry and life. 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 crystallized 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. However, when inside living cells, they express their genes and use cellular machinery to produce more virus particles.

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. 17.15). Thousands of the smallest ones could fit inside a single bacterium of average size.

Figure 17.15    Papovaviruses in a human wart. ×52,000. Courtesy of Richard S. Demaree, Jr.

A virus consists 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 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.

Bacteriophages

Viruses that attack bacteria have been studied extensively. These are called bacteriophages, or simply phages (Fig. 17.16). Some resemble the space exploration vehicles portrayed in science fiction movies. Each phage consists of a head on top of a thin, cylindrical core, which is surrounded by a sheathing coat. At the base of the core are six spiderlike, fibril “legs,” which anchor the virus in place.

Molecular

Plant Viruses

The book Hot Zone and the movie Contagion 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 $60 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.

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 caused not 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, which 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 plants they infect and by 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 TMV. Stanley, however, did not recognize the nucleic acid content of the virus, which 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 (Box Figure 17.3). Like all plant viruses, it 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 a 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.

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Box Figure 17.3    Structure of tobacco mosaic virus.

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 theplasmodesmata (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.

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 1.1% 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

Viral Reproduction

Viruses can replicate (reproduce themselves) only at the expense of their host cells. In doing so, they first become attached to a susceptible cell. Then they penetrate to the interior, some types leaving their coats on the outside. Inside the cell, their DNA or RNA directs the synthesis of new virus molecules, which are then assembled into complete viruses. These are released from the host cell, usually as it dies (Fig. 17.17).

Some viruses (e.g., those causing influenza) can mutate (see Chapter 15) very rapidly, allowing them to attack organisms that previously had been immune to them. As a result, new vaccines constantly have to be developed to combat new strains of viruses. Some viruses greatly affect the metabolism of their host cells. For example, botulism bacteria produce toxins only if specific phages are present and active. Evidence is mounting that many forms of cancer, which usually involves abnormal cell growth, are caused by viruses. Scientists also suspect that all living organisms carry viruses in an inactive form in their cells, and they are trying to discover what causes the inactive viruses to become active.

Cells of higher animals that are invaded by viruses produce a protein called interferon, which is released into the fluid around the cells or into the bloodstream. Minute amounts of interferon in contact with cells cause the cells to produce a protective protein that prevents or inhibits the propagation of many types of viruses within the protected cells and inhibits viruses from causing tumors that transform normal cells into tumor cells.

Because of these properties of interferon, it is now being produced in large quantities for use in controlling certain cancers and many other viral infections. The use of bacteria as hosts for donor DNA is especially promising for the future. This process, in effect, turns the bacteria into interferon synthesis centers. In 1981, scientists at the University of Washington and the Genentech Corporation of San Francisco announced that they had succeeded in producing a form of interferon by splicing interferon genes into yeast cells. Because yeast cells are larger than bacteria, the process can potentially produce much larger quantities of interferon at considerably lower cost than is possible using bacteria.

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Figure 17.16    Phage viruses. (a) Structural detail. (b) Digitally generated stylized scanning electron microscopic view of Enterobacteria phage T4, which infects Escherichia coli. ×25,000. (b) © MedicalRF.com RF

Figure 17.17    Stages in the development of a phage virus within a bacillus bacterium. (a) The virus becomes attached to the bacterium. (b) The DNA of the virus enters the cell. (c) Various components of the virus are synthesized by the DNA of the bacterium. (d) The viral components are assembled into units. (e) The assembled viruses are released as the bacterial wall breaks down.

Human Relevance of Viruses

The economic impact of viruses in both developing and industrialized countries is enormous. The annual loss in work time due to common cold and influenza viruses alone amounts to millions of hours. While discomfort, adverse effects on employment, and even deaths due to viral diseases, such as chicken pox, measles, German measles, mumps, and yellow fever, have declined dramatically because immunizations against the diseases became widespread, they still take their toll. Another viral disease, infectious hepatitis, periodically still claims victims. Guillain-Barré syndrome and Epstein-Barr are debilitating diseases caused by viruses that are apparently carried by nearly everybody, but what triggers them into action is as yet still unknown. Avian flu is caused by the bird flu virus. This virus is typically confined to birds, but transmission to humans has been page 321documented. Symptoms can range from fever and cough to life-threatening respiratory distress. Currently, human-to-human spread of the disease is rare. However, health officials are concerned that a mutation in the virus could allow it to spread rapidly, causing a pandemic (worldwide disease outbreak).

Explanation / Answer

Explain how viruses reproduce in plant cells?

Viruses can replicate or reproduce themselves in their host cells. For this, they first become attached to a susceptible cell. Then they penetrate to the interior, some types leaving their coats on the outside. Inside the cell, their DNA or RNA directs the synthesis of new virus molecules, which are then assembled into complete viruses. These are released from the host cell, usually after the death of the host cell.

what organelles plants have that viruses lack?

They cannot synthesize proteins, because they lack ribosomes and must use the ribosomes of their host cells to translate viral messenger RNA into viral proteins. Nonliving characteristics include the fact that they are not cells, have no cytoplasm or cellular organelles, and carry out no metabolism on their own and therefore must replicate using the host cell's metabolic machinery.

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