A virus is a small infectious particle that can reproduce only after invading living cells of other organisms, the virus’s hosts. Viruses can infect all types of life forms, from animals to bacteria, and cause a wide range of diseases. Although human viruses like those that cause the flu and common cold are best known, there is another group of viruses that also have a significant impact on people: plant viruses, which cause an estimated S60 billion worth of crop loss per year worldwide.
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 agent in the sap (plant liquid) 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 re-injected into the leaf veins of healthy plants and the disease was transmitted, he had made his point. Dr. Beijerinck’s virus was later named tobacco mosaic virus (TMV), consistent with the now-established practice of naming plant viruses by the plants they infect and by the major disease symptoms.
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 center consisting of nucleic acid (genetic material). They can be seen only with an electron microscope. They are obligate parasites,meaning they can reproduce themselves only with a living host cell. The biochemical nature of viruses remained unknown until 1935, when Dr. Wendell Stanley, an organic chemist in the United States, succeeded in crystallizing (imaging) the protein coat of TMV. Stanley, however, did not recognize the nucleic acid content of the virus, which was later shown to be RNA (a type of genetic material). The fact that RNA could exist separately from DNA (another type of genetic material) was a discovery that has had great influence on the development of molecular biology thought.
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, which 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 insects such as aphids and mites. Aphids are the most important source of virus spread, infecting healthy plants when they insert their mouthparts, called stylets, into phloem tubes (plant organs for transporting nutrients) for feeding. 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 plasmodesmata, which are cylindrical openings through cell walls. Plasmodesmata create 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 their 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.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 immense each year.
1
A virus is a small infectious particle that can reproduce only after invading living cells of other organisms, the virus’s hosts. Viruses can infect all types of life forms, from animals to bacteria, and cause a wide range of diseases. Although human viruses like those that cause the flu and common cold are best known, there is another group of viruses that also have a significant impact on people: plant viruses, which cause an estimated S60 billion worth of crop loss per year worldwide.
According to paragraph 1, how do plant viruses affect humans?
ABy making people who eat infected crops sick
BBy decreasing agricultural production
CBy increasing the risk of colds and the flu
DBy developing the ability to move from living plants to people
2
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 agent in the sap (plant liquid) 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 re-injected into the leaf veins of healthy plants and the disease was transmitted, he had made his point. Dr. Beijerinck’s virus was later named tobacco mosaic virus (TMV), consistent with the now-established practice of naming plant viruses by the plants they infect and by the major disease symptoms.
According to paragraph 2, Dr. Martinus Beijerinck made which of the following discoveries?
AHe identified the bacteria that produce mottled leaves in tobacco plants.
BHe invented a method that could be used to remove viruses from plant sap.
CHe discovered how viruses spread from one plant to another.
DHe proved that a plant disease is caused by something other than bacteria.
3
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 agent in the sap (plant liquid) 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 re-injected into the leaf veins of healthy plants and the disease was transmitted, he had made his point. Dr. Beijerinck’s virus was later named tobacco mosaic virus (TMV), consistent with the now-established practice of naming plant viruses by the plants they infect and by the major disease symptoms.
Based on the experiment discussed in paragraph 2, which of the following can be inferred about TMV?
AIt cannot enter tobacco’s sap.
BIt is unaffected by filters that can remove all bacteria from a liquid.
CIt is able to infect many plants other than tobacco.
DIt is able to infect bacteria as well as plants.
4
In paragraph 3, why does the author provide information about Dr. Stanley’s discovery?
ATo show how research on tobacco mosaic virus led to the understanding of viruses’ biochemical characteristics
BTo help explain why viruses need a living cell to reproduce themselves
CTo provide evidence that some scientists misunderstood viruses’ nucleic acid content
DTo identify one of the many ways in which Stanley’s research influenced the development of molecular biology thought
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 center consisting of nucleic acid (genetic material). They can be seen only with an electron microscope. They are obligate parasites,meaning they can reproduce themselves only with a living host cell. The biochemical nature of viruses remained unknown until 1935, when Dr. Wendell Stanley, an organic chemist in the United States, succeeded in crystallizing (imaging) the protein coat of TMV. Stanley, however, did not recognize the nucleic acid content of the virus, which was later shown to be RNA (a type of genetic material). The fact that RNA could exist separately from DNA (another type of genetic material) was a discovery that has had great influence on the development of molecular biology thought.
5
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, which 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 insects such as aphids and mites. Aphids are the most important source of virus spread, infecting healthy plants when they insert their mouthparts, called stylets, into phloem tubes (plant organs for transporting nutrients) for feeding. 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 plasmodesmata, which are cylindrical openings through cell walls. Plasmodesmata create bridges that cross cell walls to connect adjacent cells, and this transport route explains why many viral infections are systemic, affecting the entire organism.
The word “ merely ” in the passage is closest in meaning to
Ain great numbers
Bsimply
Crapidly
Ddirectly
6
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, which 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 insects such as aphids and mites. Aphids are the most important source of virus spread, infecting healthy plants when they insert their mouthparts, called stylets, into phloem tubes (plant organs for transporting nutrients) for feeding. 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 plasmodesmata, which are cylindrical openings through cell walls. Plasmodesmata create bridges that cross cell walls to connect adjacent cells, and this transport route explains why many viral infections are systemic, affecting the entire organism.
Paragraph 4 suggests which of the following reasons that TMV spreads more easily than most other plant diseases do?
ATobacco leaves commonly experience punctures and wounds.
BTobacco leaves are often eaten by aphids and mites.
CTMV can live for an unusually long time outside of a host cell.
DPlants with TMV are more commonly handled by humans than plants with other viruses are.
7
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, which 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 insects such as aphids and mites. Aphids are the most important source of virus spread, infecting healthy plants when they insert their mouthparts, called stylets, into phloem tubes (plant organs for transporting nutrients) for feeding. 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 plasmodesmata, which are cylindrical openings through cell walls. Plasmodesmata create bridges that cross cell walls to connect adjacent cells, and this transport route explains why many viral infections are systemic, affecting the entire organism.
According to paragraph 4, which of the following is the most common way in which viruses enter plants?
ABy infecting plants when the plants take in nutrients
BBy actively creating wounds and punctures on plants’ bodies
CBy traveling directly from plant to plant through the open air
DBy moving from aphids’ mouths into plants’ phloem tubes
8
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 their 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.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 immense each year.
According to paragraph 5, which of the following is NOT a common effect that viruses have on plants?
ACausing the plant to die quickly
BChanging the color, shape, or vein patterns of the plant’s leaves
CCausing the plant’s fruit to ripen too early
DMaking the plant produce less fruit
9
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 their 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. [■]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 immense each year.[■]
Look at the four squaresthat indicate where the following sentence could be added to the passage
Additionally, virus-resistant varieties of some plants have been developed.
Where would the sentence best fit？Click on a square sentence to the passage.
10
Viruses can infect all types of life forms and require living cells for reproduction.
AResearch on plant viruses, which were the first viruses identified, led to the knowledge that viruses consist of RNA and a protein coat and resulted in an improved understanding of RNA.
BViruses use stylets to cut through plants’ cell walls and create cylindrical bridges between plant parts in order to move throughout the host plant.
CViruses make plants weaker, and humans have few effective ways of protecting crops from these disease-causing agents, which cause expensive damage around the world.
DScientists used electron microscopes to establish that RNA could exist separately from DNA, a discovery that explained how viruses use living cells to reproduce.
EViruses, which are often transferred by insects, enter plants through wounds or punctures and then spread inside the plant, moving from cell to cell through plasmodesmata.
FThe most common way to control the spread of plant viruses, which includes removing and burning diseased plants, is not always effective because of the cost and environmental concerns.
