Plant Viruses

A great number of plant diseases are caused by viruses. These can be of major economic importance, particularly when they occur in crop plants such as corn, wheat, and rice. Virus infections are especially prevalent among perennial plants (those that live for many seasons), such as tulips and potatoes, and those propagated vegetatively (not by seeds), such as potatoes. Other crops in which viruses cause considerable damage are soybeans and sugar beets. A serious virus infection may reduce yields of these crops in a field by more than 50%.

Infection of plants by viruses may be recognized through various outward signs (figure 14.19). Localized abnormalities may result in a loss of green pigment, and entire leaves may turn color. In many cases, rings or irregular lines appear on the leaves and fruits of the infected plant. Individual cells or specialized organs of the plant may die, and tumors may appear. Usually, infected plants become stunted in their growth, although in a few cases growth is stimulated, leading to deformed structures. In the vast majority of cases, plants do not recover from viral infections, for unlike animals, plants are not capable of developing specific immunity to rid themselves of invading viruses. On occasion, however, infected plants produce new growth in which visible signs of infection are absent, even though the infecting virus is still present.

In severely infected plants, virions may accumulate in enormous quantities. For example, as much as 10% of the dry weight of a TMV-infected tobacco plant may consist of virus.

In a few instances, plants have been purposely maintained in a virus-infected state. The best known example of this involves tulips, in which a virus transmitted through the bulbs can cause a desirable color variegation of the flowers (figure 14.20). The infecting virus was transmitted through bulbs for a long time before the cause of the variegation was even suspected. The multiplication of viruses in plants is analogous to that of bacterial and animal viruses in most respects.

364 Chapter 14 Viruses, Prions, and Viroids: Infectious Agents of Animals and Plants

364 Chapter 14 Viruses, Prions, and Viroids: Infectious Agents of Animals and Plants

(a)
(b)

Figure 14.19 Symptoms of Viral Diseases of Plants (a) A healthy wheat leaf can be seen in the center.The yellowed leaves on either side are infected with wheat mosaic virus. (b) Typical ring lesions on a tobacco plant leaf resulting from infection by tobacco mosaic virus. (c) Stunted growth (right) in a wheat plant caused by wheat mosaic virus.

Figure 14.19 Symptoms of Viral Diseases of Plants (a) A healthy wheat leaf can be seen in the center.The yellowed leaves on either side are infected with wheat mosaic virus. (b) Typical ring lesions on a tobacco plant leaf resulting from infection by tobacco mosaic virus. (c) Stunted growth (right) in a wheat plant caused by wheat mosaic virus.

Virus Infection With Info Plant
Figure 14.20 Tulips The variegated colors of tulips result from viral infection. The virus is transmitted directly from plant to plant.

Spread of Plant Viruses

In contrast to phages and animal viruses, plant viruses do not attach to specific receptors on host plants. Instead, they enter through wound sites in the cell wall, which is very tough and rigid. Once started, infection in the plant can spread from cell to cell through openings, the plasmodesmata, that interconnect cells.

Many plant viruses are extraordinarily resistant to inactiva-tion. Tobacco mosaic virus apparently retains its infectivity for up to 50 years, which explains why it is usually difficult to eradicate the virions from a contaminated area. Smokers who garden can transmit TMV to susceptible plants from their cigarette tobacco. The virions are very stable, which is important in maintaining the virus because the usual processes of infection are generally inefficient.

Some viruses are transmitted through soil contaminated by prior growth of infected plants. Some 10% of the known plant viruses transmit disease through contaminated seeds or tubers, or by pollination of flowers on healthy plants with contaminated pollen from diseased plants. Virus infections may also spread through grafting of healthy plant tissue onto diseased plants. Another more exotic transmission mechanism is employed via the parasitic vine, dodder (figure 14.21). This vine can establish simultaneous connections with the vascular tissues of two host plants, which serve as conduits for transfer of viruses from one host plant to the other.

Other important infection mechanisms involve vectors of various types. These include insects, worms, fungi, and humans. For example, TMV, which causes a serious disease of tobacco, has no known insect vector. Humans are the major vectors of this disease. Viruses are transmitted to healthy seedlings on the hands of workers who have been in contact

Figure 14.21 Dodder Orange-brown twining stems of dodder wrap around two different hosts so that virions can pass from one host to the other through dodder.

14.9 Other Infectious Agents 365

questions about virus evolution. In particular, plant and animal viruses may not be as different as they first appeared.

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Responses

  • Flavus
    What r the economic importance of Plant viruses ?
    3 years ago
  • keeva
    WHAT ARE THE CASATIC AGENT OF PLANT?
    3 years ago
  • tesmi gebre
    What are the causative agent of plant viral diseases?
    3 years ago
  • Holfast
    What is the causative agent of plant tumor?
    3 years ago
  • Marina
    What is the causative agent of infected plant?
    2 years ago
  • burtuka
    What are the causative agents of viral diseases in plants?
    2 years ago
  • penelope
    What is the cousetive agent of nematodes in plant desease?
    2 years ago
  • Ursula
    What is the outward sign that a plant has been infected by a virus?
    1 year ago

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