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9.5E: Plant Virus Life Cycles

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    Plant viruses are often spread from plant to plant by organisms known as vectors.

    Learning Objectives
    • Outline plant virus life cycles

    Key Points

    • Plant viruses are harmless to humans and other animals because they can only reproduce in living plant cells.
    • For the virus to reproduce and thereby establish infection, it must enter cells of the host organism and use those cells’ materials.
    • A virus must take control of the host cell’s replication mechanisms. At this stage a distinction between susceptibility and permissibility of a host cell is made.
    • After control is established and the environment is set for the virus to begin making copies of itself, replication occurs quickly by the millions.

    Key Terms

    • vector: A carrier of a disease-causing agent.

    Plant viruses are viruses that affect plants. Like all other viruses, plant viruses are obligate intracellular parasites that do not have the molecular machinery to replicate without a host. Plant viruses are pathogenic to higher plants.

    There are many types of plant virus, but often they only cause a loss of yield, and it is not economically viable to try to control them. Plant viruses are often spread from plant to plant by organisms ( vectors ). These are normally insects, but some fungi, nematode worms and single-celled organisms have been shown to be vectors. When control of plant virus infections is considered economical, (for perennial fruits for example), efforts are concentrated on killing the vectors and removing alternate hosts such as weeds. Plant viruses are harmless to humans and other animals because they can only reproduce in living plant cells.

    Viral Life Cycle

    For the virus to reproduce and thereby establish infection, it must enter cells of the host organism and use those cells’ materials. To enter the cells, proteins on the surface of the virus interact with proteins of the cell. Attachment, or adsorption, occurs between the viral particle and the host cell membrane. A hole forms in the cell membrane, then the virus particle or its genetic contents are released into the host cell, where viral reproduction may commence. Next, a virus must take control of the host cell’s replication mechanisms. At this stage, a distinction between susceptibility and permissibility of a host cell is made. Permissibility determines the outcome of the infection. After control is established and the environment is set for the virus to begin making copies of itself, replication occurs quickly by the millions. After a virus has made many copies of itself, it usually has exhausted the cell of its resources. The host cell is now no longer useful to the virus, therefore the cell often dies and the newly produced viruses must find a new host. The process by which virus progeny are released to find new hosts, is called shedding. This is the final stage in the viral life cycle. Some viruses can “hide” within a cell, either to evade the host cell defenses or immune system, or simply because it is not in the best interest of the virus to continually replicate. This hiding is deemed latency. During this time, the virus does not produce any progeny, it remains inactive until external stimuli—such as light or stress—prompts it to activate.

    Viruses can be spread by direct transfer of sap, and by contact of a wounded plant with a healthy one. Such contact may occur during agricultural practices, when damage is caused by tools or hands, or naturally, when an animal feeds on the plant. Generally Tobacco mosaic virus (TMV), potato viruses, and cucumber mosaic viruses are transmitted via sap.

    Tobacco mosaic virus and Cauliflower mosaic virus (CaMV) are frequently used in plant molecular biology. Of special interest is the CaMV 35S promoter, which is a very strong promoter most frequently used in plant transformations.

    image
    Figure: Tobacco Mosaic Virus: Tobacco mosaic virus (TMV) is a positive-sense single stranded RNA virus that infects plants, especially tobacco and other members of the family Solanaceae. The infection causes characteristic patterns (mottling and discoloration) on the leaves (hence the name). TMV was the first virus to be discovered. Although it was known from the late 19th century that an infectious disease was damaging tobacco crops, it was not until 1930 that the infectious agent was determined to be a virus.

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