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| Unit 4: Demos |
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Earth formation hypothesis (1a) Protobionts, coacervate droplets, proteinoid
microspheres (3a) Timeline of life Modes of attack, infection: plant
viruses v. bacteriophages v. animal viruses (6a) Anti-viral drugs, why don't viruses respond to antibiotics? Centers for Disease Control and Prevention BSE information The evolution of complex biochemical pathways |
Modes of viral infection Bacteriophages infect their host cells by injecting their genetic material through the host bacterium's cell membrane (see animation). The bacteriophage tail apparatus is designed for this purpose. In contrast, eukaryotic animal viruses use their viral envelopes to gain entry to host cells by fusing with the host cell membrane (see animation of Herpes virus infection). Infection by plant viruses is generally blocked by the cell walls, so plant viruses must enter plant cells when plant tissues are damaged (by weather or insects, for example), or must be spread via inheritance when plants reproduce. In this latter way plant viruses can be spread by asexual propagation or can be sexually-transmitted through seeds. Over 1,000 different plant diseases are known to be caused by viruses. Almost all plant viruses are RNA viruses. Because all plant cells have a protective cell wall around them, the virus cannot reach the interior of the cell unless the cell wall is damaged in some way. Most plant viruses are transmitted from plant to plant by invertebrate animals, such as insects or nematodes. For instance, sucking insects, such as leafhoppers or aphids, insert their mouthparts into the phloem or epidermal cells of plants and suck the fluids out. If the plant is infected, the virus enters the insect’s body and can then be transmitted to another plant. Once inside the plant, the viruses can move through the plant either by short-distance cell-to-cell-transmission through plasmodesmata, or by long-distance spread through the vascular system. Recall that plasmodesmata are very small -actually too small to allow passage of the viruses. It turns out that the virus codes for certain proteins that enlarge the plasmodesmata, thereby facilitating cell-to-cell spread. Viral diseases greatly reduce the productivity of many different kinds of crops around the world and much effort is being expended to find effective ways to control these diseases. Often the approach is to try to control the vector, the animal agent, that spreads the virus from plant to plant. For more on plant viruses, click here. |
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