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plant. The effect is particularly significant when minerals are scarce. For example, in one study (Figure 10.44) tomato plants grown in sterile soil deficient in phosphate grew 6.5 times as much leaf mass when a mycorrhyzal fungus was inoculated into the soil.

Mycorrhyzal fungi are associated with some 90% of terrestrial plants. Most are zygomycetes that grow within the root, with mycelia extending out. Certain trees and shrubs have mycorrhyzal fungi that grow only on the outside of the root. Most of these are basi-diomycetes, but some are ascomycetes such as truffles. Mycorrhyzae seem to help plants grow under adverse conditions such as high altitudes or acidic soils. Plants of the heather family (Ericaceae) show increased resistance to toxic heavy metals when mycorrhyzae are present. Orchid seeds will not germinate if an associated fungus is not present.

Lichens Lichens are symbiotic associations between ascomycetes and either algae or cyanobacteria. The fungus receives carbohydrates and other nutrients from the algae, and in return, it provides moisture and protection. Lichens are familiar to hikers as the often-colorful flat growths adhering to rocks. About 17,000 lichen combinations are known. One of their ecological roles is to hasten the breakdown of the rock substrate, forming soil. They are very sensitive to sulfur dioxide, and thus can be used as an indicator of air pollution.

10.8 NONCELLULAR INFECTIVE AGENTS: VIRUSES, VIROIDS, AND PRIONS

Viruses, viroids, and prions are submicroscopic particles that are not composed of cells. They can carry out no metabolic activities on their own, and therefore are not considered "alive." However, they can infect living host cells and cause them to produce new copies of the infective agent. Thus, many of them produce disease. The viruses (from the Latin virus, meaning "poison") are the major type of such agent, but we also discuss the others briefly below.

Figure 10.45 Tableau of viruses (mostly bacteriophage) from saline wetland ponds in Saskatchewan, Canada. (Photo by David Bird; courtesy of D. Bird and R. Robarts.)

10.8.1 Viruses

A virus particle, or virion, is composed of a nucleic acid core and a protein coat, or capsid (Figure 10.45). Some also have an outer envelope composed mainly of lipid and protein. They are too small—typically 20 to 300 nm (0.02 to 0.3 mm)—to be visible with a light microscope (or to be removed by normal filtration), but they can be "seen" using electron microscopy. Although they are strictly intracellular parasites, they also are characterized by having an extracellular form in which they can be transmitted to infect other host cells. Although individual virus types often are very specific as to the organisms (often a single species, or even a single strain) they can attack, there are varieties of viruses for almost (if not) all hosts, including animals, plants, fungi, and algae. Viruses that attack bacteria are called bacteriophage, or simply phage.

The nucleic acid in a virus genome is either DNA or RNA (but not both) and is either single or double stranded. It encodes for replication of the virion by the host cell. Some RNA viruses first produce a strand of DNA during replication (the reverse of the normal production of an RNA strand from DNA); such viruses are called retroviruses. The human immunodeficiency virus (HIV), which causes the disease acquired immune deficiency syndrome (AIDS; Section 12.6.4), is a retrovirus. Other RNA viruses are able to replicate their RNA directly without the intermediate DNA.

The protein coat is composed of subunits called capsomeres, which self-assemble after production by the host to form the capsid. The two basic forms are helical and the 20-sided icosahedron (which looks almost spherical), but some capsids are combinations of the two (e.g., an icosahedron head with a helical tail).

The classification of viruses usually depends on the nature of the genetic material, the kingdom of the host, and the form of the capsid. The Baltimore classification system (Table 10.12) has established six different major groups with a variety of subgroups on this basis. A "negative" RNA strand must be reversed (by transcription) to the complementary RNA strand before it can be used as a template for the synthesis of proteins.

There are typically several stages to a virus infection. First the virus will attach to the new host cell. It must then penetrate the cell surface and inject its genome into the host. With lytic viruses, replication of the nucleic acid and synthesis of the capsid protein then occurs, followed by assembly of the new virions. Finally, the host cell lyses (ruptures), releasing the new viruses.

TABLE 10.12 Baltimore Classification Groups for Animal, Plant, and Bacteria Viruses

Class

! Form

Host

Group

Architecturea

Representative

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