Mutualistic Relationships Between Microorganisms and Eukaryotes

As described in chapter 19, mutualism is a symbiotic association in which both partners benefit. A variety of other ecologically important symbiotic relationships exist, but mutualistic relationships highlight the vital role of microorganisms to life on this planet. This section will describe three important mutu-alistic relationships between microbes and eukaryotes—mycor-rhizae and plants, symbiotic nitrogen fixers and plants, and microorganisms and herbivores. ■ symbiotic relationships, p. 461


Mycorrhizae are fungi growing in symbiotic relationships with plant roots. They enhance the competitiveness of plants by helping them take up phosphorus and other substances from the soil. In turn, the fungi gain nutrients for their own growth from root secretions. It is estimated that over 85% of vascular plants (plants with specialized water and food conducting tissues) have mycorrhizae.

There are two common types of mycorrhizal relationships:

■ Endomycorrhizae. The fungi penetrate root cells, growing as coils or tight bushlike masses within the cells. These are by far the most common mycorrhizal relationships, and are found in association with most herbaceous plants. Relatively few species of fungi are involved, perhaps only 100 or so, and most appear to be obligate symbionts. The relationship for some plants is also obligate; for example, most orchid seeds will not germinate without the activities of a fungal partner.

■ Ectomycorrhizae. The fungi grow around the plant cells, forming a fungal sheath around the root. These fungi mainly associate with certain trees, including conifers, beeches, and oaks. Over 5,000 species of fungi are involved in ectomycorrhizal relationships, but many are restricted to a single type of plant. Chanterelles and truffles are examples of commercially valuable ectomycorrhizal fungi.

Symbiotic Nitrogen-Fixers and Plants

Although free-living bacteria are potentially capable of adding a considerable amount of fixed nitrogen to the soil, symbiotic nitrogen-fixing organisms are far more significant in benefiting plant growth and crop production (table 30.1). They are important in both terrestrial and aquatic habitats.

Members of several genera including Rhizobium, Bradyrhizobium, Sinorhizobium, and Azorhizobium, collectively referred to as rhizobia, are the most agriculturally important symbiotic nitrogen-fixing bacteria. These organisms associate with leguminous plants including alfalfa, clover, peas, beans, and peanuts. The input of soil nitrogen from the microbial symbionts of these plants may be roughly 10 times the annual rate of nitrogen fixation attainable by non-symbiotic organisms in a natural ecosystem. To encourage plant growth, people often plant the seeds of certain legumes together with inocula of the appropriate symbionts.

The symbiotic association between rhizobia and plants involves chemical communication between the partners. First, root exudates of the leguminous plant attract the appropriate bacterial species, which then attach to and colonize the root cells (figure 30.14). Substances secreted by the root cells cause the bacteria to produce chemical signals called nod factors, which

30.5 Mutualistic Relationships Between Microorganisms and Eukaryotes

Table 30.1 Quantities of Nitrogen Fixed by Microorganisms


Species or Habitat

N2 Fixed per Acre per Year (lbs)



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  • judy
    Why mutualistic relationships so common in eukaryotes?
    5 months ago

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