Roots of legumes often establish mutualistic relationships with nitrogenfixing bacteria

Nitrogen is often deficient in soils; this deficiency limits the growth of plants and, as a consequence, animals. The deficiency of nitrogen in soil is somewhat paradoxical, because the atmosphere—the air you breathe—is almost 80% nitrogen gas (N2). The chemical bond holding the two atoms of nitrogen gas together makes the N2 molecule extremely stable and therefore unusable by most organisms, including humans. However, a few species of soil bacteria contain a complex enzyme called nitrogenase that can convert, or fix, atmospheric N2. These microorganisms convert the N2 to ammonia (NH3), which can be used by plants to make nitrogen-containing compounds, such as amino acids needed for growth (fig. 7.23). These nitrogen-fixing bacteria live alone as well as symbiotically with plants.

Some plants have established mutualistic relationships with nitrogen-fixing bacteria; these plants obtain nitrogen from compounds produced by the bacteria. The roots of legumes such as beans and alfalfa have a mutual-istic relationship with Rhizobium bacteria. These bacteria secrete carbohydratelike molecules that trigger the formation of nodules in the roots of the legume plants (fig. 7.24). Rhizobium invades the roots through root hairs, and legume roots form localized swellings, the nodules, in response to the bacteria. Unlike free-living nitrogen-fixing bacteria, Rhizobium fixes almost 10 times more nitrogen than it uses. The excess nitrogen is released into the legume, where it is used for growth. In return, the bacteria get sugars from the host plant.

Nitrogen-fixing bacteria form mutualistic relationships with such diverse plants as mosses (Sphagnum), liverworts (Blasia), and tropical trees (Parasponia). These associations, like those with legumes, often have important agricultural implications. For example, the cyanobacterium Anabaena grows symbiotically in leaves of water fern (Azolla) (fig. 7.25a). When infected Azolla are grown in rice paddies, some of the nitrogen fixed by Anabaena dissolves in the water of the paddy and becomes available to rice plants (fig. 7.25b). As a result, rice crops in China and southeast Asia can be grown in

Glycine Max Root

FIGURE 7.24

Nodules on the roots of these soybeans (Glycine max) consist of cortex cells infected with nitrogen-fixing bacteria.

FIGURE 7.24

Nodules on the roots of these soybeans (Glycine max) consist of cortex cells infected with nitrogen-fixing bacteria.

nitrogen-deficient soils. Botanists are now searching for new strains of Anabaena to increase rice production.

Harvesting crops removes more than 21 million metric tons of nitrogen per year from the soil. Nitrogen fixation by bacteria replenishes almost two-thirds of this loss. The remaining nitrogen is resupplied to the soil in fertilizers and other sources. Nitrogen fixation helps many farmers. For example, much of the nitrogen fixed in nod

Nitrogen Fixing Plants Southeast Asia
(a)

ules of legumes is released into the soil. Farmers exploit this by practicing crop rotation, by which legume crops are alternated with nonlegumes as a means of maintaining relatively high levels of nitrogen in the soil. Plowing under a legume crop such as alfalfa adds organic material and up to 300 kilograms of nitrogen per hectare—more than 45 times that contributed by free-living bacteria. Nitrogen fixation also reduces the need to apply expensive fertilizers (see Perspective 7.1, "Putting Things Back"). In fact, addition of nitrogen fertilizer inhibits formation of root nodules on legumes and nitrogen fixation. Molecular biologists are trying to incorporate the genes involved in nitrogen fixation into other crop plants such as corn and wheat that do not have associations with nitrogen-fixing bacteria.

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Responses

  • Elias
    Why do legumes have mutualistic bacteria on their roots?
    8 years ago
  • jolly
    How is nitrogen fixing bacteria and legumes an example of mutualism?
    8 years ago
  • Tiblets
    Why have some plants evolved mutualistic relationships with nitrogenfixing bacteria?
    6 years ago
  • iggi
    Why have some plants evolved mutualistic relationships with nitrogen fixing?
    6 years ago
  • Tom
    Is nitrogen fixing bacteria on the roots of trees a mutualistic relationship?
    5 years ago

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