Nitrogen Fixation

Symbiotic nitrogen fixation results form intricate interactions between roots and bacteria

"Many plant families include species that form symbiotic relationships with nitrogen-fixing bacteria that give roots a built-in source of fixed nitrogen for assimilation into organic compounds. Most of the research on symbiotic nitrogen fixation has focused on agriculturally important members of the legume family, including peas, beans, soybeans, peanuts, alfalfa, and clover. A legume’s roots have swellings called nodules composed of plant cells that contain nitrogen-fixing bacteria of the genus Rhizobium ("root living"). Inside the nodule, Rhizobium bacteria assume a form called bacteroids, which are contained within vesicles formed by the root cell (below, FIGURE 37.10). Each legume is associated with a particular species of Rhizobium . FIGURE 37.11(below) describes the steps in the development of root nodules after bacteria enter through what is called an infection thread." - Campbell and Reece, Biology, 6th edition.

Campbell's Fig 37-10. Root nodules on legumes. (a) Pea plant root. The bumps on this pea plant root are nodules containing symbiotic bacteria. The bacteria fix nitrogen and obtain photosynthetic products supplied by the plant. (b) Bacteroids in a soybean root nodule. In this TEM, a cell from a root nodule of soybean is filled with bacteroids in vesicles. The cell on the left is uninfected. See a macro-view of root nodules.

Campbell's Fig 37-11. Development of a soybean root nodule.

Rhizobium leguminsarum. Nitrogen-fixing bacteria in legume root tissue.

Links:

• Plant Physiology Online: Development of a root nodule

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