in turn, induce the root hairs to curl. The bacteria then invade the root hair, multiplying and moving into the root cells by means of an infection thread, a tube produced by the plant cells in response to infection, through which the bacteria invade. Once inside the root, the bacterial cells change in appearance, forming a cell type called a bacterioid. Repeated division of both root cells and bacteroids lead to the development of root nodules (figure 30.14b). The plant cells synthesize a special oxygen carrier called leghemoglobin, which binds to O2 and regulates its concentration in the nodule. This protects the O2-sensitive nitrogenase by keeping free O2 at a level low. The bacterioids in the nodule fix nitrogen, releasing ammonia that then diffuses into the root cell. There it is assimilated into amino acids for use by the plant. In return, the bacteria receive nutrients from the plant.

Although the relationship between the plant and bacterium is not obligate, it offers a distinct competitive advantage to both partners. The rhizobia do not fix nitrogen in soils lacking leguminous plants, and they compete poorly with other environmental organisms, slowly disappearing from soils in which leguminous plants are not grown. Likewise, leguminous plants compete poorly against other plants in heavily fertilized soils.

Several genera of non-leguminous trees, including alder and gingko, possess nitrogen-fixing root nodules at some stages of their life cycle. The bacteria involved in the symbiosis are members of the genus Frankia. The details of how they form a symbiotic relationship is poorly understood.

In aquatic environments, the most significant nitrogen fixers are cyanobacteria. They are especially important in flooded soils such as rice paddies. In fact, rice has been cultivated successfully for centuries without the addition of nitrogen-containing fertilizer because of the symbiotic relationship between the cyanobacterium Anabaena azollae and the aquatic fern Azolla. The bacteria grow in specialized sacs in the leaves of the fern, providing nitrogen to the fern. Before planting rice, the farmer allows the flooded rice paddy to overgrow with

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