Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: June 27, 1996
Publication Date: N/A
Technical Abstract: Judicious management of nitrogen in the environment will be essential to the development of more productive and sustainable agriculture. Biological nitrogen fixation accounts for 65% of nitrogen currently utilized in agriculture. Plants capable of symbiotic nitrogen fixation are important sources of food, fiber, forage, and firewood. Perennial forage legumes fix xabout 120 kg N per ha per season, while annual pulse legumes fix about 90 kg N per ha per season. Root nodules, the site of nitrogen fixation in forage legumes, are hyperplastic, hypertrophic tissue masses derived from root cortical cells. The formation of root nodules is a complex process involving the controlled, coordinated expression of both plant and Rhizobium bacteria genes. Specific flavonoids released by the roots of forage legumes activate Rhizobium nodulation genes to synthesize lipo oligosaccharide factors (nod factors). These nod factors induce root nodule initiation. Many of the plant responses that occur during Rhizobiu infection mimic reactions seen with plant diseases. The bacteria appears to avoid reactions that could result in incompatibility. While the Rhizobium provides nitrogen in the form of ammonia for the plant, the plant provides carbon in the form of malate for the bacterium. The nitrogen fixed in nodules is assimilated by plant gene products. Substantial progress has been made in identifying and characterizing both bacterial and plant genes involved in nitrogen fixation. Over 100 genes, about 50 from both the plant and bacterium, have been identified as contributing to effective nodules. Although little progress has been made in improving nitrogen fixation through biotechnology, significant progress will be made in future decades.