Associative N_2-fixation of Rice with Soil Microorganisms

Abstract

Recently the improvement of the biological nitrogen fixation of rice plants has been one of the important targets of biotechnology applied to agriculture. Researchers of the National Institute of Genetics in Mishima, devised an assay system using the acetylene reduction technique, and observed the variation in nitrogen-fixing activity in the rhizosphere among different rice species and strains. Ability of the associative nitrogen fixation of rice with bacteria was found to depend on the rice genotypes. And it became possible to develop a high N_2-fixing rice by breeding techniques. Several nitrogen-fixing bacteria were isolated from the rhizosphere of rice. Among these bacteria, Klebsiella oxytoca (NG13) and Azospirillum lipoferum (COC8) indicated high N_2-fixing activity associated with rice. In recent experiments, a plasmid pRD1 having multiple copies of the nif-gene of Klebsiella pneumoniae was introduced in NG13 by crossing experiment to obtain NG1325. NG1325 showed a nitrogen-fixing activity about three times higher than that of NG13. Several plasmids related to the nif-gene were identified from other nitrogen-fixing bacteria. Practical use of the bacteria is under study in the Shizuoka Agricultural Experiment Station. Some prospects of biological nitrogen fixation in plant genetic engineering are discussed.