Nitrogen fixation in legume root nodules: biochemical studies with soybean

Abstract

It is now clear from studies with soybean root nodules that the nitrogen fixing activity resides in the bacteroids which are the symbiotic form of the root nodule bacteria. These develop as a result of a complex series of changes in metabolism and structure which occur in the bacteria during the final stages of growth within membrane-enclosed vesicles in the host cytoplasm. Nitrogenase appears when these changes are complete. The primary product of nitrogen fixation is NH 3 , which in intact nodules, is rapidly transformed into α -amino compounds which are used by the host plant. In suspensions of bacteroids and in cell-free extracts prepared from them, the reaction terminates in NH 3 , which is released into the medium. Free O 2 , which is required for the production of energy for nitrogen fixation by nodules and by bacteroid suspensions, also causes inactivation of the nitrogen fixing system and exerts important kinetic influences upon the reaction. Reducing power and energy for the reduction of N 2 to NH 3 is provided by a photosynthetic product from the host in nodules; in bacteroid suspensions, a substrate such as succinate is required. In cell-free extracts, requirements for energy and reductant are met by ATP and dithionite. The natural reductant has not yet been identified. A schematic representation of various factors which affect nitrogen fixation in nodules, bacteroid suspensions and cell-free extracts is presented.