Carbon Metabolism During Symbiotic Nitrogen Fixation

Abstract

The complex interaction between legumes and the family of soil bacteria known as rhizobia results in the formation of a novel plant organ, the root nodule. Inside nodules, bacteroids reduce molecular dinitrogen into ammonia. Both partners benefit from this symbiosis, since the plant’s requirements for nitrogen are fulfilled by the bacteroids, which in turn receive organic carbon from the plant originating from photosynthesis. The carbon cost for this process is high, rendering the nodules strong carbon sinks, as they are primarily dependent on the import and metabolism of sucrose to provide the energy and carbon skeletons required for atmospheric nitrogen reduction, the assimilation of the ammonia and the export of the resulting nitrogenous compounds. Thus it is not surprising that the availability of photoassimilates is found to be an important factor controlling nodule development and function. Although carbon metabolism during symbiotic nitrogen fixation has received significant attention since the early days of research on the legume–rhizobium symbiosis, the introduction of the model legumes Lotus japonicus and Medicago truncatula and the development of genomic resources for them and various crop legumes have allowed new insights into old questions about the biochemical and molecular mechanisms involved. These include sucrose and starch metabolism, dark-CO₂ fixation and responses of carbohydrate metabolism to environmental factors and the availability of other nutrients.