New insights into the control of symbiotic nitrogen fixation by carbon supply

The interaction between legumes and rhizobium-type bacteria allows symbiotic nitrogen fixation (SNF). Symbiosis takes place in a new root organ, the nodule, in which the plant host fuels the bacteria differentiated into bacteroids with dicarboxylic acids, enabling their active carbon and energy metabolisms (Figure 1). In addition, the plant maintains the bacteroids in microoxic conditions allowing the nitrogenase to reduce atmospheric nitrogen (N2) to ammonia (NH3). Ammonia is not assimilated in the bacteroids but is subsequently transferred into the surrounding plant cells, where it combines with carbon (C) skeletons through the glutamine synthetase/glutamate synthase pathway.