The process of nitrogen fixation by the legumes imposes carbon (C) energy burden on the plant. The photosynthate translocated from the leaves provides carbon skeleton, reducing power and energy required for the symbiotic nitrogen fixation. Sucrose from the shoot is converted to organic acids, principally dicarboxylates that are supplied to bacteroids to provide reductant for the support of key enzyme nitrogenase. It has been established during the last two decades that an anaplerotic carbon dioxide fixation takes place in nodules, via phosphoenol pyruvate carboxylase (PEPC), a key enzyme for carbon dioxide fixation in plants, algae, cyanobaderia and bacteria, located in the cytoplasm of host cells. Although, it has been shown that nitrogen fixation is fuelled by recently synthesized sucrose translocated to the root nodule, neither sucrose nor hexoses are readily metabolised by isolated bacteroids at rates capable of supporting nitrogenase. In contrast, there is evidence to prove that dicarboxylates play a major and essential role in supporting nitrogen fixation. This review throws light on the relationship between nitrogen fixation and PEPC activity in legume nodules.
