Nitrogen fixation and carbon metabolism in legume nodules

A large amount of energy is utilized by legume nodules for the fixation of nitrogen and assimilation of fixed nitrogen (ammonia) into organic compounds. The source of energy is provided in the form of photosynthates by the host plant. Phosphoenol pyruvate carboxylase (PEPC) enzyme, which is responsible for carbon dioxide fixation in C4 and crassulacean acid metabolism plants, has also been found to play an important role in carbon metabolism in legume root nodule. PEPC-mediated CO2 fixation in nodules results in the synthesis of C4 dicarboxylic acids, viz. aspartate, malate, fumarate etc. which can be transported into bacteroids with the intervention of dicarboxylate transporter (DCT) protein. PEPC has been purified from the root nodules of few legume species. Information on the relationship between nitrogen fixation and carbon metabolism through PEPC in leguminous plants is scanty and incoherent. This review summarizes the various aspects of carbon and nitrogen metabolism in legume root nodules.