Some Processes Related to Nitrogen Fixation in Nodulated Legumes

We have summarized information in four areas of the broad topic of legume-Rhizobium symbiosis. These include: carbon substrates provided to nodule bacteroids by the host, assimilation of fixed nitrogen by the host, O_2 metabolism in legume nodules and involvement of H_2 in nodule metabolism. Although nodules contain a variety of carbon substrates, both biochemical and genetic evidence indicate that C4 dicarboxylates are the major carbon substrates that support N_2 fixation in nodules. The biochemical pathways for utilization of products of N_2 fixation are fairly well understood but relatively little is known about the regulation of the assimilation of fixed nitrogenous compounds at the gene level. Ureides are primary nitrogenous compounds exported from nodules of the tropical legumes. Because the catabolism of these products may involve the hydrolysis of urea by nickel-dependent urease, the possible importance of nickel as a trace element in the nutrition of legumes is raised. The O_2 supply to nodule bacteroids is regulated by a barrier to free-O_2 diffusion and by leghaemoglobin. Progress has been made in understanding of the molecular genetics and biochemistry of leghaemoglobin but little is known about the mechanisms that control the physical barrier to O_2 diffusion. Legume nodules contain mechanisms for the disposition of peroxide and free radicals of oxygen. The importance of these systems as protective mechanisms for the O_2-labile nitrogenase is discussed. Some strains of Rhizobium form nodules which recycle the H_2 produced as a byproduct of N_2 fixation. The genes necessary for H_2 oxidation have been cloned and transferred within and among species of Rhizobium. The advantages and disadvantages of H_2 recycling in legume nodules are discussed.