Some Processes Related to Nitrogen Fixation in Nodulated Legumes [and Discussion]

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.