Host Specificity in Rhizobium-Legume Interactions

Microbiome analysis revealed distinct microbial communities occupying different sized nodules in field-grown peanut

Legume nodulation is the powerhouse of biological nitrogen fixation (BNF) where host-specific rhizobia dominate the nodule microbiome. However, other rhizobial or non-rhizobial inhabitants can also colonize legume nodules, and it is unclear how these bacteria interact, compete, or combinedly function in the nodule microbiome. Under such context, to test this hypothesis, we conducted 16S-rRNA based nodule microbiome sequencing to characterize microbial communities in two distinct sized nodules from field-grown peanuts inoculated with a commercial inoculum. We found that microbial communities diverged drastically in the two types of peanut nodules (big and small). Core microbial analysis revealed that the big nodules were inhabited by Bradyrhizobium, which dominated composition (>99%) throughout the plant life cycle. Surprisingly, we observed that in addition to Bradyrhizobium, the small nodules harbored a diverse set of bacteria (~31%) that were not present in big nodules. Notably, these initially less dominant bacteria gradually dominated in small nodules during the later plant growth phases, which suggested that native microbial communities competed with the commercial inoculum in the small nodules only. Conversely, negligible or no competition was observed in the big nodules. Based on the prediction of KEGG pathway analysis for N and P cycling genes and the presence of diverse genera in the small nodules, we foresee great potential of future studies of these microbial communities which may be crucial for peanut growth and development and/or protecting host plants from various biotic and abiotic stresses.

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.