Comparative Analysis of Three Bradyrhizobium diazoefficiens Genomes Show Specific Mutations Acquired during Selection for a Higher Motility Phenotype and Adaption to Laboratory Conditions

Maximizing Photosynthesis and Plant Growth in African Legumes Through Rhizobial Partnerships: The Road Behind and Ahead

The interplay between soil rhizobial bacteria and leguminous plants, particularly in Africa, has a profound impact on photosynthetic efficiency and overall crop productivity. This review explores the critical role of rhizobia in enhancing photosynthesis through nitrogen fixation, a process crucial for sustainable agriculture. Rhizobial bacteria residing in root nodules provide legumes with symbiotic nitrogen that significantly boosts plant growth and photosynthetic capacity. Recent advances in molecular genomics have elucidated the genetic frameworks underlying this symbiosis, identifying key genes involved in root nodule formation and nitrogen fixation. Comparative genomics of Bradyrhizobium species have revealed seven distinct lineages, with diverse traits linked to nodulation, nitrogen fixation, and photosynthesis. Field studies across Africa demonstrate that rhizobial inoculation can markedly increase nodulation, nitrogen fixation, and grain yields, though outcomes vary depending on local soil conditions and legume species. Notable findings include enhanced nutrient uptake and photosynthetic rates in inoculated legumes compared with nitrate-fed plants. This review highlights the potential of utilizing indigenous rhizobia to improve photosynthesis and crop resilience. Future prospects involve leveraging genomic insights to optimize rhizobial inoculants and enhance legume productivity in water-limited environments. As climate change intensifies, integrating these advancements into agricultural practices could play a crucial role in improving food security and sustainable soil health in Africa.

Ribulose 1,5-Bisphosphate Carboxylase/Oxygenase Is Required in Bradyrhizobium diazoefficiens for Efficient Soybean Root Colonization and Competition for Nodulation

The Hyphomicrobiales (Rhizobiales) order contains soil bacteria with an irregular distribution of the Calvin-Benson-Bassham cycle (CBB). Key enzymes in the CBB cycle are ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO), whose large and small subunits are encoded in cbbL and cbbS, and phosphoribulokinase (PRK), encoded by cbbP. These genes are often found in cbb operons, regulated by the LysR-type regulator CbbR. In Bradyrhizobium, pertaining to this order and bearing photosynthetic and non-photosynthetic species, the number of cbbL and cbbS copies varies, for example: zero in B. manausense, one in B. diazoefficiens, two in B. japonicum, and three in Bradyrhizobium sp. BTAi. Few studies addressed the role of CBB in Bradyrhizobium spp. symbiosis with leguminous plants. To investigate the horizontal transfer of the cbb operon among Hyphomicrobiales, we compared phylogenetic trees for concatenated cbbL-cbbP-cbbR and housekeeping genes (atpD-gyrB-recA-rpoB-rpoD). The distribution was consistent, indicating no horizontal transfer of the cbb operon in Hyphomicrobiales. We constructed a ΔcbbLS mutant in B. diazoefficiens, which lost most of the coding sequence of cbbL and has a frameshift creating a stop codon at the N-terminus of cbbS. This mutant nodulated normally but had reduced competitiveness for nodulation and long-term adhesion to soybean (Glycine max (L.) Merr.) roots, indicating a CBB requirement for colonizing soybean rhizosphere.

Comparative genomics analysis reveals genetic characteristics and nitrogen fixation profile of Bradyrhizobium

Bradyrhizobium is a genus of nitrogen-fixing bacteria, with some species producing nodules in leguminous plants. Investigations into Bradyrhizobium have recently revealed its substantial genetic resources and agricultural benefits, but a comprehensive survey of its genetic diversity and functional properties is lacking. Using a panel of various strains (N = 278), this study performed a comparative genomics analysis to anticipate genes linked with symbiotic nitrogen fixation. Bradyrhizobium's pan-genome consisted of 84,078 gene families, containing 824 core genes and 42,409 accessory genes. Core genes were mainly involved in crucial cell processes, while accessory genes served diverse functions, including nitrogen fixation and nodulation. Three distinct genetic profiles were identified based on the presence/absence of gene clusters related to nodulation, nitrogen fixation, and secretion systems. Most Bradyrhizobium strains from soil and non-leguminous plants lacked major nif/nod genes and were evolutionarily more closely related. These findings shed light on Bradyrhizobium's genetic features for symbiotic nitrogen fixation.