Project description:The ability of Bradyrhizobium japonicum and B. elkanii strains to utilize alkane and aromatic sulfonates as sole sources of sulfur for growth was investigated. All of the strains tested were able to utilize alkane sulfonates, but not aromatic sulfonates for growth. Whole-genome transcriptional profiling was used to assess B. japonicum USDA 110 genes involved in growth on alkane sulfonates, as compared to growth on sulfate and cysteine. Two sets of genes, bll7007 to bll7011 and bll6449 to 6456 were highly expressed during growth with sulfate and sulfonates. These genes were predicted to encode alkanesulfonate monooxygenases and ABC transporter components. Reverse transcription-PCR (RT-PCR) analyses showed that these genes were organized in two operon-like structures and expressed as polycistronic messages. The sulfonate monooxygenase encoded by bll7010 (ssuD) complemented an E. coli mutant defective in utilization of sulfonates. The expression of many genes that were induced during growth on cysteine and taurine were under the control of the FixLJ-FixK2-FixK1 symbiotic nitrogen fixation cascade, indicating there is a novel linkage between sulfur metabolism and nitrogen fixation. Taken together, results of this study indicate that Bradyrhizobium sp. strains are metabolically diverse and likely use organosulfur compounds for growth and survival, and for legume nodulation and nitrogen fixation in soil systems.
Project description:Legume plants can form root organs called nodules where they house intracellular symbiotic rhizobium bacteria. Within nodule cells, rhizobia differentiate into bacteroids, which fix nitrogen for the benefit of the plant. Depending on the combination of host plants and rhizobial strains, the output of rhizobium-legume interactions is varying from non-fixing associations to symbioses that are highly beneficial for the plant. Bradyrhizobium diazoefficiens USDA110 was isolated as a soybean symbiont but it can also establish a functional symbiotic interaction with Aeschynomene afraspera. In contrast to soybean, A. afraspera triggers terminal bacteroid differentiation, a process involving bacterial cell elongation, polyploidy and membrane permeability leading to loss of bacterial viability while plants increase their symbiotic benefit. A combination of plant metabolomics, bacterial proteomics and transcriptomics along with cytological analyses was used to study the physiology of USDA110 bacteroids in these two host plants. We show that USDA110 establish a poorly efficient symbiosis with A. afraspera, despite the full activation of the bacterial symbiotic program. We found molecular signatures of high level of stress in A. afraspera bacteroids whereas those of terminal bacteroid differentiation were only partially activated. Finally, we show that in A. afraspera, USDA110 bacteroids undergo an atypical terminal differentiation hallmarked by the disconnection of the canonical features of this process. This study pinpoints how a rhizobium strain can adapt its physiology to a new host and cope with terminal differentiation when it did not co-evolve with such a host.
