Transcriptomic Characterization of Bradyrhizobium diazoefficiens Bacteroids Reveals a Post-Symbiotic, Hemibiotrophic-Like Lifestyle of the Bacteria within Senescing Soybean Nodules

The transcriptional activity of Bradyrhizobium diazoefficens isolated from soybean nodules was monitored over the period from symbiosis to late plant nodule senescence. The bacteria retained a near constant level of RNA throughout this period, and the variation in genes demonstrating increased, decreased, and/or patterned transcriptional activity indicates that the bacteria are responding to the changing environment within the nodule as the plant cells progress from an organized cellular structure to an unorganized state of internal decay. The transcriptional variation and persistence of the bacteria suggest that the bacteria are adapting to their environment and acting similar to hemibiotrophs, which survive both as saprophytes on live plant tissues and then as necrophytes on decaying plant tissues. The host plant restrictions of symbiosis make B. diazoefficiens a highly specialized, restricted hemibiotroph.