Search

Soybean bradyrhizobia (Bradyrhizobium spp.) are symbiotic root-nodulating bacteria that fix atmospheric nitrogen for the host plant. The University of Delaware Bradyrhizobium Culture Collection (UDBCC; 353 accessions) was created to study the diversity and ecology of soybean bradyrhizobia. Some UDBCC accessions produce temperate (lysogenic) bacteriophages spontaneously under routine culture conditions without chemical or other apparent inducing agents. Spontaneous phage production may promote horizontal gene transfer and shape bacterial genomes and associated phenotypes. A diverse subset (n = 98) of the UDBCC was examined for spontaneously produced virus-like particles (VLPs) using epifluorescent microscopy, with a majority (69%) producing detectable VLPs (>1 × 107 mL−1) in laboratory culture. Phages from the higher-producing accessions (>2.0 × 108 VLP mL−1; n = 44) were examined using transmission electron microscopy. Diverse morphologies were observed, including various tail types and lengths, capsid sizes and shapes, and the presence of collars or baseplates. In many instances, putative extracellular vesicles of a size similar to virions were also observed. Three of the four species examined (B. japonicum, B. elkanii, and B. diazoefficiens) produced apparently tailless phages. All species except B. ottawaense also produced siphovirus-like phages, while all but B. diazoefficiens additionally produced podovirus-like phages. Myovirus-like phages were restricted to B. japonicum and B. elkanii. At least three strains were polylysogens, producing up to three distinct morphotypes. These observations suggest spontaneously produced phages may play a significant role in the ecology and evolution of soybean bradyrhizobia. [ABSTRACT FROM AUTHOR]

Purpose: The fixation of atmospheric CO2 and N2 by soil bacteria is important to the terrestrial carbon and nitrogen cycles and can be greatly affected by anthropogenic disturbance. The application of mineral fertilizer combined with manure (MCM) has been used to maintain soil quality and improve soil C storage. However, the impacts of MCM on soil CO2-fixing bacteria (CFB) and N2-fixing bacteria (NFB), encoded separately by the cbbL and nifH genes, remain largely unknown. Methods: In this study, we examined how the community structure of soil CFB and NFB from a chronosequence of Moso bamboo forests (6, 10, 15, and 20 years of intensive management) responded to long-term intensive management with MCM and identified the critical determinants that regulated them. Quantitative PCR and high-throughput pyrosequencing of the cbbL and nifH genes were used to analyze the abundance and community composition of CFB and NFB, respectively. Results: Employing MCM enhanced the content of SOC and soil available N, K, although some fluctuating values were observed over the 20-year period. The abundance of cbbL gene decreased significantly (P < 0.05) after 6 years of management and then recovered after 20 years of management. However, the abundance of nifH gene consistently declined over 20 years of management. The diversity of CFB increased significantly after 6 years of management, while that of NFB decreased significantly after 15 years of management compared to the no-fertilizer control. The phylum Proteobacteria was predominant in the CFB and NFB communities. Furthermore, the sum abundance of keystone taxa Rhodopseudomonas, Aminobacter, and Rubrivivax for CFB increased after 20 years of management, while the keystone taxon Bradyrhizobium for NFB decreased after 20 years of management compared to no-fertilizer control. Conclusions: In conclusion, the application of MCM favors the abundance, diversity, and keystone taxa of soil CFB rather NFB, which may improve soil C fixation. [ABSTRACT FROM AUTHOR]

A number of studies have shown an association between soybean (Glycine max (L.) Merr.) seed yield and plant N accumulation, suggesting that yield could be increased by nodulation by superior strains of Bradyrhizobium japonicum. Attempts to evaluate the response of field-grown soybean to bradyrhizobia-mediated increases in N accumulation have been hindered by an inability of introduced strains to compete successfully with indigenous strains. Therefore, to determine the effect of increased N2 fixation on soybean growth and seed yield, an experiment was conducted using soybean seedlings that were nodulated under greenhouse conditions and then transplanted to the field. Two inoculation treatments, USDA 122 and a mixture of bradyrhizobia indigenous to the field site (control), were applied to the seed and seedlings were developed for 21 d in the greenhouse. Prenodulated seedlings were then transplanted to 1.8 m squared field microplots on a Matapeake silt loam (fine-loamy, mixed, mesic Typic Hapludult) with a most probable number of indigenous bradyrhizobial population of 6.1 X 10 to the eighth power cells kg to the -1 soil. Relative to the control, nodulation by USDA 122 increased both N(sub 2) fixed and total shoot N by a 2-yr average of 29%. Plants nodulated by USDA 122 also assimilated more soil N. The increase in N(sub 2) fixation and total N accumulation occurred prior to, but not during, seedfill. The USDA 122 treatment increased seed yield by an average of 31% (90 g m to the -2). The yield increase was associated with increases in pod number, seedfill rate, remobilized dry matter, remobilized N, and vegetative biomass.

The internally transcribed spacer (ITS) sequences of several different members within each of the soybean bradyrhizobial serogroups where available are determined. The analysis has provided evidence that extant bradyrhizobial genomes are the result of reticulate evolutionary events.