Gopit R. Shah,2
Michael J. Sadowsky,1
Andrew W. Vail,1 and
1Department of Soil Water and Climate, University of Minnesota, St. Paul, U.S.A.; 2Department of Biological Sciences, University of Wisconsin, Milwaukee, U.S.A.; and 3Department of Biochemistry and Molecular Biology, Wright State University, Dayton OH, U.S.A.
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Accepted 14 December 2010.
Strains of Bradyrhizobium spp. form nitrogen-fixing symbioses with many legumes, including soybean. Although inorganic sulfur is preferred by bacteria in laboratory conditions, sulfur in agricultural soil is mainly present as sulfonates and sulfur esters. Here, we show that Bradyrhizobium japonicum and B. elkanii strains were able to utilize sulfate, cysteine, sulfonates, and sulfur-ester compounds as sole sulfur sources for growth. Expression and functional analysis revealed that two sets of gene clusters (bll6449 to bll6455 or bll7007 to bll7011) are important for utilization of sulfonates sulfur source. The bll6451 or bll7010 genes are also expressed in the symbiotic nodules. However, B. japonicum mutants defective in either of the sulfonate utilization operons were not affected for symbiosis with soybean, indicating the functional redundancy or availability of other sulfur sources in planta. In accordance, B. japonicum bacteroids possessed significant sulfatase activity. These results indicate that strains of Bradyrhizobium spp. likely use organosulfur compounds for growth and survival in soils, as well as for legume nodulation and nitrogen fixation.
© 2011 The American Phytopathological Society