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Transcriptional and Physiological Responses of Bradyrhizobium japonicum to Desiccation-Induced Stress ^,

Department of Soil, Water, and Climate, BioTechnology Institute, and Microbial and Plant Genomics Institute, University of Minnesota, St. Paul, Minnesota,¹ Department of Chemical Engineering and Materials Science, BioTechnology Institute, University of Minnesota, St. Paul, Minnesota,² Department of Horticulture and Crop Science, The Ohio State University, Wooster, Ohio,³ Divisions of Plant Sciences and Biochemistry, National Center for Soybean Biotechnology, and Christopher Bond Life Sciences Center, University of Missouri, Columbia, Missouri,⁴ Department of Biochemistry, University of Missouri, Columbia, Missouri,⁵ Department of Computer Science and Christopher S. Bond Life Sciences Center, University of Missouri, Columbia, Missouri⁶

Received 7 April 2007/ Accepted 16 July 2007

The growth and persistence of rhizobia and bradyrhizobia in soils are negatively impacted by drought conditions. In this study, we used genome-wide transcriptional analyses to obtain a comprehensive understanding of the response of Bradyrhizobium japonicum to drought. Desiccation of cells resulted in the differential expression of 15 to 20% of the 8,480 B. japonicum open reading frames, with considerable differentiation between early (after 4 h) and late (after 24 and 72 h) expressed genes. While 225 genes were universally up-regulated at all three incubation times in response to desiccation, an additional 43 and 403 up-regulated genes were common to the 4/24- and 24/72-h incubation times, respectively. Desiccating conditions resulted in the significant induction (>2.0-fold) of the trehalose-6-phosphate synthetase (otsA), trehalose-6-phosphate phosphatase (otsB), and trehalose synthase (treS) genes, which encode two of the three trehalose synthesis pathways found in B. japonicum. Gene induction was correlated with an elevated intracellular concentration of trehalose and increased activity of trehalose-6-phosphate synthetase, collectively supporting the hypothesis that this disaccharide plays a prominent and important role in promoting desiccation tolerance in B. japonicum. Microarray data also indicated that ⁵⁴- and ²⁴-associated transcriptional regulators and genes encoding isocitrate lyase, oxidative stress responses, the synthesis and transport of exopolysaccharides, heat shock response proteins, enzymes for the modification and repair of nucleic acids, and the synthesis of pili and flagella are also involved in the response of B. japonicum to desiccation. Polyethylene glycol-generated osmotic stress induced significantly fewer genes than those transcriptionally activated by desiccation. However, 67 genes were commonly induced under both conditions. Taken together, these results suggest that B. japonicum directly responds to desiccation by adapting to changes imparted by reduced water activity, such as the synthesis of trehalose and polysaccharides and, secondarily, by the induction of a wide variety of proteins involved in protection of the cell membrane, repair of DNA damage, stability and integrity of proteins, and oxidative stress responses.

Published ahead of print on 27 July 2007.

Supplemental material for this article may be found at http://jb.asm.org/.

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