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Keywords:

  • bacteroid;
  • Bradyrhizobium japonicum USDA110;
  • nodule;
  • proteomic analysis;
  • soybean

Abstract

During nodule development of legumes, together with the morphological alteration of plant cells, the rhizobia undergo marked biochemical and physiological changes and differentiate to bacteroids. Bacteroids invading a nodule are no longer free-growing cells, instead depending on the plant cell for all resources. To elucidate the molecular mechanism of the bacteroid differentiation of Bradyrhizobium japonicum, a time-course analysis of bacteroid protein profiles was studied by 2-D gel electrophoresis in soybean nodules. Using proteomic analysis, protein expressions in soybean nodule bacteroids 7, 10, 14, 28, and 49 days after inoculation (DAI) were monitored. The time points coincide with the early stage of nodule formation (7–10 DAI), the onset of nitrogen fixation (14–28 DAI), and nodule senescence (49 DAI). In this study, 275 annotated protein spots were successfully identified, and a cluster analysis of their expression was performed. A large portion of putative, upregulated proteins were observed in bacteroids at 7 or 10 DAI, and these upregulated proteins were mainly related to transcription, translation, protein folding, and degradation. In the later period (14–28 DAI) of bacteroid differentiation, a number of Nif and Fix proteins were upregulated. In addition, proteins related to the chaperonin and a synthetic enzyme of the poly-beta-hydroxybutyrate were expressed at high abundance. A number of proteins related to solute transporter were upregulated in the bacteroids and dominantly detected throughout nodule development. Changes of relative abundance of these proteins are discussed in relation to symbiosis.