The symbiotic matching for nodulation of Bradyrhizobium japonicum strains is a synergy of multi-proteins and plays a key role in symbiotic nitrogen fixation in nature. Studies on mechanism of symbiotic matching are significant in both theory and practice. In this paper, B. japonicum USDA110-A with high symbiotic matching with high-oil content soybean cultivar Suinong 20 and B. japonicum 2178 with low symbiotic matching were selected for proteomic to reveal mechanism of different symbiotic nodulation. The results showed that the amount and categories of proteins identified in this test were different when the two strains were treated by symbiotic nodulation. There were 10 up-regulated proteins and 5 down-regulated proteins with significant difference for B. japonicum USDA110-A. Proteins associated with nodulation and metabolism of energy and material, which were propitious to symbiotic nodulation, were all up-regulated, such as PHDPS synthase, metal-dependent phosphohydrolase, glycosyl transferase family. In contrast, only 5 up-regulated and 7 down-regulated differential proteins were detected in B. japonicum 2178. Molecular chaperones and defensive proteins, which influence the folding of nascent polypeptide chains and the active of azotase were down-regulated. To a certain extent, the different responses of B. japonicum to daidzein were one of the most important reasons that cause varieties in symbiotic matching ability.

Competitiveness for nodulation of Bradyrhizobium japonicum strains plays a key role in symbiotic nitrogen fixation. Inorder to reveal the difference in competitiveness, B. japonicum 4534 with high competitiveness and B. japonicum 4222with low competitiveness for nodulation were analyzed by proteomic technique. The results showed that differentialproteins were fewer when two strains were treated with just daidzein. Only 24 and 10 differential proteins were detectedwith an up-regulated rate of 58 and 40% in B. japonicum 4534 and B. japonicum 4222, respectively. However, moredifferential proteins were detected upon treatment with daidzein and mutual extracellular materials simultaneously. Therewere 78 differential proteins detected in B. japonicum 4534 with 43 being up-regulated and 35 being down-regulated.These differential proteins, such as metabolism-related proteins, transporters, transcription-related proteins, translationrelatedproteins, and flagellin, were found to be associated with nodulation process. 25 up-regulated and 22 down-regulatedproteins were detected in B. japonicum 4222. Some of these proteins were not related to nodulation. More differentialproteins associated with nodulation in B. japonicum 4534 may be the reason for its high competitiveness. The results canprovide a guide to the selection and inoculation of effective strains and are significant to biological nitrogen fixation.