Proteomics-Based Investigation of Salt-Responsive Mechanisms in Roots of Bradyrhizobium japonicum-Inoculated Glycine max and Glycine soja Seedlings

Salt stress is one of the environmental factors most limiting crop productivity worldwide. Plant roots are the primary site for salt sensing; therefore, a thorough understanding of molecular mechanisms of salt response in roots is critical for improving plant salt tolerance. In this study, Glycine max Lee68 cultivar (USA, the salt-tolerant) and Glycine soja N23227 accession (the relatively salt-sensitive) were used as the experimental materials. This work aims to further understand the molecular mechanisms occurring in the roots of B. japonicum-inoculated soybean seedlings in response to salt stress from a proteomic perspective using liquid chromatography technique coupled to tandem mass spectrometry (LC–MS/MS). Proteomic analysis showed that a total of 105 differentially expressed proteins in the roots of B. japonicum-inoculated soybean seedlings were successfully identified. Among them, 42 and 46 proteins were up-regulated in Lee68 and in N23227, respectively, where 34 were commonly up-regulated in both genotypes and eight were up-regulated in Lee68 and down-regulated in N23227. All of the bacteroid proteins were down-regulated in N23227, of which four were up-regulated in Lee68. These results indicated that salt stress could change the expression level of some proteins in the roots of B. japonicum-inoculated soybean seedlings which may in turn play a role in the adaption to salt stress; meanwhile, inoculation with B. japonicum might be more advantageous to salt-tolerant genotypes than the salt-sensitive ones.