Ameliorative effects of inoculation with Bradyrhizobium japonicum on Glycine max and Glycine soja seedlings under salt stress

In order to understand whether inoculation with Bradyrhizobium japonicum can enhance soybean's ability to cope with salt stress, Glycine max Lee68 cultivar (the salt-tolerant) and Glycine soja N23227 accession (the relatively salt-sensitive) were used as the experimental materials in this study. The morphological and anatomical characteristics (including whole plant, organic, cellular and subcellular levels), physiological parameters (containing contents of chlorophyll and carotenoid, value of Fv/Fm (maximum photochemical efficiency of PSII), relative electrolytic leakage and isoflavone contents) and transcriptional pattern of a few isoflavone synthesis-related key enzyme genes (such as PAL1, CHS8, CHI and IFS2) were investigated. The results showed that, inoculation with B. japonicum on soybean seedlings under 100 mM NaCl stress could obviously increase leaf area, contents of chlorophyll and carotenoid, value of Fv/Fm and the numbers of osmiophilic globule, starch grain and well-arranged stroma thylakoids and grana thylakoids in chloroplasts of salt-stressed soybean seedlings, decrease the relative electrolytic leakage in roots and leaves, and thus demonstrated the ameliorative effects on salt injury to soybean seedlings with different salt tolerance. In comparison, the protective function of B. japonicum inoculation on salt-stressed Lee68 seedlings is mainly related to the effects on leaves, while that on N23227 is mainly on roots, which might be attribute to the transcription promotion of isoflavone biosynthesis-related key enzyme genes, such as CHI and IFS2, and more rise of isoflavone content in roots. Thus, inoculation with B. japonicum for alleviating salt stress on various soybean species with diverse salt tolerance can reflect the genotype differences or organ specificity, and may give us a meaningful pathway for salt tolerance improvement of G. max by inoculation with B. japonicum and gene engineering on isoflavone synthesis.