Nitrogen-fixing bacteria Bradyrhizobium yuanmingense SB006-1-2 and Rhizobium spp. S058-A2-3, carbohydrate types, and growth conditions alter the phytocompounds during soybean (Glycine max L.) germination: an innovative study

Soybeans are rich in isoflavones beneficial to humans and plants and affected by soil microbiota and nitrogen-fixing bacteria. Effects of three carbohydrates and two nitrogen-fixing Bradyrhizobium yuanmingense SB006-1–2 and Rhizobium spp. on the production of phytocompounds and isoflavonoid–like compounds were evaluated. After plating seeds on 1.5% agar in sterilized and nonsterilized soils, growth characteristics and phytocompound contents in the seedlings were investigated before and after inoculation with 1 × 104 and 1 × 106 colony formation units (CFUs) on day 5. Using six known isoflavones as standards, the levels of methanol purified phytocompounds were determined by reversed-phase HPLC analysis. During extraction, temperatures at 100 °C reduced the peaks 6 and 8, increased the peak 7, and formed a new compound of peak 4 (glycitein-like) in seed and filtered liquid extract and peak 5 (genistein-like) in seed. Glucose and maltose promoted the synthesis of daidzein-like compounds, and only B. yuanmingense could synthesize higher levels of daidzein- and glycitein-like compounds. Both bacterial species reduced seedling growth and moisture content, and altered phytocompound types and levels in three periods and especially better in nonsterilized soils than in sterilized soils. Furthermore, the total phytocompound content differed between RHB (roots + hypocotyls + buds) and cotyledons, and between bacterial species and doses among the three periods. The soil microbiota altered phytocompound and isoflavonoid-like compound synthesis with lower expression in nonsterilized soil than in sterilized agar and soil. In conclusion, carbohydrate types and nodule bacterial species can change phytocompounds for functional soybean seedlings.