A newly isolated Bacillus pumilus strain SH-9 modulates response to drought stress in soybean via endogenous phytohormones and gene expression (Daegu, South Korea)

Drought stress hampers plant growth and productivity. Some microorganisms mitigate stress in plants; however, the molecular mechanism by which they interact with plants in mitigating stress remains unknown. This study aimed to determine the mechanism by which plant growth–promoting rhizobacteria modulate drought stress in soybean. Results indicated that B. pumilus SH-9 (SH-9) produced indole acetic acid and siderophore, and had capacity for phosphate solubilization. The test for polyethylene glycol 6000 (PEG) tolerance, showed that SH-9 could withstand high PEG concentration (up to 30 %). The isolate was identified a B. pumilus strain using a 16 s ribosomal DNA gene sequence. Inoculation of soybeans with SH-9 enhanced soybean plant growth and biomass by 20 %, even under high drought stress. This was due to a reduction of the endogenous phytohormone (abscisic acid) and upregulation of the antioxidant defense system (SOD, POD, APX, and GSH) by SH-9. Furthermore, the transcription factors GmDREB2, GmbZIP1, and GmNCED3 were involved. Inoculation with SH-9 also improved physio-morphological characteristics such as biomass, chlorophyll, seedling length, and relative water contents in the stressed plant. Overall, the findings of this study indicated that SH-9 enhances plant growth by promoting phosphate solubilization, siderophore, and exopolysaccharides. We, therefore, conclude that SH-9 is a drought-tolerant variant that can improve plant growth even under drought stress via modulation of expression of the phytohormone gene, and antioxidant profile.