Bacillus subtilis RK027 Induces Arsenic Stress Tolerance in Rice via Gene Expression Regulation

Arsenic contamination in agriculture negatively impacts the normal growth and development of plants, limiting crop production. Although plant growth-promoting rhizobacteria (PGPR) can enhance plant growth under stress, the molecular mechanisms of induced systemic tolerance (IST) exerted by these bacteria remain unclear. Thus, this study was designed to isolate arsenic-stress-tolerant PGPR from the rice rhizosphere to evaluate its effectiveness in rice plant growth and to understand the molecular mechanisms of IST against arsenite (As^III) through jasmonic acid (JA) biosynthetic (OsAOS1), signaling (OsJAR1), and antioxidant (OsSOD-Cu/Zn, OsCATA, and OsAPX1) gene expression profiling. In this study, the arsenic-tolerant rhizobacterium RK027 was identified as Bacillus subtilis, exhibiting multiple PGP traits, including IAA, ammonia, and EPS production, phosphate solubilization, and ACCD activity, under both non-stressed and As^III-stressed conditions. The FT-IR analysis of EPS revealed the presence of functional groups that likely form As^III-EPS complexes, reducing As^III translocation. Rice seed priming with RK027 and subsequent plant inoculation exhibited significant improvements in morphophysiological and biochemical traits, including antioxidant activities, during the vegetative and reproductive phases under both conditions. The qRT-PCR analysis indicated that B. subtilis RK027 significantly upregulated OsAOS1 (38% and 25%) and OsJAR1 (35% and 30%), OsSOD-Cu/Zn (59% and 78%), OsCATA (40% and 18%), and OsAPX1 (30% and 25%) compared to non-inoculated stressed plants in vegetative and reproductive phases, respectively. These enhancements suggest that B. subtilis RK027 exerts IST by enhancing JA-biosynthesis and downstream JA-responsive antioxidant pathways under As^III-stress. Our findings highlight the potential of B. subtilis RK027 to improve rice plant health in arsenic-stressed environments.