Bacillus tropicus KH90 induces systemic drought tolerance in rice: an integrated PGPR-mediated mechanistic framework

In our previous study, Bacillus tropicus KH90 was identified as a drought-tolerant, multifunctional plant growth-promoting rhizobacterium that enhanced rice plant growth under both non-drought and drought conditions. However, the mechanisms underlying KH90-mediated induced drought tolerance remained unclear. The present study elucidates the physiological, biochemical, and molecular basis of KH90-induced drought tolerance in rice using seed priming and subsequent plant inoculation under controlled non-drought and drought conditions. Drought stress significantly impaired root architecture, reduced macro- and micronutrient content, and increased reactive oxygen species (O₂•⁻ and H₂O₂) accumulation. In contrast, the KH90 treatment improved root architecture and nutrient acquisition while markedly reducing reactive oxygen species levels in drought-stressed plants. Although total protein content declined under drought stress, antioxidant enzyme activities (SOD, CAT, APX, GPX, GR, MDHAR, and DHAR) were significantly enhanced and further stimulated by KH90 inoculation. The gene expression analysis revealed that drought stress caused downregulation of the aquaporin (OsNIP1;1) gene and only slight upregulation of AP2-EREBP, whereas KH90 treatment upregulated these genes along with key antioxidant genes (OsSOD-Cu/Zn, OsCATA, OsAPX1, and GR1), indicating coordinated molecular regulation of stress-responsive pathways. Unlike most studies on Bacillus spp., which emphasize plant growth promotion or limited biochemical responses, this study establishes a KH90-specific integrated mechanistic framework for PGPR-mediated induced systemic drought tolerance in rice, linking coordinated physiological, biochemical and molecular responses under drought stress. Collectively, KH90 mitigates drought-induced oxidative damage and restores physiological equilibrium, highlighting its potential as an effective bioinoculant for sustainable rice cultivation.