Synergistic alleviation effects of salt-tolerant plant growth-promoting rhizobacteria and hydrogen-rich water on salt stress in Pennisetum giganteum

Soil salinization severely restricts agricultural production and the sustainable use of soil. While plant growth-promoting rhizobacteria (PGPR) and hydrogen-rich water (HRW) have individually been reported to alleviate salt tolerance, their synergistic effects and molecular mechanisms remain largely unexplored. In this study, we investigated the combined application of a salt-tolerant PGPR strain <i>Cytobacillus firmus</i> L71 and HRW in <i>Pennisetum giganteum</i> under NaCl stress. A factorial pot experiment was conducted under three salt levels (0, 250, and 500 mM NaCl) with or without PGPR-HRW treatment. Growth traits, antioxidant activities, osmotic regulators, and transcriptomic responses were measured. The combined treatment significantly promoted growth under severe salinity, with shoot fresh weight increasing by 148% and root length by 54.60% compared with untreated control. Physiological measurements showed elevated activities of Superoxide Dismutase (SOD), Peroxidase (POD), and Catalase (CAT), and reduced accumulation of Malondialdehyde (MDA) and Hydrogen peroxide (H₂O₂). Transcriptome analysis indicated consistent enrichment in plant hormone signaling, mitogen-activated protein kinase (MAPK) signaling, and plant-pathogen interaction pathways. Negative regulators such as CaM/CML (induces stomatal closure), CDPK (triggers hypersensitive response), WRKY25/33 (inhibits DNA defense genes), and JAZ (accelerates stress-induced senescence) were down-regulated, while positive regulators including A-ARR (enhances cell division and shoot growth) were up-regulated, contributing to sustained stomatal function, delayed senescence, and improved reactive oxygen species (ROS) balance. These results demonstrate that PGPR-HRW synergy enhances salt tolerance through coordinated physiological and transcriptional regulation, highlighting the potential of integrating microbial inoculants with HRW for sustainable saline soil remediation and crop improvement.