Synergistic Effects of <i>Bacillus cereus</i> Mn‐5 PGPR‐Derived Silver Oxide Nanoparticles on Tomato Plant Growth, Stress Resilience and Nutritional Enhancement

ABSTRACT Introduction Agricultural crops face increasing environmental stresses, such as soil salinity and heavy metal contamination, aggravated by climate change and threatening food security. While PGPRs are eco‐friendly agrochemical substitutes, their efficacy in the field is inconsistent. Integrating microbial biotechnology with green nanotechnology presents a solution. This in vitro research investigates a multifunctional Bacillus cereus strain that functions as both a PGPR and a green synthesizer of silver oxide nanoparticles (Ag₂O NPs) to enhance early tomato seedling growth sustainably. Materials and Methods Bacterial isolates from tomato rhizosphere were tested for PGPR traits, biocontrol activities and stress tolerance. The most effective PGPR isolates were identified by 16S rRNA sequencing and also employed for Ag₂O NPs biosynthesis, followed by characterization using XRD, SEM, TEM and FTIR. Tomato seeds were treated with Mn‐5‐derived Ag₂O NPs (1–100 ppm) and evaluated for in vitro germination and seedling growth. Results The B. cereus Mn‐5 PGPR exhibited strong growth‐promoting characteristics, including phosphate solubilization (18.91–187.47 µg/mL), siderophores production, nitrogen fixation, protease production and antagonism against Rosellinia necatrix . It tolerated 8% salinity and heavy metals up to 100 µg/mL. Mn‐5 PGPR was employed for Ag₂O NP synthesis and characterization. XRD, SEM, TEM and FTIR revealed spherical, agglomerated NPs with a crystallite size of 30.26 nm and a particle diameter of 80.16 ± 1.51 nm, with FTIR bands ranging from 400 to 4000 cm − ¹. In vitro, 5 ppm Mn‐5 PGPR‐derived Ag₂O NPs concentrations notably promoted tomato seed germination, shoot–root growth, seed vigour index and biomass by 200% over controls and a 44.4% over PGPR alone. Conversely, higher concentrations (50 and 100 ppm) were phytotoxic inhibiting seedling growth. Conclusion The study demonstrates the potential of B. cereus Mn‐5 PGPR and biosynthesized Ag₂O NPs as eco‐friendly bio‐stimulants for tomato growth under stress. The findings establish a baseline for future in vivo studies on dosage optimization toward sustainable agriculture.