Evaluation of Plant-Growth-Promoting Rhizobacteria on Growth, Physiological Performance, Defense Responses, and Root Rot Resistance of Barley Under Sterile Controlled Greenhouse Conditions with Rhizoctonia solani Challenge

Barley productivity in arid regions is severely constrained by abiotic and biotic stresses. This study evaluated the interactive effects of three plant growth-promoting rhizobacteria (PGPR) strains (Pseudomonasfluorescens ATCC 13525, Bacillus subtilis ATCC 6051, and Streptomyces sp. ABU-1) applied at three inoculum concentrations (1.0 × 106, 1.0 × 108, and 1.0 × 1010 CFU mL−1) via seed treatment or soil drench under sterile controlled greenhouse conditions with Rhizoctonia solani AG-4 pathogen challenge. Results demonstrated that bacterial strain identity was the primary driver of plant response, followed by inoculum concentration and application method. P. fluorescens ATCC 13525 at 1.0 × 1010 CFU mL−1 delivered as a soil drench yielded optimal overall performance. Relative to the non-inoculated, pathogen-challenged control, main-effect evaluations revealed that P. fluorescens inoculation significantly increased root length by 15.2%, shoot dry weight by 11.9%, leaf area by 13.9%, and photosynthetic rate by 17.8%, while reducing oxidative stress markers (malondialdehyde by 17.6% and hydrogen peroxide by 15.8%) and root rot disease severity by 34.3%. This treatment achieved over 90% root colonization compared to a 5.4% baseline background presence. Overall, selecting P. fluorescens ATCC 13525, applying an optimal high dose 1.0 × 1010 CFU mL−1), and utilizing a soil drench method significantly enhanced barley growth and root rot resistance within our sterile controlled greenhouse framework. While these phenotypic, physiological, and molecular improvements are strongly associated with viable bacterial establishment, potential contributions from co-administered or secreted metabolites present in the bacterial suspension cannot be entirely excluded in the absence of cell-free filtrate controls.