Pseudomonas fluorescens improves morph-physiological characteristics, essential oil compounds, and gene expression...

Drought stress limits growth, essential oil production, and menthol biosynthesis in peppermint (Mentha piperita). Plant growth-promoting rhizobacteria (PGPR), such as Pseudomonas fluorescens, can act as a biotic factor to enhance plant growth, antioxidant defense, and secondary metabolite accumulation under water stress. This study investigated the effects of P. fluorescens strain 18 inoculation on growth, antioxidant activity, essential oil composition, and menthol biosynthesis gene expression under three drought levels (30%, 65%, and 100% field capacity). Under drought stress at 65% field capacity, bacterial inoculation resulted in a 21.4% increase in shoot dry weight and a 29.8% increase in root dry weight. Bacterial inoculation also enhanced the activity of antioxidant enzymes peroxidase (POD), superoxide dismutase (SOD), and catalase (CAT) this reducing hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) levels by 21.5% and 21%, respectively. Bacterial inoculation led to increases in essential oil (EO) content (13.5% in moderate stress, 9.3% in severe stress) and menthol content (20%, 17.6%, and 29.4% under severe, mild, and non-stress conditions, respectively). The expression of the menthonl reductase (mr) gene under moderate and severe stress conditions, compared to the control (non-stress) condition, was reduced by 4.6 and 2.9 times in the presence of bacteria and by 6.6 and 11.3 times in the absence of bacteria. The results indicate that pseudomonas fluorescens inoculation can effectively mitigate the adverse effects of drought stress and improve essential oil content and composition in peppermint by producing auxin ACC deaminase, enhancing water and nutrient uptake, and controlling pathogenic factors.