Optimizing plant growth and stimulating the production of specialized metabolites, including essential oils, are key objectives for advancing agricultural productivity and meeting the growing demands of the pharmaceutical industry. Bioelicitors, particularly endophytic bacteria, offer a sustainable strategy to promote the accumulation of these bioactive compounds. This study aimed to isolate and characterize endophytic bacterial strains with the potential to enhance plant growth and increase the synthesis of targeted phytochemicals in Zataria multiflora Boiss. A completely randomized design (CRD) with three replications was used in two phases: a laboratory experiment to assess seedling responses and a greenhouse trial to evaluate morphophysiological and biochemical traits, essential oil content, and composition. Eight endophytic bacterial isolates were screened, and Agrobacterium sp. ER40 and Paenibacillus peoriae ER11 were the most effective in enhancing seedling growth parameters. Molecular identification using the NCBI database confirmed their identities. Greenhouse experiments significantly improved growth, physiology, and biochemical traits following inoculation. Agrobacterium sp. ER40 increased chlorophyll a (16.46%), chlorophyll b (84.21%), total chlorophyll (38.73%), carotenoids (59.37%), anthocyanins (97.83%), phenylalanine ammonia-lyase (130.85%), phenols (112.68%), flavonoids (188.85%), and essential oil content (146%) compared to the control. Gas chromatography-mass spectrometry (GC-MS) analysis identified 32 essential oil components, with notable increases in thymol (10.32%), o-cymene (6.18%), carvacrol (18.17%), α-pinene (3.45%), β-myrcene (10.41%), and sesquiterpenes, including caryophyllene oxide (54.29%), caryophyllene (277.78%), and tau-cadinol (188.00%). These findings highlight the potential of endophytic bacteria as bioelicitors for improving growth and specialized metabolite production in Z. multiflora. The results support bioelicitor-based strategies for optimizing plant-derived bioactive compounds, with promising applications in the pharmaceutical, cosmetic, and nutraceutical industries.
