PGPR-mediated enhancement of growth, phytochemical diversity, and metabolites in black turmeric (Curcuma caesia Roxb)

Introduction Medicinal plants are an essential source of bioactive compounds with therapeutic potential, yet their cultivation frequently depends on chemical fertilizers and pesticides that may compromise environmental and human health. Plant growth–promoting rhizobacteria (PGPR) represent a sustainable alternative for enhancing plant productivity and phytochemical quality through beneficial plant–microbe interactions. This study evaluated the effects of rhizobacterial inoculation on growth performance, phytochemical composition, and secondary metabolite production in black turmeric ( Curcuma caesia ), a medicinal plant recognized for its anticancer properties. Methods A greenhouse experiment was conducted using a completely randomized design with five treatments: inoculation with Bacillus subtilis , Paenibacillus polymyxa , a bacterial consortium ( B. subtilis + P. polymyxa ), NPK fertilizer, and an uninoculated control. Rhizobacteria were applied at a concentration of 2%. Each treatment consisted of five replicates, with three plants per replicate (total = 75 plants). Plant growth parameters, dry biomass, phytochemical profiles, GC–MS analysis, and bacterial colony–based secondary metabolite analysis were assessed. Results Rhizobacterial treatments significantly enhanced plant growth, dry weight, and phytochemical content compared to the control and NPK treatment. B. subtilis inoculation resulted in the highest growth performance and the greatest diversity of phytochemical compounds detected by GC–MS. The consortium treatment exhibited the highest abundance and area of secondary metabolite compounds, indicating synergistic interactions between the two bacterial strains. Both B. subtilis and P. polymyxa demonstrated traits associated with PGPR activity, including indole-3-acetic acid production, nitrogen fixation, phosphate solubilization, root colonization, and induction of systemic resistance, contributing to improved nutrient uptake and suppression of wilt disease. Discussion These findings highlight the role of PGPR-mediated mechanisms in enhancing secondary metabolite biosynthesis and medicinal quality in black turmeric. The use of rhizobacterial inoculants, particularly B. subtilis and its consortium with P. polymyxa , offers a promising strategy for sustainable cultivation of medicinal plants and supports the integration of microbial biotechnology in sustainable pharmacological production.