Plant Growth-Promoting Rhizospheric Bacteria of Curcuma amada Enhance Growth, Curcumin Content, and Antioxidant Potential Under Controlled Conditions

AbstractThe excessive and imbalanced application of chemical fertilizers has led to soil degradation,declining nutrient-use efficiency, and stagnating yields, particularly in intensively farmed regionsof India. In medicinal plants such as Curcuma amada (mango ginger), continuous reliance onsynthetic inputs not only reduces productivity but may also compromise the biosynthesis ofpharmacologically important metabolites. Plant growth-promoting bacteria (PGPB) offer an eco-friendly alternative by improving nutrient availability, stress tolerance, and secondary metabolism. In this study, 14 bacterial isolates were recovered from the rhizosphere of C. amada,of which six (RCL1–RCL6) were selected for detailed characterization. Morphological andbiochemical assays identified the isolates as belonging to Bacillus, Azotobacter, Desulfovibrio,and Promicromonospora spp. Functional screening revealed that all isolates produced indole-3-acetic acid (15.8–38.6 μg/mL), while RCL3 and RCL5 additionally produced siderophores,solubilized phosphate, zinc, and potassium, and exhibited strong ACC deaminase activity(142.7–156.3 nmol α-KB mg−1 protein h−1). RCL5 and RCL3 demonstrated the highest ammoniaproduction (14.6 and 12.8 μg/mL, respectively) and were the only strains positive for hydrogencyanide, indicating biocontrol potential. Both isolates also inhibited Fusarium oxysporum (62–68%) and Rhizoctonia solani (55–62%) in dual culture assays. Greenhouse trials confirmed thatinoculation with RCL5 and RCL3 significantly enhanced plant growth, with shoot lengths of41.3 ± 2.2 cm and 39.8 ± 2.0 cm, respectively, compared with 32.1 ± 1.8 cm in controls. Drybiomass, chlorophyll content, and phosphorus/potassium uptake were also markedly improved.Importantly, curcumin content increased to 14.2 ± 0.8 mg/g DW in RCL5-treated plants, nearlydoubling that of controls. Antioxidant assays further demonstrated that RCL5 and RCL3High Technology Lettersenhanced total phenolics, reduced DPPH IC50 values, and increased ABTS scavenging activityrelative to uninoculated plants. These findings highlight the multifunctional role of indigenousrhizospheric bacteria in promoting growth, nutrient mobilization, and secondary metabolism ofC. amada. The isolates, particularly RCL5 and RCL3, hold promise as biofertilizers andbioprotectants for sustainable medicinal plant cultivation.