Synergistic Application of Humic Acid and Bacillus mobilis IJL Mitigates Nickel‐Induced Stress in Soybean Through Modulation of the ABA Pathway

This study investigated the synergistic effects of plant growth–promoting rhizobacteria (PGPR) and humic acid in mitigating nickel‐induced stress in soybean. Bacterial isolates were obtained from the Pohang beach environment, among which the Bacillus mobilis IJL strain demonstrated particularly promising performance under in vitro conditions. To further evaluate the combined effects of B. mobilis IJL and humic acid, a pot experiment was established with four treatments: control (water only), IJL, humic acid (100 mg kg −1 ), and IJL + humic acid, under both normal and nickel‐stress conditions (50 mg kg −1 ). A comprehensive set of plant physiological, biochemical, antioxidant, and computational parameters was assessed. The IJL + humic acid treatment significantly improved plant growth, chlorophyll content, leaf spectral indices, antioxidant capacity, and nutrient uptake under both normal and Ni‐stressed conditions. Additionally, endogenous abscisic acid (ABA) levels were markedly reduced in IJL + humic acid–treated plants. Computational analyses indicated that metabolites produced by IJL and humic acid may inhibit two key enzymes in the ABA biosynthetic pathway, 9‐cis‐epoxycarotenoid dioxygenase (NCED3) and abscisic aldehyde oxidase (AAO3), thereby contributing to lower ABA. These findings provide novel insight into how the synergistic application of humic acid and IJL mitigates Ni‐induced stress in soybean by modulating the ABA‐mediated stress signaling pathway. The results suggested that the potential of B. mobilis IJL combined with humic acid can be used as an organic tool to enhance plant performance and nutrient uptake in nickel‐contaminated soil.