Integrated role of biochar and PGPR (Leclercia adecarboxylata HW04) in enhancing cadmium phytoremediation and stress tolerance in Glycine max L.

Plant growth-promoting rhizobacteria (PGPR) and biochar (BC) are recognized as effective biological agents for enhancing stress tolerance and mitigating heavy metal toxicity in crops. Therefore, this study aims to investigate the effects of the cadmium (Cd)-resistant PGPR strain Leclercia adecarboxylata HW04 (>4 mM Cd resistance) on soybean plants exposed to 300 μM Cd. HW04 was observed to possess the innate ability to synthesize indole-3-acetic acid and exopolysaccharides, which facilitated the absorption of Cd in the medium. Scanning electron microscopy (SEM) images revealed that HW04 effectively colonized the porous structure of BC. Their combined treatment significantly enhanced photosynthesis and improved the morphological characteristics of soybean plants. Additionally, the Cd content in soybean shoots significantly increased following both the sole or combined treatments of BC and HW04. However, the combined treatments significantly reduced Cd content in the roots and soil by 31% and 47%, respectively. HW04 inoculation alone increased Cd content in the roots by 43.7% while lowering it in the soil by 7.4%. Additionally, the co-application of HW04 and BC significantly enhanced calcium (Ca) and magnesium (Mg) assimilation while promoting Cd/Fe accumulation in soybean shoots. The higher expression of signaling cascade genes, including GmCaMK1 and GmCDPK5 (Ca signaling) and GmMAPK4a and GmMAPK7 (MAP kinase signaling), in the co-applied crops further validate the findings. The increase in abscisic acid levels and the decrease in salicylic acid levels after HW04 and BC application were correlated with enhanced stress tolerance in crops. These findings suggest that the combined application of HW04 and BC could serve as an effective, eco-friendly approach for mitigating heavy metal toxicity in crops and promoting phytoremediation.