Mechanistic insights into chromium immobilization and uptake inhibition in lettuce mediated by endophyte-loaded...

Chromium (Cr) contamination poses severe environmental and food-safety risks due to the high mobility and toxicity of Cr(VI) in agricultural soils. Microbial immobilization technology (MIT) has emerged as a promising approach for Cr detoxification; however, the mechanisms underlying endophyte-loaded biochar systems remain insufficiently understood. In this study, we evaluated the synergistic remediation potential of biochar (BC), the endophyte Serratia sp. Y-13 (SR1), and their immobilized composite (BSR1), employing morphological, physiological, transcriptomic, and metabolomic analyses. Results showed that SR1 mitigated Cr stress via cell-wall adsorption, periplasmic accumulation, and Cr(VI) reduction, while biochar loading further strengthened oxygen-containing functional groups (C-O and OC-O) on the composite surface. Thus, BSR1 exhibited the highest efficiency, reducing soil Cr(VI) and exchangeable Cr fractions by 94.2% and 84.1%, respectively, compared to the control. This was accompanied by a significant decrease in Cr translocation from roots to shoots. In addition, BSR1 altered antioxidant-enzyme activities and induced extensive transcriptional and metabolic reprogramming. Integrated transcriptomic and root-exudate metabolomics analyses revealed enrichment of phenylpropanoid metabolism, ABC transporter pathways, and defense-related metabolites under BSR1 treatment. RT-qPCR validation of six representative DEGs showed expression trends consistent with those obtained from RNA-seq. Collectively, the results suggest that BSR1 enhances Cr immobilization and promotes plant adaptation to Cr stress through coordinated soil and plant responses. The study provides mechanistic insights into how endophyte-loaded biochar enhances Cr(VI) reduction and Cr immobilization, supporting the development of sustainable amendments for reducing Cr bioavailability in contaminated soils.