Mercapto-Functionalized Carbon Dots Reduce Cadmium Accumulation in Wheat Grains via Plant–Soil–Microbial Regulation

Sustainable strategies to reduce Cd accumulation in wheat grown on Cd-contaminated alkaline soils remain limited. Here, we developed mercapto-functionalized carbon dots (SH-CDs) from Salvia miltiorrhiza biomass via a solvent-free ball milling approach with 3-mercaptopropyltrimethoxysilane. SH-CDs exhibited superior Cd²⁺ adsorption capacity (35.9 mg·g^-1), primarily attributed to the strong affinity of surface-bound mercapto (-SH) groups, as confirmed by DFT calculations. SH-CDs applied via soil irrigation (800 mg·L^-1) and foliar spraying (1 mg·mL^-1) effectively reduced Cd content in wheat grains by 61.1% and 43.8%, respectively, compared to the control group (CK), while grain yield increased significantly by 24.5% and 12.4%. Soil irrigation with SH-CDs transformed soil Cd into less bioavailable fractions, reducing diethylenetriaminepentaacetic acid-extractable Cd (DTPA-Cd) by 13.6-45.7%. Integrated transcriptomic and metabolomic analyses revealed that SH-CDs mitigate Cd accumulation and phytotoxicity through: (1) enhancing photosynthesis (upregulating Lhca/b, Psa, Psb genes) and antioxidant defense (elevating SOD, CAT, POD, GSH; upregulating GST, APX); (2) promoting Cd sequestration in cell walls via remodeling genes involved in lignin, cellulose, and pectin biosynthesis; (3) regulating Cd transporters (upregulating ABCG3/7, ABCC2/3/4; downregulating HIPP27/45); and (4) reprogramming stress-related metabolites (amino acids, lipids, flavonoids). Additionally, SH-CDs irrigation enriched soil plant-growth-promoting and heavy-metal-resistant bacteria (e.g., Lysobacter and Vicinamibacteraceae). This work elucidates the multifaceted plant-soil-microbe mechanisms underpinning the efficacy of SH-CDs, demonstrating their potential as a nanoenabled strategy for safe crop production in Cd-contaminated farmlands.