Synergistic regulation of Cd stress tolerance in Brassica juncea: Metabolic reprogramming and nutrient-Cd co-transport under Trichoderma harzianum and polyaspartic acid

Cadmium (Cd) contamination constitutes a serious threat to plant growth and agricultural productivity. This study investigated how Trichoderma harzianum (T. harzianum) and polyaspartic acid (PASP) work together to help B. juncea handle and store more Cd from soil. The results showed that T. harzianum effectively colonized the root system and improved root volume. Compared to the control, the combined treatment (AT) significantly increased photosynthetic efficiency and total biomass (+42.38 %). Particularly, AT treatment enhanced nutrient accumulation (K by 42.28 %, Na by 63.58 % and Mg by 48.59 %). Cd was predominantly localized in the vacuoles and cell wall of both leaf and root. Meanwhile, an increase in xylem Cd flux was found to enhance leaf Cd accumulation by 71.12 %. The levels of antioxidant enzymes and glutathione were increased, which were significantly correlated with Cd in the cell wall. AT treatment reprogrammed root metabolic processes, upregulating tyrosine metabolism to stabilize cell membranes, while downregulating starch and sucrose metabolism to prioritize defense mechanisms. Two core metabolites were identified as key regulators of enhanced Cd and nutrition accumulation. Furthermore, structural equation modeling confirmed that the synergistic effects of plant resistance and metabolic reprogramming drove the co-transport of nutrients and Cd. This study proposed a microbe-chelator-based growth promotion framework for B. juncea under Cd stress. Overall, the framework enhances plant Cd tolerance and facilitates nutrient–Cd co-transport, providing a technical basis for phytoremediation. Nevertheless, given the limitations of controlled pot experiments, long-term field trials are required to evaluate its effectiveness and environmental adaptability.