Enhanced cadmium phytoextraction in Brassica juncea by a hyperaccumulator-sourced synthetic inoculant

Synthetic communities (SynComs) of plant growth-promoting bacteria (PGPB) could enhance cadmium (Cd) phytoextraction and promote plant growth in Brassica juncea L. However, the precise molecular mechanisms of facilitated Cd uptake and accumulation remain unclear. Here, using a Cd-specific fluorescent probe, we demonstrated that SynComs application markedly improved Cd distribution and uptake within plant roots. Integrated transcriptomic and metabolomic analyses under Cd stress revealed that SynComs mitigate Cd-induced stress by activating plant antioxidative defense system, encompassing antioxidants glutathione, key enzymes such as peroxidase, catalase, ascorbate peroxidase, and superoxide dismutase. Furthermore, SynComs inoculation facilitated root exudates secretion which contributed to plant growth. Weighted Gene Co-expression Network Analysis (WGCNA) identified 17 and 14 Cd- and antioxidant-related gene modules in shoots and roots, respectively, with critical hub genes functionally associated with metal transport, redox homeostasis, and secondary metabolisms. Continuous determination of gene expression levels verified that SynComs inoculation upregulated the metal transporters such as Nramp1, NRT1, ZIP4 and ZIP6 to increase Cd uptake. These results implied that SynComs could facilitate Cd uptake and eliminate Cd-induced damage through the coordinated regulation of specific hub genes and a broad spectrum of plant secondary metabolites. These findings lead to augmented plant growth and enhanced Cd phytoextraction in B. juncea, strengthened a comprehensive understanding of the molecular mechanisms underlying PGPB-mediated Cd uptake and accumulation. This research verified SynComs application as an effective agent in the future field application to enhance phytoremediation efficiency, which also contribute to soil health, and maintain sustainable, safe crop production in Cd-contaminated agricultural systems.