Functional Portability of a Hyperaccumulator-Derived Core Microbiome: Enhancing Cadmium Phytoextraction in Brassica juncea L. Through Molecular Reprogramming

Soil cadmium (Cd) contamination is a persistent threat to global food security, requiring sustainable in situ remediation strategies. While hyperaccumulating plants possess specialized traits for metal extraction, their low biomass limits large-scale application. This study investigates the potential of a core endophytic synthetic community (SynCom-NS)—characterized by heavy metal tolerance and growth-promoting traits, originally derived from the hyperaccumulator Sedum alfredii—by assessing its ability to modulate the remediation phenotype of a high-biomass non-host crop, Brassica juncea. Pot experiments revealed that SynCom-NS root-zone application significantly alleviated Cd toxicity, increasing total fresh weight by 82% and chlorophyll content by 33%. Crucially, the consortium bypassed the “growth-dilution” trade-off, facilitating a 4.07-fold increase in shoot Cd accumulation. Multi-omics analysis demonstrated a systemic modulation of the host’s defense machinery, marked by a >3-fold surge in glutathione (GSH) levels and the induction of phenylpropanoid biosynthesis for cell wall reinforcement. SynCom-NS application also mediated tissue-specific regulation of the key metal transporter HMA4, upregulating its expression in roots to accelerate long-distance translocation while downregulating it in shoots. These findings demonstrate that specialized core microbiomes function as potent bio-inoculants, offering a promising biological strategy for engineering high-efficiency phytoremediation systems.