Plants growing in heavy metal (HM)-contaminated soil have evolved a special detoxification mechanism. The rhizosphere gathers many living substances and their secretions at the center of plant roots, which has a unique ecological remediation effect. It is of great significance to thoroughly understand the ecological process of rhizosphere pollution under heavy metals (HMs) stress and develop biotechnology for joint remediation using plants and their coexisting microbial systems according to the mechanism of rhizosphere stress. Microbes can weaken the toxicity of HM pollutants by transforming the existing forms or reducing the bioavailability in the rhizosphere. Microbes survive in the HM-polluted soils through the production of stress-resistant substances, the participation of proteins, and the expression of heavy metal resistance genes, which strengthens the resistance of plants. Moreover, microbes can improve the nutritional status of plants to improve plant resistance to HMs. Plants, in turn, provide a habitat for microbes to survive and reproduce, which greatly accelerates the process of bioremediation. Briefly, the combined remediation of soil HMs pollution by plants and microbes is a promising, green, and sustainable strategy. Here, we mainly elucidate the joint remediation mechanism of plant–microbe symbiosis and introduce the coping characteristics of plants, microbes, and their symbiotic system, hoping to provide a scientific basis for the remediation of HM-contaminated soil in mining areas and the sustainable development of the ecological environment.

Iron plaques on the root surface can promote or inhibit the absorption and accumulation of heavy metals by plants. However, the mechanism by which iron regulates the response of Robinia pseudoacacia to mercury (Hg) has not been elucidated, which hinders its application in divalent Hg (Hg2+) removal from Hg-contaminated soil. In this study, association analyses between transcriptome and metabolome were used to investigate effects of iron on the rhizosphere microenvironment and performance of R. pseudoacacia to assess its potential for Hg2+ removal. The results showed that the addition of 10 mg kg−1 iron significantly increased the development of iron plaques on the root surface and reduced the secretion of low-molecular-weight organic acids by roots, thereby changing rhizosphere soil characteristics and decreasing total Hg in roots. In addition, the secretion of choline supported signal transduction and enhanced the interaction between R. pseudoacacia and rhizobia, thereby inducing resistance to Hg2+. Anti-oxidative enzyme activities were increased and Hg2+ exposure of plants was reduced. Enhanced Hg2+ resistance was indicated by improved photosynthesis and growth, despite promoted xylem loading and transport of Hg2+, resulting in its accumulation in aboveground tissues, which is essential for Hg2+ removal. These results indicate that iron addition has a great potential to improve the growth of R. pseudoacacia in Hg-contaminated soil and promote the accumulation of Hg2+ in aboveground tissues for phytoremediation approaches.

Cadmium (Cd) contamination threatens soil ecosystems and food security. Robinia pseudoacacia is a potential candidate for phytoremediation, but the molecular responses and root-associated microbial responses accompanying rhizobial symbiosis under Cd exposure remain incompletely understood. We integrated root transcriptomic, internal-root metabolomic, and 16S rRNA gene sequencing data to characterize responses to Cd exposure and rhizobial inoculation in SX, the high shoot-Cd accumulator, and HB, the low shoot-Cd accumulator. SX and HB exhibited distinct transcriptional patterns and exploratory candidate-metabolite profiles. In SX, the transcriptional responses were associated with the greater shoot Cd translocation reported previously for this genotype. In HB, rhizobial inoculation under Cd exposure was associated with broader changes in candidate metal-transport and compartmentalization genes, consistent with its previously reported greater root Cd retention. Rhizobial inoculation was also associated with higher candidate abundances of leucine, trehalose, palmitic acid, thymine, and pantothenates in HB, whereas inoculated SX showed higher candidate abundances of leucine and isoleucine but lower internal L-malic acid. Cd exposure, rhizobial inoculation, and root compartment were associated with variation in bacterial community composition, including abundance patterns involving Pseudomonas, Devosia, and Rhizobium. Exploratory Spearman correlation analyses revealed compartment-specific patterns of covariation between candidate metabolites and bacterial genera; however, no metabolite-genus association remained statistically significant after Benjamini-Hochberg correction in the rhizoplane, endosphere, or rhizosphere. None of the compartment-specific Procrustes tests indicated significant overall concordance between bacterial community composition and root-metabolite profiles. Collectively, these findings identify candidate genes, metabolites, and bacterial taxa and provide testable hypotheses that could inform future genotype selection and microbiome-assisted phytoremediation strategies.