Hyperaccumulator extracts promoting the phytoremediation of rare earth elements (REEs) by Phytolacca americana: Role of active microbial community in rhizosphere hotspots.

The increased usage of rare earth elements (REEs) leads to the extensive exploitation of rare earth mines, and the REEs pollution in soil caused by the legacy mine tailings has brought great harm to environment and human health. Although Phytolacca americana can remove REEs from contaminated soil to some extent, there is still an urgent problem to improve its efficiency. Hyperaccumulator extract is a new organic material with potential in metal phytoextraction, but its role in REEs phytoremediation and the related underlying processes remain unclear. In this study, hyperaccumulator extracts from P. americana root (PR), stem (PS), leaf (PL) and EDTA were used to improve the phytoremediation efficiency of REEs with P. americana. Soil zymography was applied to assess the enzyme hotspots' spatial distribution in the rhizosphere, and the hotspots' microbial communities were also identified. The results indicated that the application of hyperaccumulator extracts improved the biomass and REEs uptake of P. americana, and the highest REEs content in plant was observed in the treatment of PS, which increased 299% compared to that of the control. Hotspots area of ß-glucosidase, leucine aminopeptidase and acid phosphatase were concentrated in the pant rhizosphere along the roots and increased 2.2, 5.3 and 2.2 times after PS application compared to unamended soils. The PS application increased the relative abundance of Proteobacteria, Cyanobacteria, Bacteroidota and Firmicutes phyla in rhizosphere. Soil fungi have a higher contribution on promoting REEs activation than that of bacteria. Available P and extractable REEs were leading predictors for the plant biomass and REEs concentrations. The co-occurrence network showed that the application of PS creates a more efficient and stable microbial network compared to other treatments. In conclusion, stem-derived hyperaccumulator extract is excellent in stimulating REEs phytoremediation with P. americana by improving hotspots microbial activities and form a healthy rhizosphere microenvironment.

The translocation and fractionation of rare earth elements (REEs) via the phloem in Phytolacca americana L.

Rare earth elements (REEs) are considered to be emerging contaminants due to their widespread use and lack of recycling. Phytolacca americana L. has great potential for REEs phytoextraction. Our understanding of REEs in P. americana focuses mostly on root absorption and xylem translocation, but the role of phloem translocation has received little attention. In this research, the translocation and fractionation of REEs from phloem to organs in P. americana were investigated. In addition, the effect of organic acids in the REEs translocation via phloem exudates was also examined. The results showed that REEs could transport bidirectionally via the phloem, and 86% of REEs exported from old leaves could move downwards to the root, whereas only 14% of them transported upwards to the young leaves. Heavy rare earth elements (HREEs) enrichment was found in the REEs fractionation processes both from phloem to leaf and from stem to root, indicating that HREEs were preferentially transferred not only down to roots, but also up to the young leaves. The concentration of oxalic acid in phloem exudates was much higher than other organic acids. 94.7% oxalic acid in phloem exudates was preferred to combine with REEs, especially HREEs. Additionally, the concentrations of HREEs had a high positive correlation with oxalic acid in phloem exudates, which demonstrated oxalic acid may play a significant role in the long-distance transport of HREEs in phloem. In conclusion, HREEs have higher translocation ability than light rare earth elements (LREEs) in both xylem and phloem of P. americana. As far as we know, this is the first report focused on the phloem translocation and redistribution of REEs in P. americana, which provides a valuable understanding of the mechanism for phytoremediation of REEs contaminated soils.

Rare Earth Elements (REEs) Adsorption and Detoxification Mechanisms in Cell Wall Polysaccharides of Phytolacca americana L.

The cell wall (CW) is critical for the accumulation of heavy metals in metal-tolerant plants. Polysaccharides, the main component of the CW, contribute significantly to the immobilization of heavy metals. However, the mechanisms of rare earth elements (REEs) adsorption and detoxification by polysaccharides in the cell walls of Phytolacca americana L. (P. americana) remain unclear. In this work, we explored the binding sites of REEs and the modifications to polysaccharides in the cell walls of roots and leaves in P. americana, in order to elucidate the adsorption and fixation mechanism of REEs by the cell wall. Our findings indicated that up to 40.7% and 48.1% of cell-wall-bound REEs were present in the root and leaf pectin, respectively. The removal of pectin led to a 39.8% and 23.6% decrease in the maximum adsorption of REEs in the CW, suggesting that pectin was the main binding site for REEs in the cell walls of P. americana. Hydroxyl (-OH) and carboxyl (-COOH) groups in the cell wall interacted mainly with REEs ions under stress conditions, which played a key role in REEs binding. An obvious REEs fractionation was found during the various fractions of the CW, and all fractions of the root cell wall were enriched with HREEs, whereas all fractions of the leaf cell wall were enriched with LREEs. Moreover, P. americana modulated cell wall composition in reaction to REEs stress. In conclusion, cell wall pectin is the main binding site of REEs, and the functional groups on the cell wall play a significant role in the binding of REEs. At the same time, plants can control the selective adsorption and fixation of REEs by adjusting the composition of cell walls. This study offers valuable insights into the mechanisms of REEs adsorption and fixation in cell walls of P. americana, contributing to a theoretical basis for the bioremediation of REEs pollution.