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Cadmium (Cd) is highly toxic to most organisms, but some rare plant species can hyperaccumulate Cd in aboveground tissues without suffering from toxicity. The mechanism underlying Cd detoxification by hyperaccumulators is interesting but unclear. Here, the heavy metal ATPase 3 (SpHMA3) gene responsible for Cd detoxification was isolated from the Cd/zinc (Zn) hyperaccumulator Sedum plumbizincicola. RNA interference (RNAi)-mediated silencing and overexpression of SpHMA3 were induced to investigate its physiological functions in S. plumbizincicola and a nonhyperaccumulating ecotype of Sedum alfredii. Heterologous expression of SpHMA3 in Saccharomyces cerevisiae showed Cd-specific transport activity. SpHMA3 was highly expressed in the shoots and the protein was localized to the tonoplast. The SpHMA3-RNAi lines were hypersensitive to Cd but not to Zn, with the growth of shoots and young leaves being severely inhibited by Cd. Overexpressing SpHMA3 in the nonhyperaccumulating ecotype of S. alfredii greatly increased its tolerance to and accumulation of Cd, but not Zn. These results indicate that elevated expression of the tonoplast-localized SpHMA3 in the shoots plays an essential role in Cd detoxification, which contributes to the maintenance of the normal growth of young leaves of S. plumbizincicola in Cd-contaminated soils.

Aims Plant roots can significantly alter soil pH and the chemical concentration and distribution of different elements in the rhizosphere environment. Here we ask whether cadmium (Cd) bioavailability in the rhizosphere of Cd-hyperaccumulator Sedum plumbizincicola can be influenced by root-induced effects on soil pH. Methods The Cd-hyperaccumulator S. plumbizincicola and the Cd non-hyperaccumulator ecotype Sedum alfredii were both grown in four different Cd-contaminated soils. We used the planar optode imaging technique to produce two-dimensional and high-resolution measurements of soil pH. Shoot excess cation concentration, root architecture and Cd concentrations ([Cd]) in soil pore water were also measured. Spatial analyses based on kernel density estimate of roots (KDE) and a Moran’s I correlogram were performed to assess spatial patterns and potential relationships among root distribution, soil pH and [Cd]. Results Both Sedum species showed root-induced increases in soil acidification (i.e. soil pH decreases of 0.1 to 0.62 units), which were clearly associated with greater root density of these plants. Remarkable excess cation uptakes by both Sedum species were detected and likely a driving factor for the root-induced acidification. The presence of the roots of S. plumbizincicola were then related to higher [Cd] in the rhizosphere than in bulk soil in Orthic Acrisol (+342%) and in Hydragric Antrosol soils (+296%). The hyperaccumulator S. plumbizincicola had larger root systems, higher acidification ability, and was associated with greater soil [Cd] than S. alfredii . Spatial patterns of root distribution and soil pH were similar between Sedum plants, however, spatial patterns of [Cd] differed across polluted soils. Conclusion Rhizosphere acidification induced by S. plumbizincicola plants can play an important role on soil Cd mobilization, but overall effects on soil Cd bioavailability will depend on intrinsic soil biogeochemical properties.

The cadmium hyperaccumulator Sedum plumbizincicola has remarkable abilities for cadmium (Cd) transport, accumulation and detoxification, but the transcriptional regulation mechanisms responsible for its Cd hyperaccumulation remain unknown. To address this knowledge gap, we conducted a comparative transcriptome study between S. plumbizincicola and the non-hyperaccumulating ecotype (NHE) of Sedum alfredii with or without Cd treatment. Our results revealed many differentially expressed genes involved in heavy metal transport and detoxification that were abundantly expressed in S. plumbizincicola. Additionally, we identified a large number of differentially expressed transcription factor genes, highlighting the complexity of transcriptional regulatory networks. We further screened four transcription factor genes that were highly expressed in the roots of S. plumbizincicola as candidate genes for creating CRISPR/Cas9 knockout mutations. Among these, the SpARR11 and SpMYB84 mutant lines exhibited decreased Cd accumulation in their aboveground parts, suggesting that these two transcription factors may play a role in the regulation of the Cd hyperaccumulation in S. plumbizincicola. Although further research will be required to determine the precise targeted genes of these transcription factors, combined transcriptome analysis and CRISPR/Cas9 technology provides unprecedented opportunities for identifying transcription factors related to Cd hyperaccumulation and contributes to the understanding of the transcriptional regulation mechanism of hyperaccumulation in S. plumbizincicola.

Main conclusionS. plumbizincicola genetic transformation was optimized using a self-excision molecular-assisted transformation system by integrating the SpGRF4/SpGIF1 gene with XVE and Cre/loxP.Sedum plumbizincicola, despite being an excellent hyperaccumulator of cadmium and zinc with significant potential for soil pollution phytoremediation on farmland, has nonetheless trailed behind other major model plants in genetic transformation technology. In this study, different explants and SpGRF4–SpGIF1 genes were used to optimize the genetic transformation of S. plumbizincicola. We found that petiole and stem segments had higher genetic transformation efficiency than cluster buds. Overexpression of SpGRF4–SpGIF1 could significantly improve the genetic transformation efficiency and shorten the period of obtaining regenerated buds. However, molecular assistance with overexpression of SpGRF4–SpGIF1 leads to abnormal morphology, resulting in plant tissue enlargement and abnormal growth. Therefore, we combined SpGRF4–SpGIF1 with XVE and Cre/loxP to obtain DNA autocleavage transgenic plants induced by estradiol, thereby ensuring normal growth in transgenic plants. This study optimized the S. plumbizincicola genetic transformation system, improved the efficiency of genetic transformation, and established a self-excision molecular-assisted transformation system. This work also established the basis for studying S. plumbizincicola gene function, and for S. plumbizincicola breeding and germplasm innovation.

Sedum plumbizincicola (Crassulaceae), a cadmium (Cd)/zinc (Zn)/lead (Pb) hyperaccumulator native to Southeast China, is potentially useful for the phytoremediation of heavy metal-contaminated soil. Basic leucine zipper (bZIP) transcription factors play vital roles in plant growth, development, and abiotic stress responses. However, there has been minimal research on the effects of Cd stress on the bZIP gene family in S. plumbizincicola . In this study, 92 SpbZIP genes were identified in the S. plumbizincicola genome and then classified into 12 subgroups according to their similarity to bZIP genes in Arabidopsis. Gene structure and conserved motif analyses showed that SpbZIP genes within the same subgroup shared similar intron–exon structures and motif compositions. In total, eight pairs of segmentally duplicated SpbZIP genes were identified, but there were no tandemly duplicated SpbZIP genes. Additionally, the duplicated SpbZIP genes were mainly under purifying selection pressure. Hormone-responsive, abiotic and biotic stress-responsive, and plant development-related cis -acting elements were detected in the SpbZIP promoter sequences. Expression profiles derived from RNA-seq and quantitative real-time PCR analyses indicated that the expression levels of most SpbZIP genes were upregulated under Cd stress conditions. Furthermore, a gene co-expression network analysis revealed that most edge genes regulated by hub genes were related to metal transport, responses to stimuli, and transcriptional regulation. Because its expression was significantly upregulated by Cd stress, the hub gene SpbZIP60 was selected for a functional characterization to elucidate its role in the root response to Cd stress. In a transient gene expression analysis involving Nicotiana benthamiana leaves, SpbZIP60 was localized in the nucleus. The overexpression of SpbZIP60 enhanced the Cd tolerance of transgenic Arabidopsis plants by inhibiting ROS accumulation, protecting the photosynthetic apparatus, and decreasing the Cd content. These findings may provide insights into the potential roles of the bZIP family genes during the S. plumbizincicola response to Cd stress.

A cadmium (Cd) tolerance protein ( SpCTP3 ) involved in the Sedum plumbizincicola response to Cd stress was identified. However, the mechanism underlying the Cd detoxification and accumulation mediated by SpCTP3 in plants remains unclear. We compared wild-type (WT) and SpCTP3 -overexpressing transgenic poplars in terms of Cd accumulation, physiological indices, and the expression profiles of transporter genes following with 100 μmol/L CdCl 2 . Compared with the WT, significantly more Cd accumulated in the above-ground and below-ground parts of the SpCTP3 -overexpressing lines after 100 μmol/L CdCl 2 treatment. The Cd flow rate was significantly higher in the transgenic roots than in the WT roots. The overexpression of SpCTP3 resulted in the subcellular redistribution of Cd, with decreased and increased Cd proportions in the cell wall and the soluble fraction, respectively, in the roots and leaves. Additionally, the accumulation of Cd increased the reactive oxygen species (ROS) content. The activities of three antioxidant enzymes (peroxidase, catalase, and superoxide dismutase) increased significantly in response to Cd stress. The observed increase in the titratable acid content in the cytoplasm might lead to the enhanced chelation of Cd. The genes encoding several transporters related to Cd 2+ transport and detoxification were expressed at higher levels in the transgenic poplars than in the WT plants. Our results suggest that overexpressing SpCTP3 in transgenic poplar plants promotes Cd accumulation, modulates Cd distribution and ROS homeostasis, and decreases Cd toxicity via organic acids. In conclusion, genetically modifying plants to overexpress SpCTP3 may be a viable strategy for improving the phytoremediation of Cd-polluted soil.

Background and aims Soil cadmium (Cd) contamination threatens food safety and human health. Increasing Cd phytoextraction efficiency by hyperaccumulators and growing safe products during remediation remain challenges. Methods A pot experiment was conducted to explore the effects of different rates of sulfur (S) associated with alternating drying and wetting on a Sedum plumbizincicola - Oryza sativa rotation in Cd-contaminated neutral and calcareous soils. Results The oxidation of added S under aerobic conditions significantly decreased soil solution pH and increased soluble sulfate (SO 4 2− ), Cd and iron (Fe) concentrations in both soils. During the rice growing season the soil solution redox (Eh) decreased to < −200 mV and the solution pH increased to neutral during the first few days of flooding. Soluble SO 4 2− and Cd in the S treatments decreased significantly with increasing duration of flooding. Sulfur addition promoted shoot Cd concentrations of S. plumbizincicola by 1.7–5.5 times on calcareous soil and 1.7–2.3 times on neutral soil compared to the controls. Rice yields increased but Cd concentrations decreased at suitable S addition rates. Conclusions Appropriate sulfur amendment combined with water management can be a feasible strategy to enhance the Cd remediation efficiency of the hyperaccumulator and reduce the accumulation of Cd in the rice grains in this rotation.

Application of hyperaccumulator-endophyte symbiotic systems is a potential approach to improve phytoremediation efficiency, since some beneficial endophytic bacteria are able to detoxify heavy metals, alter metal solubility in soil, and facilitate plant growth. The objective of this study was to isolate multi-metal resistant and plant beneficial endophytic bacteria and to evaluate their role in enhancing plant growth and metal accumulation/translocation. The metal resistant endophytic bacterial strain E6S was isolated from stems of the Zn/Cd hyperaccumulator plant Sedum plumbizincicola growing in metalliferous mine soils using Dworkin and Foster salts minimal agar medium with 1-aminocyclopropane-1-carboxylate (ACC) as the sole nitrogen source, and identified as homologous to Achromobacter piechaudii based on morphological and biochemical characteristics, partial 16S rDNA sequence and phylogenetic analysis. Strain E6S showed high level of resistance to various metals (Cd, Zn, and Pb). Besides utilizing ACC, strain E6S exhibited plant beneficial traits, such as solubilization of phosphate and production of indole-3-acetic acid. Inoculation with E6S significantly increased the bioavailability of Cd, Zn, and Pb in soil. In addition, bacterial cells bound considerable amounts of metal ions in the following order: Zn > Cd >Pb. Inoculation of E6S significantly stimulated plant biomass, uptake and bioaccumulation of Cd, Zn, and Pb. However, E6S greatly reduced the root to shoot translocation of Cd and Zn, indicating that bacterial inoculation assisted the host plant to uptake and store heavy metals in its root system. Inoculation with the endophytic bacterium E6S homologous to A. piechaudii can improve phytostabilization of metalliferous soils due to its effective ability to enhance in situ metal rhizoaccumulation in plants.

Root exudates are carriers for the transfer of material, energy and information between plant roots and soils. Plants encountering environmental stresses such as heavy metal pollution adapt to the environment by producing and secreting root exudates. In this study, laboratory soil culture experiment and pot experiment with Sedum plumbizincicola were used to study the effects of single and combined application of three root exudates, citric acid, glycine, and fructose, on the Cd-activation and phytoremediation of Cd-contaminated paddy soil. Results from the soil culture experiment showed that for the single application of root exudates, all three root exudates significantly activated the Cd in soil as presented by the increased content of diethylenetriamine pentaacetic acid extracted Cd (DTPA-Cd). In Particular, citric acid (SC) at a relatively low concentration (2 mmol/kg) exhibited the best Cd activation efficiency by increasing DTPA-Cd in the soil by 66.12%. For the combined application of root exudates, citric acid in combination with glycine (SC + G, 1:3) had the best activation effect on the Cd in the soil. In the phytoremediation pot experiment, both the single application of citric acid at a low concentrate (1 mmol/kg) and the combined application of citric acid and glycine (1:1) significantly reduced the total Cd and DTPA-Cd in the soil and increased the biomass and the content of Cd in S. plumbizincicola ; thus, the phytoremediation efficiency of Cd-contaminated soil increased by 42.33% and 35.61%. The results from this study suggest that citric acid plays a crucial role in Cd activation and phytoremediation with single or combined applications with glycine. However, the mechanisms under the synergetic interaction between citric acid and glycine require further investigation.

Aims The rhizosphere microbiome plays an important role in plant growth and behavior during phytoremediation. This work aims to explore how hyperaccumulating plant performance (shoot biomass and Cd uptake) and rhizosphere bacterial community are driven by plant development under a Cd concentration gradient and their relationship during plant growth. Methods A pot experiment was conducted using the Cd hyperaccumulator Sedum plumbizincicola grown in soil spiked with 0, 5 and 20 mg Cd kg −1 for 13 weeks. Plant performance (shoot biomass and Cd uptake) was determined and the rhizosphere bacterial community was examined by weekly Illumina sequencing. Biomarker genera were identified by the Random Forest model and the functions were predicted by PICRUSt2. Results The rhizospheres bacterial community tended to stabilize from the 6 th or 7 th week regardless of the soil Cd concentration, coinciding with the vigorous growth stage of the hyperaccumulator. Biomarker genera Paenibacillus and Streptacidiphilus enriched in the stable stage showed significant positive correlations with Cd uptake. In addition, some predicted gene abundances of enzymes related to plant growth promotion by the rhizosphere bacterial community were influenced to various degrees by plant development and soil Cd concentration. Conclusion There exists a stable bacterial community in the rhizosphere of S . plumbizincicola during phytoremediation and biomarker taxa enriched at this stage were positively correlated with Cd uptake. Selecting microorganisms that match the developmental stages might further enhance phytoremediation. This provides some theoretical basis for modulating rhizosphere microbes at the appropriate time during phytoextraction to optimize the phytoremediation technique.