Hyperaccumulating Ecotype Research Articles

Contribution of plant growth-promoting endophytic bacteria from hyperaccumulator to non-host plant zinc nutrition and health

Application of microbial agents is a novel strategy to improve the quality and health of plant, which can be used to increase zinc (Zn) uptake and alleviate Zn toxicity. Here, endophytic bacteria with Zn solubilizing and growth-promoting properties were isolated from hyperaccumulating ecotype (HE) of Sedum alfredii Hance and their effects on Zn absorption and accumulation of non-hyperaccumulating ecotype (NHE) were studied. The results showed that most endophytic bacteria of HE have good Zn solubilizing or growth-promoting properties. Under the condition of 20 μM ZnSO4, the biomass of NHE inoculated with SaPS1, SaEN2, SaPR2, SaBA2, SaBA3 was 2.8–3.2 times higher than that of non-inoculation control, and the Zn concentration of shoots was increased by 45.9, 89.0, 53.7, 77.5, and 42.6% after inoculation with SaPA1, SaP1, SaEN2, SaBA1, and SaBA2. Under the condition of 100 μM ZnSO4, inoculation with SaVA1, SaPS3, SaB1, SaPR1, and SaEN3 alleviated Zn stress and significantly reduced Zn concentration of shoots. Therefore, endophytic bacteria can be an effective means of improving plant Zn nutrition quality in the normal condition and benefit plant health in the stress environment.

Soil water stress alters differentially relative metabolic pathways affecting growth performance and metal uptake efficiency in a cadmium hyperaccumulator ecotype of Sedum alfredii.

Modeling plants for biomass production and metal uptake from surrounding environment is strongly dependent on the moisture content of soil. Therefore, experiments were conducted to find out how soil moisture affects the phenotypic traits, photosynthetic efficiency, metabolic profile, and metal accumulation in the hyperaccumulating ecotype of Sedum alfredii (S. alfredii). A total of six water potential gradients were set: 0 ~ -15 kPa (T1), -15 ~ -30 kPa (T2), -30 ~ -45 kPa (T3), -45 ~ -60 kPa (T4), -60 ~ -75 kPa (T5), and -75 ~ -90 kPa (T6). Different water potential treatments had a significant effect on plant growth and metal uptake efficiency. Compared to T3, T2 was more effective in promoting plant growth and development, with an increase in biomass of 23% and 17% in both fresh weight (FW) and dry weight (DW), respectively. T2 and T3 had the highest cadmium (Cd) content in the shoot (280.2 mg/kg) and (283.3 mg/kg), respectively, whereas T1 had the lowest values (204.7 mg/kg). Cd availability for plants in the soil was affected by moving soil moisture cycles. Changes in soil moisture that were either too high or too low compared to the ideal soil water content for S. alfredii growth resulted in a significant reduction in Cd accumulation in shoots. Tryptophan, phenylalanine, and other amino acids were accumulated in T5, whereas only tryptophan and phenylalanine slightly increased in T1. Sugars and alcohols such as sucrose, trehalose, mannitol, galactinol, and mannobiose increased in T5, while they decreased significantly in T1. Interestingly, in contrast to T1, the two impaired metabolic pathways in T5 (galactose and starch metabolism) were identified to be glucose metabolic pathways. These findings provide scientific information (based on experiments) to improve biomass production and metal uptake efficiency in hyperaccumulating ecotype of S. alfredii for phytoremediation-contaminated agricultural fields.

Positive Effect of High Zinc on Growth of Sedum alfredii

Sedum alfredii Hance (S. alfredii) is a native hyperaccumulator plant species in China that has strong tolerance and accumulation ability for Zn and Cd. In addition, it is a good material for the phytoextraction of soil heavy metal pollutants. However, the specific effect of high Zn concentrations on the growth of S. alfredii and its metabolic mechanisms are not clear. Using an untargeted metabolomics method, we analysed the differential metabolites of the two ecotypes in S. alfredii roots under different Zn treatments. The results showed that high Zn levels significantly promoted plant growth in the hyperaccumulating ecotype (HE), while growth was inhibited in the non-hyperaccumulating ecotype (NHE). We detected 624 metabolites in the roots of S. alfredii. Under the high Zn treatment, lots of lipids and lipid-like molecules, such as glyceryl monooleate and 9,12,13-trihydroxyoctadecane-10-enoic acid, along with organic acids, such as lauramidopropylbetaine, L-malic acid, and their derivatives, decreased significantly in HE roots. Differential metabolites, such as some lipids and lipid-like molecules, were significantly upregulated in NHE roots. The above results indicate that the exogenous high Zn treatment induces the downregulation of HE differential metabolites in response to Zn, but significantly induces the upregulation of differential metabolites in NHE.

Role of SaPCR2 in Zn Uptake in the Root Elongation Zone of the Zn/Cd Hyperaccumulator Sedum alfredii.

Zn pollution is a potential toxicant for agriculture and the environment. Sedum alfredii is a Zn/Cd hyperaccumulator found in China and has been proven as a useful resource for the phytoremediation of Zn-contaminated sites. However, the molecular mechanism of Zn uptake in S. alfredii is limited. In this study, the function of SaPCR2 on Zn uptake in S. alfredii was identified by gene expression analysis, yeast function assays, Zn accumulation and root morphology analysis in transgenic lines to further elucidate the mechanisms of uptake and translocation of Zn in S. alfredii. The results showed that SaPCR2 was highly expressed in the root elongation zone of the hyperaccumulating ecotype (HE) S. alfredii, and high Zn exposure downregulated the expression of SaPCR2 in the HE S. alfredii root. The heterologous expression of SaPCR2 in yeast suggested that SaPCR2 was responsible for Zn influx. The overexpression of SaPCR2 in the non-hyperaccumulating ecotype (NHE) S. alfredii significantly increased the root uptake of Zn, but did not influence Mn, Cu or Fe. SR-μ-XRF technology showed that more Zn was distributed in the vascular buddle tissues, as well as in the cortex and epidermis in the transgenic lines. Root morphology was also altered after SaPCR2 overexpression, and a severe inhibition was observed. In the transgenic lines, the meristematic and elongation zones of the root were lower compared to the WT, and Zn accumulation in meristem cells was also reduced. These results indicate that SaPCR2 is responsible for Zn uptake, and mainly functions in the root elongation zone. This research on SaPCR2 could provide a theoretical basis for the use of genetic engineering technology in the modification of crops for their safe production and biological enhancement.

Exogenous application of Mn significantly increased Cd accumulation in the Cd/Zn hyperaccumulator Sedum alfredii.

Sedum alfredii is a Cd/Zn hyperaccumulator native to China, which was collected from a mined area where Mn content in soil was extremely high, together with Zn and Cd content. We investigated the tolerance and accumulation ability of Mn and its possible association with Cd hyperaccumulation in this plant species by using MP-AES, SR-μ-XRF, and RT-PCR. The results showed that the hyperaccumulating ecotype (HE) S.alfredii exhibited high tolerance to Mn and accumulating around 10,000 and 12,000mgkg-1 Mn in roots and shoots, respectively, without exhibiting toxicity under 5000mgkg-1 Mn treatment for 4 weeks. Exposure to Cd significantly reduced plant uptake of Mn. In contrast, exogenous Mn application significantly improved root uptake and root-to-shoot translocation of Cd, resulting in the increased Cd accumulation in the shoots of HE S.alfredii. SR-μ-XRF analysis demonstrated that high Mn (20μM) exposure resulted in higher intensities of Cd localized in both stem vascular bundles and cortex, as well as leaf mesophyll cells, than in those treated with low Mn levels (0.2μM or 2.0μM). RT-PCR analysis of several genes possibly involved in Mn/Cd transportation showed that expression of SaNramp3 in roots was significantly reduced under high Mn exposure. These results suggested a significant interaction between Cd and Mn in the HE S.alfredii plants, possibly through their competition for transporters and theoretically provided a strategy to improve the efficiency of Cd extraction from polluted soils by this plant species, after using appropriate nutrient management of Mn.

Abscisic acid-mediated modifications in water transport continuum are involved in cadmium hyperaccumulation in Sedum alfredii

Abscisic acid (ABA) play a crucial role in plant acclimation to heavy-metals stresses. Nevertheless, the effects of ABA on long-distance transport and its consequences for cadmium (Cd) accumulation are insufficiently understood. Here, we investigated the effects of ABA on the development of the whole-plant water transport pathway and implications for Cd uptake and transport to the shoot of Sedum alfredii. Exposure to Cd stimulated the production of endogenous ABA levels in the non-hyperaccumulating ecotype (NHE), but not in the hyperaccumulating ecotype (HE). Increased ABA levels in NHE significantly reduced aquaporin expressions in roots, the number of xylem vessel in stem, dimensions and densities of stomata in leaves, but induced leaf osmotic adjustment. Furthermore, the ABA-driven modifications in NHE plants showed typically higher sensitivity to ABA content in leaves compared to HE, illustrating ecotype-specific responses to ABA level. In NHE, the ABA-mediated modifications primarily affected the xylem transport of Cd ions and, at the cost of considerable water delivery limitations, significantly reduced delivery of Cd ions to shoots. In contrast, maintenance of low ABA levels in HE failed to t limit transpiration rates and maximized Cd accumulation in shoots. Our results demonstrated that ABA regulates Cd hyperaccumulation of S. alfredii through specific modifications in the water transport continuum.

Plant Cadmium Resistance 2 (SaPCR2) Facilitates Cadmium Efflux in the Roots of Hyperaccumulator Sedum alfredii Hance.

Hyperaccumulators are the preferred materials for phytoremediation. Sedum alfredii Hance is a cadmium (Cd) hyperaccumulator plant in China, although its detoxification mechanism remains unresolved. In our study, we cloned a gene belonging to the plant cadmium resistance (PCR) family, named SaPCR2, from the hyperaccumulating ecotype (HE) of S. alfredii. Sequence analysis indicated that SaPCR2 contained a cysteine-rich domain highly conserved in the PCR family and played an important role in Cd detoxification. Based on the relative quantitative results, SaPCR2 was highly expressed in the roots of HE S. alfredii, but not the shoots and Cd exposure did not significantly affect SaPCR2 expression. In contrast, the expression level of SaPCR2 was very low in plants of its non-hyperaccumulating ecotype (NHE). The subcellular localization of SaPCR2 in tobacco leaves and yeasts showed that SaPCR2 was localized on the plasma membrane and the expression of the SaPCR2 protein in a Zn/Cd-sensitive yeast Δzrc1 significantly increased its tolerance to Cd stress by decreasing the Cd content in cells. Heterologous expression of SaPCR2 in plants of both Arabidopsis thaliana and NHE S. alfredii significantly reduced the Cd levels in the roots, but not in the shoots. These results suggest that the overexpression of SaPCR2 in plants provides a route for Cd leak out of the root cells and protects the root cells against phytotoxicity of Cd stress. To the best of our knowledge, this is the first study of transporter-mediated root efflux of Cd in hyperaccumulator S. alfredii.

The effects and health risk assessment of cauliflower co-cropping with Sedum alfredii in cadmium contaminated vegetable field.

Phytoremediation coupled with co-cropping is assumed to be good for safety utilization and remediation of heavy metal contaminated farmland, which can ensure farmers' income without increasing health risks for human. In this study, the effects on plant cadmium (Cd) accumulation and health risk of consuming the vegetable plant were compared between monoculture and co-cropping of cauliflower (Brassica oleracea) with two ecotypes of Sedum alfredii in a moderately (0.82mgkg-1) Cd contaminated greenhouse vegetable field. The results showed that co-cropping with S.alfredii raised Cd concentration in edible part of cauliflower with slightly growth promotion. The health risk of consuming cauliflower to different groups of people have been evaluated by calculating Hazard Quotient (HQ) and all HQ value were less than 1.0, which indicated that eating co-cropped cauliflower would not cause health risks to adults and children. Besides, the Cd concentration of hyperaccumulating ecotype (HE) of S.alfredii was 27.3mgkg-1 in monoculture and it increased to 51.2mgkg-1 after co-cropping with cauliflower, suggesting that the co-cropping system promoted HE Cd absorption capacity. Therefore, the "Phytoextraction Coupled with Agro-safe-production" (PCA) model of cauliflower and HE can serve as an alternative sustainable strategy in the Cd moderate polluted greenhouse.

Coordination between root cell wall thickening and pectin modification is involved in cadmium accumulation in Sedum alfredii.

Root cell wall (RCW) modification is a widespread important defense strategy of plant to cope with trace metals. However, mechanisms underlying its remolding in cadmium (Cd) accumulation are still lacking in hyperaccumulators. In this study, changes of RCW structures and components between nonhyperaccumulating ecotype (NHE) and hyperaccumulating ecotype (HE) of Sedum alfredii were investigated simultaneously. Under 25μM Cd treatment, RCW thickness of NHE is nearly 2 folds than that of HE and the thickened cell wall of NHE was enriched in low-methylated pectin, leading to more Cd trapped in roots tightly. In the opposite, large amounts of high-methylated pectin were assembled around RCW of HE with Cd supply, in this way, HE S.alfredii decreased its root fixation of Cd and enhanced Cd migration into xylem. TEM and AFM results further confirmed that thickened cell wall was caused by the increased amounts of cellulose and lignin while root tip lignification was resulted from variations of sinapyl (S) and guaiacyl (G) monomers. Overall, thickened cell wall and methylated pectin have synchronicity in spatial location of roots, and their coordination contributed to Cd accumulation in S.alfredii.

Ethylene-mediated apoplastic barriers development involved in cadmium accumulation in root of hyperaccumulator Sedum alfredii

Ethylene is an important phytohormone for plant adaptation to heavy metal stress. However, the effects of ethylene on radial apoplastic transport of Cd remain elusive. This study investigated the role of ethylene on apoplastic barriers development and consequences for Cd uptake in Sedum alfredii. In response to Cd, endogenous ethylene production in hyperaccumulating ecotype (HE) roots was decreased due to the down-regulated expressions of ethylene biosynthesis genes, while the opposite result was observed in non-hyperaccumulating ecotype (NHE). Interestingly, the ethylene emission in HE was always higher than that in NHE, regardless of Cd concentrations. Results of exogenous application of ethylene biosynthesis precursor/inhibitor indicate that ethylene with high level would delay the formation of apoplastic barriers in HE through restraining phenylalanine ammonia lyase activity and gene expressions related to lignin/suberin biosynthesis. Simultaneously, correlation analyses suggest that Cd-induced apoplastic barriers formation may be also regulated by ethylene signaling. By using an apoplastic bypass tracer and scanning ion-selected electrode, we observed that the delayed deposition of apoplastic barriers significantly promoted Cd influx in roots. Taken together, high endogenous ethylene in HE postponed the formation of apoplastic barriers and thus promoted the Cd accumulation in the apoplast of roots.

Nickel tolerance, translocation and accumulation in a Cd/Zn co-hyperaccumulator plant Sedum alfredii

Multi-elements hyperaccumulators are of high scientific interest to be applied in remediation of mix-contaminated soils. Sedum alfredii is a well-known Cd/Zn co-hyperaccumulator with high Pb and Cu tolerance. This study investigated the ability of the hyperaccumulating ecotype (HE) S. alfredii to tolerate and accumulate Ni. Differed from the non-hyperaccumulating ecotype (NHE), HE plants grew healthy after 50 μM Ni exposure for 4 weeks. The HE plants translocated up to 40 % Ni to the shoots under high Ni stress and accumulated >3000 and 200 mg kg−1 Ni in roots and shoots, respectively. Micro-XRF image showed that Ni was highly restricted within the HE stem and leaf vascular bundles, especially the xylem tissues. The HE roots were of high Ni tolerance, showing much less pronounced Ni-induced phytotoxicity as compared with the NHEs. Ni-induced O2– was observed in the apoplastic part of HE root cells, but both Ni and the induced O2– were highly accumulated in the sensitive zone (root cap, meristem, and cylinder) of NHE roots. These results suggest that although low Ni mobility out of vascular tissues limits the metal accumulation in stems and leaves, HE S. alfredii is highly tolerant towards Ni stress by metal homeostasis in root cells.

A Single Amino Acid Change in Nramp6 from Sedum Alfredii Hance Affects Cadmium Accumulation.

SaNramp6 in Sedum alfredii encodes a membrane-localized metal transporter. We isolated the SaNramp6h allele from the hyperaccumulating ecotype (HE) of S. alfredii. When this allele was expressed in transgenic yeast and Arabidopsis thaliana, it enhanced their cadmium (Cd) sensitivity by increased Cd transport and accumulation. We isolated another allele, SaNramp6n, from a nonhyperaccumulating ecotype (NHE) of S. alfredii. Amino acid sequence comparisons revealed three amino acid differences between SaNramp6h and SaNramp6n. We investigated the Cd transport activity of the Nramp6 allele, and determined which residues are essential for the transport activity. We conducted structure-function analyses of SaNramp6 based on site-directed mutagenesis and functional assays of the mutants in yeast and Arabidopsis. The three residues that differed between SaNramp6h and SaNramp6n were mutated. Only the L157P mutation of SaNramp6h impaired Cd transport. The other mutations, S218N and T504A, did not affect the transport activity of SaNramp6h, indicating that these residues are not essential for metal selectivity. Transgenic plants overexpressing SaNramp6hL157P showed altered metal accumulation in shoots and roots. Our results suggest that the conserved site L157 is essential for the high metal transport activity of SaNramp6h. This information may be useful for limiting or increasing Cd transport by other plant natural resistance associated macrophage protein (NRAMP) proteins.

A Cd/Zn Co-hyperaccumulator and Pb accumulator, Sedum alfredii, is of high Cu tolerance

High sensitivity towards Cu toxicity is problematic when using some hyperaccumulator plants for phytoremediation of soils with mixed contamination of Cu. Sedum alfredii, a Cd/Zn co-hyperaccumulator and Pb accumulator, is widely used for remediation of Cd, Zn, and Pb co-contaminated soils in China. In this paper, the tolerance and accumulation ability of S. alfredii towards Cu stress and its potential for phytoremediation of multi-metal polluted soils have been studied. Both the hyperaccumulating ecotype (HE) and non-hyperaccumulating ecotype (NHE) of S. alfredii accumulated high Cu in the roots and translocated minimal Cu to the shoots, and Cu in the stems and leaves mostly restricted in the vascular tissues (phloem zone). The HE plants, however, exhibited high Cu resistance with stimulated lateral root growth and increased chlorophyll content under 10 μM Cu treatment. XANES analysis showed that Cu in HE roots comprised Cu2+ (46.7%), Cu-histidine (35.2%) and Cu-cell wall (18.1%). The NHE under Cu stress showed decreased biomass, reduced leaf chlorophyll content, altered root architecture, and higher Cu localized to root cell wall as compared with the HEs. Potted HE plants thrived six months in multi-metal contaminated soils including 3897 mg kg−1 available Cu. In conclusion, HE S alfredii is highly tolerant toward Cu due to metal homeostasis in root cells. Therefore, this plant has great potential to remediate Zn, Cd, and Pb contaminated soils those also contain high levels of Cu.

Cd-induced difference in root characteristics along root apex contributes to variation in Cd uptake and accumulation between two contrasting ecotypes of Sedum alfredii.

The root apex is the most active part for water and ions uptake, however, longitudinal alterations in root characteristics along root apex and consequences for metal uptake in hyperaccumulator are poorly understood. Here, we compared cadmium (Cd)-induced longitudinal alterations in root apex of two ecotypes of Sedum alfredii and assess their effects on Cd uptake. Under Cd treatment, cell death began from epidermis to the stele in non-hyperaccumulating ecotype (NHE) over time, and the number of dead cells was significantly higher than that in hyperaccumulating ecotype (HE). Cd-induced the presence of border-like cells (BLCs) surrounding the root tip of NHE prevented Cd from entering roots, however, almost no BLCs were observed in the root tip of in HE. Besides, Cd-treated NHE exhibited 76% and 52% decrease in the proportions of meristematic and elongation zone, respectively, resulting in lower Cd influx and less intensive Cd-fluorescence in these zones, as compared with HE. In the differentiation zone, Cd induced earlier initiation of root hairs (RHs), lower RHs-density, shorter RHs-length, thicker RHs-radius and less trichoblasts in NHE than those in HE. These remarkable variations led to less Cd influx and lower intensity of Cd-fluorescence in RHs of NHE than those of HE. Furthermore, decline in cell wall thickness under Cd exposure resulted in less cell-wall-bond Cd in the cell wall of HE. Therefore, Cd-induced alterations in root characteristics alongside root apex contributed to the difference in Cd uptake and accumulation between two ecotypes of S.alfredii.

Unique root exudate tartaric acid enhanced cadmium mobilization and uptake in Cd-hyperaccumulator Sedum alfredii

Low molecular weight organic acids (LMWOA) involved in heavy metal tolerance, translocation, and accumulation in plants. However, underlying mechanism of LMWOA secretion in metal mobilization and uptake in hyperaccumulator still need to be identified. In this study, a 13C labeling rhizobox was designed to investigate the composition and distribution of LMWOA in the rhizosphere of S. alfredii. The result showed that about 2.30%, 2.25% and 2.35% of the assimilated 13C was incorporated into oxalic acid, malic acid, and tartaric acid in rhizosphere of S. alfredii after 13CO2 assimilation, respectively. Oxalic acid, malic acid, and tartaric acid were the predominant LMWOA in rhizosphere soil solution of hyperaccumulating ecotype (HE) S. alfredii, however, almost no tartaric acid was detected for non-hyperaccumulating ecotype (NHE). Tartaric acid was identified as the unique root exudate from HE S. alfredii which was mainly distributed within the range of rhizosphere 0–6 mm. Tartaric acid significantly increased the solubility of four Cd minerals. HE S. alfredii treated with tartrate + CdCO3 had higher Cd contents and larger biomass than CdCO3 treatment. Cadmium accumulation in HE S. alfredii was promoted by the exudation of tartaric acid, which was highly efficient in Cd solubilization due to the formation of soluble Cd-tartrate complexes.

Abscisic acid-mediated modifications of radial apoplastic transport pathway play a key role in cadmium uptake in hyperaccumulator Sedum alfredii.

Abscisic acid (ABA) is a key phytohormone underlying plant resistance to toxic metals. However, regulatory effects of ABA on apoplastic transport in roots and consequences for uptake of metal ions are poorly understood. Here, we demonstrate how ABA regulates development of apoplastic barriers in roots of two ecotypes of Sedum alfredii and assess effects on cadmium (Cd) uptake. Under Cd treatment, increased endogenous ABA level was detected in roots of nonhyperaccumulating ecotype (NHE) due to up-regulated expressions of ABA biosynthesis genes (SaABA2, SaNCED), but no change was observed in hyperaccumulating ecotype (HE). Simultaneously, endodermal Casparian strips (CSs) and suberin lamellae (SL) were deposited closer to root tips of NHE compared with HE. Interestingly, the vessel-to-CSs overlap was identified as an ABA-driven anatomical trait. Results of correlation analyses and exogenous applications of ABA/Abamine indicate that ABA regulates development of both types of apoplastic barriers through promoting activities of phenylalanine ammonialyase, peroxidase, and expressions of suberin-related genes (SaCYP86A1, SaGPAT5, and SaKCS20). Using scanning ion-selected electrode technique and PTS tracer confirmed that ABA-promoted deposition of CSs and SL significantly reduced Cd entrance into root stele. Therefore, maintenance of low ABA levels in HE minimized deposition of apoplastic barriers and allowed maximization of Cd uptake via apoplastic pathway.

Effects of CO2 application and endophytic bacterial inoculation on morphological properties, photosynthetic characteristics and cadmium uptake of two ecotypes of Sedum alfredii Hance.

Plant uptake of cadmium (Cd) is affected by soil and environmental conditions. In this study, hydroponic experiments were conducted to investigate the effects of elevated CO2 coupled with inoculated endophytic bacteria M002 on morphological properties, gas exchange, photosynthetic pigments, chlorophyll fluorescence, and Cd uptake of S. alfredii. The results showed that bio-fortification processes (elevated CO2 and/or inoculated with endophytic bacteria) significantly (p < 0.05) promoted growth patterns, improved photosynthetic characteristics and increased Cd tolerance of both ecotypes of S. alfredii, as compared to normal conditions. Net photosynthetic rate (Pn) in intact leaves of hyperaccumulating ecotype (HE) and non-hyperaccumulating ecotype (NHE) were increased by 73.93 and 32.90%, respectively at the low Cd (2μM), 84.41 and 57.65%, respectively at the high Cd level (10μM). Superposition treatment increased Cd concentration in shoots and roots of HE, by 50.87 and 82.12%, respectively at the low Cd and 46.75 and 88.92%, respectively at the high Cd level. Besides, superposition treatment declined Cd transfer factor of NHE, by 0.85% at non-Cd rate, 17.22% at the low Cd and 22.26% at the high Cd level. These results indicate that elevated CO2 coupled with endophytic bacterial inoculation may effectively improve phytoremediation efficiency of Cd-contaminated soils by hyperaccumulator, and alleviate Cd toxicity to non-hyperaccumulator ecotype of Sedum alfredii.

Efficient phloem transport significantly remobilizes cadmium from old to young organs in a hyperaccumulator Sedum alfredii.

Our knowledge of cadmium (Cd) in hyperaccumulators mainly concerns root uptake, xylem translocation and foliar detoxification, while little attention has been paid to the role of phloem remobilization. We investigated Cd distribution in different organs of the hyperaccumulating ecotype (HE) of Sedum alfredii and compared its Cd phloem transport with that of the non-hyperaccumulating ecotype (NHE). In HE, results of micro X-ray fluorescence revealed that Cd preferentially accumulated in younger organs compared to the older, and its primary distribution sites changed from parenchyma to vascular/epidermal cells with increased organ age. Strong Cd signals in phloem cells were observed in HE old stems. Pre-stored Cd was readily exported from older to growing leaves, which could be accelerated by leaf senescence. Short-term feeding experiments showed that phloem-mediated Cd transport is rapid and efficient in HE. HE relocated 44% of the total leaf-labelled Cd to other organs, while over 90% Cd was retained in labelled leaves of NHE. High Cd was detected in HE phloem exudates but not in those from NHE leaves. In conclusion, Cd phloem transport is efficient and important for dominating the age-dependent Cd allocation in plants of HE S. alfredii.

Cadmium-induced nitric oxide burst enhances Cd tolerance at early stage in roots of a hyperaccumulator Sedum alfredii partially by altering glutathione metabolism

Understanding cadmium (Cd) tolerance and accumulation strategies of hyperaccumulators is crucial for promoting phytoremediation of polluted soils. Root resistance to Cd regulated by nitric oxide (NO) was investigated for the Cd hyperaccumulating ecotype (HE) of Sedum alfredii. Differed from that of its non-hyperaccumulating ecotype, Cd stress in HE roots triggered a strong NO burst mediated through both nitrate reductase and nitric oxide synthase. Elimination of endogenous NO did not affect Cd levels in roots, but greatly aggravated the metal toxicity, including increased reactive oxygen species (ROS) accumulation, oxidative damage and cell ultrastructure injury. Cadmium stress in HE triggered up-regulated SOD activities but down-regulated POD, CAT, and APX activities, which were significantly inverted by NO scavenger. The NO burst also expanded the glutathione (GSH) pool in HE roots by activation of GR, GSNOR, and γ-ECS, but had no effects on the ascorbate acid (AsA) recycle. Similar to that of NO, preferential localizations of ROS and GSH to meristem and cylinder were observed in root tips of HE. Cadmium uptake and translocation were not affected by the NO levels. These results suggest that NO burst activated a GSH-involved strategy, instead of altering Cd accumulation, to protect root tips of HE S. alfredii against Cd toxicity at early stage.

Cadmium Exposure-Sedum alfredii Planting Interactions Shape the Bacterial Community in the Hyperaccumulator Plant Rhizosphere.

Rhizospheric bacteria play important roles in plant tolerance and activation of heavy metals. Understanding the bacterial rhizobiome of hyperaccumulators may contribute to the development of optimized phytoextraction for metal-polluted soils. We used 16S rRNA gene amplicon sequencing to investigate the rhizospheric bacterial communities of the cadmium (Cd) hyperaccumulating ecotype (HE) Sedum alfredii in comparison to its nonhyperaccumulating ecotype (NHE). Both planting of two ecotypes of S. alfredii and elevated Cd levels significantly decreased bacterial alpha-diversity and altered bacterial community structure in soils. The HE rhizosphere harbored a unique bacterial community differing from those in its bulk soil and NHE counterparts. Several key taxa from Actinobacteria, Bacteroidetes, and TM7 were especially abundant in HE rhizospheres under high Cd stress. The actinobacterial genus Streptomyces was responsible for the majority of the divergence of bacterial community composition between the HE rhizosphere and other soil samples. In the HE rhizosphere, the abundance of Streptomyces was 3.31- to 16.45-fold higher than that in other samples under high Cd stress. These results suggested that both the presence of the hyperaccumulator S. alfredii and Cd exposure select for a specialized rhizosphere bacterial community during phytoextraction of Cd-contaminated soils and that key taxa, such as the species affiliated with the genus Streptomyces, may play an important role in metal hyperaccumulation.IMPORTANCESedum alfredii is a well-known Cd hyperaccumulator native to China. Its potential for extracting Cd relies not only on its powerful uptake, translocation, and tolerance for Cd but also on processes underground (especially rhizosphere microbes) that facilitate root uptake and tolerance of the metal. In this study, a high-throughput sequencing approach was applied to gain insight into the soil-plant-microbe interactions that may influence Cd accumulation in the hyperaccumulator S. alfredii Here, we report the investigation of rhizosphere bacterial communities of S. alfredii in phytoremediation of different levels of Cd contamination in soils. Moreover, some key taxa in its rhizosphere identified in the study, such as the species affiliated with genus Streptomyces, may shed new light on the involvement of bacteria in phytoextraction of contaminated soils and provide new materials for phytoremediation optimization.