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Abstract: Disposal of heavy metal contaminated biomass after phytoremediation is still unfeasible. This paper presents a viable phyto-extraction approach in which metals in contaminated soils are stabilized by nursery stocks before transplantation for greening. In this respect, two pot-experiments are reported comparing seven nursery stocks species exposed to different Cd levels. The first experiment revealed that Cd was mainly stabilized in the roots of all species studied. Greater amounts of Cd were accumulated in the epidermis than cortex plus stele. Cupressus Blue Ice showed greatest tolerance to the 100 and 200mgkg−1 Cd stresses. The second experiment additionally evaluated the possible risk of Cd release after transplanting the Cd treated plants into uncontaminated soil. After 120 days of transplantation, the relatively trace amounts of Cd in the roots of Euonymus japonicus, Pittosporum tobira and C. Blue Ice had either been partially transferred into the shoots or released into the soil. The highest Cd concentration increase in bulk soil (0.428mgkg−1), however, was much lower than the environmental quality standard for soils of China (1mgkg−1). The potential effectiveness of this technique in the use of Cd-contaminated soil and further investigation needed in the field trials were also evaluated. [Copyright &y& Elsevier]

Summary: Salicylic acid (SA) may accelerate the cell death of cadmium-stressed roots to avoid cadmium (Cd) uptake by plants or may play positive roles in protecting the stressed roots from Cd-induced damage. To test these hypotheses, we performed a series of split-root hydroponic experiments with one-half of rice (Oryza sativa L. cv. Jiahua 1) roots exposed to 50μM Cd and the other half not exposed. The objectives were to elucidate the effects of SA pretreatment on the time-dependent changes of H2O2 levels in roots, antioxidant defense system in different organs, root cell death and the dynamic distribution of Cd in the plants. In the split-root system, a higher Cd uptake rate was observed in the Cd-stressed portions of roots compared with the treatment with the whole roots exposed to Cd. Furthermore, an appreciable amount of Cd was translocated from the Cd-exposed roots to the unexposed roots and trace amounts of Cd were released into the external solution. The split-root method also caused the two root portions to respond differently to Cd stress. The activities of major antioxidant enzymes (superoxide dismutase, SOD; peroxidase, POD; and catalase, CAT) were significantly suppressed in the Cd-treated roots, hence leading to H2O2 burst, lipid peroxidation, cell death and growth inhibition. By contrast, in the non-Cd-treated roots, the activities of enzymes (SOD, CAT, and POD) and root growth were persistently stimulated during the experimental period. The H2O2 accumulation and lipid peroxidation were also induced in the non-Cd-treated roots, but they were significantly lower than those of the Cd-treated roots. The concentrations of glutathione (GSH) and non-protein thiols (NPT) in the Cd-treated roots were significantly higher than those of the untreated roots. SA pretreatment elevated enzymatic and non-enzymatic antioxidants, and the concentrations of GSH and NPT in roots and shoots, hence leading to alleviation of the oxidative damage as indicated by the lowered H2O2 and MDA levels. Furthermore, SA pretreatment mitigated the Cd-induced growth inhibition in both roots and shoots and increased transpiration compared with non-SA-pretreatment under Cd exposure. It is concluded that Cd can be partly transferred from the Cd-exposed roots to Cd-unexposed roots and that cell death can be accelerated in the Cd-stressed roots in response to Cd stress. The SA-enhanced Cd tolerance in rice can be attributed to SA-elevated enzymatic and non-enzymatic antioxidants and NPT, and to SA-regulated Cd uptake, transport and distribution in plant organs. [Copyright &y& Elsevier]

In order to understand how salicylic acid (SA) is involved in modulating rice responses to cadmium (Cd) toxicity, particularly in Cd immobilization, a series of hydroponic experiments were conducted to examine changes in cell wall composition, activities of the enzymes related to lignin synthesis including phenylalanine ammonia-lyase (PAL), peroxidase (POD), and polyphenol oxidase (PPO), subcellular Cd distribution, levels of hydrogen peroxide (H2O2), malondialdehyde (MDA), and lignin and non-protein thiols (NPT) in rice roots under Cd stress with or without the pretreatment of SA. Results showed that Cd treatment decreased root biomass by 40% compared with the control (no Cd treatment) and pretreatment with SA significantly mitigated the Cd-induced inhibition of root growth. There was no significant difference in root cell wall composition or lignification between the treatment with Cd alone and the treatment with Cd with SA. No effects were observed for SA pretreatment on the activities of PAL, POD, or PPO under Cd stress. Furthermore, soluble Cd concentrations in root cells were significantly higher in the treatment with Cd with SA than in the treatment with Cd alone. However, H2O2 and MDA concentrations in rice roots were significantly lower but NPT levels were higher in the treatment with Cd with SA than in the treatment with Cd alone, which indicated that SA alleviated Cd-induced oxidative damage. It seems to suggest that SA-mediated enhancement of Cd tolerance was not due to enhanced Cd retention in the cell wall but to enhanced Cd bindings with thiols (─SH) group. The effects of SA-enhanced Cd tolerance were discussed with regard to H2O2 signaling pathways. [ABSTRACT FROM AUTHOR]

Abstract: Pot experiments were performed to study the alleviative effects of exogenous silicon (Si) on cadmium (Cd) phytotoxicity in maize grown in an acid soil experimentally contaminated with Cd. Five treatments were investigated in the first trial consisting of a control (neither Cd nor Si added), Cd added at 20 or 40mgkg−1 Cd without or with Si added at 400mgkg−1 Si. A following-up trial was conducted with almost the same treatments as in the first trial except that Si was incorporated at 50mgkg−1 Si. The results showed that Cd treatment significantly decreased shoot and root dry weight, while addition of Si at both levels significantly enhanced biomass. Addition of Si at 400mgkg−1 Si significantly increased soil pH but decreased soil Cd availability, thus reducing Cd concentration in the shoots and roots and total Cd in the shoots. Moreover, more Cd was found to be in the form of specific adsorbed or Fe–Mn oxides-bound fraction in the Si-amended soil. In contrast, soil pH, available Cd and Cd forms were unaffected by addition of Si at 50mgkg−1 Si, but shoot Cd concentration in the Si-amended Cd treatments significantly decreased at both Cd levels used compared to the non-Si-amended Cd treatments. Total Cd in the shoots and roots was considerably and significantly higher in the Si-amended Cd treatments than in the non-Si-amended Cd treatments. The xylem sap significantly increased but Cd concentration in the xylem sap significantly decreased in the Si-amended Cd treatments compared with the non-Si-amended Cd treatments irrespective of Cd and Si levels used. The results suggest that Si-enhanced tolerance to Cd can be attributed not only to Cd immobilization caused by silicate-induced pH rise in the soils but also to Si-mediated detoxification of Cd in the plants. [Copyright &y& Elsevier]

Aims: As a vital polysaccharide related to mechanisms of plant resistance to trace metal in the root cell wall, the role of hemicellulose in cadmium (Cd) accumulation in hyperaccumulators is still unknown. We investigated hemicellulose modification in response to Cd in two populations of Sedum alfredii. Methods: Nonhyperaccumulating population (NHP) and hyperaccumulating population (HP) of S. alfredii were grown in nutrient solutions with or without 25 μM Cd for 15 d. Monosaccharide composition of root cell wall hemicellulose and its remolding mechanisms (e.g. enzyme activity and gene expression) were analyzed by using gas chromatography-mass spectrometer (GC-MS), fourier transform infrared spectrometer (FTIR), nuclear magnetic resonance (NMR) and quantitative real-time PCR techniques. Results: In 25 μM Cd treatment, root cell wall hemicellulose in the NHP significantly (P < 0.05) increased and its hemicellulose-bound Cd was nearly 2.5-fold higher than that of HP. In the presence of Cd, xylose and glucose, proved to be the main component of hemicellulose, were higher in the NHP than in the HP owing to the up-regulation of XET/XEH and encoding gene (XTH 31). 113Cd-NMR and FTIR results indicated that hemicellulose with hydroxyl and carboxyl groups of HP retained less Cd than that of NHP. Conclusion: Hemicellulose modification decreased the Cd-binding capacity of the root cell wall and increased the entry of Cd in the shoot of HP S. alfredii. [ABSTRACT FROM AUTHOR]

Salicylic acid (SA), as an enigmatic signalling molecule in plants, has been intensively studied to elucidate its role in defence against biotic and abiotic stresses. This review focuses on recent research on the role of the SA signalling pathway in regulating cadmium (Cd) tolerance in plants under various SA exposure methods, including pre-soaking, hydroponic exposure, and spraying. Pretreatment with appropriate levels of SA showed a mitigating effect on Cd damage, whereas an excessive dose of exogenous SA aggravated the toxic effects of Cd. SA signalling mechanisms are mainly associated with modification of reactive oxygen species (ROS) levels in plant tissues. Then, ROS, as second messengers, regulate a series of physiological and genetic adaptive responses, including remodelling cell wall construction, balancing the uptake of Cd and other ions, refining the antioxidant defence system, and regulating photosynthesis, glutathione synthesis and senescence. These findings together elucidate the expanding role of SA in phytotoxicology. [ABSTRACT FROM AUTHOR]

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. Abscisic acid regulated the development of endodermal Casparian strips (CSs) and suberin lamellae (SL) in roots of S. alfredii exposed to Cd. Maintenance of low ABA levels in hyperaccumulating ecotype minimized deposition of CSs/SL and allowed maximization of Cd uptake via apoplastic pathway. [ABSTRACT FROM AUTHOR]

Background and aims: The role of polysaccharide modification in metal accumulation in hyperaccumulators is still unknown. Our aim was to compare the differences in the role of root cell-wall polysaccharides in cadmium (Cd) accumulation between hyperaccumulating (HE) and non-hyperaccumulating ecotype (NHE) of Sedm alfredii. Methods: Hydroponic experiments were performed to characterize root-to-shoot Cd translocation, cadmium species and polysaccharide modification in root cell-wall of S. alfredii using stable isotope tracing, X-ray absorption near edge structure and immunofluorescence localization techniques. Results: Cd absorbed was more readily available for transport to the shoots by the HE roots than by the NHE roots, which is confirmed by a 6-fold higher Cd concentration in xylem sap. Root Cd efflux originated mainly from the cell walls. The concentration of cell-wall polysaccharides and activity of pectin methylesterase were higher in the NHE than in the HE in the absence of Cd, and even higher in the presence of Cd. More pectins were methylated in the HE than in the NHE, indicating more free pectic acid residues in the NHE. The cell-wall-bound Cd was retained more tightly in the NHE than in the HE. Conclusions: Cadmium hyperaccumulation by HE of S. alfredii is associated with its enhanced Cd flux into the xylem, which is partly regulated by cell-wall polysaccharide modification in roots. [ABSTRACT FROM AUTHOR]

Cadmium (Cd) is a highly toxic heavy metal for both plants and animals. The presence of Cd in agricultural soils is of great concern regarding its transfer in the soil-plant system. This study investigated the transfer of Cd (exogenous salts) from a wide range of Chinese soils to corn grain (Zhengdan 958). Through multiple stepwise regressions, prediction models were developed, with the combination of Cd bioconcentration factor (BCF) of Zhengdan 958 and soil pH, organic matter (OM) content, and cation exchange capacity (CEC). Moreover, these prediction models from Zhengdan 958 were applied to other non-model corn species through cross-species extrapolation approach. The results showed that the pH of the soil was the most important factor that controlled Cd uptake and lower pH was more favorable for Cd bioaccumulation in corn grain. There was no significant difference among three prediction models in the different Cd levels. When the prediction models were applied to other non-model corn species, the ratio ranges between the predicted BCF values and the measured BCF values were within an interval of 2 folds and close to the solid line of 1∶1 relationship. Furthermore, these prediction models also reduced the measured BCF intra-species variability for all non-model corn species. Therefore, the prediction models established in this study can be applied to other non-model corn species and be useful for predicting the Cd bioconcentration in corn grain and assessing the ecological risk of Cd in different soils. [ABSTRACT FROM AUTHOR]

Phytoremediation is a cost-effective and environmentally-friendly method to treat cadmium (Cd) contaminated soils, however, there is still a lack of safe disposal methods of harvested hyperaccumulators. In this study, by integrating glycolysis and pyrolysis, we investigated the possibility of bioproduct production and Cd collection from the hyperaccumulator Sedum alfredii. By means of acid-alkali pretreatment, the degree of cellulose polymerization was reduced by 36.24% while the surface accessibility was increased by 115.80%, resulting in a bioethanol yield of 9.29%. Meanwhile, 99.22% of total Cd of biomass could be reclaimed by collecting H 2 SO 4 -pretreatment waste. The saccharification residue was subsequently modified by NaOH-pretreatment-filtrate and converted into biochar at 500 °C which possessed a maximum Cd2+ sorption capacity of 60.52 mg g−1 based on the Langmuir model. Furthermore, sustainability analysis indicated that the economic input of this process is acceptable when considering its good environmental benefits. Taken together, our study provides a strategy for simultaneous bioethanol and biochar production during Cd collection from the hyperaccumulator S. alfredii , which could be a promising alternative for the suitable treatment of metal-enriched plants. [Display omitted] • Fermentation of Cd hyperaccumulator S. alfredii yielded 9.29% of bioethanol. • Mild acid-alkali pretreatment actualized effective Cd collection from biomass. • Saccharification residues derived biochar possessed great Cd sorption capacity. • The procedure had good environmental benefits and acceptable economic input. • Cd hyperaccumulators could be multiple utilized by integrating glycolysis and pyrolysis process. [ABSTRACT FROM AUTHOR]

Abstract: A series of hydroponics experiments were performed to investigate roles of silicon (Si) in enhancing cadmium (Cd) tolerance in two pakchoi (Brassica chinensis L.) cultivars: i.e. cv. Shanghaiqing, a Cd-sensitive cultivar, and cv. Hangyoudong, a Cd-tolerant cultivar. Plants were grown under 0.5 and 5mg Cd L−1 Cd stress without or with 1.5mM Si. Plant growth of the Cd-tolerant cultivar was stimulated at the lower Cd level, but was decreased at the higher Cd level when plants were treated with Cd for one week. However, Plant growth was severely inhibited at both Cd levels as stress duration lasted for up to three weeks. Plant growth of the Cd-sensitive cultivar was severely inhibited at both Cd levels irrespective of Cd stress duration. Addition of Si increased shoot and root biomass of both cultivars at both Cd levels and decreased Cd uptake and root-to-shoot transport. Superoxide dismutase, catalase and ascorbate peroxidase activities decreased, but malondialdehyde and H2O2 concentrations increased at the higher Cd level, which were counteracted by Si added. Ascorbic acid, glutathione and non-protein thiols concentrations increased at the higher Cd level, which were further intensified by addition of Si. The effects of Si and Cd on the antioxidant enzyme activity were further verified by isoenzyme analysis. Silicon was more effective in enhancing Cd tolerance in the Cd-tolerant cultivar than in the Cd-sensitive cultivar. It can be concluded that Si-enhanced Cd tolerance in B. chinensis is attributed mainly to Si-suppressed Cd uptake and root-to-shoot Cd transport and Si-enhanced antioxidant defense activity. [Copyright &y& Elsevier]

Abstract: Two contrasting rice (Oryza sativa L.) cultivars, i.e. Wuyujing 3 (WYJ3, Cd-tolerant) and Shanyou 63 (SY63, Cd-sensitive), were grown on a red soil (Ultisol) to study both individual and combined phytotoxicity of arsenic (As) and cadmium (Cd) in terms of Cd and As availability, their uptake and accumulation, antioxidant defense activity and oxidative damage. The antioxidant defense system examined in this study included enzymatic and non-enzymatic molecular antioxidants such as superoxide dismutase (SOD), peroxidase (POD), glutathione (GSH) and ascorbic acid (AsA). Results showed that As or Cd treatment decreased root and shoot biomass in both cultivars compared with their corresponding control (no Cd or As treatment), although less severe inhibition of plant growth was observed in WYJ3 than in SY63. Moreover, rice growth was inhibited more severely by Cd treatment than by As treatment, which could be explained by the higher amount of available Cd (60%) (0.1M HCl-extractable Cd) compared to the lower amount of available As (15%) (0.5M NaH2PO4-extractable As) in their postharvest soils. However, shoot biomass in cultivar SY63, and root and shoot biomass in cultivar WYJ3 were significantly higher in the As plus Cd treatment than in the Cd treatment alone, showing that the combined toxicity of these two heavy metals was not additive and on the contrary, As mitigated Cd-induced growth inhibition. The As plus Cd treatment also significantly decreased As or Cd concentrations both in roots and in shoots of the two rice cultivars compared with the As or Cd treatment alone, respectively. On the other hand, treatment with As or Cd alone significantly decreased the SOD and POD activities, and GSH and AsA concentrations, while the activities of these enzymes and the concentrations of GSH and AsA were significantly higher in the As plus Cd treatment than in the Cd treatment alone, resulting in less severe oxidative damage as indicated by the lower concentration of MDA in the As plus Cd treatment (P <0.05). However, no significant difference was observed in the antioxidant defense activity between the As plus Cd treatment and the As treatment alone. These results suggest that the combined toxicity of As and Cd in rice is lower than that of individual Cd or As, which might be attributed to the decreased uptake and accumulation of Cd and As, and the less oxidative stress caused by the interactive effects of As with Cd both in rhizosphere and in plants. [Copyright &y& Elsevier]

Cadmium (Cd) is harmful to the environment and threatens human health. With the increasing use of cerium oxide nanoparticles (CeO 2 NPs) in extensive industries, investigating the combination of CeO 2 NPs and plants has attracted research interests for phytoremediation. Here, we explored the effects of CeO 2 NPs on Cd uptake, transport and the consequent Cd accumulation in Sedum alfredii. Exposure of 50 or 500 mg L−1 CeO 2 NPs alone had no apparent damaging effects on plant growth. However, upon Cd condition, the consistent CeO 2 NPs decreased Cd concentrations in the roots and shoots by up to 37%. Furthermore, the application of a metabolic inhibitor revealed that CeO 2 NPs mainly decreased the Cd uptake in roots by the apoplastic pathway. Simultaneously, CeO 2 NPs accelerated the development of Casparian strips (CSs) and suberin, which was further proven by the elevated expression levels of genes associated with their formation, SaCASP , SaGPAT5, SaKCS20 and SaCYP86A1. Compared to CeO 2 NPs added alone, the concurrent Cd decreased the Ce contents in the roots and altered its translocation from root to shoot. Taken together, both CeO 2 NPs and Cd influence the interactional uptake of both chemicals in roots of S. alfredii mainly via the apoplastic pathway which is primarily regulated by the development of CSs and suberin. [Display omitted] • CeO 2 NPs accelerate the endodermal apoplastic barriers development. • Ce contents are promoted by Cd in shoot at low level of CeO 2 NPs. • CeO 2 NPs mainly decrease Cd accumulation in HE via apoplastic pathway. • CeO 2 NPs reduce the phytoremediation efficiency of Cd by Sedum alfredii. [ABSTRACT FROM AUTHOR]

• HE S. alfredii decreases root endogenous ethylene production in response to Cd. • HE and NHE S. alfredii respond differently in terms of ethylene emission in root. • Ethylene delays the apoplastic barriers deposition induced by Cd in HE S. alfredii. • Ethylene-regulated endodermal differentiation regulates apoplastic Cd uptake. 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. [ABSTRACT FROM AUTHOR]

Mineral nutrients play an important role in heavy metal uptake and accumulation in plant. However, the effects of calcium (Ca) supply level on apoplastic transport in roots and consequences for uptake of cadmium (Cd) in hyperaccumulators are poorly understood. Here, we investigated how Ca regulated the development of apoplastic barriers in the roots of two ecotypes of Sedum alfredii and assessed its effects on Cd uptake. Results of correlation analysis indicated that Ca content was positively correlated with the development of Casparian strips (CSs) and suberin lamellae (SL) in the absence or presence of Cd. Simultaneously, low Ca supply was proven to delay the formation of endodermis CSs and suberin accumulation by decreasing the relative expressions of genes associated with CSs localization and lignin/suberin synthesis. Moreover, Cd in apoplastic fluid and cell walls (regarding the apoplastic transport) and symplastic fractions were elevated by low Ca supply. Contrary to high Ca supply, the expression levels of genes related to Cd influx and xylem loading were increased upon low Ca addition in roots of both ecotypes. All the results above suggested that low Ca supply promotes root Cd uptake via apoplastic pathway by delaying apoplastic barriers development and also regulating Cd transport to the xylem in S. alfredii. Unlabelled Image • Low Ca supply delays CSs and SL formation in roots of Sedum alfredii. • Ca was more pronounced related with suberization, controlling the uptake of root Cd. • Less suberization induced by low Ca supply promotes the uptake of Ca and Cd in HE. • Enhanced suberization prevents Ca leakage in roots under Ca deficiency in NHE. [ABSTRACT FROM AUTHOR]