Selenium increases antimony uptake in As-hyperaccumulators Pteris vittata and Pteris cretica by promoting antimonate reduction: GSH-GSSG cycle and arsenate reductases HAC1/ACR2.

Selenium-enhanced arsenic uptake by As-hyperaccumulators Pteris vittata and Pteris cretica is known, but how it impacts antimony (Sb) uptake and associated mechanisms are unclear. Here, we investigated the effects of 2.5 µM selenate (Se2.5) on Sb uptake by two plants after growing for 10 days under hydroponics containing 10 or 50 µM antimonate (SbV) (Sb10 or Sb50). Both plants were efficient in taking up SbV, which was reduced to SbIII (17-40 %) and mainly accumulated in the roots (86-97 %). The addition of Se increased the Sb contents by 78-97 and 29-33 % to 242-1358 and 132-697 mg kg-1 in P. vittata and P. cretica roots. Compared with the Sb10 and Sb50 treatments, addition of Se increased the SbV reduction, with more increase in P. vittata than P. cretica roots (181-273 % vs. 17-29 %). Enhanced GSH-GSSG cycle mediated by glutathione peroxidase (GPX) and glutathione reductase (GR) may play an important role in SbV reduction in the roots. Compared with the Sb treatments, addition of Se increased the GPX and GR activity by 71-97 and 2-50 % in P. vittata roots, and 59-153 and 22-63 % in P. cretica roots. Besides, Se upregulated the expression of arsenate reductases PvHAC1 and PvACR2 in P. vittata roots by 1.7-3.4 folds but not in P. cretica. Se-enhanced SbV reduction in P. vittata explains why it was more effective in Sb accumulation than P. cretica. Taken together, Se is effective in increasing the Sb uptake in both plants probably by promoting SbV reduction via GSH-GSSG cycle and/or PvHAC1/PvACR2, suggesting that Se may be used to enhance phytostabilization of Sb-contaminated soils.

Foliar-selenium enhances plant growth and arsenic accumulation in As-hyperaccumulator Pteris vittata: Critical roles of GSH-GSSG cycle and arsenite antiporters PvACR3.

It is known that selenium (Se) enhances plant growth and arsenic (As) accumulation in As-hyperaccumulator Pteris vittata, but the associated mechanisms are unclear. In this study, P. vittata was exposed to 50 µM arsenate (AsV) under hydroponics plus 25 or 50 µM foliar selenate. After 3-weeks of growth, the plant biomass, As and Se contents, As speciation, malondialdehyde (MDA) and glutathione (GSH and GSSG) levels, and important genes related to As-metabolism in P. vittata were determined. Foliar-Se increased plant biomass by 17 - 30 %, possibly due to 9.1 - 19 % reduction in MDA content compared to the As control. Further, foliar-Se enhanced the As contents by 1.9-3.5 folds and increased arsenite (AsIII) contents by 64 - 136 % in the fronds. The increased AsV reduction to AsIII was attributed to 60 - 131 % increase in glutathione peroxidase activity, which mediates GSH oxidation to GSSG (8.8 -29 % increase) in the fronds. Further, foliar-Se increased the expression of AsIII antiporters PvACR3;1-3;3 by 1.6 - 2.1 folds but had no impact on phosphate transporters PvPht1 or arsenate reductases PvHAC1/2. Our results indicate that foliar-Se effectively enhances plant growth and arsenic accumulation by promoting the GSH-GSSG cycle and upregulating gene expression of AsIII antiporters, which are responsible for AsIII translocation from the roots to fronds and AsIII sequestration into the fronds. The data indicate that foliar-Se can effectively improve phytoremediation efficiency of P. vittata in As-contaminated soils.

Selenium alleviates chromium stress and promotes chromium uptake in As-hyperaccumulator Pteris vittata: Cr reduction and cellar distribution.

Arsenic-hyperaccumulator Pteris vittata exhibits remarkable absorption ability for chromium (Cr) while beneficial element selenium (Se) helps to reduce Cr-induced stress in plants. However, the effects of Se on the Cr uptake and the associated mechanisms in P. vittata are unclear, which were investigated in this study. P. vittata plants were grown for 14 days in 0.2-strength Hoagland solution containing 10 (Cr10) or 100 µM (Cr100) chromate (CrVI) and 1 µM selenate (Se1). The plant biomass, malondialdehyde contents, total Cr and Se contents, Cr speciation, expression of genes associated with Cr uptake, and Cr subcellular distribution in P. vittata were determined. P. vittata effectively accumulated Cr by concentrating 96-99% in the roots under Cr100 treatment. Further, Se substantially increased its Cr contents by 98% to 11,596 mg kg-1 in the roots, which may result from Se's role in reducing its oxidative stress as supported by 27-62% reduction in the malondialdehyde contents. Though supplied with CrVI, up to 98% of the Cr in the roots was reduced to insoluble chromite (CrIII), with 83-89% being distributed on root cell walls. Neither Cr nor Se upregulated the expression of sulfate transporters PvSultr1;1-1;2 or phosphate transporter PvPht1;4, indicating their limited role in Cr uptake. P. vittata effectively accumulates Cr in the roots mainly as CrIII on cell walls and Se effectively enhances its Cr uptake by reducing its oxidative stress. Our study suggests that Se can be used to enhance P. vittata Cr uptake and reduce its oxidative stress, which may have application in phytostabilization of Cr-contaminated soils.

Copper enhanced arsenic-accumulation in As-hyperaccumulator Pteris vittata by upregulating its gene expression for As uptake, translocation, and sequestration.

In contaminated soils, arsenic (As) often co-exists with copper (Cu). However, its effects on As accumulation and the related mechanisms in As-hyperaccumulator Pteris vittata remain unclear. In this study, P. vittata plants were exposed to 50 µM As and/or 50 µM Cu under hydroponics to investigate the effects of Cu on plant growth and As accumulation, as well as gene expression related to arsenic uptake (P transporters), reduction (arsenate reductases), and translocation and sequestration (arsenite antiporters). After 14 d of growth and compared to the As treatment, the As concentration in P. vittata fronds increased by 1.4-times from 793 to 1131 mg·kg-1 and its biomass increased by 1.2-fold from 18.0 to 21.1 g·plant-1 in the As+Cu treatment. Copper-enhanced As accumulation was probably due to upregulated gene expressions related to As-metabolisms including As uptake (1.9-fold in P transporter PvPht1;3), translocation (2.1-2.4 fold in arsenite antiporters PvACR3/3;2) and sequestration (1.5-2.0 fold in arsenite antiporters PvACR3;1/3;3). Our results suggest that moderate amount of Cu can help to increase the As accumulation efficiency in P. vittata, which has implication in its application in phytoremedation in As and Cu co-contaminated soils.

Antimony uptake and speciation, and associated mechanisms in two As-hyperaccumulators Pteris vittata and Pteris cretica.

The behaviors of antimony (Sb) and arsenic (As) in plants are different, though they are chemical analogs. Here, we examined the Sb uptake and speciation in two As-hyperaccumulators P. vittata and P. cretica, which were exposed to 0.5 or 5 mg L-1 antimonate (SbV) or antimonite (SbIII) under hydroponics for 7 d. Both plants grew better under Sb exposure, especially for P. cretica. The biomass of P. cretica roots increased by 29-46% after exposing to SbV, possibly due to increased S. Further, the Sb content in P. vittata was 17-93% greater than P. cretica, with 2-3 times more SbIII than SbV in both plants and > 92% Sb being concentrated in the roots, showing limited translocation. Under SbV exposure, SbV was dominant in P. vittata roots at 86-94%, while SbIII was predominant in P. cretica roots at 36-95%. P. cretica's stronger reducing ability than P. vittata may be due to arsenate reductases HAC1 and ACR2, which were upregulated in both plants. In short, while effective in Sb accumulation, it is mostly concentrated in the roots for both plants. The differences in their accumulation and speciation may help to better understand Sb behaviors in other plants.

Phytate and Arsenic Enhance Each Other's Uptake in As-hyperaccumulator Pteris vittata: Root Exudation of Phytate and Phytase, and Plant Uptake of Phytate-P.

Phytate as a root exudate is rare in plants as it mainly serves as a P storage in the seeds; however, As-hyperaccumulator Pteris vittata effectively secretes phytate and utilizes phytate-P, especially under As exposure. This study investigated the effects of As on its phytate and phytase exudation and the impacts of As and/or phytate on each other's uptake in P. vittata through two hydroponic experiments. Under 10-100 µM arsenate (AsV), the exudation of phytate and phytase by P. vittata was increased by 50-72% to 20.4-23.4 µmol h-1 g-1 and by 28-104% to 18.6-29.5 nmol h-1 plant-1, but they were undetected in non-hyperaccumulator Pteris ensiformis at 10 µM AsV. Furthermore, compared to 500 µM phytate, the phytate concentration in the growth media was reduced by 69% to 155 µM, whereas the P and As contents in P. vittata fronds and roots were enhanced by 68-134% and 44-81% to 2423-2954 and 82-407 mg kg-1 under 500 µM phytate plus 50 µM AsV. The increased P/As uptake in P. vittata was probably attributed to 3.0-4.5-fold increase in expressions of P transporters PvPht1;3-1;4. Besides, under As exposure, plant P may be converted to phytate in P. vittata roots, thereby increasing phytate's contents by 84% to 840 mg kg-1. Overall, our results suggest that As-induced phytate/phytase exudation and phytate-P uptake stimulate its growth and As hyperaccumulation by P. vittata.

Selenium Increased Arsenic Accumulation by Upregulating the Expression of Genes Responsible for Arsenic Reduction, Translocation, and Sequestration in Arsenic Hyperaccumulator Pteris vittata.

Selenate enhances arsenic (As) accumulation in As-hyperaccumulator Pteris vittata, but the associated molecular mechanisms are unclear. Here, we investigated the mechanisms of selenate-induced arsenic accumulation by exposing P. vittata to 50 µM arsenate (AsV50) and 1.25 (Se1.25) or 5 µM (Se5) selenate in hydroponics. After 2 weeks, plant biomass, plant As and Se contents, As speciation in plant and growth media, and important genes related to As detoxification in P. vittata were determined. These genes included P transporters PvPht1;3 and PvPht1;4 (AsV uptake), arsenate reductases PvHAC1 and PvHAC2 (AsV reduction), and arsenite (AsIII) antiporters PvACR3 and PvACR3;2 (AsIII translocation) in the roots, and AsIII antiporters PvACR3;1 and PvACR3;3 (AsIII sequestration) in the fronds. The results show that Se1.25 was more effective than Se5 in increasing As accumulation in both P. vittata roots and fronds, which increased by 27 and 153% to 353 and 506 mg kg-1. The As speciation analyses show that selenate increased the AsIII levels in P. vittata, with 124-282% more AsIII being translocated into the fronds. The qPCR analyses indicate that Se1.25 upregulated the gene expression of PvHAC1 by 1.2-fold, and PvACR3 and PvACR3;2 by 1.0- to 2.5-fold in the roots, and PvACR3;1 and PvACR3;3 by 0.6- to 1.1-fold in the fronds under AsV50 treatment. Though arsenate enhanced gene expression of P transporters PvPht1;3 and PvPht1;4, selenate had little effect. Our results indicate that selenate effectively increased As accumulation in P. vittata, mostly by increasing reduction of AsV to AsIII in the roots, AsIII translocation from the roots to fronds, and AsIII sequestration into the vacuoles in the fronds. The results suggest that selenate may be used to enhance phytoremediation of As-contaminated soils using P. vittata.

Arsenic-induced up-regulation of P transporters PvPht1;3-1;4 enhances both As and P uptake in As-hyperaccumulator Pteris vittata.

Plants often up-regulate gene expression of P transporters under P deficiency, but down-regulate them under arsenic stress. Different from other P transporters, PvPht1;3 and PvPht1;4 expressions in As-hyperaccumulator Pteris vittata are up-regulated under P deficiency and As stress, showing strong transport capacity for both As and P. This study examined the mechanisms behind As-induced up-regulation of P transporters in P. vittata after exposing to 10-50 µM arsenate (AsV) for 14 d under hydroponics, with non-hyperaccumulator P. ensiformis as a control. Under As stress, P. vittata was more efficient in taking up both As and P than P. ensiformis, showing 48-84% more P content in the fronds and roots, leading to 18-79% greater biomass. Though As enhanced the P uptake by P. vittata, the inorganic P was reduced by 25-64% from 140-347 to 65-126 mg kg-1. It is likely that, under As stress, more P was utilized by P. vittata to counter As toxicity, causing reduction in inorganic P. This was supported by As-induced conversion of inorganic P to phytate in P. vittata, with phytate-P being increased by 26-75% from 239-713 to 418-1221 mg kg-1, maintaining internal low P levels. Under As-induced low inorganic-P conditions, the expressions of P transporters PvPht1;3 and PvPht1;4 were up-regulated by 1.4- and 2.7-fold in the roots, helping greater As and P uptake by P. vittata. Clearly, As-induced overexpression of P transporters in P. vittata roots plays a critical role in taking up both As and P, thereby increasing its efficiency in As-hyperaccumulation from contaminated media.

Uptake, speciation and detoxification of antimonate and antimonite in As-hyperaccumulator Pteris Cretica L.

Antimony (Sb) and arsenic (As) are chemical analogs, but their behaviors in plants are different. To investigate the Sb uptake, translocation and speciation in As-hyperaccumulator P. cretica, a hydroponic experiment was conducted. In this study, P. cretica was exposed to 0.2-strength Hoagland nutrient solution, which contained 0.5 or 5 mg/L antimonite (SbIII) or antimonate (SbV). After 14 d exposure, P. cretica took up 1.4-2.8 times more SbIII than SbV. Since P. cretica was unable to translocate Sb, its roots accumulated >97% Sb with the highest at 7965 mg/kg. In both SbIII and SbV treatments, SbIII was the predominant species in P. cretica, with 90-100% and 46-100% SbIII in the roots. As the first barrier against Sb to enter plant cells, more Sb was accumulated in cell wall than cytosol or organelles. The results suggest that P. cretica may detoxify Sb by reducing SbV to SbIII and immobilizing it in root cell walls. Besides, the presence of SbIII significantly reduced the concentrations of dissolved organic C including organic acids in P. cretica root exudates. Further, increasing Sb levels promoted P accumulation in the plant, especially in the fronds, which may help P. cretica growth. The information from this study shed light on metabolic transformation of Sb in As-hyperaccumulators P. cretica, which helps to better understand Sb uptake and detoxification by plants.

Using rice as a remediating plant to deplete bioavailable arsenic from paddy soils.

Rice plant is efficient in arsenic (As) accumulation due to enhanced soil As release under flooded condition and its effective As uptake. Therefore, rice plant can be used to remove bioavailable As from paddy soil. In this study, the depleting dynamics of soil As with rice growth was mapped two-dimensionally with the zirconium-oxide diffusive gradients in thin films (DGT) technique. Further, the key biochemistry promoting soil As bioavailability was studied to better understand the underlying processes. Results revealed that the average DGT-As encompassing the root zone decreased steeply from 331 in the seedling stage to 136 in the heading & flowering stage and further to 118 µg l-1 at harvest, which was 26% lower than that of the control at 160 µg l-1. During this process, rhizosphere porewater As developed a dynamic profile similar to Fe and dissolved organic carbon, with the diversity of arrA gene peaking at heading & flowering stage. The data support soil As release from microbial reduction of Fe hydroxides fueled by root exudation as carbon source. Arsenic was mainly accumulated in the roots, accounting for up to 95% of total As in rice plants. Removal of rice roots resulted in ~19% lower DGT-As in post-harvest soil compared to without removing the roots. As a result, a sharp decline in As accumulation in rice plants was obtained in the second planting after removing one crop of rice roots. The results highlight that rice, as a paddy-adapted plant, is effective in As uptake in the roots, and thereby removing rice roots efficiently depletes bioavailable As from paddy soils.

Enhanced arsenic depletion by rice plant from flooded paddy soil with soluble organic fertilizer application.

In this study, the promoting effect of soluble organic fertilizer (SOF) on arsenic (As) release and depletion by rice plant in flooded paddy soil was investigated. The increased soil DOC with SOF incorporation displayed a significantly positive correlation with As in soil solution (r = 0.415, p < 0.01). Porewater As with SOF addition was higher than control from 77 days. Soil As depletion by rice plant was enhanced by SOF application, with porewater As in SM + Rice + SOF being 49% lower than that in SM + Rice at harvest. Compared to SM + Rice, the averaged soil profile of As measured by diffusive gradients in thin films (DGT) declined by 4% in tillering stage and by 16% in grain-filling stage in SM + Rice + SOF. As a result, As accumulation in each rice plant was increased by 35% at the presence of SOF, with 92% of total As retained in roots. With SOF amendment, 8% of total As in the tested soil was removed by harvesting rice biomass including root. In this way, an estimated 184 mg As m-2 can be depleted from paddy soil at the end of one rice-growing season. These results revealed the potential of SOF in enhancing soil As depletion by rice plant under flooded condition, providing a cost-effective pathway for efficient cleanup of bioavailable As from rice paddies.

Arsenic removal from flooded paddy soil with spontaneous hygrophyte markedly attenuates rice grain arsenic.

China ranks the top in global annual rice output. However, extensive mining and smelting has led to elevated arsenic (As) in paddy soils, potentially imperiling local population health and sustainable rice production in the country. Under flooded condition, reductive As mobilization generally occurs, providing a unique advantage for soil As removal. In this study, we explore the depletion magnitude of labile As from paddy soils through cycling of flooding-drainage with three distinct co-strategies, i.e. (1) no soil disturbance with spontaneously established hygrophyte plants, (2) selective fertilization to enhance soil As release, and (3) soil ploughing following each drainage. After 151 days of flooding with periodic drainage, diffusive gradients in thin film (DGT)-labile As through 0-14 cm soil profile with hygrophyte plants growing decreased from initial 292 µg l-1 to well below the required threshold level (57-77 µg l-1) for safe rice production. Correspondingly, an average of 22.9% of total soil As was removed, with up to 76.7% of As bound to amorphous Fe hydroxides being stripped in this treatment. In the following rice cultivation, inorganic As in the polished rice from the naturally vegetated treatment (0.15 mg kg-1) fell successfully below the Chinese food safety standard (0.2 mg kg-1). The results highlight that As removal from paddy soils with native hygrophyte under shallow flooded condition can decrease soil bioavailable As specifically to safe levels within a relatively short period, and thus provides a novel and quite cost-effective pathway securing rice production.