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.

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.

Arsenic induced plant growth by increasing its nutrient uptake in As-hyperaccumulator Pteris vittata: Comparison of arsenate and arsenite.

Arsenic-hyperaccumulator Pteris vittata is efficient in taking up arsenate (AsV) and arsenite (AsIII), however, their impacts on P. vittata growth and nutrient uptake remain unclear. The uptake of AsV and AsIII, their influences on nutrient uptake and plant biomass, and As speciation were investigated in P. vittata after exposing to 5 or 50 µM AsV or AsIII for 12 d under hydroponics. The results show that AsV uptake in P. vittata was 1.2 times more efficient than AsIII, corresponding to 1.7-2.1 fold greater biomass than the control at 50 µM As. While AsV was dominant in the roots at ∼60%, AsIII was more dominant in the fronds at ∼70% in all treatments. Macronutrients P, K, Ca, and S were increased by 118-185% at 50 µM As, with greater uptake of micronutrients Fe, Mn, Cu, and Zn at 5 µM As. Further, positive correlations between P. vittata biomass and its As contents (r = 0.97), and P. vittata biomass and its S, Mg, P, or Ca contents (r = 0.70-0.98) were observed. Our results suggest that its increased nutrient uptake probably enhanced P. vittata growth under As exposure.

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.

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.

Nickel bioaccessibility in soils with high geochemical background and anthropogenic contamination.

Abnormally high concentrations of metals including nickel (Ni) in soils result from high geochemical background (HB) or anthropogenic contamination (AC). Metal bioaccessibility in AC-soils has been extensively explored, but studies in HB-soils are limited. This study examined the Ni bioaccessibility in basalt and black shale derived HB-soils, with AC-soils and soils without contamination (CT) being used for comparison. Although HB- and AC-soils had similar Ni levels (123 ± 43.0 vs 155 ± 84.7 mg kg-1), their Ni bioaccessibility based on the gastric phase of the Solubility Bioaccessibility Research Consortium (SBRC) in vitro assay was different. Nickel bioaccessibility in HB-soils was 6.42 ± 3.78%, 2-times lower than the CT-soils (12.0 ± 9.71%) and 6-times lower than that in AC-soils (42.6 ± 16.3%). Based on the sequential extraction, a much higher residual Ni fractionation in HB-soils than that in CT- and AC-soils was observed (81.9 ± 9.52% vs 68.6 ± 9.46% and 38.7 ± 16.0%). Further, correlation analysis indicate that the available Ni (exchangeable + carbonate-bound + Fe/Mn hydroxide-bound) was highly correlated with Ni bioaccessibility, which was also related to the organic carbon content in soils. The difference in co-localization between Ni and other elements (Fe, Mn and Ca) from high-resolution NanoSIMS analysis provided additional explanation for Ni bioaccessibility. In short, based on the large difference in Ni bioaccessibility in geochemical background and anthropogenic contaminated soils, it is important to base contamination sources for proper risk assessment of Ni-contaminated soils.

Selenate increased plant growth and arsenic uptake in As-hyperaccumulator Pteris vittata via glutathione-enhanced arsenic reduction and translocation.

The beneficial effects of selenium on As uptake and plant growth in As-hyperaccumulator Pteris vittata are known, but the associated mechanisms remain unclear. Here, we investigated the effects of selenate on arsenic accumulation by P. vittata under two arsenate levels. P. vittata plants were exposed to 13 (As13) or 133 µM (As133) arsenate and 5 µM selenate in 0.2-strength Hoagland solution. After 14 d of growth, plant biomass, Se and As content, As speciation, and malondialdehyde (MDA), glutathione reductase (GR), glutathione peroxidase (GPX), and glutathione (GSH and GSSG) levels were determined. The results show that selenate promoted P. vittata growth and increased As concentrations in the roots and fronds by 256% from 97 to 346 mg kg-1 and 142% from 213 to 514 mg kg-1 under As13 treatment, and by 166% from 500 to 1332 mg kg-1 and 534% from 777 to 4928 mg kg-1 under As133 treatment. In addition, selenate increased the glutathione content in P. vittata roots and fronds by 75-86% under As13 treatment and 44-45% under As133 treatment. Selenate also increased the GPX activity by 161-173%, and GR activity by 72-79% in P. vittata under As13 and As133 treatments. The HPLC-ICP-MS analysis indicated that selenate increased both AsIII and AsV levels in P. vittata, with AsIII/AsV ratio being lower in the roots and higher in the fronds, i.e., more AsIII was being translocated to the fronds. Taken together, our results suggest that, via GPX-GR mediated enhancement of GSH-GSSG cycle, selenate effectively increases plant growth and As uptake in P. vittata by improving AsV reduction in the roots and AsIII translocation from the roots to the fronds.