Expression of New Pteris vittata Phosphate Transporter PvPht1;4 Reduces Arsenic Translocation from the Roots to Shoots in Tobacco Plants

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.

Do the residual metals in multiple environmental media surrounding mines pose ecological and health risks? A case of an abandoned mining area in central south China

Despite effective mining environmental regulations, residual metal pollution persists, leading to significant ecological harm and posing substantial risks to human well-being. This study employed multiple-criteria methods to investigate the ecological and health risks caused by metals in multiple environmental media (e.g., arable soil, indoor dust, PM10, homegrown vegetables, and rice) around abandoned mine areas (MA) in central south China. The study also aimed to identify predominant risk factors and the main exposure pathway. The findings revealed that metal levels and risks in the environmental media surrounding the MA were significantly higher than those in the control areas (away from abandoned mines, CA). This indicates that the accumulation of metals in the environmental media surrounding the MA was attributed to the previous mining activities. Variations in metal content were observed among different environmental media in MA, with Cd from mining source being the primary pollutant in arable soil, indoor dust, PM10, and vegetables, while As from agricultural source was the main pollutant in rice. Additionally, the consumption of Cd-contaminated vegetables and As-contaminated rice emerged as the primary routes of health hazards for the local population, leading to significant non-carcinogenic and carcinogenic risks. Consequently, it is imperative for the government and mining companies to promptly establish risk control and remedial strategies for mitigating residual metal levels in multiple environmental media surrounding the MA.

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.

Novel phosphatase PvPAP1 from the As-hyperaccumulator Pteris vittata promotes organic P utilization and plant growth: Extracellular exudation and phytate hydrolysis

Organic phosphorus (Po) is a large component of soil P, but it is often unavailable for plant uptake. Purple acid phosphatases (PAP) can hydrolyze a wide range of Po, playing an important role in Po utilization by plants. In this study, we investigated a novel secretary PvPAP1 from the As-hyperaccumulator Pteris vittata, which can effectively utilize exogenous Po, including adenosine triphosphate (ATP) and phytate. Unlike other PAP, PvPAP1 was abundantly-expressed in P. vittata roots, which was upregulated 3.5-folds under P-deprivation than P-sufficient conditions. When expressed in tobacco, its activity in the roots of PvPAP1-Ex lines was ∼8 folds greater than that in wild-type (WT) plants. Besides, PvPAP1 exhibited its secretory ability as evidenced by the sapphire-blue color on the root surface after treating with 5-bromo-4-chloro-3-indolyl phosphate. In a long-term experiment using sand media, PvPAP1-expressing tobacco plants showed 25-30 % greater root biomass than WT plants when using ATP as the sole P source. This is because PvPAP1-expression enhanced its phosphatase activity by 6.5-9.2 folds in transgenic tobacco, thereby increasing the P contents by 39-41 % in its roots under ATP treatment and 9.4-30 % under phytate treatment. The results highlight PvPAP1 as a novel secreted phosphatase crucial for external Po utilization in P. vittata, suggesting that PvPAP1 has the potential to serve as a valuable gene resource for enhancing Po utilization by crop plants.

Arsenic transport, detoxification, and recent technologies for mitigation: A systemic review

Arsenic (As) is an acute toxic metalloid that affects plant growth and development. As is found in the environment in organic and inorganic forms, but arsenite As(III) and arsenate As(V) are the most prevalent forms that negatively impact the plants. Roots exposed to As can easily absorb it mainly through transporters that carry vital mineral nutrients. As reach the food chain via crops irrigated with As-polluted water and exerts a negative impact. Even at low levels, As exposure disrupts the regular functioning of plants by generating a high level of reactive oxygen species (ROS) results into oxidative damage, and disruption of redox system. Plants have built-in defence mechanisms to combat this oxidative damage. The development of a food crop with lower As levels is dependent upon understanding the molecular process of As detoxification in plants, which will help reduce the consumption of As-contaminated food. Numerous genes in plants that may provide tolerance under hazardous conditions have been examined using genetic engineering techniques. The suppression of genes by RNA interference (RNAi) and CRISPR-Cas 9 (CRISPR associated protein 9) technology revealed an intriguing approach for developing a crop that has minimal As levels in consumable portions. This study aims to present current information on the biochemical and molecular networks associated with As uptake, as well as recent advances in the field of As mitigation using exogenous salicylic acid (SA), Serendipita indica and biotechnological tools in terms of generating As-tolerant plants with low As accumulation.

Arsenite Antiporter PvACR3 Driven by Its Native Promoter Increases Leaf Arsenic Accumulation in Tobacco

Pteris vittata is the first-reported arsenic (As) hyperaccumulator, which has been applied to phytoremediation of As-contaminated soil. PvACR3, a key arsenite (AsIII) antiporter, plays an important role in As hyperaccumulation in P. vittata. However, its functions in plants are not fully understood. In this study, the PvACR3 gene was heterologously expressed in tobacco, driven by its native promoter (ProPvACR3). After growing at 5 μM AsIII or 10 μM AsV in hydroponics for 1-5 days, PvACR3-expression enhanced the As levels in leaves by 66.4-113 and 51.8-101%, without impacting the As contents in the roots or stems. When cultivated in As-contaminated soil, PvACR3-expressed transgenic plants accumulated 47.9-85.5% greater As in the leaves than wild-type plants. In addition, PvACR3-expression increased the As resistance in transgenic tobacco, showing that enhanced leaf As levels are not detrimental to its overall As tolerance. PvACR3 was mainly expressed in tobacco leaf veins and was likely to unload AsIII from the vein xylem vessels to the mesophyll cells, thus elevating the leaf As levels. This work demonstrates that heterologously expressing PvACR3 under its native promoter specifically enhances leaf As accumulation in tobacco, which helps to reveal the As-hyperaccumulation mechanism in P. vittata and to enhance the As accumulation in plant leaves for phytoremediation.

Arsenic-Hyperaccumulator Pteris vittata Effectively Uses Sparingly-Soluble Phosphate Rock: Rhizosphere Solubilization, Nutrient Improvement, and Arsenic Accumulation

Sparingly-soluble phosphate rock (PR), a raw material for P-fertilizer production, can be effectively utilized by the As-hyperaccumulator Pteris vittata but not most plants. In this study, we investigated the associated mechanisms by measuring dissolved organic carbon (DOC) and acid phosphatase in the rhizosphere, and nutrient uptake and gene expression related to the As metabolism in P. vittata. The plants were grown in a soil containing 200 mg kg-1 As and/or 1.5% PR for 30 days. Compared to the As treatment, the P. vittata biomass was increased by 33% to 4.6 g plant-1 in the As+PR treatment, corresponding to 27% decrease in its frond oxidative stress as measured by malondialdehyde. Due to PR-enhanced DOC production in the rhizosphere, the Ca, P, and As contents in P. vittata fronds were increased by 17% to 9.7 g kg-1, 29% to 5.0 g kg-1, and 57% to 1045 mg kg-1 in the As+PR treatment, thereby supporting its better growth. Besides, PR-induced rhizosphere pH increase from 5.0 to 6.9 promoted greater P uptake by P. vittata probably via upregulating low-affinity P transporters PvPTB1;1/1;2 by 3.7-4.1 folds. Consequently, 29% lower available-P induced the 3.3-fold upregulation of high-affinity P transporter PvPht1;3 in the As+PR treatment, which was probably responsible for the 58% decrease in available-As content in the rhizosphere. Consistent with the enhanced As translocation and sequestration, arsenite antiporters PvACR3/3;3 were upregulated by 1.8-4.4 folds in the As+PR than As treatment. In short, sparingly-soluble PR enhanced the Ca, P, and As availability in P. vittata rhizosphere and improved their uptake via upregulating genes related to As metabolism, suggesting its potential application for improving phytoremediation in As-contaminated soils.

Novel Phosphate Transporter-B PvPTB1;1/1;2 Contribute to Efficient Phosphate Uptake and Arsenic Accumulation in As-Hyperaccumulator Pteris vittata

Arsenic (As) contamination in soil poses a potential threat to human health via crop uptake. As-hyperaccumulator Pteris vittata serves as a model plant to study As uptake and associated mechanisms. This study focuses on a novel P/AsV transport system mediated by low-affinity phosphate transporter-B 1 family (PTB1) in P. vittata. Here, we identified two plasma-membrane-localized PTB1 genes, PvPTB1;1/1;2, in vascular plants for the first time, which were 4.4-40-fold greater in expression in P. vittata than in other Pteris ferns. Functional complementation of a yeast P-uptake mutant and enhanced P accumulation in transgenic Arabidopsis thaliana confirmed their role in P uptake. Moreover, the expression of PvPTB1;1/1;2 facilitated the transport and accumulation of As in both yeast and A. thaliana shoots, demonstrating a comparable AsV uptake capacity. Microdissection-qPCR analysis and single-cell transcriptome analysis collectively suggest that PvPTB1;1/1;2 are specifically expressed in the epidermal cells of P. vittata roots. PTB1 may play a pivotal role in efficient P recycling during phytate secretion and hydrolysis in P. vittata roots. In summary, the dual P transport mechanisms consisting of high-affinity Pht1 and low-affinity PTB1 may have contributed to the efficient P/As uptake in P. vittata, thereby contributing to efficient phytoremediation for As-contaminated soils.

Insoluble-Phytate Improves Plant Growth and Arsenic Accumulation in As-Hyperaccumulator Pteris vittata: Phytase Activity, Nutrient Uptake, and As-Metabolism

Phytate, the principal P storage in plant seeds, is also an important organic P in soils, but it is unavailable for plant uptake. However, the As-hyperaccumulator Pteris vittata can effectively utilize soluble Na-phytate, while its ability to utilize insoluble Ca/Fe-phytate is unclear. Here, we investigated phytate uptake and the underlying mechanisms based on the phytase activity, nutrient uptake, and expression of genes involved in As metabolisms. P. vittata plants were cultivated hydroponically in 0.2-strength Hoagland nutrient solution containing 50 μM As and 0.2 mM Na/Ca/Fe-phytate, with 0.2 mM soluble-P as the control. As the sole P source, all three phytates supported P. vittata growth, with its biomass being 3.2-4.1 g plant-1 and Ca/Fe-phytate being 19-29% more effective than Na-phytate. Phytate supplied soluble P to P. vittata probably via phytase hydrolysis, which was supported by 0.4-0.7 nmol P min-1 g-1 root fresh weight day-1 phytase activity in its root exudates, with 29-545 μM phytate-P being released into the growth media. Besides, compared to Na-phytate, Ca/Fe-phytate enhanced the As contents by 102-140% to 657-781 mg kg-1 in P. vittata roots and by 43-86% to 1109-1447 mg kg-1 in the fronds, which was accompanied by 21-108% increase in Ca and Fe uptake. The increased plant As is probably attributed to 1.3-2.6 fold upregulation of P transporters PvPht1;3/4 for root As uptake, and 1.8-4.3 fold upregulation of arsenite antiporters PvACR3/3;1/3;3 for As translocation to and As sequestration into the fronds. This is the first report to show that, besides soluble Na-phytate, P. vittata can also effectively utilize insoluble Ca/Fe-phytate as the sole P source, which sheds light onto improving its application in phytoremediation of As-contaminated sites.

Expression of Sailx suchowensis SsIRT9 enhances cadmium accumulation and alters metal homeostasis in tobacco

Cadmium (Cd) contamination in soils is of great concern for plant growth and human health. Willow (Salix spp.) is a promising phytoextractor because of its high biomass production. However, as a non-hyperaccumulator, willow has a low competitive ability in extraction of Cd. Thus, improving Cd concentrations in developing tissues is one of the primary tasks. Here, our study uncovers a novel SsIRT9 gene from Sailx suchowensis which manipulates plant Cd accumulation. SsIRT9 was more highly expressed in willow roots than other SsIRT genes. As a plasma membrane-localized protein, when expressed in yeast, SsIRT9 retarded cell growth more severely than other SsIRT proteins in the presence of Cd. Furthermore, SsIRT9 was cloned and expressed in tobacco and SsIRT9 did not affect plant growth. In hydroponic experiments, SsIRT9 lines displayed higher Cd in the shoots than the wild type. When grown in Cd-contaminated soils, Cd levels in transgenic tobacco increased by 152-364% in roots and by 135-444% in shoots, demonstrating significant superiority in Cd accumulation over other functional IRT/ZIP transporters. Moreover, expressing SsIRT9 in tobacco altered metal homeostasis, especially manganese and zinc. Taken together, we envision that SsIRT9 expression in plants is a promising strategy for upgrading extraction of Cd from soils.

Arsenic in the hyperaccumulator Pteris vittata: A review of benefits, toxicity, and metabolism

Arsenic (As) is a toxic metalloid, elevated levels of which in soils are becoming a major global environmental issue that poses potential health risks to humans. Pteris vittata, the first known As hyperaccumulator, has been successfully used to remediate As-polluted soils. Understanding why and how P. vittata hyperaccumulates As is the core theoretical basis of As phytoremediation technology. In this review, we highlight the beneficial effects of As in P. vittata, including growth promotion, elemental defense, and other potential benefits. The stimulated growth of P. vittata induced by As can be defined as As hormesis, but differs from that in non-hyperaccumulators in some aspects. Furthermore, the As coping mechanisms of P. vittata, including As uptake, reduction, efflux, translocation, and sequestration/detoxification are discussed. We hypothesize that P. vittata has evolved strong As uptake and translocation capacities to obtain beneficial effects from As, which gradually leads to As accumulation. During this process, P. vittata has developed a strong As vacuolar sequestration ability to detoxify overloaded As, which enables it to accumulate extremely high As concentrations in its fronds. This review also provides insights into several important research gaps that need to be addressed to advance our understanding of As hyperaccumulation in P. vittata from the perspective of the benefits of As.

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.

Research advances in mechanisms of arsenic hyperaccumulation of Pteris vittata: Perspectives from plant physiology, molecular biology, and phylogeny

Pteris vittata, as the firstly discovered arsenic (As) hyperaccumulator, has great application value in As-contaminated soil remediation. Currently, the genes involved in As hyperaccumulation in P. vittata have been mined continuously, while they have not been used in practice to enhance phytoremediation efficiency. Aiming to better assist the practice of phytoremediation, this review collects 130 studies to clarify the progress in research into the As hyperaccumulation process in P. vittata from multiple perspectives. Antioxidant defense, rhizosphere activities, vacuolar sequestration, and As efflux are important physiological activities involved in As hyperaccumulation in P. vittata. Among related 19 genes, PHT, TIP, ACR3, ACR2 and HAC family genes play essential roles in arsenate (AsⅤ) transport, arsenite (AsⅢ) transport, vacuole sequestration of AsⅢ, and the reduction of AsⅤ to AsⅢ, respectively. Gene ontology enrichment analysis indicated it is necessary to further explore genes that can bind to related ions, with transport activity, or with function of transmembrane transport. Phylogeny analysis results implied ACR2, HAC and ACR3 family genes with rapid evolutionary rate may be the decisive factors for P. vittata as an As hyperaccumulator. A deeper understanding of the As hyperaccumulation network and key gene components could provide useful tools for further bio-engineered phytoremediation.

Enhancement of arsenic uptake and accumulation in green microalga Chlamydomonas reinhardtii through heterologous expression of the phosphate transporter DsPht1

Arsenate (AsV) is a predominant arsenic contaminant in aerobic water. Microalgae have been recently used in the phytoremediation of arsenic-contaminated water. However, the amount of AsV uptake in microalgae is limited, which hinders the application of microalgae in arsenic-contaminated water treatment. Here, we found that the expression of a novel phosphate transporter DsPht1 in Dunaliella salina was highly upregulated after AsV exposure. Fluorescent protein-tagging analysis showed the plasma membrane location of DsPht1. Furthermore, DsPht1 was overexpressed in a model microalga Chlamydomonas reinhardtii. The DsPht1 transgenetic lines accumulated up to 6.4-fold higher total arsenic than the untransformed line, and the AsV amount in total arsenic increased by 8.3-fold. Moreover, the organoarsenic content was also higher in the transgenetic lines. Overall, the DsPht1 transformants generated in this study increased arsenate uptake and transformation, which are promising for the effective phytoremediation of arsenic-contaminated water.

As-hyperaccumulator Pteris vittata and non-hyperaccumulator Pteris ensiformis under low As-exposure: Transcriptome analysis and implication for As hyperaccumulation

Soil contamination by arsenic (As) poses potential health risks to humans. As-hyperaccumulator P. vittata has been used in As-contaminated soils for phytoremediation. Clarifying the mechanisms of its As-hyperaccumulation is critical to enhance its efficiency in phytoremediation. Here, based on transcriptome analysis, we determined the concentration-dependent patterns of As-related gene families by comparing As-hyperaccumulator P. vittata and non-hyperaccumulator P. ensiformis after exposing to 20 µM arsenate (AsV). As expected, arsenic induced more stress in P. ensiformis than P. vittata. Based on gene ontology, differences in transporter activity are probably responsible for their differential As accumulation. Though As exposure induced expression of phosphate transporter PvPht1;4 for AsV absorption in both plants, stronger AsV reduction, AsIII transport, and AsIII-GSH complexation were found in P. ensiformis roots. Unlike P. ensiformis, As metabolism processes occurred mainly in P. vittata fronds. Notably, tonoplast-localized ACR3s were only present in P. vittata, making it more effective in sequestrating AsIII into frond vacuoles. Further, vesicle As transformation via PvGAPC1 (glyceraldehyde 3-phosphate dehydrogenase), PvOCT4 (organic cation transporter 4), and PvGSTF1 (glutathione S-transferase) contributed little to As-hyperaccumulation. This study provides information on critical genes responsible for As-hyperaccumulation by P. vittata, which can be applied to construct As-hyperaccumulating plants by genetic engineering to enhance their phytoremediation efficiency in As-contaminated soils.

Wheat PHT1;9 acts as one candidate arsenate absorption transporter for phytoremediation

Arsenate (AsV) is one of the most common forms of arsenic (As) in environment and plant high-affinity phosphate transporters (PHT1s) are the primary plant AsV transporters. However, few PHT1s involved in AsV absorption have been identified in crops. In our previous study, TaPHT1;3, TaPHT1;6 and TaPHT1;9 were identified to function in phosphate absorption. Here, their AsV absorption capacities were evaluated using several experiments. Ectopic expression in yeast mutants indicated that TaPHT1;9 had the highest AsV absorption rates, followed by TaPHT1;6, while not for TaPHT1;3. Under AsV stress, further, BSMV-VIGS-mediated TaPHT1;9-silencing wheat plants exhibited higher AsV tolerance and lower As concentrations than TaPHT1;6-silenced plants, whereas TaPHT1;3-silencing plants had similar phenotype and AsV concentrations to control. These suggested that TaPHT1;9 and TaPHT1;6 possessed AsV absorption capacity with the former showing higher activities. Under hydroponic condition, furthermore, CRISPR-edited TaPHT1;9 wheat mutants showed the enhanced tolerance to AsV with decreased As distributions and concentrations, whereas TaPHT1;9 ectopic expression transgenic rice plants had the opposite results. Also, under AsV-contaminated soil condition, TaPHT1;9 transgenic rice plants exhibited depressed AsV tolerance with increased As concentrations in roots, straws and grains. Moreover, Pi addition alleviated the AsV toxicity. These suggested that TaPHT1;9 should be a candidate target gene for AsV phytoremediation.

Negative Impacts of Arsenic on Plants and Mitigation Strategies

Arsenic (As) is a metalloid prevalent mainly in soil and water. The presence of As above permissible levels becomes toxic and detrimental to living organisms, therefore, making it a significant global concern. Humans can absorb As through drinking polluted water and consuming As-contaminated food material grown in soil having As problems. Since human beings are mobile organisms, they can use clean uncontaminated water and food found through various channels or switch from an As-contaminated area to a clean area; but plants are sessile and obtain As along with essential minerals and water through roots that make them more susceptible to arsenic poisoning and consequent stress. Arsenic and phosphorus have many similarities in terms of their physical and chemical characteristics, and they commonly compete to cause physiological anomalies in biological systems that contribute to further stress. Initial indicators of arsenic's propensity to induce toxicity in plants are a decrease in yield and a loss in plant biomass. This is accompanied by considerable physiological alterations; including instant oxidative surge; followed by essential biomolecule oxidation. These variables ultimately result in cell permeability and an electrolyte imbalance. In addition, arsenic disturbs the nucleic acids, the transcription process, and the essential enzymes engaged with the plant system's primary metabolic pathways. To lessen As absorption by plants, a variety of mitigation strategies have been proposed which include agronomic practices, plant breeding, genetic manipulation, computer-aided modeling, biochemical techniques, and the altering of human approaches regarding consumption and pollution, and in these ways, increased awareness may be generated. These mitigation strategies will further help in ensuring good health, food security, and environmental sustainability. This article summarises the nature of the impact of arsenic on plants, the physio-biochemical mechanisms evolved to cope with As stress, and the mitigation measures that can be employed to eliminate the negative effects of As.

Mapping QTLs and gene validation studies for Mg2+ uptake and translocation using a MAGIC population in rice

Magnesium (Mg) is an essential element for plant growth and development. Rice is an important food crop in the world, but there are few studies on the uptake and translocation of Mg²⁺ in rice. We used a multi-parent advanced generation inter-cross (MAGIC) population constructed using four parental lines and genotyped by a 55 K rice SNP array for association analysis to locate QTLs related to Mg²⁺ uptake and translocation in rice at the seedling stage. Four QTLs (qRMg1, qRMg2, qRMg7 and qRMg8) were detected for the root Mg²⁺ concentration, which explained 11.45-13.08% of the phenotypic variation. The Mg²⁺ transporter gene, OsMGT1, was within the region of qRMg1. Three QTLs (qSMg3, qSMg7 and qSMg10) were detected for the shoot Mg²⁺ concentration, which explained 4.30-5.46% of the phenotypic variation. Two QTLs (qTrMg3 and qTrMg8) were found to affect the translocation of Mg²⁺ from the roots to the shoots, and explained 10.91% and 9.63% of phenotypic variation. qSMg3 and qTrMg3 might be the same, since they are very close to each other on chromosome 3. Analysis of candidate genes in the region of qSMg3 and qTrMg3 through qRT-PCR, complementation assay in the yeast Mg²⁺ transport-defective mutant CM66, and sequence analysis of the parental lines suggested that LOC_Os03g04360 may play important roles in Mg²⁺ uptake, translocation and accumulation in rice. Overexpression of LOC_Os03g04360 can significantly increase the Mg²⁺ concentration in rice seedlings, especially under the condition of low Mg²⁺ supply.

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.

The involvement of hydrogen sulphide in melatonin-induced tolerance to arsenic toxicity in pepper (Capsicum annuum L.) plants by regulating sequestration and subcellular distribution of arsenic, and antioxidant defense system

Melatonin (MT) and hydrogen sulphide (H₂S) are recognised as vital biomolecules actively taking part in plant defence systems as free radical scavengers and antioxidants against a myriad of biotic and abiotic stressors. However, it has been yet unknown in plants subjected to arsenic (As) toxicity whether or not H₂S interacts with MT to regulate endogenous antioxidant defence system. Prior to beginning As stress (As-S) treatments, MT (0.10 mM) was applied externally to plants daily for three days. AsS was then started for two weeks with As(V) (0.1 mM as Na₂HAsO₄·7H₂O). The treatment of As reduced plant biomass (24.4%) and chlorophyll a (51.7%), chlorophyll b (25.9%), while it increased subcellular As in roots and leaves, levels of glutathione (GSH), hydrogen peroxide (H₂O₂), malondialdehyde (MDA), methylglyoxal (MG), H₂S and phytochelatins (PCs) in pepper plants. In As-stressed pepper plants, the application of MT increased plant biomass (16.3%), chlorophyll a (52.7%), chlorophyll b (28.2%), antioxidant enzymes' activities, and H₂S accumulation, while it lowered the concentrations of MDA and H₂O₂. In As-treated plants, GSH and phytochelatins (PCs) were increased by MT by regulating As sequestration to make it harmless. The addition of MT increased As accumulation in the vacuoles of roots and caused the soluble fraction of As in vacuoles to become less toxic to vital organelles. MT-induced tolerance to As stress was further enhanced using NaHS, a source of H₂S. Hypotaurine (0.1 mM HT), a H₂S scavenger, was applied to the control and As-stressed plants together with MT and MT + NaHS to determine whether H₂S was implicated in MT-induced increased As-S tolerance. By reducing H₂S generation in pepper plants, HT counteracted the beneficial effects of MT, whereas the addition of NaHS to MT + HT restored the negative effects of HT, proving that H₂S is necessary for the pepper plants As-stress tolerance caused by MT.

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 (Se_1.25) 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 Se_1.25 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 Se_1.25 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.

Novel Mycorrhiza-Specific P Transporter PvPht1;6 Contributes to As Accumulation at the Symbiotic Interface of As-Hyperaccumulator Pteris vittata

Arsenic (As) is toxic and ubiquitous in the environment, posing a growing threat to human health. As-hyperaccumulator Pteris vittata has been used for phytoremediation of As-contaminated soil. Symbiosis with arbuscular mycorrhizal fungi (AMF) enhances As accumulation by P. vittata, which is different from As inhibition in typical plants. In this study, P. vittata seedlings inoculated with or without AMF were cultivated in As-contaminated soils for 2 months. AMF-root symbiosis enhanced plant growth, with 64.5% greater As contents in the fronds. After exposure to AsV for 2 h, the arsenate (AsV) and arsenite (AsIII) contents in AMF-roots increased by 1.8- and 3.6-fold, suggesting more efficient As uptake by P. vittata with AMF-roots. Plants take up and transport AsV via phosphate transporters (Phts). Here, for the first time, we identified a novel mycorrhiza-specific Pht transporter, PvPht1;6, from P. vittata. The transcripts of PvPht1;6 were strongly induced in AMF-roots, which were localized to the plasma membrane of arbuscule-containing cells. By complementing a yeast mutant lacking 5-Phts, we confirmed PvPht1;6's transport activity for both P and AsV. In contrast to typical AMF-inducible phosphate transporter LePT4 from tomato, PvPht1;6 showed greater AsV transport capacity. The results suggest that PvPht1;6 is probably critical for AsV transport at the periarbuscular membrane of P. vittata root cells, revealing the underlying mechanism of efficient As accumulation in P. vittata with AMF-roots.

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.

Soil texture and climate limit cultivation of the arsenic hyperaccumulator Pteris vittata for phytoextraction in a long-term field study

Long term field studies are required to bridge gaps between research and practical application of arsenic phytoextraction with the arsenic-hyperaccumulating fern Pteris vittata. In a 4-year field study, we investigated the effects of nutrient application (compost, inorganic or organic nitrogen, inorganic or organic phosphorus) and soil texture (13 % and 35 % clay) on arsenic phytoextraction with P. vittata in moderately contaminated soils (74-79 mg As/kg in the 0-15 cm depth interval). We found the highest phytoextraction rates, 5 ± 1 kg As/ha/y, in a coarse-textured compost-amended soil after 2 years of phytoextraction. Phytoextraction rates decreased over time, likely due to decreased root growth in mature stands, indicating plants should be replaced every 2-3 years to maintain phytoextraction efficiency. Across soil textures, nitrogen or phosphorus application led to a 60 % decrease in mean frond arsenic concentrations, leading to mean phytoextraction rates 54 % lower than in control ferns. In the fine-textured soil, frond arsenic concentrations were 54 % lower than in the coarse-textured soil, and fewer ferns survived from year 3 to 4. Across soil textures, compost application increased fern survival. We show that phytoextraction with P. vittata is limited to specific soil and climate conditions, narrower than those under which P. vittata grows in the wild.

Rhizospheric plant-microbe synergistic interactions achieve efficient arsenic phytoextraction by Pteris vittata

Phytoextraction is a cost-effective and eco-friendly technology to remove arsenic (As) from contaminated soil using plants and associated microorganisms. Pteris vittata is the most studied As hyperaccumulator, which effectively takes up inorganic arsenate via roots. Arsenic solubilization and speciation occur prior to plant absorption in the rhizosphere, which play a key role in As phytoextraction by P. vittata. This study investigated the metabolomic correlation of P. vittata and associated rhizospheric microorganisms during As phytoextraction. Three-month pot cultivation of P. vittata in As polluted soil was conducted. In rhizosphere, an increase of water-soluble As concentration and a decrease of pH was observed in the second month, suggesting acidic metabolites as a possible cause of As solubilization. A correlation network was built to elucidate the interactions among metabolites, bacteria and fungi in the rhizosphere of P. vittata. Our results demonstrate that the plant is the major driving force of rhizospheric microbiota generation, and both microbial community and metabolites in rhizosphere of P. vittata correlate to increased bioavailable As. Multi-omics analysis revealed that pterosins enrich microbes that potentially promote As phytoextraction. This study extends the current view of rhizospheric plant-microbes synergistic effects of hyperaccumulators on phytoextraction, which provides clues for developing efficient As phytoremediation approaches.

Molecular insight into arsenic uptake, transport, phytotoxicity, and defense responses in plants: a critical review

A critical investigation into arsenic uptake and transportation, its phytotoxic effects, and defense strategies including complex signaling cascades and regulatory networks in plants. The metalloid arsenic (As) is a leading pollutant of soil and water. It easily finds its way into the food chain through plants, more precisely crops, a common diet source for humans resulting in serious health risks. Prolonged As exposure causes detrimental effects in plants and is diaphanously observed through numerous physiological, biochemical, and molecular attributes. Different inorganic and organic As species enter into the plant system via a variety of transporters e.g., phosphate transporters, aquaporins, etc. Therefore, plants tend to accumulate elevated levels of As which leads to severe phytotoxic damages including anomalies in biomolecules like protein, lipid, and DNA. To combat this, plants employ quite a few mitigation strategies such as efficient As efflux from the cell, iron plaque formation, regulation of As transporters, and intracellular chelation with an array of thiol-rich molecules such as phytochelatin, glutathione, and metallothionein followed by vacuolar compartmentalization of As through various vacuolar transporters. Moreover, the antioxidant machinery is also implicated to nullify the perilous outcomes of the metalloid. The stress ascribed by the metalloid also marks the commencement of multiple signaling cascades. This whole complicated system is indeed controlled by several transcription factors and microRNAs. This review aims to understand, in general, the plant-soil-arsenic interaction, effects of As in plants, As uptake mechanisms and its dynamics, and multifarious As detoxification mechanisms in plants. A major portion of this article is also devoted to understanding and deciphering the nexus between As stress-responsive mechanisms and its underlying complex interconnected regulatory networks.

Characterization of a novel arsenite long-distance transporter from arsenic hyperaccumulator fern Pteris vittata

Pteris vittata is an arsenic (As) hyperaccumulator that can accumulate several thousand mg As kg^-1 DW in aboveground biomass. A key factor for its hyperaccumulation ability is its highly efficient As long-distance translocation system. However, the underlying molecular mechanisms remain unknown. We isolated PvAsE1 through the full-length cDNA over-expression library of P. vittata and characterized it through a yeast system, RNAi gametophytes and sporophytes, subcellular-location and in situ hybridization. Phylogenomic analysis was conducted to estimate the appearance time of PvAsE1. PvAsE1 was a plasma membrane-oriented arsenite (AsIII) effluxer. The silencing of PvAsE1 reduced AsIII long-distance translocation in P. vittata sporophytes. PvAsE1 was structurally similar to solute carrier (SLC)13 proteins. Its transcripts could be observed in parenchyma cells surrounding the xylem of roots. The appearance time was estimated at c. 52.7 Ma. PvAsE1 was a previously uncharacterized SLC13-like AsIII effluxer, which may contribute to AsIII long-distance translocation via xylem loading. PvAsE1 appeared late in fern evolution and might be an adaptive subject to the selection pressure at the Cretaceaou-Paleogene boundary. The identification of PvAsE1 provides clues for revealing the special As hyperaccumulation characteristics of P. vittata.

Heavy metal transporters: Functional mechanisms, regulation, and application in phytoremediation

Heavy metal pollution in soil is a global problem with serious impacts on human health and ecological security. Phytoextraction in phytoremediation, in which plants uptake and transport heavy metals (HMs) to the tissues of aerial parts, is the most environmentally friendly method to reduce the total amount of HMs in soil and has wide application prospects. However, the molecular mechanism of phytoextraction is still under investigation. The uptake, translocation, and retention of HMs in plants are mainly mediated by a variety of transporter proteins. A better understanding of the accumulation strategy of HMs via transporters in plants is a prerequisite for the improvement of phytoextraction. In this review, the biochemical structure and functions of HM transporter families in plants are systematically summarized, with emphasis on their roles in phytoremediation. The accumulation mechanism and regulatory pathways related to hormones, regulators, and reactive oxygen species (ROS) of HMs concerning these transporters are described in detail. Scientific efforts and practices for phytoremediation carried out in recent years suggest that creation of hyperaccumulators by transgenic or gene editing techniques targeted to these transporters and their regulators is the ultimate powerful path for the phytoremediation of HM contaminated soils.

A Panax notoginseng phosphate transporter, PnPht1;3, greatly contributes to phosphate and arsenate uptake

The crisis of arsenic (As) accumulation in rhizomes threatens the quality and safety of Panax notoginseng (Burk.) F.H. Chen, which is a well-known traditional Chinese herb with a long clinical history. The uptake of arsenate (AsV) could be suppressed by supplying phosphate (Pi), in which Pi transporters play important roles in the uptake of Pi and AsV. Herein, the P . notoginseng Pi transporter-encoding gene PnPht1;3 was identified and characterised under Pi deficiency and AsV exposure. In this study, the open reading frame (ORF) of PnPht1;3 was cloned according to RNA-seq and encoded 545 amino acids. The relative expression levels revealed that PnPht1;3 was significantly upregulated under phosphate deficiency and AsV exposure. Heterologous expression in Saccharomyces cerevisiae MB192 demonstrated that PnPht1;3 performed optimally in complementing the yeast Pi-transport defect and accumulated more As in the cells. Combined with the subcellular localisation prediction, it was concluded that PnPht1;3 encodes a functional plasma membrane-localised transporter protein that mediates putative high-affinity Pi/H+ symport activity and enhances the uptake of Pi and AsV. Therefore, a better understanding of the roles of the P . notoginseng Pi transporter could provide new insight for solving As accumulation in medicinal plants.

Biochar composite with microbes enhanced arsenic biosorption and phytoextraction by Typha latifolia in hybrid vertical subsurface flow constructed wetland

Arsenic contamination of ground water is a worldwide issue, causing a number of ailments in humans. As an engineered and integrated solution, a hybrid vertical subsurface flow constructed wetland (VSSF-CW) amended with BCXZM composite (Bacillus XZM immobilized on rice husk biochar), was found effective for the bioremediation of arsenic contaminated water. Biological filter was prepared by amending top 3 cm of VSSF-CW bed with BCXZM. This filter scavenged ∼64% of total arsenic and removal efficiency of ∼95% was achieved by amended and planted (As + P + B) VSSF-CW, while non-amended (As + P) VSSF-CW showed a removal efficiency of ∼55%. The unplanted and amended (As + B) VSSF-CW showed a removal efficiency of ∼70%. The symbiotic association of Bacillus XZM, confirmed by SEM micrographs, significantly (p ≤ 0.05) reduced reactive oxygen species (ROS) and malondialdehyde (MDA) accumulation in Typha latifolia, hence, increasing the plant growth (2 folds). An increase in the indole acetic acid (IAA) and arsenic accumulation in plant was also observed in As + P + B system. The removal efficiency of the system was compromised after 4th consecutive cycle and 48 h was observed as optimum retention time. The FTIR-spectra showed the involvement of -N-H bond, carboxylic acids, -CH₂ stretching of -CH₂ and -CH_3, carbonyl groups, -C-H, C-O-P and C-O-C, sulphur/thiol and phosphate functional groups in the bio-sorption of arsenic by BCXZM filter. Our study is a first reported on the simultaneous phytoextraction and biosorption of arsenic in a hybrid VSSF-CW. It is proposed that BCXZM can be applied effectively in CWs for the bioremediation of arsenic contaminated water on large scale.

Metalloid transporters and their regulation in plants

Transport of metalloids including B, Si, and As is mediated by a combination of channels and efflux transporters in plants, which are strictly regulated in response to environmental changes.

Arsenic acquisition, toxicity and tolerance in plants - From physiology to remediation: A review

Globally, environmental contamination by potentially noxious metalloids like arsenic is becoming a critical concern to the living organisms. Arsenic is a non-essential metalloid for plants and can be acclimatised in plants to toxic levels. Arsenic acquisition by plants poses serious health risks in human due to its entry in the food chain. High arsenic regimes disturb plant water relations, promote the generation of reactive oxygen species (ROS) and induce oxidative outburst in plants. This review evidences a conceivable tie-up among arsenic levels, speciation, its availability, uptake, acquisition, transport, phytotoxicity and arsenic detoxification in plants. The role of different antioxidant enzymes to confer plant tolerance towards the enhanced arsenic distress has also been summed up. Additionally, the mechanisms involved in the modulation of different genes coupled with arsenic tolerance have been thoroughly discussed. This review is intended to present an overview to rationalise the contemporary progressions on the recent advances in phytoremediation approaches to overcome ecosystem contamination by arsenic.

Arsenic Remediation through Sustainable Phytoremediation Approaches

Arsenic contamination of the environment is a serious problem threatening the health of millions of people exposed to arsenic (As) via drinking water and crops grown in contaminated areas. The remediation of As-contaminated soil and water bodies needs to be sustainable, low-cost and feasible to apply in the most affected low-to-middle income countries, like India and Bangladesh. Phytoremediation is an aesthetically appreciable and successful approach that can be used for As decontamination with use of the best approach(es) and the most promising plant(s). However, phytoremediation lacks the required speed and sometimes the stress caused by As could diminish plants’ potential for remediation. To tackle these demerits, we need augment plants’ potential with appropriate technological methods including microbial and nanoparticles applications and genetic modification of plants to alleviate the As stress and enhance As accumulation in phytoremediator plants. The present review discusses the As phytoremediation prospects of soil and water bodies and the usefulness of various plant systems in terms of high biomass, high As accumulation, bioenergy potential, and economic utility. The potential and prospects of assisted phytoremediation approaches are also presented.

Rhizobium symbiosis modulates the accumulation of arsenic in Medicago truncatula via nitrogen and NRT3.1-like genes regulated by ABA and linalool

Arsenic (As) contamination is a worldwide problem and threatens human health. Here, we found that Rhizobium symbiosis can improve the tolerance to arsenate [As(V)], and a wild type R. meliloti Rm5038 symbiosis can significantly decrease the accumulation of As in Medicago truncatula shoots. The As content in plants could be decreased by nitrogen and the mutation of nitrate transporter NRT3.1. The expression of M. truncatula NRT3.1-like gene NRT3.1L1 could reverse the As(V)-tolerance phenotype of the Arabidopsis nrt3.1 mutant. Rm5038 symbiosis significantly increased the level of nitrogen in the shoot and reduced the expression of NRT3.1Ls in plants afflicted by As(V). The genetic analyses of aba2-1, pyr1/pyl1/2/4/5/8, and abi1-2/abi2-2/hab1-1/pp2ca-1 mutants revealed that abscisic acid (ABA) signaling regulates the tolerance of plants to As(V). ABA and linalool could promote the expression of NRT3.1Ls, however, their root biosynthesis was inhibited by ammonium, the first form of nitrogen fixed by Rhizobium symbiosis. Moreover, ABA and linalool may also control As and nitrate accumulation in Rhizobium symbionts via signaling pathways other than ammonia and NRT3.1Ls. Thus, Rhizobium symbiosis modulates the accumulation of As in plants via nitrogen and NRT3.1Ls regulated by ABA and linalool, which provides novel approaches to reduce As accumulation in legume crops.

Novel PvACR3;2 and PvACR3;3 genes from arsenic-hyperaccumulator Pteris vittata and their roles in manipulating plant arsenic accumulation

Arsenite (AsIII) antiporter ACR3 is crucial for arsenic (As) translocation and sequestration in As-hyperaccumulator Pteris vittata, which has potential for phytoremediation of As-contaminated soils. In this study, two new ACR3 genes PvACR3;2 and PvACR3;3 were cloned from P. vittata and studied in model organism yeast (Saccharomyces cerevisiae) and model plant tobacco (Nicotiana tabacum). Both ACR3s mediated AsIII efflux in yeast, decreasing its As accumulation and enhancing its As tolerance. In addition, PvACR3;2 and PvACR3;3 were expressed in tobacco plant. Localized on the plasma membrane, PvACR3;2 mediated both AsIII translocation to the shoots and AsIII efflux from the roots in tobacco, resulting in 203 - 258% increase in shoot As after exposing to 5 μM AsIII under hydroponics. In comparison, localized to the vacuolar membrane, PvACR3;3 sequestrated AsIII in tobacco root vacuoles, leading to 18 - 20% higher As in the roots and 15 - 36% lower As in the shoots. Further, based on qRT-PCR, both genes were mainly expressed in P. vittata fronds, indicating PvACR3;2 and PvACR3;3 may play roles in AsIII translocation and sequestration in the fronds. This study provides not only new insights into the functions of new ACR3 genes in P. vittata, but also important gene resources for manipulating As accumulation in plants for phytoremediation and food safety.

New evidence of arsenic translocation and accumulation in Pteris vittata from real-time imaging using positron-emitting 74As tracer

Pteris vittata is an arsenic (As) hyperaccumulator plant that accumulates a large amount of As into fronds and rhizomes (around 16,000 mg/kg in both after 16 weeks hydroponic cultivation with 30 mg/L arsenate). However, the sequence of long-distance transport of As in this hyperaccumulator plant is unclear. In this study, we used a positron-emitting tracer imaging system (PETIS) for the first time to obtain noninvasive serial images of As behavior in living plants with positron-emitting ⁷⁴As-labeled tracer. We found that As kept accumulating in rhizomes as in fronds of P. vittata, whereas As was retained in roots of a non-accumulator plant Arabidopsis thaliana. Autoradiograph results of As distribution in P. vittata showed that with low As exposure, As was predominantly accumulated in young fronds and the midrib and rachis of mature fronds. Under high As exposure, As accumulation shifted from young fronds to mature fronds, especially in the margin of pinna, which resulted in necrotic symptoms, turning the marginal color to gray and then brown. Our results indicated that the function of rhizomes in P. vittata was As accumulation and the regulation of As translocation to the mature fronds to protect the young fronds under high As exposure.

Arsenic Accumulation in Hydroponically Grown Schizachyrium scoparium (Little Bluestem) Amended with Root-Colonizing Endophytes

We integrated microscopy, spectroscopy, culturing and molecular biology, and aqueous chemistry techniques to evaluate arsenic (As) accumulation in hydroponically grown Schizachyrium scoparium inoculated with endophytic fungi. Schizachyrium scoparium grows in historically contaminated sediment in the Cheyenne River Watershed and was used for laboratory experiments with As(V) ranging from 0 to 2.5 mg L^-1 at circumneutral pH. Arsenic accumulation in regional plants has been a community concern for several decades, yet mechanisms affecting As accumulation in plants associated with endophytic fungi remain poorly understood. Colonization of roots by endophytic fungi supported better external and vascular cellular structure, increased biomass production, increased root lengths and increased P uptake, compared to noninoculated plants (p value <0.05). After exposure to As(V), an 80% decrease of As was detected in solution and accumulated mainly in the roots (0.82-13.44 mg kg^-1) of noninoculated plants. Endophytic fungi mediated intracellular uptake into root cells and translocation of As. Electron microprobe X-ray mapping analyses detected Ca-P and Mg-P minerals with As on the root surface of exposed plants, suggesting that these minerals could lead to As adsorption on the root surface through surface complexation or coprecipitation. Our findings provide new insights regarding biological and physical-chemical processes affecting As accumulation in plants for risk assessment applications and bioremediation strategies.

Metalloid hazards: From plant molecular evolution to mitigation strategies

Metalloids such as boron and silicon are key elements for plant growth and crop productivity. However, toxic metalloids such as arsenic are increasing in the environment due to inputs from natural sources and human activities. These hazardous metalloids can cause serious health risks to humans and animals if they enter the food chain. Plants have developed highly regulated mechanisms to alleviate the toxicity of metalloids during their 500 million years of evolution. A better understanding the molecular mechanisms underlying the transport and detoxification of toxic metalloids in plants will shed light on developing mitigation strategies. Key transporters and regulatory proteins responsive to toxic metalloids have been identified through evolutionary and molecular analyses. Moreover, knowledge of the regulatory proteins and their pathways can be used in the breeding of crops with lower accumulation of metalloids. These findings can also assist phytoremediation by the exploration of plants such as fern species that hyperaccumulate metalloids from soils and water, and can be used to engineer plants with elevated uptake and storage capacity of toxic metalloids. In summary, there are solutions to remediate contamination due to toxic metalloids by combining the research advances and industrial technologies with agricultural and environmental practices.

Arsenic accumulation and speciation in two cultivars of Pteris cretica L. and characterization of arsenate reductase PcACR2 and arsenite transporter PcACR3 genes in the hyperaccumulating cv. Albo-lineata

Pollution and poisoning with carcinogenic arsenic (As) is of major concern globally. Interestingly, there are ferns that can naturally tolerate remarkably high As concentrations in soils while hyperaccumulating this metalloid in their fronds. Besides Pteris vittata in which As-related traits and molecular determinants have been studied in detail, the As hyperaccumulation status has been attributed also to Pteris cretica. We thus inspected two P. cretica cultivars, Parkerii and Albo-lineata, for As hyperaccumulation traits. The cultivars were grown in soils supplemented with 20, 100, and 250 mg kg-1 of inorganic arsenate (iAsV). Unlike Parkerii, Albo-lineata was confirmed to be As tolerant and hyperaccumulating, with up to 1.3 and 6.4 g As kg-1 dry weight in roots and fronds, respectively, from soils amended with 250 mg iAsV kg-1. As speciation analyses rejected that organoarsenical species and binding with phytochelatins and other proteinaceous ligands would play any significant role in the biology of As in either cultivar. While in Parkerii, the dominating As species, particularly in roots, occurred as iAsV, in Albo-lineata the majority of the root and frond As was apparently converted to iAsIII. Parkerii markedly accumulated iAsIII in its fronds when grown on As spiked soils. Considering the roles iAsV reductase ACR2 and iAsIII transporter ACR3 may have in the handling of iAs, we isolated Albo-lineata PcACR2 and PcACR3 genes closely related to P. vittata PvACR2 and PvACR3. The gene expression analysis in Albo-lineata fronds revealed that the transcription of PcACR2 and PcACR3 was clearly As responsive (up to 6.5- and 45-times increase in transcript levels compared to control soil conditions, respectively). The tolerance and uptake assays in yeasts showed that PcACRs can complement corresponding As-sensitive mutations, indicating that PcACR2 and PcACR3 encode functional proteins that can perform, respectively, iAsV reduction and membrane iAsIII transport tasks in As-hyperaccumulating Albo-lineata.

Expressing Phosphate Transporter PvPht2;1 Enhances P Transport to the Chloroplasts and Increases Arsenic Tolerance in Arabidopsis thaliana

Arsenic (As) contamination in soils is of great concerns due to its toxicity to plants. As an analogue, phosphorus plays an important role in protecting plants from As toxicity. In this study, we identified a new phosphate transporter 2 (PHT2), PvPht2;1, from As-hyperaccumulator Pteris vittata and analyzed its functions in As and P transport in a yeast mutant, and model plant Arabidopsis thalian. PvPht2;1 contained 12 transmembrane domains, sharing high identity with PHT2 genes in diverse plants. Further, independent of external P or As levels, PvPht2;1 was mainly expressed in P. vittata fronds with the expression being 3-4 folds higher than that in the roots and rhizomes. Localized to the chloroplasts based on GFP-fused PvPht2;1 in model plant tobacco, PvPht2;1 functioned as a low-affinity P transporter. Under As exposure, PvPht2;1 yeast transformants showed comparable growth with the control while high-affinity P transporter PvPht1;3 transformants showed better growth, suggesting that PvPht2;1 transported P but slower than PvPht1;3 transporter. Expressing PvPht2;1 in A. thaliana increased its shoot P concentration without influencing its As accumulation. Further, the chloroplasts' P content in transgenic A. thaliana increased by 37-59% than wild-type (WT) plants. Under As exposure, the photosynthesis of PvPht2;1-expressing A. thaliana remained stable but that of WT plants decreased. The data indicate that, under As stress, expressing PvPht2;1 in A. thaliana enhanced its P transport to the chloroplasts and protected its photosynthesis. In short, highly expressed in the fronds and not impacted by As exposure, chloroplast-located PvPht2;1 may have protected As-hyperaccumulator P. vittata from As toxicity by efficiently transporting only P to its chloroplasts.

Evolution of Abscisic Acid Signaling for Stress Responses to Toxic Metals and Metalloids

Toxic heavy metals and metalloids in agricultural ecosystems are crucial factors that limit global crop productivity and food safety. Industrial toxic heavy metals and metalloids such as cadmium, lead, and arsenic have contaminated large areas of arable land in the world and their accumulation in the edible parts of crops is causing serious health risks to humans and animals. Plants have co-evolved with various concentrations of these toxic metals and metalloids in soil and water. Some green plant species have significant innovations in key genes for the adaptation of abiotic stress tolerance pathways that are able to tolerate heavy metals and metalloids. Increasing evidence has demonstrated that phytohormone abscisic acid (ABA) plays a vital role in the alleviation of heavy metal and metalloid stresses in plants. Here, we trace the evolutionary origins of the key gene families connecting ABA signaling with tolerance to heavy metals and metalloids in green plants. We also summarize the molecular and physiological aspects of ABA in the uptake, root-to-shoot translocation, chelation, sequestration, reutilization, and accumulation of key heavy metals and metalloids in plants. The molecular evolution and interaction between the ABA signaling pathway and mechanisms for heavy metal and metalloid tolerance are highlighted in this review. Therefore, we propose that it is promising to manipulate ABA signaling in plant tissues to reduce the uptake and accumulation of toxic heavy metals and metalloids in crops through the application of ABA-producing bacteria or ABA analogues. This may lead to improvements in tolerance of major crops to heavy metals and metalloids.