The arsenic hyperaccumulating Pteris vittata expresses two arsenate reductases

From genes to ecosystems: Decoding plant tolerance mechanisms to arsenic stress

Arsenic (As), a metalloid of considerable toxicity, has become increasingly bioavailable through anthropogenic activities, raising As contamination levels in groundwater and agricultural soils worldwide. This bioavailability has profound implications for plant biology and farming systems. As can detrimentally affect crop yield and pose risks of bioaccumulation and subsequent entry into the food chain. Upon exposure to As, plants initiate a multifaceted molecular response involving crucial signaling pathways, such as those mediated by calcium, mitogen-activated protein kinases, and various phytohormones (e.g., auxin, methyl jasmonate, cytokinin). These pathways, in turn, activate enzymes within the antioxidant system, which combat the reactive oxygen/nitrogen species (ROS and RNS) generated by As-induced stress. Plants exhibit a sophisticated genomic response to As, involving the upregulation of genes associated with uptake, chelation, and sequestration. Specific gene families, such as those coding for aquaglyceroporins and ABC transporters, are key in mediating As uptake and translocation within plant tissues. Moreover, we explore the gene regulatory networks that orchestrate the synthesis of phytochelatins and metallothioneins, which are crucial for As chelation and detoxification. Transcription factors, particularly those belonging to the MYB, NAC, and WRKY families, emerge as central regulators in activating As-responsive genes. On a post-translational level, we examine how ubiquitination pathways modulate the stability and function of proteins involved in As metabolism. By integrating omics findings, this review provides a comprehensive overview of the complex genomic landscape that defines plant responses to As. Knowledge gained from these genomic and epigenetic insights is pivotal for developing biotechnological strategies to enhance crop As tolerance.

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.

Arsenic accumulation in Pteris vittata: Time course, distribution, and arsenic-related gene expression in fronds and whole plantlets

In this work, arsenic (As) accumulation and distribution over time in Pteris vittata young fronds from adult plants and in whole plantlets, grown on a highly contaminated As-soil, was determined by μ-XRF. A linear increase in As content up to 60 days was found in young fronds at different times, and a progressive distribution from the apex to the base of the fronds was observed. In whole plantlets, As signal was detectable from 9 to 20 days in the apex of a few fronds and fiddleheads. Later, up to 60 days, As was localized in all fronds, in the rhizome and in basal part of the roots. The dynamics of expression of As-related genes revealed a good correlation between As content and the level of the As (III)-antiporter PvACR3 transcript in plantlets roots and fronds and in young fronds. Moreover, the transcription of As (V)-related gametophytic genes PvGAPC1, PvOCT4 increases over time during As accumulation while PvGSTF1 is expressed only in roots. Here, we demonstrate the suitability of the μ-XRF technique to monitor As accumulation, which allowed us to propose that As is initially directly transported to fiddleheads and apex of fronds, is later distributed to the whole fronds and simultaneously accumulated in the rhizome and roots. We also provide indications on the expression of candidate genes possibly involved in As (hyper)accumulation.

Arsenic transport and interaction with plant metabolism: Clues for improving agricultural productivity and food safety

Arsenic (As) is a ubiquitous metalloid that is highly toxic to all living organisms. When grown in As-contaminated soils, plants may accumulate significant amounts of As in the grains or edible shoot parts which then enter a food chain. Plant growth and development per se are also both affected by arsenic. These effects are traditionally attributed to As-induced accumulation of reactive oxygen species (ROS) and a consequent lipid peroxidation and damage to cellular membranes. However, this view is oversimplified, as As exposure have a major impact on many metabolic processes in plants, including availability of essential nutrients, photosynthesis, carbohydrate metabolism, lipid metabolism, protein metabolism, and sulfur metabolism. This review is aimed to fill this gap in the knowledge. In addition, the molecular basis of arsenic uptake and transport in plants and prospects of creating low As-accumulating crop species, for both agricultural productivity and food safety, are discussed.

Root responses to localised soil arsenic enrichment in the fern Pityrogramma calomelanos var. austroamericana grown in rhizoboxes

The terrestrial fern Pityrogramma calomelanos, a cosmopolitan tropical species, is one of the strongest known arsenic (As) hyperaccumulator plants. This study aimed to determine whether P. calomelanos preferentially forages for arsenite (As³⁺) or arsenate (As⁵⁺) in As-contaminated soils, and whether a positive root response to As enhances accumulation in P. calomelanos. Therefore, an experiment using rhizoboxes divided in two halves were constructed with a control soil (C) and As³⁺ or As⁵⁺ dosed soil at either 50 and 100 μg g^-1 As. Micro-X-ray Fluorescence elemental mapping (μXRF) was employed to analyze the distribution of As in roots and fronds, and Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS) was used to determine As distribution in the reproductive tissues of P. calomelanos. The results showed that Pityrogramma roots do not specifically forage for As-contaminated soil; the area based on pixel counts was similar across all the treatments with no statistical differences. However, frond biomass was slightly higher in the treatments C ǀ As³⁺ and C ǀ As⁵⁺, and the highest accumulation of As in fronds was in the As⁵⁺ ǀ As³⁺ (100 μg g^-1) treatment, with 3418 and 2370 μg g^-1 in old and young fronds respectively. Arsenic cycling across the roots was observed by the μXRF mapping; in C ǀ As⁵⁺ (100) the As was higher and evenly distributed in both sections, whilst in C ǀ As³⁺ (50), the As was higher in the As³⁺ side. The μXRF mapping showed a broader As distribution in older fronds, where As was highest in the rachis and extended into the pinnule through the midrib. Pityrogramma calomelanos does not specifically root forage for As-enriched zones in the soil and grows healthily without signs of toxicity at lower (50 μg g^-1) and higher (100 μg g^-1) concentrations of As³⁺ and As⁵⁺ in the soil.

Synchrotron micro-X-ray fluorescence imaging of arsenic in frozen-hydrated sections of a root of Pteris vittata

We performed micro-X-ray fluorescence imaging of frozen-hydrated sections of a root of Pteris vittata for the first time, to the best of our knowledge, to reveal the mechanism of arsenic (As) uptake. The As distribution was successfully visualized in cross sections of different parts of the root, which showed that (i) the major pathway of As uptake changes from symplastic to apoplastic transport in the direction of root growth, and (ii) As and K have different mobilities around the stele before xylem loading, despite their similar distributions outside the stele in the cross sections. These data can reasonably explain As reduction, axially observed around the root tip in the direction of root growth and radially observed in the endodermis in the cross sections, as a consequence of the incorporation of As into the cells or symplast of the root. In addition, previous observations of As species in the midrib can be reconciled by ascribing a reduction capacity to the root cells, which implies that As reduction mechanisms at the cellular level may be an important control on the peculiar root-to-shoot transport of As in P. vittata.

Efficient arsenate reduction in As-hyperaccumulator Pteris vittata are mediated by novel arsenate reductases PvHAC1 and PvHAC2

Arsenic-hyperaccumulator Pteris vittata is efficient in As absorption, reduction, and translocation. But the molecular mechanisms and locations of arsenate (AsV) reduction in P. vittata are still unclear. Here, we identified two new arsenate reductase genes from P. vittata, PvHAC1 and PvHAC2. Two PvHAC genes encoded a rhodanase-like protein, which were localized in the cytoplasm and nucleus. Both recombinant Escherichia coli strains and transgenic Arabidopsis thaliana lines showed arsenate reductase ability after expressing PvHAC genes. Further, expressing PvHAC2 enhanced As tolerance and reduced As accumulation in A. thaliana shoots under AsV exposure. Based on expression pattern analysis, PvHAC1 and PvHAC2 were predominantly expressed in the rhizomes and fronds of P. vittata. Different from those of HAC homologous genes in non-hyperaccumulators, little PvHAC was expressed in the roots. Besides, PvHAC1 expression was strongly upregulated under AsV exposure but not AsIII. The data suggest that arsenate reductase PvHAC1 in the rhizomes coupled with arsenate reductase PvHAC2 in the fronds of P. vittata played a critical role in As-hyperaccumulation by P. vittata, which helps to further improve its utility in phytoremediation of As-contaminated soils.

Arsenic redox transformations and cycling in the rhizosphere of Pteris vittata and Pteris quadriaurita

Pteris vittata (PV) and Pteris quadriaurita (PQ) are reported to hyperaccumulate arsenic (As) when grown in Asrich soil. Yet, little is known about the impact of their unique As accumulation mechanisms on As transformations and cycling at the soil-root interface. Using a combined approach of two-dimensional (2D), sub-mm scale solute imaging of arsenite (As^III), arsenate (As^V), phosphorus (P), manganese (Mn), iron (Fe) and oxygen (O₂), we found localized patterns of As^III/As^V redox transformations in the PV rhizosphere (As^III/As^V ratio of 0.57) compared to bulk soil (As^III/As^V ratio of ≤0.04). Our data indicate that the high As root uptake, translocation and accumulation from the As-rich experimental soil (2080 mg kg^-1) to PV fronds (6986 mg kg^-1) induced As detoxification via As^V reduction and As^III root efflux, leading to As^III accumulation and re-oxidation to As^V in the rhizosphere porewater. This As cycling mechanism is linked to the reduction of O2 and Mn^III/IV (oxyhydr)oxides resulting in decreased O2 levels and increased Mn solubilization along roots. Compared to PV, we found 4-fold lower As translocation to PQ fronds (1611 mg kg^-1), 2-fold lower As^V depletion in the PQ rhizosphere, and no As^III efflux from PQ roots, suggesting that PQ efficiently controls As uptake to avoid toxic As levels in roots. Analysis of root exudates obtained from soil-grown PV showed that As acquisition by PV roots was not associated with phytic acid release. Our study demonstrates that two closely-related As-accumulating ferns have distinct mechanisms for As uptake modulating As cycling in As-rich environments.

Arsenic phytovolatilization and epigenetic modifications in Arundo donax L. assisted by a PGPR consortium

Arsenic-(As) pollution is an increasing threat across the globe and it is reaching harmful values in several areas of the world. In this perspective, we assayed bio-phyto-remediation technology using Arundo donax L., assisted by Plant Growth Promoting Bacteria (PGPB) consortium (BC) constituted of two strains of Stenotrophomonas maltophilia sp. and one of Agrobacterium sp.; furthermore, we assayed the epigenetic response to As pollution. The three bacterial strains initially evaluated for their As tolerance, revealed different resistance to both forms of As[As(III) and As(V)] however at concentration greater than those foreseen in the phytoremediation experiment (2.0, 10.0, 20.0 mgL^-1 of NaAsO₂). At the end of the trial plant biomass and As concentration were measured. Plants did not show any visible signs of toxicity, rather the leaf and stem biomass slightly increased in the presence of As and/or PGPBs; moreover, although the Bioaccumulation Factor was double in the presence of BC, the absolute values of As accumulation in the Arundo plants were very low, both in the presence or absence of BC and only detectable in the presence of the highest As dose (20 mgL^-1 As). In this case, regardless the presence of PGPB, ≈25% of As remained in the sand and ≈0.15% was accumulated in the plant, whilst the remaining 75% was volatilized by transpiration. Finally, the methylation sensitive amplified polymorphisms (MSAP) of leaves were analyzed in order to investigate their epigenetic response to As and/or BC. Our results suggest that epigenetic modifications are involved in stress response and As detoxification.

A review on global metal accumulators-mechanism, enhancement, commercial application, and research trend

The biosphere is polluted with metals due to burning of fossil fuels, pesticides, fertilizers, and mining. The metals interfere with soil conservations such as contaminating aqueous waste streams and groundwater, and the evidence of this has been recorded since 1900. Heavy metals also impact human health; therefore, the emancipation of the environment from these environmental pollutants is critical. Traditionally, techniques to remove these metals include soil washing, removal, and excavation. Metal-accumulating plants could be utilized to remove these metal pollutants which would be an alternative option that would simultaneously benefit commercially and at the same time clean the environment from these pollutants. Commercial application of pollutant metals includes biofortification, phytomining, phytoremediation, and intercropping. This review discusses about the metal-accumulating plants, mechanism of metal accumulation, enhancement of metal accumulation, potential commercial applications, research trends, and research progress to enhance the metal accumulation, benefits, and limitations of metal accumulators. The review identified that the metal accumulator plants only survive in low or medium polluted environments with heavy metals. Also, more research is required about metal accumulators in terms of genetics, breeding potential, agronomics, and the disease spectrum. Moreover, metal accumulators' ability to uptake metals need to be optimized by enhancing metal transportation, transformation, tolerance to toxicity, and volatilization in the plant. This review would benefit the industries and environment management authorities as it provides up-to-date research information about the metal accumulators, limitation of the technology, and what could be done to improve the metal enhancement in the future.

Hyperaccumulator Plants from China: A Synthesis of the Current State of Knowledge

Hyperaccumulator plants are the material basis for phytoextraction research and for practical applications in decontaminating polluted soils and industrial wastes. China's high biodiversity and substantial mineral resources make it a global hotspot for hyperaccumulator plant species. Intensive screening efforts over the past 20 years by researchers working in China have led to the discovery of many different hyperaccumulators for a range of elements. In this review, we present the state of knowledge on all currently reported hyperaccumulator species from China, including Cardamine hupingshanensis (selenium, Se), Dicranopteris dichotoma (rare earth elements, REEs), Elsholtzia splendens (copper, Cu), Phytolacca americana (manganese, Mn), Pteris vittata (arsenic, As), Sedum alfredii, and Sedum plumbizincicola (cadmium/zinc, Cd/Zn). This review covers aspects of the ecophysiology and molecular biology of tolerance and hyperaccumulation for each element. The major scientific advances resulting from the study of hyperaccumulator plants in China are summarized and synthesized.

Speciation and uptake of antimony and arsenic by two populations of Pteris vittata L. and Holcus lanatus L. from co-contaminated soil

This study aimed to investigate the Sb and As co-accumulating processes of Pteris vittata under soil culture condition, including the transformation of Sb and As, and the difference in co-accumulating ability among different plant species/populations. Two populations of P. vittata and one population of As-tolerant species Holcus lanatus L. were grown on soil co-contaminated by Sb and As. Sb and As speciation in plants was assessed by X-ray absorption near-edge structure (XANES) spectroscopy. P. vittata displayed strong As- but limited Sb-accumulating ability, with the highest shoot concentrations of As and Sb reaching 455 and 26 mg kg^-1, respectively. After 28 days culture, the concentrations of Sb and As in the soil solution were reduced by up to 22% and 36% in the P. vittata treatments, respectively. Holcus lanatus showed limited uptake for both metalloids. In P. vittata, the reduction of arsenate to arsenite occurred (with As in shoots all existing as arsenite), but limited reduction of antimonate to antimonite (with more than 90% of Sb in shoots existing as antimonate) was observed. In terms of the differences in metalloid uptake between the two P. vittata populations, the population from the habitat with higher soil As concentration showed 35% higher As uptake than the population from the habitat with lower As concentration. This populational difference may partly result from varying As transformation efficiencies. However, no significant difference was observed in Sb accumulation between the two populations.

The first complete chloroplast genome of Pteris vittata (Pteridaceae), an arsenic hyperaccumulating fern

It is the first report on complete chloroplast genome of Pteris vittata, an arsenic hyperaccumulating fern. Its genome size is 154,130 bp, with a typical circular structure including a large single-copy (LSC) (82,623 bp) and a small single-copy (SSC) (20,957 bp) regions separated by a pair of inverted repeats (25,275 bp each). The plastome encodes 132 genes, including 87 protein-coding genes, 35 tRNA genes, eight rRNA genes, and two pseudogenes. The overall Guanine+Cytosine (GC) content is 41.7% and GC content in the IR regions is higher than in the LSC and SSC regions. Maximum likelihood (ML) tree indicated that P. vittata was clustered with Ceratopteris richardii.

An Endophytic Bacterial Consortium modulates multiple strategies to improve Arsenic Phytoremediation Efficacy in Solanum nigrum

Endophytic microbes isolated from plants growing in contaminated habitats possess specialized properties that help their host detoxify the contaminant/s. The possibility of using microbe-assisted phytoremediation for the clean-up of Arsenic (As) contaminated soils of the Ganga-Brahmaputra delta of India, was explored using As-tolerant endophytic microbes from an As-tolerant plant Lantana camara collected from the contaminated site and an intermediate As-accumulator plant Solanum nigrum. Endophytes from L. camara established within S. nigrum as a surrogate host. The microbes most effectively improved plant growth besides increasing bioaccumulation and root-to-shoot transport of As when applied as a consortium. Better phosphate nutrition, photosynthetic performance, and elevated glutathione levels were observed in consortium-treated plants particularly under As-stress. The consortium maintained heightened ROS levels in the plant without any deleterious effect and concomitantly boosted distinct antioxidant defense mechanisms in the shoot and root of As-treated plants. Increased consortium-mediated As(V) to As(III) conversion appeared to be a crucial step in As-detoxification/translocation. Four aquaporins were differentially regulated by the endophytes and/or As. The most interesting finding was the strong upregulation of an MRP transporter in the root by the As + endophytes, which suggested a major alteration of As-detoxification/accumulation pattern upon endophyte treatment that improved As-phytoremediation.

Root transcripts associated with arsenic accumulation in hyperaccumulator Pteris vittata

Hyperaccumulation of arsenic (As) by brake fern Pteris vittata has been described as an important genetic trait that provides an option for development of a sustainable phytoremediation process for As mitigation. Accumulation of very high concentration of arsenic in above-ground tissues may be the result of arsenic vacuole compartmentalization, but the mechanism(s) of arsenic uptake and transport by underground tissues are largely unknown. In this study, we made an attempt towards understanding the molecular mechanism of As hyperaccumulation in this plant. A time-dependent As accumulation study indicates an exponential accumulation of As from 7 to 30 days of arsenic exposure in fronds, and day 3-7 in roots. Root transcriptome analysis identified 554,973 transcripts. Further, subsets of 824 transcripts were differentially expressed between treated and control samples. Many of the genes of critical As-stress response, transcription factors and metal transporters, biosynthesis of chelating compounds involved in uptake and accumulation mechanisms were identified. The genes that were highly expressed such as cysteine-rich RLK, and ABC transporter G family member 26 needs further studies along with arsenite transmembrane transporter. The analysis of generated transcriptome dataset has provided valuable information and platform for further functional studies.

Mechanisms of efficient As solubilization in soils and As accumulation by As-hyperaccumulator Pteris vittata

Arsenic (As) in soils is of major environmental concern due to its ubiquity and carcinogenicity. Pteris vittata (Chinese brake fern) is the first known As-hyperaccumulator, which is highly efficient in extracting As from soils and translocating it to the fronds, making it possible to be used for phytoremediation of As-contaminated soils. In addition, P. vittata has served as a model plant to study As metabolisms in plants. Based on the recent advances, we reviewed the mechanisms of efficient As solubilization and transformation in rhizosphere soils of P. vittata and effective As uptake, translocation and detoxification in P. vittata. We also provided future research perspectives to further improve As phytoremediation by P. vittata.

Evidence for exocellular Arsenic in Fronds of Pteris vittata

The arsenic (As) hyperaccumulating fern species Pteris vittata (PV) is capable of accumulating large quantities of As in its aboveground tissues. Transformation to AsIII and vacuolar sequestration is believed to be the As detoxification mechanism in PV. Here we present evidence for a preponderance of exocellular As in fronds of Pteris vittata despite numerous reports of a tolerance mechanism involving intracellular compartmentalization. Results of an extraction experiment show that 43-71% of the As extruded out of the fronds of PV grown in 0.67, 3.3 and 6.7 mM AsV. SEM-EDX analysis showed that As was localized largely on the lower pinna surface, with smaller amounts on the upper surface, as crystalline deposits. X-ray fluorescence imaging of pinna cross-sections revealed preferential localization of As on the pinna surface in the proximity of veins, with the majority localized near the midrib. Majority of the As in the pinnae is contained in the apoplast rather than vacuoles. Our results provide evidence that exocellular sequestration is potentially a mechanism of As detoxification in PV, particularly at higher As concentrations, raising concern about its use for phytoremediation.

Micro-distribution of arsenic species in tissues of hyperaccumulator Pteris vittata L

Arsenic (As) contamination and its harmful consequences have gained increasing attention in research. Phytoextraction, which uses the As hyperaccumulator Pteris vittata L., is a well-established technology adopted in many countries. However, the hyperaccumulation mechanisms of this plant remain controversial. This study investigated the species and the micro-distribution of As species in three P. vittata L. ecotypes after exposure to arsenite (AsIII) and arsenate (AsV) for 7d. Arsenic-accumulating abilities and preferences to As species varied among different ecotypes. The reduction of AsV into AsIII, oxidation of AsIII into AsV, and chelation of AsIII with thiols were all observed in P. vittata. The reduction of As mainly occurred in the rhizoid, whereas oxidation and chelation mainly occurred in the aboveground parts. Correlation analyses showed that the As concentration in pinna was significantly correlated with the AsV percentage in paraxial and abaxial epidermis (positive), AsIII-GSH percentage in paraxial epidermis (positive), and AsIII percentage in paraxial and abaxial epidermis (negative). Results indicated that oxidation and chelation reactions contributed to the accumulation of As in P. vittata.

Isolation and identification of flavonoids components from Pteris vittata L

Background

Pteris vittata L. is rich in flavonoids which exhibit different bioactivities. In order to investigate the flavonoids components of P. vittata L., extracts of the plant were isolated by column chromatography on silica gel and Sephadex LH-20.

Results

Four flavonoids compounds were obtained, the structures were identified as kaempferol (1), quercetin (2), kaempferol-3-O-d-glucopyranoside (3) and rutin (4), respectively, on the basis of NMR spectroscopic analyses.

Conclusions

Compound 3 was reported for the first time from P. vittata L.

Electronic supplementary material

The online version of this article (doi:10.1186/s40064-016-3308-9) contains supplementary material, which is available to authorized users.