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

Sparingly-soluble CaCO3 promotes plant growth and arsenic accumulation in As-hyperaccumulator Pteris vittata: Oxidative stress and gene expression in As metabolisms

Being a lithophytic plant and arsenic-hyperaccumulator, Pteris vittata can efficiently utilize sparingly-soluble CaCO3, which enhances its arsenic (As) uptake and plant growth with the underlying mechanisms being unclear. Here, after growing P. vittata for 14 days under hydroponics containing 50 μM As and 8.0 mM CaCO3 or 0.8 mM CaCl2, the plant biomass, Ca, As and malondialdehyde (MDA) contents and As-metabolizing gene expressions in P. vittata were determined. Compared to CaCl2 control, CaCO3 increased plant biomass by 44-57 % to 2.4-3.9 g plant-1. Consistent with its better growth, P. vittata effectively solubilized CaCO3, with Ca being increased from 42-57 to 356-369 μM in the growth media, resulting in 38-76 % greater Ca uptake in the roots. Due to its continued Ca supply, CaCO3+As treatment enhanced the As content in P. vittata fronds by 39 % to 1460 mg kg-1 compared to the CaCl2+As control, with MDA being decreased by 16 %. The increased As accumulation was probably attributed to 1.3-1.6 fold upregulation of phosphate transporters PvPht1;3/1;4 for As uptake in P. vittata roots, and 1.4-fold upregulation of arsenite antiporters PvACR3;2/3;3 for As translocation to and sequestration in P. vittata fronds. Overall, the efficient CaCO3 utilization and its enhanced As accumulation in P. vittata shed light on its potential application in phytoremediation of As-contaminated soils.

Arsenic accumulation in two Pteris vittata ecotypes: Insights into transpiration, spore As/P ratios, and arsenic metabolism gene expression

Arsenic (As) contamination in soils poses a potential risk to human health. The arsenic-hyperaccumulator Pteris vittata has been utilized for phytoremediation of As-contaminated soils. Though P. vittata from different regions all accumulates As, their accumulation ability varies among ecotypes. Typically, ecotypes with greater transpiration and lower As levels in their habitat show greater As accumulation, but the underlying molecular mechanisms remain unclear. Here, we assessed the As contents, As speciation, and As metabolism-related genes in two ecotypes of P. vittata from Kunming and Hangzhou in Southwest and Southeast China. After 30 days of growth in sand media with 50 µM As, the Kunming ecotype with greater transpiration accumulated 1.5-fold more As in the fronds than Hangzhou, which was negatively correlated with its spore As contents but positively correlated with its spore P contents. Besides, the arsenite (AsIII) content in P. vittata fronds of Kunming ecotype was 1.5-fold greater than Hangzhou ecotype. The greater As and AsIII contents in the fronds of Kunming ecotype were probably due to the greater expression of As metabolism genes, including 2.8-fold greater in P transporter PvPht1;3 (As uptake), 1.8-fold greater in arsenate reductase PvHAC2 (As reduction) and 2.9-fold greater in arsenite antiporter PvACR3;2 (As translocation), being critical genes for As accumulation by P. vittata. This is the first examination of genes related to As metabolism in different P. vittata ecotypes, providing valuable insights to select P. vittata with greater As accumulation for more efficient phytoremediation.

Arsenic Uptake and Metabolism in Mycorrhizal As-Hyperaccumulator Pteris vittata: Symbiotic P Transporters and As Reductases

Arbuscular mycorrhiza (AM) often protect host plants from As accumulation under arsenic stress; however, the opposite is true for the As-hyperaccumulator Pteris vittata. With non-hyperaccumulator Pteris ensiformis as a comparison, the AM colonization, P and As uptake, and genes associated with As metabolism were investigated in P. vittata after growing 60-day with Rhizophagus irregularis inoculation under 0 (As0), 10 (As10), or 100 μM As (As100) treatments. Based on the As-induced increase in AM colonization (up to 21%), AM symbiosis promoted P. vittata growth by 24% and frond P content by 22% in the AM+As100 treatment than As100 treatment. These increases corresponded to 4.2- to 5.4-fold upregulation in symbiotic P transporter RiPT1/7 in AM fungi and PvPht1;6 in P. vittata roots, which probably supported 37% greater As accumulation at 4980 mg kg-1 in the fronds. Besides total As, enhanced arsenate reduction was evidenced by 19% greater arsenite and 15-fold upregulation of fungal arsenate reductase RiArsC in mycorrhizal roots. Further, the 2.1-fold upregulation of arsenite antiporters PvACR3/3;3 contributed to greater arsenite translocation to and sequestration in the fronds. Unlike P. ensiformis symbiont, which suffers from As stress, the mycorrhiza-specific P transporters (RiPT1/7 and PvPht1;6), arsenate reductases (RiArsC and PvHAC2), and arsenite antiporters (PvACR3/3;3) all benefited AM symbiosis and As accumulation in P. vittata.

New evidence of the timing of arsenic accumulation and expression of arsenic-response genes in field-grown Pteris vittata plants under different arsenic concentrations

The timing and efficiency of arsenic (As) accumulation is crucial for using the hyperaccumulator P. vittata in remediation of As-contaminated soils. In this study, through an innovative microXRF-based approach, using a new "pinna powder" sampling method, we monitored As accumulation over time in fronds of individual P. vittata plants grown in the greenhouse and in the field on two natural soils, with high (750 mg/kg) and moderate (58.4 mg/kg) As concentrations. Results, validated by multivariant statistical analysis show that the peak of As occurs on both soils at 45/60 days and at 100/120 days in greenhouse and field grown plants, respectively. Furthermore, in field trials, the timing of As accumulation in both soils was similar during the first autumn-winter and the second spring-summer phytoextraction cycle. After the two cycles, soil As content was reduced by 70.4% in the high-As soil and 26.4% in the moderate one. Moreover, candidate genes involved in As hyperaccumulation -the arsenite antiporter PvACR3, the As (V)-reductases Pv2.5-8 and the organic cation transporter PvOCT4- are expressed in response to As in field-grown plants with similar kinetics in both soils. In conclusion, we established by this innovative technique, the timing of maximum As accumulation that is linked to the intrinsic hyperaccumulation mechanism and represents a highly powerful tool to set up the duration of phytoextraction cycles.

Claroideglomus etunicatum enhances Pteris vittata L. arsenic resistance and accumulation by mediating the rapid reduction and transport of arsenic in roots

Arbuscular mycorrhizal fungi (AMF) have been widely shown to significantly promote the growth and recovery of Pteris vittata L. growth and repair under arsenic stress; however, little is known about the molecular mechanisms by which AMF mediate the efficient uptake of arsenic in this species. To understand how AMF mediate P. vittata arsenic metabolism under arsenic stress, we performed P. vittata root transcriptome analysis before and after Claroideglomus etunicatum (C. etunicatum) colonization. The results showed that after C. etunicatum colonization, P. vittata showed greater arsenic resistance and enrichment, and its dry weight and arsenic accumulation increased by 2.01-3.36 times. This response is attributed to the rapid reduction and upward translocation of arsenic. C. etunicatum enhances arsenic uptake by mediating the MIP, PHT, and NRT transporter families, while also increasing arsenic reduction (PvACR2 direct reduction and vesicular PvGSTF1 reduction). In addition, it downregulates the expression of ABC and P-type ATPase protein families, which inhibits the compartmentalization of arsenic in the roots and promotes its translocation to the leaves. This study revealed the mechanism of C. etunicatum-mediated arsenic hyperaccumulation in P. vittata, providing guidance for understanding the regulatory mechanism of P. vittata.

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

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

Elucidation of arsenic detoxification mechanism in Marchantia polymorpha: The role of ACR3

The arsenic-specific ACR3 transporter plays pivotal roles in As detoxification in yeast and a group of ancient tracheophytes, the ferns. Despite putative ACR3 genes being present in the genomes of bryophytes, whether they have the same relevance also in this lineage is currently unknown. In this study, we characterized the MpACR3 gene from the bryophyte Marchantia polymorpha L. through a multiplicity of functional approaches ranging from phylogenetic reconstruction, expression analysis, loss- and gain-of-function as well as genetic complementation with an MpACR3 gene tagged with a fluorescent protein. Genetic complementation demonstrates that MpACR3 plays a pivotal role in As tolerance in M. polymorpha, with loss-of-function Mpacr3 mutants being hypersensitive and MpACR3 overexpressors more tolerant to As. Additionally, MpACR3 activity regulates intracellular As concentration, affects its speciation and controls the levels of intracellular oxidative stress. The MpACR3::3xCitrine appears to localize at the plasma membrane and possibly in other endomembrane systems. Taken together, these results demonstrate the pivotal function of ACR3 detoxification in both sister lineages of land plants, indicating that it was present in the common ancestor to all embryophytes. We propose that Mpacr3 mutants could be used in developing countries as low-cost and low-technology visual bioindicators to detect As pollution in water.

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.

Transcriptome analysis of Gossypium reveals the molecular mechanisms of Ca2+ signaling pathway on arsenic tolerance induced by arbuscular mycorrhizal fungi

Introduction

Arbuscular mycorrhizal fungi (AMF) have been demonstrated their ability to enhance the arsenic (As) tolerance of host plants, and making the utilization of mycorrhizal plants a promising and practical approach for remediating As-contaminated soils. However, comprehensive transcriptome analysis to reveal the molecular mechanism of As tolerance in the symbiotic process between AMF and host plants is still limited.

Methods

In this study, transcriptomic analysis of Gossypium seedlings was conducted with four treatments: non-inoculated Gossypium under non-As stress (CK0), non-inoculated Gossypium under As stress (CK100), F. mosseae-inoculated Gossypium under non-As stress (FM0), and F. mosseae-inoculated Gossypium under As stress (FM100).

Results

Our results showed that inoculation with F. mosseae led to a reduction in net fluxes of Ca2+, while increasing Ca2+ contents in the roots and leaves of Gossypium under the same As level in soil. Notably, 199 and 3129 differentially expressed genes (DEGs) were specially regulated by F. mosseae inoculation under As stress and non-As stress, respectively. Through Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) annotation and enrichment analyses, we found that under As stress, F. mosseae inoculation up-regulated a significant number of genes related to the Ca2+ signaling pathway genes, involved in cellular process, membrane part, and signal transduction. This suggests a potential role in mitigating As tolerance in Gossypium seedlings. Furthermore, our analysis identified specific DEGs in transcription factor families, including ERF, MYB, NAC, and WRKY, that were upregulated by F. mosseae inoculation. Conversely, MYB and HB-other were down-regulated. The ERF and MYB families exhibited the highest number of up- and down-regulated DEGs, respectively, which were speculated to play an important role in alleviating the As toxicity of Gossypium.

Discussion

Our findings provided valuable insights into the molecular theoretical basis of the Ca2+ signaling pathway in improving As tolerance of mycorrhizal plants in the future.

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.