Pteris melanocaulon Fée is an As hyperaccumulator

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.

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.

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.

Closely-related species of hyperaccumulating plants and their ability in accumulation of As, Cd, Cu, Mn, Ni, Pb and Zn

Soil contamination by heavy metals is widespread. Heavy metals of concern include As, Cd, Cu, Cr, Mn, Ni, Pb, and Zn. Hyperaccumulating plants are efficient in accumulating metals, which have potential to remediate metal-contaminated soils. Species of closely-related hyperaccumulating plants have been used to screen their ability in metal accumulation. However, there is limited evidence to show that closely-related plant species have similar ability in metal accumulation. Using a global database of 664 hyperaccumulating plants, we constructed a phylogeny of hyperaccumulating plants of As, Cd, Cu, Cr, Mn, Ni, Pb, and Zn. We evaluated the phylogenetic randomness of plants hyperaccumulating different metals by comparing the minimum number of trait-state changes across the phylogenetic tree to a null model. Based on the D value, we evaluated whether closely-related plants tend to accumulate similar metals. Based on the Blomberg's K and Pagel's λ, we tested whether closely-related plants have similar ability in metal accumulation. Excluding Cd and Pb, closely-related plant species tend to accumulate similar metal, however, its ability cannot be predicted based on phylogenetic relations except Ni. Therefore, we concluded that focusing on species of closely-related hyperaccumulating plants can help to screen new hyperaccumulators although their ability could be different.