Novel phytase from Pteris vittata resistant to arsenate, high temperature, and soil deactivation

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.

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.

Intercropping efficiency of Pteris vittata with two legume plants: Impacts of soil arsenic concentrations

Intercropping of hyperaccumulators with crops has emerged as a promising method for remediating arsenic (As)-contaminated soil in agroecosystems. However, the response of intercropping hyperaccumulators with different types of legume plants to diverse gradients of As-contaminated soil remains poorly understood. In this study, we assessed the response of plant growth and accumulation of an As hyperaccumulator (Pteris vittata L.) intercropped with two legume plants to three gradients of As-contaminated soil. Results indicated that soil As concentration had a substantial effect on the As uptake by plants. P. vittata growing in slightly As-contaminated soil (80 mg kg^-1) exhibited higher As accumulation (1.52-5.49 folds) than those in higher As-contaminated soil (117 and 148 mg kg^-1), owing to the lower soil pH in high As-contaminated soil. Intercropping with Sesbania cannabina L. increased As accumulation in P. vittata by 19.3%- 53.9% but decreased in intercropping with Cassia tora L. This finding was attributed to S. cannabina providing more NO₃^--N to P. vittata to support its growth, and higher resistance to As. The decreased rhizosphere pH in the intercropping treatment also resulted in the increased As accumulation in P. vittata. Meanwhile, the As concentrations in the seeds of the two legume plants met the national food standards(<0.5 mg kg^-1). Therefore, the intercropping P. vittata with S. cannabina is a highly effective intercropping system in slightly As-contaminated soil and provides a potent method for As phytoremediation.

Vanadium-resistant endophytes modulate multiple strategies to facilitate vanadium detoxification and phytoremediation in Pteris vittata

Vanadium (V) contamination of soils poses potential risks to humans and ecosystems. This study was conducted to evaluate the effects of endophyte-assisted phytoremediation and to determine the mechanisms involved in V detoxification and plant growth promotion. Results showed that the endophytic bacterium Serratia marcescens PRE01 could successfully colonize the roots and increase the total V uptake of Pteris vittata by 25.4 %, with higher plant biomass and V accumulation in roots. Endophyte inoculation significantly improved the secretion of phytic, malic, and oxalic acids and accelerated FeVO₄ dissolution and subsequent Fe and V uptake in the rhizosphere. Under V stress without inoculation, V removed by shoot uptake, root uptake, and root surface adsorption accounted for 21.76 %, 42.14 %, and 30.93 % of the total V removal efficiency, respectively. To detoxify excess V, PRE01 effectively strengthened the adsorption of V on the root surface, with an increase in its contribution to the total V removal efficiency from 30.93 % to 38.10 %. Furthermore, beneficial endophytes could alleviate oxidative damage caused by V stress by reinforcing the plant antioxidant system and promoting V(V) reduction in root tissues. These findings clearly reveal that inoculation with endophytes is a promising method for modulating multiple strategies to enhance the phytoremediation of V-contaminated soils.

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.

Effects of Claroideoglomus etunicatum Fungi Inoculation on Arsenic Uptake by Maize and Pteris vittata L

The intercropping of arsenic (As) hyperaccumulator Chinese brake fern (Pterisvittata L.) with maize (Zea mays L.) is being widely utilized to enhance phytoremediation without impeding agricultural production. Arbuscular mycorrhizal (AM) fungi can regulate the physiological and molecular responses of plants in tolerating heavy metal stress. We studied the effects of inoculation with AM fungi on As uptake by maize and P. vittata grown in soil contaminated with As. The results show that infection with the fungus Claroideoglomus etunicatum (Ce) increased the biomass of maize and P. vittata. Moreover, infection with Ce significantly reduced the accumulation of As and the coefficient for root-shoot transport of As in maize, whereas it enhanced the accumulation of As and coefficient for root-shoot transport of As in P. vittata. Infection with Ce led to a high content of available As in the soil planted with P. vittata, while there was a lower content of available As in the soil planted with maize. The different concentrations of available As in the soils suggest that inoculation with Ce may enhance the secretion of organic acids, particularly citric acid and tartaric acid, by maize roots and promote rhizosphere acidification, which then causes a decrease in As uptake by maize. Inoculation with Ce decreased the secretion of citric acid from P. vittata and promoted rhizosphere alkalization, which then caused an increase in As uptake by P. vittata and maize. Thus, co-combining AM fungi in the intercropping of the hyperaccumulator P. vittata with maize could be a promising approach to improving the efficiency of remediating As-contaminated soil.

Enhancing Phytate Availability in Soils and Phytate-P Acquisition by Plants: A Review

Phytate (myo-inositol hexakisphosphate salts) can constitute a large fraction of the organic P in soils. As a more recalcitrant form of soil organic P, up to 51 million metric tons of phytate accumulate in soils annually, corresponding to ∼65% of the P fertilizer application. However, the availability of phytate is limited due to its strong binding to soils via its highly-phosphorylated inositol structure, with sorption capacity being ∼4 times that of orthophosphate in soils. Phosphorus (P) is one of the most limiting macronutrients for agricultural productivity. Given that phosphate rock is a finite resource, coupled with the increasing difficulty in its extraction and geopolitical fragility in supply, it is anticipated that both economic and environmental costs of P fertilizer will greatly increase. Therefore, optimizing the use of soil phytate-P can potentially enhance the economic and environmental sustainability of agriculture production. To increase phytate-P availability in the rhizosphere, plants and microbes have developed strategies to improve phytate solubility and mineralization by secreting mobilizing agents including organic acids and hydrolyzing enzymes including various phytases. Though we have some understanding of phytate availability and phytase activity in soils, the limiting steps for phytate-P acquisition by plants proposed two decades ago remain elusive. Besides, the relative contribution of plant- and microbe-derived phytases, including those from mycorrhizas, in improving phytate-P utilization is poorly understood. Hence, it is important to understand the processes that influence phytate-P acquisition by plants, thereby developing effective molecular biotechnologies to enhance the dynamics of phytate in soil. However, from a practical view, phytate-P acquisition by plants competes with soil P fixation, so the ability of plants to access stable phytate must be evaluated from both a plant and soil perspective. Here, we summarize information on phytate availability in soils and phytate-P acquisition by plants. In addition, agronomic approaches and biotechnological strategies to improve soil phytate-P utilization by plants are discussed, and questions that need further investigation are raised. The information helps to better improve phytate-P utilization by plants, thereby reducing P resource inputs and pollution risks to the wider environment.

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.

The wheat growth-promoting traits of Ochrobactrum and Pantoea species, responsible for solubilization of different P sources, are ensured by genes encoding enzymes of multiple P-releasing pathways

Production and release of organic acids and phosphatase enzymes by microbes are important for inorganic and organic phosphorus cycling in soil. The presence of microorganisms with corresponding traits in the plant rhizosphere lead to improved plant P uptake and ultimately growth promotion. We studied the potential of two rhizosphere-competent strains, Pantoea sp. MR1 and Ochrobactrum sp. SSR, for solubilization of different organic and inorganic P sources in vitro. In a pot experiment we further revealed the impact of the two strains on wheat seedling performance in soil amended with either phytate, rock phosphate or K₂HPO₄ as solely P source. To directly link P-solubilizing activity to the strain-specific genetic potential, we designed novel primers for glucose dehydrogenase (gcd), phosphatase (pho) and phytase (phy) genes, which are related to the organic and inorganic P solubilization potential. Quantitative tracing of these functional genes in the inoculated soils of the conducted pot experiment further allowed to compare strain abundances in the soil in dependency on the present P source. We observed strain- and P source-dependent patterns of the P solubilization in vitro as well as in the pot experiment, whereby P release, particularly from phytate, was linked to the strain abundance. We further revealed that the activity of microbial phosphatases is determined by the interplay between functional gene abundance, available soil P, and substrate availability. Moreover, positive impacts of microbial seed inoculation on wheat root architecture and aboveground growth parameters were observed. Our results suggest that screening for rhizosphere-competent strains with gcd, pho and phy genes may help to identify new microbial taxa that are able to solubilize and mineralize inorganic as well as organic bound P. Subsequently, the targeted use of corresponding strains may improve P availability in agricultural soils and consequently reduce fertilizer application.

Phytate promoted arsenic uptake and growth in arsenic-hyperaccumulator Pteris vittata by upregulating phosphorus transporters

While phosphate (P) inhibits arsenic (As) uptake by plants, phytate increases As uptake by As-hyperaccumulator Pteris vittata. Here we tried to understand the underling mechanisms by investigating the roles of phytate in soil As desorption, P transport in P. vittata, short-term As uptake, and plant growth and As accumulation from soils. Sterile soil was used to exclude microbial degradation on phytate. Results showed that inorganic P released 3.3-fold more As than that of phytate from soil. However, P. vittata accumulated 2-2.5 fold more As from soils with phytate than that in control and P treatment. In addition, different from P suppression on As uptake, solution uptake experiment showed that As uptake in phytate treatment was comparable to that of control under 0.1-7.5 μM As after 1-24 h. Moreover, responding to phytate, P. vittata P transporter PvPht1;3 increased by 3-fold while PvPht1;1 decreased by 65%. The data suggested that phytate upregulated PvPht1;3, thereby contributing to As uptake in P. vittata. Our results showed that, though with lower As release from soil compared to P, phytate induced more As uptake and better growth in P. vittata by upregulating P transporters.

Arsenic removal by As-hyperaccumulator Pteris vittata from two contaminated soils: A 5-year study

The ability of As-hyperaccumulator Pteris vittata to remove As from two contaminated soils (CCA from an As-treated wood facility and DVA from a cattle-dipping vat) over 5 years was investigated for the first time. The goal was to evaluate P. vittata's ability to continuously remove As during 10 harvests and identify how soil As was affected by P. vittata under P-sufficient (P-fertilizer) and P-limiting (phosphate rock) conditions. Sequential extraction was used to determine changes in metal distribution among different soil fractions. The high frond biomass production occurred on the 9th (62.1-63.9 and 35.6-63.5 g plant^-1) and 10th harvest (58.6-60.7 and 51.9-57.1 g plant^-1) for CCA and DVB soils, though frond As concentration decreased. Soil arsenic removal averaged 7-10% per harvest during the 1-6th harvests and was reduced to 0-3% during the 7-10th harvests for DVA and CCA soils. Arsenic from all fractions, excluding the residual fraction, was affected by plant uptake. The largest reduction occurred in the amorphous fraction of CCA-soil at 64-66% (61.2-61.5 to 20.8-21.8 mg kg^-1) and in the crystalline fraction of DVA-soil at 50-86% (2.18-4.35 to 0.61-1.10 mg kg^-1). Soil As concentrations were reduced by 37-47% from 26.7 to 129 to 15.6-16.8 and 68.9-70.1 mg kg^-1 for the DVA and CCA soils, respectively. Our data indicated that P. vittata efficiently solubilized non-labile As under P-limiting conditions without impacting its As depletion.

Arsenic-induced nutrient uptake in As-hyperaccumulator Pteris vittata and their potential role to enhance plant growth

It is known that arsenic (As) promotes growth of As-hyperaccumulator Pteris vittata (PV), however, the associated mechanisms are unclear. Here we examined As-induced nutrient uptake in P. vittata and their potential role to enhance plant growth in sterile agar by excluding microbial effects. As-hyperaccumulator P. multifida (PM) and non-hyperaccumulator P. ensiformis (PE) belonging to the Pteris genus were used as comparisons. The results showed that, after 40 d of growth, As induced biomass increase in hyperaccumulators PV and PM by 5.2-9.4 fold whereas it caused 63% decline in PE. The data suggested that As played a beneficial role in promoting hyperaccumulator growth. In addition, hyperaccumulators PV and PM accumulated 7.5-13, 1.4-3.6, and 1.8-4.4 fold more As, Fe, and P than the non-hyperaccumulator PE. In addition, nutrient contents such as K and Zn were also increased while Ca, Mg, and Mn decreased or unaffected under As treatment. This study demonstrated that As promoted growth in hyperaccumulators and enhanced Fe, P, K, and Zn uptake. Different plant growth responses to As among hyperaccumulators PV and PM and non-hyperaccumulator PE may help to better understand why hyperaccumulators grow better under As-stress.

Phosphate Transporter PvPht1;2 Enhances Phosphorus Accumulation and Plant Growth without Impacting Arsenic Uptake in Plants

Phosphorus is an important macronutrient for plant growth and is acquired by plants mainly as phosphate (P). Phosphate transporters (Phts) are responsible for P and arsenate (AsV) uptake in plants including arsenic-hyperaccumulator Pteris vittata. P. vittata is efficient in AsV uptake and P utilization, but the molecular mechanism of its P uptake is largely unknown. In this study, a P. vittata Pht, PvPht1;2, was cloned and transformed into tobacco ( Nicotiana tabacum). In hydroponic experiments, all transgenic lines displayed markedly higher P content and better growth than wild type, suggesting that PvPht1;2 mediated P uptake in plants. In addition, expressing PvPht1;2 also increased the shoot/root ³²P ratio by 69-92% and enhanced xylem sap P by 46-62%, indicating that PvPht1;2 also mediated P translocation in plants. Unlike many Phts permeable to AsV, PvPht1;2 showed little ability to transport AsV. In soil experiments, PvPht1;2 also significantly increased shoot biomass without elevating As accumulation in PvPht1;2 transgenic tobacco. Taken together, our results demonstrated that PvPht1;2 is a specific P transporter responsible for P acquisition and translocation in plants. We envisioned that PvPht1;2 can enhance crop P acquisition without impacting AsV uptake, thereby increasing crop production without compromising food safety.

Phytate induced arsenic uptake and plant growth in arsenic-hyperaccumulator Pteris vittata

Phytate is abundant in soils, which is stable and unavailable for plant uptake. However, it occurs in root exudates of As-hyperaccumulator Pteris vittata (PV). To elucidate its effect on As uptake and growth, P. vittata were grown on agar media (63 μM P) containing 50 μM As and/or 50 or 500 μM phytate with non As-hyperaccumulator Pteris ensiformis (PE) as a congeneric control for 60 d. Phytate induced efficient As and P uptake, and enhanced growth in PV, but had little effects on PE. The As concentrations in PV fronds and roots were 157 and 31 mg kg^-1 in As₅₀+phytate₅₀, 2.2- and 3.1-fold that of As₅₀ treatment. Phosphorus uptake by PV was reduced by 27% in As treatment than the control (P vs. P+As) but increased by 73% comparing phytate₅₀₀ to phytate₅₀₀+As, indicating that PV effectively took up P from phytate. Neither As nor phytate affected Fe accumulation in PV, but phytate reduced root Fe concentration in PE (46-56%). As such, the increased As and P and the unsuppressed Fe uptake in PV probably promoted PV growth. Thus, supplying phytate to As-contaminated soils may promote As uptake and growth in PV and its phytoremediation ability.

Arsenic-hyperaccumulator Pteris vittata efficiently solubilized phosphate rock to sustain plant growth and As uptake

Phosphorus (P) is one of the most important nutrients for phytoremediation of arsenic (As)-contaminated soils. In this study, we demonstrated that As-hyperaccumulator Pteris vittata was efficient in acquiring P from insoluble phosphate rock (PR). When supplemented with PR as the sole P source in hydroponic systems, P. vittata accumulated 49% and 28% higher P in the roots and fronds than the -P treatment. In contrast, non-hyperaccumulator Pteris ensiformis was unable to solubilize P from PR. To gain insights into PR solubilization by plants, organic acids in plant root exudates were analyzed by HPLC. The results showed that phytic acid was the predominant (>90%) organic acid in P. vittata root exudates whereas only oxalic acid was detected in P. ensiformis. Moreover, P. vittata secreted more phytic acid in -P and PR treatments. Compared to oxalic acid, phytic acid was more effective in solubilizing PR, suggesting that phytic acid was critical for PR utilization. Besides, secretion of phytic acid by P. vittata was not inhibited by arsenate. Our data indicated that phytic acid played an important role in efficient use of insoluble PR by P. vittata, shedding light on using insoluble PR to enhance phytoremediation of As-contaminated soils.

Microbial siderophores and root exudates enhanced goethite dissolution and Fe/As uptake by As-hyperaccumulator Pteris vittata

Arsenic (As) in soils is often adsorbed on Fe-(hydro)oxides surface, rendering them more resistant to dissolution, which is undesirable for phytoremediation of As-contaminated soils. Arsenic hyperaccumulator Pteris vittata prefers to grow in calcareous soils where available Fe and As are low. To elucidate its mechanisms of acquiring Fe and As from insoluble sources in soils, we investigated dissolution of goethite with pre-adsorbed arsenate (AsV; As-goethite) in presence of four organic ligands, including two root exudates (oxalate and phytate, dominant in P. vittata) and two microbial siderophores (PG12-siderophore and desferrioxamine B). Their presence increased As solubilization from As-goethite from 0.03 to 0.27-5.33 mg L^-1 compared to the control. The siderophore/phytate bi-ligand treatment released 7.42 mg L^-1 soluble Fe, which was 1.2-fold that of the sum of siderophore and phytate, showing a synergy in promoting As-goethite dissolution. In the ligand-mineral-plant system, siderophore/phytate was most effective in releasing As and Fe from As-goethite. Moreover, the continuous plant uptake induced more As-goethite dissolution. The continued release of As and Fe significantly enhanced their plant uptake (from 0.01 to 0.43 mg plant^-1 As and 2.7-14.8 mg plant^-1 Fe) and plant growth (from 1.2 to 3.1 g plant^-1 fw) in P. vittata. Since microbial siderophores and root exudates often coexist in soil rhizosphere, their synergy in enhancing dissolution of insoluble As-Fe minerals may play an important role in efficient phytoremediation of As-contaminated soils.

Arsenic-enrichment enhanced root exudates and altered rhizosphere microbial communities and activities in hyperaccumulator Pteris vittata

Phytoremediation of arsenic (As)-contaminated soil by hyperaccumulator Pteris vittata is promising. A better understanding of the rhizosphere microbial dynamics that regulate As availability and plant growth is important to optimize the phytoremediation process. In this study, Illumina sequencing of 16S rRNA genes was applied to assess the rhizosphere microbial community structure of P. vittata. Microbial functionality was monitored by soil enzyme activities and MPN-PCR targeting genes of interest. Arsenic (100mgkg^-1 AsV) addition to soil significantly increased DOC, root exudates, As and P uptake and the frond biomass of P. vittata. Moreover, As-enrichment significantly increased soil enzyme activities involved in N, P and S cycling and the gene abundance of As transforming bacteria, Fe- and S-reducing bacteria and N and C fixing bacteria in the rhizosphere of P. vittata. Together, the results revealed that the combined selective pressure of As and rhizosphere resulted in stimulation of microbial community, which most likely has a role in reductive dissolution of Fe and S, As and P mobilization, C degradation and fixation, and N fixation. These changes appeared to have a role in mitigation of As toxicity and to promote growth and the As uptake ability of P. vittata under As-enriched conditions.

Arsenic Induced Phytate Exudation, and Promoted FeAsO4 Dissolution and Plant Growth in As-Hyperaccumulator Pteris vittata

Arsenic hyperaccumulator Pteris vittata (PV) is efficient in taking up As and nutrients from As-contaminated soils. We evaluated the mechanisms used by PV to mobilize As and Fe by examining the impacts of As and root exudates on FeAsO4 solubilization, and As and Fe uptake in four plants: As-hyperaccumulators PV and Pteris multifida (PM), nonhyperaccumulator Pteris ensiformis (PE), and angiosperm plant tomato (Solanum lycopersicum). Phytate and oxalate were dominant in fern plants (>93%), which were 50-83, 15-42, and 0-32 mg kg(-1) phytate and 10-15, 7-26, and 4-12 mg kg(-1) oxalate for PV, PM, and PE respectively, with higher As inducing greater phytate exudation and no phytate being detected in tomato exudates. PV treated with phytate+FeAsO4 had higher As and Fe contents and larger biomass than phytate or FeAsO4 treatment, which were 340 vs 20 and 130 mg kg(-1) As in the fronds and 7900 vs 1600 and 4100 mg kg(-1) Fe in the roots. We hypothesized that As-induced phytate exudation helped PV to take up Fe and As from insoluble FeAsO4 and promoted PV growth. Our study suggests that phytate exudation may be special to fern plants, which may play an important role in enhancing As and nutrient uptake by plants, thereby increasing their efficiency in phytoremediation of As-contaminated soils.

Phosphorus solubilization and plant growth enhancement by arsenic-resistant bacteria

Phosphorus is an essential nutrient, which is limited in most soils. The P solubilization and growth enhancement ability of seven arsenic-resistant bacteria (ARB), which were isolated from arsenic hyperaccumulator Pteris vittata, was investigated. Siderophore-producing ARB (PG4, 5, 6, 9, 10, 12 and 16) were effective in solubilizing P from inorganic minerals FePO4 and phosphate rock, and organic phytate. To reduce bacterial P uptake we used filter-sterilized Hoagland medium containing siderophores or phytase produced by PG12 or PG6 to grow tomato plants supplied with FePO4 or phytate. To confirm that siderophores were responsible for P release, we compared the mutants of siderophore-producing bacterium Pseudomonas fluorescens Pf5 (PchA) impaired in siderophore production with the wild type and test strains. After 7d of growth, mutant PchA solubilized 10-times less P than strain PG12, which increased tomato root biomass by 1.7 times. For phytate solubilization by PG6, tomato shoot biomass increased by 44% than control bacterium Pseudomonas chlororaphis. P solubilization by ARB from P. vittata may be useful in enhancing plant growth and nutrition in other crop plants.