Exploring the potential: Can arsenic (As) resistant silicate-solubilizing bacteria manage the dual effects of silicon on As accumulation in rice?

Rice (Oryza sativa L.) cultivation in regions marked by elevated arsenic (As) concentrations poses significant health concerns due to As uptake by the plant and its subsequent entry into the human food chain. With rice serving as a staple crop for a substantial share of the global population, addressing this issue is critical for food security. In flooded paddy soils, where As availability is pronounced, innovative strategies to reduce As uptake and enhance agricultural sustainability are mandatory. Silicon (Si) and Si nanoparticles have emerged as potential candidates to mitigate As accumulation in rice. However, their effects on As uptake exhibit complexity, influenced by initial Si levels in the soil and the amount of Si introduced through fertilization. While low Si additions may inadvertently increase As uptake, higher Si concentrations may alleviate As uptake and toxicity. The interplay among existing Si and As availability, Si supplementation, and soil biogeochemistry collectively shapes the outcome. Adding water-soluble Si fertilizers (e.g., Na2SiO3 and K2SiO3) has demonstrated efficacy in mitigating As toxicity stress in rice. Nonetheless, the expense associated with these fertilizers underscores the necessity for low cost innovative solutions. Silicate-solubilizing bacteria (SSB) resilient to As hold promise by enhancing Si availability by accelerating mineral dissolution within the rhizosphere, thereby regulating the Si biogeochemical cycle in paddy soils. Promoting SSB could make cost-effective Si sources more soluble and, consequently, managing the intricate interplay of Si's dual effects on As accumulation in rice. This review paper offers a comprehensive exploration of Si's nuanced role in modulating As uptake by rice, emphasizing the potential synergy between As-resistant SSB and Si availability enhancement. By shedding light on this interplay, we aspire to shed light on an innovative attempt for reducing As accumulation in rice while advancing agricultural sustainability.

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.

Effect of phosphate on arsenic species uptake in plants under hydroponic conditions

Monothioarsenate (MTA) is a newly discovered arsenic (As) compound that can be formed under reduced sulfur conditions, mainly in paddy soil pore waters. It is structurally similar to arsenate As(V) and inorganic phosphate (Pi), which is taken up through phosphate transporters. Due to the similarity between As(V) and Pi, As(V) enters into plants instead of Pi. The important role played by phytochelatin (PC), glutathione (GSH), and the PC-vacuolar transporters ABCC1 and ABCC2 under As stress in plants is well known. However, the plant uptake and mechanisms surrounding MTA still have not been completely addressed. This investigation was divided in two stages: first, several hydroponic assays were set up to establish the sensibility-tolerance of wild-type Arabidopsis thaliana (accession Columbia-0, Col-0). Then Col-0 was used as a control plant to evaluate the effects of As(V) or MTA in (PC)-deficient mutant (cad1-3), glutathione biosynthesis mutant (cad2), and PC transport (abcc1-2). The inhibitory concentration (IC50) root length was calculated for both As species. According to the results, both arsenic species (As(V) and MTA) exhibited high toxicity for the genotypes evaluated. This could mean that these mechanisms play a constitutive role in MTA detoxification. Second, for the Pi-MTA and As(V)-Pi competition assays, a series of experiments on hydroponic seedlings of A. thaliana were carried out using Col-0 and a pht1;1. The plants were grown under increasing Pi concentrations (10 μM, 0.1 mM, or 1 mM) at 10 μM As(V) or 50 μM MTA. The total As concentration in the roots was significantly lower in plants exposed to MTA, there being less As content in the pht1;1 mutant at the lowest Pi concentrations tested compared with the As(V)/Pi treatments. In addition, a higher rate of As translocation from the roots to the shoots under MTA was observed in comparison to the As(V)-treatments.

Pantoea conspicua promoted sunflower growth and engulfed rhizospheric arsenate by secreting exopolysaccharide

A rhizobacterium, Pantoea conspicua, was examined against sunflower seedlings' growth under arsenate stress. Sunflower upon exposure to arsenate resulted in compromised growth that might be due to the accumulation of higher concentrations of arsenate and reactive oxygen species (ROS) in seedlings' tissues. The deposited arsenate led to oxidative damage and electrolyte leakage, making the sunflower seedlings vulnerable to compromise its growth and development. However, inoculation of sunflower seedlings with P. conspicua alleviated arsenate stress in host by initiating a multilayered defence mechanism. In fact, P. conspicua filtered out 75.1% of the arsenate from growth medium that were available to the plant roots in the absence of the said strain. To accomplish such activity, P. conspicua secreted exopolysaccharides as well as altered lignification in host roots. The arsenate (24.9%) that made its way to plant tissues was countered by helping the host seedlings to produce higher levels of indole acetic acid, non enzymatic antioxidants (phenolics and flavonoids) and antioxidant enzymes (catalase, ascorbte peroxidase, peroxidase, superoxide dismutase). As a result, ROS accumulation and electrolyte leakage were brought back to normal levels as observed in control seedlings. Hence, the rhizobacterium associated host seedlings achieved higher net assimilation (127.7%) and relative growth rate (113.5%) under 100 ppm of arsenate stress. The work concluded that P. conspicua alleviated arsenate stress in the host plants by imposing physical barrier as well as improving host seedlings' physiology and biochemistry.

Sustainable Management of Arsenic Translocation in the Paddy Plants (Oryza sativa L) Cultivated in the Alluvial Soil of Gangetic West Bengal, India

Rice plants are known to be more susceptible to arsenic (As) contamination during the cultivation process. Arsenic is genotoxic and can be a big threat to the rice eating people at large. Studies on an effective mitigation mechanism are the need of the hour. This work was an approach using iron (Fe³⁺) to form Fe-plaque in the plant root that could trap As. The present research was designed with several experimental set ups for rice cultivation in pot culture using different Fe doses with fertilizer in the soil, and finally, the optimum dose was selected considering the translocation ability, plant health, and molecular and stress biomarkers. The study revealed that on an increase in Fe dose, translocation factor (TF) and stress marker (malondialdehyde content) of the plant decreased gradually and encountered minimum (0.12 and 0.03 mg/kg, respectively) at the dose of 4.5gm/kg. In contrast, higher values of chlorophyll (2.5 mg/kg) and carbohydrate (2.2 mg/kg) and intact DNA content were recorded highlighting the rich health condition of the plant. Thus, the experiment supported well the fact that the dose of Fe as fortified fertilizer can be considered the most effective in reducing soil arsenic accumulation in the rice plants. This approach might save the rice eating people from harmful effects of As contamination in this region of India.

Preventative effect of crop straw-derived biochar on plant growth in an arsenic polluted acidic ultisol

In different regions of the world, arsenic (As) contaminated soils poses a serious threat to plant growth and its physiological processes. Organic amendments are a cost-effective and environmentally friendly way to improve plant growth under stress conditions in contaminated soils. In As polluted acidic ultisol, a greenhouse trial was conducted to investigate the protective effects of peanut straw biochar (PSB) and canola straw biochar (CSB) on soybean mineral nutrition, antioxidant enzymes, and physiological growth parameters. The current study used eighteen treatments with different levels of As ((1) 0 mg kg^-1, (2) 30 mg kg^-1, (3) 60 mg kg^-1) and biochar (PSB and CSB) (0%, 1%, and 2%). The result suggests that biochar addition under As stress in highly weathered acidic ultisol soil increased soybean growth attributes and defense mechanisms. The PSB was more effective than the CSB in a dose-dependent manner. The application of 2% PSB in polluted soil resulted in significant increases in soybean height (58%), biomass production (root (44%) and shoot length (52%)), chlorophyll contents (92%), soybean functional leaves (62%), total soluble sugars (TSS) (71%) and base cations (Ca²⁺, Mg²⁺, K⁺, Na⁺). However, biochar application decreased proline, MDA, H₂O₂, and O₂^- by 64%, 82%, 49%, and 45% respectively. Furthermore, biochar application increased (Phosphate) P and As uptake in soybean, with PSB application exhibiting a greater increase than CSB application. As a result, crop straw-derived biochar can reduce As-induced soybean plant damage and insert a protective effect in As-contaminated acidic ultisol soils.

Influence of environmental factors on arsenite transformation and fate in the Hydrilla verticillata (L.f.) royle - Medium system

Hydrilla verticillata (L.f.) Royle has a great ability to accumulate large amounts of arsenic (As). We studied the influence of phosphorus (P), nitrogen (N), pH, and arsenite (As(III)) on As transformation and fate in the H. verticillata - medium system via orthogonal experimental design. The results showed highest plant growth was under intermediate As(III) in the medium, with Chlorophyll a and Chlorophyll b contents in plant diminishing after 96 h treatment. Exposure to high N, high As(III), intermediate P, and low pH in the medium, the highest total arsenic uptake by plants were 169.1 ± 5.5 μg g^-1 dry weight, with As(III) as the predominant speciation (49.1 ± 4.8% to 88.5 ± 0.2%) in plants. Meanwhile, trace As (mainly arsenate (As(V))) was adsorbed on the surface of H. verticillata, and the adsorption amounts of As(V) increased with increasing As(III) concentrations in the medium. The dominant As species was As(V) in the medium although plant was supplied with As(III), and highest As(III) oxidation proportion in the medium would occur when low N and pH associated with high P and As(III). Collectively, As(III) uptake and transformation by H. verticillata cannot be overlooked in the biogeochemical cycling of As in aquatic environment.

Influence of surface chemistry of activated carbon electrodes on electro-assisted adsorption of arsenate

In this work, the influence of oxygen-containing surface groups of activated carbon electrodes on the charge efficiency of electro-assisted adsorption of As(V) was investigated. It was distinguished between activated carbons modified through acidic (oxidation) and thermal (reduction) treatments, starting with a granular pristine commercial activated carbon of bituminous origin. The textural characterization of the three materials showed that the treatments did not produce significant changes in the surface area and in the distribution of pores. The three carbon samples were used to fabricate packed electrodes with stainless-steel mesh as electric current collector. This work report that the application of anodic potentials (1.01 and 1.41 V vs. NHE) increased the adsorption capacity and rate of arsenate uptake in solutions containing only this contaminant (2.5 mg L^-1) at pH 7. The oxidized carbon electrode presented the lowest capacitance and adsorption capacity during electroadsorption (0.33 mg g^-1), compared to pristine material (1.77 mg g^-1). On the other hand, the reduced electrode displayed the highest adsorption capacity of arsenate (3.14 mg g^-1) when applying a potential of 1.01 V. The results were correlated with the potential of zero charge values. In addition, for this material, the rate of kinetics increased 26.7 % compared to experiments without applied potential.

Influence of organic management on As bioavailability: Soil quality and tomato As uptake

The research studied the effects of organic vs. conventional management of soil quality and tomato yield quality, cultivated in a geogenic arsenic contaminated soil. The chemical and biochemical properties were analyzed to evaluate soil quality, arsenic mobility and its phyto-availability, as well as arsenic accumulation in the tomato plant tissues and if tomatoes cultivated in arsenic rich soil represents a risk for human health. A general improvement of tomato growth and soil quality was observed in the organic management, where soil organic carbon increased from 1.24 to 1.48% and total nitrogen content. The arsenic content of the soil in the organic management increased from 57.0 to 65.3 mg kg^-1, probably due to a greater content of organic matter which permitted the soil to retain the arsenic naturally present in irrigation water. An increase of bioavailable arsenic was observed in the conventional management compared to the organic one (7.05 vs 6.18 mg kg^-1). The bioavailable form of metalloid may affect soil microbial community structure assessed using El-FAME analysis. The increase of the total arsenic concentration in the organic management did not represent a stress factor for soil microbial biomass carbon (Cmic), which was higher in the organic management than in the conventional one (267 vs. 132 μg Cmic g^-1). Even if the organic management caused an increase of total arsenic concentration in the soil due to the enhanced organic matter content, retaining arsenic from irrigation water, this management mitigates the arsenic uptake by tomato plants reducing the mobility of the metalloid.

Phytofiltration of arsenic by aquatic moss (Warnstorfia fluitans)

This work investigates whether aquatic moss (Warnstorfia fluitans) originating from an arsenic (As)-contaminated wetland close to a mine tailings impoundment may be used for phytofiltration of As. The aim was to elucidate the capacity of W. fluitans to remove As from arsenite and arsenate contaminated water, how nutrients affect the As uptake and the proportion of As adsorption and absorption by the moss plant, which consists of dead and living parts. Arsenic removal from 0, 1, or 10% Hoagland nutrient solution containing 0-100 μM arsenate was followed over 192 h, and the total As in aquatic moss after treatment was analysed. The uptake and speciation of As in moss cultivated in water containing 10 μM arsenate or arsenite were examined as As uptake in living (absorption + adsorption) and dead (adsorption) plant parts. Results indicated that W. fluitans removed up to 82% of As from the water within one hour when 1 μM arsenate was added in the absence of nutrients. The removal time increased with greater nutrient and As concentrations. Up to 100 μM As had no toxic effect on the plant biomass. Both arsenite and arsenate were removed from the solution to similar extents and, independent of the As species added, more arsenate than arsenite was found in the plant. Of the As taken up, over 90% was firmly bound to the tissue, a possible mechanism for resisting high As concentrations. Arsenic was both absorbed and adsorbed by the moss, and twice as much As was found in living parts as in dead moss tissue. This study revealed that W. fluitans has potential to serve as a phytofilter for removing As from As-contaminated water without displaying any toxic effects of the metalloid.

Remediation of arsenic in mung bean (Vigna radiata) with growth enhancement by unique arsenic-resistant bacterium Acinetobacter lwoffii

Arsenic, a carcinogenic and toxic contaminant of soil and water, affects human health adversely. During last few decades, it has been an important global environmental issue. Among several arsenic detoxification methods remediation using arsenic resistant microbes is proved to be environment-friendly and cost-effective. This study aimed to test the effects of arsenic utilizing bacterial strain Acinetobacter lwoffii (RJB-2) on arsenic uptake and growth of mung bean plants (Vigna radiata). RJB-2 exhibited tolerance up to 125mM of arsenic (V) and 50mM of arsenic (III). RJB-2 produced plant growth promoting substances e.g. indole acetic acid (IAA), siderophores, exopolysaccharide (EPS) and phosphate solubilization in the absence and in presence of arsenic. Pot experiments were used to scrutinize the role of RJB-2 on arsenic uptake and growth of mung bean plants grown in soil amended with 22.5mgkg^-1 of sodium arsenate (Na₂HAsO₄·7H₂O). RJB-2 could arrest arsenic uptake in just 7days and increase plant growth, number of plants per pot, chlorophyll and carotenoid content of the mung bean plants. RJB-2 formed biofilm and its root-association helped to abate arsenic uptake in mung bean. Confocal and light microscopic studies also revealed the abatement of arsenic uptake and increase in chlorophyll content in mung bean plants in presence of RJB-2. RJB-2 was also responsible for less production of reactive oxygen species (ROS) in mung bean plants reducing the oxidative damage caused by arsenic. The lower percentage of electrolytic leakage (EL) in RJB-2 inoculated mung bean plants proved arsenic abatement. The study also reported the distribution of arsenic in various parts of mung bean plant. RJB-2 owing to its intrinsic abilities of plant growth promotion even in presence of high concentrations of arsenic could inhibit arsenic uptake completely and therefore it could be used in large-scale cultivation for phytostabilization of plants.