Effect of arsenite-oxidizing bacterium B. laterosporus on arsenite toxicity and arsenic translocation in rice seedlings

Effects of the combined regulation of nitrogen, phosphorus, and potassium nutrients on the migration and transformation of arsenic species in paddy soil

Nitrogen (N), phosphorus (P) and potassium (K) are three macroelements in agriculture production, but their combined effects on arsenic (As) toxicity and its translocation in rice plants are not clear. In this study, an orthogonal rotation combination based on different N, P and K (NPK) concentration was first designed to examine their combined effect on the As toxicity, its transformation and migration in rice plants based on the hydroponic culture and pot soil culture. The results showed that 2.0 mg/L arsenite (As(III)) had obvious toxicity on the growth of indica LuYouMingZhan (LYMZ) and the optimal NPK concentration was 28.41, 6 and 50 mg/L based on the quadratic regression of the recovery rate of chlorophyll SPAD value of indica LYMZ. The optimal NPK combination significantly alleviated the physiological toxicity of As(III) on indica LYMZ rice seedling and decreased the accumulation of inorganic As in their roots and shoots by 23.8±1.8 % and 33.4±2.4 % respectively; further pot culture from different As(III) polluted soil showed that the optimal NPK combination significantly increased the dry weight of roots, stems, sheaths and leaves of indica LYMZ rice plants as well as yield indicators by 6.4 %-61.7 % and 7.1 %-89.8 % respectively, decreased the accumulation of As(III) and arsenate by 6.25 %-100 % and 12.36 %-100 % respectively in their roots, stems, sheaths, leaves, brans and kernels except As(III) concentration in their sheaths, decreased the accumulation of dimethylarsenate in their sheaths, leaves, brans and kernels, and had the best repair effect on the translocation of As species in 50 mg/kg As(III)-added soil. Our study provided a desirable strategy for alleviating As toxicity in paddy soil and reducing As pollution in rice plants.

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.

As(III)-Oxidizing Bacteria Alleviate Arsenite Toxicity via Reducing As Accumulation, Elevating Antioxidative Activities and Modulating Ionome in Rice (Oryza sativa L.)

Paddy rice trends to accumulate more arsenic (As) from soils than other terrestrial crops. The toxicity and mobility of As mainly depend on its chemical species. Transformation of arsenite [As(III)] into arsenate [As(V)] would be a promising method to mitigate As toxicity. In the current study, As(III)-oxidizing strain SMS11 isolated from As-contaminated soils was employed for As remediation. Co-cultured with SMS11 alleviated As(III) stress to the rice plants by increasing the length and biomass of rice shoots up to 10% and 15%, respectively. Evaluation of oxidative stress indices showed that the activity of catalase in the rice shoots was weakened when exposed to As(III), increasing the risk of hydroxyl radical (·OH) formation. When co-cultivated with the bacteria, ·OH formation was significantly inhibited in the rice shoots. The ionomes of the rice plants were impacted by the external conditions. As(III) stress significantly disturbed ionome homeostasis in the rice plants. Uptake of As simultaneously elevated the levels of macro and nutrient elements such as Mg, P, K, Ca, and Zn in the rice shoots. The ionomic variation in the rice plants under As(III) stress was mitigated by inoculated with SMS11. The results represented that the As(III)-oxidizing bacteria alleviated external As(III) stress to the rice plants through elevating antioxidative activities and modulating ionome homeostasis.

Ionome profiling and arsenic speciation provide evidence of arsenite detoxification in rice by phosphate and arsenite-oxidizing bacteria

Arsenite (As(III)) as the most toxic and mobile form is the dominant arsenic (As) species in flooded paddy fields, resulting in higher accumulation of As in paddy rice than other terrestrial crops. Mitigation of As toxicity to rice plant is an important way to safeguard food production and safety. In the current study, As(III)-oxidizing bacteria Pseudomonas sp. strain SMS11 was inoculated with rice plants to accelerate conversion of As(III) into lower toxic arsenate (As(V)). Meanwhile, additional phosphate was supplemented to restrict As(V) uptake by the rice plants. Growth of rice plant was significantly inhibited under As(III) stress. The inhibition was alleviated by the introduction of additional P and SMS11. Arsenic speciation showed that additional P restricted As accumulation in the rice roots via competing common uptake pathways, while inoculation with SMS11 limited As translocation from root to shoot. Ionomic profiling revealed specific characteristics of the rice tissue samples from different treatment groups. Compared to the roots, ionomes of the rice shoots were more sensitive to environmental perturbations. Both extraneous P and As(III)-oxidizing bacteria SMS11 could alleviate As(III) stress to the rice plants through promoting growth and regulating ionome homeostasis.

Arsenic (As) resistant bacteria with multiple plant growth-promoting traits: Potential to alleviate As toxicity and accumulation in rice

A currently serious agronomic concern for paddy soils is arsenic (As) contamination. Paddy soils are mostly utilized for rice cultivation. Arsenite (As(III)) is prevalent in paddy soils, and its high mobility and toxicity make As uptake by rice substantially greater than that by other food crops. Globally, interest has increased towards using As-resistant plant growth-promoting bacteria (PGPB) to improve plant metal tolerance, promote plant growth, and immobilize As to prevent its uptake and accumulation in the edible parts of rice as much as possible. This review focuses on the As-resistant PGPB characteristics influencing rice growth and the mechanisms by which they function to alleviate As toxicity stress in rice plants. Several recent examples of mechanisms responsible for decreasing the availability of As to rice and coping with As stresses facilitated by the PGPB with multiple PGP traits (e.g., phosphate and silicate solubilization, the production of 1-aminocyclopropane-1-carboxylate deaminase, phytohormones, and siderophore, N₂ fixation, sulfate reduction, the biosorption, bioaccumulation, methylation, and volatilization of As, and arsenite oxidation) are also reviewed. In addition, future research needs about the application of As-resistant PGPB with PGP traits to mitigate As accumulation in rice plants are described.

Mitigation of arsenic toxicity in rice by the co-inoculation of arsenate reducer yeast with multifunctional arsenite oxidizing bacteria

The study aimed to explicate the role of microbial co-inoculants for the mitigation of arsenic (As) toxicity in rice. Arsenate (AsV) reducer yeast Debaryomyces hansenii NBRI-Sh2.11 (Sh2.11) with bacterial strains of different biotransformation potential was attempted to develop microbial co-inoculants. An experiment to test their efficacy (yeast and bacterial strains) on plant growth and As uptake was conducted under a stressed condition of 20 mg kg-1 of arsenite (AsIII). A combination of Sh2.11 with an As(III)-oxidizer, Citrobacter sp. NBRI-B5.12 (B5.12), resulted in ∼90% decrease in grain As content as compared to Sh2.11 alone (∼40%). Reduced As accumulation in rice roots under co-treated condition was validated with SEM-EDS analysis. Enhanced As expulsion in the selected combination under in vitro conditions was found to be correlated with higher As content in the soil during their interaction with plants. Selected co-inoculant mediated enhanced nutrient uptake in association with better production of indole acetic acid (IAA) and gibberellic acid (GA) in shoot, support microbial co-inoculant mediated better biomass under stressful condition. Boosted defense response in association with enhanced glutathione-S-transferase (GST) and glutathione reductase (GR), activities under in vitro and in vivo conditions were observed. These results indicated that the As(III) oxidizer-B5.12 accelerated the As detoxification property of the As(V) reducer-Sh2.11. Henceforth, the results confer that the coupled reduction-oxidation process of the co-inoculant reduces the accumulation of As in rice grain. These co-inoculants can be further developed for field trials to achieve higher biomass with alleviated As toxicity in rice.

Potential of methyltransferase containing Pseudomonas oleovorans for abatement of arsenic toxicity in rice

Arsenic (As) has become natural health hazard for millions of people across the world due to its distribution in the food chain. Naturally, it is present in different oxidative states of inorganic [As(V) and As(III)] and organic (DMA, MMA and TMA) forms. Among different mitigation approaches, microbe mediated mitigation of As toxicity is an effective and eco-friendly approach. The present study involves the characterization of bacterial strains containing arsenite methyltransferase (Pseudomonas oleovorans, B4.10); arsenate reductase (Sphingobacterium puteale, B4.22) and arsenite oxidase (Citrobacter sp., B5.12) activity with plant growth promoting (PGP) traits. Efficient reduction of grain As content by 61 % was observed due to inoculation of methyltransferase containing B4.10 as compared to B4.22 (47 %) and B5.12 (49 %). Reduced bioaccumulation of As in root (0.339) and shoot (0.166) in presence of B4.10 was found to be inversely related with translocation factor for Mn (3.28), Fe (0.073), and Se (1.82). Bioaccumulation of these micro elements was found to be associated with the modulated expression of different mineral transporters (OsIRT2, OsFRO2, OsTOM1, OsSultr4;1, and OsZIP2) in rice shoot. Improved dehydrogenase (407 %), and β-glucosidase (97 %) activity in presence of P. oleovorans (B4.10) as compared to arsenate reductase (198 and 50 %), and arsenite oxidase (134 and 69 %) containing bacteria was also observed. Our finding confers the potential of methyltransferase positive P. oleovorans (B4.10) for As stress amelioration. Reduced grain As uptake was found to be mediated by improved plant growth and nutrient uptake associated with enhanced soil microbial activity.

Responses to arsenic stress of rice varieties coinoculated with the heavy metal-resistant and rice growth-promoting bacteria Pseudomonas stutzeri and Cupriavidus taiwanensis

Arsenic (As)-contaminated rice paddy fields are spreading globally, and thus, rice grains with low As accumulation at a safe level for consumption is profoundly needed. Rice is highly susceptible to As accumulation, and the responses to As vary among rice varieties. Here, combinations of the As^III-oxidizing bacteria Pseudomonas stutzeri strains 4.25, 4.27, or 4.44 and Cupriavidus taiwanensis KKU2500-3 were investigated with respect to their responses to As toxicity and rice growth promotion during the early growth stage. All bacterial strains enhanced antioxidant enzyme activities, including SOD, CAT, APX, GPX, and GR, under As stress in vitro. Uninoculated and coinoculated rice seedlings of three rice varieties (KDML105, RD6, RD10) were cultivated in hydroponic solution without and with a combination of toxic As^III and less toxic As^V for 30 days. Compared with uninoculated seedlings, the inoculated seedlings showed higher growth parameters and lower As contents in roots, shoots and throughout the plants. The bioconcentration factor (BCF) and translocation factor were reduced in inoculated seedlings. The effective response of rice to As toxicity influenced by bacteria was highest in KDML105, followed by RD6 and RD10. The root sulfide content was correlated with As accumulation in roots, shoots, and total seedlings and the BCFs. P. stutzeri 4.44 and C. taiwanensis KKU2500-3 were the most promising combinations for application in KDML105 cultivation under As-contaminated conditions. Understanding the basic response of rice coinoculated with effective bacteria at the early stage will provide guidelines for rice cultivation under As conditions at other scales.

Improved grain yield and lowered arsenic accumulation in rice plants by inoculation with arsenite-oxidizing Achromobacter xylosoxidans GD03

Arsenite is the predominant arsenic species in flooded paddy soil, and arsenite bioaccumulation in rice grains has been identified as a major problem in many Asian countries. Lowering arsenite level in rice plants and grain via accelerating arsenite oxidation is a potential strategy to help populations, who depended on rice consumption, to reduce the internal exposure level of arsenic. We herein isolated a strain, Achromobacter xylosoxidans GD03, with the high arsenite-oxidizing ability and plant growth-promoting traits. We observed that arsenite exposure could promote A. xylosoxidans GD03 to excrete indole-3-acetic acid and thus promoted rice growth. The pot culture experiments of Indica rice cultivar Guang You Ming 118 (GYM118) demonstrated that A. xylosoxidans GD03 inoculation of paddy soil (4.5-180 × 10⁸ CFU GD03/kg soil) significantly accelerated arsenite oxidation in flooded soil. The daily arsenic oxidation rate with GD03 inoculation was 1.5-3.3 times as that without strain GD03 inoculation within the whole growth period of Indica GYM118 in the presence of the native microflora. It thus led to a 34-69%, 43-74%, 24-76% and 35-57% decrease in arsenite concentration of the stems, leaves, bran and grain of Indica GYM118 respectively and a 59-96% increase in rice grain yield. The paddy soil inoculated with 40.0 mL/kg of A. xylosoxidans GD03 resulted in a lowest As(III) concentrations in all rice organs of Indica GYM118, which equivalent to only 24-50% of the As(III) concentrations in the group without GD03 inoculation. The results highlight that a highly arsenite-oxidizing bacterium could accelerate arsenite oxidation of paddy soil when facing competition with the native microflora, thus decrease arsenic toxicity and bioavailable soil arsenic.

Metal(loid)s (As, Hg, Se, Pb and Cd) in paddy soil: Bioavailability and potential risk to human health

Rice is one of the principal staple foods, essential for safeguarding the global food and nutritional security, but due to different natural and anthropogenic sources, it also acts as one of the biggest reservoirs of potentially toxic metal(loids) like As, Hg, Se, Pb and Cd. This review summarizes mobilization, translocation and speciation mechanism of these metal(loids) in soil-plant continuum as well as available cost-effective remediation measures and future research needs to eliminate the long-term risk to human health. High concentrations of these elements not only cause toxicity problems in plants, but also in animals that consume them and gradual deposition of these elements leads to the risk of bioaccumulation. The extensive occurrence of contaminated rice grains globally poses substantial public health risk and merits immediate action. People living in hotspots of contamination are exposed to higher health risks, however, rice import/export among different countries make the problem of global concern. Accumulation of As, Hg, Se, Pb and Cd in rice grains can be reduced by reducing their bioavailability, and controlling their uptake by rice plants. The contaminated soils can be reclaimed by phytoremediation, bioremediation, chemical amendments and mechanical measures; however these methods are either too expensive and/or too slow. Integration of innovative agronomic practices like crop establishment methods and improved irrigation and nutrient management practices are important steps to help mitigate the accumulation in soil as well as plant parts. Adoption of transgenic techniques for development of rice cultivars with low accumulation in edible plant parts could be a realistic option that would permit rice cultivation in soils with high bioavailability of these metal(loid)s.

Effect of selenium in soil on the toxicity and uptake of arsenic in rice plant

Selenium can regulate arsenic toxicity by strengthening antioxidant potential, but the antagonism between selenite or selenate nutrient and the translocation of arsenic species from paddy soil to different rice organs are poorly understood. In this study, a pot experiment was designed to investigate the effect of selenite or selenate on arsenite or arsenate toxicity to two indica rice cultivars (namely Ming Hui 63 and Lu You Ming Zhan), and the uptake and transportation of arsenic species from paddy soil to different rice organs. The results showed that selenite or selenate could significantly decrease the arsenate concentration in pore water of soils, and thus inhibited arsenate uptake by rice roots. However, the existence of selenite or selenate didn't decrease arsenate concentration in rhizosphere pore water of two indica rice cultivars. There existed good antagonistic effect between selenite or selenate and the uptake of arsenite and arsenate in rice plant in the case of low arsenic paddy soil. However, this antagonism depended on rice cultivars, arsenic species and arsenic level in soil. There existed both synergistic and inhibiting effects between the addition of selenite or selenate and the uptake of trimethylarsinoxide and dimethylarsinic acid by two indica rice cultivars, but the mechanism was unclear. Both selenite and selenate are all effective to decrease the translocation of inorganic arsenic from the roots to their above-ground rice organs in arsenite/arsenate-spiked paddy soil, but selenate had stronger inhibiting effect on their transfer factors than selenite.

Measuring and Achieving Equity in Multiperiod Emergency Material Allocation

Emergency material allocation is an important part of postdisaster emergency logistics that is significant for improving rescue effectiveness and reducing disaster losses. However, the traditional single-period allocation model often causes local surpluses or shortages and high cost, and prevents the system from achieving an equitable or optimal multiperiod allocation. To achieve equitable allocation of emergency materials in the case of serious shortages relative to the demand by victims, this article introduces a multiperiod model for allocation of emergency materials to multiple affected locations (using an exponential utility function to reflect the disutility loss due to material shortfalls), and illustrates the relationship between equity of allocations and the cost of emergency response. Finally, numerical examples are presented to demonstrate both the feasibility and the usefulness of the proposed model for achieving multiperiod equitable allocation of emergency material among multiple disaster locations. The results indicate that the introduction of a nonlinear utility function to reflect the disutility of large shortfalls can make the material allocation fairer, and minimize large losses due to shortfalls. We found that achieving equity has a significant but not unreasonable impact on emergency costs. We also illustrate that using differing utility functions for different types of materials adds an important dimension of flexibility.

Evaluation of uptake, translocation, and accumulation of arsenic species by six different Brazilian rice (Oryza sativa L.) cultivars

Rice is a significant source of arsenic (As) exposure. The accumulation of the plant depends on several factors, including environmental conditions and genetic factors. The differences in As uptake, translocation, and grains filling in different cultivars are a focus on studies to mitigate the grains contamination. This study assessed the pattern of As species accumulation in different Brazilian rice cultivars (Oryza sativa L.). Thus, pot experiments were conducted with 6 different cultivars (white rice: EPAGRI 109, EPAGRI 108, BRS Tiotaka SCS, and SCS 114 Andosan and red rice: Maranhão and Cáqui) cultivated in soils at low (As-) (0.65 mg kg^-1) and high (As+) (12.1 mg kg^-1) As levels. All cultivars in As+ group presented total As (t-As) in grains more elevated than the maximum limit of inorganic arsenic (i-As) recommended by Codex Alimentarius Commission. The As speciation disclose that Maranhão, Caqui, and SCS 114 Andosan cultivars presented the lowest % i-As (27%, 25% and 31%, respectively) at the highest As exposure condition. On the other hand, higher i-As concentration and % i-As (91%) were observed in EPAGRI 108. Moreover, EPAGRI 108 and EPAGRI 109 had the highest transference factor soil-to-grain (TFsoil-grain = 0.22 and 0.20, respectively). Interestingly, for the cultivars EPAGRI 108 and Maranhão, the levels of some essential elements (Co and Mn) in grains were modulated by the levels of As in the soil. This study shows that levels of i-As were modulated by the type of Brazilian rice cultivar, the range of As levels in soil, As phytotoxicity and the transference factor of As from soil to root straw and grains. Moreover, SCS 114 Andosan is the promising cultivar that exhibits low t-As and % i-As in grains and low TF soil-grain.

Reduction in arsenic toxicity and uptake in rice (Oryza sativa L.) by As-resistant purple nonsulfur bacteria

This study aimed to investigate the potential of Rhodopseudomonas palustris C1 and Rubrivivax benzoatilyticus C31 to ameliorate As toxicity and to reduce As uptake in rice. Strain C1 was superior to strain C31 for siderophore production. The mixed culture (1: 1) was most effective in reducing the toxicity of As species [As(III) and/or As(V), each 30 mg/l] by yielding maximal germination index that related to α- and β-amylase activities in two Thai rice cultivars (HomNil: HN and PathumThani 1: PT). Arsenic toxicity to the seed germination followed the order: mixed As species > As(III) > As(V); and the toxicity was reduced in inoculated sets, particularly with a mixed culture. The mixed culture significantly enhanced rice growth under As stress in both rice cultivars as indicated by an increase in the production of chlorophyll a and b, and also supporting the non-enzymatic (carotenoids, lipid oxidation, and nitric oxide) and enzymatic (superoxide dismutase, ascorbate peroxidase, catalase, and glutathione reductase) activities. These were concomitant with productions of 5-aminolevulinic acid, indole-3-acetic acid, exopolymeric substances, and siderophores which significantly reduced As accumulation in treated rice. It can be concluded that the mixed culture has great potential to ameliorate rice from As toxicity by preventing As species entry into rice for enhancing rice growth and also for reducing As accumulation to produce safe rice from rice grown in contaminated paddy fields.

Mitigation of arsenic toxicity and accumulation in hydroponically grown rice seedlings by co-inoculation with arsenite-oxidizing and cadmium-tolerant bacteria

Arsenic (As) contamination of rice grain is a serious problem worldwide. The objective of this study was to mitigate As toxicity and accumulation in hydroponically grown KDML105 rice seedlings using bacteria isolated from heavy metal-contaminated soils. Seven strains (KKU2500-1, -2, -3, -9, -12, -16 and -22) of 24 cadmium (Cd)-tolerant bacteria produced high levels of inorganic sulfide and thiol-rich compounds in As-supplemented media. The strains were allowed to colonize rice seedlings growing in arsenite [As(III)]- or arsenate [As(V)]-supplemented Hoagland's nutrient solutions. Colonization by strains KKU2500-3 and -12 led to increases in plant growth parameters and similarly reduced As translocation into shoots [translocation factor (TF) = 0.05] in the As(V)-supplemented solution. Strains KKU2500-1 and - 12 also greatly reduced As translocation into shoots (TF = 0.16-0.20) in As(III)-supplemented solution. KKU2500-3 and - 12 co-colonized onto seedlings with the As(III)-oxidizing isolates 4.25, 4.27, 4.40 and 4.44, and the strain combinations KKU2500-12/4.25, KKU2500-3/4.25, KKU2500-3/4.27 and KKU2500-3/4.44 resulted in higher growth parameters for plants grown in As [As(III)+As(V)]-supplemented solution than other combinations. Moreover, the combinations KKU2500-3/4.25 and KKU2500-3/4.44 greatly reduced As translocation (TF = 0.15 and 0.12, respectively), and this decreased As accumulation in shoots was significantly correlated with increased sulfide stimulation in roots and nutrient solution. These results indicate that these co-inoculated bacteria can mitigate As toxicity, translocation and accumulation in KDML105 seedlings and thus demonstrate synergistic activity in rice plants, and this effect can be further developed in field trials.

Arsenic accumulation and speciation in rice grown in arsanilic acid-elevated paddy soil

P-arsanilic acid (AsA) is a emerging but less concerned contaminant used in animal feeding operations, for it can be degraded to more toxic metabolites after being excreted by animals. Rice is the staple food in many parts of the world, and also more efficient in accumulating arsenic (As) compared to other cereals. However, the uptake and transformation of AsA by rice is unclear. This study aimed to evaluate the potential risk of using AsA as a feed additive and using the AsA contaminated animal manure as a fertilizer. Five rice cultivars were grown in soil containing 100mg AsA/kg soil, after harvest, As species and their concentrations in different tissues were determined. Total As concentration of the hybrid rice cultivar was more than conventional rice cultivars for whole rice plant. For rice organs, the highest As concentration was found in roots. AsA could be absorbed by rice, partly degraded and converted to arsenite, monomethylarsonic acid, dimethylarsinic acid, arsenate. The number of As species and their concentrations in each cultivar were related to their genotypes. The soil containing 100mg AsA/kg or more is unsuitable for growing rice. The use of AsA and the disposal of animal manure requires detailed attention.