Integrated application of selenium and silica reduce arsenic accumulation and enhance the level of metabolites in rice grains

In this study, rice plants were co-exposed to selenium (Se) and silica (Si) under arsenic (As) stress to evaluate As accumulation in rice grains, associated cancer risk, and its impact on the types and numbers of grain metabolites. A total of 58 metabolites were identified, of which, 19 belong to sugars, and drastically altered during different treatments. Arsenic exposure significantly reduced monosaccharides, i.e., D-glucose (83%) >D-galactose (60%) >D-fructose (57%) >D-ribose (29%) but increased that monosaccharide units which have antioxidant properties (i.e. α-D-glucopyranoside and melibiose). However, the levels of D-galactose, fructose, and ribose were significantly increased during co-supplementation of selenite (SeIV) and Si under As stress. Other groups of rice grain metabolites, like sugar alcohols, organic acids, polyphenols, carboxylic acids, fatty acids, and phytosterols, were also significantly altered by As exposure and increased in grains of SeIV and Si supplemented rice compared to alone As exposure. In brief, rice growing in As-affected areas may have a low level of different metabolites. However, supplementation by selenite (SeIV) with Si not only increased metabolites and amylose/amylopectin ratio but also reduced ∼90% of As accumulation in grains. Thus, the use of SeIV with Si might be advantageous for the locals to provide a healthy diet of rice and limit As-induced cancer risk up to 10-fold.

Conventional and biodegradable microplastics affected arsenic mobility and methylation in paddy soils through distinct chemical-microbial pathways

The presence of microplastics (MPs) in paddy soil has become a growing concern, yet the influence of MPs on arsenic (As) dynamics in paddy soil remains largely unexplored. A 98-day microcosm experiment was conducted to investigate the impact of MPs on As behavior in As-contaminated paddy soil. The results revealed that conventional microplastics (CMPs) reduced As concentration in porewater by 25-38 %, but substantially increased the percentage of methylated As (% MeAs) in soil by 8-23 times under 5 % dosages after 98-day incubation. In contrast, at the end of incubation, biodegradable microplastics (BMPs) at 5 % dosages notably increased As concentration in porewater and % MeAs in soil by 2-9 times and 11-395 times, respectively. The combination of network analysis and Random-Forest analysis implied that CMPs might inhibit As mobility through enhancing microbial As(III) oxidation and promote As methylation by enriching arsM-carrying microbes. However, BMPs promoted As release mainly accompanying with microbial iron reduction, and enhanced As methylation through enriching fermenting bacteria (i.e., Clostridiaceae) and arsM-carrying organic matter degrading bacteria (i.e., Gemmatimonas and Nocardia). These findings might provide broaden insights into As cycling induced by MPs and contribute to the prevention of combined pollution from As and MPs in paddy soil.

Predicting the risk of arsenic accumulation in soil-rice system in Asian monsoon region

Rice is a staple food for a significant portion of the global population. Arsenic (As) accumulated in rice grains influences rice quality which threatens human health. In this study, we used three machine learning models to predict arsenic accumulation in rice based on over 300 surveys. The prediction results of soil arsenic indicate that high-arsenic soil areas are mainly distributed in South and Southeast Asia such as India, China, and Thailand. In addition, higher bioaccumulation factors (BAF), associated with higher temperature, are predominantly observed in eastern India and southern Myanmar. However, arsenic content in soil is relatively lower in these areas. About 5.5 billion population may be threatened by the consumption of high-arsenic rice. It can be concluded that temperatures may influence the BAF except for soil arsenic, and soil physicochemical properties. Further research on the relationship between climate parameters and BAF should be conducted to address and adapt to future climate change. Additionally, understanding the mechanism of arsenic accumulation under different climatic conditions is crucial for developing agricultural technologies to reduce arsenic accumulation in rice.

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.

Synergistic effects of silicon and goethite co-application in alleviating cadmium stress in rice (Oryza sativa L.): Insights into plant growth and iron plaque formation mechanisms

Rice is one of the most important staple food crops; however, it is prone to cadmium (Cd) accumulation, which has negative health effects. Therefore, methods to reduce Cd uptake by rice are necessary. At present, there is limited research on the effects of co-application of silicon (Si) and goethite in mitigating Cd stress in rice. Furthermore, the specific mechanisms underlying the effects of their combined application on iron plaque formation in rice roots remain unclear. Therefore, this study analyzed the effects of the combined application of Si and goethite on the biomass, physiological stress indicators, Cd concentration, and iron plaques of rice using hydroponic experiments. The results revealed that co-treatment with both Si and goethite increased the plant height and dry weight, superoxide dismutase and catalase activities, photosynthetic pigment concentration, and root activity. Moreover, this treatment decreased the malondialdehyde concentration, repaired epidermal cells, reduced the Cd concentration in the roots by 57.2 %, and increased the number of iron plaques and Cd concentration by 150.9 % and 266.2 % in the amorphous and crystalline fractions, respectively. The Cd/Fe ratio in amorphous iron plaques also increased. Our findings suggest that goethite serves as a raw material for iron plaque formation, while Si enhances the oxidation capacity of rice roots. The application of a combination of Si and goethite increases the quantity and quality of iron plaques, enhancing its Cd fixation capacity. This study provides theoretical evidence for the effective inhibition of Cd uptake by iron plaques in rice, providing insights into methods for the remediation of Cd contamination.

Nutrient and mycoremediation of a global menace 'arsenic': exploring the prospects of phosphorus and Serendipita indica-based mitigation strategies in rice and other crops

KEY MESSAGE

Serendipita indica induced metabolic reprogramming in colonized plants complements phosphorus-management in improving their tolerance to arsenic stress on multifaceted biological fronts. Restoration of the anthropic damage done to our environment is inextricably linked to devising strategies that are not only economically sound but are self-renewing and ecologically conscious. The dilemma of heavy metal (HM) dietary ingestion, especially arsenic (As), faced by humans and animals alike, necessitates the exploitation of such technologies and the cultivation of healthy and abundant crops. The remarkable symbiotic alliance between plants and 'mycorrhizas' has evolved across eons, benefiting growth/yield aspects as well as imparting abiotic/biotic stress tolerance. The intricate interdependence of Serendipita indica (S. indica) and rice plant reportedly reduce As accumulation, accentuating the interest of microbiologists, agriculturists, and ecotoxicological scientists apropos of the remediation mechanisms of As in the soil-AMF-rice system. Nutrient management, particularly of phosphorus (P), is also praised for mitigating As phytotoxicity by deterring the uptake of As molecules due to the rhizospheric cationic competition. Taking into consideration the reasonable prospects of success in minimizing As acquisition by rice plants, this review focuses on the physiological, metabolic, and transcriptional alterations underlying S. indica symbiosis, recuperation of As stress together with nutritional management of P by gathering case studies and presenting successful paradigms. Weaving together a volume of literature, we assess the chemical forms of As and related transport pathways, discuss As-P-rice interaction and the significance of fungi in As toxicity mitigation, predominantly the role of mycorrhiza, as well as survey of the multifaceted impacts of S. indica on plants. A potential strategy for simultaneous S. indica + P administration in paddy fields is proposed, followed by future research orientation to expand theoretic comprehension and encourage field-based implementation.

Physiological and transcriptomic analyses reveal that phytohormone pathways and glutathione metabolism are involved in the arsenite toxicity response in tomatoes

The main forms of inorganic arsenic (As) in soil are arsenate [As(V)] and arsenite [As(III)]. Both forms inhibit plant growth. Here, we investigate the effects of As(III) toxicity on the growth of tomatoes by integrating physiological and transcriptomic analyses. As(III) toxicity induces oxidative damage, inhibits photosynthetic efficiency, and reduces soluble sugar levels. As(III) toxicity leads to reductions in auxin, cytokinin and jasmonic acid contents by 29 %, 39 % and 55 %, respectively, but leads to increases in the ethylene precursor 1-amino-cyclopropane carboxylic acid, abscisic acid and salicylic acid contents in roots, by 116 %, 79 % and 39 %, respectively, thereby altering phytohormone signalling pathways. The total glutathione, reduced glutathione (GSH) and oxidized glutathione (GSSG) contents are reduced by 59 %, 49 % and 94 % in roots; moreover, a high GSH/GSSG ratio is maintained through increased glutathione reductase activity (increased by 214 %) and decreased glutathione peroxidase activity (decreased by 40 %) in the roots of As(III)-treated tomato seedlings. In addition, As(III) toxicity affects the expression of genes related to the endoplasmic reticulum stress response. The altered expression of aquaporins and ABCC transporters changes the level of As(III) accumulation in plants. A set of hub genes involved in modulating As(III) toxicity responses in tomatoes was identified via a weighted gene coexpression network analysis. Taken together, these results elucidate the physiological and molecular regulatory mechanism underlying As(III) toxicity and provide a theoretical basis for selecting and breeding tomato varieties with low As(III) accumulation. Therefore, these findings are expected to be helpful in improving food safety and to developing sustainable agricultural.

Cadmium, arsenic, and mineral nutrients in rice and potential risks for human health in South China

Rice (Oryza sativa L.) is one of the most important staple food crops worldwide. For people fed on rice, toxic elements cadmium (Cd) and arsenic (As) and mineral nutrients in rice are pivotal to evaluate potential risks of harmful element intake and malnutrition. We collected rice samples of 208 cultivars (83 inbred and 125 hybrid) from fields in South China and determined Cd, As, As species, and mineral elements in brown rice. Chemical analysis shows that the average content of Cd and As in brown rice were 0.26 ± 0.32 and 0.21 ± 0.08 mg·kg^-1, respectively. Inorganic As (iAs) was the dominative As species in rice. Rice Cd and iAs in 35.1% and 52.4% of the 208 cultivars exceeded rice Cd and iAs limits, respectively. Significant variations of rice subspecies and regions were found for Cd, As, and mineral nutrients in rice (P < 0.05). Inbred rice had lower As uptake and more balanced mineral nutrition than hybrid species. Significant correlation was observed between Cd, As versus mineral elements like Ca, Zn, B, and Mo (P < 0.05). Health risk assessment indicates that high risks of non-carcinogenic and carcinogenic of Cd and iAs, and malnutrition, in particular Ca, protein and Fe deficiencies, might be caused by rice consumption in South China.

Different levels of arsenic exposure through cooked rice and its associated benefit-risk assessment from rural and urban populations of West Bengal, India: a probabilistic approach with sensitivity analysis

Rice arsenic (As) contamination and its consumption poses a significant health threat to humans. The present study focuses on the contribution of arsenic, micronutrients, and associated benefit-risk assessment through cooked rice from rural (exposed and control) and urban (apparently control) populations. The mean decreased percentages of As from uncooked to cooked rice for exposed (Gaighata), apparently control (Kolkata), and control (Pingla) areas are 73.8, 78.5, and 61.3%, respectively. The margin of exposure through cooked rice (MoE_{cooked rice}) < 1 signifies the existence of health risk for all the studied exposed and control age groups. The respective contributions of iAs (inorganic arsenic) in uncooked and cooked rice are nearly 96.6, 94.7, and 100% and 92.2, 90.2, and 94.2% from exposed, apparently control, and control areas. LCR analysis for the exposed, apparently control, and control populations (adult male: 2.1 × 10^-3, 2.8 × 10^-4, 4.7 × 10^-4; adult female: 1.9 × 10^-3, 2.1 × 10^-4, 4.4 × 10^-4; and children: 5.8 × 10^-4, 4.9 × 10^-5, 1.1 × 10^-4) through cooked rice is higher than the recommended value, i.e., 1 × 10^-6, respectively, whereas HQ > 1 has been observed for all age groups from the exposed area and adult male group from the control area. Adults and children from rural area showed that ingestion rate (IR) and concentration are the respective influencing factors towards cooked rice As, whereas IR is solely responsible for all age groups from urban area. A vital suggestion is to reduce the IR of cooked rice for control population to avoid the As-induced health risks. The average intake (μg/day) of micronutrients is in the order of Zn > Se for all the studied populations and Se intake is lower for the exposed population (53.9) compared to the apparently control (140) and control (208) populations. Benefit-risk assessment supported that the Se-rich values in cooked rice are effective in avoiding the toxic effect and potential risk from the associated metal (As).

The role of sulfur nutrition in plant response to metal(loid) stress: Facilitating biofortification and phytoremediation

Metal(loid)s contamination poses a serious threat to ecosystem biosafety and human health. Phytoremediation is a cost-effective and eco-friendly technology with good public acceptance, although the process does require a significant amount of time for success. To enhance the phytoremediation efficiency, numerous approaches have been explored, including soil amendments application with chelators to facilitate remediation. Sulfur (S), a macronutrient for plant growth, plays vital roles in several metabolic pathways that can actively affect metal(loid)s phytoextraction, as well as attenuate metal(loid) toxicity. In this review, different forms of S-amendments (fertilizers) on uptake and translocation in plants upon exposure to various metal(loid) are evaluated. Possible mechanisms for S application alleviating metal(loid) toxicity are documented at the physiological, biochemical and molecular levels. Furthermore, this review highlights the crosstalk between S-assimilation and other biomolecules, such as phytohormones, polyamines and nitric oxide, which are also important for metal(loid) stress tolerance. Given the effectiveness and potential of S amendments on phytoremediation, future studies should focus on optimizing phytoremediation efficiency in long-term field studies and on investigating the appropriate S dose to maximize the food safety and ecosystem health.

How different nitrogen fertilizers affect arsenic mobility in paddy soil after straw incorporation?

In straw return fields, nitrogen-fertilizers are added to mitigate microbial competition for nitrogen with plants. However, in arsenic (As)-contaminated paddy fields, the specific effects of different nitrogen fertilizers on As mobility after straw incorporation and the interactions among iron(Fe)/carbon(C)/nitrogen(N)/As are not well understood. In the reported microcosm experiment we monitored As-mobility as a function of different dosages of KNO₃, NH₄Cl and rice straw incorporation. Addition of both KNO₃ and NH₄Cl significantly inhibited the As mobilization induced by straw incorporation. Following the KNO₃ addition, the As concentration in porewater dropped by 51-66% after 2 days of the incubation by restraining Fe reduction and enhancing Fe oxidation. High-dose NH₄Cl addition reduced As in porewater by 22-43% throughout the incubation by decreasing porewater pH. High-throughput sequencing results demonstrated that KNO₃ addition enriches both the denitrifying and Fe-oxidizing bacteria, while diminishing Fe-reducing bacteria; NH₄Cl addition has the opposite effect on Fe-reducing bacteria. Network analysis revealed that As and Fe concentrations in porewater were positively correlated with the abundance of denitrifying and Fe-reducing bacteria. This study broadens our insight into the As biogeochemistry associated with the N/C/Fe balance in soil, which are of great significance for agronomic management and mitigation the risk of As-contaminated paddy fields.

Investigating resistance levels to cyhalofop-butyl and mechanisms involved in Chinese sprangletop (Leptochloa chinensis L.) from Anhui Province, China

Chinese sprangletop (Leptochloa chinensis (L.) Nees) is a common grass species that severely threatens rice (Oryza sativa L.) cropping systems globally. Cyhalofop-butyl is a highly efficient acetyl-CoA carboxylase (ACCase)-inhibiting herbicide widely used for control of this species in China. However, some L. chinensis populations have gradually evolved resistance to this herbicide in recent years. To better understand the cyhalofop-butyl resistance status of L. chinensis in the major rice planting area of the middle-lower Yangtze River basin, 73 populations collected from the rice fields across Anhui Province were investigated for cyhalofop-butyl susceptibility and potential herbicide resistance-conferring mutations. Single-dose testing indicated that of the 73 populations, 25 had evolved resistance to cyhalofop-butyl and were separately classified as "RRR" and "RR" populations according to their fresh weight reductions, 8 had a high risk of evolving cyhalofop-butyl resistance and were classified as "R?" populations, and 40 were susceptible and classified as "S" populations. Whole-plant dose-response experiments showed that the resistance index (RI) of these R?, RR, and RRR populations to cyhalofop-butyl ranged from 2.47 to 36.94. Target gene sequencing identified seven ACCase resistance mutations (I1781L, W1999C, W2027S, W2027L, W2027C, I2041N, and D2078G), with W1999C and W2027C the two most common detected in about three quarters of all the resistant populations. Seven populations including LASC3, BBHY1, AQQS1, HFFD3, HFFD4, AQWJ1, and HFLJ6 each carrying a specific ACCase mutation were tested for their cross- and multiple-resistance patterns. Compared with a standard susceptible population HFLY1, the seven resistant populations showed distinct cross-resistance. All had low- to high-level cross-resistance to metamifop (RIs ranging from 6.16 to 17.65), fenoxaprop-P-ethyl (RIs ranging from 6.39 to 24.08), and quizalofop-P-ethyl (RIs ranging from 2.20 to 10.25), but responded differently to clodinafop-propargyl and clethodim. Multiple-resistance testing suggested that the seven resistant populations were all susceptible to the 4-hydroxyphenylpyruvate dioxygenase inhibitor tripyrasulfone, the protoporphyrinogen oxidase inhibitor oxyfluorfen, and the auxin mimic herbicide florpyrauxifen. In conclusion, this study has shown that cyhalofop-butyl resistance was prevalent in L. chinensis in Anhui Province, China, and target site mutation was one of the most common resistance mechanisms.

An In Vitro and In Silico Perspective Study of Seed Priming with Zinc on the Phytotoxicity and Accumulation Pattern of Arsenic in Rice Seedlings

Arsenic (As) contamination of the rice agro-ecosystem is a major concern for rice farmers of South East Asia as it imposes a serious threat to human and animal life; thus, there is an unrelenting need to explore the ways by which arsenic stress mitigation could be achieved. In the present investigation, we explore the effect of zinc (Zn²⁺) supplementation using the seed priming technique for the mitigation of As-induced stress responses in developing rice seedlings. In addition to the physiological and biochemical attributes, we also studied the interactive effect of Zn²⁺ in regulating As-induced changes by targeting antioxidant enzymes using a computational approach. Our findings suggest that Zn²⁺ and As can effectively modulate redox homeostasis by limiting ROS production and thereby confer protection against oxidative stress. The results also show that As had a significant impact on seedling growth, which was restored by Zn²⁺ and also minimized the As uptake. A remarkable outcome of the present investigation is that the varietal difference was significant in determining the efficacy of the Zn²⁺ priming. Further, based on the findings of computational studies, we observed differences in the surface overlap of the antioxidant target enzymes of rice, indicating that the Zn²⁺ might have foiled the interaction of As with the enzymes. This is undoubtedly a fascinating approach that interprets the mode of action of the antioxidative enzymes under the metal/metalloid-tempted stress condition in rice by pointing at designated targets. The results of the current investigation are rationally significant and may be the pioneering beginning of an exciting and useful method of integrating physiological and biochemical analysis together with a computational modelling approach for evaluating the stress modulating effects of Zn²⁺ seed priming on As-induced responses in developing rice seedlings.

Alleviative mechanisms of silicon solubilizing Bacillus amyloliquefaciens mediated diminution of arsenic toxicity in rice

Silicon (Si) has gained considerable attention for its utility in improved plant health under biotic and abiotic stresses through alteration of physiological and metabolic processes. Its interaction with arsenic (As) has been the compelling area of research amidst heavy metal toxicity. However, microbe mediated Si solubilization and their role for reduced As uptake is still an unexplored domain. Foremost role of Bacillus amyloliquefaciens (NBRISN13) in impediment of arsenite (AsIII) translocation signifies our work. Reduced grain As content (52-72%) during SN13 inoculation under feldspar supplementation (Si+SN+As) highlight the novel outcome of our study. Upregulation of Lsi1, Lsi2 and Lsi3genes in Si+SN+As treated rice plants associated with restricted As translocation, frames new propositions for future research on microbemediated reduced As uptake through increased Si transport. In addition to low As accumulation, alleviation of oxidative stress markers by modulation of defense enzyme activities and differential accumulation of plant hormones was found to be associated with improved growth and yield. Thus, our findings confer the potential role of microbe mediated Si solubilization in mitigation of As stress to restore plant growth and yield.

Molecular insight into arsenic uptake, transport, phytotoxicity, and defense responses in plants: a critical review

A critical investigation into arsenic uptake and transportation, its phytotoxic effects, and defense strategies including complex signaling cascades and regulatory networks in plants. The metalloid arsenic (As) is a leading pollutant of soil and water. It easily finds its way into the food chain through plants, more precisely crops, a common diet source for humans resulting in serious health risks. Prolonged As exposure causes detrimental effects in plants and is diaphanously observed through numerous physiological, biochemical, and molecular attributes. Different inorganic and organic As species enter into the plant system via a variety of transporters e.g., phosphate transporters, aquaporins, etc. Therefore, plants tend to accumulate elevated levels of As which leads to severe phytotoxic damages including anomalies in biomolecules like protein, lipid, and DNA. To combat this, plants employ quite a few mitigation strategies such as efficient As efflux from the cell, iron plaque formation, regulation of As transporters, and intracellular chelation with an array of thiol-rich molecules such as phytochelatin, glutathione, and metallothionein followed by vacuolar compartmentalization of As through various vacuolar transporters. Moreover, the antioxidant machinery is also implicated to nullify the perilous outcomes of the metalloid. The stress ascribed by the metalloid also marks the commencement of multiple signaling cascades. This whole complicated system is indeed controlled by several transcription factors and microRNAs. This review aims to understand, in general, the plant-soil-arsenic interaction, effects of As in plants, As uptake mechanisms and its dynamics, and multifarious As detoxification mechanisms in plants. A major portion of this article is also devoted to understanding and deciphering the nexus between As stress-responsive mechanisms and its underlying complex interconnected regulatory networks.

The chemical-microbial release and transformation of arsenic induced by citric acid in paddy soil

Citric acid (CA) is the major exudate of rice roots, yet the effects of CA on arsenic (As) transformation and microbial community in flooded paddy soil have not been clearly elucidated. In this study, microcosms were established by amending CA to As contaminated paddy soils, mimicking the rhizosphere environment. Results showed that 0.5% CA addition significantly enhanced As mobilization after one-hour incubation, increased total As in porewater by about 20-fold. CA addition induced arsenate release into porewater, and subsequently formed ternary complex of As, iron and organic matters, inhibiting further As transformation (including arsenate reduction and arsenite methylation). Furthermore, the results of linear discriminant analysis (LDA) effect size (LEfSe) and network analysis revealed that CA addition significantly enriched bacteria associated with arsenic and iron reductions, such as Clostridium (up to 35-fold) and Desulfitobacterium (up to 4-fold). Our results suggest that CA exhibits robust ability to mobilize As through both chemical and microbial processes, increasing the risk of As accumulation by rice. This study sheds light on our understanding of As mobilization and transformation in rhizosphere soil, potentially providing effective strategies to restrict As accumulation in food crops by screening or cultivating varieties with low CA exuding.

Arsenic contamination, impact and mitigation strategies in rice agro-environment: An inclusive insight

Arsenic (As) contamination and its adverse consequences on rice agroecosystem are well known. Rice has the credit to feed more than 50% of the world population but concurrently, rice accumulates a substantial amount of As, thereby compromising food security. The gravity of the situation lays in the fact that the population in theAs uncontaminated areas may be accidentally exposed to toxic levels of As from rice consumption. In this review, we are trying to summarize the documents on the impact of As contamination and phytotoxicity in past two decades. The unique feature of this attempt is wide spectrum coverages of topics, and that makes it truly an interdisciplinary review. Aprat from the behaviour of As in rice field soil, we have documented the cellular and molecular response of rice plant upon exposure to As. The potential of various mitigation strategies with particular emphasis on using biochar, seed priming technology, irrigation management, transgenic variety development and other agronomic methods have been critically explored. The review attempts to give a comprehensive and multidiciplinary insight into the behaviour of As in Paddy -Water - Soil - Plate prospective from molecular to post-harvest phase. From the comprehensive literature review, we may conclude that considerable emphasis on rice grain, nutritional and anti-nutritional components, and grain quality traits under arsenic stress condition is yet to be given. Besides these, some emerging mitigation options like seed priming technology, adoption of nanotechnological strategies, applications of biochar should be fortified in large scale without interfering with the proper use of biodiversity.

A predictive model for arsenic accumulation in rice grains based on bioavailable arsenic and soil characteristics

Arsenic (As) is a well-known human carcinogen, and rice consumption is the main way Chinese people are exposed to As. In this study, 14 kinds of paddy soils were collected from the main rice-producing areas in China. The results showed that rice roots and leaves accumulated more As than stems and grains in the following sequence: As_root> As_leaf> As_stem> As_grain. The accumulation of As by rice grains mainly depends on the total As and bioavailable As (0.43 mol/L HNO₃ extractable As), which explained 32.2% and 22.2% of the variation in the grain As, respectively. In addition, soil pH, organic matter (OM) and clay contents were the major factors affecting grain As, explaining 13.1%, 7.9% and 5.3% of the variation, respectively. An effective prediction model was established via multiple linear regression as As_grain= 0.024 BAs - 0.225 pH+ 0.013 OM+ 0.648 EC - 0.320 TN - 0.088 TP - 0.002 AS+ 2.157 (R² =0.68, P < 0.01). Through the verification of the samples from both pot experiments and paddy fields, the model successfully provided accurate predictions for rice grain As.

Influence of agricultural amendments on arsenic biogeochemistry and phytotoxicity in a soil polluted by the destruction of arsenic-containing shells

Agricultural soils can contain high arsenic (As) concentrations due to specific geological contexts or pollution. Fertilizer amendments could influence As speciation and mobility thus increasing its transfer to crops and its toxicity. In the present study, field-relevant amounts of fertilizers were applied to soils from a cultivated field that was a former ammunition-burning site. Potassium phosphate (KP), ammonium sulfate and organic matter (OM) were applied to these soils in laboratory experiments to assess their impact on As leaching, bioavailability to Lactuca sativa and microbial parameters. None of the fertilizers markedly influenced As speciation and mobility, although trends showed an increase of mobility with KP and a decrease of mobility with ammonium sulfate. Moreover, KP induced a small increase of As in Lactuca sativa, and the polluted soil amended with ammonium sulfate was significantly less phytotoxic than the un-amended soil. Most probable numbers of AsIII-oxidizing microbes and AsIII-oxidizing activity were strongly linked to As levels in water and soils. Ammonium sulfate negatively affected AsIII-oxidizing activity in the un-polluted soil. Whereas no significant effect on As speciation in water could be detected, amendments may have an impact in the long term.

Differing effects of inorganic and organic arsenic on uptake and distribution of multi-elements in Rice grain

Arsenic (As) pollution can lead to an element imbalance in rice. A hydroponic study was carried out to examine the influence of inorganic (arsenate) and organic (dimethylarsinic acid (DMA)) arsenic compounds on the concentration and distribution of iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), nickel (Ni), carbon (C), nitrogen (N), and sulfur (S) in rice caryopsis at maturity using laser confocal microscopy and synchrotron X-ray fluorescence (SXRF). Results showed that treatments with inorganic (iAs) and organic (DMA) arsenic did not change the distribution characteristics of the above elements in rice grains. Fe, Mn, and iAs were mainly limited to the ventral ovular vascular trace, while Cu, Zn, and DMA extended into the endosperm. This implies that milling processes are likely to remove a majority of Fe, Mn, and iAs, but not Cu, Zn, and DMA. With regard to the average fluorescent intensity of the rice endosperm, iAs exposure caused significant reductions in Mn (53%), Fe (40%), Cu (27%), and Zn (74%) while DMA treatments decreased Mn (49%), Fe (37%), and Zn (21%). Compared with DMA, iAs exerted more influence on the reduction of these elements in rice caryopsis. In addition, the elemental analysis revealed a significant 12.7% increase for N and 8% reduction for S in DMA-treated rice caryopsis while a significant decrease of 24.0% for S in iAs-exposed rice caryopsis. These findings suggest that Cu, Zn, and S are more easily impacted by iAs, while N is mostly affected by DMA.

Coordinated cytokinin signaling and auxin biosynthesis mediates arsenate-induced root growth inhibition

Interactions between plant hormones and environmental signals are important for the maintenance of root growth plasticity under ever-changing environmental conditions. Here, we demonstrate that arsenate (As^V), the most prevalent form of arsenic (As) in nature, restrains elongation of the primary root through transcriptional regulation of local auxin biosynthesis genes in the root tips of Arabidopsis (Arabidopsis thaliana) plants. The ANTHRANILATE SYNTHASE ALPHA SUBUNIT 1 (ASA1) and BETA SUBUNIT 1 (ASB1) genes encode enzymes that catalyze the conversion of chorismate to anthranilate (ANT) via the tryptophan-dependent auxin biosynthesis pathway. Our results showed that As^V upregulates ASA1 and ASB1 expression in root tips, and ASA1- and ASB1-mediated auxin biosynthesis is involved in As^V-induced root growth inhibition. Further investigation confirmed that As^V activates cytokinin signaling by stabilizing the type-B ARABIDOPSIS RESPONSE REGULATOR1 (ARR1) protein, which directly promotes the transcription of ASA1 and ASB1 genes by binding to their promoters. Genetic analysis revealed that ASA1 and ASB1 are epistatic to ARR1 in the As^V-induced inhibition of primary root elongation. Overall, the results of this study illustrate a molecular framework that explains As^V-induced root growth inhibition via crosstalk between two major plant growth regulators, auxin and cytokinin.

Monsoonal paddy cultivation with phase-wise arsenic distribution in exposed and control sites of West Bengal, alongside its assimilation in rice grain

The present study mainly deals with monsoonal paddy farming with respect to its phase-wise arsenic (As) accumulation and distribution throughout cultivation in As exposed sites and control areas of West Bengal for two consecutive years, 2017 and 2018. Arsenic uptake in paddy depends on the watering pattern with the help of groundwater (Madhusudhankati: 171 μg/l, Teghoria: 493 μg/l in Gaighata and Pingla: 10 μg/l in Medinipur), soil As phase-wise movement with its enrichment pattern and the variation of rainfall. Arsenic mobility is the highest in root and decreases with height of a plant. However, the synergistic effect of groundwater and rainwater makes a diffused approach to the nature of As flow in plants, because rainwater has a pivotal role in diluting the As content available for translocation. Reproductive phase accumulates maximum As compared to vegetative and ripening phases. Sequential extraction and SEM studies re-confirm no possibility of iron (Fe) plaque formation in root soils which sequestered As. Finally, we conclude that monsoonal cultivation provides least As enriched grain (exposed area: 350 μg/kg, control area: 224 μg/kg) irrespective of the variety of cultivar and area of cultivation, which amounts to one-third of pre-monsoonal grain (1120 μg/kg) and so, it is much safer for consumption with respect to As and micro-nutrient status.

Melatonin and calcium function synergistically to promote the resilience through ROS metabolism under arsenic-induced stress

The interplay between melatonin (Mel) and calcium (Ca²⁺) in enhancing tolerance to metalloid toxicity and underlying physiological and biochemical mechanisms of this relationship still remains unknown. The present study reveals that the signaling molecules Mel and/or Ca²⁺ enhanced tolerance of Vicia faba (cv. Tara) plant to metalloid arsenic (As) toxicity. However, a combination of Mel and Ca²⁺ was more efficient than alone. Plants grew with As exhibited enhanced hydrogen peroxide, superoxide anion, electrolyte leakage, lipid peroxidation together with increased reactive oxygen species (ROS) producing enzymes, such as NADPH oxidase and glycolate oxidase (GOX). On the contrary, an inhibition in chlorophyll (Chl) biosynthesis and gas exchange parameters (net photosynthetic rate, stomatal conductance, intercellular carbon dioxide concentration) was observed. Under As toxicity conditions, the application of Mel and Ca²⁺ synergistically suppressed the plants' program cell death features (nucleus condensation and nucleus fragmentation) in guard cells of stomata, DNA damage, and formation of ROS in guard cells, leaves and roots. Moreover, it enhanced gas exchange parameters and activity of enzymes involved in photosynthesis process (carbonic anhydrase and RuBisco), Chl biosynthesis (δ-aminolevulinic acid dehydratase), and decreased activity of Chl degrading enzyme (chlorophyllase) under As toxicity conditions. Our investigation evidently established that expression of ATP synthase, Ca²⁺-ATPase, Ca²⁺-DPKase, Hsp17.6 and Hsp40 was found maximum in the plants treated with Mel + Ca²⁺, resulting in higher tolerance of plants to As stress. Also, increased total soluble carbohydrates, cysteine, and Pro accumulation with increased Pro synthesizing enzyme (Δ¹-pyrroline-5-carboxylate synthetase (P5CS) and decreased Pro degrading enzyme (proline dehydrogenase) in Mel + Ca²⁺ treated plants conferred As toxicity tolerance. The obtained results postulate strong evidence that the application of Mel along with Ca²⁺ enhances resilience against As toxicity by upregulating the activity of plasma membrane H⁺-ATPase, enzymes involved in antioxidant system, and ascorbate-glutathione pathway.

Remodeling of Root Growth Under Combined Arsenic and Hypoxia Stress Is Linked to Nutrient Deprivation

Root architecture responds to environmental stress. Stress-induced metabolic and nutritional changes affect the endogenous root development program. Transcriptional and translational changes realize the switch between stem cell proliferation and cell differentiation, lateral root or root hair formation and root functionality for stress acclimation. The current work explores the effects of stress combination of arsenic toxicity (As) and hypoxia (Hpx) on root development in Arabidopsis thaliana. As revealed previously, combined As and Hpx treatment leads to severe nutritional disorder evident from deregulation of root transcriptome and plant mineral contents. Both As and Hpx were identified to pose stress-specific constraints on root development that lead to unique root growth phenotype under their combination. Besides inhibition of root apical meristem (RAM) activity under all stresses, As induced lateral root growth while root hair density and lengths were strongly increased by Hpx and HpxAs-treatments. A dual stimulation of phosphate (Pi)-starvation response was observed for HpxAs-treated plant roots; however, the response under HpxAs aligned more with Hpx than As. Transcriptional evidence along with biochemical data suggests involvement of PHOSPHATE STARVATION RESPONSE 1; PHR1-dependent systemic signaling. Pi metabolism-related transcripts in close association with cellular iron homeostasis modulate root development under HpxAs. Early redox potential changes in meristematic cells, differential ROS accumulation in root hair zone cell layers and strong deregulation of NADPH oxidases, NADPH-dependent oxidoreductases and peroxidases signify a role of redox and ROS signaling in root architecture remodeling under HpxAs. Differential aquaporin expression suggests transmembrane ROS transport to regulate root hair induction and growth. Reorganization of energy metabolism through NO-dependent alternate oxidase, lactate fermentation, and phosphofructokinase seems crucial under HpxAs. TOR and SnRK-signaling network components were potentially involved in control of sustainable utilization of available energy reserves for root hair growth under combined stress as well as recovery on reaeration. Findings are discussed in context of combined stress-induced signaling in regulation of root development in contrast to As and Hpx alone.

Arsenic speciation and elemental composition of rice samples from the Slovenian market

A survey of highly toxic arsenic compounds, together with some other elements was carried out on 40 polished rice samples (white, basmati and parboiled) and 10 brown rice samples from the Slovenian market. The average total As concentration was 157 ± 60 μg kg^-1; highest levels were found in parboiled and brown rice and lowest in basmati. The average inorganic As concentration was 90 ± 35 μg kg^-1. Dimethylarsinic acid and monomethylarsonic acid, which also exhibit high toxicity levels in some cases constitute >50% of total arsenic and might deserve more attention. Contrary to other foods, the total arsenic concentration in rice may even be a better health hazard indicator than the inorganic arsenic concentration. Elemental analysis of rice revealed large differences between polished and brown rice, especially for Mg, Mn, P, Fe and K, which were 2-4 times higher in brown rice than in polished rice.

Flow of arsenic between rice grain and water: Its interaction, accumulation and distribution in different fractions of cooked rice

Arsenic (As) contaminated water is a major threat to human health when used for drinking, cooking and irrigational purposes. Rice being consumed by 50% of the world's population, supplies considerable amount of As to the human body. Our study provides a detailed understanding of As distribution in each fraction of rice while cooking (viz. uncooked rice, cooking water, cooked rice and gruel/total discarded water), ultimately leading to a better explanation of As movement between rice grain and water. A significant decrease of As was observed in cooked rice (34-89% and 23-84% for sunned and parboiled rice respectively) when cooked with low-As containing water, <3 μg/l and moderate As-contaminated water, 36-58 μg/l (3-50% and 12-61% for sunned and parboiled rice respectively) with increasing selenium (Se) concentration. Movement of As from water to rice grain has been inferred with increasing water As (84-105 μg/l), which results in a significant increase of As in cooked rice (24-337% and 114% for sunned and parboiled rice, respectively) with decreasing Se concentration. Arsenic speciation study emphasizes the fact of similar reduction percentage of As (III), As (V) and total As in wet cooked rice when cooked with low-As containing water. The SAMOE value in 'risk thermometer' supports the higher risk of suffering from wet cooked rice (class 4) with increasing cooking water As concentration (class 3 to class 5).

Reactive Oxygen Species (ROS) Metabolism and Nitric Oxide (NO) Content in Roots and Shoots of Rice (Oryza sativa L.) Plants under Arsenic-Induced Stress

Arsenic (As) is a highly toxic metalloid for all forms of life including plants. Rice is the main food source for different countries worldwide, although it can take up high amounts of As in comparison with other crops, showing toxic profiles such as decreases in plant growth and yield. The induction of oxidative stress is the main process underlying arsenic toxicity in plants, including rice, due to an alteration of the reactive oxygen species (ROS) metabolism. The aim of this work was to gain better knowledge on how the ROS metabolism and its interaction with nitric oxide (NO) operate under As stress conditions in rice plants. Thus, physiological and ROS-related biochemical parameters in roots and shoots from rice (Oryza sativa L.) were studied under 50 μM arsenate (AsV) stress, and the involvement of the main antioxidative systems and NO in the response of plants to those conditions was investigated. A decrease of 51% in root length and 27% in plant biomass was observed with 50 μM AsV treatment, as compared to control plants. The results of the activity of superoxide dismutase (SOD) isozymes, catalase, peroxidase (POD: total and isoenzymatic), and the enzymes of the ascorbate–glutathione cycle, besides the ascorbate and glutathione contents, showed that As accumulation provoked an overall significant increase of most of them, but with different profiles depending on the plant organ, either root or shoot. Among the seven identified POD isozymes, the induction of the POD-3 in shoots under As stress could help to maintain the hydrogen peroxide (H2O2) redox homeostasis and compensate the loss of the ascorbate peroxidase (APX) activity in both roots and shoots. Lipid peroxidation was slightly increased in roots and shoots from As-treated plants. The H2O2 and NO contents were enhanced in roots and shoots against arsenic stress. In spite of the increase of most antioxidative systems, a mild oxidative stress situation appears to be consolidated overall, since the growth parameters and those from the oxidative damage could not be totally counteracted. In these conditions, the higher levels of H2O2 and NO suggest that signaling events are simultaneously occurring in the whole plant.

Interactions of Dimethylarsinic Acid, Total Arsenic and Zinc Affecting Rice Crop Management and Human Health in Cambodia

BACKGROUND

In parts of Cambodia and in many other parts of the world, irrigation of rice with groundwater results in arsenic (As) accumulation in soil and rice, leading to health concerns associated with rice consumption. At times, some As is present as relatively nontoxic, non-regulated, dimethylarsinic acid (DMA). Low levels of zinc (Zn) have been found in rice from Bangladesh, Cambodia, and China where As levels in rice are high. Furthermore, there have been claims that Zn deficiency is responsible for stunting the growth of children in Cambodia and elsewhere, however in rural Asia, rice is the major source of Zn. Current data are inadequate for both Zn and DMA in Cambodian rice.

OBJECTIVES

The present study aimed to provide a preliminary evaluation of the relationship between the content of Zn and DMA in rice grain in Preak Russey, an area with elevated levels of As in groundwater and to improve the management of Zn deficiency in rice.

METHODS

Rice agriculture was evaluated along the Mekong River in Cambodia. Analyses for metals, total As, and As species in rice and water were conducted by inductively coupled plasma mass spectrometry. Analysis of total Zn and As in soils and total Zn in rice were analyzed using X-ray fluorescence (XRF) spectrometry.

RESULTS

Rice in Preak Russey had Zn concentrations less than a third the level recommended by the United Nations World Food Programme. There was a significant (p < 0.05) negative correlation between the Zn content of rice and DMA in rice with the lowest Zn and highest DMA levels occurring near irrigation wells, the source of As.

CONCLUSIONS

The highest levels of DMA in rice were associated with Zn deficiency in rice.

COMPETING INTERESTS

The authors declare no competing financial interests.

Microbiomes inhabiting rice roots and rhizosphere

Land plants directly contact soil through their roots. An enormous diversity of microbes dwelling in root-associated zones, including endosphere (inside root), rhizoplane (root surface) and rhizosphere (soil surrounding the root surface), play essential roles in ecosystem functioning and plant health. Rice is a staple food that feeds over 50% of the global population. Its root is a unique niche, which is often characterized by an oxic region (e.g. the rhizosphere) surrounded by anoxic bulk soil. This oxic-anoxic interface has been recognized as a pronounced hotspot that supports dynamic biogeochemical cycles mediated by various functional microbial groups. Considering the significance of rice production upon global food security and the methane budget, novel insights into how the overall microbial community (i.e. the microbiome) of the rice root system influences ecosystem functioning is the key to improving crop health and sustainable productivity of paddy ecosystems, and alleviating methane emissions. This mini-review summarizes the current understanding of microbial diversity of rice root-associated compartments to some extent, especially the rhizosphere, and makes a comparison of rhizosphere microbial community structures between rice and other crops/plants. Moreover, this paper describes the interactions between root-related microbiomes and rice plants, and further discusses the key factors shaping the rice root-related microbiomes.

Effects of adding selenium on different remediation measures of paddy fields with slight-moderate cadmium contamination

A number of remediation measures have been used in paddy fields to alleviate serious cadmium (Cd) contamination, which may pose a public health risk through the food chain. In this study, a field trial was conducted in paddy fields with slight-moderate Cd contamination to investigate the remediation effects of combined remediation measures (CRMs), including the use of Cd-safe rice (Oryza sativa L.) cultivars, water management modes (WMMs), lime application (LA), soil amendment application (SAA), and foliar silicon (Si) fertilizers. Two groups of field trials were designed including CRMs with selenium (Se) and without selenium (non-Se) application. The results show that soil measures (LA + SAA) can increase the soil pH by 0.99 and decrease the soil DTPA-extracted Cd content by 34.19% (p < 0.05). All measures used in the present study significantly decreased the Cd content in husked rice and yield, except for the WMMs; the CRMs achieved the best results, and Se application enhanced the effects of all measures. This study shows that CRMs significantly decreased the Cd content in husked rice by 58.10%; this value increased to 72.69% after Se application (p < 0.05). These results provide useful information for selecting remediation measures in paddy fields with slight-moderate Cd contamination.

Arsenic and nutrient absorption characteristics and antioxidant response in different leaves of two ryegrass (Lolium perenne) species under arsenic stress

Arsenic (As), a heavy metal element, causes soil environmental concerns in many parts of the world, and ryegrass has been considered as an effective plant species for bioremediation of heavy metal pollution including As. This study was designed to investigate As content, nutrient absorption and antioxidant enzyme activity associated with As tolerance in the mature leaves, expanded leaves and emerging leaves of perennial ryegrass (Lolium perenne) and annual ryegrass (Lolium multiflorum) under 100 mg·kg-1 As treatment. The contents of As, calcium (Ca), magnesium (Mg), manganese (Mn) in the leaves of both ryegrass species were greatest in the mature leaves and least in the emerging leaves. The nitrogen (N), phosphorus (P), potassium (K) contents of both ryegrass species were greatest in the emerging leaves and least in the mature leaves. The As treatment reduced biomass more in the mature leaves and expanded leaves relative to the emerging leaves for annual ryegrass and reduced more in emerging leaves relative to the mature and expanded leaves for perennial ryegrass. Perennial ryegrass had higher As content than annual ryegrass in all three kinds of leaves. The As treatment increased hydrogen peroxide (H2O2) in expanded leaves of two ryegrass species, relative to the control. The As treatment increased the ascorbate peroxidase (APX) activity in the expanded leaves of perennial ryegrass and the mature leaves of annual ryegrass, the catalase (CAT) activity in the mature and expanded leaves of perennial ryegrass and the emerging leaves of annual ryegrass, relative to the control. The As treatment reduced peroxidase (POD) activity in all three kinds of leaves of annual ryegrass and superoxide dismutase (SOD) activity in expanded leaves of perennial ryegrass, relative to the control. The results of this study suggest that As tolerance may vary among different ages of leaf and reactive oxygen species (ROS) and antioxidant enzyme activity may be associated with As tolerance in the ryegrass.

Selenium impedes cadmium and arsenic toxicity in potato by modulating carbohydrate and nitrogen metabolism

Past studies have already determined that selenium (Se) is very effective in alleviating cell oxidative damage caused by various abiotic stresses in plants. Past studies have also indicated other physiological pathways by which Se may benefit plants. In order to better understand the full array of potential applications for Se in agriculture, this study investigated the influence of Se on carbohydrate and nitrogen (N) metabolism in potato plants (Solanum tuberosum L. cv. Sante) grown under cadmium (Cd) and/or arsenic (As) toxicity. Potato plants were grown in a growth chamber and fertigated with Hoagland nutrient solution with or without Se (9 μM). After 48-d of growth under Cd (40 μM) and/or As (40 μM) stress, carbohydrate and N metabolism in leaves, roots and stolons were measured. For carbohydrate metabolism, various sugars-i.e., sucrose, starch, glucose, fructose, and total soluble sugar contents (TSSC)-and the activities of enzymes associated with sucrose metabolism and glycolysis-i.e., acid invertase (AI), neutral invertase (NI), sucrose-synthetase (SS), sucrose phosphatesynthetase (SPS), fructokinase (FK), hexokinase (HK), phosphofructokinase (PFK), and pyruvatekinase (PK)-were measured. For N metabolism, NO₃^-, NO₂^- and NH₄⁺ contents along with the enzymatic activities of nitrate reductase (NRA), nitrite reductase (NiRA), glutamine-synthetase (GS), and glutamate-synthetase (GOGAT) were measured. Overall, Cd and/or As treatments had reduced plant growth relative to those plants grown without heavy metal toxicity, due to hindered photosynthesis and alterations in N metabolism and glycolysis. Regarding N metabolism, heavy metal toxicity caused a reduction in NO₃^- and NO₂^- content and NRA and NiRA enzymatic activity and enhanced NH₄⁺ content and GDH activity in leaves, roots and stolons. Regarding glycolysis, the activity of enzymes of glycolysis-i.e., FK, HK, PFK, and PK-were also reduced. In the C metabolism study, plants combatted Cd and As toxicity naturally by an adaptation mechanism which caused an increase in soluble sugars (fructose, glucose, sucrose) by increasing NI, SS and SSP enzymatic activity. Supplementation with Se in the Cd and/or As treatments in the carbohydrate and N metabolism studies improved plant growth. Selenium supplementation in the Cd and As treatments decreased Cd and/or As content in the plant tissue and alleviating the Cd- and/or As-induced toxicity by enhancing the C-metabolism adaptation mechanism. Applying Se to Cd and As treatments also decreased nitrogen losses by hindering Cd- and As-induced changes in the N-metabolism. Se also limited Cd and As accumulation in the plant tissue by the antagonistic effect between Cd/Se and As/Se in the roots. The results of this study indicate that in the presence of Cd and/or As. soil toxicity, Se may be a powerful tool for promoting plant growth.

Differential impacts of copper oxide nanoparticles and Copper(II) ions on the uptake and accumulation of arsenic in rice (Oryza sativa)

Arsenic (As) in rice grains is a serious food safety concern. Some coexisting engineered nanoparticles (ENPs) were shown to alter the accumulation and speciation of As in rice grains. However, investigation on the effects of copper oxide nanoparticles (CuO NPs), a popular ingredient in pesticides, on the uptake and accumulation of As is rare. We explored the potentially different impact of CuO NPs and corresponding Cu(II) ions on the accumulation of two As species in rice seedlings in a hydroponic system. Rice seedlings were treated with a combinations of 1 mg/L of arsenite (As(III)) or arsenate (As(V)) and 100 mg/L of CuO NPs or Cu(II) for 6 days. Both forms of Cu significantly reduced the accumulation of total As in rice tissues, with Cu(II) exhibiting significantly greater effect than CuO NPs. As speciation in rice roots was markedly affected by both forms of Cu, and the impacts were Cu-form dependent. For example, the co-existence of As(V) with CuO NPs led to a 45% decrease of As(V) in rice roots, while the co-existence of As(V) with Cu(II) caused a 47% increase in As(V) in rice roots. As speciation in rice shoots was less affected by co-present Cu than in rice roots. Co-occurring As(III) or As(V) lowered Cu concentration in rice roots by 40% and 50% in treatments with CuO NPs, but did not affect Cu content in rice roots co-exposed to Cu(II). The study confirmed the reciprocal effect of co-occurring CuO NPs or Cu(II) and As in rice paddies and highlighted the unique "nano-effect" of CuO NPs. The results alsos showed that the initial oxidation state of As plays an important role in the interactions between As and Cu. The results shed light on the current debate on the safe applications of nano-enabled agrichemicals vs. conventional metal salts in agriculture.

NH4H2PO4-extractable arsenic provides a reliable predictor for arsenic accumulation and speciation in pepper fruits (Capsicum annum L.)

Dietary arsenic (As) intake from food is of great concern, and developing a reliable model capable of predicting As concentrations in plant edible parts is desirable. In this study, pot experiments were performed with 16 Chinese upland soils spiked with arsenate [As(V)] to develop a predictive model for As concentrations in pepper fruits (Capsicum annum L.). Our results showed that after three months' aging, concentrations of bioavailable As (extracted by 0.05 M NH₄H₂PO₄) in various soils varied widely, depending on soil total As concentrations and soil properties such as soil pH and amorphous iron (Fe) contents. Furthermore, both the bioconcentration factor (BCF, denoted as the ratio of fruit As to soil As) and total As concentrations in pepper fruits were largely determined by concentrations of bioavailable As, which explained 27% and 69% variations in the BCF and fruit As concentrations, respectively. Apart from bioavailable As, soil pH and Fe contents were another two important factors influencing As accumulation in pepper fruits. Taking the three factors into account, concentrations of fruit As can be well predicted using a stepwise multiple linear regression (SMLR) analysis (R² = 0.80, RMSE = 0.17). Arsenic species in soils and edible parts were also analyzed. Although As(V) predominated in soils (>96%), As in pepper fruits presented as As(V) (46%) and arsenite [As(III)] (39%) with small amount of methylated As (<15%). Aggregated boosted tree (ABT) analysis revealed that inorganic As concentrations in pepper fruits were determined by concentrations of bioavailable As, phosphorus (P) and Fe in soils. In contrast to inorganic As, methylated As concentrations were not correlated with those factors in soils. Taken together, this study established an empirical model for predicting As concentrations in pepper fruits. The predictive model can be used for establishing the As threshold in fruit vegetable farming soils.

Environmental and Health Implications of the Correlation Between Arsenic and Zinc Levels in Rice from an Arsenic-Rich Zone in Cambodia

BACKGROUND

In parts of Cambodia, irrigation of rice with groundwater results in arsenic accumulation in soils and rice, leading to health concerns associated with rice consumption. In Bangladesh and China, low zinc levels in rice have been found in regions where arsenic levels in rice are high. Furthermore, there have been claims that zinc deficiency is responsible for stunting of children in Cambodia. There are limited data on zinc in Cambodian rice, but in rural Asia, rice is the major source of zinc.

OBJECTIVES

To provide a preliminary evaluation of the zinc content in rice grain in Preak Russey, an area with elevated levels of arsenic. The importance of zinc in rice for infants was also assessed.

METHODS

Rice cultivation was evaluated in sixty farms along the Mekong River in Cambodia. Analyses for metals, total arsenic, and arsenic species in the water and rice were conducted at the University of Ottawa, Canada by inductively coupled plasma - mass spectrometry. Analysis of total zinc and arsenic in soils were analyzed in Phnom Penh using X-ray fluorescence spectrometry (XRF). Total zinc in rice was also measured by XRF analysis.

RESULTS

Rice in the Preak Russey area contained zinc with ½ to ¼ of the 1987 Codex standard for rice in Infant Formula. Moreover, our average zinc concentration in rice samples was less than a third that recommended for zinc fortification in rice by the United Nations World Food Programme. There was a significant (α=0.05) negative correlation between the arsenic and zinc content of rice with the lowest zinc levels occurring near the irrigation wells, the source of arsenic. There was a significantly higher content of zinc in rice from farms that fertilized with cow manure.

CONCLUSIONS

Handheld XRF spectrometers are useful tools for detection of zinc levels in rice. The potential for zinc deficiency in farmers in areas of Cambodia with arsenic toxicity is high.

COMPETING INTERESTS

The authors declare no competing financial interests.

Selenium amelioration of arsenic toxicity in rice shows genotypic variation: A transcriptomic and biochemical analysis

The toxic metalloid arsenic (As) is consumed mostly through contaminated rice. Therefore, reducing its accumulation and maintaining nutrient homeostasis in crop plants are imperative to ensure food safety. However, there is a dearth of information on the interrelationship between nutrient homeostasis and the regulatory mechanisms of arsenic-selenium (As-Se) interactive pathways responsible for stress tolerance. In the present study, experiments were conducted in hydroponically grown 12-day-old seedlings of rice (Oryza sativa L.) varieties (Pusa Basmati1 and IR64) treated with arsenite (AsIII) (150 μM), selenium (SeVI) (20 μM), and As + Se. It was observed that selenium supplementation ameliorated As toxicity by reducing its accumulation and retrieving As-induced nutrient deficiency. Significant decrease in As accumulation, H₂O₂ content, and fluorescent intensity of nitric oxide (NO), reactive oxygen species (ROS), and superoxide radical (O₂^.-) along with cell death with Se supplementation in both rice varieties demonstrated the protective role of Se as a probable ROS quencher. Addition of Se increased the enzyme activities of thiol metabolism and induced differential transcript accumulation patterns of sulfur-related genes. Nutrient level positively correlated with the differential expression pattern of NPK-related genes that play roles in metabolism and nutrient availability in both varieties. Though Pusa Basmati1 (PB1) showed higher tolerance to As, IR64 overcomes As toxicity more efficiently than PB1 in the presence of Se, which highlights that IR64 is a better performer in the presence of Se. Overall, this study provides novel insight into the role of Se in As-stressed rice genotypes through alteration of nutrient transporters and thiol-related genes.

Impacts of silicon addition on arsenic fractionation in soils and arsenic speciation in Panax notoginseng planted in soils contaminated with high levels of arsenic

Arsenic (As) is a well-known carcinogenic substance whose biological toxicity in soils and plants depends on its concentration and chemical forms. Silicon (Si) generally can alleviate biotic and abiotic stresses, including As stress. However, its effects vary depending on As chemical form, plant species and other factors. A pot experiment was performed to investigate the effects of Si addition on the content and forms of As in red soil and its uptake, transport and speciation in Panax notoginseng. The results showed that additions of 25 and 75 mg kg^-1 of Si both significantly decreased the concentrations of water-soluble As and exchangeable As in soil and therefore decreased the bioavailability of soil As. However, the As uptake by Panax notoginseng (PN) was increased, which resulted in increases in As concentration by 18.5% and 2.3% in roots and by 56.7% and 58.3% in shoots, respectively, when compared with the control. Arsenate (As(V)) was the dominant As species in all the treatment soils (99.8-100%), whereas arsenite (As(III)) was prevalent in plant roots (75.2-92.4%), shoots (74.1-87.9%) and leaves (73.9-84.3%). Si addition (25 and 75 mg kg^-1) significantly increased As(III) concentration in roots by 167.5% and 83.3%, respectively. Monomethylarsonic acid (MMA) was the only detected methylated As but at low concentrations (0.01-0.29 mg kg^-1) and only in PN leaves. Si addition (25 and 75 mg kg^-1) significantly increased the copy number of the arsenite methyltransferase (arsM) gene by 31.0% and 47.2% but did not increase the methylated As species content in PN leaves. The detected copy number of the arsM gene did not represent the capacity of soil to methylate As, and the sources of MMA in leaves need to be explored in further research.

Elucidating the Effects of Cerium Oxide Nanoparticles and Zinc Oxide Nanoparticles on Arsenic Uptake and Speciation in Rice ( Oryza sativa) in a Hydroponic System

The accumulation of arsenic (As) in rice grains depends greatly on the redox chemistry in rice rhizosphere. Intentional or accidental introduction of strong oxidizing or reducing agents, such as metallic engineered nanoparticles (ENPs) into the plant-soil ecosystem, can change As speciation and plant uptake. However, investigation on the effects of ENPs on plant uptake of co-occurring redox sensitive heavy metals and their speciation in plant tissues is scarce. We investigated the mutual effects of two commonly encountered ENPs, cerium oxide nanoparticles (CeO₂ NPs) and zinc oxide nanoparticles (ZnO NPs), and two inorganic species of As on their uptake and accumulation in rice seedlings in a hydroponic system. Rice seedlings were exposed to different combinations of 1 mg/L of As(III) or As(V) and 100 mg/L of CeO₂ NPs and ZnO NPs for 6 days about 40 days after germination. ZnO NPs significantly reduced the accumulation of As(III) in rice roots by 88.1 and 96.7% and in rice shoots by 71.4 and 77.4% when the initial As was supplied as As(III) and As(V), respectively. ZnO NPs also reduced As(V) in rice roots by 68.3 and 52.3% when the As was provided as As(III) and As(V), respectively. However, the As(V) in rice shoots was unaffected by ZnO NPs regardless of the initial oxidation state of As. Neither the total As nor the individual species of As in rice tissues was significantly changed by CeO₂ NPs. The co-presence of As(III) and As(V) increased Ce in rice shoots by 6.5 and 2.3 times but did not affect plant uptake of Zn. The results confirmed the active interactions between ENPs and coexisting inorganic As species, and the extent of their interactions depends on the properties of ENPs as well as the initial oxidation state of As.

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.

Microbe mediated arsenic release from iron minerals and arsenic methylation in rhizosphere controls arsenic fate in soil-rice system after straw incorporation

Arsenic (As) contamination is a global problem. Straw incorporation is widely performed in As contaminated paddy fields. To understand how straw and straw biochar incorporation affect As transformation and translocation in the soil-microbe-rice system, a pot experiment was carried out with different dosages of rice straw and straw biochar application. Results showed that both straw biochar and straw application significantly increased As mobility. Straw biochar mobilized As mainly through increasing soil pH and DOM content. Straw incorporation mainly through enhancing As release from iron (Fe) minerals and arsenate (As(V)) reduction to arsenite (As(III)). Straw biochar didn't significantly affect As methylation, while straw incorporation significantly enhanced As methylation, elevated dimethylarsenate (DMA) concentration in soil porewater and increased As volatilization. Straw biochar didn't significantly change total As accumulation in rice grains, but decreased As(III) accumulation by silicon (Si) inhibition. Straw incorporation significantly increased DMA, but decreased As(III) concentration in rice grains. After biochar application, dissolved As was significantly positively correlated with the abundance of Bacillus, indicating that Bacillus might be involved in As release, and As(III) concentration in polished grains was negatively correlated with Si concentration. The significant positive correlation between dissolved As with Fe and the abundance of iron-reducing bacteria suggested the coupling of As and Fe reduction mediated by iron-reducing bacteria. The significant positive correlation between DMA in rice grains and the abundance of methanogenic bacteria indicated that methanogenic bacteria could be involved in As methylation after straw application. The results of this study would advance the understanding how rice straw incorporation affects As fate in soil-microbe-rice system, and provide some guidance to straw incorporation in As contaminated paddy soil. This study also revealed a wealth of microorganisms in the soil environment that dominate As mobility and transformation after straw incorporation.

Regulation of oxidative stress and mineral nutrient status by selenium in arsenic treated crop plant Oryza sativa

The present study was conducted to examine the impact of selenium (Se) on mineral nutrient status and oxidative stress in crop plant Oryza sativa treated with arsenic (As). Scanning electron microscopy (SEM) coupled with Energy dispersive x-ray spectroscopy (EDS) study revealed the morphological deformities in leaf veins along with granular deposition on the leaf surface. The EDS analysis exhibited loss of elements (S, Si, Cl, K, Ca, Fe and Cu) in As(III) treatment in rice roots as compared to untreated root. In the case of As(III) treated shoot, changes in elements content in term of percent atomic weight was K (1.17-0.90%), Cl (1.04-24.75%), Na (0.65-3.52%) and S (0.49-2.52%) when compared with untreated shoot. The result of EDS analysis showed that As limits the concentration of important mineral elements present in the rice root and shoot. Rice plant treated with Se (10µM) and sub lethal dose of As(III) (60µM) showed better growth responses in term of root, shoot length (11.4% and 10.71%, respectively), biomass (11.7%), reduced malonyldialdehyde content (35.14%) and stimulated antioxidant level indicating better As tolerance potential against As. Further, a selenium dependent significant reduction in As accumulation was also observed in root (14.24%) and shoot (23.78%) of rice plant when compared with plant treated with As alone. This study highlights the potential of Se to ameliorate the ecotoxicological risks associated with the As buildup in agricultural land.

Mechanism study of sulfur fertilization mediating copper translocation and biotransformation in rice (Oryza sativa L.) plants

Metabolism of sulfur (S) is suggested to be an important factor for the homeostasis and detoxification of Cu in plants. We investigated the effects of S fertilizers (S⁰, Na₂SO₄) on Cu translocation and biotransformation in rice plants by using multiple synchrotron-based techniques. Fertilization of S increased the biomass and yield of rice plants, as well as the translocation factor of Cu from root to shoot and shoot to grain, resulting in enhanced Cu in grain. Sulfur K-edge X-ray near edge structure (XANES) analysis showed that fertilization of S increased the concentration of glutathione in different rice tissues, especially in rice stem and leaf. Copper K-edge XANES results indicated that a much higher proportion of Cu (I) species existed in rice grain than husk and leaf, which was further confirmed by soft X-ray scanning transmission microscopy results. Sulfur increased the proportion of Cu (I) species in rice grain, husk and leaf, suggesting the inducing of Cu (II) reduction in rice tissues by S fertilization. These results suggested that fertilization of S in paddy soils increased the accumulation of Cu in rice grain, possibly due to the reduction of Cu (II) to Cu (I) by enhancing glutathione synthesis and increasing the translocation of Cu from shoot to grain.

Additive toxicity of zinc and arsenate on barley (Hordeum vulgare) root elongation

Zinc (Zn) and arsenic (As) are typically present as mixed contaminants in mining-impacted areas; however, their joined effects have rarely been evaluated. The present study was set up to test whether the Zn²⁺ and H₂ AsO₄^- (hereafter, As) mixture toxicity to plants is additive or whether interactions occur. Barley (Hordeum vulgare) root elongation was measured in resin buffered nutrient solutions. The design included ranges of single-element concentrations and combinations at 3 different Ca²⁺ concentrations (0.5 mM, 2.2 mM, and 15.0 mM) to vary the relative toxicity of Zn²⁺ . Increasing Ca concentrations decreased Zn toxicity, whereas As toxicity was unaffected by Ca. Root elongation was generally more affected in Zn-As mixtures than in corresponding single-element treatments. This is merely a joint additive effect, as 96% of the root elongation data were within a factor of 1.2 from predictions using the independent action (IA) or concentration addition (CA) model. The CA and IA predictions were similar, and data did not allow identification of equal or dissimilar modes of action. Small but significant Zn-As antagonisms were only found at high effects (>50% inhibition). The present study suggests that mixture effects of Zn and As are environmentally relevant and that current risk assessment underestimates toxicity in multielement-contaminated environments. The CA model can be used as a conservative model for risk assessment; however, for soil-grown plants, soil-exposed studies are needed. Environ Toxicol Chem 2017;36:1556-1562. © 2016 SETAC.

Phosphorus deficiency modifies As translocation in the halophyte plant species Atriplex atacamensis

Most arsenic in surface soil and water exists primarily in its oxidized form, as arsenate (As(V); AsO₄^3-), which is an analog of phosphate (PO₄^3-). Arsenate can be taken up by phosphate transporters. Atriplex atacamensis Phil. is native to northern Chile (Atacama Desert), and this species can cope with high As concentrations and low P availability in its natural environment. To determine the impact of P on As accumulation and tolerance in A. atacamensis, the plants were cultivated in a hydroponic system under four treatments: no As(V) addition with 323µM phosphate (control); 1000µM As(V) addition with 323µM phosphate; no As(V) and no phosphate; 1000µM As(V) addition and no phosphate. Phosphate starvation decreased shoot fresh weight, while As(V) addition reduced stem and root fresh weights. Arsenate addition decreased the P concentrations in both roots and leaves, but to a lesser extent than for P starvation. Phosphorus starvation increased the As concentrations in roots, but decreased it in shoots, which suggests that P deficiency reduced As translocation from roots to shoots. Arsenate addition increased total glutathione, but P deficiency decreased oxidized and reduced glutathione in As(V)-treated plants. Arsenate also induced an increase in S accumulation and nonprotein thiol and ethylene synthesis, and a decrease in K concentrations, effects that were similar for the P-supplied and P-starved plants. In contrast, in As(V)-treated plants, P starvation dramatically decreased total soluble protein content and increased lipid peroxidation, compared to plants supplied with P. Phosphorus nutrition thus appears to be an important component of A. atacamensis response to As toxicity.

Arsenic-induced stress activates sulfur metabolism in different organs of garlic (Allium sativum L.) plants accompanied by a general decline of the NADPH-generating systems in roots

Arsenic (As) contamination is a major environmental problem which affects most living organisms from plants to animals. This metalloid poses a health risk for humans through its accumulation in crops and water. Using garlic (Allium sativum L.) plants as model crop exposed to 200μM arsenate, a comparative study among their main organs (roots and shoots) was made. The analysis of arsenic, glutathione (GSH), phytochelatins (PCs) and lipid peroxidation contents with the activities of antioxidant enzymes (catalase, superoxide dismutase, ascorbate-glutathione cycle), and the main components of the NADPH-generating system, including glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH), NADP-malic enzyme (NADP-ME) and NADP-isocitrate dehydrogenase (NADP-ICDH) was carried out. Data showed a correlation among arsenic accumulation in the different organs, PCs content and the antioxidative response, with a general decline of the NADPH-generating systems in roots. Overall, our results demonstrate that there are clear connections between arsenic uptake, increase of their As-chelating capacity in roots and a decline of antioxidative enzyme activities (catalase and the ascorbate peroxidase) whose alteration provoked As-induced oxidative stress. Thus, the data suggest that roots act as barrier of arsenic mediated by a prominent sulfur metabolism which is characterized by the biosynthesis of high amount of PCs.

A Simple Metallothionein-Based Biosensor for Enhanced Detection of Arsenic and Mercury

Metallothioneins (MTs) are a family of cysteine-rich proteins whose biological roles include the regulation of essential metal ions and protection against the harmful effects of toxic metals. Due to its high affinity for many toxic, soft metals, recombinant human MT isoform 1a was incorporated into an electrochemical-based biosensor for the detection of As³⁺ and Hg²⁺. A simple design was chosen to maximize its potential in environmental monitoring and MT was physically adsorbed onto paper discs placed on screen-printed carbon electrodes (SPCEs). This system was tested with concentrations of arsenic and mercury typical of contaminated water sources ranging from 5 to 1000 ppb. The analytical performance of the MT-adsorbed paper discs on SPCEs demonstrated a greater than three-fold signal enhancement and a lower detection limit compared to blank SPCEs, 13 ppb for As³⁺ and 45 ppb for Hg²⁺. While not being as low as some of the recommended drinking water limits, the sensitivity of the simple MT-biosensor would be potentially useful in monitoring of areas of concern with a known contamination problem. This paper describes the ability of the metal binding protein metallothionein to enhance the effectiveness of a simple, low-cost electrochemical sensor.

Inorganic Arsenic Determination in Food: A Review of Analytical Proposals and Quality Assessment Over the Last Six Years

Here we review recent developments in analytical proposals for the assessment of inorganic arsenic (iAs) content in food products. Interest in the determination of iAs in products for human consumption such as food commodities, wine, and seaweed among others is fueled by the wide recognition of its toxic effects on humans, even at low concentrations. Currently, the need for robust and reliable analytical methods is recognized by various international safety and health agencies, and by organizations in charge of establishing acceptable tolerance levels of iAs in food. This review summarizes the state of the art of analytical methods while highlighting tools for the assessment of quality assessment of the results, such as the production and evaluation of certified reference materials (CRMs) and the availability of specific proficiency testing (PT) programmes. Because the number of studies dedicated to the subject of this review has increased considerably over recent years, the sources consulted and cited here are limited to those from 2010 to the end of 2015.