Assessment of arsenic toxicity in rice plants in areas of West Bengal

24-Epibrassinolide mitigates arsenate stress in seedlings of Oryza sativa (IR-20) via the induction of phenylpropanoid pathway

The introduction of arsenic, a hazardous metalloid, into the soil system due to heavy industrialization has negatively affected agricultural productivity, resulting in limited crop yields. A recent breakthrough in stress-responsive hormones, specifically brassinosteroids, has extensively covered the role of antioxidant enzyme defense systems in heavy metal stress mitigation. Considering the antioxidant properties and metal complex formation abilities of polyphenols, our study focuses on examining their role in arsenate toxicity amelioration by 24-epibrassinolide. We demonstrate enhanced growth parameters of sodium arsenate-stressed seedlings upon application of 24-epibrassinolide, with increased root and shoot polyphenol levels analyzed by high-performance liquid chromatography. Specifically, the concentration of catechin, sinapic acid, 4-hydroxy benzoic acid, protocatechuic acid, 4-coumaric acid, and myricetin were elevated, indicating induction of phenylpropanoid signaling pathway. Further, we also report a decrease in the generation of superoxide anions and hydrogen peroxide validated the antioxidant effects of these metabolites through the nitrobluetetrazolium and diaminobenzidine staining method. In addition, evaluation of transcript level of genes encoding for specific enzymes of the phenylpropanoid pathway in shoot and root showed a significant upregulation in mRNA expression of phenylalanine ammonia-lyase-1, cinnamate-4-hydroxylase, and caffeic acid o-methyltransferase-1 upon exogenous application of 24-epibrassinolide in arsenate stressed Oryza sativa.

Application of glutamic acid improved As tolerance in aromatic rice at early growth stage

To alleviate the arsenic (As) toxicity in aromatic rice, a hydroponic experiment of two As concentrations (0 and 100 μM sodium arsenite: A0, A1), three glutamic acid (Glu) concentrations (0, 100, and 500 μM l-glutamic acid: G0, G1, and G2) with Xiangyaxiangzhan and Meixiangzhan 2 was conducted. Results showed that the root As content were increased under A1G2 but reduced under A1G1 for Xiangyaxiangzhan as compared with A1G0. A decrement of As was transported from root to shoot caused by up-regulated OsABCC1 relative expression in Meixiangzhan 2. Likewise, As stress enhanced the H₂O₂ and malondialdehyde content, resulting in the impaired cell wall observed by transmission electron microscopy. However, compared with A1G0, the superoxide dismutase activity, ascorbic acid, glutathione, proline, and soluble sugar content were increased under A1G1. Additionally, arsenate reductase, monodehydroascorbate reductase activity, Glu, proline, and soluble sugar content were found positively associated with the As accumulation. Further, the metabolome analysis indicated that the pathway of amino acid and arginine biosynthesis were notably enriched after Glu application. Generally, 100 μM Glu application was the better treatment to enhance As tolerance in aromatic rice through up-regulating amino acid biosynthesis with increasing antioxidants and osmolytes to scavenge excessive reactive oxygen species.

Identification of Promising Genotypes Through Systematic Evaluation for Arsenic Tolerance and Exclusion in Rice (Oryza sativa L.)

Rice remains a major staple food source for the rapidly growing world population. However, regular occurrences of carcinogenic arsenic (As) minerals in waterlogged paddy topsoil pose a great threat to rice production and consumers across the globe. Although As contamination in rice has been well recognized over the past two decades, no suitable rice germplasm had been identified to exploit in adaptive breeding programs. Therefore, this current study identified suitable rice germplasm for As tolerance and exclusion based on a variety of traits and investigated the interlinkages of favorable traits during different growth stages. Fifty-three different genotypes were systematically evaluated for As tolerance and accumulation. A germination screening assay was carried out to identify the ability of individual germplasm to germinate under varying As stress. Seedling-stage screening was conducted in hydroponics under varying As stress to identify tolerant and excluder genotypes, and a field experiment was carried out to identify genotypes accumulating less As in grain. Irrespective of the rice genotypes, plant health declined significantly with increasing As in the treatment. However, genotype-dependent variation in germination, tolerance, and As accumulation was observed among the genotypes. Some genotypes (WTR1-BRRI dhan69, NPT-IR68552-55-3-2, OM997, and GSR IR1-5-Y4-S1-Y1) showed high tolerance by excluding As in the shoot system. Arsenic content in grain ranged from 0.12 mg kg-1 in Huang-Hua-Zhan (indica) from China to 0.48 mg kg-1 in IRAT 109 (japonica) from Brazil. This current study provides novel insights into the performance of rice genotypes under varying As stress during different growth stages for further use in ongoing breeding programs for the development of As-excluding rice varieties for As-polluted environments.

Determination of safe limit for arsenic contaminated irrigation water using solubility free ion activity model (FIAM) and Tobit Regression Model

Irrigation water contaminated with arsenic acts as a potent source of contamination to humans through water-soil-crop-food transfer so quantification of safe limit for irrigation water is also critical. A pot experiment was conducted to determine the safe limit for As contaminated irrigation water with two soil types (alluvial and red) using ten levels of contaminated irrigation water (0, 0.25, 0.5, 0.75, 1.0, 1.25, 1.50, 1.75, 2.0, 2.25 mg L^-1), applied 5 times in rice (Variety: Sushak Samrat),used as a test crop. The results reveal that the different fractions of arsenic in terms of its profusion followed the order F4 > F2 > F5 > F3 > F1 and F4 > F3 > F2 > F5 > F1 across all the doses of As for alluvial soil and red soil respectively. The safe limit of irrigation water in terms of risk assessment expressed as Hazard Quotient (HQ) was at 0.75 mg L^-1 and the solubility FIAM can effectively predict the As content in rice grain in both the soils. The Tobit Regression Model in alluvial soil quantified the safe limit for As in irrigation water from 1.20 to 0.10 mg L^-1 for available soil As 0.25-3.0 mg kg^-1 and in red soil, the range was from 0.10 to 0.40 mg L^-1 for soil As 1.0 to 0.25 mg kg^-1 provided that the As content in rice grain is < 0.4 mg kg^-1. This proved to be an effective protocol for estimation of safe limits after proper validation and calibration.

Assessing human health risk of arsenic for rice consumption by an iron plaque based partition ratio model

A field experiment was conducted to study the transport and uptake of arsenic (As) from soil to rice roots and the subsequent translocation from roots to shoots and grains. Twelve rice cultivars were used in the field experiment. The amount of As accumulated in rice grains and sequestered by root iron plaque and rhizosphere soil, were determined to establish the relationship between As concentrations in brown rice and As sequestration by iron oxides. Human health risk was then assessed for Taiwan's population exposed to As through rice consumption. The result of this study showed that the mean total As concentrations in the experimental site and in brown rice were 93.02 mg/kg and 0.158 mg/kg, respectively. The As sequestration by iron oxides on root plaque (3.48-9.51) was higher than that of the rhizosphere soil (1.86-4.09) for all tested rice cultivars. Therefore, the partition ratio (PR) representing the relative tendency of As sequestration by rhizosphere soil to that in root iron plaque was all less than 1. In addition, there was a significant negative linear relationship between inorganic As concentration (iAs) in brown rice and PR value (r² = 0.38, p < 0.05). Based on the iAs in brown rice, the median value of hazard quotient (HQ) and target cancer risk (TR) was 1.13 and 5.10 × 10^-4, respectively, indicating potential cancer and non-cancer risk for Taiwan residents exposed to As through the consumption of rice grown on the studied site. Various PR values were then successfully used for estimating risk, implying that screening the PR of the rice plant before harvest could serve as an early warning signal for protecting consumers' health. However, more experiments with different rice cultivars for the paddy soils were suggested in the future to establish a comprehensive relationship between iAs in brown rice and PR value.

Effect of Arsenic Stress on Expression Pattern of a Rice Specific miR156j at Various Developmental Stages and Their Allied Co-expression Target Networks

In plants, arsenic (As) stress modulates metabolic cascades at various developmental stages by influencing the pattern of gene expressions mediated by small non-coding RNAs, especially Micro-RNAs, involved in the moderation of a myriad of cellular processes needed for plant adaptation upon oxidative stress. miR156j of miR156 gene family, involved mainly in the regulation of growth and development in plants. This study was designed to find out the role of arsenic toxicity on Osa-miR156j expression in all physiological growth stages. To better understand the functional role of Osa-miR156j in rice, we observed the expression in different developmental stages (seedlings, tillering and flowering) and various tissues of leaf, stem and root tissues (at 0, 24, 48, and 72 h) under 25 μM arsenite [As (III)] exposure. Additionally, using bioinformatic tools to target genes of Osa-miR156j and the potential co-expressed genes were explored at different development stages in the various tissues of rice under stress conditions. The expression of Osa-miR156j showed its temporal downregulation in various tissues in different developmental stages. Of note, the downregulation was more pronounced in root tissues at seedlings, tillering, and flowering stages during 0-72 h under arsenite exposure as compared to other tissues. Overall, the As stress altered the gene expression more prominently at seedlings developmental stage followed by flowering and tillering. Additionally, through the In silico approach, the target functions and presence of oxidative stress-responsive cis-acting regulatory elements/motifs also confirmed Osa-miR156j involvement in the regulation of arsenic stress in rice. The findings of this study demonstrate the prominent role of Osa-miR156j in rice under arsenite stress, which was found to modulate the metabolic activities in rice plants at different developmental stages, and thus it might be useful for the development of arsenic tolerant varieties.

Arsenic accumulation in lentil (Lens culinaris) genotypes and risk associated with the consumption of grains

Arsenic (As) is a toxic metalloid. As phyto-toxicity is manifested by its accumulation in different tissue types and subsequent growth inhibition in plants. Despite the vital role of leguminous crops in providing proteins to human diets, a little is known about the As accumulation in lentil. In this study, the rate of As uptake and transport from soil to root, shoot and grain of lentil as well as associated risks with the consumption of As contaminated food were examined. Biomass accumulation of lentil genotypes pardina, red chief and precoz drastically decreased when treated with As at 6 mg kg⁻¹ concentration in comparison to 0 and 3 mg kg⁻¹ As. Quantification of As concentrations following different treatment periods showed that As accumulation in roots and shoots of 0, 3 and 6 mg kg⁻¹ As-treated lentil genotypes was statistically different. Arsenic content in grains of red chief genotype was found significantly lower than pardina and precoz. Moreover, As transport significantly increased in roots and shoots compared to the grains. Due to the high concentrations of As in biomass of lentil genotypes, animal as well as human health risk might be associated with the consumption of the As contaminated legume crops.

Arsenic accumulation in paddy plants at different phases of pre-monsoon cultivation

Geogenic arsenic (As) contamination in Bengal Delta Plain is a growing environmental and research concern. Cultivation of staple crops like paddy on these contaminated fields is one of the major routes for human dietary exposure. The present study investigates changes of arsenic concentrations in paddy plant parts, root soil and surface soil throughout the various phases of pre-monsoon (boro) cultivation. Arsenic uptake property of paddy plants collected from 10 fields was found to be dependent on the variety of paddy plant (like Minikit, Jaya) rather than arsenic levels in groundwater (0.074-0.301 mg/l) or soil (25.3-60 mg/kg). Arsenic is translocated from root to aerial parts in descending order. Leaf, stem, root, root soil and surface soil showed a similar trend in their change of arsenic concentration throughout the cultivation period. Arsenic concentration was highest in vegetative phase; sharply declined in reproductive phase; followed by moderate increase in ripening phase. The young root tissues in vegetative (primary) phase could uptake arsenic at a much faster rate than the older tissues in later phases. With the growth of the plant, higher concentrations of iron in root soil in the reproductive phase confirmed the formation of iron plaques on the surface of the root, which sequester arsenic and prevented its uptake by plants. Finally, co-precipitation of arsenic with iron released from crystallized iron plaques results in loosening of the iron plaques from root surface. Thus, soil arsenic concentration increases in the final phase of cultivation which in turn contributes to increased concentration in plant parts.

Temporal dynamics of total and free-living nitrogen-fixing bacterial community abundance and structure in soil with and without history of arsenic contamination during a rice growing season

Despite the fact that the nitrogen (N) fixers act as the key regulator of ecosystem process, a detailed study of their abundance, diversity, and dynamics in arsenic (As)-contaminated rice fields is missing so far. DNA extracted from soil followed by 16S rRNA and nifH gene-based real-time qPCR, clone library analysis, and DNA sequencing were used to examine the status of the total and diazotrophic communities in two agricultural fields with and without arsenic contamination history during one rice cultivation season. In general, higher nifH and 16S rRNA gene copy numbers were observed in rice growing soils with lesser As than that with higher As. Elevated levels of 16S rRNA and nifH genes in soil is directly associated with total and nitrogen fixers abundance in the agricultural land without As contamination history through the cultivation period, but the copy number of 16S rRNA gene was decreased, and the nifH gene remained unchanged in the As-contaminated land. Additionally, Canonical Correspondence Analysis (CCA) indicated the possible suppression of nifH gene abundance by soil pH, phosphate, and As content. Increased abundance of total and Acidobacterial lineages in low As-containing soil and the detection of several uncultured groups among nifH gene sequence in higher frequency indicated the presence of novel nifH bearing bacterial groups. Conversely, the abundance of copiotrophic Proteobacterial lineages gradually increased in soil with higher As. Herein, our study demonstrated that the dynamics of free-living nitrogen-fixing bacterial communities were perturbed due to As contamination in agricultural land.

Assessment of Availability and Human Health Risk Posed by Arsenic Contaminated Well Waters from Timis-Bega Area, Romania

Mobilization of As from geological materials into ground and drinking water sources may represent an important threat to human health. The objective of this study was to assess the As concentration and availability in underground water used as drinking water sources. Water samples were collected from public and private wells in Timis-Bega area of Pannonian Basin, West Romania. Total-dissolved As measured after "classical" filtration of water samples was in the range of 0.10-168 μg L^-1, thus exceeding the guideline value in majority of the samples. The aim of this study was also to assess the "truly dissolved" concentrations of As considered as available concentrations, in well waters, after passive sampling by Diffusive Gradients in Thin-films (DGT). The results showed that over 70% of total-dissolved As is in available forms. The obtained data were used to evaluate the risks of using the wells as drinking water source. Hazard quotients for ingestion and dermal pathways and hazard index (HI) for exposure to As were calculated. The HI values > 1 found that majority of samples indicated a health risk for local residents.