Nitrogen supply influences arsenic accumulation and stress responses of rice (Oryza sativa L.) seedlings

N-acetyl-cysteine mitigates arsenic stress in lettuce: Molecular, biochemical, and physiological perspective

Agricultural land contaminated with heavy metals such as non-biodegradable arsenic (As) has become a serious global problem as it adversely affects agricultural productivity, food security and human health. Therefore, in this study, we investigated how the administration of N-acetyl-cysteine (NAC), regulates the physio-biochemical and gene expression level to reduce As toxicity in lettuce. According to our results, different NAC levels (125, 250 and 500 μM) significantly alleviated the growth inhibition and toxicity induced by As stress (20 mg/L). Shoot fresh weight, root fresh weight, shoot dry weight and root dry weight (33.05%, 55.34%, 17.97% and 46.20%, respectively) were decreased in plants grown in As-contaminated soils compared to lettuce plants grown in soils without the addition of As. However, NAC applications together with As stress increased these growth parameters. While the highest increase in shoot fresh and dry weight (58.31% and 37.85%, respectively) was observed in 250 μM NAC application, the highest increase in root fresh and dry weight (75.97% and 63.07%, respectively) was observed in 125 μM NAC application in plants grown in As-polluted soils. NAC application decreased the amount of ROS, MDA and H2O2 that increased with As stress, and decreased oxidative damage by regulating hormone levels, antioxidant and enzymes involved in nitrogen metabolism. According to gene expression profiles, LsHIPP28 and LsABC3 genes have shown important roles in reducing As toxicity in leaves. This study will provide insight for future studies on how NAC applications develop resistance to As stress in lettuce.

Evaluation of efficacy of non-coding RNA in abiotic stress management of field crops: Current status and future prospective

Abiotic stresses are responsible for the major losses in crop yield all over the world. Stresses generate harmful ROS which can impair cellular processes in plants. Therefore, plants have evolved antioxidant systems in defence against the stress-induced damages. The frequency of occurrence of abiotic stressors has increased several-fold due to the climate change experienced in recent times and projected for the future. This had particularly aggravated the risk of yield losses and threatened global food security. Non-coding RNAs are the part of eukaryotic genome that does not code for any proteins. However, they have been recently found to have a crucial role in the responses of plants to both abiotic and biotic stresses. There are different types of ncRNAs, for example, miRNAs and lncRNAs, which have the potential to regulate the expression of stress-related genes at the levels of transcription, post-transcription, and translation of proteins. The lncRNAs are also able to impart their epigenetic effects on the target genes through the alteration of the status of histone modification and organization of the chromatins. The current review attempts to deliver a comprehensive account of the role of ncRNAs in the regulation of plants' abiotic stress responses through ROS homeostasis. The potential applications ncRNAs in amelioration of abiotic stresses in field crops also have been evaluated.

A critical analysis of various post-harvest arsenic removal treatments of rice and their impact on public health due to nutrient loss

Rice (Oryza sativa L.) is particularly susceptible to arsenic (As) accumulation. Currently, to decrease the level of As accumulated in rice, various post-harvest methods, i.e., polishing, parboiling, pH-dependent soaking, washing, and cooking at different rice-to-water ratios (r/w), are being focused, because it removes significant amount of As from rice grain. Depending upon the rice variety and type, i.e., rough (with husk), husked (without husk/brown), or polished rice, these methods can remove 39-54% As by parboiling, 38-55% by polishing, 37-63% by soaking, and 6-80% by washing and cooking. Infants are highly vulnerable to As exposure; thus, these methods can be helpful for the production of rice-based infant foods. Although concern arises during the use of these methods that apart from decreasing the level of As in rice grain, they also lead to a significant loss of nutrients, such as macro- and micro-elements present in rice. Among these discussed methods, parboiling curtails 5-59%, polishing curtails 6-96%, soaking curtails 33-83%, and washing and cooking in different r/w reduce 8-81% of essential nutrients resulting in 2-90% reduction in contribution to the RDI of these nutrients through rice-based diet. Thus, these post-harvest arsenic removal methods, although reduce arsenic induced health hazard, but may also lead to malnutrition and compromised health in the population based on rice diet. There is a need to explore another way to reduce As from rice without compromising the nutrient availability or to supplement these nutrients through grain enrichment or by introducing additional dietary sources by changing eating habits; however, this may impose an extra economic burden on people.

Nitrate_dependent suberization regulates cadmium uptake and accumulation in maize

In this study, effect of nitrate-dependent suberization in maize root on cadmium (Cd) uptake and accumulation was investigated. Suberization in maize roots was significantly lower in plants grown with a high nitrate supply compared with low nitrate. This decrease was seen in the total amount of suberin, in which the aliphatic suberin amount was significantly decreased, whereas no difference in aromatic suberin content between different N-treatments. RNA-sequencing showed that suberin biosynthesis genes were upregulated in low nitrate treatment, which correlated well with the increased suberin content. Bioimaging and xylem sap analysis showed that reduced exodermal and endodermal suberization in roots of plants grown under high nitrate promoted radial Cd transport along the crown root. The enhanced suberization in crown roots of plants grown in low nitrate restricted the radial transport of Cd from epidermis to cortex via decreased accessibility to Cd related transporters at the plasmalemma. Also, under low nitrate supply, the Cd transport gene ZmNramp5 was upregulated in the crown root, which may enhance Cd uptake by root tip where exodermis and endodermis were not fully suberized. These results suggest that high nitrate supply enhances Cd uptake and radial transport in maize roots by reducing exodermal and endodermal suberization.

Molecular and biochemical characterization of rice developed through conventional integration of nDart1-0 transposon gene

Mutations, the genetic variations in genomic sequences, play an important role in molecular biology and biotechnology. During DNA replication or meiosis, one of the mutations is transposons or jumping genes. An indigenous transposon nDart1-0 was successfully introduced into local indica cultivar Basmati-370 from transposon-tagged line viz., GR-7895 (japonica genotype) through conventional breeding technique, successive backcrossing. Plants from segregating populationsshowed variegated phenotypes were tagged as BM-37 mutants. Blast analysis of the sequence data revealed that the GTP-binding protein, located on the BAC clone OJ1781_H11 of chromosome 5, contained an insertion of DNA transposon nDart1-0. The nDart1-0 has "A" at position 254 bp, whereas nDart1 homologs have "G", which efficiently distinguishes nDart1-0 from its homologs. The histological analysis revealed that the chloroplast of mesophyll cells in BM-37 was disrupted with reduction in size of starch granules and higher number of osmophillic plastoglobuli, which resulted in decreased chlorophyll contents and carotenoids, gas exchange parameters (Pn, g, E, Ci), and reduced expression level of genes associated with chlorophyll biosynthesis, photosynthesis and chloroplast development. Along with the rise of GTP protein, the salicylic acid (SA) and gibberellic acid (GA) and antioxidant contents(SOD) and MDA levels significantly enhanced, while, the cytokinins (CK), ascorbate peroxidase (APX), catalase (CAT), total flavanoid contents (TFC) and total phenolic contents (TPC) significantly reduced in BM-37 mutant plants as compared with WT plants. These results support the notion that GTP-binding proteins influence the process underlying chloroplast formation. Therefore, it is anticipated that to combat biotic or abiotic stress conditions, the nDart1-0 tagged mutant (BM-37) of Basmati-370 would be beneficial.

Calcium and L-glutamate present the opposite role in managing arsenic in barley

Arsenic contamination in agricultural soils has posed tremendous threat to sustainable crop production and human health via food chain. Calcium and Glutamate have been well-documented in metal(loid)s detoxification, but it is poorly understood how they regulate arsenic-induced toxicity to plants. In this study, the effect of glutamate and calcium at high concentration on arsenic toxicity and accumulation in barley seedling was accessed in terms of plant growth, photosynthetic efficacy, arsenic uptake, translocation and accumulation, antioxidant defense, nutrient uptake and the expression of As transporters. Our results have demonstrated that calcium could effectively ameliorate arsenic toxicity to barley seedlings, which is mainly attributed to its beneficial effect on increasing nutrient uptake, reducing the aboveground arsenic accumulation and enhancing antioxidative defense capacity. However, it is unexpected that glutamate considerably exacerbated the arsenic toxicity to barley seedlings. More importantly, for the first time, glutamate was observed to tremendously facilitate the root-to-shoot translocation of arsenic by 41.8- to 60.8-fold, leading to 90% of the total amount of As accumulating in barley shoots. The reason of this phenomenon can be well explained by the glutamate-triggered enormous upregulation of genes involved in arsenic uptake (HvPHT1;1, HvPHR2 and HvNIP3;2), reduction (HvHAC1;1), translocation (HvABCC7, HvNIP1;1 and HvNIP3;3) and intracellular sequestration (HvABCC1). These findings suggest that calcium and glutamate function as the opposite player in managing arsenic, with calcium being an effective alleviator of arsenic stress to ensure the safe production of crops; while glutamate being a highly efficient phytoextraction agent for phytoremediation of arsenate-contaminated soils.

Dark Side of Ammonium Nitrogen in Paddy Soil with Low Organic Matter: Stimulation of Microbial As(V) Reduction and As(III) Transfer from Soil to Rice Grains

The bioavailability of arsenic (As) is influenced by ammonium (NH₄⁺-N) fertilization, but the underlying mechanisms controlling As transformation in soil-rice systems are still not fully understood. The effects of two NH₄⁺-N fertilizers, urea and NH₄HCO₃, on the transformation of As in a paddy soil with low organic matter content and transfer in rice plants were investigated. Treatments with urea and NH₄HCO₃ significantly increased arsenite (As(III)) concentration in porewater, bioavailable As in rhizosphere soil, and the relative abundance of the As(V) respiratory reductase gene (arrA) and As(III) methyltransferase gene (arsM). Furthermore, the relative expression of As transporter genes in rice roots, such as OsLsi1, OsLsi2, and OsLsi3, was upregulated, and the translocation efficiency of As(III) from rice roots to brown rice was promoted. Subsequently, As(III) accumulation in brown rice significantly increased. Therefore, attention should be paid to As-contaminated paddy fields with NH₄⁺-N fertilization.

Co-application of biochar and nitrogen fertilizer promotes rice performance, decreases cadmium availability, and shapes rhizosphere bacterial community in paddy soil

Cadmium (Cd) contamination in soil has posed a great threat to crop safety and yield as well as soil quality. Biochar blended with nitrogen fertilizer have been reported to be effective in remediating Cd-contaminated soil. However, the influence of co-application of biochar and nitrogen fertilizer on the Cd bioavailability, rice yield and soil microbiome remains unclear. In this study, eight different treatments including control (CK), 5% biochar (B), 2.6, 3.5, 4.4 g/pot nitrogen fertilizers (N1, N2 and N3), and co-application of biochar and nitrogen fertilizers (BN1, BN2, BN3) were performed in a pot experiment with paddy soil for observations in an entire rice cycle growth period. Results showed single N increased soil available Cd content and Cd uptake in edible part of rice, while the soil available Cd content significantly decreased by 14.8% and 7.4%-11.1% under the B, BN treatments, and the Cd content in edible part of rice was significantly reduced by 35.1% and 18.5%-26.5%, respectively. Besides, B, N and BN treatments significantly increased the yield of rice by 14.3%-86.6% compared with CK, and the highest yield was gained under BN3 treatment. Soil bacterial diversity indices (Shannon, Chao1, observed species and PD whole tree index) under N2, N3 were generally improved. Cluster analysis indicated that bacterial community structures under BN treatments differed from those of CK and single N treatments. BN treatments enhanced the abundances of key bacterial phylum such as Acidobacteria, positively associated with yield, and increased the abundance of Spirochaetes, negatively correlated to soil available Cd and Cd uptake of rice. Furthermore, the regression path analysis (RPA) revealed that pH, organic matter (OM), alkaline hydrolysis of nitrogen (AHN) and available Cd were the major properties influencing Cd content in edible part of rice. Redundancy analysis (RDA) revealed that pH and available Cd played key role in shaping soil bacterial community. Thus, BN is a feasible practice for the improvements of rice growth and remediation of Cd-polluted soil.

Thiourea mediated ROS-metabolites reprogramming restores root system architecture under arsenic stress in rice

Arsenic (As) is a ubiquitous carcinogenic metalloid that enters into human food chain, through rice consumption. To unravel the conundrum of oxidative vs. reductive stress, the differential root-system architecture (RSA) was studied under As (a ROS producer) and thiourea (TU; a ROS scavenger) alone treatments, which indicated 0.80- and 0.74-fold reduction in the number of lateral roots (NLR), respectively compared with those of control. In case of As+TU treatment, NLR was increased by 4.35-fold compared with those of As-stress, which coincided with partial restoration of redox-status and auxin transport towards the root-tip. The expression levels of 16 ROS related genes, including RBOHC, UPB-1 C, SHR1, PUCHI, were quantified which provided the molecular fingerprint, in accordance with endogenous ROS signature. LC-MS based untargeted and targeted metabolomics data revealed that As-induced oxidative stress was metabolically more challenging than TU alone-induced reductive stress. Cis/trans-ferruloyl putrescine and γ-glutamyl leucine were identified as novel As-responsive metabolites whose levels were decreased and increased, respectively under As+TU than As-treated roots. In addition, the overall amino acid accumulation was increased in As+TU than As-treated roots, indicating the improved nutritional availability. Thus, the study revealed dynamic interplay between "ROS-metabolites-RSA", to the broader context of TU-mediated amelioration of As-stress in rice.

Arsenite phytotoxicity and metabolite redistribution in lettuce (Lactuca sativa L.)

Arsenic (As) contamination has become a global problem, especially in developing countries, where a significant percentage of the population depends on groundwater for drinking. Arsenic toxicity depends on its chemical form. Herein, we evaluated the phytotoxicity of arsenite [As(III)], including As accumulation and adverse physiological responses (e.g., growth inhibition, oxidative stress, and metabolic disturbances). Furthermore, this result was compared with the mechanism of the phytotoxicity of arsenate [As(V)] that we previously explored. As accumulated mainly in the roots (29.33-88.73 mg/kg) of lettuce, only a small amount was transferred to the leaves (0.08-0.22 mg/kg); arsenic mainly existed in the form of As(III) in plants. As(III) was positively correlated with Mn in the leaves and roots and negatively correlated with Ca in roots and Mg in leaves, consistent with the increase in SOD activity and the destruction of the chloroplast membrane. Plants responded differently to As(III) and As(V) in terms of the antioxidant response and metabolic response. CAT activity in leaves was reduced following As(III) exposure and increased upon As(V) exposure. Furthermore, As(III) decreased the levels of some products of the tricarboxylic acid cycle and induced abnormal metabolism of secondary metabolites, such as phenol and niacin. The present study explored arsenic accumulation induced by As(III), the related physiological and biochemical responses and subsequent metabolite redistribution, and provided insights into the effects of different As species on plants.

Arsenic and cadmium induced macronutrient deficiencies trigger contrasting gene expression changes in rice

Arsenic (As) and cadmium (Cd), two major carcinogenic heavy metals, enters into human food chain by the consumption of rice or rice-based food products. Both As and Cd disturb plant-nutrient homeostasis and hence, reduces plant growth and crop productivity. In the present study, As/Cd modulated responses were studied in non-basmati (IR-64) and basmati (PB-1) rice varieties, at physiological, biochemical and transcriptional levels. At the seedling stage, PB-1 was found more sensitive than IR-64, in terms of root biomass; however, their shoot phenotype was comparable under As and Cd stress conditions. The ionomic data revealed significant nutrient deficiencies in As/Cd treated-roots. The principal component analysis identified NH₄⁺ as As-associated key macronutrient; while, NH₄⁺/NO₃^- and K⁺ was majorly associated with Cd mediated response, in both IR-64 and PB-1. Using a panel of 21 transporter gene expression, the extent of nutritional deficiency was ranked in the order of PB-1(As)<IR-64(As)<PB-1(Cd)<IR-64(Cd). A feed-forward model is proposed to explain nutrient deficiency induced de-regulation of gene expression, as observed under Cd-treated IR-64 plants, which was also validated at the level of sulphur metabolism related enzymes. Using urea supplementation, as nitrogen-fertilizer, significant mitigation was observed under As stress, as indicated by 1.018- and 0.794-fold increase in shoot biomass in IR-64 and PB-1, respectively compared to that of control. However, no significant amelioration was observed in response to supplementation of urea under Cd or potassium under As/Cd stress conditions. Thus, the study pinpointed the relative significance of various macronutrients in regulating As- and Cd-tolerance and will help in designing suitable strategies for mitigating As and/or Cd stress conditions.

Antioxidant enzymes and transporter genes mediate arsenic stress reduction in rice (Oryza sativa L.) upon thiourea supplementation

Thiourea (TU) is a chemo-priming agent and non-physiological reactive oxygen species (ROS) scavenger whose application has been found to reduce As accumulation in rice grains along with improved growth and yield. The present field study explored TU-mediated mechanistic changes in silicon (Si) assimilation in root/shoot, biochemical and molecular mechanisms of arsenic (As) stress amelioration in rice cultivars. Gosai and Satabdi (IET-4786) rice cultivars were selected for field experiment at three different places; control field and two other As contaminated experimental fields (EF1 and EF2) in West Bengal, India. The average As reduction was observed to be 9.5% and 19.8% whereas the yield increment was 8.8% and 17.7% for gosai and satabdi, respectively among all the three experimental fields. The positive interrelation was also observed between improved internal ultrastructure anatomy and enhanced Si assimilation (36%-423%) upon TU application. The level of photosynthetic pigments was increased by 29.8%-99.2%. Further, activities of antioxidant enzymes were harmonically altered in TU supplemented plants. The expression of various As related transporter genes in flag leaf and developing grains (inflorescence) was changed in both the rice cultivars (gosai and satabdi). It was also presumably responsible for observed As reduction in grains. Thus, TU application was found to be an efficient and sustainable agronomic practice for amelioration of As toxicity in rice plants in As contaminated field conditions.

Effect of sulfate application on inhibition of arsenic bioaccumulation in rice (Oryza sativa L.) with consequent health risk assessment of cooked rice arsenic on human: A pot to plate study

Arsenic (As) in rice is posing a serious threat worldwide and consumption of As contaminated rice by human is causing health risks. A pot experiment with different levels of sulfate dosage (0, 20, 40, 60 and 80 mg/kg) was set up in this study to explore the influence of sulfate fertilizer on rice plant growth, yield, and As accumulation in rice grain. Apart from As bioaccumulation in rice grains, the As fraction of cooked rice was quantified, and the health risks associated with cooked rice consumption were also investigated. The sulfate application significantly (p ≤ 0.05) enhanced the chlorophyll, tiller number, grains per panicle, grain and biomass yield under As stressed condition. The sulfate application also reduced the oxidative stress and antioxidant activity in rice plants. Sulfate fertigation improved the accumulation of total sulfur (S) and reduced the uptake and translocation of As in rice plants. Arsenic concentration in rice grain was reduced by 50.1% in S80 treatment (80 mg of sulfate/kg of soil) as compared to S0 set. The reduction percentage of As in cooked parboiled and sunned rice with correspond to raw rice ranged from 55.9 to 74% and 40.3-60.7%, respectively. However, the sulfate application and cooking of parboiled rice reduced the potential non-cancer and cancer risk as compared to sunned rice. The S80 treatment and cooking of parboiled rice reduce the As exposure for both children and adults by 51% as compared to cooked sunned rice under S80 treatment and this trend was similar for all treatments. Therefore, sulfate application in soil can be recommended to produce safer rice grains and subsequent cooking of parboiled rice grain with low-As contaminated water need to be done to avoid any potential health risk in As endemic areas.

Ammonium-nitrogen addition at the seedling stage does not reduce grain cadmium concentration in two common wheat (Triticum aestivum L.) cultivars

High cadmium (Cd) concentration in common wheat (Triticum aestivum L.) grains poses potential health risks. Several management strategies have been used to reduce grain Cd concentration. However, limited information is available on the use of ammonium-nitrogen (NH₄⁺-N) as a strategy to manage Cd concentration in wheat grains. In this study, NH₄⁺-N addition at the seedling stage unchanged the grain Cd concentration in the high-Cd accumulator, Zhoumai 18 (ZM18), but dramatically increased that in the low-Cd accumulator, Yunmai 51 (YM51). Further analysis revealed that the effects of NH₄⁺-N addition on whole-plant Cd absorption, root-to-shoot Cd translocation, and shoot-to-grain Cd remobilization were different between the two wheat cultivars. In ZM18, NH₄⁺-N addition did not change whole-plant Cd absorption, but inhibited root-to-shoot Cd translocation and Cd remobilization from lower internodes, lower leaves, node 1, and internode 1 to grains via the down-regulation of yellow stripe-like transporters (YSL), zinc transporters (ZIP5, ZIP7, and ZIP10), and heavy-metal transporting ATPases (HMA2). This inhibition decreased the grain Cd content by 29.62%, which was consistent with the decrease of the grain dry weight by 23.26%, leading to unchanged grain Cd concentration in ZM18. However, in YM51, NH₄⁺-N addition promoted continuous Cd absorption during grain filling, root-to-shoot Cd translocation and whole-plant Cd absorption. The absorbed Cd was directly transported to internode 1 via the xylem and then re-transported to grains via the phloem by up-regulated YSL, ZIP5, and copper transporters (COPT4). This promotion increased the grain Cd content by 245.35%, which was higher than the increased grain dry weight by 132.89%, leading to increased grain Cd concentration in YM51. Our findings concluded that the addition of NH₄⁺-N fertilizer at the seedling stage is not suitable for reducing grain Cd concentration in common wheat cultivars.

MicroRNAs: Tiny, powerful players of metal stress responses in plants

Metal contamination of the environment is a widespread problem threatening sustainable and safe crop production. Physio-biochemical and molecular mechanisms of plant responses to metal exposure have been studied to establish the best possible agronomical or biotechnological methods to tackle metal contamination. Metal stress tolerance is regulated by several molecular effectors among which microRNAs are one of the key master regulators of plant growth and stress responses in plants. MicroRNAs are known to coordinate multitude of plant responses to metal stress through antioxidant functions, root growth, hormonal signalling, transcription factors and metal transporters. The present review discusses integrative functions of microRNAs in the regulation of metal stress in plants, which will be useful for engineering stress tolerance traits for improved plant growth and productivity in metal stressed situations.

Rhizobium symbiosis modulates the accumulation of arsenic in Medicago truncatula via nitrogen and NRT3.1-like genes regulated by ABA and linalool

Arsenic (As) contamination is a worldwide problem and threatens human health. Here, we found that Rhizobium symbiosis can improve the tolerance to arsenate [As(V)], and a wild type R. meliloti Rm5038 symbiosis can significantly decrease the accumulation of As in Medicago truncatula shoots. The As content in plants could be decreased by nitrogen and the mutation of nitrate transporter NRT3.1. The expression of M. truncatula NRT3.1-like gene NRT3.1L1 could reverse the As(V)-tolerance phenotype of the Arabidopsis nrt3.1 mutant. Rm5038 symbiosis significantly increased the level of nitrogen in the shoot and reduced the expression of NRT3.1Ls in plants afflicted by As(V). The genetic analyses of aba2-1, pyr1/pyl1/2/4/5/8, and abi1-2/abi2-2/hab1-1/pp2ca-1 mutants revealed that abscisic acid (ABA) signaling regulates the tolerance of plants to As(V). ABA and linalool could promote the expression of NRT3.1Ls, however, their root biosynthesis was inhibited by ammonium, the first form of nitrogen fixed by Rhizobium symbiosis. Moreover, ABA and linalool may also control As and nitrate accumulation in Rhizobium symbionts via signaling pathways other than ammonia and NRT3.1Ls. Thus, Rhizobium symbiosis modulates the accumulation of As in plants via nitrogen and NRT3.1Ls regulated by ABA and linalool, which provides novel approaches to reduce As accumulation in legume crops.

Arsenic biogeochemical cycling in paddy soil-rice system: Interaction with various factors, amendments and mineral nutrients

Arsenic (As) contamination is a well-recognized environmental and health issue, threatening over 200 million people worldwide with the prime cases in South and Southeast Asian and Latin American countries. Rice is mostly cultivated under flooded paddy soil conditions, where As speciation and accumulation by rice plants is controlled by various geo-environmental (biotic and abiotic) factors. In contrast to other food crops, As uptake in rice has been found to be substantially higher due to the prevalence of highly mobile and toxic As species, arsenite (As(III)), under paddy soil conditions. In this review, we discussed the biogeochemical cycling of As in paddy soil-rice system, described the influence of critical factors such as pH, iron oxides, organic matter, microbial species, and pathways affecting As transformation and accumulation by rice. Moreover, we elucidated As interaction with organic and inorganic amendments and mineral nutrients. The review also elaborates on As (im)mobilization processes and As uptake by rice under the influence of different mineral nutrients and amendments in paddy soil conditions, as well as their role in mitigating As transfer to rice grain. This review article provides critical information on As contamination in paddy soil-rice system, which is important to develop suitable strategies and mitigation programs for limiting As exposure via rice crop, and meet the UN's key Sustainable Development Goals (SDGs: 2 (zero hunger), 3 (good health and well-being), 12 (responsible consumption and production), and 13 (climate action)).

Thiourea supplementation mediated reduction of grain arsenic in rice (Oryza sativa L.) cultivars: A two year field study

The present study delineates the interactions of arsenic (As), a carcinogenic metalloid, and thiourea (TU), a non-physiological reactive oxygen species (ROS) scavenger, in rice plants grown in As contaminated fields in West Bengal, India. The study was performed for four consecutive seasons (two boro and two aman) in 2016 and 2017 with two local rice cultivars; Gosai and Satabdi (IET-4786) in a control and two As contaminated experimental fields. Thiourea (0.05% wt/vol) treatment was given in the form of seed priming and foliar spray. Thiourea significantly improved growth and yield of rice plants and reduced As concentration in root, shoot, husk and grains in both cultivars and fields. The reduction in As concentration ranged from 10.3% to 27.5% in four seasons in different fields. The average (four seasons) increase in yield was recorded about ~8.1% and ~11.5% in control, ~20.2% and ~18.6% in experimental field 1, and ~16.2% and ~24.1% in experimental field 2, for gosai and satabdi, respectively. Mean hazard quotient (HQ) and incremental lifetime cancer risk (ILCR) values of As reduced upon TU supplementation for both cultivars as compared to that of non-TU plants. Hence, TU can be effectively used to cultivate rice safely in As contaminated fields.

Tracking the time-dependent and tissue-specific processes of arsenic accumulation and stress responses in rice (Oryza sativa L.)

The present study analysed time (0.5 h to 24 h) and tissue [roots, old leaves (OL) and young leaves (YL)] dependent nature of arsenic (As) accumulation and ensuing responses in two contrasting varieties of rice (Oryza sativa L.); Pooja (tolerant) and CO-50 (moderately sensitive). Arsenic accumulation was 5.4-, 4.7- and 7.3-fold higher at 24 h in roots, OL and YL, respectively of var. CO-50 than that in var. Pooja. Arsenic accumulation in YL depicted a delayed accumulation; at 2 h onwards in var. Pooja (0.23 µg g^-1 dw) while at 1 h onwards in var. CO50 (0.26 µg g^-1 dw). The responses of oxidative stress parameters, antioxidant enzymes, metabolites and ions were also found to be tissue- and time-dependent and depicted differential pattern in the two varieties. Among hormone, salicylic acid and abscisic acid showed variable response in var. Pooja and var. CO-50. Metabolite analysis depicted an involvement of various metabolites in As stress responses of two varieties. In conclusion, an early sensing of the As stress, proper coordination of hormones, biochemical responses, ionic and metabolic profiles allowed var. Pooja to resist As stress and reduce As accumulation more effectively as compared to that of var. CO-50.

Different nitrogen forms differentially affect Cd uptake and accumulation in dwarf Polish wheat (Triticum polonicum L.) seedlings

This study investigated the effects of different nitrogen (N) forms on Cadmium (Cd) uptake and accumulation in dwarf Polish wheat (DPW) seedlings, which were grown under Cd stress with N-Null, NH₄⁺-N, NO₃^--N and NH₄⁺-N + NO₃^--N. We measured plant growth and determined Cd uptake, translocation, accumulation, subcellular distribution and chemical forms in the roots and shoots of DPW seedlings. We also analyzed saccharide concentrations, and the transcript levels of genes encoding metal transporters in the roots of DPW seedlings. In the absence of NO₃^--N, addition of NH₄⁺-N reduced roots Cd concentration, F_CW (Cd in cell wall), F_S (Cd in soluble fraction) and F_E (inorganic Cd) concentrations, and induced the expression of four genes encoding metal transporters in roots, while it promoted Cd translocation to shoots. In the presence of NO₃^--N, addition of NH₄⁺-N increased roots Cd concentration, F_CW and F_W concentrations, and induced the expression of 22 genes encoding metal transporters in roots. Regardless of NH₄⁺-N level, addition of NO₃^--N increased roots Cd concentration, F_CW, F_S, F_W (water-soluble Cd), F_NaCl (pectates and protein Cd), F_HAc (undissolved Cd phosphate) and lactose concentrations, and also induced the expression of genes encoding metal transporters in roots. Overall, NH₄⁺-N differently regulated Cd uptake and accumulation in DPW seedlings in the absence or presence of NO₃^--N, while NO₃^--N greatly increased Cd uptake and accumulation in the presence of NH₄⁺-N compared to the absence of NH₄⁺-N. These patterns of Cd alteration likely arose due to different N forms altering Cd subcellular distribution and chemical forms, lactose concentration and the expression of metal transporter genes.

Assessment of Water Mimosa (Neptunia oleracea Lour.) Morphological, Physiological, and Removal Efficiency for Phytoremediation of Arsenic-Polluted Water

Arsenic is considered to be a toxic and heavy metal that exists in drinking water and can lead to acute biotoxicity. Water mimosa (Neptunia oleracea) has been widely identified as a feasible phytoremediator to clean up aquatic systems. In the current study, the phytoremediation potential of water mimosa exposed to different concentrations of sodium heptahydrate arsenate (Na₂HAsO₄·7H₂O) was tested. A number of plant physiological and growth responses such as height of frond, existence of green leaves, relative growth rate, relative water content, tolerance index, decrease in ratio of biomass and ratio of dry weight, chlorophyll content, photosynthesis rate, intercellular CO₂ concentrations, stomatal conductance, air pressure deficit, transpiration rate, proline and lipid peroxidation, as well as arsenic accumulation and removal efficacy were analyzed. The micromorphological analysis results confirmed water mimosa’s tolerance of up to 30 ppm of arsenic treatment. The results obtained from the chlorophyll and gas exchange content also showed severe damage by arsenic at doses higher than 30 ppm. In addition, the highest arsenic accumulation and arsenic removal efficacy were observed at the range of 30–60 ppm. An analysis of proline and lipid peroxidation content confirmed water mimosa’s tolerance of up to 30 ppm of arsenic. The scanning electron microscopy (SEM) and X-ray spectroscopy (EDX) and analysis also confirmed the accumulation of arsenic as shown by the deformation of water mimosa tissues. The results showed that water mimosa is a reliable bioremediator for removing arsenic from aquatic systems.

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

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

Excess copper inhibits the growth of rice seedlings by decreasing uptake of nitrate

Copper (Cu) toxicity has a deleterious effect on plant growth, and different nitrogen (N) forms have significantly different impacts on the uptake and accumulation of heavy metals by plants. However, it remains unclear how excess Cu inhibits the growth of rice seedlings under different N forms. Here, we examined the mechanism of Cu toxicity inhibiting the growth of rice supplied with different N forms. Rice seedlings were grown in a nutrient solution with 0.81 mmol L^-1 N, as ammonium (NH₄⁺), nitrate (NO₃^-) and NH₄⁺ + NO₃^-, or without N (0 N) in the presence of 0.2 μmol L^-1 CuSO₄ or 10 μmol L^-1 CuSO₄. The inhibition of shoot growth under excess Cu was more pronounced in plants that were supplied with NO₃^- than NH₄⁺; such inhibition was not induced by higher Cu concentration in shoots. Applied with NO₃^- alone increased solution pH value up to 6.2, but supplied with NH₄⁺ alone and NH₄⁺ + NO₃^- decreased solution pH value to 4.0 and 4.2, respectively. The increment of solution pH reduced Cu concentration in shoots of rice supplied with NO₃^- alone. Copper toxicity decreased NO₃^- concentrations in rice seedlings that were supplied with NO₃^- alone but increased the NH₄⁺ concentrations in plants that were supplied with NH₄⁺ or NH₄⁺ + NO₃^-. High Cu levels reduced the uptake of NO₃^- in roots by the analysis of net NO₃^- flux and NO₃^- assimilation enzymes activity. Under excess Cu, the transcript levels of OsNPF6.5, OsNPF2.2 and OsNPF2.4 genes were suppressed, while OsNRT2.1, OsNRT2.2 and OsNAR2.1 were raised in roots. In conclusion, Cu toxicity inhibits NO₃^- uptake and upward translocation by modulating the expression level of NO₃^- transporter genes. The reduction in the concentrations of NO₃^- and total N decreased shoot growth of rice seedlings when N was supplied as NO₃^-. Hence, rice seedlings supplied with NO₃^- had lower shoot biomass than those with NH₄⁺ under Cu stress.

Remediation of cadmium-contaminated soils using Brassica napus: Effect of nitrogen fertilizers

The physico-chemical characteristics of N fertilizers remain poorly understood with respect to their use with rape (Brassica napus L.) to remediate Cd-contaminated soil. In this work, eight types of fertilizer (comprising physico-chemical alkaline, neutral, and acidic N fertilizers) were employed to assess the effect of soil remediation via rape at different levels of Cd contamination (0, 5, and 10 mg kg^-1 Cd). The results show that the pH of rhizosphere soils was significantly higher under physico-chemical alkaline N fertilizer treatments than under physico-chemical acidic and neutral N fertilizer treatments. The physico-chemical characteristics of N fertilizers affected the rhizosphere soil pH and promoted Cd phytoextraction and accumulation by rape. In the 5 mg kg^-1 Cd-contaminated soil, the Cd accumulation and bioconcentration factor value in the shoots and the Cd translocation factor value were highest with the addition of NH₄Cl, a physico-chemical acidic N fertilizer. Among the physico-chemical alkaline N fertilizers, Ca(NO₃)₂ enabled the highest accumulation of Cd in rape shoots when soil was contaminated with 10 mg kg^-1 Cd. Thus, administering physico-chemical acidic N fertilizer to soils with lower Cd concentrations provides better remediation effects by rape, whereas physico-chemical alkaline N fertilizers are more effective in soils with higher Cd concentrations. These results show that physico-chemical N fertilizers can be employed to enhance the remediation of Cd-contaminated soil by rape and simultaneously improve the yield of this crop, with implications for environmental health and sustainable agricultural development.

Arsenic Uptake and Accumulation Mechanisms in Rice Species

Rice consumption is a source of arsenic (As) exposure, which poses serious health risks. In this study, the accumulation of As in rice was studied. Research shows that As accumulation in rice in Taiwan and Bangladesh is higher than that in other countries. In addition, the critical factors influencing the uptake of As into rice crops are defined. Furthermore, determining the feasibility of using effective ways to reduce the accumulation of As in rice was studied. AsV and AsIII are transported to the root through phosphate transporters and nodulin 26-like intrinsic channels. The silicic acid transporter may have a vital role in the entry of methylated As, dimethylarsinic acid (DMA) and monomethylarsonic acid (MMA), into the root. Amongst As species, DMA(V) is particularly mobile in plants and can easily transfer from root to shoot. The OsPTR7 gene has a key role in moving DMA in the xylem or phloem. Soil properties can affect the uptake of As by plants. An increase in organic matter and in the concentrations of sulphur, iron, and manganese reduces the uptake of As by plants. Amongst the agronomic strategies in diminishing the uptake and accumulation of As in rice, using microalgae and bacteria is the most efficient.