Arsenate exposure affects amino acids, mineral nutrient status and antioxidants in rice (Oryza sativa L.) genotypes

Selenium treatment alters the accumulation of osmolytes in arsenic-stressed rice (Oryza sativa L.)

Arsenic (As), one of the major pollutants in the soil, is an important environmental concern as its consumption can cause adverse health symptoms in living organisms. Its contamination of rice grown over As-contaminated areas is a serious concern in South Asian countries. Selenium (Se) has been reported to influence various osmolytes under metal stress in plants. The present study reports the role of Se in mitigating As stress in rice by modulating osmolyte metabolism. Rice plants grown in As-amended soil (2.5-10 mg kg-1) in pots were treated with sodium selenate (0.5-1.0 mg Se kg-1 soil) in glass house conditions and leaf samples were collected at 60 and 90 days after sowing (DAS). As-treated rice leaves displayed a reduction in relative water content (RWC) and dry weight than control with a maximum reduction of 1.68- and 2.47-fold in RWC and 1.95- and 1.69-fold in dry weight in As10 treatment at 60 and 90 DAS, respectively. Free amino acids (1.38-2.26-fold), proline (3.88-3.93-fold), glycine betaine (GB) (1.27-1.72-fold), choline (1.67-3.1-fold), total soluble sugars (1.29-1.61-fold), and reducing sugars (1.67-2.19-fold) increased in As-treated rice leaves as compared to control at both stages. As stress increased the γ-aminobutyric acid (GABA), putrescine content, and glutamate decarboxylase activity whereas diamine oxidase and polyamine oxidase activities declined by 1.69-1.88-fold and 1.52-1.86-fold, respectively. Se alone or in combination with As improved plant growth, RWC, GB, choline, putrescine, and sugars; lowered proline and GABA; and showed a reverse trend of enzyme activities related to their metabolism than respective As treatments. As stress resulted in a higher accumulation of osmolytes to combat its stress which was further modulated by the Se application. Hence, the current investigation suggested the role of osmoprotectants in Se-induced amelioration of As toxicity in rice plants.

Biomolecular insights into the inhibition of heavy metals on reductive dechlorination of 2,4,6-trichlorophenol in Pseudomonas sp. CP-1

Influences of heavy metal exposure to the organohalide respiration process and the related molecular mechanism remain poorly understood. In this study, a non-obligate organohalide respiring bacterium, Pseudomonas sp. strain CP-1, was isolated and its molecular response to the five types of commonly existed heavy metal ions were thoroughly investigated. All types of heavy metal ions posed inhibitory effects on 2,4,6-trichlorophenol dechlorination activity and cell growth with the varied degree. Exposure to Cu (II) showed the most serious inhibitive effects on dechlorination even at the lowest concentration of 0.05 mg/L, while the inhibition by As (V) was the least with the removal kinetic constant k decreased to 0.05 under 50 mg/L. Further, multi-omics analysis found compared with Cu (II), As (V) exposure led to the insignificant downregulation of a variety of biosynthesis processes, which would be one possible account for the less inhibited activity. More importantly, the inhibited mechanisms on the organohalide respiration catabolism of strain CP-1 were firstly revealed. Cu (II) stress severely downregulated NADH generation during TCA cycle and electron donation of organohalide respiration process, which might decrease the reducing power required for organohalide respiration. While both Cu (II) and As (Ⅴ) inhibited substrate level phosphorylation during TCA cycle, as well as electron transfer and ATP generation during organohalide respiration. Meanwhile, CprA-2 was confirmed as the responsible reductive dehalogenase in charge of 2,4,6-TCP dechlorination, and transcriptional and proteomic studies confirmed the directly inhibited gene transcription and expression of CprA-2. The in-depth reveal of inhibitory effects and mechanism gave theoretical supports for alleviating heavy metal inhibition on organohalide respiration activity in groundwater co-contaminated with organohalides and heavy metals.

A comprehensive review on spatial and temporal variation of arsenic contamination in Ghaghara basin and its relation to probable incremental life time cancer risk in the local population

Spatial and temporal variations have been found in the levels of arsenic (As) throughout the groundwater of the Ghaghara basin. Fifteen out of twenty-five districts in this basin are reported to be affected by As, where the levels of As in groundwater and soil exceed the permissible limits set by the WHO (10 μgl-1) and FAO (20 mgkg-1) respectively. These districts include a total of four municipalities in Nepal and eighty-six blocks in India, all of which have varying degrees of As contamination. Approximately 17 million people are at risk of As poisoning, with more than two orders of magnitude higher potential lifetime incremental cancer risk, constituting over 153 thousand potential additional cases of cancer due to As-contaminated drinking water. Out of the 90 As-contaminated blocks in the Ghaghara basin, 4 blocks have about 7-fold higher potential risk of developing cancer, 49 blocks have 8-37-fold higher risk, and 37 blocks have up to 375-fold higher risk compared to the upper limit of the USEPA acceptable range, which is 1 × 10-6-1 × 10-4. High accumulation of As has been reported in the nails, hair, and urine of local inhabitants, with higher levels observed in females than males. The toxicity of As is manifested in terms of a higher occurrence of various diseases. Reproductive endpoints, such as increased incidences of preterm birth, spontaneous abortion, stillbirth, low-birth weight, and neonatal death, have also been reported in the basin. The level of As in tube wells has been found to be negatively correlated with the depth (r = -0.906), and tube wells with high levels of As (>150 μgl-1) are generally located within close proximity (<10 km) to abandoned or present meander channels in the floodplain areas of the Ghaghara river. In addition to As contamination, the water quality index (WQI) in the Ghaghara basin is poor according to the BIS standards for drinking water. Groundwater in six out of fifteen districts is unsuitable for drinking purposes, with a WQI exceeding 100. The levels of As in agricultural soil in many villages of Ballia, Bahraich, and Lakhimpur Kheri districts have exceeded the FAO limit. Water from deep tube wells has been found to be relatively safe in terms of As content, and thus can be recommended for drinking purposes. However, the use of surface water needs to be encouraged for irrigation purposes in order to preserve soil health and reduce As contamination in the food chain, thereby minimizing the risk of cancer.

Unraveling the ameliorative potentials of native lichen Pyxine cocoes (Sw.) Nyl., during COVID 19 phase

Due to the rapid increase in the novel coronavirus virulence, the entire world implemented the practice of lockdown along with the constraint of human movement. The obligation of quarantine halted most of the commercial and industrial movement that prominently disturbed the distinct key environmental parameters directly associated with the plant's and animal's health conditions. In this regard, the research aims to study the sudden shut-off of vehicular activity impact on the naturally growing lichen of the genus Pyxine cocoes. The results showed an increase in the pigments, Fv/Fm ratio, and phytohormones during the lockdown and concurrently the decreasing levels in the post-lockdown period. Interestingly, modulations in the phytohormones occur in the lockdown period as compared to the post-lockdown period. The metals Al, Cr, and Fe show the highest increasing trends in the unlocking period, whereas As, Cd, Pb, Cu, Hg, Mn, and Zn show very little variation during the running and post-lockdown phases. The lichen photosynthetic activity justifies further examination as initial biological indicators of the abrupt environmental variations prompted by such types of atmospheric situations and, to a greater extent, for the risk assessment in the near future. In conclusion, stress-phytohormone and amino acids play a significant role as stress reducers. Although lichens are well known for long environmental assessment, the present study will provide qualitative and quantitative variation in physiochemical changes in the short term and sudden environmental fluctuations. HIGHLIGHTS: • Qualitative and quantitative variation in biochemical parameters in lichen during and post-lockdown period was analyzed. • Stress-phytohormone and amino acids play a significant role as stress reducers. • Selectivity sequence reflection in heavy metal accumulation may be used in future studies.

Bacillus amyloliquefaciens modulate sugar metabolism to mitigate arsenic toxicity in Oryza sativa L. var Saryu-52

In the current study, plant growth-promoting rhizobacterium Bacillus amyloliquefaciens SN13 (SN13) was evaluated for arsenic (As) toxicity amelioration potential under arsenate (AsV) and arsenite (AsIII) stress exposed to rice (Oryza sativa var Saryu-52) plants for 15 days. The PGPR-mediated alleviation of As toxicity was demonstrated by modulated measures such as proline, total soluble sugar, malondialdehyde content, enzymatic status, relative water content, and electrolytic leakage in treated rice seedlings under arsenic-stressed conditions as compared to the respective control. SN13 inoculation not only improved the agronomic traits but also modulated the micronutrient concentrations (Fe, Mo, Zn, Cu, and Co). The desirable results were obtained due to a significant decrease in the AsIII and AsV accumulation in the shoot (47 and 10 mg kg-1 dw), and the root (62 and 26 mg kg-1 dw) in B. amyloliquefaciens inoculated seedlings as compared to their uninoculated root (98 and 43 mg kg-1 dw) and shoot (57 and 12 mg kg-1 dw), respectively. Further, metabolome (GC-MS) analysis was performed to decipher the underlying PGPR-induced mechanisms under arsenic stress. A total of 67 distinct metabolites were identified, which influence the metabolic and physiological factors to modulate the As stress. The expression analysis of metabolism- and stress-responsive genes further proclaimed the involvement of SN13 through modulating the carbohydrate metabolism in rice seedlings, to enable improved growth and As stress tolerance.

The significance of eighteen rice genotypes on arsenic accumulation, physiological response and potential health risk

Rice is an important food crop that is susceptible to arsenic (As) contamination under paddy soil conditions depending on As uptake characteristics of the rice genotypes. Here we unveiled the significance of eighteen (fine and coarse) rice genotypes against As accumulation/tolerance, morphological and physiological response, and antioxidant enzymes-enabled defense pathways. Arsenic significantly affected rice plant morphological and physiological attributes, with relatively more impacts on fine compared to coarse genotypes. Grain, shoot, and root As uptake were lower in fine genotypes (0.002, 0.020, and 0.032 mg pot^-1 DW, respectively) than that of coarse (0.031, 0.60, and 1.2 mg pot^-1 DW, respectively). Various biochemical (pigment contents, hydrogen peroxide, lipid peroxidation) and defense (antioxidant enzymes) plant parameters indicated that the fine genotypes, notably Kainat and Basmati-385, possessed the highest As tolerance. Arsenic-induced risk indices exhibited greater hazard quotient (up to 1.47) and carcinogenic risk (up to 0.0066) for coarse genotypes compared to the fine ones, with the greatest risk for KSK-282. This study elaborates the pivotal role of genotypic variation among rice plants in As accumulation, which is crucial for mitigating the associated human health risk. Further research is required on molecular aspects, e.g., genetic sequencing, to examine rice genotypes variation in defense mechanisms to As contamination.

Effects of copper and arsenic on their uptake and distribution in As-hyperaccumulator Pteris vittata

Arsenic (As) and copper (Cu) are common co-contaminates in soils. However, their interactive effects on their accumulation and distribution in As-hyperaccumulator Pteris vittata are poorly understood. A hydroponic experiment was conducted with As being 0, 5, or 50 μM and Cu being 0.32, 3.2, or 32 μM to evaluate their phytotoxicity, accumulation, and distribution in P. vittata. In addition, As and Cu uptake kinetics were examined using the Michaelis-Menten kinetics model. Total As and Cu concentrations in P. vittata were up to 487 and 1355 mg kg^-1. About 39-81% of the As was in the fronds compared to 0.6-18% for Cu. At 50 μM As, increasing Cu concentration from 0.32 to 32 μM increased root As while decreasing frond As concentrations, with the translocation factor (ratio of As in fronds to roots) being reduced from 4.0 to 0.31. In contrast, As did not affect Cu accumulation in P. vittata. Michaelis constant K_m value for As was higher than that of Cu (6.49-24.9 vs. 0.43-3.36), consistent with higher Cu uptake than As. Besides, Cu reduced root K but increased P levels in the roots, whereas As increased the K and P concentrations in the fronds. Our results suggest that P. vittata accumulated more Cu than As in the roots, contributing to its low As translocation. As such, high levels of Cu are likely to reduce As uptake by P. vittata during phytoremediation of As-contaminated sites.

Goethite modified biochar simultaneously mitigates the arsenic and cadmium accumulation in paddy rice (Oryza sativa) L

Cadmium (Cd) and arsenic (As) contamination of paddy soils is a serious global issue because of the opposite geochemical behavior of Cd and As in paddy soils. Rice plant (Oryza sativa L.) cultivation in Cd- and As- contaminated paddy soil is regarded as one of the main dietary cause of Cd and As entry in human beings. This study aimed to determine the impact of goethite-modified biochar (GB) on bioavailability of both Cd and As in Cd- and As- polluted paddy soil. Contrary to control and biochar (BC) amendments, the application of GB amendments significantly impeded the accumulation of both Cd and As in rice plants. The results confirmed an obvious reduction in Cd and As content of rice grains by 85% and 77%, respectively after soil supplementation with GB 2% amendment. BC 3% application minimized the Cd uptake by 59% in the rice grains as compared to the control but exhibited a little impact on As accumulation in rice grains. Sequential extraction results displayed an increase in immobile Cd and As fractions of the soil by decreasing the bioavailable fractions of both elements after GB treatments. Fe-plaque formation on the root surfaces was significantly variable (P ˂ 0.05) among all the amendments. GB 2% treatment significantly increased the Fe content (10 g kg^-1) of root Fe-plaque by 48%, which ultimately enhanced the sequestration of Cd and As by Fe-plaque and minimized the transport of Cd and As in rice plants. Moreover, GB treatments significantly changed the relative abundance of the microbial community in the rice rhizosphere and minimized the metal(loid)s mobility in the soil. The relative abundance of Acidobacteria, Firmicutes and Verrucomicrobia increased with GB 2% treatment while those of Bacteroidetes and Choloroflexi decreased. Our findings confirmed improvement in the rice grains quality regarding enhanced amino acid contents with GB application. Overall, the results of this study demonstrated that GB amendment simultaneously alleviated the Cd and As concentrations in edible parts of rice plant and provided a new valuable method to protect the public health by effectively remediating the co-occurrence of Cd and As in paddy soils.

Grain starch, fatty acids, and amino acids determine the pasting properties in dry cultivation plus rice cultivars

A field experiment was conducted to explore the effects of cultivars under flooding irrigation and dry cultivation (D) on starch, fatty acids, and amino acids metabolism, starch physicochemical traits, and pasting properties of rice flour. In this study, high-quality cultivar (HH) had better pasting properties among all other cultivars in D treatment. DHH supported higher short-branch chain amylopectin to develop the crystalline regions. Besides, DHH increased C16:0, C16:1, C18:1, C18:2, glutamate, aspartate, lysine, and threonine, and reduced glutelin and prolamine levels in head rice. Higher pasting properties in DHH was also supported by higher CO in esters and ketones, CO in carboxylic acid, esters, alcohols, and ethers, OH in alcohols before pasting and lower CO and CO in carboxylic acid, CO in aldehydes, and CO, CO and OH in carboxylic acid after pasting. Overall, these findings improve pasting properties to maintain a higher cooking quality in dry cultivation.

Nitric oxide-mediated alleviation of arsenic stress involving metalloid detoxification and physiological responses in rice (Oryza sativa L.)

Rice is a staple crop, and food chain contamination of arsenic in rice grain possesses a serious health risk to billions of population. Arsenic stress negatively affects the rice growth, yield and quality of the grains. Nitric oxide (NO) is a major signaling molecule that may trigger various cellular responses in plants. The protective role of NO during arsenite (AsIII) stress and its relationship with plant physiological and metabolic responses is not explored in detail. Exogenous NO, supplemented through the roots in the form of sodium nitroprusside, has been shown to provide protection vis-à-vis AsIII toxicity. The NO-mediated variation in physiological traits such as stomatal density, size, chlorophyll content and photosynthetic rate maintained the growth of the rice plant during AsIII stress. Besides, NO exposure also enhanced the lignin content in the root, decreased total arsenic content and maintained the activities of antioxidant isoenzymes to reduce the ROS level essential for protecting from AsIII mediated oxidative damage in rice plants. Further, NO supplementation enhanced the GSH/GSSG ratio and PC/As molar ratio by modulating PC content to reduce arsenic toxicity. Further, NO-mediated modulation of the level of GA, IAA, SA, JA, amino acids and phenolic metabolites during AsIII stress appears to play a central role to cope up with AsIII toxicity. The study highlighted the role of NO in AsIII stress tolerance involving modulation of metalloid detoxification and physiological pathways in rice plants.

Dry cultivation with ratoon system impacts rice quality using rice flour physicochemical traits, fatty and amino acids contents

A field experiment was conducted to explore the impact on rice quality using high-quality (HH) or drought-resistant (HY) cultivars under flooding irrigation (F) or dry cultivation (D) in ratooning rice system by evaluating the metabolism or physicochemical traits of starch, fatty acids, and amino acids affecting grain quality. Compared to FHY and DHY in the main or ratoon season, DHH in ratoon season (DHHR) exhibited a higher appearance and processing quality but lower cooking quality. DHHR mainly synthesized long branch chain amylopectin to construct the crystalline regions with increased crystallinity, crystallites size, interplanar spacing, dislocation density, Asp and Thr in brown and head rice. Also, it accumulated more of C16:0, C18:0, C18:1, C18:2, and C18:3 but reduced glutelin in head rice. An increase in functional groups and diversity was seen in brown and head rice, respectively. Overall, these traits improved the processing, appearance, and pasting quality of DHHR.

Modulation of Cellular Redox Status and Antioxidant Defense System after Synergistic Application of Zinc Oxide Nanoparticles and Salicylic Acid in Rice (Oryza sativa) Plant under Arsenic Stress

The objective of this research was to determine the effect of zinc oxide nanoparticles (ZnONPs) and/or salicylic acid (SA) under arsenic (As) stress on rice (Oryza sativa). ZnONPs are analyzed for various techniques viz., X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). All of these tests established that ZnONPs are pure with no internal defects, and can be potentially used in plant applications. Hence, we further investigated for better understanding of the underlying mechanisms and the extent of ZnONPs and SA induced oxidative stress damages. More restricted plant growth, gas exchange indices, significant reduction in the SPAD index and maximum quantum yield (Fv/Fm) and brutal decline in protein content were noticed in As-applied plants. In contrast, foliar fertigation of ZnONPs and/or SA to As-stressed rice plants lessens the oxidative stress, as exposed by subordinate levels of reactive oxygen species (ROS) synthesis. Improved enzymatic activities of catalase (CAT), peroxidase (POX), and superoxide dismutase (SOD), proline and total soluble protein contents under ZnONPs and SA treatment plays an excellent role in the regulation of various transcriptional pathways participated in oxidative stress tolerance. Higher content of nitrogen (N; 13%), phosphorus (P; 10%), potassium (K; 13%), zinc (Zn; 68%), manganese (Mn; 14%), and iron (Fe; 19) in ZnONPs and SA treated plants under As-stress, thus hampered growth and photosynthetic efficiency of rice plants. Our findings suggest that toxicity of As was conquering by the application of ZnONPs and SA in rice plants.

Nanoscale Sulfur Improves Plant Growth and Reduces Arsenic Toxicity and Accumulation in Rice (Oryza sativa L.)

Rice is known to accumulate arsenic (As) in its grains, posing serious health concerns for billions of people globally. We studied the effect of nanoscale sulfur (NS) on rice seedlings and mature plants under As stress. NS application caused a 40% increase in seedling biomass and a 26% increase in seed yield of mature plants compared to untreated control plants. AsIII exposure caused severe toxicity to rice; however, coexposure of plants to AsIII and NS alleviated As toxicity, and growth was significantly improved. Rice seedlings treated with AsIII + NS produced 159 and 248% more shoot and root biomass, respectively, compared to plants exposed to AsIII alone. Further, AsIII + NS-treated seedlings accumulated 32 and 11% less As in root and shoot tissues, respectively, than the AsIII-alone treatment. Mature plants treated with AsIII + NS produced 76, 110, and 108% more dry shoot biomass, seed number, and seed yield, respectively, and accumulated 69, 38, 18, and 54% less total As in the root, shoot, flag leaves, and grains, respectively, compared to AsIII-alone-treated plants. A similar trend was observed in seedlings treated with AsV and NS. The ability of sulfur (S) to alleviate As toxicity and accumulation is clearly size dependent as NS could effectively reduce bioavailability and accumulation of As in rice via modulating the gene expression activity of As transport, S assimilatory, and glutathione synthesis pathways to facilitate AsIII detoxification. These results have significant environmental implications as NS application in agriculture has the potential to decrease As in the food chain and simultaneously enable crops to grow and produce higher yields on marginal and contaminated lands.

Impact of organic and inorganic amendments on arsenic accumulation by rice genotypes under paddy soil conditions: A pilot-scale investigation to assess health risk

In this study, we investigated the distinct effects of organic (farmyard manure (FYM), cow dung (CD), biogas slurry (BGS), sugarcane bagasse (SCB)) and inorganic (gypsum and lignite) amendments on arsenic (As) accumulation by two rice genotypes, Kainat (fine) and Basmati-385 (coarse), under As stress. Results showed that shoot As concentration was ~2-time greater in Kainat compared to Basmati-385 (3.1-28 vs. 1.7-16 mg kg^-1 DW, respectively), with the minimum shoot As content observed with CD and SCB. In contrast to gypsum and lignite, grain As concentration was significantly reduced with CD and SCB for Kainat (0.29 and 0.24 mg kg^-1 DW) and Basmati-385 (0.04 and 0.09 mg kg^-1 DW). Data indicated that the CD and SCB also improved chlorophyll a and b contents, reduced lipid peroxidation and hydrogen peroxide production in both rice genotypes. Significantly, the CD and SCB decreased grain As concentration below the FAO safe As limit in rice grain (0.2 mg kg^-1 DW), especially in coarse rice genotype (Basmati-385), resulting in negligible As-induced human health risk. This study highlights the significance of amendments and rice genotypes controlling As accumulation in rice grain, which should be considered prior to As remediation program of paddy soils for limiting exposure of humans to As via rice grain.

Effect of arsenic-contaminated irrigation water on growth and elemental composition of tomato and cabbage cultivated in three different soils, and related health risk assessment

This study was carried out to determine the effect of arsenic on tomato and cabbage cultivated in sand, sandy silt, and silt soil, and irrigated with water containing arsenic at concentrations 0.05 and 0.2 mg/L. Increasing arsenic in irrigation water did not affect the photosynthetic machinery. The chlorophyll content index increased in case of all soils and was dependent on the soil nitrogen, phosphorous, and plant biomass. Arsenic concentrations of 0.05 and 0.2 mg/L did not display any phytotoxic symptoms other than reduction in biomass in some cases. In cabbage, arsenic treatment of 0.2 mg/L increased the overall plant biomass production, while in tomato there was a decrease in aerial part and fruit biomass. The biomass production of both plants treated with different concentrations of arsenic, in the three soils was in the following order: silt > sand > sandy silt. Increase of arsenic in the irrigation water resulted in increase in arsenic concentration in the root and aerial part of both plants, at the same cultivation parameters. But tomato fruits displayed a decrease in arsenic accumulation with higher arsenic treatment. In both plants, the arsenic concentration in the plant parts changed in the following order: root > aerial part > fruit. Cabbage accumulated approximately twenty-fold more arsenic in the edible part (0.10-0.25 mg/kg DW) as compared to tomato (0.006-0.011 mg/kg DW) and displayed a good correlation with soil extractable arsenic. When cabbage was cultivated in three different soils applying the same irrigation water, it accumulated arsenic in the following order: sand > sandy silt > silt (p < 0.001 at 0.05 mg/L and p < 0.01 at 0.2 mg/L arsenic treatment). In tomato, the difference in arsenic accumulation among different soil types was highly significant (p < 0.001) but the accumulation pattern varied with the arsenic treatment applied. Sandy soil with the lowest total soil arsenic (4.32 mg/kg) resulted in the highest arsenic concentration in both plants. Among all soils and plants, the transfer factors and bioaccumulation factors were higher in sandy soil, and in cabbage. The estimated daily intake and hazard quotient values for arsenic were lower than 1 in all cases, implying no non-cancerous health risks at the arsenic concentrations applied in our study. Among nutrients only P showed a slight decline with increasing arsenic concentration while all other elements (Mg, K, Ca, S, Si, Fe, Mn, Cu, Zn) did not display any significant changes.

Arsenic uptake, speciation and physiological response of tree species (Acer pseudoplatanus, Betula pendula and Quercus robur) treated with dimethylarsinic acid

The aim of the study was to evaluate the effect of dimethylarsinic acid (DMA) on growth parameters and levels of stress-related metabolites in Acer pseudoplatanus, Betula pendula and Quercus robur. The increase of DMA concentration in the solution led to a notable growth retardation of trees. An intense As accumulation (mainly As(III) and As(V)) expressed as BCF and TF > 1 was recorded only for Q. robur. Generally a decrease in contents of cellulose, hemicellulose and holocellulose with a simultaneous increase in lignin content were recorded. Phenolic composition of leaf extracts was modified by DMA, while root and rhizosphere extracts were poor in phenolics. Toxicity of DMA leads to a significant drop in salicylic acid content in leaves observed at lower doses. Higher DMA levels caused a second, probably ROS-derived depletion of the metabolite accompanied with a severe growth retardation, most pronounced in the case of B. pendula. DMA caused the inhibition of LMWOA biosynthesis in roots of A. pseudoplatanus, B. pendula and their exudation into the rhizosphere, while in Q. robur roots and leaves a stimulation of their accumulation was observed. Disturbances in the activity of enzymatic antioxidants were observed for all the species following the increasing level of DMA.

Evaluation of Multiple Responses Associated with Arsenic Tolerance and Accumulation in Pteris vittata L. Plants Exposed to High As Concentrations under Hydroponics

Chinese brake fern (Pteris vittata L.) is recognized as an arsenic hyperaccumulating plant. Mechanisms underlying this capability and the associated hypertolerance have been described even if not completely elucidated. In this study, with the aim to expand the knowledge on the matter, an experimental trial was developed to investigate an array of responses, at the morphological, physiological, and biochemical level, in P. vittata plants exposed to high As concentrations in a long-term experiment under hydroponics. Results confirmed the ability of fern plants to both tolerate and accumulate a remarkable amount of As, especially in fronds. Notably, in As-treated plants, a far higher As content was detected in young fronds compared to old fronds, with bioaccumulation (BCF) and translocation (Tf) factors in accordance. At the biochemical level, As treatment affected macro and micronutrient, thiol, and phytochelatin concentrations in fronds of treated plants differently than that of the control. Physiological measurements accounted for a reduction in the photosynthetic activity of As-treated plants in the absence of visual symptoms of damage. Overall, the observed As tolerance and accumulation processes were discussed, evidencing how young fronds developed during As treatment maintain their physiological status while accumulating a high As content. Such indications could be very useful to improve the effective utilization of this plant species for phytofiltration of As-polluted water.

Mitigating arsenic accumulation in rice (Oryza sativa L.) using Fe-Mn-La-impregnated biochar composites in arsenic-contaminated paddy soil

Arsenic (As) is a prominent metal contaminant of the soil in China. Pot experiments were conducted to examine the effects of corn stem powder biochar (BC) and Fe-Mn-La-impregnated biochar composites (FMLBC₁, FMLBC₂, and FMLBC₃; BC:Fe:Mn:La at different weight ratios) on As accumulation in an indica cultivar of rice (Oryza sativa L.). The application of FMLBCs and BC improved the dry weight of the grains, leaves, stems, and roots of rice. The As uptake in different rice organs was significantly reduced in the FMLBC-amended soils (FMLBC₃ > FMLBC₂ > FMLBC₁) compared with the BC treatment. Compared to the concentration of As in the control, the concentration of As in rice grains decreased by 56.0-89.4% with the addition of 2% FMLBC₃. The application of FMLBCs significantly increased the ratio of essential amino acids in grains and the contents of Fe and Mn plaques on root surfaces. The reduction in As accumulation can be ascribed to the Fe, Mn, and La oxides that enhance the adsorption and retention of As, as well as the FMLBCs that provide nutrients and create a rhizosphere environment, promoting rice growth. This study demonstrated that applications of 2% FMLBC₂ and FMLBC₃ have the potential to remediate As-contaminated soils, reduce As accumulation in rice plants, and improve rice grain quality.

Protective role of nitric oxide on nitrogen-thiol metabolism and amino acids profiling during arsenic exposure in Oryza sativa L

Nitric oxide (NO) being a signaling molecule inside the plant cells, play significant role in signaling cascades and protection against environmental stresses. However, the protective role of NO in alleviating As toxicity in rice plants is currently not available. In the present study, the level of NO, nitrogen (N), inorganic N (nitrate, ammonium), thiols {TT (Total thiols), NPT (Nonprotein thiol)} and AAs contents along with N assimilating enzymes (NR, GDH, GOGAT) were analyzed after exposure of As^III/NO treatment alone, and in combination. NO supplementation enhanced the content of N, inorganic N & thiol contents, NR, GOGAT activities, when compared with As^III exposure alone. In As^III exposed rice seedlings, content of AAs (except His, Arg, Met) reduced over the control, while supplementation of SNP improved AAs contents, compared to As^III treatment alone. In conclusion, rice seedlings supplemented with NO tolerate the As^III toxicity by reducing the N related parameters, thiol contents, altering the AA profile and enhanced the nutritional quality by increasing EAAs (essential amino acids) and NEAAs (non-essential amino acids).

Rice grains alleviate cadmium toxicity by expending glutamate and increasing manganese in the cadmium contaminated farmland

The accumulation of cadmium (Cd) in rice grains is closely associated with the content of mineral nutrients and amino acid metabolism, but the causal link among them is unclear. Profiles of amino acids (AAs) and quantities of essential nutrients in grains from early and late rice cultivars grown at four sites with different Cd levels were analyzed in the present study. Hazard quotients (HQs) for consumers by intake of rice from late cultivars were much higher than that from early cultivars at sites with soil Cd content of 0.25, 0.61 and 0.84 mg kg^-1. Cadmium accumulation in grains resulted in a sharp reduction of total essential AAs and non-essential AAs in both early and late rice cultivars. High-Cd-accumulating (HCA) cultivars had significantly higher level of glutamate (Glu) than low-Cd-accumulating (LCA) cultivars when rice Cd content was less than 0.20 mg kg^-1. However, Glu level in grains dramatically declined with the accumulation of Cd, which subsequently leaded to the reduction of other AAs. Cadmium content was well predicted by five amino acids (i.e., Glu, Alanine, Phenylalanine, Glycine and Threonine) or four essential elements (Ca, Fe, Mn and Zn) when rice Cd was less than 0.80 mg kg^-1. Amino acids played more important roles than nutrients in Cd accumulation. When Cd content was in the range of 0.40-1.16 mg kg^-1, the Mn content in rice increased significantly with the increase of Cd content, while the Glu content dropped down synchronously. Remarkably, the ratio between Mn and Glu displayed the highest direct path coefficient on Cd accumulation than any single cation or amino acid. These results indicate that high capacity in synthesizing Glu and concentrating Mn is the determinant factor for Cd accumulation in rice grains, and abundant Glu in aleurone layer may alleviate Cd toxicity by forming Glu-Cd complex.

Orthosilicic acid (OSA) reduced grain arsenic accumulation and enhanced yield by modulating the level of trace element, antioxidants, and thiols in rice

Arsenic (As), a toxic metalloid, is finding its route to human through intake of As-contaminated water and consumption of food grown on contaminated soil. Rice is the most As-affected crop. Present study is aimed to assess the impact of stabilized orthosilicic acid (a proprietary formulation for plant-available silicon (Si) and earlier used as fertilizer for rice to enhance growth and yield) in reducing the accumulation of As in rice grains. Application of arsenic in the form of arsenate (As^V) and arsenite (As^III) significantly affected plant growth in a dose-dependent manner. Higher doses of As^V and As^III (50 and 25 mg L^-1 respectively) significantly decreased the yield attributes leading to lower yield. A significant accumulation of As in grain was observed in both As^V- and As^III-exposed plants in a dose-dependent manner. Arsenic exposure also increased the level of Si in rice grains. Application of Si, either in soil or on leaves (foliar), greatly reduced grain As accumulation (up to 67% in As^V and 78% in As^III) and enhanced the growth and yield of plants under As stress. The level of thiols and activities of antioxidant enzymes were also enhanced under Si application. Foliar Si application was more effective in increasing grain Si level and reducing grain As than soil Si. The level of other trace elements was also significantly enhanced by Si application irrespective of the presence or absence of As in comparison with control. Arsenic exposure constrained some of the trace elements, such as Zn and Co, which were restored by Si application. Results of the present study showed that the application of currently used Si formulation may effectively reduce grain As level even in highly As-contaminated soil and improve grain quality of rice.

Transcriptome and proteome analyses reveal selenium mediated amelioration of arsenic toxicity in rice (Oryza sativa L.)

Arsenic (As), a chronic poison and non-threshold carcinogen, is a food chain contaminant in rice, posing yield losses as well as serious health risks. Selenium (Se), a trace element, is a known antagonist of As toxicity. In present study, RNA seq. and proteome profiling, along with morphological analyses were performed to explore molecular cross-talk involved in Se mediated As stress amelioration. The repair of As induced structural deformities involving disintegration of cell wall and membranes were observed upon Se supplementation. The expression of As transporter genes viz., NIP1;1, NIP2;1, ABCG5, NRAMP1, NRAMP5, TIP2;2 as well as sulfate transporters, SULTR3;1 and SULTR3;6, were higher in As + Se compared to As alone exposure, which resulted in reduced As accumulation and toxicity. The higher expression of regulatory elements like AUX/IAA, WRKY and MYB TFs during As + Se exposure was also observed. The up-regulation of GST, PRX and GRX during As + Se exposure confirmed the amelioration of As induced oxidative stress. The abundance of proteins involved in photosynthesis, energy metabolism, transport, signaling and ROS homeostasis were found higher in As + Se than in As alone exposure. Overall, present study identified Se responsive pathways, genes and proteins involved to cope-up with As toxicity in rice.

Effects of Fe-Mn-Ce oxide-modified biochar on As accumulation, morphology, and quality of rice (Oryza sativa L.)

The fluidity of arsenic (As) in soil used for rice cultivation under flooding conditions is the main reason for its high accumulation in rice, which poses a serious threat to human's health. Biochar can immobilize heavy metal (for example lead) of soil because of the strong binding of heavy metals to the inner biochar particles. We conducted a pot experiment to evaluate the effects of biochar (BC) and Fe-Mn-Ce oxide-modified biochar composites (FMCBCs) on the morphology, As accumulation, and grain quality of rice grown in As-contaminated soils. The biochar and FMCBC treatments significantly increased the dry weight of roots, stems, leaves, and rice grains grown in As-contaminated soil (P < 0.05). The As concentration in different parts of rice was significantly lower with treatment FMCBC_3-2 (BC, Fe, Mn, and Ce weight ratio of 24:2:3:10) than with the BC and control (no BC) treatments. The application of FMCBC_3-2 maximized the yield and quality of rice grains: rice grain yields were 61.45-68.41% higher over control and the proportion of essential amino acids in the rice grains was 31.01-44.62%. The application of FMCBCs also increased the concentration of Fe-Mn plaques, which prevent the uptake of As by rice, thereby mitigating the toxic effects of As-contaminated soil on rice. In summary, Fe-Mn-Ce oxide-modified BC composites fixed As, reducing its fluidity and the As concentration in rice. Our results show that FMCBC₃ could play an important role in reducing As accumulation and increasing the grain yield and quality of rice, thus ensuring food safety in regions contaminated with As.

Distribution of elements and their correlation in bran, polished rice, and whole grain

The relationship of toxic elements (As, Cd, Cr) and trace elements (Cu, Se, Ni, Zn, Mn) in rice bran and corresponding polished rice is not well known. A total of 446 rice grains were collected from paddy fields distributed across China, and the concentrations of 8 elements in rice bran and their corresponding polished rice were measured. The levels of As, Cd, Cr, and Se have a good linear relationship between rice bran and polished rice (R 2: .79, .97, .82, .99, respectively; all p < .001). Polishing rice could effectively remove the average contents of 44.4% As, 19.8% Cd, and 15.4% Cr in the whole grain, but caused the substantial losses of more than half of Mn and Ni (57.7% and 56.9%), and nearly one-third (30.9%, 31.5%, and 29.1%) of Cu, Se, and Zn in brown rice although only about 10% of rice bran was milled. The "L" type correlation exists not only between As and Cd, but also between the nutrients Se, Mn, Ni, and the toxic elements As, Cd. These results indicated that As accumulation in rice could reduce the levels of essential mineral nutrients Mn, Ni, and Se. On the contrary, improving nutrient elements by fertilization could decrease the accumulation of some toxic elements. This provides a practical new idea for the prevention and control of rice As or Cd, and concomitantly improves the deficiency of nutrient elements in rice.

Mixed plantation of wheat and accumulators in arsenic contaminated plots: A novel way to reduce the uptake of arsenic in wheat and load on antioxidative defence of plant

Wheat (W) and accumulators (A) were planted in plots (arsenic amended soil and without arsenic) designed with ecotoxicological concern for arsenic safe-grains. For the study sixteen plots of 2 × 2 × 0.5 m (l × b × h) size were prepared. Arsenic (As) in the form of sodium arsenate was applied at 50 mg/kg in plots. Out of these sixteen plots eight plots had arsenic amended soil and rest 8 without any arsenic (C). Accumulator's viz. Pteris vittata (PV), Phragmites australis (PA) and Vetiveria zizanioides (VZ) were planted along with wheat in combination (W + PV, W + PA and W + VZ) in twelve plots (6 AWAs plots and 6 AWC plots). In the rest 4 plots (2 WAs plots and 2 WC plots), only wheat was planted. The study was conducted for two cropping seasons, where accumulators were left in the plots between the cropping seasons except that before 2nd cropping accumulators were properly pruned and extra tillers were removed. The germination % of wheat in WAs in 1st and the 2nd cropping season was found to be 55 and 57%, while in AWAs and AWC plots it was between 86 and 92% (W + VZ, 56 and 73%). The physiological activity was found to be reduced in WAs plots compared to AWAs (except for vetiver combination) and AWC plots in both cropping seasons. The antioxidant activity was enhanced in WAs compared with AWAs. The arsenic concentration in grains of wheat was within the permissible limit set by WHO and GOI in AWAs plots while it exceeded the limit in W + VZ (in 1st cropping) and WAs in both cropings.

Nitric oxide alters nitrogen metabolism and PIN gene expressions by playing protective role in arsenic challenged Brassica juncea L

Plants have ability to adapt themselves through altering their growth process. In the present study, we examined exogenous application of nitric oxide (NO) on nitrogen metabolism and auxin (PIN) gene expression, and its possible role in alleviation of arsenic (As) toxicity in Brassica juncea seedlings. Seven days old hydroponically grown B. juncea seedlings were exposed to As^III (150 μM), Sodium nitroprusside (NO donor, 100 μM), As^III + SNP and control (without metal)for 48 h. Experimental results revealed that As^III stress: enhanced the level of nitrite, NiR activity, NO₃^- and NH₄⁺content as well as NADH-GOGAT activity; but GDH level decreased; enhanced content of amino acids; upregulated gene expression level of N metabolism and downregulated polar auxin transporter genes (PIN); inhibited plant growth and morphological parameters; increased MDA, H₂O_2, cysteine, proline content, enzymatic antioxidants (SOD, CAT, APX; GSH, TT, NPT); and decreased nutrient content. As^III + SNP combination reduced the accumulation of As; improved growth; chlorophyll, protein and mineral nutrient content by scavenging ROS generation; maintained amino acids content; downregulated expression of N metabolism genes and upregulated expression of auxin transporter (PIN) genes . Additional biochemical data depicts reduction in the level of nitrogen related enzymatic activities, and other stress related parameters. Overall, this study provides an integrated view that exogenous SNP (NO donor) supplementation alleviated the inhibitory role of As^III in B. juncea seedlings by altering nutrients, amino acids and auxin redistribution via expression of nitrogen and PIN gene profiling.

Assessment of potential dietary toxicity and arsenic accumulation in two contrasting rice genotypes: Effect of soil amendments

High concentration of arsenic (As) in rice is a serious problem worldwide. Pot experiments were conducted to assess the potential dietary toxicity of arsenic and effect of various soil amendments on arsenic accumulation in rice grains. Two basmati rice genotypes were used to conduct pot experiments using various levels of arsenic (10, 25, 50 and 100 mg kg^-1 soil). In addition, plants were exposed to soil collected from a well documented arsenic contaminated site. Contrasting results for growth, yield and grain arsenic concentration were obtained for basmati-385 (Bas-385), exhibiting tolerance (56% yield improvement at 10 mg As kg^-1), while genotype BR-1 showed 18% yield decline under same conditions. Furthermore, application of soil amendments such as iron (Fe), phosphate (PO₄) and farmyard manure (FYM) at 50 mg kg^-1, 80 kg ha^-1 and 10 t ha^-1, respectively improved the plant height and biomass in both genotypes. Accumulation of arsenic in rice grain followed a linear trend in BR-1 whereas a parabolic relationship was observed in Bas-385. Both genotypes exhibited a positive response to iron sulfate amendment with significant reduction in grain arsenic concentrations. Regression analysis gave soil arsenic threshold values of 12 mg kg^-1 in Bas-385 and 10 mg kg^-1 in BR-1 for potential dietary toxicity. This study suggests that genotype Bas-385 can be used for safe rice production in areas with soil arsenic contamination up to 12 mg kg^-1 and that appropriate dose of iron sulfate for soil amendment can be used effectively to reduce translocation of arsenic to rice grain.

Arsenic accumulation in rice (Oryza sativa L.) is influenced by environment and genetic factors

Arsenic (As) elevation in paddy soils will have a negative impact on both the yield and grain quality of rice (Oryza sativa L.). The mechanistic understanding of As uptake, translocation, and grain filling is an important aspect to produce rice grains with low As concentrations through agronomical, physico-chemical, and breeding approaches. A range of factors (i.e. physico-chemical, biological, and environmental) govern the speciation and mobility of As in paddy soil-water systems. Major As uptake transporters in rice roots, such as phosphate and aquaglyceroporins, assimilate both inorganic (As(III) and As(V)) and organic As (DMA(V) and MMA(V)) species from the rice rhizosphere. A number of metabolic pathways (i.e. As (V) reduction, As(III) efflux, and As(III)-thiol complexation and subsequent sequestration) are likely to play a key role in determining the translocation and substantial accumulation of As species in rice tissues. The order of translocation efficiency (caryopsis-to-root) for different As species in rice plants is comprehensively evaluated as follows: DMA(V) > MMA(V) > inorganic As species. The loading patterns of both inorganic and organic As species into the rice grains are largely dependent on the genetic makeup and maturity stage of the rice plants together with environmental interactions. The knowledge of As metabolism in rice plants and how it is affected by plant genetics and environmental factors would pave the way to develop adaptive strategies to minimize the accumulation of As in rice grains.

Seed priming with Se mitigates As-induced phytotoxicity in rice seedlings by enhancing essential micronutrient uptake and translocation and reducing As translocation

We laid down this investigation to explore the promotive and antagonistic aspect of selenium (Se) when supplemented through seed priming technology in rice before sowing into arsenic (As) free and As spiked soil. Findings suggest that As stress inhibits germination (35.38%), seedling growth (38.19%), chlorophyll content by 42.31%, and reduced translocation of iron, zinc, manganese by 19.40, 17.33, and 18.40% respectively, in the seedlings of unprimed seeds. Seedlings of unprimed seeds also had greater As translocation into the aerial part beside repressing micronutrient translocation, significantly. On the contrary, Se-primed seeds had higher germination (27.82%), longer root length (20.14%), greater chlorophyll content beside having greater translocation of iron, zinc, manganese in shoots along with restricting As translocation in rice seedlings by confining more As in the root, in a significant manner (p < 0.05 level) than the unprimed seedlings grown in identical stress. On the other hand, seedlings of Se-primed seeds grown alike the control also had higher germination % (7.08%), root and shoot length with significantly less proline, and hydrogen peroxide content in root and shoot. Findings indicate that seed priming with Se executes dual role, a growth promoting and antagonism in a more practical and farmer-friendly way to mitigate As-induced toxicity and enhance growth in rice seedlings.

Effects of copper and arsenic stress on the development of Norway spruce somatic embryos and their visualization with the environmental scanning electron microscope

Somatic embryogenesis is an important biotechnological technique which can be used in studies associated with environmental stress. Four embryogenic cell lines of Norway spruce were grown on media enriched with copper and arsenic in concentration ranges 50-500 μM and 10-50 μM, respectively. The effects were observed during subsequent stages of somatic embryogenesis, the characteristics evaluated being proliferation potential, average number of somatic embryos obtained per g/fresh weight, morphology of developed somatic embryos, metal uptake, and microanalysis of macro- and micronutrients uptake. Copper and arsenic at higher concentrations significantly reduced the growth of early somatic embryos. In almost all treatments, the cell line V-1-3 showed the best performance compared with the other lines tested. Environmental scanning electron microscopy was used to visualize and identify morphological abnormalities in the development of somatic embryos. Abnormalities observed were classified into several categories: meristemless somatic embryos, somatic embryos with disrupted meristem, reduced number of cotyledons, single cotyledon and fused cotyledons. With the application of a low temperature method for the environmental scanning electron microscope, samples were stabilized and whole meristems could be investigated in their native state. As far as we are aware, this is the first report of the effect of copper and arsenic during the process of somatic embryogenesis and the first to evaluate the content of macro and micronutrients uptake in Norway spruce.

Application of glycine reduces arsenic accumulation and toxicity in Oryza sativa L. by reducing the expression of silicon transporter genes

The present study was intended to investigate the role of amino acid glycine in detoxification of As in Oryza sativa L. The growth parameters such as, shoot length and fresh weight were decreased during As(III) and As(V) toxicity. However, the application of glycine recovered the growth parameters against As stress. The application of glycine reduced the As accumulation in all the treatments, and it was more effective against As(III) treatment and reduced the accumulation by 68% in root and 71% in shoot. Similarly, the translocation of As from root to shoot, was higher against As(III) and As(V) treatments, whereas, reduced upon glycine application. The translocation of Fe and Na was also affected by As, which was lower under As(III) and As(V) treatments. However, the application of glycine significantly enhanced the translocation of Fe and Na in the shoot. Besides, the expression of lower silicon transporters i.e. Lsi-1 and Lsi-2 was observed to be significantly suppressed in the root with the application of glycine against As treatment. Similarly, the expression of three GRX and two GST gene isoforms were found to be significantly increased with glycine application. Simultaneously, the activities of antioxidant enzymes i.e. l-arginine dependent NOS, SOD, NTR and GRX were found to be significantly enhanced in the presence of glycine. Increased activities of antioxidant enzymes coincided with the decreased level of TBARS and H₂O₂ in rice seedlings. Overall, the results suggested that the application of glycine reduces As accumulation through suppressing the gene expression of lower silicon transporters and ameliorates As toxicity by enhancing antioxidants defense mechanism in rice seedlings.

Arsenic Uptake, Toxicity, Detoxification, and Speciation in Plants: Physiological, Biochemical, and Molecular Aspects

Environmental contamination with arsenic (As) is a global environmental, agricultural and health issue due to the highly toxic and carcinogenic nature of As. Exposure of plants to As, even at very low concentration, can cause many morphological, physiological, and biochemical changes. The recent research on As in the soil-plant system indicates that As toxicity to plants varies with its speciation in plants (e.g., arsenite, As(III); arsenate, As(V)), with the type of plant species, and with other soil factors controlling As accumulation in plants. Various plant species have different mechanisms of As(III) or As(V) uptake, toxicity, and detoxification. This review briefly describes the sources and global extent of As contamination and As speciation in soil. We discuss different mechanisms responsible for As(III) and As(V) uptake, toxicity, and detoxification in plants, at physiological, biochemical, and molecular levels. This review highlights the importance of the As-induced generation of reactive oxygen species (ROS), as well as their damaging impacts on plants at biochemical, genetic, and molecular levels. The role of different enzymatic (superoxide dismutase, catalase, glutathione reductase, and ascorbate peroxidase) and non-enzymatic (salicylic acid, proline, phytochelatins, glutathione, nitric oxide, and phosphorous) substances under As(III/V) stress have been delineated via conceptual models showing As translocation and toxicity pathways in plant species. Significantly, this review addresses the current, albeit partially understood, emerging aspects on (i) As-induced physiological, biochemical, and genotoxic mechanisms and responses in plants and (ii) the roles of different molecules in modulation of As-induced toxicities in plants. We also provide insight on some important research gaps that need to be filled to advance our scientific understanding in this area of research on As in soil-plant systems.

Modulation of Spartina densiflora plant growth and metal accumulation upon selective inoculation treatments: A comparison of gram negative and gram positive rhizobacteria

Metal contamination of estuaries is a severe environmental problem, for which phytoremediation is gaining momentum. In particular, the associations between halophytes-autochthonous rhizobacteria have proven useful for metal phytostabilization in salt marshes. In this work, three bacterial strains (gram-negative and gram-positive) were used for Spartina densiflora inoculation. All three bacteria, particularly Pantoea strains, promoted plant growth and mitigated metal stress on polluted sediments, as revealed from functionality of the photosynthetic apparatus (PSII) and maintenance of nutrient balance. Pantoea strains did not significantly affect metal accumulation in plant roots, whereas the Bacillus strain enhanced it. Metal loading to shoots depended on particular elements, although in all cases it fell below the threshold for animal consumption. Our results confirm the possibility of modulating plant growth and metal accumulation upon selective inoculation, and the suitability of halophyte-rhizobacteria interactions as biotechnological tools for metal phytostabilization in salt marshes, preventing metal transfer to the food chain.

Reduced arsenic accumulation in indica rice (Oryza sativa L.) cultivar with ferromanganese oxide impregnated biochar composites amendments

The effects of biochar (BC) and ferromanganese oxide biochar composites (FMBC₁ and FMBC₂) on As (Arsenic) accumulation in rice were determined using a pot experiment. Treatments with BC or FMBC improved the dry weights of rice roots, stems, leaves, and grains in soils containing different As contamination levels. Compared to BC treatment, FMBC treatments significantly reduced As accumulation in different parts of the rice plants (P < 0.05), and FMBC₂ performed better than FMBC₁ did. Furthermore, exposure to 2% FMBC₂ decreased the total As concentration in the grain by 68.9-78.3%. The addition of FMBC increased the ratio of essential amino acids in the grain, decreased As availability in the soil, and significantly increased the Fe and Mn plaque contents. The reduced As accumulation in rice can be attributed to As(III) to As(V) oxidation by ferro - manganese binary oxide, which increased the As adsorbed by FMBC. Furthermore, Fe and Mn plaques on the rice root surface decreased the transport of As in rice. Taken together, our results demonstrated the applicability of FMBC as a potential measure for reducing As accumulation in rice, improving the amino acid content of rice grains, and effectively remediating As-polluted soil.

Nitric oxide mediated transcriptional modulation enhances plant adaptive responses to arsenic stress

Arsenic (As) contamination in rice leads to yield decline and causes carcinogenic risk to human health. Although the role of nitric oxide (NO) in reducing As toxicity is known, NO-mediated genetic modulation in the plant during arsenic toxicity has not yet been established. We analyzed the key components of NO metabolism and the correlations between NO interaction and arsenic stress using rice as a relevant model plant. Illumina sequencing was used to investigate the NO-mediated genome-wide temporal transcriptomic modulation in rice root upon AsIII exposure during 12 days (d) of the growth period. Sodium nitroprusside (SNP) was used as NO donor. SNP supplementation resulted in marked decrease in ROS, cell death and As accumulation during AsIII stress. NO was found to modulate metal transporters particularly NIP, NRAMP, ABC and iron transporters, stress related genes such as CytP450, GSTs, GRXs, TFs, amino acid, hormone(s), signaling and secondary metabolism genes involved in As detoxification. We detected NO-mediated change in jasmonic acid (JA) content during AsIII stress. The study infers that NO reduces AsIII toxicity through modulating regulatory networks involved in As detoxification and JA biosynthesis.

Hexavalent chromium stress enhances the uptake of nitrate but reduces the uptake of ammonium and glycine in pak choi (Brassica chinensis L.)

Chromium (Cr) pollution affects plant growth and biochemical processes, so, the relative uptake of glycine, nitrate, and ammonium by pak choi (Brassica chinensis) seedlings in treatments with 0mgL^-1 and 10mgL^-1 Cr (VI) were detected by substrate-specific ¹⁵N-labelling in a sterile environment. The short-term uptake of ¹⁵N-labelled sources and ¹⁵N-enriched amino acids were detected by gas chromatography mass spectrometry to explore the mechanism by which Cr stress affects glycine uptake and metabolism, which showing that Cr stress hindered the uptake of ammonium and glycine but increased significantly the uptake of nitrate. Cr stress did not decrease the active or passive uptake of glycine, but it inhibited the conversion of glycine to serine in pak choi roots, indicating that the metabolism of glycine to serine in roots, rather than the root uptake, was the limiting step in glycine contribution to total N uptake in pak choi. Since Cr affects the relative uptake of different N sources, a feasible way to reduce Cr-induced stress is application of selective fertilization, in particular nitrate, in pak choi cultivation on Cr-polluted soil.

Selenite modulates the level of phenolics and nutrient element to alleviate the toxicity of arsenite in rice (Oryza sativa L.)

Arsenic (As) contamination of paddy rice is a serious threat all over the world particularly in South East Asia. Selenium (Se) plays important role in protection of plants against various abiotic stresses including heavy metals. Moreover, arsenite (AsIII) and selenite (SeIV) can be biologically antagonistic due to similar electronic configuration and sharing the common transporter for their uptake in plant. In the present study, the response of oxidative stress, phenolic compounds and nutrient elements was analyzed to investigate Se mediated As tolerance in rice seedlings during AsIII and SeIV exposure in hydroponics. Selenite (25µM) significantly decreased As accumulation in plant than As (25µM) alone treated plants. Level of oxidative stress related parameters viz., reactive oxygen species (ROS), lipid peroxidation, electrical conductivity, nitric oxide and pro-oxidant enzyme (NADPH oxidase), were in the order of As>As+Se>control>Se. Selenium ameliorated As phytotoxicity by increased level of phenolic compounds particularly gallic acid, protocatechuic acid, ferulic acid and rutin and thiol metabolism related enzymes viz., serine acetyl transferase (SAT) and cysteine synthase (CS). Selenium supplementation enhanced the uptake of nutrient elements viz., Fe, Mn, Co, Cu, Zn, Mo, and improved plant growth. The results concluded that Se addition in As contaminated environment might be an important strategy to reduce As uptake and associated phytotoxicity in rice plant by modulation of phenolic compounds and increased uptake of nutrient elements.

Accumulation and transformation of inorganic and organic arsenic in rice and role of thiol-complexation to restrict their translocation to shoot

Environmental contamination of arsenic (As) and its accumulation in rice (Oryza sativa L.) is of serious human health concern. In planta speciation of As is an important tool to understand As metabolism in plants. In the present study, we investigated root to shoot As translocation and speciation in rice exposed to inorganic and methylated As. Arsenate (AsV) and methylarsonate (MAV) were efficiently reduced to arsenite (AsIII) and MAIII, respectively in rice root and shoot but no trivalent form of dimethylarsinate (DMAV) was detected. Further, up to 48 and 83% of root As in AsV and MAV exposed plants, respectively were complexed with various thiols showing up to 20 and 16 As species, respectively. Several mixed As- and MA-complexes with hydroxymethyl-phytochelatin, DesGly-phytochelatin, hydroxymethyl-GSH and cysteine were identified in rice. Despite high complexation in roots, more As was translocated to shoots in MAV exposed plants than AsV, with shoot/root As transfer factor being in order DMAV > MAV > AsV. Moreover, in shoots 78% MAIII and 71% AsIII were present as weakly bound species which is alarming, as MAIII has been found to be more cytotoxic than AsIII for human and it could also be an important factor inducing straighthead (spikelet sterility disorder) in rice.

Genetic variation in Fe toxicity tolerance is associated with the regulation of translocation and chelation of iron along with antioxidant defence in shoots of rice

Excess iron (Fe) is phytotoxic and causes reduced growth and productivity in rice. In this study we elucidated the mechanisms conferring differential tolerance to Fe-toxicity in rice seedlings. Excess Fe caused retardation in roots of both Pokkali and BRRI 51, but it caused no significant changes on growth parameters, Fe accumulation and OsIRT1 expression in shoots of Pokkali only compared with control plants. These results suggest that the Pokkali genotype does have mechanisms in shoots to withstand Fe toxicity. Pokkali maintained membrane stability and total soluble protein in shoots due to Fe toxicity, further confirming its ability to tolerate excess Fe. Furthermore, a significant decrease of Fe-chelate reductase activity and OsFRO1 expression in shoots of Pokkali suggests that limiting Fe accumulation is possibly regulated by Fe-reductase activity. Our extensive expression analysis on the expression pattern of three chelators (OsDMAS1, OsYSL15, OsYSL2 and OsFRDL1) showed no significant changes in expression in shoots of Pokkali due to Fe toxicity, whereas these genes were significantly upregulated under Fe-toxicity in sensitive BRRI 51. These results imply that regulation of Fe chelation in shoots of Pokkali contributes to its tolerance to Fe toxicity. Finally, increased catalase (CAT), peroxidase (POD), glutathione reductase (GR) and superoxide dismutase (SOD), along with elevated ascorbic acid, glutathione, cysteine, methionine and proline in shoots of Pokkali caused by Fe toxicity suggests that strong antioxidant defence protects rice plants from oxidative injury under Fe toxicity. Taking these results together, we propose that genetic variation in Fe-toxicity tolerance in rice is shoot based, and is mainly associated with the regulation of translocation and chelation of Fe together with elevated antioxidant metabolites in shoots.

Selenate mitigates arsenite toxicity in rice (Oryza sativa L.) by reducing arsenic uptake and ameliorates amino acid content and thiol metabolism

Arsenic (As) is a toxic element with the potential to cause health effects in humans. Besides rice is a source of both amino acids (AAs) and mineral nutrients, it is undesired source of As for billions of people consuming rice as the staple food. Selenium (Se) is an essential metalloid, which can regulate As toxicity by strengthening antioxidant potential. The present study was designed to investigate As(III) stress mitigating effect of Se(VI) in rice. The level of As, thiolic ligands and AAs was analyzed in rice seedlings after exposure to As(III)/Se(VI) alone and As(III)+Se(VI) treatments. Selenate supplementation (As(III) 25μM+Se(VI) 25μM) decreased total As accumulation in both root and shoot (179 & 144%) as compared to As(III) alone treatment. The As(III)+Se(VI) treatment also induced the levels of non-protein thiols (NPTs), glutathione (GSH) and phytochelatins (PCs) as compared to As(III) alone treatment and also modulated the activity of enzymes of thiol metabolism. The content of amino acids (AAs) was significantly altered with Se(VI) supplementation. Importantly, essential amino acids (EAAs) were enhanced in As(III)+Se(VI) treatment as compared to As(III) alone treatment. In contrast, stress related non-essential amino acids (NEAAs) like GABA, Glu, Gly, Pro and Cys showed enhanced levels in As(III) alone treatment. In conclusion, rice supplemented with Se(VI) tolerated As toxicity with reduced As accumulation and increased the nutrition quality by increasing EAAs.

Increasing CO2 differentially affects essential and non-essential amino acid concentration of rice grains grown in cadmium-contaminated soils

Environmental pollution by both ambient CO2 and heavy metals has been steadily increasing, but we do not know how fluctuating CO2 concentrations influence plant nutrients under high Cd pollution, especially in crops. Here, we studied the effects of elevated CO2 and Cd accumulation on proteins and amino acids in rice under Cd stress. In this pot experiment, we analyzed the amino-acid profile of 20 rice cultivars that accumulate Cd differently; the plants were grown in Cd-containing soils under ambient conditions and elevated CO2 levels. We found that although Cd concentrations appeared to be higher in most cultivars under elevated CO2 than under ambient CO2, the effect was significant only in seven cultivars. Combined exposure to Cd and elevated CO2 strongly decreased rice protein and amino acid profiles, including essential and non-essential amino acids. Under elevated CO2, the ratios of specific amino acids were either higher or lower than the optimal ratios provided by FAO/WHO, suggesting that CO2 may flatten the overall amino-acid profile, leading to an excess in some amino acids and deficiencies in others when the rice is consumed. Thus, Cd-tainted rice limits the concentration of essential amino acids in rice-based diets, and the combination with elevated CO2 further exacerbates the problem.

Beneficial behavior of nitric oxide in copper-treated medicinal plants

Despite numerous reports implicating nitric oxide (NO) in the environmental-stress responses of plants, the specific metabolic and ionic mechanisms of NO-mediated adaptation to metal stress remain unclear. Here, the impacts of copper (Cu) and NO donor (SNP, 50μM) alone or in combination on the well-known medicinal plant Catharanthus roseus L. were investigated. Our results showed that Cu markedly increased Cu(2+) accumulation, decreased NO production, and disrupted mineral equilibrium and proton pumps, thereby stimulating a burst of ROS; in addition, SNP ameliorates the negative toxicity of Cu, and cPTIO reverses this action. Furthermore, the accumulations of ROS and NO resulted in reciprocal changes. Interestingly, nearly all of the investigated amino acids and the total phenolic content in the roots were promoted by the SNP treatment but were depleted by the Cu+SNP treatment, which is consistent with the self-evident increases in phenylalanine ammonia-lyase activity and total soluble phenol content induced by SNP. Unexpectedly, leaf vincristine and vinblastine as well as the total alkaloid content (ca. 1.5-fold) were decreased by Cu but markedly increased by SNP (+38% and +49% of the control levels). This study provides the first evidence of the beneficial behavior of NO, rather than other compounds, in depleting Cu toxicity by regulating mineral absorption, reestablishing ATPase activities, and stimulating secondary metabolites.

Change in atmospheric deposition during last half century and its impact on lichen community structure in Eastern Himalaya

Climatic fluctuations largely affects species turnover and cause major shifts of terrestrial ecosystem. In the present study the five decade old herbarium specimens of lichens were compared with recent collection from Darjeeling district with respect to elements, PAHs accumulation and carbon isotope composition (δ(13)C) to explore the changes in climatic conditions and its impact on lichen flora. The δ(13)C has increased in recent specimens which is in contrast to the assumption that anthropogenic emission leads to δ(13)C depletion in air and increased carbon discrimination in flora. Study clearly demonstrated an increase in anthropogenic pollution and drastic decrease in precipitation while temperature showed abrupt changes during the past five decades resulting in significant change in lichen community structure. The Usneoid and Pertusorioid communities increased, while Physcioid and Cyanophycean decreased, drastically. Lobarian abolished from the study area, however, Calcicoid has been introduced in the recent past. Probably, post-industrial revolution, the abrupt changes in the environment has influenced CO2 diffusion and/C fixation of (lower) plants either as an adaptation strategy or due to toxicity of pollutants. Thus, the short term studies (≤5 decades) might reflect recent micro-environmental condition and lichen community structure can be used as model to study the global climate change.

Biochemical and molecular responses underlying differential arsenic tolerance in rice (Oryza sativa L.)

The arsenic (As) is a toxic element causing major health concern worldwide. Arsenate stress caused no significant reduction in growth parameters and shoot electrolyte leakage but showed increased root arsenate reductase activity along with relatively lower root As content and shoot translocation rate in As-tolerant BRRI 33 than in As-sensitive BRRI 51. It indicates that As inhibition and tolerance mechanisms are driven by root responses. Interestingly, As stress showed consistent decrease in phosphate content and expression of phosphate transporters (OsPT8, OsPT4, OsPHO1;2) under both high and low phosphate conditions in roots of BRRI 33, suggesting that limiting phosphate transport mainly mediated by OsPHO1;2 directs less As accumulation in BRRI 33. Further, BRRI 33 showed simultaneous increase in OsPCS1 (phytochelatin synthase) expression and phytochelatins (PCs) content in roots under As exposure supporting the hypothesis that root As sequestration acts as 'firewall system' in limiting As translocation in shoots. Furthermore, increased CAT, POD, SOD, GR, along with elevated glutathione, methionine, cysteine and proline suggests that strong antioxidant defense plays integral part to As tolerance in BRRI 33. Again, BRRI 33 self-grafts and plants having BRRI 33 rootstock combined with BRRI 51 scion had no adverse effect on morphological parameters but showed reduced As translocation rate, increased root arsenate reductase activity, shoot PC synthesis and root OsPHO1;2 expression due to As stress. It confirms that signal driving As tolerance mechanisms is generated in the roots. These findings can be implemented for As detoxification and As-free transgenic rice production for health safety.

Biochemical and molecular changes in rice seedlings (Oryza sativa L.) to cope with chromium stress

Chromium (Cr) is very toxic to both humans and plants. This investigation aimed to understand the physiological and molecular responses of rice seedlings to Cr stress. Cr toxicity did not significantly affect morphological features and Cr accumulation in roots and shoots in Pokkali but not in BRRI 51, although there was a reduction in chlorophyll concentration in leaves of both genotypes. These results imply that Pokkali has mechanisms to cope with Cr supplementation. We therefore performed quantitative real-time PCR on the expression pattern of two chelator genes, OsPCS1 and OsMT1, but there were no significant changes in expression in roots and shoots of Pokkali and BRRI 51 following Cr stress. This suggests that there was no metal sequestration following heavy metal stress in roots of these genotypes. Moreover, no expression of two heavy metal transporter genes, OsHMA3 and OsNRAMP1, was induced after Cr stress in roots and shoots, suggesting that these transporter genes are not induced by Cr stress or might not be involved in Cr uptake in rice. We also performed a targeted study on the effect of Cr on Fe uptake mechanisms. Our studies showed a consistent reduction in Fe uptake, Fe reductase activity and expression of Fe-related genes (OsFRO1 and OsIRT1) under Cr stress in both roots and leaves of Pokkali. In contrast, these parameters and genes were significantly increased in Cr-sensitive BRRI 51 under Cr stress. The results confirm that limiting Fe uptake through the down-regulation of Fe reductase and Fe transporter genes is the main strategy of Cr-tolerant Pokkali to cope with Cr stress. Finally, increased CAT, POD and GR activity and elevated glutathione and proline synthesis might provide strong antioxidant defence against Cr stress in Pokkali. Taken together, our findings reveal that Cr stress tolerance in rice (Pokkali) is not related to metal sequestration but is associated with reduced Fe transport and increased antioxidant defence.

Arsenic hyperaccumulation induces metabolic reprogramming in Pityrogramma calomelanos to reduce oxidative stress

Arsenic (As) pollution is a major environmental concern due to its worldwide distribution and high toxicity to organisms. The fern Pityrogramma calomelanos is one of the few plant species known to be able to hyperaccumulate As, although the mechanisms involved are largely unknown. This study aimed to investigate the metabolic adjustments involved in the As-tolerance of P. calomelanos. For this purpose, ferns with five to seven fronds were exposed to a series of As concentrations. Young fronds were used for biochemical analysis and metabolite profiling using gas chromatography-mass spectrometry. As treatment increased the total concentration of proteins and soluble phenols, enhanced peroxidase activities, and promoted disturbances in nitrogen and carbon metabolism. The reduction of the glucose pool was one of the striking responses to As. Remarkable changes in amino acids levels were observed in As-treated plants, including those related to biosynthesis of glutathione and phenols, osmoregulation and two photorespiratory intermediates. In addition, increases in polyamines levels and antioxidant enzyme activities were observed. In summary, this study indicates that P. calomelanos tolerates high concentration of As due to its capacity to upregulate biosynthesis of amino acids and antioxidants, without greatly disturbing central carbon metabolism. At extremely high As concentrations, however, this protective mechanism fails to block reactive oxygen species production, leading to lipid peroxidation and leaf necrosis.