Arsenate induced differential response in rice genotypes

Evaluation of arsenic phytoremediation potential in Azolla filiculoides Lam. plants under low pH stress conditions

The Azolla filiculoides plants were challenged with different arsenic (As) concentration under low pH stress conditions. The growth rate and doubling time of the plants were severely affected by higher As treatments at pH 5.00 when compared with stress pH 4.75 treatments. Hence, pH 5.00 was considered for further studies. In 10-30 μM As treated cultures, after 6 days, the relative growth rate (RGR) of Azolla plants was significantly reduced and in higher concentration of As, the RGR was negatively regulated. The root trait parameters were also significantly affected by increasing concentrations of As. Further, photosynthetic performance indicators also show significant decline with increasing As stress. Overall, the plants treated with 40 and 50 μM of As displayed stress phenotypes like negative RGR, reduced doubling time and root growth, browning of leaves and root withering. The total proline, H2O2, POD and Catalase activities were significantly affected by As treatments. Meantime, 30 μM of As treated cultures displayed 15 μg/g/Fw As accumulation and moderate growth rate. Thus, the Azolla plants are suitable for the phytoremediation of As (up to 30 μM concentration) in the aquatic environment under low pH conditions (5.00). Furthermore, the transcriptome studies on revealed that the importance of positively regulated transporters like ACR3, AceTr family, ABC transporter super family in As (10 μM) stress tolerance, uptake and accumulation. The transporters like CPA1, sugar transporters, PiT were highly down-regulated. Further, expression analysis showed that the MATE1, CIP31, HAC1 and ACR3 were highly altered during the As stress conditions.

Assessment of rice genotypes through the lens of morpho-physiological and biochemical traits in response to arsenic stress

Rice is a globally important food crop which is sensitive to the presence of a metalloid, arsenic (As). There is limited research pertaining to identifying relevant As-tolerant rice germplasm in adaptive breeding research initiatives, despite the fact that As contamination in rice has long been known. This study served to identify the growth performance of different rice genotypes under high levels of As. Rice seed germination analysis (germination percentage, GP) was performed to categorize the eight different rice genotypes and growing under varying As levels including As25, 25 μM and As50, 50 μM. The Zhenong 41 was identified as the highly tolerant genotypes with lowest decrease in GP by 87 %, plant height (PH) by 26 %, and dry weight (DW) by 16 %; while 9311 was observed to be the most sensitive genotype with highest reduction in GP by 44 %, PH by 48 % and DW by 54 % under As25 stress conditions, compared to control treatment. The higher As50 stress treatment delivered more adverse growth inhibitory effects than the rice plants cultivated under As25. Specifically, the As-sensitive rice genotype 9311 showed significantly higher decrease in foliar chlorophyll contents relative to the other genotypes, especially Zhenong 41 (As-tolerant). During exposure to high As levels, the rice genotype 9311 significantly modulated and augmented the production of MDA and H2O2 by stimulating the activities of POD, SOD, and CAT. This study revealed interesting insights into the responses of rice genotypes to variable As stresses throughout the various growth stages. Overall, the findings of this study could be harnessed to support any ongoing As-tolerant rice breeding agendas for cultivation in As-polluted environments.

Arsenic-induced plant stress: Mitigation strategies and omics approaches to alleviate toxicity

Arsenic (As) is a metalloid pollutant that is extensively distributed in the biosphere. As is among the most prevalent and toxic elements in the environment; it induces adverse effects even at low concentrations. Due to its toxic nature and bioavailability, the presence of As in soil and water has prompted numerous agricultural, environmental, and health concerns. As accumulation is detrimental to plant growth, development, and productivity. Toxicity of As to plants is a function of As speciation, plant species, and soil properties. As inhibits root proliferation and reduces leaf number. It is associated with defoliation, reduced biomass, nutrient uptake, and photosynthesis, chlorophyll degradation, generation of reactive oxygen species, membrane damage, electrolyte leakage, lipid peroxidation and genotoxicity. Plants respond to As stress by upregulating genes involved in detoxification. Different species have adopted avoidance and tolerance responses for As detoxification. Plants also activate phytohormonal signaling to mitigate the stressful impacts of As. This review addresses As speciation, uptake, and accumulation by plants. It describes plant morpho-physiological, biochemical, and molecular changes and how phytohormones respond to As stress. The review closes with a discussion of omic approaches for alleviating As toxicity in plants.

Hesperidin and chlorogenic acid mitigate arsenic-induced oxidative stress via redox regulation, photosystems-related gene expression, and antioxidant efficiency in the chloroplasts of Zea mays

The ubiquitous metalloid arsenic (As), which is not essential, can be found extensively in the soil and subterranean water of numerous nations, raising substantial apprehensions due to its impact on both agricultural productivity and sustainability. Plants exposed to As often display morphological, physiological, and growth-related abnormalities, collectively leading to reduced productivity. Polyphenols, operating as secondary messengers within the intricate signaling networks of plants, assume integral functions in the acquisition of resistance to diverse environmental stressors, including but not limited to drought, salinity, and exposure to heavy metals. The pivotal roles played by polyphenols in these adaptive processes underscore their profound significance in plant biology. This study aims to elucidate the impact of hesperidin (HP) and chlorogenic acid (CA), recognized as potent bioactive compounds, on maize plants exposed to As. To achieve this objective, the study examined the physiological and biochemical impacts, including growth parameters, photosynthesis, and chloroplastic antioxidants, of HP (100 μM) and CA (50 μM) on Zea mays plants exposed to arsenate stress (AsV, 100 μM - Na2HAsO4⋅7H2O). As toxicity led to reductions in fresh weight (FW) and dry weight (DW) by 33% and 26%, respectively. However, the application of As+HP and As + CA increased FW by 22% and 40% and DW by 14% and 17%, respectively, alleviating the effects of As stress. As toxicity resulted in the up-regulation of PSII genes (psbA and psbD) and PSI genes (psaA and psaB), indicating a potential response to the re-formation of degraded regions, likely driven by the heightened demand for photosynthesis. Exogenous HP or/and CA treatments effectively counteracted the adverse effects of As toxicity on the photochemical quantum efficiency of PSII (Fv/Fm). H2O2 content showed a 23% increase under As stress, and this increase was evident in guard cells when examining confocal microscopy images. In the presence of As toxicity, the chloroplastic antioxidant capacity can exhibit varying trends, with either a decrease or increase observed. After the application of CA and/or HP, a significant increase was observed in the activity of GR, APX, GST, and GPX enzymes, resulting in decreased levels of H2O2 and MDA. Additionally, the enhanced functions of MDHAR and DHAR have modulated the redox status of ascorbic acid (AsA) and glutathione (GSH). The HP or CA-mediated elevated levels of AsA and GSH content further contributed to the preservation of redox homeostasis in chloroplasts facing stress induced by As. In summary, the inclusion of HP and CA in the growth medium sustained plant performance in the presence of As toxicity by regulating physiological and biochemical characteristics, chloroplastic antioxidant enzymes, the AsA-GSH cycle and photosynthesis processes, thereby demonstrating their significant potential to confer resistance to maize through the mitigation of As-induced oxidative damage and the safeguarding of photosynthetic mechanisms.

Nitric oxide and spermidine alleviate arsenic-incited oxidative damage in Cicer arietinum by modulating glyoxalase and antioxidant defense system

Anthropogenic activities such as mining, fossil fuel combustion, fertilisers and pesticides utilisation in agriculture, metallurgic processes and disposal of industrial wastes have contributed an exponential rise in arsenic content in environment. The present paper deals with arsenate (AsV) incited stress in chickpea (Cicer arietinum L.) plants and its alleviation through the application of nitric oxide (NO) and spermidine (SPD). The exposure of C. arietinum to AsV reduced seedling length, biomass, relative water content and biochemical constituents. All the above-mentioned parameters were escalated when sodium nitroprusside (SNP) or SPD were utilised alone or in combination with AsV. The electrolyte leakage and malondialdehyde content were increased in chickpea treated with AsV, but reduced in combine treatment (As+SNP+SPD). In chickpea seedlings, 89.4, 248.4 and 333.3% stimulation were recorded in sugar, proline and glycine betaine contents, respectively, with As+SNP+SPD treatment in comparison to control. SNP and SPD modulated function of glyoxalase enzymes by which methylglyoxal (MG) was significantly detoxified in C. arietinum . Maximum reduction 45.2% was observed in MG content in SNP+SPD treatment over AsV stress. Hence, synergistic application of NO and SPD protected chickpea plants against AsV-generated stress by strengthening the antioxidant defence and glyoxalase system, which helped in regulation of biochemical pathways.

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.

Arsenic-Induced Oxidative Stress and Antioxidant Defense in Plants

The non-essential metalloid arsenic (As) is widely distributed in soil and underground water of many countries. Arsenic contamination is a concern because it creates threat to food security in terms of crop productivity and food safety. Plants exposed to As show morpho-physiological, growth and developmental disorder which altogether result in loss of productivity. At physiological level, As-induced altered biochemistry in chloroplast, mitochondria, peroxisome, endoplasmic reticulum, cell wall, plasma membrane causes reactive oxygen species (ROS) overgeneration which damage cell through disintegrating the structure of lipids, proteins, and DNA. Therefore, plants tolerance to ROS-induced oxidative stress is a vital strategy for enhancing As tolerance in plants. Plants having enhanced antioxidant defense system show greater tolerance to As toxicity. Depending upon plant diversity (As hyperaccumulator/non-hyperaccumulator or As tolerant/susceptible) the mechanisms of As accumulation, absorption or toxicity response may differ. There can be various crop management practices such as exogenous application of nutrients, hormones, antioxidants, osmolytes, signaling molecules, different chelating agents, microbial inoculants, organic amendments etc. can be effective against As toxicity in plants. There is information gap in understanding the mechanism of As-induced response (damage or tolerance response) in plants. This review presents the mechanism of As uptake and accumulation in plants, physiological responses under As stress, As-induced ROS generation and antioxidant defense system response, various approaches for enhancing As tolerance in plants from the available literatures which will make understanding the to date knowledge, knowledge gap and future guideline to be worked out for the development of As tolerant plant cultivars.

Comparative physiological and transcriptomic analyses illuminate common mechanisms by which silicon alleviates cadmium and arsenic toxicity in rice seedlings

The inessential heavy metal/loids cadmium (Cd) and arsenic (As), which often co-occur in polluted paddy soils, are toxic to rice. Silicon (Si) treatment is known to reduce Cd and As toxicity in rice plants. To better understand the shared mechanisms by which Si alleviates Cd and As stress, rice seedlings were hydroponically exposed to Cd or As, then treated with Si. The addition of Si significantly ameliorated the inhibitory effects of Cd and As on rice seedling growth. Si supplementation decreased Cd and As translocation from roots to shoots, and significantly reduced Cd- and As-induced reactive oxygen species generation in rice seedlings. Transcriptomics analyses were conducted to elucidate molecular mechanisms underlying the Si-mediated response to Cd or As stress in rice. The expression patterns of the differentially expressed genes in Cd- or As-stressed rice roots with and without Si application were compared. The transcriptomes of the Cd- and As-stressed rice roots were similarly and profoundly reshaped by Si application, suggesting that Si may play a fundamental, active role in plant defense against heavy metal/loid stresses by modulating whole genome expression. We also identified two novel genes, Os01g0524500 and Os06g0514800, encoding a myeloblastosis (MYB) transcription factor and a thionin, respectively, which may be candidate targets for Si to alleviate Cd and As stress in rice, as well as for the generation of Cd- and/or As-resistant plants. This study provides valuable resources for further clarification of the shared molecular mechanisms underlying the Si-mediated alleviation of Cd and As toxicity in rice.

Piriformospora indica augments arsenic tolerance in rice (Oryza sativa) by immobilizing arsenic in roots and improving iron translocation to shoots

Arsenic (As) toxicity can be a hazardous threat to sustainable agriculture and human health. Piriformospora indica (P. indica), as a beneficial endophytic fungus, is involved in the plant tolerance to stressful conditions. Here, the biochemical and molecular responses of rice plants to As (50 μM) phytotoxicity and P. indica inoculation as well as the role of P. indica in improving rice adaptation to As stress were evaluated. The results showed that As stress reduced chlorophylls content, chlorophyll fluorescence yield (Fv/Fm), electron transport rate (ETR) and growth. However, P. indica restored chlorophyll content and growth. P. indica decreased the contents of methylglyoxal and malondialdehyde by improving the activity of enzymes involved in the glyoxalase pathway and modulating the redox state of the ascorbic acid-glutathione cycle, and consequently, increased the plant tolerance to As toxicity. P. indica, by downregulating Lsi2 expression (involved in As translocation to the shoot) and upregulating PCS1 and PCS2 expression (involved in As sequestration in vacuoles), immobilized As in the roots and reduced damage to photosynthetic organs. P. indica increased iron (Fe) accumulation in the shoot under As toxicity by upregulating the expression of IRO2, YSL2 and FRDL1 genes. The results of the present study augmented our knowledge in using P. indica symbiosis in improving the tolerance of rice plants against As toxicity for sustainable agriculture.

Methyl jasmonate alleviates arsenic toxicity in rice

Methyl jasmonate improved yield of both rice varieties under arsenic toxicity by alleviating oxidative stress through increasing the activity of antioxidant enzymes and decreasing arsenic accumulation by modulating arsenic transporters. Human health and rice cultivation are threatened by arsenic (As) contamination. Methyl jasmonate (MJ), as a regulator of plant growth, plays an important role in response to environmental stresses. In the present study, the effects of MJ (0, 0.5 and 1 µM) on yield, biochemical and molecular traits of two rice varieties (T. hashemi and Fajr) under As treatments (0, 25 and 50 µM) were investigated. The results showed that As decreased chlorophyll content, chlorophyll fluorescence and biomass production; however, MJ improved photosynthetic pigments and plant growth. As also induced oxidative stress (H₂O₂ and MDA) in both rice varieties; however, MJ reduced the As-induced oxidative stress by regulating the activity of antioxidant enzymes and the ASA-GSH cycle. As treatment increased As accumulation in the roots and leaves, which is in line with the increased expression of Lsi1, Lsi2 and Lsi6 genes. However, MJ reduced As accumulation by decreasing the expression of Lsi1, Lsi2 and Lsi6. Fe translocation to leaves reduced under As treatments; while, MJ increased Fe accumulation in the leaves by increasing expression of FRDL1 and YSL2 transporters under As toxicity. As treatments, especially 50 μM, decreased yield and yield components of both rice varieties; however, MJ improved yield and yield components of both rice varieties. The findings of the present study indicate that MJ improved the growth and yield of both rice varieties under As toxicity by alleviating oxidative stress through increasing the activity of antioxidant enzymes and ASA-GSH cycle and decreasing As accumulation by modulating As transporters.

Multi-Component Antioxidative System and Robust Carbohydrate Status, the Essence of Plant Arsenic Tolerance

Arsenic (As) contaminates the food chain and decreases agricultural production through impairing plants, particularly due to oxidative stress. To better understand the As tolerance mechanisms, two contrasting tobacco genotypes: As-sensitive Nicotiana sylvestris and As-tolerant N.tabacum, cv. 'Wisconsin' were analyzed. The most meaningful differences were found in the carbohydrate status, neglected so far in the As context. In the tolerant genotype, contrary to the sensitive one, net photosynthesis rates and saccharide levels were unaffected by As exposure. Importantly, the total antioxidant capacity was far stronger in the As-tolerant genotype, based on higher antioxidants levels (e.g., phenolics, ascorbate, glutathione) and activities and/or appropriate localizations of antioxidative enzymes, manifested as reverse root/shoot activities in the selected genotypes. Accordingly, malondialdehyde levels, a lipid peroxidation marker, increased only in sensitive tobacco, indicating efficient membrane protection in As-tolerant species. We bring new evidence of the orchestrated action of a broad spectrum of both antioxidant enzymes and molecules essential for As stress coping. For the first time, we propose robust carbohydrate metabolism based on undisturbed photosynthesis to be crucial not only for subsidizing C and energy for defense but also for participating in direct reactive oxygen species (ROS) quenching. The collected data and suggestions can serve as a basis for the selection of plant As phytoremediators or for targeted breeding of tolerant crops.

Facets of iron in arsenic exposed Oryza sativa varieties: A manifestation of plant's adjustment at morpho-biochemical and enzymatic levels☆

Rice consumption is one of the primary sources of arsenic (As) exposure as the grains contain relatively higher concentration of inorganic As. Abundant studies on the ability of iron (Fe) plaque in hampering As uptake by plants has been reported earlier. However, little is known about its role in the mitigation of As mediated oxidative damage in rice plants. The present study highlights the effect of As and Fe co-supplementation on growth response, oxidative stress, Fe uptake related enzymes and nutrient status in rice varieties. Eight different Indica rice varieties were screened and finally four varieties (Varsha, Jaya, PB-1 and IR-64) were selected for detailed investigations. Improved germination and chlorophyll/protein levels during As+Fe co-exposure indicate healthier plants than As(III) treated ones. Interestingly Fe was found act both as an antagonist and also as a synergist of As treatments. It acted by reducing As translocation and improving the nutritional levels and enhancing the oxidative stress. Fe uptake related enzymes (nitrite reductase and ferric chelate reductase) and phytosiderophores analysis revealed that Fe supplementation can reduce its deficiency in rice plants. Morpho-biochemical, oxidative stress and nutrient analysis symbolizes higher tolerance of PB-1 towards As, while Varsha being most sensitive, efficiently combated the As(III) stress in the presence of Fe.

Comparative transcriptome profiling reveals the reprogramming of gene networks under arsenic stress in Indian mustard

Arsenic is a widespread toxic metalloid that is classified as a class I carcinogen known to cause adverse health effects in humans. In the present study, we investigated arsenic accumulation potential and comparative gene expression in Indian mustard. The amount of arsenic accumulated in shoots varied in the range of 15.99-1138.70 mg/kg on a dry weight basis among five cultivars. Comparative expression analysis revealed 10 870 significantly differentially expressed genes mostly belonging to response to stress, metabolic processes, signal transduction, transporter activity, and transcription regulator activity to be up-regulated, while most of the genes involved in photosynthesis, developmental processes, and cell growth were found to be down-regulated in arsenic-treated tissues. Further, pathway analysis using the KEGG Automated Annotation server (KAAS) revealed a large-scale reprogramming of genes involved in genetic and environmental information processing pathways. Top pathways with maximum KEGG orthology hits included carbon metabolism (2.5%), biosynthesis of amino acids (2.1%), plant hormone signal transduction (1.4%), and glutathione metabolism (0.6%). A transcriptomic investigation to understand the arsenic accumulation and detoxification in Indian mustard will not only help to improve its phytoremediation efficiency but also add to the control measures required to check bioaccumulation of arsenic in the food chain.

A transcriptomic (RNA-seq) analysis of genes responsive to both cadmium and arsenic stress in rice root

Cadmium (Cd) and arsenic (As) are nonessential and toxic elements in rice that often occur together in contaminated paddy field soils. To understand whether rice has a common molecular response mechanism against Cd and As toxicity, 30-day seedlings (Oryza sativa L. indica) were exposed separately to Cd and As³⁺ in hydroponic cultures for up to 7 days. Root transcriptomic analysis of plants exposed to Cd and As for 3 days revealed that a total of 2224 genes in rice roots responded to Cd stress, while 1503 genes responded to As stress. Of these, 841 genes responded to both stressors. The genes in common to Cd and As stress were associated with redox control, stress response, transcriptional regulation, transmembrane transport, signal transduction, as well as biosynthesis and metabolism of macromolecules and sulfur compounds. In plants exposed to Cd and As separately or in combination for 3 and 7 days, qRT-PCR verification revealed that the glutathione metabolism associated gene Os09g0367700 was significantly up-regulated with respect to unexposed controls and had a positive synergistic effect under combined Cd and As stress. In addition, the redox control related genes Os06g0216000, Os07g0638300 and Os01g0294500, the glutathione metabolism related gene Os01g0530900, the cell wall biogenesis related genes Os05g0247800, Os11g0592000 and Os03g0416200, the expression regulation related genes Os07g0597200 and Os02g0168200, and the transmembrane transport related genes Os04g0524500, also varied significantly with respect to an unexposed control and displayed synergistic effects after 7 days of simultaneous exposure to Cd and As. Our identification of a novel set of genes in rice which responded to both Cd and As³⁺ stress may be of value in mitigating the toxicity of co-contaminated soils. These results also provide a deeper understanding of the molecular mechanisms involved in response to multi-metal/loids stress, and may be used in the genetic improvement of rice varieties.

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

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

Exogenous Glutathione Increases Arsenic Translocation Into Shoots and Alleviates Arsenic-Induced Oxidative Stress by Sustaining Ascorbate-Glutathione Homeostasis in Rice Seedlings

Glutathione (GSH) plays diverse roles in the physiological processes, stress defense, growth, and development of plants. This study investigated the effects of exogenous GSH on the biochemical responses of reactive oxygen species and antioxidant levels in rice (Oryza sativa L. cv. Dasan) seedlings under arsenic (As) stress. As treatment inhibited growth; increased the level of superoxide, hydrogen peroxide, and malondialdehyde; and enhanced the uptake of As by the roots and shoots in hydroponically grown 14-day-old seedlings. Furthermore, it reduced GSH content and GSH redox ratios, which have been correlated with the decrease in ascorbate (AsA) redox state. Whereas the exogenous application of GSH in As-treated seedlings reduced As-induced oxidative stress, improved antioxidant defense systems by maintaining antioxidant and/or redox enzyme homeostasis, and increased the AsA and GSH contents, the GSH application also increased the As translocation from the roots to the shoots. These results indicated that the increase in GSH redox state can be linked to an increase in the AsA redox ratio via the induction of the AsA-GSH cycle. Therefore, the results suggest that exogenous GSH application should be a promising approach to enhance As stress resistance in rice plants.

Simultaneous exposure of sulphur and calcium hinder As toxicity: Up-regulation of growth, mineral nutrients uptake and antioxidants system

The current study was carried out to investigate the role of exogenous sulphur (K₂SO₄: S; 60 mg S kg^-1 sand) and calcium (CaCl₂: Ca; 250 mg Ca kg^-1 sand) individually as well as in combination (S + Ca) in ameliorating the inhibitory effect of As (Na₂HAsO₄·7H₂O: As₁; 15 mg As kg^-1 sand and As₂; 30 mg As kg^-1 sand) by analyzing biomass accumulation, mineral nutrients uptake, photosynthetic pigments content, redox status of the cell, enzymatic and non-enzymatic defense system in Brassica juncea L. seedlings. Biomass accumulation, uptake of mineral nutrients, photosynthetic pigments (chlorophyll a, b and carotenoids) content and the activity of proline dehydrogenase (ProDH) declined with increasing accumulation of As in root as well as leaves in As dose dependent manner. Contrary to this, exogenous application of S, Ca and S + Ca, markedly reduced the negative impact of As on above captioned traits except ProDH activity. On the other hand, ROS and their biomarkers (superoxide radical; O₂˙-, hydrogen peroxide; H₂O_2, malondialdehyde; MDA equivalents content and membrane damage; electrolyte leakage), activities of enzymatic (superoxide dismutase; SOD, peroxidase; POD, catalase; CAT and glutathione-S-transferase; GST) and non-enzymatic antioxidant i.e. proline (Pro) content and its enzyme pyrroline-5-carboxylate synthetase; P5CS activity were increased in root and leaves under As stress. While, exogenous application of S, Ca and S + Ca, further enhanced the activities of above mentioned enzymes and Pro content thereby causing considerable reduction in O₂˙-, H₂O₂, MDA equivalents content and electrolyte leakage. This study suggests that exogenous application of S and/or Ca efficiently (particularly S + Ca) lowered the negative impact of As on biomass accumulation in Brassica seedlings by improving the uptake of essential mineral nutrients', content of photosynthetic pigments, activities of enzymatic and content of non-enzymatic antioxidants.

Arsenic-silicon priming of rice (Oryza sativa L.) seeds influence mineral nutrient uptake and biochemical responses through modulation of Lsi-1, Lsi-2, Lsi-6 and nutrient transporter genes

Silicon (Si) has attracted substantial attention because of its beneficial effect on plants during abiotic stress, including stress due to arsenic (As). We here report that priming rice seeds with As and Si together, helped the plant to sustain As stress for longer period. We examined Si induced tolerance against As in rice seedlings at short (7 d) and long (15 d) exposure periods under As^(III) and Si treatments since their germinating stage. Results showed that the expression of As^(III) transporter genes OsLsi1, OsLsi2 and OsLsi6 was more in As^(III) + Si treatment as compared to control and Si treatment, but lower than As^(III) alone treatments. The gene expression was maximum in shoot and root at 15 d over 7 d under both As^(III) and As^(III) + Si treatment, which ultimately leads to decreased accumulation of As in the presence of Si. Morphological characters, antioxidant capacity, oxidative stress marker (MDA), stress modulators (cysteine, proline), and enzymes related with ascorbate-glutathione cycle significantly altered during As^(III) + Si treatment at both exposure periods. Further, macro and micronutrient contents also improved with Si, and differentially regulated 12 key genes (NR, NiR, AMT, NR, GS, GOGAT, PT, PHT1, PHT2, APase, KAT1 and HAK10) related with NPK transport and utilization. Results highlight that Si priming of seeds along with As^(III) influences growth positively of As-stressed rice.

Sulfur and Calcium Simultaneously Regulate Photosynthetic Performance and Nitrogen Metabolism Status in As-Challenged Brassica juncea L. Seedlings

In the present study, the role of sulfur (K2SO4: S; 60 mg S kg-1 sand) and/or calcium (CaCl2: Ca; 250 mg Ca kg-1 sand) applied alone as well as in combination on growth, photosynthetic performance, indices of chlorophyll a fluorescence, nitrogen metabolism, and protein and carbohydrate contents of Indian mustard (Brassica juncea L.) seedlings in the absence and presence of arsenic (Na2HAsO4.7H2O: As1; 15 mg As kg-1 sand and As2; 30 mg As kg-1 sand) stress was analyzed. Arsenic with its rising concentration negatively affected the fresh weight, root/shoot ratio, leaf area, photosynthetic pigments content, photosynthetic oxygen yield, and chlorophyll a fluorescence parameters: the O-J, J-I and I-P rise, QA- kinetic parameters, i.e., ΦP0, Ψ0, ΦE0, and PIABS, along with Fv/F0 and Area while increased the energy flux parameters, i.e., ABS/RC, TR0/RC, ET0/RC, and DI0/RC along with F0/Fv and Sm due to higher As/S and As/Ca ratio in test seedlings; however, exogenous application of S and Ca and their combined effect notably counteracted on As induced toxicity on growth and other important growth regulating processes. Moreover, inorganic nitrogen contents, i.e., nitrate (NO3-) and nitrite (NO2-) and the activities of nitrate assimilating enzymes, viz., nitrate reductase (NR) and nitrite reductase (NiR) and ammonia assimilating enzymes, viz., glutamine synthetase (GS) and glutamate synthase (GOGAT) along with protein and carbohydrate contents were severely affected with As toxicity; while under similar condition, ammonium (NH4+) content and glutamate dehydrogenase (GDH) activity in both root and leaves showed reverse trend. Furthermore, S and Ca supplementation alone and also in combination to As stressed seedlings ameliorated these parameters except NH4+ content and GDH activity, which showed an obvious reduction under similar conditions. These findings point out that exogenous application of S and/or Ca particularly S+Ca more favorably regulated the photosynthesis, contents of protein, carbohydrate and inorganic nitrogen, and the activities of nitrate and ammonia assimilating enzymes, which might be linked with the mitigation of As stress. Our results suggest that exogenous application of S+Ca more efficiently defends Brassica seedlings by declining As accumulation in root and shoot tissues and by maintaining the photosynthesis and nitrogen metabolism as well.

Growth-inhibition patterns and transfer-factor profiles in arsenic-stressed rice (Oryza sativa L.)

Arsenic (As) accumulation in rice owing to uptake from the soil is a critical human health issue. Here, we studied the chemical properties of As-treated soils, growth inhibition patterns of As-stressed rice plants, changes in the As content of soil and soil solutions, and the relationship between As accumulation and As transfer factor from the soil to the rice organs. Rice plants were cultivated in a greenhouse under four concentrations of As: 0 (control), 25, 50, and 75 mg kg^-1. A significant positive correlation was found between available P₂O₅ and exchangeable K and between As concentration and available P₂O₅ or exchangeable K. The As concentration for 50% shoot growth inhibition was 50 mg kg^-1. As levels in roots and shoots were positively correlated with the growth stages of rice. The transfer factor (TF)_root/soil increased with As concentration at the tillering stage but decreased at the heading stage. TF_root/soil and TF_shoot/soil were higher at the heading stage than at the tillering stage. As accumulation in the 25 mg kg^-1 treatment was higher during the heading stage, whereas no difference was found at the tillering stage. As accumulation was related to plant biomass and soil As concentration. We found that As accumulation was greater at As concentrations that allowed for plant growth and development. Thus, species-specific threshold concentrations must be determined based on As phytotoxicity for the phytoremediation of As-contaminated soils. Hence, developing practical approaches for managing safe crop production in farmlands with an As contamination of 25 mg kg^-1 or less is necessary.

A protective role for nitric oxide and salicylic acid for arsenite phytotoxicity in rice (Oryza sativa L.)

Nitric oxide (NO) and salicylic acid (SA) are important signaling molecules in plant system. In the present study both NO and SA showed a protective role against arsenite (As^III) stress in rice plants when supplied exogenously. The application of NO and SA alleviated the negative impact of As^III on plant growth. Nitric oxide supplementation to As^III treated plants greatly decreased arsenic (As) accumulation in the roots as well as shoots/roots translocation factor. Arsenite exposure in plants decreased the endogenous levels of NO and SA. Exogenous supplementation of SA not only enhanced endogenous level of SA but also the level of NO through enhanced nitrate reductase (NR) activity, whether As^III was present or not. Exogenously supplied NO decreased the NR activity and level of endogenous NO. Arsenic accumulation was positively correlated with the expression level of OsLsi1, a transporter responsible for As^III uptake. The endogenous level of NO and SA were positively correlated to each other either when As^III was present or not. This close relationship indicates that NO and SA work in harmony to modulate the signaling response in As^III stressed plants.

Arsenic affects the production of glucosinolate, thiol and phytochemical compounds: A comparison of two Brassica cultivars

Arsenic (As), a non-essential metalloid, severely affects the normal functioning of plants, animals and humans. Plants play a crucial role in metabolic, physiological and numerous detoxification mechanisms to cope up with As induced stress. This study aimed to examine the differential response in two Brassica juncea cultivars, Varuna and Pusa Jagannath (PJn) exposed to different doses of As (50, 150, 300 μM) for 48 h duration. Change in morphological traits, concentration of individual as well as total GSL, sulfur related thiol proteins, sulfur content, and phytochemicals were analyzed in both cultivars. Accumulation pattern of As showed dose dependent accumulation in both the cultivars, being more in PJn. Our finding revealed that both cultivars were tolerant at low concentrations of As, while at higher concentration Varuna excelled over PJn. The increased tolerance of Varuna cultivar exposed to 150 and 300 μM concentration of As, correlated with its increased thiol related proteins, sulfur content and phytochemicals, which serves as defence strategy in the plant against oxidative stress. Differential pattern of total as well as individual GSLs content was observed in both Varuna and PJn cultivars. Varuna cultivar showed higher level of total and aliphatic GSLs, which serves as defence compound with other detoxification machineries to combat As stress. Our findings provide foundation for developing metalloid tolerant crops by analyzing the role of different genes involved in GSL mechanism and signaling pathways in different organs of plant.

Impact of silicon on Indian mustard (Brassica juncea L.) root traits by regulating growth parameters, cellular antioxidants and stress modulators under arsenic stress

Arsenic (As) is an emerging pollutant causing inhibition in growth and development of plants resulting into phytotoxicity. On the other hand, silicon (Si) has been suggested as a modulator in abiotic and biotic stresses that, enhances plant's physiological adaptations in response to several stresses including heavy metal stress. In this study, we used roots of hydroponically grown 14 day old seedlings of Brassica juncea var. Varuna treated with 150 μM As, 1.5 mM Si and both in combination for 96 h duration. Application of Si modulated the effect of As by improving morphological traits of root along with the development of both primary and lateral roots. Changes observed in root traits showed positive correlation with As induced cell death, accumulation of reactive oxygen species (ROS), nitric oxide (NO) and intracellular superoxide radicals (O2(-)). Addition of 1.5 mM Si during As stress increased accumulation of As in roots. Mineral nutrient analysis was done using energy-dispersive X-ray fluorescence (EDXRF) technique and positively correlated with increased cysteine, proline, MDA, H2O2 and activity of antioxidant enzymes (SOD, CAT and APX). The results obtained from the above biochemical approaches support the protective and active role of Si in the regulation of As stress through the changes in root developmental process.

Nitric Oxide Alleviated Arsenic Toxicity by Modulation of Antioxidants and Thiol Metabolism in Rice (Oryza sativa L.)

Nitric oxide (NO) is a gaseous signaling molecule and has a profound impact on plant growth and development. It is reported to serve as pro oxidant as well as antioxidant in plant system. In the present study, we evaluated the protective role of NO against arsenate (As(V)) toxicity in rice plants. As(V) exposure has hampered the plant growth, reduced the chlorophyll content, and enhanced the oxidative stress, while the exogenous NO supplementation has reverted these symptoms. NO supplementation has reduced the arsenic (As) accumulation in root as well as shoot. NO supplementation to As(V) exposed plants has reduced the gene expression level of OsLsi1 and OsLsi2. As(V) stress significantly impacted thiol metabolism, it reduced GSH content and GSH/GSSG ratio, and enhanced the level of PCs. NO supplementation maintained the GSH/GSSG ratio and reduced the level of PCs. NO supplementation reverted As(V) induced iron deficiency in shoot and had significant impact of gene expression level of various iron transporters (OsYSL2, OsFRDL1, OsIRT1, and OsIRO2). Conclusively, exogenous application of NO could be advantageous against As(V) toxicity and could confer the tolerance to As(V) stress in rice.

Exogenous proline application ameliorates toxic effects of arsenate in Solanum melongena L. seedlings

Hydroponic experiments were conducted to investigate an effect of exogenous application of proline (Pro; 25 µM) in alleviating arsenate (As(V); 5 and 25 µM) toxicity in Solanum melongena L. (eggplant) seedlings. Exposure of As(V) declined growth of eggplant, which was coincided with an enhanced accumulation of As. However, exogenous Pro application alleviated As(V) toxicity in eggplant seedlings by reducing the accumulation of As. The fluorescence characteristics (JIP-test): φP0, Ψ0, φE0, PIABS, ABS/RC, TR0/RC, ET0/RC, DI0/RC, NPQ and qP were also affected by As(V). However, the effects of As(V) were more prominent on PIABS DI0/RC and NPQ. In Pro treated seedlings, following parameters viz. φP0, Ψ0, φE0 and PIABS were stimulated, while, energy flux parameters (ABS/RC, TR0/RC, ET0/RC and DI0/RC) were inhibited. Toxic effects of As(V) on photochemistry of photosystem II (PS II) were ameliorated by an exogenous application of Pro. Oxidative stress markers: superoxide radical, hydrogen peroxide and malondialdehyde (lipid peroxidation) were enhanced by As(V) exposure, however, their levels were significantly diminished by an exogenous application of Pro. Treatment of As(V) stimulated the activities of superoxide dismutase, peroxidase and catalase except that of glutathione-S-transferase. Exogenous Pro application improved the activities of enzymatic antioxidants. The level of endogenous Pro was higher in As(V) treated as well as in Pro fed seedlings. The activity of a key enzyme of Pro biosynthesis: Δ(1)-pyrroline-5-carboxylate synthetase was higher in Pro fed seedlings. The activity of Pro dehydrogenase was inhibited under As(V) stress, and its activity was minimum in case of Pro+As(V) combination. These results indicate that Pro metabolism could play a key role in regulating the accumulation of As and levels of antioxidants, which concomitantly result into a better growth of eggplant seedlings when compared to the As(V) treatments alone.

Omics and biotechnology of arsenic stress and detoxification in plants: current updates and prospective

Arsenic (As), a naturally occurring metallic element, is a dreadful health hazard to millions of people across the globe. Arsenic is present in low amount in the environment and originates from anthropogenic impact and geogenic sources. The presence of As in groundwater used for irrigation is a worldwide problem as it affects crop productivity, accumulates to different tissues and contaminates food chain. The consumption of As contaminated water or food products leads to several diseases and even death. Recently, studies have been carried out to explore the biochemical and molecular mechanisms which contribute to As toxicity, accumulation, detoxification and tolerance acquisition in plants. This information has led to the development of the biotechnological tools for developing plants with modulated As tolerance and detoxification to safeguard cellular and genetic integrity as well as to minimize food chain contamination. This review aims to provide current updates about the biochemical and molecular networks involved in As uptake by plants and the recent developments in the area of functional genomics in terms of developing As tolerant and low As accumulating plants.