Arsenic uptake, translocation and speciation in pho1 and pho2 mutants of Arabidopsis thaliana

Arsenate [As (V)] is taken up by phosphate [P (V)] transporters in the plasma membrane of roots cells, but the translocation of As from roots to shoots is not well understood. Two mutants of Arabidopsis thaliana (L.) [( pho1 , P deficient) and ( pho2 , P accumulator)], with defects in the regulation and translocation of P (V) from roots to shoots, were therefore used in this study to investigate uptake, translocation and speciation of As in roots and shoots of plants grown in soil or nutrient solution. The shoots of the pho2 mutant contained higher P concentrations, but similar or slightly higher As concentrations, in comparison with the wild type. In the pho1 mutant, the P concentration in the shoots was lower, and the As concentration was higher, in comparison with the wild type. Both pho2 and the wild type contained mainly As (III) in roots and shoot (67–90% of total As). Arsenic was likely to be translocated by a different pathway to P (V) in the pho2 and pho1 mutants . Therefore, it is suggested that As (III) is the main As species translocated from roots to shoots in Arabidopsis thaliana.     

Enhanced arsenate reduction by a CDC25-like tyrosine phosphatase explains increased phytochelatin accumulation in arsenate-tolerant Holcus lanatus

Decreased arsenate [As(V)] uptake is the major mechanism of naturally selected As(V) hypertolerance in plants. However, As(V)-hypertolerant ecotypes also show enhanced rates of phytochelatin (PC) accumulation, suggesting that improved sequestration might additionally contribute to the hypertolerance phenotype. Here, we show that enhanced PC-based sequestration in As(V)-hypertolerant Holcus lanatus is not due to an enhanced capacity for PC synthesis as such, but to increased As(V) reductase activity. Vacuolar transport of arsenite-thiol complexes was equal in both ecotypes. Based on homology with the yeast As(V) reductase, Acr2p, we identified a Cdc25-like plant candidate, HlAsr, and confirmed the As(V) reductase activity of both HlAsr and the homologous protein from Arabidopsis thaliana. The gene appeared to be As(V)-inducible and its expression was enhanced in the As(V)-hypertolerant H. lanatus ecotype, compared with the non-tolerant ecotype. Homologous ectopic overexpression of the AtASR cDNA in A. thaliana produced a dual phenotype. It improved tolerance to mildly toxic levels of As(V) exposure, but caused hypersensitivity to more toxic levels. Arabidopsis asr T-DNA mutants showed increased As(V) sensitivity at low exposure levels and enhanced arsenic retention in the root. It is argued that, next to decreased uptake, enhanced expression of HlASR might act as an additional determinant of As(V) hypertolerance and As transport in H. lanatus.     

Relationship between plant phosphorus status and the kinetics of arsenate influx in clones ofdeschampsia cespitosa (L.) beauv. that differ in their tolerance to arsenate

Uptake kinetics of arsenate were determined in arsenate tolerant and non-tolerant clones of the grassDeschampsia cespitosa under differing root phosphorus status to investigate the mechanism controlling the suppression of arsenate influx observed in tolerant clones. Influx was always lower in tolerants compared to non-tolerants. Short term influx of arsenate by the high affinity uptake system in both tolerant clones was relatively insensitive to root phosphorus status. This was in contrast to the literature where the regulation of the phosphate (arsenate) uptake system is normally much more responsive to plant phosphorus status. The low affinity uptake system in both tolerant and non-tolerant clones, unlike the high affinity uptake system, was more closely regulated by root phosphate status and was repressed to a much greater degree under increasing root phosphorus levels than the high affinity system.     

Arsenic tolerance in mesquite (Prosopis sp.): Low molecular weight thiols synthesis and glutathione activity in response to arsenic

The effects of arsenic stress on the production of low molecular weight thiols (LMWT), glutathione S-transferase activity (GST) and sulfur metabolism of mesquite plant (Prosopis sp.) were examined in hydroponic culture at different arsenic [As(III) and (V)] concentrations. The production of LMWT was dependent on As speciation and concentration in the growth medium. The roots of As(III) treated plants produced significantly higher LMWT levels than As(V) treated roots at the same concentration of As applied. In leaves, the thiols content increased with increasing As(III) and (V) concentrations in the medium. Hypersensitivity of the plant to high As concentrations was observed by a significant decrease of LMWT produced in the roots at 50 mg/L treatment in both As(III) and (V) treatments. Sulfur was translocated from roots and accumulated mainly in the shoots. In response to As-induced phytotoxicity, the plants slightly increased the sulfur content in the roots at the highest As treatment. Compared with As(V)-treated plants, As(III)-treated roots and leaves showed significantly higher GST activity. The roots of both As(III) and (V) treated plants showed an initial increase in GST at low As concentration (5 mg/L), followed by significant inhibition up to 50 mg/L. The leaves had the highest GST activity, an indication of the ability of the plant to detoxify As in the leaves than in the roots. The correlation between LMWT content, S content and GST activity may be an indication these parameters may be used as biomarkers of As stress in mesquite.     

Knocking Out ACR2 Does Not Affect Arsenic Redox Status in Arabidopsis thaliana: Implications for As Detoxification and Accumulation in Plants

Many plant species are able to reduce arsenate to arsenite efficiently, which is an important step allowing detoxification of As through either efflux of arsenite or complexation with thiol compounds. It has been suggested that this reduction is catalyzed by ACR2, a plant homologue of the yeast arsenate reductase ScACR2. Silencing of AtACR2 was reported to result in As hyperaccumulation in the shoots of Arabidopsis thaliana. However, no information of the in vivo As speciation has been reported. Here, we investigated the effect of AtACR2 knockout or overexpression on As speciation, arsenite efflux from roots and As accumulation in shoots. T-DNA insertion lines, overexpression lines and wild-type (WT) plants were exposed to different concentrations of arsenate for different periods, and As speciation in plants and arsenite efflux were determined using HPLC-ICP-MS. There were no significant differences in As speciation between different lines, with arsenite accounting for >90% of the total extractable As in both roots and shoots. Arsenite efflux to the external medium represented on average 77% of the arsenate taken up during 6 h exposure, but there were no significant differences between WT and mutants or overexpression lines. Accumulation of As in the shoots was also unaffected by AtACR2 knockout or overexpression. Additionally, after exposure to arsenate, the yeast (Saccharomyces cerevisiae) strain with ScACR2 deleted showed similar As speciation as the WT with arsenite-thiol complexes being the predominant species. Our results suggest the existence of multiple pathways of arsenate reduction in plants and yeast.     

Toxicity of arsenic (III) and (V) on plant growth, element uptake, and total amylolytic activity of mesquite (Prosopis juliflora x P. velutina)

The effects of arsenite [As(III)] and arsenate [As(V)] on the growth of roots, stems, and leaves and the uptake of arsenic (As), micro- and macronutrients, and total amylolytic activity were investigated to elucidate the phytotoxicity of As to the mesquite plant (Prosopis juliflora x P. velutina). The plant growth was evaluated by measuring the root and shoot length, and the element uptake was determined using inductively coupled plasma optical emission spectroscopy. The root and leaf elongation decreased significantly with increasing As(III) and As(V) concentrations; whereas, stem elongation remained unchanged. The As uptake increased with increasing As(III) or As(V) concentrations in the medium. Plants treated with 50 mg/L As(III) accumulated up to 920 mg/kg dry weight (d wt) in roots and 522 mg/kg d wt in leaves, while plants exposed to 50 mg/L As(V) accumulated 1980 and 210 mg/kg d wt in roots and leaves, respectively. Increasing the As(V) concentration up to 20 mg/L resulted in a decrease in the total amylolytic activity. On the contrary, total amylolytic activity in As(III)-treated plants increased with increasing As concentration up to 20 mg/L. The macro- and micronutrient concentrations changed in As-treated plants. In shoots, Mo and K were reduced but Ca was increased, while in roots Fe and Ca were increased but K was reduced. These changes reduced the size of the plants, mainly in the As(III)-treated plants; however, there were no visible sign of As toxicity.     

Use of X-ray absorption spectroscopy and biochemical techniques to characterize arsenic uptake and reduction in pea (Pisum sativum) plants.

Arsenite (As(III)) and arsenate (As(V)) uptake by peas was investigated using inductively coupled plasma/optical emission spectroscopy (ICP-OES) at pH below 4 and at pH 5.8. Additionally, total amylolitic activity and alpha-amylase (1,4-alpha-d-glucan glucanohydrolase; EC 3.2.1.1) activity was assayed in plants exposed to arsenic treatments. At pH below 4, the uptake for As(III) and As(V) in roots was 137 and 124 mg As kg(-1) dry weight (d wt), respectively. Translocation of arsenic to the aerial part was relatively low ( approximately 5mg As kg(-1) d wt). The uptake for As(III) and As(V) in roots at pH 5.8 was about 43 and 30 mg As kg(-1) d wt, respectively, and translocation of As to the aerial part was not detectable. None of the arsenic treatments affected the total amylolitic activity in roots; however, the shoots from all treatments showed an increase in the total amylolitic activity. Alpha-amylase activity in the pea leaves was not significantly affected by arsenic treatments. X-ray absorption spectroscopy (XAS) studies showed a reduction of As(V) to As(III) in the roots. From linear combination X-ray absorption near edge structure (LC-XANES) fittings, it was determined that arsenic was present as a mixture of As(III) oxide and sulfide in pea roots.     

Application of X-ray absorption near edge spectroscopy to the study of the effect of sulphur on selenium uptake and assimilation in wheat seedlings

Selenium (Se) is an essential trace element for humans and animals. A hydroponic experiment was performed to study the effects of sulphur (S) on Se uptake, translocation, and assimilation in wheat (Triticum aestivum L.) seedlings. Sulphur starvation had a positive effect on selenate uptake and the form of Se supplied greatly influenced Se speciation in plants. Compared with the control plants, Se uptake by the S-starved plants was enhanced by 4.81-fold in the selenate treatment, and selenate was readily transported from roots to shoots. By contrast, S starvation had no significant effect on selenite uptake, and selenite taken up by roots was rapidly converted to organic forms and tended to accumulate in roots. X-ray absorption near edge spectroscopy (XANES) analysis showed that organic forms of selenium, including selenocystine, Se-methyl-selenocysteine (MeSeCys), and selenomethionine-Se-oxide, were dominant in the plants exposed to selenite and accounted for approximately 90 % of the total Se. Whereas selenate remained as the dominant species in the roots and shoots exposed to selenate, with little selenate converted to selenite and MeSeCys. Besides, sulphur starvation increased the proportion of inorganic Se species in the selenate-supplied plants, but had no significant effects on Se speciation in plants exposed to selenite. The present study provides important knowledge to understand the associated mechanism of Se uptake and metabolism in plants.     

Role of Copper Binding, Absorption, and Translocation in Copper Tolerance of Agrostis gigantea Roth

The capacity of roots to accumulate and retain copper was examinedin two clones of Agrostis gigantea which differ in their toleranceto excess copper. Root elongation growth in the non-tolerantclone was completely inhibited by 16 mmol m^–3 Cu whereas40 mmol m^–3 was required for inhibition in the tolerantclone. The amount of readily exchangeable copper was greaterin roots of the tolerant clone than in the non-tolerant clone.The higher capacity for binding copper did not prevent the entryof copper into the cells of intact or excised roots of the tolerantclone. Roots of both clones contained similar amounts of copperafter removal of the readily exchangeable fraction. More copperwas translocated to the shoots of the tolerant than the non-tolerantclone. The explanation of copper tolerance in Agrostis giganteamust be sought in areas other than those of differences in grosscopper absorption and retention by roots.     

Phytochelatins are involved in differential arsenate tolerance in Holcus lanatus

Arsenate tolerance is conferred by suppression of the high-affinity phosphate/arsenate uptake system, which greatly reduces arsenate influx in a number of higher plant species. Despite this suppressed uptake, arsenate-tolerant plants can still accumulate high levels of As over their lifetime, suggesting that constitutive detoxification mechanisms may be required. Phytochelatins are thiol-rich peptides, whose production is induced by a range of metals and metalloids including arsenate. This study provides evidence for the role of phytochelatins in the detoxification of arsenate in arsenate-tolerant Holcus lanatus. Elevated levels of phytochelatin were measured in plants with a range of tolerance to arsenate at equivalent levels of arsenate stress, measured as inhibition of root growth. The results suggest that arsenate tolerance in H. lanatus requires both adaptive suppression of the high-affinity phosphate uptake system and constitutive phytochelatin production.     

Comparative analysis of the contribution of phytochelatins to cadmium and arsenic tolerance in soybean and white lupin

The biosynthesis of phytochelatins (PCs) plays a crucial role in the detoxification and homeostasis of heavy metals and metalloids in plants. However, in an increasing number of plant species metal(loid) tolerance is not well correlated with the accumulation of PCs: tolerant ecotypes frequently contain lower levels of PCs than non-tolerant ecotypes. In this study we have compared the responses of soybean (Glycine max L. cv. Resnik) and white lupin (Lupinus albus L. cv. Marta) to cadmium and arsenate in order to assess the role of homophytochelatins (hPCs) in the tolerance of soybean to these toxic elements. Soybean plants treated with Cd and As showed a high contribution of homo-glutathione (hGSH) to the pool of thiols in shoots in comparison to white lupin. Higher levels of hPCs in Cd-treated soybeans compared to PCs in lupins did not prevent growth inhibition. In contrast, the role of hPCs in the detoxification mechanism to arsenate in soybean seems to be clearer, showing higher thiol concentrations and lower growth reductions than those present in lupin plants.     

Expressing ScACR3 in rice enhanced arsenite efflux and reduced arsenic accumulation in rice grains

Arsenic (As) accumulation in rice grain poses a serious health risk to populations with high rice consumption. Extrusion of arsenite [As(III)] by ScAcr3p is the major arsenic detoxification mechanism in Saccharomyces cerevisiae. However, ScAcr3p homolog is absent in higher plants, including rice. In this study, ScACR3 was introduced into rice and expressed under the control of the Cauliflower mosaic virus (CaMV) 35S promoter. In the transgenic lines, As concentrations in shoots and roots were about 30% lower than in the wild type, while the As translocation factors were similar between transgenic lines and the wild type. The roots of transgenic plants exhibited significantly higher As efflux activities than those of the wild type. Within 24 h exposure to 10 μM arsenate [As(V)], roots of ScACR3-expressing plants extruded 80% of absorbed As(V) to the external solution as As(III), while roots of the wild type extruded 50% of absorbed As(V). Additionally, by exposing the As-containing rice plants to an As-lacking solution for 24 h, about 30% of the total As derived from pre-treatment was extruded to the external solution by ScACR3-expressing plants, while about 15% of As was extruded by wild-type plants. Importantly, ScACR3 expression significantly reduced As accumulation in rice straws and grains. When grown in flooded soil irrigated with As(III)-containing water, the As concentration in husk and brown rice of the transgenic lines was reduced by 30 and 20%, respectively, compared with the wild type. This study reports a potential strategy to reduce As accumulation in the food chain by expressing heterologous genes in crops.     

Arsenic speciation in turnip as affected by application of chicken manure bearing roxarsone and its metabolites

Roxarsone (ROX) is widely used as a feed additive in intensive animal production. While animals are fed with ROX, the most commonly detectable As forms in fresh manures include ROX and small quantities of its metabolites such as arsenate (As(V)), arsenite (As(III)), monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA). A pot experiment was conducted to investigate the uptake, translocation and distribution of ROX, As(V), As(III), MMA and DMA in turnips, with the soil amended with 2% and 3% (w/w) chicken manure (CM) bearing ROX and its metabolites. Soil without any fertilizer was the control. The results show that only As(V) and As(III) were detected in turnip control samples. As(V), As(III) and DMA were found in all CM applied samples, but not ROX or MMA. This implies that turnip cannot take up ROX directly and accumulate MMA at detectable levels. The contents of DMA in tubers and the three As species in shoots increased with the CM rate in contrast to reduced levels of As(V) and As(III) in tubers. Increased CM rate enhanced the translocation of the three As species, especially for DMA, from tubers to shoots. DMA was the major form (42.9–61.4% in tubers and 38.1–76.3% in shoots), followed by As(III), in turnip plants fertilized with CM. The results indicate that ROX and its metabolites in animal manures can be introduced into human food chain by the way ROX → animal → manure → soil → crop.     

Differential antioxidant enzyme and thiol responses of tolerant and non-tolerant clones of <Emphasis Type="Italic">Chloris barbata</Emphasis> to cadmium-stress

Tolerant and non-tolerant clones of Chloris barbata Sw. obtained, respectively, from an erstwhile mercury contaminated solid waste dump site near a chloralkali plant and a non-contaminated (control) site were subjected to cadmium-stress by growing the rooted cuttings in water containing CdSO₄, 13 and 130 μM. Differences between the two clones in their response to cadmium-stress were noted in root growth, and also with respect to certain biochemical parameters. Whereas catalase activity decreased and non protein-thiol levels increased in the non-tolerant clone, the level of protein-thiol alone increased significantly in the tolerant clone in response to cadmium-stress. No remarkable differences between the clones, however, were noted with respect to total soluble protein, peroxidase activity and lipid peroxidation. Remarkably the two clones responded differently to buthionine sulfoximine, an inhibitor of glutathione and/or phytochelatin synthesis, which inhibited root growth significantly in non-tolerant clone but not in the tolerant clone. Buthionine sulfoximine, nonetheless, could potentate cadmium toxicity in either of the clones, but more effectively in the tolerant clone. The high sensitivity of tolerant-clone to the combined treatment of BSO and Cd in the present study could, therefore, be attributed to the cumulative oxidative stress generated synergistically by BSO and Cd.     

Uptake, translocation and transformation of arsenate and arsenite in sunflower (Helianthus annuus): formation of arsenic-phytochelatin complexes during exposure to high arsenic concentrations

The aim of the study was to determine the time-dependent formation of arsenic-phytochelatin (As-PC) complexes in the roots, stems and leaves of an arsenic-nontolerant plant (Helianthus annuus) during exposure to 66 mol l(-1) arsenite (As(III)) or arsenate (As(V)). We used our previously developed method of simultaneous element-specific (inductively coupled plasma mass spectrometry, ICP-MS) and molecular-specific (electrospray-ionization mass spectrometry, ES-MS) detection systems interfaced with a suitable chromatographic column and eluent conditions, which enabled us to identify and quantify As-PC complexes directly. Roots of As-exposed H. annuus contained up to 14 different arsenic species, including the complex of arsenite with two (gamma-Glu-Cys)(2)-Gly molecules [As((III))-(PC(2))(2)], the newly identified monomethylarsonic phytochelatin-2 or (gamma-Glu-Cys)(2)-Gly CH(3)As (MA((III))-PC(2)) and at least eight not yet identified species. The complex of arsenite with (gamma-Glu-Cys)(3)-Gly (As((III))-PC(3)) and the complex of arsenite with glutathione (GSH) and (gamma-Glu-Cys)(2)-Gly (GS-As((III))-PC(2)) were present in all samples (roots, stems and leaves) taken from plants exposed to As. The GS-As((III))-PC(2) complex was the dominant complex after 1 h of exposure. As((III))-PC(3) became the predominant As-PC complex after 3 h, binding up to 40% of the As present in the exposed plants. No As-PC complexes were found in sap (mainly xylem sap from the root system), in contrast to roots, stems and leaves, which is unequivocal evidence that As-PC complexes are not involved in the translocation of As from root to leaves of H. annuus.     

Polymorphism and physiology of arsenate tolerance in Holcus lanatus L. from an uncontaminated site

The polymorphism of arsenate tolerance in a Holcus lanatus L. population from an uncontaminated soil was investigated and a high percentage of tolerant individuals (65%) was found in the population studied. Influx of arsenate was highly correlated to arsenate tolerance within the population, with the most tolerant individuals having the lowest rates of arsenate influx. Isotherms for the high affinity arsenate uptake systems were determined in six tolerant and six non-tolerant genotypes. Tolerant plants had the lowest rates of arsenate influx. This was achieved by adaptation of the V_max of arsenate influx with the V_max of the high affinity uptake system saturating at lower substrate concentrations in the tolerant plants. The polymorphism is discussed with relation to adaptation to the extreme environments to which the plants are subjected on mine-spoil soils.     

Methylated arsenic species in plants originate from soil microorganisms

? Inorganic arsenic (iAs) is a ubiquitous human carcinogen, and rice (Oryza sativa) is the main contributor to iAs in the diet. Methylated pentavalent As species are less toxic and are routinely found in plants; however, it is currently unknown whether plants are able to methylate As. ? Rice, tomato (Solanum lycopersicum) and red clover (Trifolium pratense) were exposed to iAs, monomethylarsonic acid (MMA(V)), or dimethylarsinic acid (DMA(V)), under axenic conditions. Rice seedlings were also grown in two soils under nonsterile flooded conditions, and rice plants exposed to arsenite or DMA(V) were grown to maturity in nonsterile hydroponic culture. Arsenic speciation in samples was determined by HPLC-ICP-MS. ? Methylated arsenicals were not found in the three plant species exposed to iAs under axenic conditions. Axenically grown rice was able to take up MMA(V) or DMA(V), and reduce MMA(V) to MMA(III) but not convert it to DMA(V). Methylated As was detected in the shoots of soil-grown rice, and in rice grain from nonsterile hydroponic culture. GeoChip analysis of microbial genes in a Bangladeshi paddy soil showed the presence of the microbial As methyltransferase gene arsM. ? Our results suggest that plants are unable to methylate iAs, and instead take up methylated As produced by microorganisms.     

Arsenic biotransformation and volatilization in transgenic rice

? Biotransformation of arsenic includes oxidation, reduction, methylation, and conversion to more complex organic arsenicals. Members of the class of arsenite (As(III)) S-adenosylmethyltransferase enzymes catalyze As(III) methylation to a variety of mono-, di-, and trimethylated species, some of which are less toxic than As(III) itself. However, no methyltransferase gene has been identified in plants. ? Here, an arsM gene from the soil bacterium Rhodopseudomonas palustris was expressed in Japonica rice (Oryza sativa) cv Nipponbare, and the transgenic rice produced methylated arsenic species, which were measured by inductively coupled plasma mass spectrometry (ICP-MS) and high-performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICP-MS). ? Both monomethylarsenate (MAs(V)) and dimethylarsenate (DMAs(V)) were detected in the roots and shoots of transgenic rice. After 12 d exposure to As(III), the transgenic rice gave off 10-fold greater volatile arsenicals. ? The present study demonstrates that expression of an arsM gene in rice induces arsenic methylation and volatilization, theoretically providing a potential stratagem for phytoremediation.     

In vivo speciation of zinc in Noccaea caerulescens in response to nitrogen form and zinc exposure

Nitrate has been shown to enhance Zn hyperaccumulation in the shoots of Noccaea caerulescens (formerly Thlaspi caerulescens) (Prayon); however, the mechanisms beyond the effect of nitrogen form are unknown. This study used synchrotron X-ray absorption near-edge spectroscopy (XANES) on alive and intact plants at room temperature to examine whether enhanced Zn hyperaccumulation in nitrate-fed plants was associated with differences in Zn speciation, and to correlate Zn species with mechanisms of Zn uptake, translocation and hyperaccumulation. The higher Zn concentration in plants supplied with nitrate compared to ammonium, or with high Zn exposure (300 ???), was not due to differences in Zn speciation. The importance of carboxylates for Zn hyperaccumulation in the shoots was supported by a predominance of Zn-malate or Zn-citrate. Zinc-phytate was detected for the first time in this species and may assist Zn-tolerance in the roots. The feasible presence of Zn-histidine in the roots but not in the xylem sap suggests a mechanism for Zn binding and non-toxic transport through the cytoplasm and release of aqueous Zn into the xylem vessels. Zinc was translocated in the xylem as Zn-malate and weakly complexed or aqueous Zn forms. Zinc speciation in roots, shoots and xylem did not differ between nitrate- and ammonium-fed plants.     

H2O2 pretreated rice seedlings specifically reduces arsenate not arsenite: difference in nutrient uptake and antioxidant defense response in a contrasting pair of rice cultivars

The study investigated the reduction in metalloid uptake at equimolar concentrations (~53.3 μM) of As(III) and As(V) in contrasting pair of rice seedlings by pretreating with H₂O₂ (1.0 μM) and SA (1.0 mM). Results obtained from the contrasting pair (arsenic tolerant vs. sensitive) of rice seedlings (cv. Pant Dhan 11 and MTU 7029, respectively) shows that pretreatment of H₂O₂ and H₂O₂ + SA reduces As(V) uptake significantly in both the cultivars, while no reduction in the As(III) uptake. The higher growth inhibition, higher H₂O₂ and TBARS content in sensitive cultivar against As(III) and As(V) treatments along with higher As accumulation (~1.2 mg g⁻¹ dw) than in cv. P11, unravels the fundamental difference in the response between the sensitive and tolerant cultivar. In the H₂O₂ pretreated plants, the translocation of As increased in tolerant cultivar against AsIII, whereas, it decreased in sensitive cultivar both against AsIII and AsV. In both the cultivars translocation of Mn increased in the H₂O₂ pretreated plants against As(III), whereas, the translocation of Cu increased against As(V). In tolerant cultivar the translocation of Fe increased against As(V) with H₂O₂ pretreatment whereas, it decreased in the sensitive cultivar. In both the cultivars, Zn translocation increased against As(III) and decreased against As(V). The higher level of H₂O₂ and SOD (EC 1.15.1.1) activity in sensitive cultivar whereas, higher, APX (EC 1.11.1.11), GR (EC 1.6.4.2) and GST (EC 1.6.4.2) activity in tolerant cultivar, also demonstrated the differential anti-oxidative defence responses between the contrasting rice cultivars.

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