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Direct evidence showing the effect of root surface iron plaque on arsenite and arsenate uptake into rice ( Oryza sativa ) roots

2004; Wiley; Volume: 165; Issue: 1 Linguagem: Inglês

10.1111/j.1469-8137.2004.01241.x

1469-8137

Zheng Chen, Yong‐Guan Zhu, Wenju Liu, Andrew A. Meharg,

Mine drainage and remediation techniques

New PhytologistVolume 165, Issue 1 p. 91-97 Free Access Direct evidence showing the effect of root surface iron plaque on arsenite and arsenate uptake into rice (Oryza sativa) roots Zheng Chen, Zheng Chen Research Center for Eco-environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China;Search for more papers by this authorYong-Guan Zhu, Corresponding Author Yong-Guan Zhu Research Center for Eco-environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China;Author for correspondence: Yong-Guan Zhu Tel: +86 10 62936940 Fax: +86 10 62925563 Email: [email protected]Search for more papers by this authorWen-Ju Liu, Wen-Ju Liu Research Center for Eco-environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China; College of Natural Resources and Environment, Hebei Agricultural University, Baoding, Hebei Province, China;Search for more papers by this authorAndy A. Meharg, Andy A. Meharg School of Biological Sciences, University of Aberdeen, Cruickshank Building, St Machar Drive, Aberdeen AB25 2TE, UKSearch for more papers by this author Zheng Chen, Zheng Chen Research Center for Eco-environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China;Search for more papers by this authorYong-Guan Zhu, Corresponding Author Yong-Guan Zhu Research Center for Eco-environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China;Author for correspondence: Yong-Guan Zhu Tel: +86 10 62936940 Fax: +86 10 62925563 Email: [email protected]Search for more papers by this authorWen-Ju Liu, Wen-Ju Liu Research Center for Eco-environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China; College of Natural Resources and Environment, Hebei Agricultural University, Baoding, Hebei Province, China;Search for more papers by this authorAndy A. Meharg, Andy A. Meharg School of Biological Sciences, University of Aberdeen, Cruickshank Building, St Machar Drive, Aberdeen AB25 2TE, UKSearch for more papers by this author First published: 09 December 2004 https://doi.org/10.1111/j.1469-8137.2004.01241.xCitations: 245AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Summary • The present study aimed to investigate the effects of root surface iron plaque on the uptake kinetics of arsenite and arsenate by excised roots of rice (Oryza sativa) seedlings. • The results demonstrated that the presence of iron plaque enhanced arsenite and decreased arsenate uptake. • Arsenite and arsenate uptake kinetics were adequately fitted by the Michaelis–Menten function in the absence of plaque, but produced poor fits to this function in the presence of plaque. • Phosphate in the uptake solution did not have a significant effect on arsenite uptake irrespective of the presence of iron plaque; however phosphate had a significant effect on arsenate uptake. Without iron plaque, phosphate inhibited arsenate uptake. The presence of iron plaque diminished the effect of phosphate on arsenate uptake, possibly through a combined effect of arsenate desorption from iron plaque. Introduction Irrigation of agricultural land with arsenic-contaminated wastewater or groundwater, particularly in Bangladesh, India and South-East Asia, has caused accumulation of As in both soils and plants, and poses long-term risks to soil and human health (Marin et al., 1992; Meharg & Rahman, 2003). Elevated As concentrations in soils and plants also occur in areas affected by mining (Xie & Huang, 1998). The environmental fate and behaviour of As has thus received increasing attention in recent years. Paddy rice (Oryza sativa) is the staple food for South-East Asian countries (Abedin et al., 2002a). In some areas of Bangladesh and West Bengal, the worst-affected region, groundwater As concentrations reach 2 mg l−1 (Tondel et al., 1999; British Geological Survey, 2000), while the WHO provisional guideline value for drinking water is only 0.01 mg l−1. A recent study indicated that the concentration of As in rice straw could be up to 92 µg g−1 when rice plants were irrigated with As-contaminated groundwater (Abedin et al., 2002b). Rice grain As levels of 1.8 mg kg−1 have been recorded in the As-affected areas of Bangladesh (Meharg & Rahman, 2003). The chronic As poisoning by drinking tube-well water and the consumption of As-contaminated food is a disaster on a mammoth scale (Meharg & Rahman, 2003). Understanding how As is taken up by rice is of vital importance for the conventional breeding or genetic engineering of varieties with reduced As accumulation in edible parts. Arsenate is the dominant As species in aerobic soils, whereas arsenite dominates under anaerobic conditions such as paddy soil (Masscheleyn et al., 1991; Marin et al., 1993; Onken & Hossner, 1995, 1996; Smith et al., 1998). Arsenate is an analogue of phosphate, competing for the same uptake carriers in the root plasmalemma (Meharg & Macnair, 1992; Meharg & Hartley-Whitaker, 2002). Meharg and coworkers recently carried out a series of experiments to elucidate As uptake mechanisms by rice plants (Abedin et al., 2002b, 2002c; Meharg & Jardine, 2003), and were the first to show that arsenite is actively translocated across plant plasma membranes. However, for paddy rice, one of the most common aquatic crop plants, iron plaque is commonly formed on root surfaces and may subsequently affect As dynamics in the rhizosphere and As accumulation by rice plants (Liu et al., 2004a, 2004b). The formation of iron plaque on rice roots is thought to be facilitated by the release of oxygen and oxidants into the rhizosphere (Armstrong, 1964, 1967; Chen et al., 1980; Taylor & Crowder, 1983; Taylor et al., 1984). Iron oxides are generally considered to have great adsorption capacity for inorganic anions, especially for arsenate and phosphate (Meng et al., 2002), and a possible capacity to oxidize arsenite to arsenate (Otte et al., 1991). Due to the ubiquity of iron plaque on rice roots and the potential As sequestration in iron plaque, the direct role of iron plaque in As (both arsenite and arsenate) uptake into rice roots needs further investigation. In a recent study on phosphorus–arsenic interactions in the soil–rice system, Liu et al. (2004a) revealed that P nutrition of rice plants might to some extent control the formation of iron plaque, and iron plaque, in turn may influence arsenate uptake by rice plants. The aims of this study were therefore to investigate the effects of iron plaque on As (arsenate and arsenite) uptake by the plant, and the interactions between phosphate and arsenate/arsenite when sorbed to roots with or without iron plaque. Materials and Methods Plants growth Rice (Oryza sativa L.) seeds of variety YY-1, which was previously identified to form abundant iron plaque on its root, were germinated and grown on moist perlite for 6 wk in a growth chamber, then plants were transplanted into bags (30 µm nylon mesh, 4 cm diameter, 7 cm height, two plants per bag) filled with acid-washed quartz sand. The nylon bag was then transferred to a PVC pot (7.5 cm diameter, 14 cm height, two plants per pot) and the gap between nylon bag and PVC pot was filled with 300 g soil (collected from Huzhou, Zhejiang province, China). Phosphorus as CaHPO4·2H2O at 0.15 g P2O5 kg−1; potassium as KCl at 0.2 g K2O kg−1; and nitrogen as CO(NH2)2 at 0.2 g N kg−1 (as powders) were thoroughly mixed with the soil at the start of the experiment to ensure adequate mineral nutrition. Plants were then allowed to grow under submerged conditions for another 6 wk in a glasshouse at room temperature. Each nylon bag was then removed carefully with the PVC pot, and roots were washed under tap water to remove any quartz particles adhering to the root surface. Roots all appeared reddish after the 6 wk growth period. Roots were then excised at the basal node and were used in the following experiments. Care was taken during this process to ensure the roots were exposed to open air for a minimal period. Transport assays Transport assays were conducted for excised rice roots with or without iron plaque. To obtain rice roots without iron plaque, excised rice roots (0.15–0.4 g d. wt) were incubated in deionized water for 20 min at room temperature. Then each of the roots was incubated in 50 ml DCB solution (containing 0.03 M Na3C6H5O7·2H2O, 0.125 M NaHCO3 and 0.06 M Na2S2O4) for 10 min to remove the iron plaque on rice root. Roots were quickly rinsed in deionized water three times and blotted with tissue paper, placed in 250 ml conical flasks containing 200 ml test solution with 0.024 mM arsenate, and shaken for 10, 20, 30 and 60 min. According to the results of an experiment conducted by Meharg and Jardine (2003) and our results (Fig. 1), 20 min was chosen as the set time for uptake in the subsequent experiments. Figure 1Open in figure viewerPowerPoint Time-dependent uptake of 0.024 mM arsenate by rice (Oryza sativa) root after iron plaque removal. The data showed a linear correlation between As (V) influx and uptake time (r = 0.996). Each point is the average of three replicates. Bars, ± 1 SEM. Kinetics of As uptake by rice roots without iron plaque The rice roots without iron plaque were transferred to the conical flasks containing test solutions. The test solutions with different concentrations of arsenite or arsenate were 0.0067, 0.0133, 0.0267, 0.0533 and 0.1067 mM. The test solutions contained an additional 0.5 mM phosphate for the phosphate competition experiment. Stock solutions of arsenite and arsenate were prepared from sodium arsenite (NaAsO2) and sodium arsenate (Na2HAsO4· 7H2O). All test solutions contained 5.0 mM 2-(N-morpholin) ethansulfonic acid (MES) and 0.5 mM Ca(NO3)2 adjusted to pH 5 using KOH. The last step in all experiments was rinsing the roots in ice-cold phosphate solution containing 1 mM K2HPO4, 5 mM MES and 0.5 mM Ca(NO3)2 for 20 min to remove absorbed As species from the root free space. The roots were oven-dried at 70°C for 2 d and weighed. Kinetics of As uptake by rice roots with iron plaque The experiment on As uptake by rice roots with iron plaque was similar to the previous experiment for roots without iron plaque. After incubation in deionized water, roots were transferred directly to a conical flask containing test solutions with 0.0067, 0.0133, 0.0267, 0.0533 and 0.1067 mM As, and incubated for 20 min. After incubation, iron plaque was removed from the surface of the rice roots with 50 ml DCB solution. Finally, the roots were rinsed in ice-cold phosphate solution for 20 min. Roots were oven-dried at 70°C for 2 d and weighed. Digestion and analysis of plant tissues The root samples were placed in 100 ml Teflon tubes and digested by 5 ml concentrated Analar HNO3 using the CEM Microwave Sample Preparation System (Matthews, NC, USA). The temperature was raised to 160°C within 15 min and held for 15 min. A reagent blank and standard reference plant material (GBW07603 from the National Research Center for Standards in China) were included to verify the accuracy and precision of the digestion procedure and subsequent analysis. Concentrations of P and As in the digest solutions and of P, Fe and As in DCB solutions were analysed by inductively coupled plasma optical emission spectrometry (ICP-OES), Optima 2000 DV (Perkin Elmer, Wellesley, MA, USA) and atomic fluorescence spectrometry (A2022; Beijing Haiguang Analytical Instrument Co., Beijing, China), respectively. Statistical analysis ANOVA on plant biomass and concentrations of metals were performed using Windows-based genstat (6th edition, VSN Ltd, Hemel Hempstead, UK). Results Arsenic uptake by rice roots after iron plaque removal Preliminary study using α-naphthylamine demonstrated that the DCB extraction procedure did not cause significant damage to roots (data not shown). Figure 1 shows that after iron plaque removal by DCB extraction, rice roots could still effectively absorb As (V) from the solution, and As (V) influx increased linearly with time. Effect of iron plaque on arsenic uptake kinetics After iron plaque removal, both arsenite and arsenate influx into rice roots followed a hyperbolic pattern, indicating that As influx increased with increasing concentrations of arsenite and arsenate in the incubating solutions (Fig. 2). However, for roots with iron plaque intact, both arsenite and arsenate influx were significantly different from those roots without iron plaque (Fig. 2). For arsenite, with external concentrations between 0.0067 and 0.1067 mM, arsenite influx with iron plaque was slightly higher than without iron plaque. For arsenate, except at an external concentration of 0.0067 mM, iron plaque decreased arsenate influx into roots substantially. Figure 2Open in figure viewerPowerPoint Concentration-dependent kinetics for arsenite and arsenate uptake by rice (Oryza sativa) roots with and without iron plaque. Each point is the average of three replicates. Bars, ± 1 SEM. Uptake kinetics of arsenite and arsenate by roots with no iron plaque were adequately described by the Michaelis–Menten function (Table 1) with R2 values of 0.931 and 0.872 for arsenite and arsenate, respectively. However, for As uptake by roots with iron plaque remaining intact, the uptake kinetics could not be described by the Michaelis–Menten function. For comparison, the results for rice roots with iron plaque are shown in Table 1. Table 1. Kinetic parameters for arsenite and arsenate influx into rice (Oryza sativa) roots with or without iron plaque Arsenic species Without iron plaque With iron plaque V max † K m ‡ R 2 V max † K m ‡ R 2 As (III) 38.7 ± 1.4 0.0037 ± 0.0008 0.931* 54.3 ± 9.6 0.0057 ± 0.0048 0.456 As (V) 55.5 ± 4.7 0.0084 ± 0.0029 0.872* 31.0 ± 4.0 0.0018 ± 0.0022 0.202 * Significant at P < 0.05. † V max units = nmol g−1 d. wt min−1. ‡ K m units = mM. Phosphate and arsenic in iron plaque after uptake kinetics study Although there was no P in the incubating solution, P was detected in the DCB extracts (Fig. 3) due to the inherent P from the preculture system. Phosphorus concentrations in DCB extracts of roots treated with arsenate were significantly lower than in those of roots treated with arsenite (P < 0.001). Arsenic (both arsenite and arsenate) concentrations in DCB extracts were linearly correlated with As concentrations in the incubating solution with R2 values of 0.959 and 0.952 for arsenite and arsenate, respectively. Concentrations of arsenite in the DCB extracts were significantly lower than those of arsenate (Fig. 4). Figure 3Open in figure viewerPowerPoint Concentration-dependent adsorption by iron plaque for phosphate when arsenite, As (III) and arsenate, As(V) exist in the solution. Each point is the average of three replicates. Bars, ± 1 SEM. Figure 4Open in figure viewerPowerPoint Concentration-dependent adsorption for arsenite, As (III) and arsenate, As (V) in iron plaque with phosphate. Each point is the average of three replicates. Bars, ± 1 SEM. Effect of phosphorus on arsenic uptake by roots with or without iron plaque The addition of P at 0.5 mM (Abedin et al., 2002c) to the incubating solution significantly reduced arsenate influx into roots with no iron plaque for both external concentrations (P < 0.001; Fig. 5b). However, for roots with iron plaque the addition of P had a marginal effect on arsenate influx. With no P addition, iron plaque again inhibited arsenate influx, whereas the addition of P at 0.5 mM iron plaque slightly increased arsenate influx into roots with iron plaque. For arsenite, the addition of P to the incubating solution did not have a significant effect on arsenite influx irrespective of external As concentrations and the presence of iron plaque (P > 0.1; Fig. 5a). Iron plaque had a marginal effect on arsenite influx for both external As concentrations. Figure 5Open in figure viewerPowerPoint Effects of external phosphate concentration and iron plaque on influx of (a) arsenite, As (III); (b) arsenate, As (V) into rice (Oryza sativa) roots. *, Significantly different from neighbouring column (P < 0.001) according to a two-way ANOVA analysis. Each column represents the mean of three replicates. Bars, ± 1 SEM. Arsenic concentrations in iron plaque after the phosphorus-effect experiment It was again shown that increasing As concentrations in the incubating solution increased As concentration in the DCB extracts (Fig. 6). Arsenite concentrations in DCB extracts were generally lower than those of arsenate. The addition of P to the incubating solution had marginal effect on arsenite concentrations in the DCB extracts. However, the addition of P significantly reduced arsenate concentrations in DCB extracts irrespective of external arsenate concentrations. Figure 6Open in figure viewerPowerPoint Arsenic concentrations in iron plaque with effects of phosphate addition and species of As. *, Significantly different from the neighbouring column (P < 0.001) according to a two-way ANOVA analysis. Each column represents the mean of three replicates. Bars, ± 1 SEM. Discussion The importance of iron plaque in As attenuation in the rhizosphere has been demonstrated in wetland plants such as Phalaris arundinacea and Typha latifolia (Hansel et al., 2002). Arsenic and iron concentrations on the root surface were revealed by X-ray absorption spectroscopy and X-ray fluorescence microtomography to be spatially and temporally correlated. However, little information is known about the impacts of iron plaque on As accumulation by rice plants. Due to the ubiquity of iron plaque on rice roots it has been difficult to obtain direct evidence of the effect of iron plaque on plant uptake of As or any other element. In the present study, we successfully developed a novel system to culture rice plants in a mesh bag filled with inert quartz sand, surrounded by soil. This system allowed easy separation of the growth medium and roots with iron plaques. The α-naphthylamine test showed that iron plaque removal by DCB extraction did not cause significant damage to roots. The vitality of roots after iron plaque removal by DCB extraction was further demonstrated by the fact that As uptake into roots after DCB extraction (Fig. 1) was similar to that observed without DCB extraction (Abedin et al., 2002c). This was also proved by Otte et al. (1989), who showed that after incubation in DCB solution, K+ leakage from plant roots did not increase compared with incubation in water. The contrasting effect of iron plaque on the uptake of arsenite and arsenate into rice roots treated by DCB also provided further evidence that DCB did not cause substantial damage to roots. Arsenate/arsenite uptake kinetics by rice roots with no iron plaque were similar to those obtained by Meharg and coworkers (Meharg & Hartley-Whitaker, 2002; Meharg & Jardine, 2003). However, due to the ubiquity of iron plaque on the surface of rice roots, parameters obtained in the absence of iron plaque may not be realistic. The results obtained in this study unequivocally demonstrated that the presence of iron plaque on rice roots significantly influenced both arsenite and arsenate uptake into roots (Fig. 2) and that, in the presence of iron plaque, uptake kinetics could no longer be fitted with the Michaelis–Menten function. Iron plaque had contrasting effects on the uptake of arsenite and arsenate: uptake of arsenite was enhanced by the presence of iron plaque, but that of arsenate was dramatically inhibited by iron plaque. These results demonstrated that iron plaque played different roles in arsenite and arsenate uptake. This is probably caused by the different affinities of arsenite and arsenate adsorption by iron plaque. It has been shown by adsorption studies that iron oxides had much higher affinity for arsenate than arsenite (Meng et al., 2002); in other words, arsenite associated with iron plaque may be much more easily desorbed than arsenate. Arsenate associated with iron plaque may be largely ‘locked up’ because of its high affinity with iron oxides. Thus iron plaque may act as a ‘buffer’ for arsenate in the rhizosphere, leading to a lower influx into root cells. The higher affinity of iron plaque for arsenate than for arsenite was further supported by the fact that, with the same external concentrations in the incubation solutions, concentrations of arsenate in DCB extracts were much higher than those of arsenite (Fig. 4). The interactions between phosphate and arsenite/arsenate were also influenced by iron plaque. Our previous study showed that P nutrition influenced arsenate uptake by rice plants through a possible three-way interaction between iron plaque, arsenate and phosphate (Liu et al., 2004a). It was demonstrated that arsenite uptake by rice roots was hardly affected by phosphate, which was also supported by results obtained from the current study (Fig. 5a). For arsenate uptake there were strong interactions between phosphate and iron plaque. The inhibitory effect of phosphate on arsenate uptake was verified when iron plaque was removed (Fig. 5b). In the presence of iron plaque, the effect of phosphate on arsenate uptake was rather complicated. There are two principal mechanisms controlling the effect of phosphate on arsenate uptake into rice roots with the presence of iron plaque. (1) Phosphate may enhance the desorption of arsenate from iron plaque to both the external solution and the apoplastic space, demonstrated by the reduced arsenate concentration in DCB extracts with the addition of phosphate to the external solution (Fig. 3); the increase in arsenate in the apoplastic space may thus lead to higher arsenate uptake into rice roots. (2) At the cell membrane level, phosphate inhibits arsenate transport into cells. Thus the overall effect of phosphate on arsenate uptake into root cells with the presence of iron plaque may depend on the balance of these two mechanisms. With the addition of phosphate to the incubation solution, iron plaque tended slightly to increase arsenate influx into roots, which suggested that arsenate desorption by phosphate may be partly responsible for the slight increase in arsenate influx into roots. The combined effects of arsenate ‘lock-up’ in iron plaque and phosphate-induced desorption from iron plaque may contribute to the insignificant difference in arsenate influx into roots between P treatments (Fig. 6). The other issue related to the dynamics of As in the rhizosphere of rice plants is the possibility of the oxidation of arsenite to arsenate by the presence of iron plaque or the release of oxygen/oxidants from rice roots. In the present study, changes in As speciation were not investigated. It is believed that that under short-term solution culture experimental conditions, oxidation of arsenite to arsenate is unlikely to occur to a significant extent. Nevertheless, there is little information available so far regarding the speciation of As in the rhizosphere of rice plants. In wetland plants Hansel et al. (2002) found that As existed as a combination of two species in the iron plaque, being comprised predominantly of arsenate (c. 82%) with lesser amounts of arsenite, suggesting that a plant-induced oxic–anoxic interface may exist at the root surface. The possible oxidation of arsenite to arsenate in the rhizosphere is an important issue and requires further investigation under soil-pot experimental conditions. Soil solution pH is another factor likely to influence the behaviour of As in the rhizosphere, as the adsorption of arsenite and arsenate is strongly altered by solution pH (Jain et al., 1999). Further experiments to investigate the effect of pH on As sequestration in iron plaque and subsequent uptake into rice roots are under way. 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