Characteristics of the Time-Dependent Selenium Biofortification of Rice

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Cite This: J. Agric. Food Chem. XXXX, XXX, XXX−XXX

Characteristics of Time-Dependent Selenium Biofortification of Rice (Oryza sativa L.) Gaoxiang Huang,†,§ Changfeng Ding,† Xiangyang Yu,⊥ Zhen Yang,†,¶ Taolin Zhang,† and Xingxiang Wang*,†,‡

J. Agric. Food Chem. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 11/22/18. For personal use only.

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CAS Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China ‡ Ecological Experimental Station of Red Soil, Chinese Academy of Sciences, Yingtan 335211, China § University of Chinese Academy of Sciences, Beijing 100049, China ⊥ Institute of Food Quality and Safety, Jiangsu Academy of Agricultural Sciences, Nanjing 210014, China ¶ College of Life Sciences, Nanjing Normal University, Nanjing, Jiangsu 210046, China S Supporting Information *

ABSTRACT: The application of selenite to soil has increasingly been used to produce Se-enriched food. This study investigated the biofortification characteristics of Se in rice after application of selenite to soil at different growth stages. The results showed that the application of Se during booting stage resulted in the highest concentration of Se in brown rice due to the highest upward translocation of Se. More than 90% of Se in the brown rice was organic species, with selenomethionine predominated. The proportion of selenomethionine in the brown rice decreased with the delay in application time. The rice grown in the acidic soil had higher Se concentrations than in the neutral soil. With increasing soil Cd level, Se accumulation and the proportion of Se-methylselenocysteine in the brown rice were reduced. This study provides a theoretical basis for the production of Se-enriched rice in clean soil or slightly to moderately Cd-contaminated soil. KEYWORDS: selenium, rice, iron plaque, translocation, speciation, cadmium

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the application time of Se, Deng et al.14 found that the concentration of Se in brown rice was three-times higher when inorganic Se was sprayed at the full heading stage compared to the late tillering stage. Because of the thermodynamical instability of Se in the soil,4 Banuelos et al.17 found a significant reduction in Se concentrations in the topsoil as the duration of cultivation increased. Therefore, the effects of the soil application of Se may also be influenced by the application time. In general, Se was applied to the soil as a base fertilizer.12,18 The biofortification characteristics of the application of Se at different growth stages have rarely been studied. Inorganic Se can be partially assimilated to organic Se using the sulfur (S) metabolic pathway in plant tissues due to the chemical similarity between Se and S.19 Several organic Se species include selenocysteine (SeCys2 or SeCys), Semethylselenocysteine (SeMeCys), selenomethionine (SeMet), and Se-containing proteins (such as albumin, globulin, glutelin, and prolamin)20 have been identified in many plants, including rice,21 wheat,22,23 broccoli, and carrots.12,24 In addition, the biotransformation ability of inorganic Se to organic Se species has been found to be related to the plant species21,25,26 and application forms of Se.23 As reported, the bioavailability and nutritive value of organic Se in food is higher than that of

INTRODUCTION Selenium (Se) is an essential micronutrient for the human body, and this is primarily due to its incorporation into selenoproteins.1 Several of these proteins are enzymes including antioxidative and anticancer agents.2,3 However, because of the inadequate Se in crops in many Se-deficient regions, such as China, Egypt, Scandinavia Peninsula, Libya, and the UK,4 approximately 0.5−1 billion people in the world have insufficient dietary intake of Se.5 The consumption of food products with insufficient Se can induce many diseases including Keshan disease,6 heart and skeletal muscle pathologies,7 and liver and pancreatic disorders.8 In addition, the deficiency of Se in the human body induces vulnerability to cancer and cancer-causing stress.9 Therefore, it is critical to increase the dietary intake of Se in Se-deficient regions. Cereals are the dominant Se source for those on low protein diets, as typified by the global malnourished population.10 Se in cereals is derived primarily from the soil, and the concentrations of Se in the grain differed significantly among different regions due to the variations in soil Se contents, such as Enshi City (soil average Se level was higher than 3.0 mg kg−1) and Keshan County (soil average Se level was lower than 0.13 mg kg−1) in China.11 In Se-deficient regions, the application of inorganic Se, Se-enriched organic fertilizers,12,13 and foliar spray of Se solution to the cereals14,15 have proven to be efficient ways to improve the Se concentrations in the grain. For example, the concentration of Se in brown rice reached as high as 0.30 mg kg−1 with 0.75 mg kg−1 selenite applied to the soil.16 While the Se biofortification of cereals was affected by © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

August 19, 2018 October 25, 2018 November 7, 2018 November 7, 2018 DOI: 10.1021/acs.jafc.8b04502 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Journal of Agricultural and Food Chemistry inorganic Se.27 Therefore, the organic Se species and concentrations in brown rice were more appropriate to be used to assess the benefits of Se-biofortified food than the total Se used in most recent studies.13,14 In addition, because of the antagonistic effect of Se and cadmium (Cd), Se fertilizers have been increasingly applied to reduce Cd levels in rice.28−30 Our previous study found that the tillering application of 0.5 mg kg−1 Se resulted in the highest reduction of Cd in brown rice (63%) among different application times, but Se accumulation in brown rice also decreased with the increase of soil Cd level.31 Whether soil Cd could affect the Se metabolism in rice or not is still unknown. Therefore, the objectives of this study were (1) to investigate the uptake, translocation and distribution characteristics of Se in rice plants when Se was applied to the soil at different growth stages in different soils and Cd levels, (2) to study the Se metabolism status in brown rice by determining Se speciation, and (3) to clarify whether Cd affects the Se biofortification of rice.

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and rinsed slightly using tap and deionized water to extract the iron plaque on the root surface. The roots without iron plaque and the shoots from the mature stage were oven-dried at 75 °C until their dry weights were constant. The rice grain was freeze-dried, and the husk was separated from the grain using a sheller (JLG-II, Institute of Grain Storage in Chengdu, China). To determine the total Se concentration, the preweighed dry roots, shoots, husks, and brown rice were all milled into powder using a blender (A11 basic, IKA, Germany). Chemical Analysis of Plants. The iron plaque on the root surface was extracted using the DCB (dithionite−citrate−bicarbonate) method:32 the fresh roots were incubated in the mixed solution, including 0.03 M Na3C6H5O7·2H2O and 0.125 M NaHCO3 for half an hour, and the mixed system was reduced using Na2S2O4 powder as previously described.30 Two milliliters extraction solution were added to a tube containing 2 mL high purity HNO3 and 2 mL 6 M HCl and incubated in a water bath at 100 °C for 2 h. The concentration of Se in the solution was determined using atom fluorescence spectroscopy (AFS-930, JiTian Instruments, Beijing). The concentrations of Se and Cd in different parts of the rice were determined using AFS-930 and atomic absorption spectroscopy (SpectrAA 220Z, Varian, USA), respectively, after the plant samples were digested in the HNO3 and H2O2 solution (GR) as previously described.30 To determine the Se speciation in brown rice, 2.5 g of brown rice powder was mixed with 50 mM Tris-HCl buffer (pH 7.5) in an ultrasonic tank for 10 min. The enzyme Protease XIV was added, and the mixture was shaken overnight at 37 °C. After homogenization, the mixtures were centrifuged at 10 000 rpm min−1 for 30 min at 4 °C. The supernatant was filtered through a 0.22 μm filter to determine the Se speciation using a SAX-HPLC-ICPMS (Agilent 1200 HPLC equipped with a Hamilton PRP-X100 strong anion exchange analytical column (10 μm particle size, 25 cm length and 4.1 mm internal diameter) coupled to an Agilent 7500 ICPMS, Agilent, USA),12,33 and the spectra of the samples are shown in Figure S1. The following Se standards were used to test the retention time of each Se speciation: L-selenocystine (SeCys2), Se-methylseleno-L-cysteine (SeMeCys), and DL-selenomethionine (SeMet) (Tokyo Chemical Industry, Co., Japan); SeO42− (Se6+); and SeO32− (Se4+) (Sigma− Aldrich, St. Louis, MO, USA). The percentages of total Se recovered in the extracts of brown rice ranged from 60% to 80%, which was consistent with the results of Bañuelos et al.12 Data Analysis. The concentration of Se (CSe) in the iron plaque was calculated using the following equation:

MATERIALS AND METHODS

Soil Preparation and Pot Trial. The tested soils included a neutral paddy soil and two acidic paddy soils (acidic soils I and II). The background concentrations of Cd and Se in the neutral soil were 0.33 and 0.30 mg kg−1, respectively, and the pH value was 7.41. The acidic soils I and II had the same basic physical and chemical properties (pH: 5.02, background concentration of Se: 0.37 mg kg−1), except for the total concentration of Cd; that is, the concentrations of Cd in the acidic soils I and II were 0.41 (background concentration) and 1.28 mg kg−1 (after the addition of 0.9 mg kg−1 Cd), respectively. The neutral soil and acidic soil I were air-dried and milled to fine particles to prepare for the pot trial, additionally, half of the acidic soil I were spiked with 0.9 mg kg−1 Cd (3CdSO4·8H2O), and then maintained for three months in the condition of 80% of the field water-holding capacity to obtain the acidic soil II. Other properties of the neutral and acidic soils are shown as follows: organic matter, 2.3% and 3.0%; cation exchange capacity, 15.2 and 10.6 cmol kg−1; clay content, 15.1% and 20.6%, respectively. The detailed information can be referred to our previous study.31 From May to October 2015, the greenhouse pot trial was conducted in the Institute of Soil Science, Chinese Academy of Sciences, Nanjing, Jiangsu Province, China. Eight kilograms of dry soil was weighed into each pot, and the basal fertilizers (0.2 g N kg−1 DWsoil (dry weight of soil) (CO (NH2)2), 0.15 g P2O5 kg−1 DWsoil (CaH2PO4 H2O) and 0.2 g K2O kg−1 DWsoil (KCl)) were added to all the pots. Because the predominant form of Se in paddy soils is selenite,4 the selenite was used as Se fertilizer in this study. A total of 0.5 mg kg−1 DWsoil Se (Na2SeO3) was applied to the soil at different growth stages of the rice. The detailed information on each treatment is shown as follows: for CK, no Se addition, sampling at the seedling (20 d), tillering (60 d), booting (100 d), and mature stages (140 d); for BS-Se, basal addition of Se, sampling at the seedling, tillering, booting, and mature stages; for TL-Se, tillering addition of Se (45 d), sampling at the booting and mature stages; and for BT-Se, booting addition of Se (85 d), sampling at the mature stage. The treatments in each sampling stage had three replicates. Plant Culture and Collection. The tested cultivar “Suxiangjing 1” was provided by the Suzhou Academy of Agricultural Sciences, China. The seeds were sterilized using 30% (v/v) H2O2 for 15 min, then soaked in deionized water for 1 d, followed by germinating in moist gauze, as previously described,31 and 15 seeds with uniform germination status were evenly planted in each pot. At the three-leaf stage, all the seedlings in each pot were thinned to six seedlings. During the entire growing period, the soils were kept submerged in water at a depth of 2−3 cm. As described above, the rice plants in the different treatments were sampled at each stage. The roots were separated from the rice plants

CSe = Tironplaque − Se/DWroot where Tiron plaque‑Se represents the total amount of Se in the iron plaque, and DWroot represents the dry weight of the root. The translocation factors (TFs) of the Se from the root to the shoot, the shoot to the husk, and the husk to brown rice were calculated using the following equation: TFA − B = CB/CA where A represents the root, shoot, or husk; B represents the shoot, husk, or brown rice, respectively, and CA and CB represent the concentration of Se in A and B, respectively. Statistical Analysis. A one-way analysis of variance (ANOVA) was used to test the statistical relationship among all the data, and the significant difference was tested using the least significant difference test at the 0.05 level of probability using SPSS 19.0. All the data were expressed as the mean ± SD (n = 3). All the figures were generated using SigmaPlot 10.0 software.

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RESULTS Effects on Yield of Brown Rice. In the same treatment, the yield of brown rice did not differ significantly between the neutral soil and acidic soil I, except for the control, as shown in Figure 1. Compared with that in the acidic soil I, a slight

B

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were approximately 2.5-fold higher than those on the rice grown in the neutral soil, and (2) in the treatments of CK and BS-Se, the DCB-extractable Se concentration peaked at the booting stage in the neutral soil, while in the acidic soil I it peaked at the tillering stage (Figure 2a,b). Compared with the rice grown in the acidic soil I, the rice grown in the acidic soil II (higher Cd level) had a bit lower sequestration of Se in the iron plaque on the root surface by 10.5% to 39.6% during the entire growth period, but the difference is significant only in the BS-Se treatment (except in the mature stage) (Figure 2b and Table S1). The sequestration of Se in the iron plaque was also significantly affected by the Se application time. The BS-Se treatment had the highest DCB-extractable Se concentrations among all the treatments during the entire growth period. The values ranged from 0.26 to 0.89 mg kg−1 and from 0.47 to 2.41 mg kg−1, on average, in the neutral and acidic soils, respectively. Moreover, Se application at the early stage induced the higher concentration of DCB-extractable Se on the root surface than Se application at the late stage. Se Accumulation in Root, Shoot, and Husk of Mature Rice. As shown in Figure 3, Se concentrations in the different tissues (root without iron plaque, shoot, and husk) of rice grown in the acidic soil I were much higher than that of the rice grown in the neutral soil (except the CK treatment in the root and shoot), especially in the shoot and husk. Compared with the rice grown in the acidic soil I, the Se concentrations in the shoot and husk of rice grown in the acidic soil II were significantly lower, and the reductions were 16.0%−35.3%, and 30.9%−41.7% in the shoots and husks, respectively. Compared with the CK, the application of Se significantly increased the concentration of Se in the roots, shoots, and husks, while the increasing rates differed among the different application stages. In the roots, Se concentration in the TL-Se treatment was significantly higher than that in the BT-Se treatment in the neutral soil; the TL-Se treatment had the highest concentration of Se among different Se application stages in both acidic soils. In the shoots and husks, no significant difference was observed among different Se application stages in the neutral soil, while Se concentrations

decrease of the yield occurred in the treatments of CK and TLSe in the acidic soil II.

Figure 1. Yield of brown rice grown in the neutral soil and acidic soils I and II. BS-Se, TL-Se, and BT-Se indicate the basal, tillering, and booting applications of Se, respectively. In the same soil, bars with the same lowercase letter(s) indicate no significant differences between different treatments at P < 0.05; in the same treatment, bars with the same capital letter(s) indicate no significant differences between different soils at P < 0.05.

In the three soils, the basal application of 0.5 mg kg−1 Se (BS-Se) significantly increased the yield by 11.9 to 16.2%; the treatment of TL-Se significantly increased the yield by 12.6% in the neutral soil, while the BT-Se treatment had no significant impact on the yield (Figure 1). Dynamics of Se Sequestration in Iron Plaque during Entire Growth Period. During the growing period of rice, the DCB-extractable Se concentrations on the root surfaces first increased and then decreased at the mature stage in all soils, except for the BT-Se treatment, which increased constantly in the neutral soil and first increased, then decreased, and finally increased at the mature stage in the acidic soils (Figure 2). In the soils with a similar Cd level (neutral soil and acidic soil I), some differences were observed: (1) the DCB-extractable Se concentrations on the roots of the rice grown in acidic soil I

Figure 2. Dynamics of DCB-extractable Se concentration on rice roots grown in the (a) neutral soil and (b) acidic soils I (solid lines) and II (dashed lines). BS-Se, TL-Se, and BT-Se indicate the basal, tillering, and booting applications of Se, respectively. C

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Figure 3. Concentrations of Se in the root, shoot, and husk of mature rice grown in the neutral soil and acidic soils I and II. BS-Se, TL-Se, and BTSe indicate the basal, tillering, and booting applications of Se, respectively. In the same soil, bars with the same lowercase letter(s) indicate no significant differences between different treatments at P < 0.05; in the same treatment, bars with the same capital letter(s) indicate no significant differences between different soils at P < 0.05.

than that grown in the neutral soil. The greatest difference occurred in the BT-Se treatment. Compared with the rice grown in acidic soil I, the total protease XIV-extractable Se in the brown rice significantly decreased by 40.0%−54.8% in acidic soil II. Compared with the CK, the concentrations of the SeMet and Se6+ in the brown rice were significantly increased by the application of Se at different stages in all the soils (except the Se6+ in the BS-Se treatment). In the neutral soil, the concentrations of SeCys2 in the treatments of TL-Se and BT-Se were lower than that in the treatments of CK and BS-Se, while the SeMet and the Se6+ had the contrary laws. In two acidic soils, the BT-Se treatment had the highest concentrations of SeCys2, SeMeCys, SeMet, and Se6+ (Table S2). SeMet was the primary species of Se in the brown rice, and its concentrations ranged from 15.2 to 604.4 μg kg−1, which accounted for 61.6%−95.0% of the total protease XIVextractable Se (Figures 4 and 5). Compared with the CK, the application of Se increased not only the concentration, but also the proportion of SeMet in brown rice (Figure 5). However, the proportions of SeCys2 (1.3%−19%) in different Se treatments were significantly lower than those in the CK. SeMeCys, accounting for 0.5%−4.0% of the total protease XIV-extractable Se, was significantly reduced in the acidic soil II compared with that in the acidic soil I. In terms of inorganic Se, only Se6+ was observed in the samples, accounting for 0.9%−6.0% of the total protease XIV-extractable Se; interestingly, the BT-Se treatment had the highest Se6+ concentration and proportion among all the treatments. In addition, approximately 1.2%−14.2% unknown Se species was detected in all the samples (Figure 5).

in the TL-Se and BT-Se treatments were significantly higher than that in the BS-Se treatment in both acidic soils. Se Speciation in Brown Rice. As shown in Figure 4, the concentrations of SeCys2, SeMeCys, SeMet, Se6+, and Se4+ in the brown rice were measured, and no Se4+ was detected in any of the samples. The total protease XIV-extractable Se in the brown rice grown in acidic soil I was 1.82−5.25-fold higher

Figure 4. Concentration of each Se speciation in the protease XIVdigested aqueous phase of brown rice. BS-Se, TL-Se, and BT-Se indicate the basal, tillering, and booting applications of Se, respectively. The bars represent the standard errors of total Se in the protease XIV digestion, and bars with the same letter(s) indicate no significant differences between different treatments at P < 0.05. D

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In addition, the upward translocation of Se from the root to the shoot, from the shoot to the husk, and from the husk to the brown rice in the acidic soil II decreased by 6.1%−27.3%, 9.0%−22.7%, and 22.3%−35.4%, respectively, compared with that in the acidic soil I.

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DISCUSSION Because of the positive role of Se in photosynthesis and antioxygenation, beneficial effects of the application of Se on yield have been reported in many crops, such as wheat,13 rice,14 and lentils,34 and the basal application of Se in this study also had the similar effects in all soils (Figure 1). However, the present results also showed that Se application at the booting stage had no significant yield-increasing effect (Figure 1), indicating that the application time of Se had a significant impact on this effect. Chu et al.35 also found that Se application at the blooming stage had a smaller effect on the yield of wheat grain than at earlier stages, which supports the results in this study. The form of Se in the soil (selenite, selenite, and selenide) primarily depends on the soil pH and Eh (redox potential), and selenite is considered to be the predominant form in paddy soils.4 However, selenite is readily adsorbed by iron oxides or hydroxides in the soil;36 thus, the phytoavailability of Se would gradually decrease after the application of selenite to the paddy soil. Despite this, in most treatments (excluding the CK in the root and shoot, and the BT-Se treatment in the root), rice tissues grown in the acidic soil I had higher Se concentrations than that grown in the neutral soil (Figure 3), which was related to the iron plaque on the roots. Our previous study found that the amount of iron plaque in the acidic soil I was 3−10-fold higher than that in the neutral soil31 due to the higher mobility of iron in the soil with lower pH value.37 This resulted in the much higher sequestration of Se in the acidic soil I (Figure 2) because the primary components of the iron plaque were iron oxides or hydroxides. Unlike the barrier effect of iron plaque on some metal ions with stable valence, such as Cd,38 the iron plaque may act as a pool of Se to be taken up by the rice after the tillering stage. The dynamics of Se concentration in the iron plaque was inconsistent with that of the iron plaque during the growth period of rice.31 This phenomenon could be related to the variable valence of Se. Selenite (Se4+) is the predominant form of Se adsorbed by iron oxides or hydroxides in the iron plaque,36 and a proportion is oxidized to selenate (Se6+) by radial oxygen loss (ROL) with the fast development in the aerenchyma of rice after the tillering stage.38 Compared with selenite, less selenate can be adsorbed by iron oxides or hydroxides,36,39 and thus, selenate is readily absorbed by the rice, indicating that Se in the iron plaque becomes a pool after the tillering stage. Therefore, the DCB-extractable Se decreased at the late stage of rice grown in the acidic soils. However, the formation of iron plaque in the neutral soil was very weak before the tillering stage and rose quickly from the tillering to the booting stage,31 resulting in the continuous rise of DCB exactable-Se after the tillering stage. The uptake of Se by plant roots from the soil was deeply affected by the rhizosphere processes, such as ROL, involved in the formation of iron plaque and the oxidation of selenite on the root surfaces of wetlands.40,41 Because of the dynamic changes in the ROL and iron plaque on the rice roots,38 the Se sequestration in the iron plaque changed dynamically during the growth period (Figure 2). The iron plaque primarily

Figure 5. Percentage of each Se speciation in the brown rice grown in the neutral soil and acidic soils I and II. BS-Se, TL-Se, and BT-Se indicate the basal, tillering, and booting applications of Se, respectively.

Translocation Characteristics of Se within Rice Plant. The upward translocation of Se within the rice grown in the acidic soil was significantly affected by the time of the Se application (Figure 6). The TFs from the roots to the shoots in

Figure 6. Translocation factors of Se from the lower to the upper tissues of rice grown in acidic soils I and II. R-S, S-H, and H-BR indicate the translocation from the root to the shoot, the shoot to the husk, and the husk to brown rice, respectively; I and II indicate acidic soil I and II; BS-Se, TL-Se, and BT-Se indicate the basal, tillering, and booting applications of Se, respectively. ∗ and ∗∗ indicate significant differences between the Se treatments and CK at P < 0.05 and P < 0.01, respectively.

the BT-Se treatment were significantly higher than those in the other treatments, and they were also significantly increased by the TL-Se treatment compared with that in the CK. From the shoot to the husk, the BT-Se treatment still had the highest TF. From the husk to the brown rice, the BT-Se treatment significantly increased the TF by 41.7%−63.0% compared with the CK, while the BS-Se and TL-Se treatments had no significant impact on this TF. The change rule of the TFs in the neutral soil was similar to that in the acidic soil I (Figure S2). E

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conducive to the upward translocation of Se in the rice (Figure S3) as well as to Se accumulation in the brown rice (Figure 4). In addition, when Se was applied to the soil at the booting stage, the brown rice required large amounts of nutrients for filling at the moment, so the minerals absorbed by the stem and leaves might be used first to fill the grains.14 As a result, Se application at the later stage resulted in more effective Se biofortification in the rice. In addition, Figures 6 and S3 show that a higher Cd level in the soil inhibited the upward transport of Se. In one sense, the antagonistic effects between Se and Cd play an important role in this inhibition.18,29 Alternatively, some insoluble Se−Cd complexes could be formed in the cell of the roots or shoots,48 thus inhibiting the upward transport of the Se. The Se speciation in brown rice was crucial to assess the benefits of brown rice Se to the human body because organic Se forms, such as SeMet, confer additional health benefits compared to inorganic Se by either being more anticarcinogenic or used more effectively in the body.49 Inorganic Se is readily assimilated to organic forms after rice uptake through the metabolic pathway,4 but the speciation of Se varies with plant species due to the different metabolic pathway.12,21,50 The present study found that SeMet was the predominant Se species in the brown rice (Figure 5), which was consistent with the results of Li et al.51 SeMet can be incorporated directly or nonspecifically into proteins through the replacement of methionine, so it is readily adsorbed by humans.52 The application of Se at the booting stage had a high nutritive value of Se due to the highest concentration of SeMet (Figure 4). Perhaps because selenite can be rapidly converted to organicSe in the roots,23 no Se(IV) was observed in brown rice (Figure 5); by contrast, the assimilation rate of selenate is slow in cereal crops;23 thus, a small proportion of Se(VI) remained in the brown rice (Figure 6). Unfortunately, booting application of Se resulted in the highest concentration of Se(VI) in the brown rice (Figure 5). This could occur because the later application of Se had a shorter assimilation time for Se than the early application. It was also suggested that health risks could occur when Se was applied too late. In addition, the inorganic Se in plants can be assimilated to SeMeCys mainly through the pathway of the methylation of hydrogen selenide (H2Se) and the replacement of proton by cysteamine; SeMet can also be converted to SeMeCys by γ-glutamyl-Semethylselenocysteine (γ-glutamyl-CH3SeCys).52 However, the synthesis of SeMeCys was inhibited by soil Cd (Figure 5), indicating that the one or more of the above processes of the synthesis of SeMeCys could be inhibited by soil Cd, while the detailed information and underlying mechanisms merit further study. Therefore, the booting stage was the optimal application time of selenite for the Se biofortification of rice in Cduncontaminated soils because of the higher Se concentration in brown rice and the similar proportion of organic Se compared to the earlier application time. The higher phytoavailability and easier upward translocation of Se were considered to be two of the primary underlying reasons for this. Although the soil application of Se at optimal time is effective in enriching Se in brown rice, the Se fertilizer was applied to the soil with irrigation or using a sprinkling, the application way need to be optimized in the future. In addition, the environmental risks of long-term repeated application of Se to the soil need further studies.

formed at the early middle growth stages and decreased at the later growth stage,38 resulting in a stronger Se sequestration ability of iron plaque at the early middle stages than at the later stages (Figure 2). Therefore, more Se is sequestered in the iron plaque when Se is applied to the soil at an early stage (Figure 2), and the Se pool in the iron plaque benefits the uptake of Se at the later stages by the rice. However, the basal application of Se resulted in a lower accumulation of Se in the rice than the tillering application (Figure 3), which was related to the root morphology and phytoavailability of Se. The rice roots were too small to absorb enough Se at the early stage. Most of the selenite applied at the early stage was adsorbed by iron oxides and or hydroxides,4 so the phytoavailability of exogenous Se was inhibited at the later stage. As the rice root at the tillering stage was developed enough to absorb microelements, and the phytoavailability of Se in the soils was relatively high, when selenite was applied to the soil at the tillering stage, the soil Se was the primary source for the uptake of Se by the rice. In addition, perhaps because Se−Cd complexes were formed in the soil,42 the mobility of Se in the soil was inhibited with the increase in the soil Cd level, and the concentration of Se in the iron plaque was decreased, as well as Se accumulation in the rice tissues (Figure 3). For this reason, a moderate application of Se also decreased the uptake of Cd by rice in Cdcontaminated soils.18,29 In China, the risk screening and intervention values of Cd in the soil depends on soil pH, the values are 0.3 and 1.5 mg kg−1, 0.4 and 2.0 mg kg−1, 0.6 and 3.0 mg kg−1, and 0.8 and 4.0 mg kg−1 in the paddy soils with pH value 7.5, respectively; the safety risk of the agricultural products in the soils with Cd level between the screening and intervention value should be assessed, and some feasible remediation methods should be conducted to control the risk (GB15618−2018).43 We previously found that the application of 0.5 mg kg−1 Se to the soil contaminated with 1.28 mg kg−1 Cd at the different stages significantly reduced the concentrations of Cd in brown rice to a lower level than the food safety standard (0.2 mg kg−1) in China (GB2762−2017);44 the tillering application of Se induced the highest reduction of Cd in brown rice.31 Therefore, the present method in this study has practical significance and application value in slightly to moderately Cdcontaminated acidic paddy soils. Under anaerobic soil conditions, such as paddy soil, roots can absorb selenite using phosphate transporters such as OsPT2.45 Once the roots have taken up the Se, upward translocation and distribution of Se in the different tissues plays an important role in the accumulation of Se in brown rice. The application of Se significantly increased Se concentration in rice (Figure 3), which was consistent with previous studies.14,18 However, the effects of Se biofortification in rice tissues were closely associated with the application time of Se (Figure 3). With the basal application of selenite to the soil, most of the Se absorbed in the roots was in the form of selenite due to the underdeveloped aerenchyma in the early stage. While little selenite in the root can be transported to the aerial parts through the xylem sap, most of the selenite is readily converted to other water-insoluble form such as SeMet.23 However, with the application of selenite at the tillering or booting stages, most of the selenite in the root can be oxidized to selenate due to the well-developed aerenchyma, and selenate is readily transported to the aerial parts through the xylem.46,47 Therefore, compared with the basal application, the tillering or booting application of selenite is more F

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jafc.8b04502. HPLC-ICPMS spectra; translocation factors of Se from lower to upper tissues of rice grown in neutral soil; translocation factors of Cd from root to brown rice in acidic soils I and II; concentration of DCB-extractable Se in different treatments; concentration of different Se speciation in different treatments (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86−25−86881200. Fax: +86−25−86881000. ORCID

Xiangyang Yu: 0000-0002-2831-172X Xingxiang Wang: 0000-0002-0781-5566 Funding

This work was jointly sponsored by the National Key Technology Research and Development Program of China (2015BAD05B04), Jiangsu Agricultural Science and Technology Innovation Fund (CX(18)2023), Agricultural Synergy Innovation Alliance Program of Jiangxi Province, and AgroEnvironmental Protection program of Jiangxi province. Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.jafc.8b04502 J. Agric. Food Chem. XXXX, XXX, XXX−XXX