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Differences in Selenium Uptake, Distribution and Expression of Selenium Metabolism Genes in Tomatoes.

Byline: Wanyi Zhao, Weihong Xu, Yourong Chai, Xinbin Zhou, Mingzhong Zhang and Wenwen Xie

Abstract

Effects of different levels of selenium (0, 5 and 10 mg*kg-1) on selenium uptake, distribution, and the expression levels of Se metabolism genes were compared with three tomato (Solanum lycopersicum L.) varieties (KT30, Defumm-8 and Luobeiqi). The results showed that Se inhibited the expression levels of APS reductase in varieties of KT30 and Luobeiqi; however, there was no significant difference in variety Defumm-8. Selenium also promoted expression levels of ATP sulfurylase of the tomato varieties. The expression levels of cysteine desulfurase of variety KT30 and variety Defumm-8 first decreased and then increased with the increasing Se concentration; however, expression levels of Se were lower than the control, and expression levels of Se in leaves of variety Luobeiqi increased. The Se concentration markedly decreased the expression levels of S-adenosyl-l-Met:l-Met S-methyltransferase in the three varieties, except variety Defumm-8 with Se at 5 mg*kg-1.

Furthermore, the Se concentration markedly decreased expression levels of serine acetyltransferase of variety KT30 and variety Defumm-8, but expression levels of leaves of variety Luobeiqi increased. Expression levels of serine acetyltransferase in variety Luobeiqi increased with the increasing Se concentration but decreased in KT30 and Defumm-8. Expression levels of SeCys methyltransferase (SMT) in Luobeiqi and KT30 were reduced by Se, while promoted in Defumm-8. The Se treatment promoted expression levels of APS in three tomato varieties. (c) 2017 Friends Science Publishers.

Keywords: Tomato varieties; Selenium uptake; Distribution; Selenium metabolism genes

Introduction

Selenium (Se), a vital trace element for humans, is important for preventing disease, improving health, and preventing aging and has been considered a life-protecting agent. Selenium deficiency can cause a series of symptoms, such as cardiovascular disease, Kaschin-Beck disease and Keshan disease. The Se content of the soil is typically low, and there are more than 40 countries and regions worldwide where people experience Se deficiency. In China, about 72% county (city) are Se deficiency areas, of which nearly 1/3 are severe Se deficiency areas. Improved living standards have resulted in more attention being given to health, and people have begun taking Se supplements. Selenium supplements can be manufactured from inorganic chemicals, such as sodium selenite, but the absorption rate in the human body is low, with the risk of side effects (Wen, 2006).

Researches have shown that consumption of plant products containing a high Se content, is the most efficient and safe way of Se supplementation (Finley and Davis, 2001; Chen et al., 2008).

The contents of selenium in plants varied greatly, because of plant species, planting environment, and the way of cultivation, etc. There is a great difference in the content of selenium in species, and there is a significant difference in the contents of selenium in different genotypes (Zhang et al, 2006). Glutathione peroxidase (GSH-Px) was the first enzyme reported in the literature as a selenium containing enzyme, which has been found out to have four categories kinds of GSH-Px family members (Miao et al., 2005). GSH-Px may be one of family member of the plant, which may or may be different vary in different developmental stages and in different stress conditions. The mRNA levels of GSH-Px increased steadily under different environmental stresses e.g., pathogen infection, high salt and heavy metals, etc. (Avsian-Kretchmer and Lev-Yadun, 2004) Therefore, GSH-Px may play a vital part in the process of oxidative signal transduction in plants (Freeman et al., 2006, 2010).

In recent studies, APS play modulates the reduction of selenite and Se assimilation because of a strong similarity between sulfur and Se; it has also been illustrated that the limit of Se assimilation is selenate reduction (Du et al., 2007). Overexpressed APS in transgenic plants promote the rate of this process and increases accumulation and tolerance of Se (Sors et al., 2005). Many studies have proven that cysteine desulfurase (CysD), S-adenosyl-l-Met:l-Met S-methyltransferase (MMT) (Berken et al., 2002), serine acetyltransferase (SAT) (Van Hoewyk et al., 2008), and SeCys methyltransferase (SMT) were also the main genes for plant metabolism and Se accumulation (Berken et al., 2002). Apart from possessing Cys desulfurase activity, NifS-like proteins can also be characterized as SeCys lyases able to cleave selenocysteine to form alanine and elemental selenium (Mihara et al., 1997).

The synthesis of Se-Met is involved in possibly by homo-Cys S-methyltransferase or Met synthase. Regarding to the sulfur assimilation, Terry et al. (2000) reported that Se-Met is methylated to Se-methyl Se-Met (SeMM) by the Sadenosyl-l-Met:Met-S-methyltransferase.

Selenocysteine methyltransferase (SMT), a central enzyme in Se hyperaccumulators, which can methylate SeCys and inhibit toxicity by preventing SeCys from being misincorporated into proteins (Neuhierl and Bock, 1996). Therefore, selenate can be converted into methylselenocysteine and g-glutamylmethylselenocysteine rapidly by the hyperaccumulator Astragalus bisulcatus (Dunnill and Fowden, 1967; Zhang et al., 2006). The gene encoding SMT not only has been cloned from A. bisulcatus (Neuhierl and Bock, 1996) but overexpressed in Arabidopsis thaliana and Brassica juncea; The strengthen Se tolerance and accumulation were brought about by this when plants were given selenite (SeO 2-) instead of selenate (Sors et al., 2005). In hyperaccumulators, accumulation of total Se was evidently higher than that was discovered in the SMT transgenic plants, which indicates that SMT enzyme is seemly related that have a synergistic interactions of Se hyperaccumulators.

Ellis et al. (2004) reported that it cannot be proved that the capacity differences in enzyme are related to reduction or assimilattion S, which is significant for Se hyperaccumulated A. bisulcatus.

Sors et al. (2005) also proposed transgenic A. thaliana overexpressing APR (APS reductase) and ATP sulfurylase played significant roles in a greater selenate reduction of total Se, however the overexpressing in SAT (serine acetyl transferase) led to a tiny reduction in organic selenium. Generally, total Se accumulation in shoots was relatively low in the transgenic plants overexpressing ATPS, SAT and PaAPR (P. aeruginosa APR). The plant root growth was affected by selenate treatment within ATPS and SAT overexpressors, besides, less in the PaAPR transgenic plants. Thereby, it draws a conclusion that ATPS and APR enzymes are vital correlated with selenate reduction in plants (Sors et al., 2005). In this research, the effect of various Se concentrations on growth and on the Se uptake and accumulation in three tomato varieties were determined.

To explore differences in Se accumulations in these tomato varieties, the genes connected with Se metabolism and absorption in tomato and the expression levels during growth were also examined.

Materials and Methods

Plant Material, Soil and Se Treatments

Soil was collected from the Jiulongpo district in Chongqing municipality, China. The soil pH was 6.03, organic matter content was 37.35 g*kg-1, total nitrogen (TN) content was 2.62 g*kg-1, available nitrogen content in soil was 105 mg*kg-1, available phosphorous content 13.1 mg*kg-1, available potassium content was 100.9 mg*kg-1, cation exchange capacity (CEC) was 0.192 mol*kg-1, total Se content was 0.26 mg*kg-1, and available Se was 0.017 mg*kg-1. Seedlings of three tomato varieties (KT30, Defumm-8 and Luobeiqi) were purchased from Chongqing Academy of Agricultural Sciences.

Three levels s of Se (0, 5 and 10 mg*kg-1) prepared from Na2SeO3 were set up for the pot experiment. Moreover, air-dried soil (5 kg) was sieved using a 40-mesh sieve, and homogeneously mixed with the Na2SeO3 solution, put in a plastic pot (diameter, 25 cm; height, 17 cm), all treatments were performed in triplicate and arranged at random. The tomato seedlings were transplanted at three or four leaf stage on March 2, 2013. Nitrogen was applied basically in 180 mg*L-1 and in three installments: 40% for basal and 30% each for seedling and flowering stages, P fertilizer (NH4H2PO4) and K (KCl) were applied in 100 and 150 mg*L-1 respectively in the basic fertilizer, productivity was recorded since the first tomato fruit was born. The process of the greenhouse experiment was took three months, and the production was recorded (Xie et al., 2014).

Analysis of Soil Physicochemical Properties

The soil pH was measured in 1:5 (soil:water). Extractable P and cation exchange capacity (CEC) were measured according to Rayment and Higginson (1992), while the soil organic matter was determined with the method of McCleod (1975). The concentration of extractable Se was determined via KH2PO4 (0.016 mol*L-1) extraction [10 g soil (dry weight) to 30 mL of KH2PO4 (w/v)] followed by atomic fluorescence spectrometry analysis (Zhang et al., 2014).

Analysis of Se Concentrations in the Plants

Dried plant samples were ground and digested at 170degC using 10 mL (8 mL of ultrapure nitric acid and 2 mL of perchloric acid) of acid mixture (Kyodan et al., 1988). The acid mixture was heated until white smoke appeared; further, 10 mL of 6 mol*L-1 hydrochloric acid was added, and the mixture was heated until the white smoke reappeared. The residual sample was then diluted in 25 mL of ultrapure water. The concentration of Se in the solution was analyzed using hydride generation atomic fluorescence spectrometry (PF6.3, Beijing Purkinje General Instrument Co. Ltd., China).

Detection of Gene Expression

Total RNA extraction: Total RNA was extracted from the plant material with RNA extraction kit purchased from Huashun Biotechnology Company. RLP lysate: RLA and Plant-PLUS were mixed at 4:1 (V/V), adding 2% of the total volume of 2-ME. Fresh samples (100 mg) kept at -80degC were powdered under liquid nitrogen. The samples were added to 1.5 mL centrifuge tubes pre-filled with 600 uL RLP lysates and vibrated for 2 min for mixing the samples before the liquid nitrogen completely evaporated. Let sit for five minutes, then subjected to reverse oscillation for 30 s after adding 200 uL chloroform, followed by 3 min at room temperature. Centrifuged for 5 min at 12 000 rpm, the supernatant was transferred to another 1.5 mL centrifuge tube of and mixed with 250 uL 75% ethanol after through mixing and then put them to the RNA adsorption column. Then centrifuged for 1 min at 12 000 rpm, the liquid was discarded in the collection tube, and the adsorption column was put back in the original collection tube.

Then 500 uL RP (protein-free solution) was added, placed for 1 min and centrifuged for 30 s at 12 000 rpm. Next, the liquid was discarded in the collection tube and the adsorption column was put back in the original collection tube. 500 uL liquid of W3 (75% ethanol) was added, placed for 1 min, and centrifuged for 15 s at 12 000 rpm; further, the supernatant was discarded in the collection tube, the centrifuge tube was washed with 1 mL 75% ethanol and centrifuged for 1 min at 12,000 rpm. After that the adsorption column was put into another clean 1.5 mL centrifuge tube. Finally, 50 uL of DEPC water was added to the center of the membrane adsorption and air dried for 2 min, then centrifuged for 1 min at 13 000 rpm. Lastly, the eluent contained total RNA in the samples. Two tubes were taken for electrophoresis and spectrophotometer detection after packing, and the rest were quickly preserved in refrigerator at -80degC.

Purification of total RNA: RNase-free DNase I (Code No. D2215) (TARALA) was used for removing trace amounts of DNA in the RNA sampl. 20-50 ug total RNA, 5 uL in 10x DNase I buffer, 2 uL DNase I (RNase-free, 5 U/uL), and 0.5 uL RNase inhibitor (40 U/uL)were added in microcentrifugal tubes, and then added DEPC-treated water to a total volume of 50 uL. The solutions were reacted for 30 min at 37degC. 50 uL DEPC-treated water was added to make a total volume of 100 uL. The same volume (100 uL) of phenol/chloroform/isoamyl alcohol (25:24:1) was added and mixed thoroughly. The solutions were centrifuged for 5 min at 12 000 rpm, and the upper supernatant was removed into another microcentrifugal tube. The same volume (100 uL) of chloroform/isoamyl alcohol (v:v=24:1) was added and mixed thoroughly. The solutions were allowed to stand at room temperature and centrifuged for 5 min at 12 000 rpm; the upper supernatant was transferred to another microcentrifugal tube.

The 1/10 volume (10 uL) of 3 M NaOAC (pH 5.2) and 250 uL precooling ethanol were added and placed for 30-60 min at -20degC. The solutions were centrifuged for 10 min at 12 000 rpm at 4degC; the supernatant was abandoned and the precipitate was recovered. The precipitation was washed with 2-4degC 70% ethanol and centrifuged for 10 min at 12 000 rpm at 4degC and the supernatant was then discarded and the precipitate was air or blow dried. The precipitate was then dissolved with a proper amount of RNase-free H2O, providing purified RNA.

RNA detection: RNA (1 uL) was taken, and the OD values were set up at 260 nm and 280 nm by Nanodrop 2 000 UV-vis spectrophotometer (Thermo Scientific, USA). Concentration calculation: For single-stranded RNA, the RNA concentration was 40 ug*mL-1 when OD260 = 1.0. The RNA concentration is calculated according to the method. Purity analysis: The ideal OD260/280 of RNA samples should be 1.7-2.0. Further, 1% agarose gel and 1x TAE electrophoretic buffer were prepared; 5 uL of RNA was taken, and 2 uL bromophenol blue was added to it. This mixture was then thoroughly mixed, electrophoresed (90 V, 1-2 h), and photos were captured with E-Gel Imager.

Synthesis of Reverse Transcription cDNA

A 20 uL reaction mixture was put in the microcentrifuge tubes, and cooled on ice, added 2 ug total RNA and 1 uL Olig (dT)18 solutions, then add to 11 uL with DEPC-treated H2O, mixed softly and gently centrifuged for 5 s. The solutions were kept 70degC in a water bath for 5 min, quickly transferred to an ice bath for 5 min, and gently centrifuged for 5 s. 4 uL 5x Buffer, 1 uL RNase Inhibitor (40 U/uL), 1 uL MMLV (200 U/uL) and 2 uL dNTP (10 mM) were added to the tubes. The solutions were gently shaken and centrifuged at a low speed for 5 s. The solutions were kept at 42degC water bath for 1 h. The temperature of solutions was maintained in 70degC for 15 min, following which, the reaction was terminated, and the solutions stored at -20degC.

Primer Design and Synthesis

Six genes associated with Se metabolism were examined. According to cDNA sequence in NCBI GenBank of tomato (Solanum lycopersicum L) and other solanaceae crops (Kabelka et al., 2002; Barry and Pandey, 2009) designing the RT-PCR gene-specific primers by Oligo 6.0 software (Xiao et al., 2015).

PCR Amplification of cDNA

Take the acquired cDNA, specific amplified with ABI-9700 instrument, reaction system (total volume is 25 uL) included with 2.5 uL 10x Buffer (including Mg2+), 0.5 uL dNTP (10 mM), 0.5 uL forward primer (10 uM), 0.5 uL reverse primer (10 uM), 0.3 uL Taq DNA polymerase (5 U/uL), 0.25 uL cDNA template, and then add dd H2O to a total volume of 25 uL. PCR reaction program included 94degC initial denaturation for 2 min, and 94degC denaturation for 40 s, and 52degC-60degC annealing temperature (depends on the specific reaction, APR 56degC, APS 60degC, CysD 60degC, MMT 60degC, SAT 52degC, and SMT 58degC, etc. ) for 1 min, and 72degC for 45 s, and 30 cycles, and then 72degC for 10 min. PCR products with detected with electrophoresis 1.2% agarose gel (including GoldView, GV), and images were taken with ultraviolet transilluminator.

Real-time Quantitative PCR

The cDNA obtained by reverse transcription was diluted 25-fold with dd H2O, and its specificity was detected with qRT-PCR with the 25S rRNA gene as the standard (Levesque et al., 1990) using SYBR Premix Ex TaqTM II (Perfect Real Time) kit provided by TaKaRa. The PCR amplification reaction system include 10 ulSYBR(r) Premix Ex Taq(tm) II (2x), 0.8 uL F-primer (10 umol/L), 0.8 uL R-primer (10 umol/L), 0.4 uL ROX Reference Dye II (50x), and 100 ng cDNA template, then added ddH2O up to a total volume of 20 uL. Amplification reaction conditions were 95degC pre-denatured for 30 s, 95degC denatured for 5 s, corresponding annealing temperature annealing for 30 s, extension of 30 s, 40 cycles, and then 72degC extension of 10 min. PCR products were assayed using a rapid automated capillary electrophoresis system. Each sample was repeated three times, while 25S rRNA gene was used as the standard gene.

Statistical Analysis

Three-way analysis of univariate ANOVA and correlation analysis were performed using SPSS version 21.0 package (SPSS, 2009). The variables analyzed separately were Se concentration and Se uptake in tomato. The transfer factors (TF; proportion of Se concentration in shoot to that in roots) were also analyzed to three-way univariate ANOVA. The level of significance was 0.05 (Xu et al., 2010).

Results

Concentration of Se in Fruit, Leaf, Stem and Root

The Se concentrations in fruit, leaf, stem and root were significantly different (p[?]0.05) between Se treatments and between varieties (Fig. 1). Considering the effects of Se treatments on all varieties, Se contents in plant tissues were in the order root (9.898 mg*kg-1) > leaf (2.299 mg*kg-1) > stem (1.297 mg*kg-1) > fruit (1.176 mg*kg-1). Furthermore, Se concentrations in root, stem, leaf, and fruit increased with an increase in the applied Se fertilizer. The Se concentration in the root of 10 mg*kg-1 Se treatment was 2.25 times that of the 5 mg*kg-1 Se treatment for variety KT30. In the stem, leaf and fruit were 3.04, 1.81, and 2.73 times that of the 5 mg*kg-1 Se treatment. For variety Defumm-8, the Se content in the root was 1.64 times that of 5 mg*kg-1 Se, whereas in the stem, leaf, and fruit were 3.28, 1.83 and 2.10 that of 5 mg*kg-1 Se treatment.

For variety Luobeiqi, the Se concentration in the root was 1.61 times that of the 5 mg*kg-1 Se treatment, whereas the stem, leaf and fruit were 2.82, 2.63 and 3.04 times that of the 5 mg*kg-1 Se treatment. Highest fruit Se concentration was 2.778 mg*kg-1 in variety Luobeiqi at the 10 mg*kg-1 Se treatment.

Se Accumulation in Fruit, Leaf, Stem and Root

A marked difference was found in the Se content in the leaves, stem, fruit, and roots between the three varieties and between the different Se concentrations (Fig. 2). The Se content in different tissues of tomato increased with the increasing Se concentration in the soil and reached a maximum at 10 mg*kg-1 Se treatment. The Se accumulation (per pot) in the root was 16.19-30.92 times, for the stem was 17.03-21.69, for the leaves was 12.80-25.01, and for the fruit was 18.02-25.21 times that of the control. Moreover, the Se accumulation in the different tissues of tomato were different and was in order of stem leaf > root fruit (p [?] 0.05). In this experimental condition, after treated with Se, fruit Se accumulation in variety Defumm-8 was higher than that in variety KT30 and variety Luobeiqi. On the other hand, whether treated with Se or not, total Se concentration in variety KT30 and variety Luobeiqi were higher than that in variety Defumm-8.

Highest total Se concentration was 206.10 ug*pot-1 in variety KT30 at 10 mg*kg-1 Se treatment.

Detection of Se Metabolism Gene Expression

Real-time PCR results of APR and six gene expression levels between the different tomato varieties and concentration are shown in Fig. 3 and Fig. 4. The expression level of APR in variety KT30 was the highest in the treatment without Se, and it markedly decreased with Se, whereas there was no marked difference of APR expression in in different treatments of variety Defumm-8. There was marked difference of APR expression in different treatments of variety Luobeiqi, and it was decreased with the increasing Se concentration and reached a minimum at 10 mg*kg-1. Expression levels of ATP sulfurylase (APS) of variety KT30 first decreased and then increased and reached maximum at 10 mg*kg-1 Se treatment; it was 3.67 times that of the control. The Se concentration markedly promoted expressions of APS of variety Defumm-8 and variety Luobeiqi, which increased with the increasing soil Se concentration.

Moreover, expression levels of CysD of variety KT30 and variety Defumm-8 revealed the same trend with the increasing Se concentration: first decreased, then increased, and reached minimum at 5 mg*kg-1 Se treatment, which decreased by 21.43% and 30.77%, respectively, compared with the control. However, Se expression levels at 10 mg kg-1 Se treatments were lower than that of the control. The Se treatment promoted the expression of CysD of variety Luobeiqi, and it reached a maximum at 5 mg*kg-1 Se treatment. The Se concentration markedly decreased expression levels of MMT of variety Luobeiqi and variety KT30, both increased with the increasing Se concentration; expression of SAT of variety Defumm-8 reached a maximum at 5 mg kg-1 Se treatment, while reduced at 10 mg*kg-1 Se treatment. Expression levels of SAT of variety KT30 and variety Defumm-8 revealed the same trend with the increasing Se concentration: first decreased and then increased; however, they were lower than the control.

The expression level of SAT of variety Luobeiqi increased with the increasing Se concentration and reached a maximum at 10 mg*kg-1 Se treatment. Expression levels of SMT of variety Luobeiqi and variety KT30 were inhibited by Se treatment, whereas it was promoted in variety Defumm-8.

PsNumber 1, 2 and 3 indicate varieties KT30, Defumm-8 and Luobeiqi at no Se treatment; 4, 5 and 6 indicated varieties KT30, Defumm-8 and Luobeiqi at 5 mg*kg-1 Se treatment; 7, 8 and 9 indicated varieties KT30, Defumm-8 and Luobeiqi at 5 mg*kg-1 Se treatment, respectively.

Discussion

In this experiment, the Se content in roots of tomato was the highest, followed by leaf, stem and then fruit was the lowest, which was similar to the results of many scholars. For example, Shi et al. (1993) studied the absorption, distribution, and transformation of Se in tomato and found that the Se content in various organs in the plant in the order of root > leaf > stem > fruit > flowers. Tian et al. (2005) had a similar report, with different Se concentrations; the Se content in different parts of ryegrass was in order of roots > leaves > stems (Tian et al., 2005). The selenium transfering from soil to plant depends upon plant species, form of Se supply, soil type, concentration and so forth (Munierlamy et al., 2007). Furthermore, the content of Se in the organs is related with Se chemical form.

In soil, selenite adsorbed by carbonate, amorphous iron or aluminum content mainly exsists in exchangeable speciation and carbonate bound form that cannot be uptaken largely by plants (Li et al., 2015), whereas Se (VI) has high mobility and validity in soil which indicates that the selenate accumulation and absorption capacity of plants is stronger than selenite. There is a great difference in the plant uptake, transformation and distribution of Se (VI) and Se (IV), and Se (VI) and Se (IV) absorption and transport mechanism are relatively independent. Eustice et al. (1980) found that selenate was more easily transported than selenite and the organic Se selenomethionine (SMT), Terry et al. (2000) demonstrated the same conclusion. When selenate was absorbed, the ratio of plant Se concentration and root Se concentration was 1.4-1.72, while the ratio range was 0.6-1 when they absorbed selenomethionine (SMT); the ratio range was KT30 > Defumm-8. Different expression levels of APR, APS, CysD, MMT, SAT and SMT genes were found in different tomato varieties and different concentration. The higher expression levels of CysD, MMT, and SAT were found in Luobeiqi variety.

Acknowledgments

This work was supported by Fund of China Agriculture Research System (Nycytx-35-gw16), the National Science and Technology Pillar Program of China (No. 2007BAD87B10), the National Natural Science Foundation of China (No.31372141; 31672238), and the Ministry of Science and Technology of China (grant No. 2016YFD0100506); all are acknowledge. The first author gratefully thanks Dr. Hong Li at Lancaster Environment Centre, Lancaster University for reviewing the paper.

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Author:Zhao, Wanyi; Xu, Weihong; Chai, Yourong; Zhou, Xinbin; Zhang, Mingzhong; Xie, Wenwen
Publication:International Journal of Agriculture and Biology
Article Type:Report
Date:Jun 30, 2017
Words:5378
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