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Cite This: Environ. Sci. Technol. 2019, 53, 9252−9259
Enantioselective Phytotoxic Disturbances of Fatty Acids in Arabidopsis thaliana by Dichlorprop Siyu Chen,† Hui Chen,‡ Zunwei Chen,§ Yuezhong Wen,*,† and Weiping Liu† †
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MOE Key Laboratory of Environmental Remediation & Ecosystem Health, Institute of Environmental Health, College of Environmental and Resource Sciences, Zhejiang University, Hangzhou 310058, China ‡ College of Science and Technology, Ningbo University, Ningbo 315211, China § Department of Veterinary Integrative Biosciences, Texas A&M University, College Station, Texas 77843, United States S Supporting Information *
ABSTRACT: Plant fatty acids have indispensable physiological functions and nutritional value. However, the overuse of herbicides may cause phytotoxic disturbances of fatty acids in nontarget plants while spraying for weeds. Evidence has shown that the herbicide dichlorprop can inhibit the activity of acetylCoA carboxylase (ACCase), a key enzyme involved in fatty acid synthesis. However, the enantioselective phytotoxic effects of dichlorprop enantiomers ((R)-dichlorprop and (S)-dichlorprop) on fatty acids and their related mechanisms remain unclear. To solve this issue, the enantioselective phytotoxicity of dichlorprop in the model plant species Arabidopsis thaliana (A. thaliana) with a focus on fatty acids was evaluated for the first time. The results indicated a significant difference in enantioselectivity and that exposure to (R)-dichlorprop can cause marked fatty acid disturbances in nontarget plant species. Specifically, (R)dichlorprop decreased the content of three fatty acids by more than 50% by inhibiting the activity of ACCase. In addition, increased malondialdehyde (MDA) and lipid hydroperoxides (LOOHs) contents and membrane permeability reflected herbicide-induced lipid peroxidation, which decreased the unsaturation of fatty acids in membranes and further influenced membrane composition and function. Moreover, an increased level of glutathione peroxidase (GPX) and cytochrome P450 (CYP450) reflected a plant stress-induced response. To summarize, fatty acids represent a new perspective for evaluating the toxicity of chiral pesticides, contributing to a better understanding of the enantioselective phytotoxicity and mechanisms of dichlorprop, and providing evidence for herbicide security and risk assessments.
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INTRODUCTION Fatty acids carry out essential physiological functions for biological growth and metabolism. Composing the main component of cell membrane lipids, fatty acids are also important energy sources and precursors of bioactive molecules.1,2 In addition, in plants, fatty acids are closely related to resistance to cold and disease.3−5 However, studies have shown that environmental pollutants may induce disturbances in biological fatty acids. For example, exposure to fine process particles enriched with metals and metalloids led to alterations in Lactuca sativa leaf fatty acid composition.6 In addition, the toxicity of organic pollutants such as pesticides can also influence fatty acids in living organisms. The herbicide dimethachlor was reported to inhibit very long-chain fatty acid (VLCFA) synthesis in barley leaves and was suggested to be an effective inhibitor of fatty acid elongation.7,8 It was reported that pinoxaden dione can inhibit the de novo synthesis of fatty acids via the inhibition of acetyl-CoA carboxylase (ACCase).8 Plant fatty acids are considered one of the targets of herbicide phytotoxicity. The Herbicide Resistance Action Committee (HRAC) has classified herbicides into several categories based on their target sites. For instance, herbicides classified as group © 2019 American Chemical Society
K3 can inhibit the elongation of fatty acid chains such that they affect VLCFA synthesis. In addition, herbicides that hinder de novo fatty acid synthesis and lipids are classified as group N.8−10 In China, the average amount of pesticides used per hectare is approximately 1.5- to 4-fold higher than the world average.11 A large number of agricultural chemicals are put into agricultural production but are often accompanied by certain adverse effects due to improper application. Herbicides are among the most extensively used agrochemicals for controlling weeds; uncontrolled applications may cause phytotoxicity to nontarget plants and may even pose a risk to human health via the food chain.12,13 Additionally, as many as 30% of the herbicides currently in use are characterized by chirality. As more complex compounds are put into use, this proportion is expected to increase.13,14 The enantioselective effects of chiral herbicides have received increasing attention in recent decades Received: Revised: Accepted: Published: 9252
June 24, 2019 July 5, 2019 July 10, 2019 July 10, 2019 DOI: 10.1021/acs.est.9b03744 Environ. Sci. Technol. 2019, 53, 9252−9259
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Environmental Science & Technology
heated at 80 °C for 2 h. Once the samples returned to normal temperature, 5 mL of 0.9% (w/v) NaCl and 2 mL of hexane were added. Fatty acid methyl esters (FAMEs) were extracted from the hexane phase and analyzed on an HP-5MS GC column (30 m, 0.25 mm, 0.25 μm, Agilent) via GC-MS (7890B, 7000C, Agilent). The column was operated with helium as the carrier gas, an injection temperature of 270 °C, and a detector temperature of 250 °C. The oven temperature setting was initially 50 °C for 2 min, followed by an increase of 20 °C/min to 150 °C, an increase of 10 °C/min to 180 °C for 12 min, an increase of 5 °C/min to 215 °C for 8 min, and then an increase of 10 °C/min to 270 °C. The Supelco 37 component FAME mix (CRM47885 Supelco) was used to make the standard curves, and heptadecylic acid (H3500, Sigma-Aldrich) was added as an internal standard. The average recovery efficacy of the fatty acids was 93.20 ± 1.30% in preliminary tests. Determination of Lipid Peroxidation Products. The level of lipid peroxidation was measured by the production of malondialdehyde (MDA) and lipid hydroperoxides (LOOHs) after 18, 22, 26, and 30 days of plant culture. The MDA content was measured based on the thiobarbituric acid (TBA) reaction as described by Heath and Packer.27 Fresh A. thaliana tissues were collected and ground into a homogenate with 10 mL of 10% (w/v) trichloroacetic acid (TCA) in water. After centrifugation at 4000g for 10 min, 2 mL of the supernatant extract of each treatment group was collected (replaced by 2 mL of distilled water in the control group), with 2 mL of the 0.6% (w/v) TBA solution subsequently added. The mixture was then heated in a boiling water bath for 15 min and cooled immediately. The absorbance was recorded at 532, 600, and 450 nm on a Cary 100 UV−vis spectrophotometer (Agilent Technology, Tokyo, Japan), and the MDA concentration of each treatment group was calculated according to the following formula
because enantiomers of most chiral herbicides can exhibit different biological toxicity and activity on organisms.15−17 Dichlorprop is a typical chiral herbicide commonly applied in different parts of the world;18 the structures of the two enantiomers are provided in Figure S1. As a widely used aryloxy phenoxy propionic (AOPP) herbicide, dichlorprop mainly targets acetyl-CoA carboxylase (ACCase) in gramineous plants, a critical enzyme in fatty acid biosynthesis.19−21 However, according to our previous study, when exposed to (R)-dichlorprop, the gene expression related to ACCase was suppressed in the nontarget plant species A. thaliana.13 Whether ACCase-inhibiting herbicides can cause plant fatty acid disturbances is poorly understood. Therefore, whether the excess exposure of dichlorprop can disturb fatty acids in nontarget plant or crop species by inhibiting the key enzyme and related mechanisms is still unknown. In addition, although (R)-dichlorprop has been indicated to be the enantiomer with herbicidal activity,22 the toxic effects related to fatty acids of this chiral herbicide on nontarget plants at the enantiomeric level remain unclear. Focusing on plant fatty acids at the enantiomeric level, the present study aimed to clarify the phytotoxic effects and mechanism of dichlorprop with A. thaliana as the nontarget model plant species. Due to the essential physiological functions of plant fatty acids, the results from this study will be of great significance to plant growth and metabolism. From the perspective of chiral herbicides, this study will assist in better understanding the enantioselective phytotoxicity and the involved mechanisms of dichlorprop as well as other similar herbicides, providing basic evidence for herbicide security and risk assessments.
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MATERIALS AND METHODS Plant Culture and Herbicide Exposure. Seeds of A. thaliana (ecotype Columbia) were first sterilized with 4% (v/ v) sodium hypochlorite for 1 min, after which they were rinsed two times with 75% (v/v) ethanol and finally with sterile water three times. The sterilized seeds were sown in a 24-well culture plate, with each well containing 1 mL of presterilized Murashige and Skoog medium23 with herbicide. According to a previous study, enantiomers of dichlorprop with 99% purity were synthesized from 2,4-dichlorophenol and 2chloropropionic acid followed by GC-MS and circular dichroism to obtain the molecular weight, purity, and specific configuration.24 (R)- and (S)-dichlorprop were dissolved in acetone with a final solvent concentration of 1‰ (v/v) acetone for each treatment. The exposure concentration (0.2 μM) was set on the basis of our previous study.25 The control group received 1‰ (v/v) acetone alone. After 2 days of vernalization at 4 °C, the test plants were transferred to an incubator and cultivated at 23 ± 2 °C under 16 h of light and 8 h of darkness per day before harvest. The required glassware was sterilized in an autoclave before the cultivation experiments. Analysis of Fatty Acids. After 3 weeks of cultivation, plant fatty acids were analyzed by gas chromatography-mass spectrometry (GC-MS) after trans-esterification according to a previously described method with some modifications.26 In brief, fresh plant samples were collected and ground into a homogenate with 10 mL of 2.5% (v/v) H2SO4:methanol used as a methylation reagent. After centrifugation, the supernatant extract was transferred to a 20 mL glass tube. The tubes containing the plant samples in acidified methanol were then
MDA concentration(μmol/L) = 6.45(OD532 − OD600) − 0.56OD450
A ferrous oxidation−xylenol orange (FOX) assay was applied for quantification of LOOHs in the plant extracts.28,29 Fresh plant tissues of each treatment group were ground into a homogenate with 10 mL of 80% ethanol containing 0.01% (w/ v) butylated hydroxytoluene (BHT), and the supernatant was preserved after centrifugation. Afterward, 1 mL of 10 mM trimethylphosphine (TPP) in methanol was added to 2 mL of the plant extract. Following vortex mixing, the mixture was incubated for 30 min for complete reduction of LOOHs. Plant extracts without TPP addition were combined with 1 mL of methanol alone. After incubation, 1 mL of FOX reagent was added to each sample. Preparation of the FOX reagent is described in detail in previous studies.28 The absorbance of the samples with or without TPP was then read at 560 nm approximately 10 min after the FOX reagent addition, and the absorbance difference reflected the level of LOOHs. The LOOHs value was expressed as micromoles of H 2 O 2 equivalents according to the established standard curve. Determination of Cell Membrane Permeability. Membrane permeability was reflected by electrolyte leakage and was measured according to a previous study with slight modifications.1 Fresh plant leaves of each treatment group cultivated for 3 weeks were added to a clean test tube containing reverse osmosis water. After vacuuming for 10 min, 9253
DOI: 10.1021/acs.est.9b03744 Environ. Sci. Technol. 2019, 53, 9252−9259
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Environmental Science & Technology the leaves sunk to the bottom as air slowly entered. Following shaking for 1 h, the electrical conductivity (C1) was determined using a conductivity meter. Each tube was then capped and boiled in a water bath for 10 min. The electrical conductivity (C2) was measured again after cooling to room temperature. The membrane relative permeability (electrolyte leakage) was calculated as follows, where C1 and C2 represent the electrical conductivity measured before and after boiling, respectively membrane relative permeability =
C1 × 100% C2
ELISA for ACCase, FAS, GPX, and CYP450 Detection. After 3 weeks of cultivation, the detection of plant ACCase, fatty acid synthase (FAS), glutathione peroxidase (GPX), and cytochrome p450 (CYP450) was carried out by ELISA kits (JL22799, JL45292, JL45704, JL45130, Jianglai Biotechnology Co. Ltd., Shanghai, China, http://www.jonln.com/). Specific operation steps, including the addition of standards, samples, and enzymes as well as the incubation, washing, coloration, termination reaction and determination, were performed according to the kit instructions. Statistical Analysis. The data were analyzed using Origin 8.5 software (OriginLab, Northampton, MA, USA). Normality tests of the data were carried out, and the comparisons were made using one-way analyses of variance30 followed by multiple comparison tests of the means (Tukey’s test). The differences were considered statistically significant when the P value was less than 0.05. All experiments were performed with three biological replicates and two or three technical replicates.
Figure 1. Effects on fatty acids of A. thaliana treated with different dichlorprop enantiomers. (A) Contents of three typical fatty acids. (B) Fatty acid unsaturation. (Different letters above the bars represent statistically significant differences, P < 0.05.)
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RESULTS AND DISCUSSION Analysis of Fatty Acids. According to the GC-MS results of the FAMEs as well as the converted fatty acid content, three kinds of the most abundant fatty acids in A. thaliana were selected for analysis, namely, palmitic acid (C16:0), linoleic acid (C18:2), and linolenic acid (C18:3). The chromatograms and mass spectra of the fatty acids in A. thaliana by GC/MS are available in Figure S2. As shown in Figure 1A, compared with those in the control group and the (S)-dichlorprop treatment group, the contents of the three typical fatty acids in A. thaliana treated with (R)-dichlorprop decreased significantly. Specifically, the contents of C16:0, C18:2, and C18:3 in the (R)-dichlorprop treatment group were 62.18 ± 1.01%, 64.49 ± 2.79%, and 73.81 ± 1.92% lower than those in the control group, respectively; when compared with the (S)dichlorprop treatment group, the percent decreases were 48.74 ± 1.37%,50.48 ± 3.90%, and 55.38 ± 3.28%, indicating significant enantiomeric differences of dichlorprop. Therefore, the content of C18:3 and C18:2 in plants exposed to (R)dichlorprop decreased more than that of C16:0, regardless of whether the comparison was made with the control group or the (S)-dichlorprop group. The fatty acid unsaturation within the membrane lipids was measured by the ratio of (C18:2 + C18:3) to C16:0, thereby reflecting changes in fatty acid composition. As shown in Figure 1B, the fatty acid unsaturation of A. thaliana exposed to (R)-dichlorprop markedly decreased in comparison with the control group. The results above indicate that the phytotoxicity of dichlorprop, especially (R)-dichlorprop, is able to cause a significant reduction in plant fatty acid contents, among which
the polyunsaturated fatty acids (PUFAs), including C18:2 and C18:3, decreased to a markedly greater extent. The herbicide-induced reduced content and lower unsaturation of plant fatty acids may be caused by two reasons. On one hand, fatty acid biosynthesis may be inhibited due to the exposure of (R)-dichlorprop. According to our previous study, gene expression related to ACCase, which is a critical enzyme in fatty acid biosynthesis, was suppressed in A. thaliana when exposed to (R)-dichlorprop.13 In addition, herbicides such as spiro-decanedione A and pinoxaden dione are reported to be inhibitors of lipid synthesis by inhibiting ACCase.8 On the other hand, the synthesized PUFAs are likely to be peroxidized due to the toxicity of this herbicide. Lipid peroxidation has been suggested to be one of the molecular mechanisms involved in pesticide-induced toxicity.31,32 Studies have shown that the exposure to 2,4-dichlorophenoxyacetic as well as organophosphate pesticides can result in lipid peroxidation in rat tissues.33−35 However, few studies have been conducted on pesticide-induced lipid peroxidation in plant tissues. Therefore, both factors were verified in the follow-up experiments of this study. Effects on Fatty Acid Synthesis. To verify whether plant fatty acid synthesis was influenced by dichlorprop, the activity of key related enzymes was measured. The results presented in Figure 2A show that the activity of ACCase in A. thaliana exposed to (R)-dichlorprop decreased by 51.88 ± 2.92% compared with that in the control group, while the inhibitory effect did not appear in plants treated with (S)-dichlorprop, reflecting an obvious difference in toxic effects between the two enantiomers. However, plant FAS activity was influenced 9254
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Effects on Lipid Peroxidation Levels. In addition to the inhibition of the target enzyme, oxidative damage is also one of the toxic effects of dichlorprop, as reported in our previous study.13 Excessive reactive oxygen species (ROS) produced in plants due to environmental stress can interact with various biological molecules, and lipid peroxidation occurs when ROS react with lipids.39,40 Usually, lipid peroxidation in plants mainly refers to the peroxidation of unsaturated fatty acids (UFAs) in cell membrane lipids. Considering the decrease in fatty acid unsaturation in A. thaliana resulting from the toxicity of (R)-dichlorprop, it can be speculated that lipid peroxidation of UFAs may occur in A. thaliana. The occurrence of lipid peroxidation is inevitably accompanied by lipid peroxidation products. To verify this speculation, the lipid peroxidation products in A. thaliana were determined in this study. To reflect the dynamic change in herbicide phytotoxic effects, the growth process of A. thaliana was divided into time points corresponding to 18, 22, 26, and 30 days after sowing. As presented in Figure 4,
Figure 2. Activity of key enzymes related to fatty acid synthesis in A. thaliana treated with different dichlorprop enantiomers. (A) Content of ACCase. (B) Activity of FAS. (Different letters above the bars represent statistically significant differences, P < 0.05.)
by neither (R)-dichlorprop nor (S)-dichlorprop as presented in Figure 2B. Fatty acid synthesis plays a critical role in biological growth and metabolism. The first committed step in de novo fatty acid biosynthesis is the carboxylation of acetyl-CoA to form malonyl-CoA, which is catalyzed by the enzyme ACCase.36 A series of successive reactions then occurs under the catalysis of FAS. Different kinds of fatty acids are ultimately synthesized by elongation and desaturation.37 The results here suggest that (R)-dichlorprop inhibits the activity of the target enzyme ACCase, thereby affecting the crucial and rate-limiting step in de novo fatty acid synthesis, thus hindering fatty acid production (Figure 3). Moreover, (S)-dichlorprop is known
Figure 4. Lipid peroxidation products in A. thaliana treated with different dichlorprop enantiomers during the growth process. (A) Content of MDA. (B) Content of LOOHs. (Different letters above the bars represent statistically significant differences, P < 0.05.) Figure 3. Simplified process of the de novo synthesis of fatty acids.
regardless of which time point was analyzed, the contents of MDA and LOOHs in the plants in the (R)-dichlorprop treatment group were significantly higher than those in the (S)-dichlorprop and the control groups, indicating enantioselectivity between the two enantiomers. This finding is consistent with a previous study in which (R)-dichlorprop led to higher levels of OH radicals in maize.41 In combination
to be the enantiomer with no herbicidal activity,38 and no significant inhibitory effect on ACCase was observed. However, compared with the control group, fatty acid content in A. thaliana treated with (S)-dichlorprop also decreased significantly, and the reason for the decrease remains to be investigated. 9255
DOI: 10.1021/acs.est.9b03744 Environ. Sci. Technol. 2019, 53, 9252−9259
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Environmental Science & Technology with the dynamic tracking of morphological changes (Figure 5), it can be seen in the (R)-dichlorprop treatment groups that
Figure 6. Membrane relative permeability of A. thaliana leaves exposed to different dichlorprop enantiomers. (Different letters above the bars represent statistically significant differences, P < 0.05.) Figure 5. Morphological dynamic tracking of A. thaliana exposed to different dichlorprop enantiomers.
The above results demonstrate that the occurrence of lipid peroxidation caused by the toxicity of (R)-dichlorprop can damage UFAs, thus interfering with the structure and function of the plant cell membrane. First, fatty acids are one of the critical components of the cell membrane. The disturbances of fatty acids unavoidably influence the composition of the plant membrane. In addition, the fatty acid composition affects essential properties of the cell membrane, including the stability, permeability, and fluidity, which are closely related to many cellular functions.42,44 Thus, the changes in fatty acid composition altered the basic membrane properties, thereby causing damage to membrane functions. Second, the decomposition products of lipid peroxides, such as MDA and 4-hydroxynonenal (4-HNE), are considered biochemical markers of membrane damage.45 Studies have shown that these aldehydes can be dispersed in cells and can even escape from cells, attacking substances far from their targets.42,46 With high reactivity and biological effects, these toxicity messengers can further spread and amplify the initial radical reactions. Biomolecules such as proteins and lipids can be covalently modified with these lipid peroxidation products, disrupting membrane structure and affecting their physical properties, thereby leading to cytotoxicity and even cell death.46 Therefore, in consideration of the large extent of MDA and LOOHs in A. thaliana disturbed by (R)-dichlorprop in the above-mentioned results, the membrane damage could be explained in terms of lipid peroxidation products. Effects on GPX and CYP450. Once subjected to adverse conditions, organisms are capable of responding with certain defensive measures, such as changes in the activity of related enzymes. In the present study, we analyzed GPX and CYP450 from the perspective of the antioxidant system and the metabolism of the herbicide, respectively. As shown in Figure 7A, the activity of GPX in the test plants exhibited notable enantioselectivity. Specifically, the GPX activity of A. thaliana in the (R)-dichlorprop treatment group was 45.41 ± 1.90% higher than that in the control group, while the activity of this enzyme was not disturbed by (S)-dichlorprop. Furthermore, a similar situation occurred with respect to the CYP450 enzyme content, as shown in Figure 7B. The enantiomeric difference was reflected in the fact that the CYP450 enzyme content in the (R)-dichlorprop group increased by 68.48 ± 5.60% compared with that in the (S)-dichlorprop group. The above results can be analyzed from the perspective of the function of the two enzymes. As one of the indispensable peroxidases and free radical scavengers, GPX is capable of
the contents of MDA and LOOHs were greatest in plants at 22 days. Moreover, the enantiomeric difference between the two enantiomers was most pronounced at this time. MDA is one of the cytotoxic products of UFAs in phospholipids, and the MDA content usually reflects the level of lipid peroxidation.39 However, MDA can be formed only from fatty acids with three or more double bonds, and merely measuring the MDA content may underestimate the level of lipid peroxidation.29 Thus, the LOOHs content was measured at the same time. The notably elevated levels of MDA and LOOHs indicate that the UFAs in A. thaliana affected by (R)-dichlorprop underwent severe peroxidation, explaining the reduction in the content and proportion of UFAs, as well as a decrease in unsaturation. Moreover, PUFAs with more double bonds such as C18:2 and C18:3 are more susceptible to ROS-mediated peroxidation, leading to a greater decline in content, leading to a greater decline in content. In addition, in terms of the dynamic changes of lipid peroxidation products, it can be concluded that plants experiencing early growth are more sensitive and vulnerable to the toxicity of dichlorprop. Effects on Cell Membranes. Lipid molecules make up 30−80% of biological membranes by mass. Membrane lipids, mainly phospholipids, constitute the main component of the phospholipid bilayer, which is the basic structure of cell membranes, along with other molecules including proteins, glycolipids, and cholesterol. Additionally, fatty acids are a major component of lipids, including fats, phospholipids, and glycolipids.42,43 In summary, fatty acidsespecially PUFAs, which are essential to the membrane propertiesare one of the indispensable components of cell membranes. Therefore, oxidative damage to PUFAs unavoidably poses a threat to cell membrane structure and function. As one of the essential properties of the cell membrane, permeability was reflected by the relative conductivity of plant tissues. It can be seen clearly from Figure 6 that the relative conductivity of plant leaves adversely influenced by (R)-dichlorprop increased as expected; the levels were 61.10 ± 3.46% and 52.8 ± 3.29% higher than those in the control group and the (S)-dichlorprop group, respectively, that is, in plant cells affect by the toxicity of (R)dichlorprop, the extravasation of water-soluble substances and electrolytes was more severe, thereby indicating heavier damage to plant cell membrane. 9256
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protection. For the first time in the present study, the disturbances of (R)-dichlorprop to plant fatty acids and cell membrane were explained from the perspective of fatty acid synthesis as well as ROS-induced lipid peroxidation. The phytotoxic mechanisms of dichlorprop on plant fatty acids and cell membranes were revealed at the enantiomeric level in terms of the target enzyme ACCase as well as oxidative damage. In summary, herein, fatty acids provide a new angle to evaluate the toxicity of chiral pesticides, contributing to a better understanding of the enantioselective phytotoxicity and mechanisms of dichlorprop and providing evidence for herbicide security and risk assessments.
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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.est.9b03744. Two enantiomers of the chiral herbicide dichlorprop; chromatograms and mass spectra of the studied fatty acids in A. thalianaby GC/MS (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86-571-8898-2421. Fax: +86-571-8898-2421. E-mail:
[email protected]. ORCID
Zunwei Chen: 0000-0001-7773-1635 Yuezhong Wen: 0000-0002-7871-7699 Weiping Liu: 0000-0002-1173-892X
Figure 7. Activity of related enzymes in A. thaliana treated with different dichlorprop enantiomers. (A) Activity of GPX. (B) Content of CYP450. (Different letters above the bars represent statistically significant differences, P < 0.05.)
Notes
The authors declare no competing financial interest.
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catalyzing the conversion of reduced glutathione to its oxidized form as well as the reduction of toxic lipid peroxides to nontoxic hydroxyl compounds, protecting the structure and function of the cell membrane from lipid peroxides.47 Changes in GPX activity indicate the oxidative stress in plants. Therefore, the activity of this membrane repair enzyme was stimulated by the toxicity of (R)-dichlorprop. The enhancement of other antioxidant enzyme activities, including superoxide dismutase (SOD) and catalase (CAT), in A. thaliana exposed to (R)-dichlorprop was also observed in our previous study.13,25 In addition, plant CYP450 monooxygenases constitute one of the largest families of protein genes involved in plant growth, development, and acclimation to biotic and abiotic stresses.48 These enzymes are involved in the metabolism of endogenous compounds and exogenous substances such as drugs and environmental pollutants.35,49 The changes in the activity of CYP450 reflected the plant stress response and indicated an important role in herbicide metabolism or detoxification in A. thaliana. In addition, according to a previous study, the activity of CYP450 in atrazine-exposed rice plants was significantly higher than that in the control, indicating that CYP450 may be involved in atrazine metabolism or detoxification in rice.48 Therefore, in the present study, the elevated level of CYP450 content in plants exposed to (R)-dichlorprop suggests that the toxic effects of (R)-dichlorprop induce the metabolism of the herbicide by CYP450. Implications. These results have important implications for herbicide toxicology, public health, and environmental
ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC, Nos. 21876150 and 21677124). REFERENCES
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DOI: 10.1021/acs.est.9b03744 Environ. Sci. Technol. 2019, 53, 9252−9259
