Methylation and Demethylation of Emerging Contaminants in Plants.

Many contaminants of emerging concern (CECs) have reactive functional groups and may readily undergo biotransformations, such as methylation and demethylation. These transformations have been reported to occur during human metabolism and wastewater treatment, leading to the propagation of CECs. When treated wastewater and biosolids are used in agriculture, CECs and their transformation products (TPs) are introduced into soil-plant systems. However, little is known about whether transformation cycles, such as methylation and demethylation, take place in higher plants and hence affect the fate of CECs in terrestrial ecosystems. In this study, we explored the interconversion between four common CECs (acetaminophen, diazepam, methylparaben, and naproxen) and their methylated or demethylated TPs in Arabidopsis thaliana cells and whole wheat seedlings. The methylation-demethylation cycle occurred in both plant models with demethylation generally taking place at a greater degree than methylation. The transformation rate of demethylation or methylation was dependent on the bond strength of R-CH3, with demethylation of methylparaben or methylation of acetaminophen being more pronounced. Although not explored in this study, these interconversions may exert influences on the behavior and biological activity of CECs, particularly in terrestrial ecosystems. The study findings demonstrated the prevalence of transformation cycles between CECs and their methylated or demethylated TPs in higher plants, contributing to a more complete understanding of risks of CECs in the human-wastewater-soil-plant continuum.

Contaminants of emerging concerns in recycled water: Fate and risks in agroecosystems.

Recycled water (RW) has been increasingly recognized as a valuable source of water for alleviating the global water crisis. When RW is used for agricultural irrigation, many contaminants of emerging concern (CECs) are introduced into the agroecosystem. The ubiquity of CECs in field soil, combined with the toxic, carcinogenic, or endocrine-disrupting nature of some CECs, raises significant concerns over their potential risks to the environment and human health. Understanding such risks and delineating the fate processes of CECs in the water-soil-plant continuum contributes to the safe reuse of RW in agriculture. This review summarizes recent findings and provides an overview of CECs in the water-soil-plant continuum, including their occurrence in RW and irrigated soil, fate processes in agricultural soil, offsite transport including runoff and leaching, and plant uptake, metabolism, and accumulation. The potential ecological and human health risks of CECs are also discussed. Studies to date have shown limited accumulation of CECs in irrigated soils and plants, which may be attributed to multiple attenuation processes in the rhizosphere and plant, suggesting minimal health risks from RW-fed food crops. However, our collective understanding of CECs is rather limited and knowledge of their offsite movement and plant accumulation is particularly scarce for field conditions. Given a large number of CECs and their occurrence at trace levels, it is urgent to develop strategies to prioritize CECs so that future research efforts are focused on CECs with elevated risks for offsite contamination or plant accumulation. Irrigating specific crops such as feed crops and fruit trees may be a viable option to further minimize potential plant accumulation under field conditions. To promote the beneficial reuse of RW in agriculture, it is essential to understand the human health and ecological risks imposed by CEC mixtures and metabolites.

Conjugation of Di-n-butyl Phthalate Metabolites in Arabidopsis thaliana and Potential Deconjugation in Human Microsomes.

Plasticizers, due to the widespread use of plastics, occur ubiquitously in the environment. The reuse of waste resources (e.g., treated wastewater, biosolids, animal waste) and other practices (e.g., plastic mulching) introduce phthalates into agroecosystems. As a detoxification mechanism, plants are known to convert phthalates to polar monophthalates after uptake, which are followed by further transformations, including conjugation with endogenous biomolecules. The objective of this study was 2-fold: to obtain a complete metabolic picture of the widely used di-n-butyl phthalate (DnBP) by using a suite of complementary techniques, including stable isotope labeling, 14C tracing, and high-resolution mass spectrometry, and to determine if conjugates are deconjugated in human microsomes to release bioactive metabolites. In Arabidopsis thaliana cells, the primary initial metabolite of DnBP was mono-n-butyl phthalate (MnBP), and MnBP was rapidly metabolized via hydroxylation, carboxylation, glycosylation, and malonylation to seven transformation products. One of the conjugates, MnBP-acyl-ß-d-glucoside (MnBP-Glu), was incubated in human liver (HLM) and intestinal (HIM) microsomes and was found to undergo rapid transformations. Approximately 15% and 10% of MnBP-Glu were deconjugated to the free form MnBP in HIM and HLM, respectively. These findings highlight that phthalates, as diesters, are susceptible to hydrolysis to form monoesters that can be readily conjugated via a phase II metabolism in plants. Conjugates may be deconjugated to release bioactive compounds after human ingestion. Therefore, an accurate assessment of the dietary exposure of phthalates and other contaminants must consider plant metabolites, especially including conjugates, to better predict their potential environmental and human health risks.

Metabolism of mono-(2-ethylhexyl) phthalate in Arabidopsis thaliana: Exploration of metabolic pathways by deuterium labeling.

Mono-(2-ethylhexyl) phthalate (MEHP) is the primary monoester transformation product of the commonly used plasticizer, di-2-ethylhexyl phthalate (DEHP), and has been frequently detected in various environmental compartments (e.g., soil, biosolids, plants). Plants growing in contaminated soils can take up MEHP, and consumption of the contaminated plants may result in unintended exposure for humans and other organisms. The metabolism of MEHP in plants is poorly understood, but critical for evaluating the potential human and environmental health risks. The present study represents the first attempt to explore the metabolic fate of MEHP in plants. We used Arabidopsis thaliana cells as a plant model and explored metabolic pathways of MEHP using deuterium stable isotope labelling (SIL) coupled with time-of-flight high resolution mass spectrometer (TOF-HRMS). A. thaliana rapidly took up MEHP from the culture medium and mediated extensive metabolism of MEHP. Combining SIL with TOF-HRMS analysis was proved as a powerful method for identification of unknown MEHP metabolites. Four phase Ⅰ and three phase Ⅱ metabolites were confirmed or tentatively identified. Based on the detected transformation products, hydroxylation, oxidation, and malonylation are proposed as the potential MEHP metabolism pathways. In cells, the maximum fraction of each transformation product accounted for 2.8-56.5% of the total amount of metabolites during the incubation. For individual metabolites, up to 2.9-100% was found in the culture medium, suggesting plant excretion. The results in the cell culture experiments were further confirmed in cabbage and A. thaliana seedlings. The findings suggest active metabolism of MEHP in plants and highlight the need to include metabolites in refining environmental risk assessment of plasticizers in the agro-food systems.