Metabolism of [14C]-2,4-dichlorophenol in edible plants

Four-Chlorophenoxyacetic Acid Treatment Induces the Defense Resistance of Rice to White-Backed Planthopper Sogatella furcifera

Chemical elicitors can increase plant defense against herbivorous insects and pathogens. The use of synthetic chemical elicitors is likely to be an alternative to traditional pesticides for crop pest control. However, only a few synthetic chemicals are reported to protect plants by regulating signaling pathways, increasing the levels of defense metabolites and interfering with insect feeding. Here, we found that the exogenous application of a phenoxycarboxylic compound, 4-chlorophenoxyacetic acid (4-CPA), can induce chemical defenses to protect rice plants from white-backed planthoppers (WBPH, Sogatella furcifera). Four-CPA was rapidly taken up by plant roots and degraded to 4-chlorophenol (4-CP). Four-CPA treatment modulated the activity of peroxidase (POD) and directly induced the deposition of lignin-like polymers using hydrogen peroxide (H2O2) as the electron acceptor. The polymers, which are thought to prevent the planthopper's stylet from reaching the phloem, were broken down by WBPH nymphs. Meanwhile, 4-CPA increased the levels of flavonoids and phenolamines (PAs). The increased flavonoids and PAs, together with the degradation product of the polymers, avoided nymphal feeding and prolonged the nymphal period for 1 day. These results indicate that 4-CPA has the potential to be used as a chemical elicitor to protect rice from planthoppers. Moreover, these findings also open a pathway for molecule structure design of phenoxycarboxylic compounds as chemical elicitors.

Biokinetic Analysis and Metabolic Fate of 2,4-D in 2,4-D-Resistant Soybean (Glycine max)

The Enlist weed control system allows the use of 2,4-D in soybean but slight necrosis in treated leaves may be observed in the field. The objectives of this research were to measure and compare uptake, translocation, and metabolism of 2,4-D in Enlist (E, resistant) and non-AAD-12 transformed (NT, sensitive) soybeans. The adjuvant from the Enlist Duo herbicide formulation (ADJ) increased 2,4-D uptake (36%) and displayed the fastest rate of uptake (U50= 0.2 h) among treatments. E soybean demonstrated a faster rate of 2,4-D metabolism (M50= 0.2 h) compared to NT soybean, but glyphosate did not affect 2,4-D metabolism. Metabolites of 2,4-D in E soybean were qualitatively different than NT. Applying 2,4-D-ethylhexyl ester instead of 2,4-D choline (a quaternary ammonium salt) eliminated visual injury to E soybean, likely due to the time required for initial de-esterification and bioactivation. Excessive 2,4-D acid concentrations in E soybean resulting from ADJ-increased uptake may significantly contribute to foliar injury.

Metabolism and Residues of 2,4-Dichlorophenoxyacetic Acid in DAS-40278-9 Maize (Zea mays) Transformed with Aryloxyalkanoate Dioxygenase-1 Gene

DAS-40278-9 maize, which is developed by Dow AgroSciences, has been genetically modified to express the aryloxyalkanoate dioxygenase-1 (AAD-1) protein and is tolerant to phenoxy auxin herbicides, such as 2,4-dichlorophenoxyacetic acid (2,4-D). To understand the metabolic route and residue distribution of 2,4-D in DAS-40278-9 maize, a metabolism study was conducted with ¹⁴C-radiolabeled 2,4-D applied at the maximum seasonal rate. Plants were grown in boxes outdoors. Forage and mature grain, cobs, and stover were collected for analysis. The metabolism study showed that 2,4-D was metabolized to 2,4-dichlorophenol (2,4-DCP), which was then rapidly conjugated with glucose. Field-scale residue studies with 2,4-D applied at the maximum seasonal rate were conducted at 25 sites in the U.S. and Canada to measure the residues of 2,4-D and free and conjugated 2,4-DCP in mature forage, grain, and stover. Residues of 2,4-D were not detectable in the majority of the grain samples and averaged <1.0 and <1.5 μg/g in forage and stover, respectively. Free plus conjugated 2,4-DCP was not observed in grain and averaged <1.0 μg/g in forage and stover.

"Rational" management of dichlorophenols biodegradation by the microalga Scenedesmus obliquus

The microalga Scenedesmus obliquus exhibited the ability to biodegrade dichlorophenols (dcps) under specific autotrophic and mixotrophic conditions. According to their biodegradability, the dichlorophenols used can be separated into three distinct groups. Group I (2,4-dcp and 2,6 dcp – no meta-substitution) consisted of quite easily degraded dichlorophenols, since both chloride substituents are in less energetically demanding positions. Group II (2,3-dcp, 2,5-dcp and 3,4-dcp – one meta-chloride) was less susceptible to biodegradation, since one of the two substituents, the meta one, required higher energy for C-Cl-bond cleavage. Group III (3,5-dcp – two meta-chlorides) could not be biodegraded, since both chlorides possessed the most energy demanding positions. In general, when the dcp-toxicity exceeded a certain threshold, the microalga increased the energy offered for biodegradation and decreased the energy invested for biomass production. As a result, the biodegradation per cell volume of group II (higher toxicity) was higher, than group I (lower toxicity) and the biodegradation of dichlorophenols (higher toxicity) was higher than the corresponding monochlorophenols (lower toxicity). The participation of the photosynthetic apparatus and the respiratory mechanism of microalga to biodegrade the group I and the group II, highlighted different bioenergetic strategies for optimal management of the balance between dcp-toxicity, dcp-biodegradability and culture growth. Additionally, we took into consideration the possibility that the intermediates of each dcp-biodegradation pathway could influence differently the whole biodegradation procedures. For this reason, we tested all possible combinations of phenolic intermediates to check cometabolic interactions. The present contribution bring out the possibility of microalgae to operate as “smart” bioenergetic “machines”, that have the ability to continuously “calculate” the energy reserves and “use” the most energetically advantageous dcp-biodegradation strategy. We tried to manipulate the above fact, changing the energy reserves and as a result the chosen strategy, in order to take advantage of their abilities in detoxifying the environment.

Metabolic fate of 2,4-dichlorophenol and related plant residues in rats

This study compared the metabolic fate of [(14)C]-DCP, [(14)C]-residues from radish plants, and purified [(14)C]-DCP-(acetyl)glucose following oral administration in rats. A rapid excretion of radioactivity in urine occurred for [(14)C]-DCP, [(14)C]-DCP-(acetyl)glucose, and soluble residues, 69, 85, and 69% within 48 h, respectively. Radio-HPLC profiles of 0-24 h urine from rats fed [(14)C]-DCP and [(14)C]-DCP-(acetyl)glucose were close and qualitatively similar to those obtained from plant residues. No trace of native plant residues was detected under the study conditions. The structures of the two major peaks were identified by MS as the glucuronide and the sulfate conjugates of DCP. The characterization of a dehydrated glucuronide conjugate by MS and NMR of DCP was unusual. In contrast to soluble residues, bound residues were mainly excreted in feces, 90% within 48 h, whereas total residues were eliminated in both urine and feces. For total residues, the radioactivity in feces was higher than expected from the percentage of soluble and bound residues in radish plants. This result highlighted that less absorption took place when residues were present in the plant matrix as compared to plant-free residues and DCP.

Phytoremediation of chlorpyrifos by Populus and Salix

Chlorpyrifos is one of the commonly used organophosphorus insecticides that are implicated in serious environmental and human health problems. To evaluate plant potential for uptake of chlorpyrifos, several plant species of poplar (Populus sp.) and willow (Salix sp.) were investigated. Chlorpyrifos was taken up from nutrient solution by all seven plant species. Significant amounts of chlorpyrifos accumulated in plant tissues, and roots accumulated higher concentrations of chlorpyrifos than did shoots. Chlorpyrifos did not persist in the plant tissues, suggesting further metabolism of chlorpyrifos in plant tissue. To our knowledge, this work represents the first report for phytoremediation of chlorpyrifos using poplar and willow plants.

Metabolic fate of [14C] chlorophenols in radish (Raphanus sativus), lettuce (Lactuca sativa), and spinach (Spinacia oleracea)

Chlorophenols are potentially harmful pollutants that are found in numerous natural and agricultural systems. Plants are a sink for xenobiotics, which occur either intentionally or not, as they are unable to eliminate them although they generally metabolize them into less toxic compounds. The metabolic fate of [ (14)C] 4-chlorophenol (4-CP), [ (14)C] 2,4-dichlorophenol (2,4-DCP), and [ (14)C] 2,4,5-trichlorophenol (2,4,5-TCP) was investigated in lettuce, spinach, and radish to locate putative toxic metabolites that could become bioavailable to food chains. Radish plants were grown on sand for four weeks before roots were dipped in a solution of radiolabeled chlorophenol. The leaves of six-week old lettuce and spinach were treated. Three weeks after treatments, metabolites from edible plant parts were extracted and analyzed by high performance liquid chromatography (HPLC) and characterized by mass spectrometry (MS), and nuclear magnetic resonance spectroscopy (NMR). Characterization of compounds highlighted the presence of complex glycosides. Upon hydrolysis in the digestive tract of animals or humans, these conjugates could return to the toxic parent compound, and this should be kept in mind for registration studies.

Metabolic fate of [(14)C]-2,4-dichlorophenol in tobacco cell suspension cultures

In plant tissues, xenobiotics often are conjugated with natural constituents such as sugars, amino acids, glutathione, and malonic acid. Usually, conjugation processes result in a decrease in the reactivity and toxicity of xenobiotics by increasing the water solubility and polarity of conjugates, and reducing their mobility. Due to their lack of an efficient excretory system, xenobiotic conjugates finally are sequestered in plant storage compartments or cell vacuoles, or are integrated as bound residues in cell walls. Chlorophenols are potentially harmful pollutants that are found in numerous natural and agricultural systems. We studied the metabolic fate of 2,4-dichlorophenol (DCP) in cell-suspension cultures of tobacco (Nicotiana tabacum L.). After a standard metabolism experiment, 48 h of incubation with a [U-phenyl-(14)C]-DCP solution, aqueous extracts of cell suspension cultures were analyzed by high-performance liquid chromatography (HPLC). Metabolites then were isolated and their chemical structures determined by enzymatic and chemical hydrolyses, electrospray ionization-mass spectrometry in negative mode (ESI-NI), and (1)H nuclear magnetic resonance analyses. The main terminal metabolites identified were DCP-glycoside conjugates, DCP-(6-O-malonyl)-glucoside, DCP-(6-O-acetyl)-glucoside, and their precursor, DCP-glucoside. More unusual and complex DCP conjugates such as an alpha(1-->6)-glucosyl-pentose and a triglycoside containing a glucuronic acid were further characterized. All the metabolites identified were complex glycoside conjugates. However, these conjugates still may be a source of DCP in hydrolysis reactions caused by microorganisms in the environment or in the digestive tract of animals and humans. Removal of xenobiotics by glycoside conjugation thus may result in underestimation of the risk associated with toxic compounds like DCP in the environment or in the food chain.