Activity of mesotrione on resistant weeds in maize

Design, Synthesis, and Biological Activity of Novel Triketone-Containing Phenoxy Nicotinyl Inhibitors of HPPD

4-Hydroxyphenylpyruvate dioxygenase (HPPD) is a crucial target enzyme in albino herbicides. The inhibition of HPPD activity interferes with the synthesis of carotenoids, blocking photosynthesis and resulting in bleaching and necrosis. To develop herbicides with excellent activity, a series of 3-hydroxy-2-(6-substituted phenoxynicotinoyl)-2-cyclohexen-1-one derivatives were designed via active substructure combination. The title compounds were characterized via infrared spectroscopy, 1H and 13C nuclear magnetic resonance spectroscopies, and high-resolution mass spectrometry. The structure of compound III-17 was confirmed via single-crystal X-ray diffraction. Preliminary tests demonstrated that some compounds had good herbicidal activity. Crop safety tests revealed that compound III-29 was safer than the commercial herbicide mesotrione in wheat and peanuts. Moreover, the compound exhibited the highest inhibitory activity against Arabidopsis thaliana HPPD (AtHPPD), with a half-maximal inhibitory concentration of 0.19 μM, demonstrating superior activity compared with mesotrione (0.28 μM) in vitro. A three-dimensional quantitative structure-activity relationship study revealed that the introduction of smaller groups to the 5-position of cyclohexanedione and negative charges to the 3-position of the benzene ring enhanced the herbicidal activity. A molecular structure comparison demonstrated that compound III-29 was beneficial to plant absorption and conduction. Molecular docking and molecular dynamics simulations further verified the stability of the complex formed by compound III-29 and AtHPPD. Thus, this study may provide insights into the development of green and efficient herbicides.

Physiological effects of maize stressed by HPPD inhibitor herbicides via multi-spectral technology and two-dimensional correlation spectrum technology

Understanding the physiological effects of herbicides on crops is crucial for crop production and environmental management. The effects of 4-hydroxyphenylpyruvate dioxygenase inhibitor (HPPDi) herbicides at different concentrations on chlorophyll content in maize leaves, fresh weight of roots, stems and leaves, and fluorescence substances and functional groups in root exudates (REs) were studied by UV-Vis absorption spectroscopy, fluorescence spectroscopy, Fourier transform infrared spectroscopy (FTIR) and two-dimensional correlation analysis (2D-COS). The results showed that 5 mg/L and 10 mg/L HPPDi herbicides inhibited the synthesis of chlorophyll in maize leaves. The weight of roots, stems and leaves of maize after application was lighter than that of the control group. HPPDi herbicides affected the early growth of maize seedlings, and the effect was most obvious at high concentration. Synchronous fluorescence spectrum and three-dimensional (3D) fluorescence spectrum revealed that the fluorescence intensity of protein, fulvic acid and humic acid in maize REs changed prominently. With the increase of HPPDi herbicides concentration, the fluorescence intensity decreased gradually. Through FTIR and 2D-COS, functional groups such as C-H, CO, Cl, NO3-, C-O and O-H were found to participate in the interaction between HPPDi herbicides and maize REs as binding sites. C-O, C-Cl and C-C have the strongest binding ability, while CC and CO of aromatic rings, quinones or ketones first take part in the binding between HPPDi herbicides and maize REs. The results can provide a theoretical basis for evaluating the safety of HPPDi herbicides on maize and a method for discovering the effects of pesticides on environmental media and plant physiological effects.

Sense and Shoot: Unveiling the Electro-/Photocatalytic Potential of 2D White Graphene-Supported Perovskite Strontium Cobaltite from Detection to Remediation of Oxidative Stress Herbicide (Mesotrione)

In this research, we employed a strategy akin to "Feeding Two Birds with One Stone" aiming for the dual objectives of highly selective electrochemical detection and photocatalytic degradation of the environmentally hazardous herbicide mesotrione (MTN). We achieved this by utilizing hexagonal boron nitride (BN)-supported strontium cobaltite perovskite nanocomposites (SrCoO3/BN). The fabrication of the innovative bifunctional SrCoO3/BN nanocomposites involved a straightforward process of precipitation, followed by an annealing treatment and ultrasonication. The successful formation of these nanocomposites was corroborated through the application of diverse spectroscopic tools. Notably, as-prepared SrCoO3/BN nanocomposites exhibited a remarkable sensing platform for MTN, characterized by a notably low detection limit (11 nm), considerable sensitivity (3.782 μA μM-1 cm-2), and outstanding selectivity, alongside remarkable stability. Concurrently, these SrCoO3/BN nanocomposites demonstrated exceptional visible-light-driven photocatalytic efficacy for MTN degradation (99%) and complete mineralization. Our investigation systematically delved into the influence of operational parameters, including catalyst loading and the involvement of reactive oxidative species, in both the electrocatalytic and photocatalytic reactions. Drawing from these comprehensive studies, we have proposed plausible mechanisms for detecting and degrading MTN. Our findings pave the way for catalyst development, offering a unified solution for detecting and eliminating toxic organic compounds from the environment.

First Report on Resistance to HPPD Herbicides Mediated by Nontarget-Site Mechanisms in the Grass Leptochloa chinensis

The emergence of 4-hydroxyphenylpyruvate dioxygenase (HPPD) herbicides as efficacious target-site herbicides has been noteworthy. In recent years, only four species of broadleaf weeds have developed resistance due to the long-term widespread use of HPPD herbicides. This study represents the first reported instance of a grass weed exhibiting resistance to HPPD inhibitors. We identified a new HPPD-resistant Chinese sprangletop [Leptochloa chinensis (L.) Nees] population (R population). At the recommended dose of tripyrasulfone, the inhibition rate of the R population was only half that of the sensitive population (S). The mechanism underlying resistance does not involve target-site resistance triggered by amino acid mutations or depend on disparities within the HPPD INHIBITOR SENSITIVE 1 (HIS1) gene. The impetus for resistance appears to be interlinked with the metabolic activities of cytochrome P450 monooxygenase (P450) and glutathione S-transferase (GST) family genes. Following RNA sequencing (RNA-seq) and quantitative real-time PCR (qRT-PCR) validation, the study suggests that five P450 genes, CYP71C1, CYP74A2, CYP72A1, CYP84A1, and CYP714C2, alongside a single GST gene GSTF1, may be implicated in the process of metabolic detoxification.

Identification and Characterization of Mesotrione-Resistant Grain Sorghum [Sorghum bicolor (L.) Moench]: A Viable Option for Postemergence Grass Weed Control

Mesotrione is effective in controlling a wide spectrum of weeds in corn but not registered for postemergence use in sorghum because of crop injury. We screened a sorghum germplasm collection and identified two mesotrione-resistant sorghum genotypes (G-1 and G-10) and one susceptible genotype (S-1) in an in vitro plate assay. A mesotrione dose-response assay under greenhouse and field conditions confirmed that G-1 and G-10 are highly resistant compared to S-1. We found enhanced metabolism of mesotrione in G-1 and G-10 using HPLC assay, and a significant reduction in biomass accumulation was found in G-1 and G-10 plants pretreated with cytochrome P450 (CYP)-inhibitors malathion or piperonyl butoxide, indicating the involvement of CYPs in the metabolism of mesotrione. Genetic analyses using F₁ and F₂ progenies generated by crossing G-1 and G-10 separately with S-1 revealed that mesotrione resistance in sorghum is controlled by a single dominant gene along with several genes with minor effects.

Essential oil of Piper purusanum C.DC (Piperaceae) and its main sesquiterpenes: biodefensives against malaria and dengue vectors, without lethal effect on non-target aquatic fauna

The mosquito vectors of the genera Aedes and Anopheles present resistance to several commercial insecticides, which are also toxic to non-predator targets. On the other hand, essential oils are a promising source of insecticides. Thus, in this work, the essential oil from the leaves of Piper purusanum was characterized by gas chromatography-based approaches and evaluated as biodefensive against malaria and dengue vectors. The main compounds of P. purusanum essential oil were β-caryophyllene (57.05%), α-humulene (14.50%), and germacrene D (8.20%). The essential oil inhibited egg hatching (7.6 ± 1.5 to 95.6 ± 4.5%), caused larval death (LC₅₀ from 49.84 to 51.60 ppm), and inhibited the action of acetylcholinesterase (IC₅₀ of 2.29 µg/mL), which can be related to the mechanisms of action. On the other hand, the biological activities of β-caryophyllene, α-humulene, and germacrene D were higher than that of essential oil. In addition, these sesquiterpenes and essential oil did not show a lethal effect on Toxorhynchites splendens, Anisops bouvieri, Gambusia affinis, and Diplonychus indicus (LC₅₀ from 2098.80 to 7707.13 ppm), although D. indicus is more sensitive (SI/PSF from 48.56 to 252.02 ppm) to essential oil, representing a natural alternative against these relevant vectors.

Comparison of the binding interactions of 4-hydroxyphenylpyruvate dioxygenase inhibitor herbicides with humic acid: Insights from multispectroscopic techniques, DFT and 2D-COS-FTIR

4-Hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor is one of the important herbicides to solve the problem of weed control. With the widespread and continued use of HPPD inhibitor (HPPDi) herbicides, it may inevitably put pressure on the environment. Humic acid (HA) can effectively interact with pesticides through sorption or covalent bond formation and promote the degradation of pesticides, which can reduce the risk of pesticides in the environment. In the present study, the interactions of four HPPDi herbicides (sulcotrione, tembotrione, topramezone and mesotrione) with HA were reported and comparative assessment of the binding using multispectral technology, density functional theory (DFT) calculation and two-dimensional correlation spectroscopy (2D-COS). Time-resolved measurements and the Stern-Volmer constant at different temperature verified that HPPDi can bind with HA through the static quenching mechanism. From the thermodynamic parameters, the interaction force between HA and sulcotrione, tembotrione, topramezone and mesotrione was provided by electrostatic force. DFT, binding constant and three-dimensional (3D) fluorescence peak variation all indicated that the order of the binding ability of the four HPPDi and HA was mesotrione > tembotrione > sulcotrione > topramezone. According to dynamic light scattering (DLS), pH 7 is most conducive to the formation of HA-HPPDi complexes. Fourier transform infrared spectroscopy (FTIR) and 2D-COS showed that HA combined with HPPDi through aromatic C-H, CO and C-X, and the first binding group to HA was almost all CO. Sulcotrione, tembotrione, topramezone and mesotrione quench the endogenous fluorescence of HA by a static quenching mechanism and bind to HA through electrostatic interaction to form a complex. These results provide important insights into the combination of environmental pollutants with HA.

Charge-transfer complex-based spectrophotometric method for the determination of mesotrione in environmental samples

A simple, accurate, sensitive, and selective spectrophotometric method has been developed for the determination of mesotrione. This method is based on the reaction of mesotrione with Fe(III) to form a charge transfer metal complex having λ_max at 348 nm. Beer's law was obeyed in the concentration range of 0.2-10.0 µg mL^-1 with limit of detection (LOD) and limit of quantification (LOQ) equal to 0.053 and 0.162 µg mL^-1, respectively. The percent recovery of mesotrione from different environmental and agricultural samples was found to be 95.00-106.50% at various levels. Notably, the developed method was successfully employed for the determination of mesotrione in environmental (pond water, canal water, tap water, and soil) and agricultural (maize grains) samples.

Recurrent Selection with Sub-Lethal Doses of Mesotrione Reduces Sensitivity in Amaranthus palmeri

Amaranthus palmeri, ranked as the most prolific and troublesome weed in North America, has evolved resistance to several herbicide sites of action. Repeated use of any one herbicide, especially at lower than recommended doses, can lead to evolution of weed resistance, and, therefore, a better understanding of the process of resistance evolution is essential for the management of A. palmeri and other difficult-to-control weed species. Amaranthus palmeri rapidly developed resistance to 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors such as mesotrione. The objective of this study was to test the potential for low-dose applications of mesotrione to select for reduced susceptibility over multiple generations in an A. palmeri population collected from an agricultural field in 2001. F₀ plants from the population were initially treated with sub-lethal mesotrione rates and evaluated for survival three weeks after treatment. All F₀ plants were controlled at the 1× rate (x = 105 g ai ha⁻¹). However, 2.5% of the F₀ plants survived the 0.5× treatment. The recurrent selection process using plants surviving various mesotrione rates was continued until the F₄ generation was reached. Based on the GR₅₀ values, the sensitivity index was determined to be 1.7 for the F₄ generation. Compared to F₀, HPPD gene expression level in the F₃ population increased. Results indicate that after several rounds of recurrent selection, the successive generations of A. palmeri became less responsive to mesotrione, which may explain the reduced sensitivity of this weed to HPPD-inhibiting herbicides. The results have significance in light of the recently released soybean and soon to be released cotton varieties with resistance to HPPD inhibitors.

Identification of key residues determining the binding specificity of human 4-hydroxyphenylpyruvate dioxygenase

4-Hydroxyphenylpyruvate dioxygenase (HPPD, EC 1.13.11.27) is the second enzyme of the tyrosine catabolic pathway. Its physiological function is to catalyze the conversion of 4-hydroxyphenylpyruvic acid to homogentisic acid, which displays different physiological effects in mammals and plants. Insights on the selective inhibition of human HPPD (hHPPD) by triketone inhibitors were furnished by the integrated application of molecular simulation and biological testing. The binding free energy of hHPPD and inhibitors was obtained through molecular dynamics (MD) simulations, and the result was in agreement with the inhibition experiment in vitro. The binding free energy contribution demonstrated that the formation of hHPPD-inhibitor complexes was mainly driven by van der Waals energy. Ser226, Asn241, Gln265, Phe336, Phe359 and Phe364 made great contributions to binding affinities of all the systems. Among the residues involved in the interaction between nitisinone (NTBC) and hHPPD, Tyr221 and Leu224, whose mutation into Ala caused significant decrease of NTBC binding ability, were two key residues in determining the selective binding affinity of inhibitor and hHPPD. This work provides valuable theoretical basis for rational design of highly selective inhibitors targeting hHPPD.

The response of Chenopodium album L. and Abutilon theophrasti Medik. to reduced doses of mesotrione

The application of minimal doses of herbicides is very popular due to concerns about the negative impacts of herbicides on the environment and public health. Studies were conducted to estimate the possibility of using quick and non- destructive methods to investigate Chenopodium album L. and Abutilon theophrasti Medik. response to mesotrione. The studies were conducted in a controlled environment to determine the response of C. album and A. theophrasti to mesotrione using dose-response curves created based on plant dry weight, chlorophyll fluorescence parameters and chlorophyll content. The obtained effective dose values showed that the studied weeds were susceptible to reduced doses of mesotrione. ED₉₅ values estimated for both species for dry weight and chlorophyll fluorescence parameters were lower than the recommended dose rate (120 g a.i. ha^-1), with less than 85 g a.i. ha^-1 needed to achieve a reduction of 95%, compared with untreated plants, while ED₉₅ value (A. theophrasti: 182 g a.i. ha^-1 and C. album: 180 g a.i. ha^-1) for chlorophyll content for both species was above the recommended dose rates. Consequently, dry weight and the chlorophyll fluorescence parameters are suitable for estimating the plant response to mesotrione, while chlorophyll content is not.

Combination of Virtual Screening Protocol by in Silico toward the Discovery of Novel 4-Hydroxyphenylpyruvate Dioxygenase Inhibitors

4-Hydroxyphenylpyruvate dioxygenase (EC 1.13.11.27, HPPD) is a potent new bleaching herbicide target. Therefore, in silico structure-based virtual screening was performed in order to speed up the identification of promising HPPD inhibitors. In this study, an integrated virtual screening protocol by combining 3D-pharmacophore model, molecular docking and molecular dynamics (MD) simulation was established to find novel HPPD inhibitors from four commercial databases. 3D-pharmacophore Hypo1 model was applied to efficiently narrow potential hits. The hit compounds were subsequently submitted to molecular docking studies, showing four compounds as potent inhibitor with the mechanism of the Fe(II) coordination and interaction with Phe360, Phe403, and Phe398. MD result demonstrated that nonpolar term of compound 3881 made great contributions to binding affinities. It showed an IC₅₀ being 2.49 μM against AtHPPD in vitro. The results provided useful information for developing novel HPPD inhibitors, leading to further understanding of the interaction mechanism of HPPD inhibitors.

Inheritance of Mesotrione Resistance in an Amaranthus tuberculatus (var. rudis) Population from Nebraska, USA

A population of Amaranthus tuberculatus (var. rudis) evolved resistance to 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor herbicides (mesotrione, tembotrione, and topramezone) in Nebraska. The level of resistance was the highest to mesotrione, and the mechanism of resistance in this population is metabolism-based likely via cytochrome P450 enzymes. The increasing number of weeds resistant to herbicides warrants studies on the ecology and evolutionary factors contributing for resistance evolution, including inheritance of resistance traits. In this study, we investigated the genetic control of mesotrione resistance in an A. tuberculatus population from Nebraska, USA. Results showed that reciprocal crosses in the F1 families exhibited nuclear inheritance, which allows pollen movement carrying herbicide resistance alleles. The mode of inheritance varied from incomplete recessive to incomplete dominance depending upon the F1 family. Observed segregation patterns for the majority of the F2 and back-cross susceptible (BC/S) families did not fit to a single major gene model. Therefore, multiple genes are likely to confer metabolism-based mesotrione resistance in this A. tuberculatus population from Nebraska. The results of this study aid to understand the genetics and inheritance of a non-target-site based mesotrione resistant A. tuberculatus population from Nebraska, USA.

3D Pharmacophore-Based Virtual Screening and Docking Approaches toward the Discovery of Novel HPPD Inhibitors

p-Hydroxyphenylpyruvate dioxygenase (HPPD) is not only the useful molecular target in treating life-threatening tyrosinemia type I, but also an important target for chemical herbicides. A combined in silico structure-based pharmacophore and molecular docking-based virtual screening were performed to identify novel potential HPPD inhibitors. The complex-based pharmacophore model (CBP) with 0.721 of ROC used for screening compounds showed remarkable ability to retrieve known active ligands from among decoy molecules. The ChemDiv database was screened using CBP-Hypo2 as a 3D query, and the best-fit hits subjected to molecular docking with two methods of LibDock and CDOCKER in Accelrys Discovery Studio 2.5 (DS 2.5) to discern interactions with key residues at the active site of HPPD. Four compounds with top rankings in the HipHop model and well-known binding model were finally chosen as lead compounds with potential inhibitory effects on the active site of target. The results provided powerful insight into the development of novel HPPD inhibitors herbicides using computational techniques.

Physiological and Molecular Characterization of Hydroxyphenylpyruvate Dioxygenase (HPPD)-inhibitor Resistance in Palmer Amaranth (Amaranthus palmeri S.Wats.)

Herbicides that inhibit hydroxyphenylpyruvate dioxygenase (HPPD) such as mesotrione are widely used to control a broad spectrum of weeds in agriculture. Amaranthus palmeri is an economically troublesome weed throughout the United States. The first case of evolution of resistance to HPPD-inhibiting herbicides in A. palmeri was documented in Kansas (KS) and later in Nebraska (NE). The objective of this study was to investigate the mechansim of HPPD-inhibitor (mesotrione) resistance in A. palmeri. Dose response analysis revealed that this population (KSR) was 10-18 times more resistant than their sensitive counterparts (MSS or KSS). Absorbtion and translocation analysis of [¹⁴C] mesotrione suggested that these mechanisms were not involved in the resistance in A. palmeri. Importantly, mesotrione (>90%) was detoxified markedly faster in the resistant populations (KSR and NER), within 24 hours after treatment (HAT) compared to sensitive plants (MSS, KSS, or NER). However, at 48 HAT all populations metabolized the mesotrione, suggesting additional factors may contribute to this resistance. Further evaluation of mesotrione-resistant A. palmeri did not reveal any specific resistance-conferring mutations nor amplification of HPPD gene, the molecular target of mesotrione. However, the resistant populations showed 4- to 12-fold increase in HPPD gene expression. This increase in HPPD transcript levels was accompanied by increased HPPD protein expression. The significant aspects of this research include: the mesotrione resistance in A. palmeri is conferred primarily by rapid detoxification (non-target-site based) of mesotrione; additionally, increased HPPD gene expression (target-site based) also contributes to the resistance mechanism in the evolution of herbicide resistance in this naturally occurring weed species.

Fate and ecotoxicological impact of new generation herbicides from the triketone family: An overview to assess the environmental risks

Triketones, derived chemically from a natural phytotoxin (leptospermone), are a good example of allelochemicals as lead molecules for the development of new herbicides. Targeting a new and key enzyme involved in carotenoid biosynthesis, these latest-generation herbicides (sulcotrione, mesotrione and tembotrione) were designed to be eco-friendly and commercialized fifteen-twenty years ago. The mechanisms controlling their fate in different ecological niches as well as their toxicity and impact on different organisms or ecosystems are still under investigation. This review combines an overview of the results published in the literature on β-triketones and more specifically, on the commercially-available herbicides and includes new results obtained in our interdisciplinary study aiming to understand all the processes involved (i) in their transfer from the soil to the connected aquatic compartments, (ii) in their transformation by photochemical and biological mechanisms but also to evaluate (iii) the impacts of the parent molecules and their transformation products on various target and non-target organisms (aquatic microorganisms, plants, soil microbial communities). Analysis of all the data on the fate and impact of these molecules, used pure, as formulation or in cocktails, give an overall guide for the assessment of their environmental risks.

Monopersulfate photocatalysis under 365 nm radiation. Direct oxidation and monopersulfate promoted photocatalysis of the herbicide tembotrione

Oxone(®) (potassium monopersulfate, MPS) has been used to oxidize the herbicide tembotrione in aqueous solution. Tembotrione elimination kinetics by MPS direct oxidation has been studied. The influence of the main operating variables affecting the process (MPS concentration, temperature and pH) has been evaluated. The process follows 2/3 and first orders in MPS and tembotrione concentrations, respectively. Optimal pH is located around circumneutral conditions. MPS decomposition in the presence of 365 nm UVA radiation and titanium dioxide has also been studied. A kinetic mechanism that simulates MPS decomposition has been proposed, showing the positive effect of titania load and MPS concentration. The system MPS/UVA/TiO2 significantly improves tembotrione and mineralization rate abatement if compared to runs conducted in the absence of MPS. Tembotrione total abatement was achieved in 20 min when 0.05 g L(-1) of titania and 10(-4) M of Oxone(®) were used. TOC conversion was roughly 70% in 90 min under similar operating conditions. An experimental design (Plackett-Burman) has been considered to study the influence of the main variables affecting tembotrione photocatalytic oxidation promoted by MPS.

Sugarcane bagasse as support for immobilization of Bacillus pumilus HZ-2 and its use in bioremediation of mesotrione-contaminated soils

The degrading microorganisms isolated from environment usually fail to degrade pollutants when used for bioremediation of contaminated soils; thus, additional treatments are needed to enhance biodegradation. In the present study, the potential of sugarcane bagasse as bacteria-immobilizing support was investigated in mesotrione biodegradation. A novel isolate Bacillus pumilus HZ-2 was applied in bacterial immobilization, which was capable of degrading over 95 % of mesotrione at initial concentrations ranging from 25 to 200 mg L(-1) within 4 days in flask-shaking tests. Scanning electron microscope (SEM) images showed that the bacterial cells were strongly absorbed and fully dispersed on bagasse surface after immobilization. Specially, 86.5 and 82.9 % of mesotrione was eliminated by bacteria immobilized on bagasse of 100 and 60 mesh, respectively, which indicated that this immobilization was able to maintain a high degrading activity of the bacteria. Analysis of the degradation products determined 2-amino-4-methylsulfonylbenzoic acid (AMBA) and 4-methylsulfonyl-2-nitrobenzoic acid (MNBA) as the main metabolites in the biodegradation pathway of mesotrione. In the sterile soil, approximately 90 % of mesotrione was degraded after supplementing 5.0 % of molasses in bacteria-bagasse composite, which greatly enhanced microbial adaptability and growth in the soil environment. In the field tests, over 75 % of mesotrione in soil was degraded within 14 days. The immobilized preparation demonstrated that mesotrione could be degraded at a wide range of pH values (5.0-8.0) and temperatures (25-35 °C), especially at low concentrations of mesotrione (5 to 20 mg kg(-1)). These results showed that sugarcane bagasse might be a good candidate as bacteria-immobilizing support to enhance mesotrione degradation by Bacillus p. HZ-2 in contaminated soils.

Physiological and Molecular Mechanisms of Differential Sensitivity of Palmer Amaranth (Amaranthus palmeri) to Mesotrione at Varying Growth Temperatures

Herbicide efficacy is known to be influenced by temperature, however, underlying mechanism(s) are poorly understood. A marked alteration in mesotrione [a 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor] efficacy on Palmer amaranth (Amaranthus palmeri S. Watson) was observed when grown under low- (LT, 25/15 °C, day/night temperatures) and high (HT, 40/30° C) temperature compared to optimum (OT, 32.5/22.5 °C) temperature. Based on plant height, injury, and mortality, Palmer amaranth was more sensitive to mesotrione at LT and less sensitive at HT compared to OT (ED50 for mortality; 18.5, 52.3, and 63.7 g ai ha-1, respectively). Similar responses were observed for leaf chlorophyll index and photochemical efficiency of PSII (Fv/Fm). Furthermore, mesotrione translocation and metabolism, and HPPD expression data strongly supported such variation. Relatively more mesotrione was translocated to meristematic regions at LT or OT than at HT. Based on T50 values (time required to metabolize 50% of the 14C mesotrione), plants at HT metabolized mesotrione faster than those at LT or OT (T50; 13, 21, and 16.5 h, respectively). The relative HPPD:CPS (carbamoyl phosphate synthetase) or HPPD:β-tubulin expression in mesotrione-treated plants increased over time in all temperature regimes; however, at 48 HAT, the HPPD:β-tubulin expression was exceedingly higher at HT compared to LT or OT (18.4-, 3.1-, and 3.5-fold relative to untreated plants, respectively). These findings together with an integrated understanding of other interacting key environmental factors will have important implications for a predictable approach for effective weed management.

4-Hydroxyphenylpyruvate dioxygenase and hydroxymandelate synthase: exemplars of the α-keto acid dependent oxygenases

4-Hydroxyphenylpyruvate dioxygenase (HPPD) and hydroxymandelate synthase (HMS) are outliers within the α-keto acid dependent oxygenase (αKAO) family. HPPD and HMS catalyze the chemistry of the majority of enzymes within the αKAO family but are clearly mechanistically convergent, having a grossly different structural topology. Some of the unique characteristics of HPPD and HMS have elucidated select parts of the catalytic cycle that are obscured in other family members. Moreover, the inhibitory chemistry of HPPD is a phenomenon with ever-expanding relevance across all kingdoms of life. This review is a synopsis of the literature pertaining to HPPD and HMS. It is not intended as an exhaustive compilation of all observations made for these enzymes but rather a condensed narrative that connects those studies that have advanced the understanding of the chemistry of both enzymes.

Photolysis of tembotrione and its main by-products under extreme artificial conditions: comparison with another β-triketone herbicide

The photolytic behaviour of tembotrione, a new chemical herbicide intended for foliar application in corn, was investigated under unnatural and extreme photochemical exposure in aqueous solutions in the laboratory. It appeared that degradation was dependent on pH and occurred more rapidly under acidic and neutral conditions, leading predominantly to the formation of a xanthenedione type compound by intramolecular cyclisation with loss of HCl. Trace amounts of benzoic acid by-products appeared also during UV-C irradiation (λ=254 nm) of the parent compound. Results were comparable to those obtained with sulcotrione, another β-triketone herbicide. These extreme irradiation conditions clearly accelerated the phototransformation of sulcotrione vs. simulated sunlight irradiation. Furthermore, the photolysis of the degradation by-products, resulting from either photolysis, hydrolysis or biotic pathways of the two active ingredients, was also carried out. The benzoic acid by-products appeared more stable to photolysis than their parent molecules. Xanthenedione derivatives were degraded more rapidly with several differences depending on the pH value.

Role of soil sorption and microbial degradation on dissipation of mesotrione in plant-available soil water

Mesotrione is a carotenoid biosynthesis-inhibiting herbicide labeled for pre-emergence and postemergence weed control in corn production. Understanding the factors that influence the dissipation of mesotrione in soil and in the plant-available water (PAW) is important for the environmental fate assessment and optimal weed management practices. The present research investigated the role of soil properties and microbial activities on the interrelated sorption and degradation processes of mesotrione in four soils by direct measurements of PAW. We found that mesotrione bound to the soils time dependently, with approximately 14 d to reach equilibrium. The 24-h batch-slurry equilibrium experiments provided the sorption partition coefficient ranging from 0.26 to 3.53 L kg(-1), depending on soil organic carbon and pH. The dissipation of mesotrione in the soil-bound phase was primarily attributed to desorption to the PAW. Degradation in the PAW was rapid and primarily dependent on microbial actions, with half-degradation time (DT(50)) <3 d in all four soils tested. The rapid degradation in the PAW became rate limited by sorption as more available molecules were depleted in the soil pore water, resulting in a more slowed overall process for the total soil-water system (DT(50) <26 d). The dissipation of mesotrione in the PAW was due to microbial metabolism and time-dependent sorption to the soils. A coupled kinetics model calibrated with the data from the laboratory centrifugation technique provided an effective approach to investigate the interrelated processes of sorption and degradation in realistic soil moisture conditions.

Resistance to HPPD-inhibiting herbicides in a population of waterhemp (Amaranthus tuberculatus) from Illinois, United States

BACKGROUND

A population of waterhemp in a seed maize production field in central Illinois, United States, was not adequately controlled after post-emergence applications of herbicides that inhibit 4-hydroxyphenylpyruvate dioxygenase (HPPD).

RESULTS

Progeny from the field population survived following treatment with mesotrione, tembotrione or topramezone applied to the foliage either alone or in combination with atrazine in greenhouse experiments. Dose-response experiments indicated that the level of resistance to the HPPD inhibitor mesotrione is at least tenfold relative to sensitive biotypes.

CONCLUSION

These studies confirm that waterhemp has evolved resistance to HPPD-inhibiting herbicides.

The Interaction of Hydroxymandelate Synthase with the 4-Hydroxyphenylpyruvate Dioxygenase Inhibitor: NTBC

Hydroxymandelate synthase (HMS) catalyzes the committed step in the formation of para-hydroxyphenylglycine, a recurrent substructure of polycyclic non-ribosomal peptide antibiotics such as vancomycin. HMS uses the same substrates as 4-hydroxyphenylpyruvate dioxygenase (HPPD), 4-hydroxyphenylpyruvate (HPP) and O(2), and also conducts a dioxygenation reaction. The difference between the two lies in the insertion of the second oxygen atom, HMS directing this atom onto the benzylic carbon of the substrate while HPPD hydroxylates the aromatic C1 carbon. We have shown that HMS will bind NTBC, a herbicide/therapeutic whose mode of action is based on the inhibition of HPPD. This occurs despite the difference in residues at the active site of HMS from those known to contact the inhibitor in HPPD. Moreover, the minimal kinetic mechanism for association of NTBC to HMS differs only slightly from that observed with HPPD. The primary difference is that three charge-transfer species are observed to accumulate during association. The first reversible complex forms with a weak dissociation constant of 520 microM, the subsequent two charge-transfer complexes form with rate constants of 2.7 s(-1) and 0.67 s(-1). As was the case for HPPD, the final complex has the most intense charge-transfer, is not observed to dissociate, and is unreactive towards dioxygen.

Behaviour of sulcotrione and mesotrione in two soils

The behaviour of sulcotrione, a recently introduced triketone herbicide, in various soil types was studied under laboratory conditions. In particular, degradation and sorption processes were examined on Ghent and Perpignan soils. Kinetics showed that the degradation of sulcotrione was influenced by biotic and/or abiotic factors. Half-lives ranged between 45 and 65 days. Among the degradation compounds identified were 1,3-cyclohexanedione (CHD) and 2-chloro-4-mesyl benzoic acid (CMBA), previously described as hydrolysis products, and, under special conditions, a derivative of phenylheptanoic acid (PHD). This new degradation product suggested that sulcotrione could follow two possible pathways in the soil, as in water. During the sorption study, a moderate retention of sulcotrione and CMBA relative to CHD and PHD, which were highly adsorbed whatever the soil type, was reported. Experiments carried out under the same conditions for sulcotrione and mesotrione, another triketone herbicide recommended in maize culture, made it possible to compare the two triketones and to conclude that they exhibited relatively similar behaviour in the soil, i.e. that their leaching potential needs to be properly addressed and risks evaluated.

4-Hydroxyphenylpyruvate dioxygenase

4-Hydroxyphenylpyruvate dioxygenase (HPPD) is an Fe(II)-dependent, non-heme oxygenase that catalyzes the conversion of 4-hydroxyphenylpyruvate to homogentisate. This reaction involves decarboxylation, substituent migration and aromatic oxygenation in a single catalytic cycle. HPPD is a member of the alpha-keto acid dependent oxygenases that typically require an alpha-keto acid (almost exclusively alpha-ketoglutarate) and molecular oxygen to either oxygenate or oxidize a third molecule. As an exception in this class of enzymes HPPD has only two substrates, does not use alpha-ketoglutarate, and incorporates both atoms of dioxygen into the aromatic product, homogentisate. The tertiary structure of the enzyme would suggest that its mechanism converged with that of other alpha-keto acid enzymes from an extradiol dioxygenase progenitor. The transformation catalyzed by HPPD has both agricultural and therapeutic significance. HPPD catalyzes the second step in the pathway for the catabolism of tyrosine, that is common to essentially all aerobic forms of life. In plants this pathway has an anabolic branch from homogentisate that forms essential isoprenoid redox cofactors such as plastoquinone and tocopherol. Naturally occurring multi-ketone molecules act as allelopathic agents by inhibiting HPPD and preventing the production of homogentisate and hence required redox cofactors. This has been the basis for the development of a range of very effective herbicides that are currently used commercially. In humans, deficiencies of specific enzymes of the tyrosine catabolism pathway give rise to a number of severe metabolic disorders. Interestingly, HPPD inhibitor/herbicide molecules act also as therapeutic agents for a number of debilitating and lethal inborn defects in tyrosine catabolism by preventing the accumulation of toxic metabolites.