Unravelling Phytotoxicity and Mode of Action of Tripyrasulfone, a Novel Herbicide

Advances in Understanding the Toxicity of 4-Hydroxyphenylpyruvate Dioxygenase-Inhibiting Herbicides

4-Hydroxyphenylpyruvate dioxygenase inhibiting herbicides (HIHs) represent a recent class (HRAC group 27) of herbicides that offer many advantages, such as broad-spectrum activity, crop selectivity, and low resistance rates. However, emerging studies have highlighted the potential toxicity of HIHs in the environment. This review aims to provide a comprehensive summary of the toxicity of HIHs toward nontarget organisms, including plants, microorganisms, animals, and humans. Furthermore, the present work discusses the ecological roles of these organisms in the environment and their significance in agriculture. By shedding light on the toxicity of HIHs, this study seeks to raise awareness among end users, including environmentalists, researchers, and farmers, regarding the potential ecological implications of these herbicides. Hopefully, this knowledge can contribute to informed decision-making and sustainable practices in green agriculture and environmental management.

What can reactive oxygen species (ROS) tell us about the action mechanism of herbicides and other phytotoxins?

Reactive oxygen species (ROS) are formed in plant cells continuously. When ROS production exceeds the antioxidant capacity of the cells, oxidative stress develops which causes damage of cell components and may even lead to the induction of programmed cell death (PCD). The levels of ROS production increase upon abiotic stress, but also during pathogen attack in response to elicitors, and upon application of toxic compounds such as synthetic herbicides or natural phytotoxins. The commercial value of many synthetic herbicides is based on weed death as result of oxidative stress, and for a number of them, the site and the mechanism of ROS production have been characterized. This review summarizes the current knowledge on ROS production in plants subjected to different groups of synthetic herbicides and natural phytotoxins. We suggest that the use of ROS-specific fluorescent probes and of ROS-specific marker genes can provide important information on the mechanism of action of these toxins. Furthermore, we propose that, apart from oxidative damage, elicitation of ROS-induced PCD is emerging as one of the important processes underlying the action of herbicides and phytotoxins.

Physiological Basis for the Mechanism of Selectivity of Tripyrasulfone between Rice (Oryza sativa) and Barnyard Grass (Echinochloa crus-galli)

Tripyrasulfone is currently the only HPPD-inhibiting herbicide that possesses outstanding selectivity even for direct-seeded rice (Oryza sativa) when applied POST to control grass weeds; however, the underlying mechanisms remain unclear. In this study, the inhibitory effects of the real active HDT of tripyrasulfone on recombinant 4-hydroxyphenylpyruvate dioxygenase (HPPDs) from rice and barnyard grass (Echinochloa crus-galli) were similar, with consistent structural interactions and similar binding energies predicted by molecular docking. However, the HPPD expression level in rice was significantly greater than that in barnyard grass after tripyrasulfone treatment. Tripyrasulfone was rapidly taken up and hydrolyzed into HDT, which was similarly distributed within the whole plants of rice and barnyard grass at 24 h after treatment. Compared with barnyard grass, rice has more uniform epicuticular wax in the cuticle of its leaves, absorbing less tripyrasulfone and metabolizing much more tripyrasulfone. Overall, to a greater extent, the different sensitivities to tripyrasulfone between barnyard grass and rice resulted from metabolic variations.

Design, synthesis and herbicidal activity of novel cyclohexanedione derivations containing pyrazole and pyridine groups as potential HPPD inhibitors

4-Hydroxyphenylpyruvate dioxygenase (EC 1.13.11.27; HPPD) is one of the important target enzymes in the development of herbicides. To discover novel HPPD inhibitors with unique molecular, 39 cyclohexanedione derivations containing pyrazole and pyridine groups were designed and synthesized. The preliminary herbicidal activity test results showed that some compounds had obvious inhibitory effects on monocotyledon and dicotyledonous weeds. The herbicidal spectrums of the highly active compounds were further determined, and the compound G31 exhibited the best inhibitory rate over 90% against Plantago depressa Willd and Capsella bursa-pastoris at the dosages of 75.0 and 37.5 g ai/ha, which is comparable to the control herbicide mesotrione. Moreover, compound G31 showed excellent crop safety, with less than or equal to 10% injury rates to corn, sorghum, soybean and cotton at a dosage of 225 g ai/ha. Molecular docking and molecular dynamics simulation analysis revealed that the compound G31 could stably bind to Arabidopsis thaliana HPPD (AtHPPD). This study indicated that the compound G31 could be used as a lead molecular structure for the development of novel HPPD inhibitors, which provided an idea for the design of new herbicides with unique molecular scaffold.

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.

Butyl benzyl phthalate exposure impact on the gut health of Metaphire guillelmi

Agricultural films are extensively utilized in high-intensity agriculture, with China's annual usage reaching 1.5 million tons. Unfortunately, the recovery rate is less than 60%, leading to an inevitable accumulation of plastic mulch in agricultural soils. This accumulation primarily introduces butyl benzyl phthalate (BBP) into soil ecosystems, whose specific effects remain largely unclear, thereby posing potential risks. The present study focuses on the exposure impact of BBP on earthworms, Metaphire guillelmi, a commonly found endogenic earthworm within real farmland, as it provides insight into the direct interaction between biota gut health and contaminants. Specifically, we studied the biomarkers related to oxidative stress, the digestive system, and neurotoxicity within the gut of Metaphire guillelmi, and the integrated biological response (IBR) index was utilized to track these markers at different timeframes after BBP exposures. Our findings indicate that BBP exposures lead to oxidative damage, digestive system inhibition, and neurotoxicity, with IBR indexes of 14.6 and 17.3 on the 14th and 28th days, respectively. Further, the underlying mechanisms at a molecular level through molecular docking were investigated. The results showed that the most unstable interaction was with the Na+K+-ATPase (binding energy: -2.25 kcal-1), while BBP displayed stable bonds with superoxide dismutase and 8-hydroxydeoxyguanosine via hydrogen bonds and hydrophobic interaction. These interactions resulted in changes in protein conformation and their normal physiological functions, offering new insights into the molecular mechanism underlying enzymatic activity changes. This study has significant implications for the prediction of toxicity, environmental risk assessment, and the establishment of regulations related to BBP.

Isolation and Identification of Herbicidal Active Compounds from Brassica oleracea L. and Exploration of the Binding Sites of Brassicanate A Sulfoxide

Brassica oleracea L. has strong allelopathic effects on weeds. However, the allelochemicals with herbicidal activity in B. oleracea L. are still unknown. In this study, we evaluated the activity of allelochemicals isolated from Brassica oleracea L. based on the germination and growth of model plant Lactuca sativa Linn., grass weed Panicum miliaceum, and broadleaf weed Chenopodium album. Additionally, we employed molecular docking to predict the binding of brassicanate A sulfoxide to herbicide targets. The results of this study showed that eight compounds with herbicidal activity were isolated from B. oleracea L., and the predicted results indicated that brassicanate A sulfoxide was stably bound to dihydroxyacid dehydratase, hydroxymethylpyruvate dioxygenase, acetolactate synthase, PYL family proteins and transport inhibitor response 1. This research provides compound sources and a theoretical foundation for the development of natural herbicides.

Cypyrafluone, a 4-Hydroxyphenylpyruvate Dioxygenase Inhibitor to Control Weed in Wheat Fields

As a bleaching herbicide, cypyrafluone was applied postemergence in wheat fields for annual weed control; especially, this herbicide possesses high efficacy against cool-season grass weed species such as Alopecurus aequalis and Alopecurus japonicus. In this study, the target of action of cypyrafluone on 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibition was confirmed. This herbicide caused severe foliar whitening symptoms at 5-7 days after treatment (DAT) and death of the whole plant within 10 DAT. Significant increases in phytoene content and significant decreases in kinds of carotenoid and chlorophyll pigments were observed. The content of chlorophyll pigments in cypyrafluone-treated Spirodela polyrhiza decreased upon the addition of homogentisic acid (HGA), which indicated that cypyrafluone prevents the HGA production, possibly by inhibiting the catalytic activity of 4-HPPD. Indeed, cypyrafluone strongly inhibited the catalytic activity of Arabidopsis thaliana HPPD produced by Escherichia coli, which was approximately 2 times less effective than mesotrione. In addition, overexpression of Oryza sativa HPPD in rice and A. thaliana both conferred a high tolerance level to cypyrafluone on them. Molecular docking found that cypyrafluone bonded well to the active site of the HPPD and formed a bidentate coordination interaction with the Fe²⁺ atom, with distances of 2.6 and 2.7 Å between oxygen atoms and the Fe²⁺ atom and a binding energy of -8.0 kcal mol^-1.

Extreme environmental doses of diisobutyl phthalate exposure induce oxidative stress and DNA damage in earthworms (Eisenia fetida): Evidence at the biochemical and molecular levels

Diisobutyl phthalate (DIBP), as a plasticizer, is widely used and has caused many extreme soil contamination scenarios, posing potential risks to soil fauna. However, the toxic effects and mechanisms of DIBP on soil fauna remain unclear. In this study, earthworms (Eisenia fetida) were used as model animals to explore the subchronic toxicity of extreme DIBP soil exposure (300, 600, and 1200 mg/kg) for 28 days. The results showed that the level of reactive oxygen species (ROS) and the contents of malondialdehyde (MDA) and 8-hydroxydeoxyguanosine (8-OHdG) in E. fetida were significantly increased during continuous DIBP exposure. In addition, the activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) were significantly inhibited while glutathione S-transferase (GST) activity was activated during continuous exposure. Integrated biological response (IBR) analysis showed that DIBP had positive dose-dependent toxicity and negative time-dependent toxicity to E. fetida, and SOD/CAT were selected as sensitive biomarkers. The molecular docking study found that DIBP could stably bind to SOD/CAT through hydrogen bonding, which further proved its sensitivity. This study provides primary data for ecological and environmental risk assessment of extreme dose DIBP soil pollution.

Modern Approaches for the Development of New Herbicides Based on Natural Compounds

Weeds are a permanent component of anthropogenic ecosystems. They require strict control to avoid the accumulation of their long-lasting seeds in the soil. With high crop infestation, many elements of crop production technologies (fertilization, productive varieties, growth stimulators, etc.) turn out to be practically meaningless due to high yield losses. Intensive use of chemical herbicides (CHs) has led to undesirable consequences: contamination of soil and wastewater, accumulation of their residues in the crop, and the emergence of CH-resistant populations of weeds. In this regard, the development of environmentally friendly CHs with new mechanisms of action is relevant. The natural phytotoxins of plant or microbial origin may be explored directly in herbicidal formulations (biorational CHs) or indirectly as scaffolds for nature-derived CHs. This review considers (1) the main current trends in the development of CHs that may be important for the enhancement of biorational herbicides; (2) the advances in the development and practical application of natural compounds for weed control; (3) the use of phytotoxins as prototypes of synthetic herbicides. Some modern approaches, such as computational methods of virtual screening and design of herbicidal molecules, development of modern formulations, and determination of molecular targets, are stressed as crucial to make the exploration of natural compounds more effective.

Binding Properties of Photosynthetic Herbicides: Photosynthetic Activity and Molecular Docking Approach towards 1,4-Dihydropyridines Derivatives

In the current work, we describe the synthesis of 1,4-dihydropyridine (1,4-DHP) derivatives via Hantzsch multicomponent reaction and their evaluation as photosystem II (PSII) inhibitors through chlorophyll a fluorescence bioassay. Among all the compounds tested, 1,1'-(2,4,6-trimethyl-1,4-dihydropyridine-3,5-diyl)bis(ethan-1-one) (4b) showed best results, reducing the parameters performance index on absorption basis (PI_abs ) and electron transport per reaction center by 61 % and 49 %, respectively, as compared to the control. These results indicate the inhibitory activity of PSII over the electron transport chain. Additionally, a molecular docking approach using the protein D1 (PDB code 4V82) was performed in order to assess the structure-activity relationship among the 1,4-DHP derivatives over the PSII, which revealed that both, size of the group at position 4 and the carbonyl groups at the dihydropyridine ring are important for the ligand's interaction, particularly for the hydrogen-bonding interaction with the residues His215, Ser264, and Phe265. Thus, the optimization of these molecular features is the aim of our research group to extend the knowledge of PSII electron chain inhibitors and the establishment of new potent bioactive molecular scaffolds.

Toxicity of dibutyl phthalate to pakchoi (Brassica campestris L.): Evaluation through different levels of biological organization

Dibutyl phthalate (DBP) is a typical persistent organic pollutant with a high load in the agricultural soils of vegetable crops. Currently, studies on the toxicity of DBP in vegetable crops are limited. Therefore, in this study, pakchoi (Brassica campestris L.), a typical vegetable crop, was used to evaluate the toxic effects of DBP. Pakchoi was exposed to DBP for 24 d at three doses (2, 20, and 200 mg/kg), and the phenotypic, biochemical, and molecular indicators were determined. The results revealed that DBP could reduce the emergence of pakchoi and inhibit plant height, root length, fresh weight, and leaf area. At the biochemical level, DBP exposure could reduce the content of three typical photosynthetic pigments (chlorophyll a and b and carotenoids). The effects of DBP exposure on the quality of pakchoi were primarily through reduced soluble sugar and increased proline contents. In addition, O₂·^- and H₂O₂ levels increased after DBP stress, and the corresponding antioxidant enzymes (SOD, POD, and CAT) were activated to resist oxidative damage. The dose- and time-dependent toxicities of DBP to pakchoi were demonstrated using an integrated biological response index. Finally, the molecular-level results on Day 24 showed that the three antioxidant enzyme genes (sod, pod, and cat) were significantly downregulated, and the antioxidant enzyme genes were more sensitive biomarkers than the enzyme activities. However, the expression level of enzyme genes was opposite to that of enzyme activity (SOD and POD); thus, DBP might directly interact with these enzymes. Molecular docking showed that DBP could stably bind near the SOD/POD active center through intermolecular interaction forces. This study provides essential information on the risk of DBP toxicity to vegetable crops.

Phytochemical Profile, In Vitro Bioactivity Evaluation, In Silico Molecular Docking and ADMET Study of Essential Oils of Three Vitex Species Grown in Tarai Region of Uttarakhand

A comparative study of volatiles, antioxidant activity, phytotoxic activity, as well as in silico molecular docking and ADMET study, was conducted for essential oils from three Vitex species, viz., V. agnus-castus, V. negundo, and V. trifolia. Essential oils (OEs) extracted by hydrodistillation were subjected to compositional analysis using GC-MS. A total number of 37, 45, and 43 components were identified in V. agnus-castus, V. negundo, and V. trifolia, respectively. The antioxidant activity of EOs, assessed using different radical-scavenging (DPPH, H₂O₂ and NO), reducing power, and metal chelating assays, were found to be significant as compared with those of the standards. The phytotoxic potential of the EOs was performed in the receptor species Raphanusraphanistrum (wild radish) and the EOs showed different levels of intensity of seed germination inhibition and root and shoot length inhibition. The molecular docking study was conducted to screen the antioxidant and phytotoxic activity of the major and potent compounds against human protein target, peroxiredoxin 5, and 4-hydroxyphenylpyruvate dioxygenase protein (HPPD). Results showed good binding affinities and attributed the strongest inhibitory activity to 13-epi-manoyl oxide for both the target proteins.

A Competitive Edge: Competitor Inspired Scaffold Hopping in Herbicide Lead Optimization

Over the years, scaffold hopping has proven to be a powerful tool in the agrochemical optimization process. It offers the opportunity to modify known molecular lead structures to improve a range of parameters, including biological efficacy and spectrum, physicochemical properties, toxicity, stability, and to secure new intellectual property. Very often the disclosure of a new chemical structure can spark a multitude of competitor activities, where scaffold hopping plays a crucial role in the optimization process as well as for the generation of new intellectual property. Herein, recent examples of scaffold hopping in early phase herbicide research based on competitor inspired activities will be discussed using examples of how these research campaigns can often result in the registration of new crop protection products.

Phytochemical Profile and Herbicidal (Phytotoxic), Antioxidants Potential of Essential Oils from Calycolpus goetheanus (Myrtaceae) Specimens, and in Silico Study

The essential oil (EO) of Calycolpus goetheanus (Myrtaceae) specimens (A, B, and C) were obtained through hydrodistillation. The analysis of the chemical composition of the EOs was by gas chromatography coupled with mass spectrometry CG-MS, and gas chromatography coupled with a flame ionization detector CG-FID. The phytotoxic activity of those EOs was evaluated against two weed species from common pasture areas in the Amazon region: Mimosa pudica L. and Senna obtusifolia (L.) The antioxidant capacity of the EOs was determined by (DPPH•) and (ABTS•+). Using molecular docking, we evaluated the interaction mode of the major EO compounds with the molecular binding protein 4-hydroxyphenylpyruvate dioxygenase (HPPD). The EO of specimen A was characterized by β-eudesmol (22.83%), (E)-caryophyllene (14.61%), and γ-eudesmol (13.87%), while compounds 1,8-cineole (8.64%), (E)-caryophyllene (5.86%), δ-cadinene (5.78%), and palustrol (4.97%) characterize the chemical profile of specimen B’s EOs, and specimen C had α-cadinol (9.03%), δ-cadinene (8.01%), and (E)-caryophyllene (6.74%) as the majority. The phytotoxic potential of the EOs was observed in the receptor species M. pudica with percentages of inhibition of 30%, and 33.33% for specimens B and C, respectively. The EOs’ antioxidant in DPPH• was 0.79 ± 0.08 and 0.83 ± 0.02 mM for specimens A and B, respectively. In the TEAC, was 0.07 ± 0.02 mM for specimen A and 0.12 ± 0.06 mM for specimen B. In the results of the in silico study, we observed that the van der Waals and hydrophobic interactions of the alkyl and pi-alkyl types were the main interactions responsible for the formation of the receptor–ligand complex.

Survey on the Recent Advances in 4-Hydroxyphenylpyruvate Dioxygenase (HPPD) Inhibition by Diketone and Triketone Derivatives and Congeneric Compounds: Structural Analysis of HPPD/Inhibitor Complexes and Structure-Activity Relationship Considerations

The serendipitous discovery of the HPPD inhibitors from allelopathic plants opened the way for searching new and effective herbicidal agents by application of classical hit-to-lead optimization approaches. A plethora of active and selective compounds were discovered that belong to three major classes of cyclohexane-based triketones, pyrazole-based diketones, and diketonitriles. In addition, to enhance inhibitory constant and herbicidal activity, many efforts were also made to gain broader weed control, crop safety, and eventual agricultural applicability. Moreover, HPPD inhibitors emerged as therapeutic agents for inherited and metabolic human diseases as well as vector-selective insecticides in the control of hematophagous arthropods. Given the large set of experimental data available, structure-activity relationship analysis could be used to derive suggestions for next generation optimized compounds.

Design, Synthesis, Herbicidal Activity, and Molecular Docking Study of 2-Thioether-5-(Thienyl/Pyridyl)-1,3,4-Oxadiazoles as Potent Transketolase Inhibitors

Transketolase (TK) has been regarded as a new target for the development of novel herbicides. In this study, a series of 2-thioether-5-(thienyl/pyridyl)-1,3,4-oxadiazoles were designed and synthesized based on TK as the new target. The preliminary bioassay results indicated that compounds 4l and 4m displayed the best herbicidal activities against Amaranthus retroflexus (AR) and Digitaria sanguinalis (DS), with the inhibition exceeding 90% at 100-200 mg/L in vitro. Moreover, they also displayed higher postemergence herbicidal activities (90% control) against AR and DS than all of the positive controls at 45-90 g [active ingredient (ai)]/ha in a greenhouse. Notably, compounds 4l and 4m showed a broad spectrum of weed control at 90 g ai/ha. More significantly, compound 4l exhibited good crop selectivity against maize at 90 g ai/ha. Both fluorescent binding experiments and molecular docking analyses indicated that compounds 4l and 4m exhibited strong TK inhibitory activities with superior binding affinities than the others. Preliminary mechanism studies suggested that they might exert their TK inhibitory effects by occupying the active cavity of At TK and forming more strong interactions with amino acids in the active site. Taken together, these results suggested that compound 4l was a potential herbicide candidate for weed control in maize fields targeting TK.