(S)-Metolachor

Mechanism of Resistance to S-metolachlor in Palmer amaranth

Herbicides are major tools for effective weed management. The evolution of resistance to herbicides in weedy species, especially contributed by non-target-site-based resistance (NTSR) is a worrisome issue in crop production globally. Glyphosate-resistant Palmer amaranth (Amaranthus palmeri) is one of the extremely difficult weeds in southern US crop production. In this study, we present the level and molecular basis of resistance to the chloroacetamide herbicide, S-metolachlor, in six field-evolved A. palmeri populations that had survivors at the recommended field-dose (1.1 kg ai ha-1). These samples were collected in 2014 and 2015. The level of resistance was determined in dose-response assays. The effective dose for 50% control (ED50) of the susceptible population was 27 g ai ha-1, whereas the ED50 of the resistant populations ranged from 88 to 785 g ai ha-1. Therefore, A. palmeri resistance to S-metolachlor evolved in Arkansas as early as 2014. Metabolic-inhibitor and molecular assays indicated NTSR in these populations, mainly driven by GSTs. To understand the mechanism of resistance, selected candidate genes were analyzed in leaves and roots of survivors (with 1 × S-metolachlor). Expression analysis of the candidate genes showed that the primary site of S-metolachlor detoxification in A. palmeri is in the roots. Two GST genes, ApGSTU19 and ApGSTF8 were constitutively highly expressed in roots of all plants across all resistant populations tested. The expression of both GSTs increased further in survivors after treatment with S-metolachlor. The induction level of ApGSTF2 and ApGSTF2like by S-metolachlor differed among resistant populations. Overall, higher expression of ApGSTU19, ApGSTF8, ApGSTF2, and ApGSTF2like, which would lead to higher GST activity in roots, was strongly associated with the resistant phenotype. Phylogenetic relationship and analysis of substrate binding site of candidate genes suggested functional similarities with known metolachlor-detoxifying GSTs, effecting metabolic resistance to S-metolachlor in A. palmeri. Resistance is achieved by elevated baseline expression of these genes and further induction by S-metolachlor in resistant plants.

Combined effects of S-metolachlor and benoxacor on embryo development in zebrafish (Danio rerio)

It is necessary to study the combined toxicity of an herbicide and its safener because the two are often used in combination. S-metolachlor and its safener benoxacor have been detected in aquatic environments and can individually damage the oxidative stress system in zebrafish embryos (Danio rerio). However, only their separate toxicity in zebrafish (Danio rerio) embryo development has been reported. This study assessed the combined toxicity of benoxacor and S-metolachlor in zebrafish embryo development, including acute toxicity, developmental toxicity, oxidative damage, and cell apoptosis. The 96-h LC₅₀ values were higher in mixtures of benoxacor and S-metolachlor than in benoxacor alone. The treatments included S-metolachlor, Mix-1 (0.1 mg/L benoxacor + 0.1 mg/L S-metolachlor), Mix-2 (0.1 mg/L benoxacor + 0.3 mg/L S-metolachlor) and benoxacor alone. Embryos exposed to Mix-1 and Mix-2 had lower developmental toxicities, superoxide dismutase (SOD) activity, osx and cat expression levels than those exposed to benoxacor alone. Moreover, glutathione S-transferase (GST), catalase (CAT), glutathione reductase (GR), glutathione peroxidase (GPx) activities, and the expressions of tbx16, nrf2, bcl2, and caspase9 were higher in the mixtures than in the benoxacor group. High-throughput RNA sequencing revealed that benoxacor had a greater effect on gene regulation than Mix-1 and Mix-2. The malformation rate, different enrichment gene numbers, and gene expression levels of hatched embryos were higher in Mix-1 than in Mix-2. The results indicate that a mixture of S-metolachlor and benoxacor has antagonistic effects in the early stage of embryo development. The mixtures can break the reactive oxygen species balance, causing abnormal cell apoptosis and developmental malformation in embryos. Besides investigating the combined toxicity of benoxacor and S-metolachlor in zebrafish embryo development, this study provides a risk assessment basis for a herbicide combined with its safener.

Metabolic Pathways for S-Metolachlor Detoxification Differ Between Tolerant Corn and Multiple-Resistant Waterhemp

Herbicide resistance in weeds can be conferred by target-site and/or non-target-site mechanisms, such as rapid metabolic detoxification. Resistance to the very-long-chain fatty acid-inhibiting herbicide, S-metolachlor, in multiple herbicide-resistant populations (CHR and SIR) of waterhemp (Amaranthus tuberculatus) is conferred by rapid metabolism compared with sensitive populations. However, enzymatic pathways for S-metolachlor metabolism in waterhemp are unknown. Enzyme assays using S-metolachlor were developed to determine the specific activities of glutathione S-transferases (GSTs) and cytochrome P450 monooxygenases (P450s) from CHR and SIR seedlings to compare with tolerant corn and sensitive waterhemp (WUS). GST activities were greater (?2-fold) in CHR and SIR compared to WUS but much less than corn. In contrast, P450s in microsomal extracts from CHR and SIR formed O-demethylated S-metolachlor, and their NADPH-dependent specific activities were greater (>20-fold) than corn or WUS. Metabolite profiles of S-metolachlor generated via untargeted and targeted liquid chromatography-mass spectrometry from CHR and SIR differed from WUS, with greater relative abundances of O-demethylated S-metolachlor and O-demethylated S-metolachlor-glutathione conjugates formed by CHR and SIR. In summary, our results demonstrate that S-metolachlor metabolism in resistant waterhemp involves Phase I and Phase II metabolic activities acting in concert, but the initial O-demethylation reaction confers resistance.

Spray drift potential of dicamba plus S-metolachlor formulations

Background: Early-postemergence herbicide applications in the USA often include residual herbicides such as S-metolachlor to suppress late late-emerging Amaranthus spp. Although this practice benefits weed control, herbicide tankmixes can influence spray droplet size and drift potential during applications. The addition of S-metolachlor products to dicamba spray solutions generally decreases spray droplet size and increases spray drift potential. Advances in formulation technology fostered the development of products with reduced spray drift potential, especially for herbicide premixes containing multiple active ingredients. The objective of this study was to compare the drift potential of a novel dicamba plus S-metolachlor premix formulation (capsule suspension) against a tankmix containing dicamba (soluble liquid) and S-metolachlor (emulsifiable concentrate) using different venturi nozzles.
Results: The MUG nozzle had greater D_V0.5 (1128.6 μm) compared to the ULDM (930.3 μm), TDXL-D (872.9 μm), and TTI nozzles (854.8 μm). The premix formulation had greater D_V0.5 (971.0 μm) compared to the tankmix (922.3 μm). Nozzle influenced spray drift deposition (P < 0.0001) and soybean biomass reduction (P = 0.0465). Herbicide formulation influenced spray drift deposition (P < 0.0001), and biomass reduction of soybean (P < 0.0001) and cotton (P = 0.0479). The novel capsule suspension formulation (premix) of dicamba plus S-metolachlor had reduced area under the drift curve (AUDC) (577.6) compared to the tankmix (913.7). Applications using the MUG nozzle reduced AUDC (459.9) compared to the other venturi nozzles (ranging from 677.4 to 1141.7).
Conclusion: Study results evidence that advances in pesticide formulation can improve pesticide drift mitigation. ? 2021 Society of Chemical Industry.

Tuned S-Scheme Cu2S_TiO2_WO3 Heterostructure Photocatalyst toward S-Metolachlor (S-MCh) Herbicide Removal

A dual S-scheme Cu₂S_TiO₂_WO₃ heterostructure was constructed by sol-gel method using a two-step procedure. Due to the synthesis parameters and annealing treatment the heterostructure is characterized by sulfur deficit and oxygen excess allowing the passivation of oxygen vacancies. The photocatalytic activity was evaluated under UV and UV-Vis irradiation scenarios using S-MCh as reference pollutant. The heterostructure is composed on orthorhombic Cu₂S, anatase TiO₂ and monoclinic WO₃ with crystallite sizes varying from 65.2 ? for Cu₂S to 97.1 ? for WO₃. The heterostructure exhibit a dense morphology with pellets and particle-like morphology closely combined in a relatively compact assembly. The surface elemental composition indicate that the heterostructure maintain a similar atomic ratio as established during the synthesis with a slight sulfur deficit due to the annealing treatments. The results indicate that the three-component heterostructure have higher photocatalytic efficiency (61%) comparing with two-component heterostructure or bare components. Moreover, Cu₂S_TiO₂_WO₃ exhibit a superior constant rate (0.114 s^-1) due to the high concentration of photogenerated charge carriers, efficient charge separation and migration.