Exploratory Analysis on Herbicide Metabolism and Very-Long-Chain Fatty Acid Production in Metolachlor-Resistant Palmer Amaranth (Amaranthus palmeri S. Wats.)

Development and Application of a New QuEChERS Method Coupled with UPLC-QTOF-MS/MS for Analysis of Tiafenacil and Its Photolysis Products in Water

This research centered on the novel pyrimidinedione herbicide, tiafenacil. Residues of tiafenacil and its three photolysis products (PP1 to PP3) in water were analyzed using advanced QuEChERS and UPLC-QTOF-MS/MS techniques, reaching a low limit of quantitation (LOQ) of 10 μg/L. Calibration curves exhibited a high degree of linearity (R2 ≥ 0.993) over a concentration range of 0.01 to 1.00 mg/L. Method validation demonstrated high precision, with intraday relative standard deviation RSDr ≤7.9% and interday RSDR ≤ 6.1%, along with high accuracy (recoveries from 94.4% to 105.0%). Using density functional theory (DFT) at the B3LYP/6-311g (d) level, we calculated the electronic properties of tiafenacil and its PPs (PP1 to PP3). Additionally, frontier molecular orbital (FMO) and fukui function analyses were conducted to explore HOMO-LUMO energies, determine energy band gaps for these substances, and predict reactive sites for their electrophilic, nucleophilic, and radical reactions. Significantly, ecotoxicity assessment, including ECOSAR predictions and acute toxicity tests, revealed that the PPs exhibited higher ecotoxicity to aquatic organisms than tiafenacil. Field experiments showed a half-life of 18.9 days for tiafenacil in water, fitting a first-order kinetic model (R2 = 0.999), with a degradation of 41.5% after 14 days and approximately 89.2% after 60 days. This study significantly advances our understanding of tiafenacil's environmental fate, evaluates its associated risks, and offers valuable insights for its responsible application.

Toxicity and Glutathione S-Transferase-Catalyzed Metabolism of R-/S-Metolachlor in Rice

Metolachlor, the chiral herbicide, inhibits the very-long-chain fatty acid (VLCFA) synthesis; elucidating the enantioselectivity between R- and S-metolachlor in the toxicological difference will facilitate the understanding of the site of action. We found that the endogenous accumulation of C22 VLCFAs decreased in both R-/S-metolachlor -treated plants by 6, 12, and 24 h after treatment, with more significant reduction in the S isomer group. Gene expression of glutathione S-transferase OsGST Tau members were obviously induced upon treatments with S or R isomer; both OsGSTU1 and OsGSTU4 can metabolize metolachlor effectively, with S isomer as the preference by directly catalyzing the conjugation between S-metolachlor and glutathione. In the current study, we provide the first evidence in rice seedlings that S-metolachlor showed herbicidal toxicity by blocking the synthesis of C22-type fatty acid, which eventually affects the whole elongation chain of (V)LCFA. Meanwhile, OsGSTU1 and 4 metabolize the metolachlor with the S isomer as preference. All of these discoveries broaden our knowledge about metolachlor toxicology and enantioselectivity.

An intronless tau class glutathione transferase detoxifies several herbicides in flufenacet-resistant ryegrass

Resistance to preemergence herbicides, e.g. inhibitors of the biosynthesis of very-long-chain fatty acids (VLCFAs), is evolving in response to increased use of these compounds. Grass weeds such as ryegrasses (Lolium spp.) have accumulated resistance to various herbicide modes of action. Here, an RNA-seq analysis was conducted using 3 ryegrass populations resistant to the VLCFA biosynthesis inhibitor flufenacet to investigate this phenomenon. Besides various transcripts, including putative long noncoding RNAs (lncRNAs), a single putatively functional tau class glutathione transferase (GST) was constitutively differentially expressed. It was further induced by herbicide application. This GST was expressed as a recombinant protein in Escherichia coli along with other GSTs and detoxified flufenacet rapidly in vitro. Detoxification rates of other herbicides tested in vitro were in accordance with cross-resistance patterns previously determined in vivo. A genome-wide GST analysis revealed that the candidate GST was located in a cluster of 3 intronless GSTs. Their intronless nature possibly results from the retroposition of cellular mRNAs followed by tandem duplication and may affect gene expression. The large number of GSTs (≥195) in the genome of rigid ryegrass (Lolium rigidum) compared with other plant organisms is likely a key factor in the ability of this weed to evolve resistance to different herbicide chemistries. However, in the case of flufenacet resistance, a single upregulated GST with high affinity for the substrate flufenacet possibly contributes overproportionally to rapid herbicide detoxification in planta. The regulation of this gene and the role of differentially expressed transcripts, including various putative lncRNAs, require further investigation.

Role of glutathione S-transferases in the mode of action of herbicides that inhibit amino acid synthesis in Amaranthus palmeri

Acetolactate synthase inhibitors (ALS inhibitors) and glyphosate are two classes of herbicides that act by inhibiting an enzyme in the biosynthetic pathway of branched-chain or aromatic amino acids, respectively. Besides amino acid synthesis inhibition, both herbicides trigger similar physiological effects in plants. The main aim of this study was to evaluate the role of glutathione metabolism, with special emphasis on glutathione S-transferases (GSTs), in the mode of action of glyphosate and ALS inhibitors in Amaranthus palmeri. For that purpose, plants belonging to a glyphosate-sensitive (GLS) and a glyphosate-resistant (GLR) population were treated with different doses of glyphosate, and plants belonging to an ALS-inhibitor sensitive (AIS) and an ALS-inhibitor resistant (AIR) population were treated with different doses of the ALS inhibitor nicosulfuron. Glutathione-related contents, GST activity, and related gene expressions (glutamate-cysteine ligase, glutathione reductase, Phi GST and Tau GST) were analysed in leaves. According to the results of the analytical determinations, there were virtually no basal differences between GLS and GLR plants or between AIS and AIR plants. Glutathione synthesis and turnover did not follow a clear pattern in response to herbicides, but GST activity and gene expression (especially Phi GSTs) increased with both herbicides in treated sensitive plants, possibly related to the rocketing H2O2 accumulation. As GSTs offered the clearest results, these were further investigated with a multiple resistant (MR) population, compressing target-site resistance to both glyphosate and the ALS inhibitor pyrithiobac. As in single-resistant plants, measured parameters in the MR population were unaffected by herbicides, meaning that the increase in GST activity and expression occurs due to herbicide interactions with the target enzymes.