Germination characteristics associated with nicosulfuron resistance in Amaranthus retroflexus L.

Nicosulfuron-resistant biotype (R) and -sensitive biotype (S) Amaranthus retroflexus L. seeds were subjected to different temperature, light, salt, osmotic potential, pH value and burial depth treatments. The difference in germination response of two populations to the above abiotic environmental factors was used to study the fitness cost of nicosulfuron-resistance evolution in A. retroflexus. The aim is to find a powerful tool for weed control in the presence of evolutionary resistance selection. The results of this experiment showed that the germination rate and germination index in S population were generally higher than that in R population. When the salt stress was 80 mM, the water potential was -0.1 Mpa ~ -0.4 Mpa, and under strong acid and alkali conditions, the germination index in S population was prominently higher than that in R population (p<0.05). The delayed seed germination in R population indicated that its nicosulfuron resistance may be linked to seed biochemical compositions that altered seed germination dynamics. The resistant and sensitive biotype of A. retroflexus had differently favourable adaptability in diverse environments. Salt, osmotic potential and pH value are not the major constraints for A. retroflexus germination, however, A. retroflexus are strongly responsive to temperature, light and burial depth. Considering that seeds of A. retroflexus are unable to reach the soil surface beyond the depth of 6 cm, deep inversion tillage before sowing may be an effective and economical weed management tool for the control of nicosulfuron resistant A. retroflexus.

Molecular Mechanisms of Resistance against PSII-Inhibiting Herbicides in Amaranthus retroflexus from the Czech Republic.

Amaranthus retroflexus L. (redroot pigweed) is one of the most problematic weeds in maize, sugar beet, vegetables, and soybean crop fields in Europe. Two pigweed amaranth biotypes (R1 and R2) from the Czech Republic resistant to photosystem II (PSII)-inhibiting herbicides were analyzed in this study. This study aimed to identify the genetic mechanisms that underlie the resistance observed in the biotypes. Additionally, we also intended to establish the use of chlorophyll fluorescence measurement as a rapid and reliable method for confirming herbicide resistance in this weed species. Both biotypes analyzed showed high resistance factors in a dose-response study and were thus confirmed to be resistant to PSII-inhibiting herbicides. A sequence analysis of the D1 protein revealed a well-known Ser-Gly substitution at amino acid position 264 in both biotypes. Molecular docking studies, along with the wild-type and mutant D1 protein's secondary structure analyses, revealed that the S264G mutation did not reduce herbicide affinity but instead indirectly affected the interaction between the target protein and the herbicides. The current study identified the S264G mutation as being responsible for conferring herbicide resistance in the pigweed amaranth biotypes. These findings can provide a strong basis for future studies that might use protein structure and mutation-based approaches to gain further insights into the detailed mechanisms of resistance in this weed species. In many individuals from both biotypes, resistance at a very early stage (BBCH10) of plants was demonstrated several hours after the application of the active ingredients by the chlorophyll fluorescence method. The effective PS II quantum yield parameter can be used as a rapid diagnostic tool for distinguishing between sensitive and resistant plants on an individual level. This method can be useful for identifying herbicide-resistant weed biotypes in the field, which can help farmers and weed management practitioners develop more effective weed control tactics.

Pre-emergence herbicides mixture in soybeans: Amaranthus hybridus control and crop selectivity on cover crops soil.

The objective was to evaluate the efficacy of pre-emergence herbicides mixture applied to the soil with and without dead cover crops (Sorghum bicolor) for the control of Amaranthus hybridus L. (smooth pigweed) and its selectivity in soybeans. This study was structured in split plot (2 × 6 + 2), where factor A plots (with and without dead cover) and factor B six herbicides mixture: flumioxazin + S-metolachlor (50.4 + 1,008 g a.i. ha-1), flumioxazin + imazethapyr (60 + 127.2 g a.i. ha-1), pyroxasulfone + sulfentrazone (137.6 + 160 g a.i. ha-1), diuron + sulfentrazone (400 + 200 g a.i. ha-1), metribuzin + S-metolachlor (326.4 + 1,344 g a.i. ha-1) and sulfentrazone + imazethapyr (200 + 100 g a.i. ha-1) and two untreated control plots. As for the results, the herbicides flumioxazin + S-metolachlor, flumioxazin + imazethapyr and pyroxasulfone + sulfentrazone showed excellent control (97-99%) and were not influenced by the plot with and without dead cover. They also showed higher yield soybeans (<2,244 kg ha-1). All herbicides were selective to the soybeans. Overall, pre-emergence herbicides and cover crops were efficient methods for the control of A. hybridus, which farmers should use to avoid losses in yield soybeans due to weed competition.

Design, Synthesis, and Herbicidal Activity of Novel Tetrahydrophthalimide Derivatives Containing Oxadiazole/Thiadiazole Moieties.

Protoporphyrinogen oxidase (PPO, EC 1.3.3.4) has a high status in the development of new inhibitors. To develop novel and highly effective PPO inhibitors, active substructure linking and bioisosterism replacement strategies were used to design and synthesize novel tetrahydrophthalimide derivatives containing oxadiazole/thiadiazole moieties, and their inhibitory effects on Nicotiana tobacco PPO (NtPPO) and herbicidal activity were evaluated. Among them, compounds B11 (Ki = 9.05 nM) and B20 (Ki = 10.23 nM) showed significantly better inhibitory activity against NtPPO than that against flumiclorac-pentyl (Ki = 46.02 nM). Meanwhile, compounds A20 and B20 were 100% effective against three weeds (Abutilon theophrasti, Amaranthus retroflexus, and Portulaca oleracea) at 37.5 g a.i./ha. It was worth observing that compound B11 was more than 90% effective against three weeds (Abutilon theophrasti, Amaranthus retroflexus, and Portulaca oleracea) at 18.75 and 9.375 g a.i./ha. It was also safer to rice, maize, and wheat than flumiclorac-pentyl at 150 g a.i./ha. In addition, the molecular docking results showed that compound B11 could stably bind to NtPPO and it had a stronger hydrogen bond with Arg98 (2.9 Å) than that of flumiclorac-pentyl (3.2 Å). This research suggests that compound B11 could be used as a new PPO inhibitor, and it could help control weeds in agricultural production.

A Phytotoxin with Selective Herbicidal Activity and Related Metabolites from the Phytopathogenic Fungus Bipolaris cookei SYBL03.

Weeds are a serious threat to crop production, and the utilization of secondary metabolites of phytopathogenic fungi is considered to be an effective method of weed control. In this study, eight compounds were isolated and purified from the mycelium and fermentation broth extracts of Bipolaris cookei SYBL03. The compounds (1-8), except 2 and 6, are reported for the first time from this genus. The herbicidal activities of compounds 1-8 were studied by evaluating their effects on the seed germination and seedling growth of monocotyledonous and dicotyledonous weeds. The results indicated that compound 7 (Cyclo-N-methylphenylalanyltryptophenyl, cNMPT) exhibited a concentration-dependent dual effect on the growth of weed seedlings and selective herbicidal activity against dicotyledonous weeds. We further investigated the morphological and physiological responses of roots of Amaranthus retroflexus, a dicotyledonous weed, to compound 7. Some changes were found in seedlings grown in 400 µg/mL compound 7 solution for 96 h, such as shortening and swelling of elongation zone cells, reduced number and length of root hairs, damage and wrinkling of the root surface, occurrence of electrolyte leakage, and an increase in ethylene content. These results suggest that compound 7 may exert herbicidal activity by causing stress to weed seedlings. Increased ethylene production could be involved in the response of plants to compound 7.

Microplastics increase cadmium absorption and impair nutrient uptake and growth in red amaranth (Amaranthus tricolor L.) in the presence of cadmium and biochar.

Microplastic (MP) pollution in terrestrial ecosystems is gaining attention, but there is limited research on its effects on leafy vegetables when combined with heavy metals. This study examines the impact of three MP types-polyethylene (PE), polyethylene terephthalate (PET), and polystyrene (PS)-at concentrations of 0.02, 0.05, and 0.1% w/w, along with cadmium (Cd) and biochar (B), on germination, growth, nutrient absorption, and heavy metal uptake in red amaranth (Amaranthus tricolor L.). We found that different MP types and concentrations did not negatively affect germination parameters like germination rate, relative germination rate, germination vigor, relative germination vigor, and germination speed. However, they increased phytotoxicity and decreased stress tolerance compared to an untreated control (CK1). The presence of MPs, particularly the PS type, reduced phosphorus and potassium uptake while enhancing Cd uptake. For example, treatments PS0.02CdB, PS0.05CdB, and PS0.1CdB increased Cd content in A. tricolor seedlings by 158%, 126%, and 44%, respectively, compared to the treatment CdB (CK2). Additionally, MP contamination led to reduced plant height, leaf dry matter content, and fresh and dry weights, indicating adverse effects on plant growth. Moreover, the presence of MPs increased bioconcentration factors and translocation factors for Cd, suggesting that MPs might act as carriers for heavy metal absorption in plants. On the positive side, the addition of biochar improved several root parameters, including root length, volume, surface area, and the number of root tips in the presence of MPs, indicating potential benefits for plant growth. Our study shows that the combination of MPs and Cd reduces plant growth and increases the risk of heavy metal contamination in food crops. Further research is needed to understand how different MP types and concentrations affect various plant species, which will aid in developing targeted mitigation strategies and in exploring the mechanisms through which MPs impact plant growth and heavy metal uptake. Finally, investigating the potential of biochar application in conjunction with other amendments in mitigating these effects could be key to addressing MP and heavy metal contamination in agricultural systems.

Unraveling the reasons for failure to control Amaranthus albus: insights into herbicide application at different growth stages, temperature effect, and herbicide resistance on a regional scale.

BACKGROUND: This study investigates factors contributing Amaranthus albus control failure in processing tomato fields in northern Israel. The study region is characterized by a significant climate gradient from east to west, providing the opportunity to investigate the effect of critical elements of the agricultural environment, e.g., temperature. Eight populations were collected from commercial fields in this region. Post-emergence herbicide efficacy of metribuzin, a photosystem II inhibitor, and rimsulfuron, an acetolactate synthase (ALS) inhibitor, was assessed through dose-response analyses at various growth stages. Temperature effects on control efficacy and resistance mechanisms were also explored. RESULTS: Standard metribuzin dose (X) was ineffective on A. albus plants with more than six true-leaves, whereas 2X dose proved effective. Rimsulfuron at 16X dose was ineffective on plants with more than four true-leaves. We report here the first case of target site resistance to ALS inhibitors in A. albus, due to point mutation in the ALS gene (Pro197 to Leu). Furthermore, our findings suggest potential involvement of CYT P450 enzymes in enhanced metabolizing of rimsulfuron. An overall decrease in dry weight was observed in response to both herbicides at 16/22 °C (P < 0.0001). Rimsulfuron was effective against only one population when applied at 28/34 °C. A possible fitness cost associated with target site-resistant biotypes was observed under low temperature conditions, leading to effective control. CONCLUSION: This regional-scale study highlights the challenges faced by growers, emphasizes the need for adapting management practices to the local climatic conditions and lays the groundwork for implementing location-specific weed management strategies in commercial fields. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Dissipation and residues of imidacloprid in amaranth under greenhouse and open field cultivations.

Despite the extensive exposure to imidacloprid residues in food plants, there has been little research on imidacloprid residues in amaranth. The dissipation trend and residue behavior of imidacloprid were evaluated to provide guidelines for imidacloprid application on amaranth under open field and greenhouse. The dissipation rate of imidacloprid in amaranth conformed to the first-order kinetic equation, and the half-lives of imidacloprid in amaranth ranged from 0.29 days in open field to 1.29 days in the greenhouse. After 7 and 14 days from the application of imidacloprid (pesticide dosage, 45 or 67.5 g a.i./ha), the amaranth under the open field and greenhouse growth could be consumed safely with average residues of 0.19 and 0.38 mg/kg, respectively. This result demonstrated that the cultivation has the dominant influence on imidacloprid residue, and the residue of imidacloprid in amaranth planting on open field was much lower than that in the greenhouse, indicating a significant difference in the pesticide residues between the two cultivations with a p-value less than 0.05.

Amaranth's Growth and Physiological Responses to Salt Stress and the Functional Analysis of AtrTCP1 Gene.

Amaranth species are C4 plants that are rich in betalains, and they are tolerant to salinity stress. A small family of plant-specific TCP transcription factors are involved in the response to salt stress. However, it has not been investigated whether amaranth TCP1 is involved in salt stress. We elucidated that the growth and physiology of amaranth were affected by salt concentrations of 50-200 mmol·L-1 NaCl. The data showed that shoot and root growth was inhibited at 200 mmol·L-1, while it was promoted at 50 mmol·L-1. Meanwhile, the plants also showed physiological responses, which indicated salt-induced injuries and adaptation to the salt stress. Moreover, AtrTCP1 promoted Arabidopsis seed germination. The germination rate of wild-type (WT) and 35S::AtrTCP1-GUS Arabidopsis seeds reached around 92% by the seventh day and 94.5% by the second day under normal conditions, respectively. With 150 mmol·L-1 NaCl treatment, the germination rate of the WT and 35S::AtrTCP1-GUS plant seeds was 27.0% by the seventh day and 93.0% by the fourth day, respectively. Under salt stress, the transformed 35S::AtrTCP1 plants bloomed when they grew 21.8 leaves after 16.2 days of treatment, which was earlier than the WT plants. The transformed Arabidopsis plants flowered early to resist salt stress. These results reveal amaranth's growth and physiological responses to salt stress, and provide valuable information on the AtrTCP1 gene.

Biochar enhances the growth and physiological characteristics of Medicago sativa, Amaranthus caudatus and Zea mays in saline soils.

Biochar is a promising solution to alleviate the negative impacts of salinity stress on agricultural production. Biochar derived from food waste effect was investigated on three plant species, Medicago sativa, Amaranthus caudatus, and Zea mays, under saline environments. The results showed that biochar improved significantly the height by 30%, fresh weight of shoot by 35% and root by 45% of all three species compared to control (saline soil without biochar adding), as well as enhanced their photosynthetic pigments and enzyme activities in soil. This positive effect varied significantly between the 3 plants highlighting the importance of the plant-biochar interactions. Thus, the application of biochar is a promising solution to enhance the growth, root morphology, and physiological characteristics of plants under salt-induced stress.

Design, Synthesis, and Herbicidal Evaluation of Pyrrolidinone-Containing 2-Phenylpyridine Derivatives as Novel Protoporphyrinogen Oxidase Inhibitors.

In this work, a series of pyrrolidinone-containing 2-phenylpyridine derivatives were synthesized and evaluated as novel protoporphyrinogen IX oxidase (PPO, EC 1.3.3.4) inhibitors for herbicide development. At 150 g ai/ha, compounds 4d, 4f, and 4l can inhibit the grassy weeds of Echinochloa crus-galli (EC), Digitaria sanguinalis (DS), and Lolium perenne (LP) with a range of 60 to 90%. Remarkably, at 9.375 g ai/ha, these compounds showed 100% inhibition effects against broadleaf weeds of Amaranthus retroflexus (AR) and Abutilon theophrasti (AT), which were comparable to the performance of the commercial herbicides flumioxazin (FLU) and saflufenacil (SAF) and better than that of acifluorfen (ACI). Molecular docking analyses revealed significant hydrogen bonding and π-π stacking interactions between compounds 4d and 4l with Arg98, Asn67, and Phe392, respectively. Additionally, representative compounds were chosen for in vivo assessment of PPO inhibitory activity, with compounds 4d, 4f, and 4l demonstrating excellent inhibitory effects. Notably, compounds 4d and 4l induced the accumulation of reactive oxygen species (ROS) and a reduction in the chlorophyll (Chl) content. Consequently, compounds 4d, 4f, and 4l are promising lead candidates for the development of novel PPO herbicides.

Low-concentration repeated exposure and high-concentration single exposure of cyanamide suppressed the growth of redroot pigweed in the alfalfa field.

The inclination toward natural products has led to the onset of the discovery of new bioactive metabolites that could be targeted for specific therapeutic or agronomic applications. Despite increasing knowledge coming to light of plant-derived materials as leads for new herbicides, relatively little is known about the mode of action on herbicide-resistant weeds. Cyanamide (CA) is a naturally occurring herbicide synthesized by hairy vetch (Vicia villosa Roth.). However, it has not been experimentally verified whether CA suppresses target plants via sustained discharge at low concentrations, as is often the case with most plant-derived materials. This study aimed to detect the toxicity and the mode of action of CA to alfalfa (Medicago sativa L.) and redroot pigweed (Amaranthus retroflexus L.). The toxicity of CA toward the alfalfa and redroot pigweed by three different exposure patterns was compared: low-concentration repeated exposure with 0.3 g/L CA (LRE), high-concentration single exposure with 1.2 g/L CA (HSE), and distilled water spray as control. The results showed that CA had a stronger inhibitory effect on redroot pigweed growth compared to alfalfa under both LRE and HSE exposure modes, with leaves gradually turning yellow and finally wilting. Beyond that, field trials were conducted to corroborate the toxicity of CA to alfalfa and redroot pigweed. The results have also shown that CA could inhibit the growth of redroot pigweed without significant adverse effects on alfalfa. The outcomes concerning electrolyte permeability, root activity, and malondialdehyde (MDA) content indicated that CA suppressed the growth of redroot pigweed by interfering with the structure of the cell membrane and impacting cellular osmotic potential. CA could destroy the cell membrane structure to inhibit the growth of the redroot pigweed by both LRE and HSE exposure modes, which provides a theoretical basis for preventing and controlling redroot pigweed in alfalfa fields.

Development of a rapid detection assay for acetolactate synthase inhibitors resistance in three Amaranthus weed species through loop-mediated isothermal amplification.

BACKGROUND: The early detection of herbicide resistance in weeds is a key factor to avoid herbicide waste and improve agriculture sustainability. The present study aimed to develop and validate an allele-specific loop-mediated isothermal amplification (AS-LAMP) assay for the quick on-site detection of the resistance-endowing point mutation Trp-574-Leu in the acetolactate synthase (ALS) gene in three widely diffused Amaranthus weed species: Amaranthus retroflexus, Amaranthus hybridus and Amaranthus tuberculatus. RESULTS: The AS-LAMP protocol was developed on wild-type and ALS-mutant plants of the three species and revealed that the amplification approach with only the primer set specific for the mutant allele (574-Leu) was the most promising. The validation and estimation of the AS-LAMP performance evaluated by comparing the results with those of the molecular marker (cleaved amplified polymorphic sequences) indicated that, although the sensitivity and specificity were relatively high in all species (overall 100 and > 65%, respectively), precision was high for A. hybridus L. and A. retroflexus L. (75 and 79%, respectively), but quite low for A. tuberculatus (Moq.) J. D. Sauer (59%). The LAMP assay was also effective on crude genomic DNA extraction, allowing the quick detection of mutant plants in field situation (on site resistance detection). CONCLUSION: The proposed AS-LAMP method has proven to be a promising technique for rapid detection of resistance as a result of Trp-574-Leu on the two monoecious weedy Amaranthus species but resulted less effective in the genetically variable dioecious species A. tuberculatus. © 2024 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Dual plastid targeting of protoporphyrinogen oxidase 2 in Amaranthaceae promotes herbicide tolerance.

Plant tetrapyrrole biosynthesis (TPB) takes place in plastids and provides the chlorophyll and heme required for photosynthesis and many redox processes throughout plant development. TPB is strictly regulated, since accumulation of several intermediates causes photodynamic damage and cell death. Protoporphyrinogen oxidase (PPO) catalyzes the last common step before TPB diverges into chlorophyll and heme branches. Land plants possess two PPO isoforms. PPO1 is encoded as a precursor protein with a transit peptide, but in most dicotyledonous plants PPO2 does not possess a cleavable N-terminal extension. Arabidopsis (Arabidopsis thaliana) PPO1 and PPO2 localize in chloroplast thylakoids and envelope membranes, respectively. Interestingly, PPO2 proteins in Amaranthaceae contain an N-terminal extension that mediates their import into chloroplasts. Here, we present multiple lines of evidence for dual targeting of PPO2 to thylakoid and envelope membranes in this clade and demonstrate that PPO2 is not found in mitochondria. Transcript analyses revealed that dual targeting in chloroplasts involves the use of two transcription start sites and initiation of translation at different AUG codons. Among eudicots, the parallel accumulation of PPO1 and PPO2 in thylakoid membranes is specific for the Amaranthaceae and underlies PPO2-based herbicide resistance in Amaranthus species.

Chemical Composition and Phytotoxic Activity of Artemisia selengensis Turcz. Volatiles.

The chemical profile and allelopathic action of the volatiles produced by Artemisia selengensis were studied. Artemisia selengensis was found to release volatile chemicals to the environment to influence other plants' growth, which suppressed the root length of Amaranthus retroflexus and Poa annua by 50.46 % and 87.83 % under 80 g/1.5 L treatment, respectively. GC/MS analysis led to the identification of 41 compounds (by hydrodistillation, HD) and 48 compounds (by headspace solid-phase microextraction, HS-SPME), with eucalyptol (15.45 % by HD and 28.09 % by HS-SPME) being detected as the most abundant constituent. The essential oil (EO) of A. selengensis completely inhibited the seed germination of A. retroflexus and P. annua at 1 mg/mL and 0.5 mg/mL, respectively. However, eucalyptol displayed much weaker activity compared with the EO, indicating that other less abundant constituents might contribute significantly to the EO's activity. Our study is the first report on the phytotoxicity of A. selengensis EO, suggesting that A. selengensis might release allelopathic volatile agents into the environment that negatively affect other plants' development so as to facilitate its own dominance; the potential value of utilizing A. selengensis EO as an environmentally friendly herbicide is also discussed.

Evaluation of Pomelo Seed Extracts as Natural Antioxidant, Antibacterial, Herbicidal Agents, and Their Functional Components.

Pomelo seeds (PS) are important by-product of pomelo fruits (Citrus grandis Osbeck). The value-added utilization of PS remains highly challenged. This study aimed to investigate the utilization potential of PS as natural antioxidant, antibacterial, herbicidal agents, and their functional components. The ethanolic extract (EE) of PS and its four fractions as PEE (petroleum ether extract), AcOEtE (ethyl acetate extract), BTE (butanol extract), and WE (water extract), were prepared and biologically evaluated. BTE exhibited the best antioxidant activity among all these extracts, in both ABTS (2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt) and FRAP (ferric reducing antioxidant power) assays. AcOEtE was superior to other extracts in herbicidal assay against both Festuca elata Keng (IC50 of 0.48 mg mL-1 ) and Amaranthus retroflexus L. (IC50 of 0.94 mg mL-1 ). Meanwhile, both AcOEtE and BTE demonstrated inhibitory effects against Bacillus subtilis, Escherichia coli, and Xanthomonas citri subsp. citri, with MIC ranging 2.5-5.0 mg mL-1 . Furthermore, the primary chemical components involving naringin, deacetylnomilin, limonin, nomilin, and obacunone, were quantified in all these extracts. PCA (principal component analysis) suggested that naringin might highly contribute to the antioxidant activity of PS, and the herbicidal activity should be ascribed to limonoids. This study successfully identified AcOEtE and BTE as naturally occurring antioxidant, antibacterial, and herbicidal agents, showing application potential in food and cosmetics industries, and organic farming agriculture.

Phytotoxic Activity of Alkaloids in the Desert Plant Sophora alopecuroides.

Sophora alopecuroides is known to produce relatively large amounts of alkaloids; however, their ecological consequences remain unclear. In this study, we evaluated the allelopathic potential of the main alkaloids, including aloperine, matrine, oxymatrine, oxysophocarpine, sophocarpine, sophoridine, as well as their mixture both in distilled H2O and in the soil matrix. Our results revealed that all the alkaloids possessed inhibitory activity on four receiver species, i.e., Amaranthus retroflexus, Medicago sativa, Lolium perenne and Setaria viridis. The strength of the phytotoxicity of the alkaloids was in the following order: sophocarpine > aloperine > mixture > sophoridine > matrine > oxysophocarpine > oxymatrine (in Petri dish assays), and matrine > mixture > sophocarpine > oxymatrine > oxysophocarpine > sophoridine > aloperine (in pot experiments). In addition, the mixture of the alkaloids was found to significantly increase the IAA content, MDA content and POD activity of M. sativa seedlings, whereas CTK content, ABA content, SOD activity and CAT activity of M. sativa seedlings decreased markedly. Our results suggest S. alopecuroides might produce allelopathic alkaloids to improve its competitiveness and thus facilitate the establishment of its dominance; the potential value of these alkaloids as environmentally friendly herbicides is also discussed.

Understanding Resistance Mechanisms to Trifluralin in an Arkansas Palmer Amaranth Population.

Amaranthus palmeri S. Watson (Palmer amaranth) is considered a problematic and troublesome weed species in many crops in the USA, partly because of its ability to evolve resistance to herbicides. In this study, we explored the mechanism of resistance in a trifluralin-resistant A. palmeri accession collected from Arkansas, USA. Dose-response assays using agar plates demonstrated an EC50 (effective concentration that reduces root length by 50%) of 1.02 µM trifluralin compared to 0.39 µM obtained in the susceptible accession. Thus, under these conditions, the resistant accession required 2.6 times more trifluralin to inhibit root length by 50%. Seeds in the presence or absence of the cytochrome P450-inhibitior malathion displayed a differential response with no significant influence on root length, suggesting that resistance is not P450-mediated. In addition, application of 4-chloro-7-nitrobenzofurazan (NBD-Cl), a glutathione S-transferase (GST) inhibitor, showed significant differences in root length, indicating that GSTs are most likely involved in the resistance mechanism. Sequencing of α- and ß-tubulin genes revealed no single nucleotide polymorphisms (SNPs) previously described between accessions. In addition, relative gene copy number of α- and ß-tubulin genes were estimated; however, both resistant and susceptible accessions displayed similar gene copy numbers. Overall, our results revealed that GST-mediated metabolism contributes to trifluralin resistance in this A. palmeri accession from Arkansas.

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.

Enantioselective and Synergistic Herbicidal Activities of Common Amino Acids Against Amaranthus tricolor and Echinochloa crus-galli.

Amino acids have a wide range of biological activities, which usually rely on the stereoisomer presented. In this study, glycine and 21 common α-amino acids were investigated for their herbicidal property against Chinese amaranth (Amaranthus tricolor L.) and barnyard grass (Echinochloa crus-galli (L.) Beauv.). Both d- and l-isomers, as well as a racemic mixture, were tested and found that most compounds barely inhibited germination but moderately suppressed seedling growth. Various ratios of d:l-mixture were studied and synergy between enantiomers was found. For Chinese amaranth, the most toxic d:l-mixtures were at 3:7 (for glutamine), 8:2 (for methionine), and 5:5 (for tryptophan). For barnyard grass, rac-glutamine was more toxic than the pure forms; however, d-tryptophan exhibited greater activity than racemate and l-isomer, indicating the sign of enantioselective toxicity. The mode of action was unclear, but d-tryptophan caused bleaching of leaves, indicating pigment synthesis of the grass was inhibited. The results highlighted the enantioselective and synergistic toxicity of some amino acids, which relied upon plant species, chemical structures, and concentrations. Overall, our finding clarifies the effect of stereoisomers, and provides a chemical clue of amino acid herbicides, which may be useful in the development of herbicides from natural substances.