Multiple Herbicide Research Articles

First case of glyphosate resistance in Polypogon fugax: possible involvement of P450-based mechanisms.

Polypogon fugax has evolved resistance to multiple herbicides in China, yet there has been no documented case of glyphosate resistance. A putative glyphosate-resistant P. fugax (HN-R) population was collected from canola fields in Hunan Province, China, surviving glyphosate treatment at the field-recommended rate [540 g acid equivalent (a.e.) ha-1]. The aims of this study were to elucidate the resistance level and mechanism of HN-R population. Dose-response and shikimic acid assays indicated that the HN-R population has developed a low-level resistance (up to 4.6-fold) to glyphosate, compared to two glyphosate-susceptible populations (HN-S and SC-S). No evidence of EPSPS gene mutations and differential EPSPS gene expression were found in association with the resistance. However, pre-treatment with the known cytochrome P450 monooxygenases (P450s) inhibitor, malathion, reversed glyphosate resistance in the HN-R population. Transcriptomic and reverse transcription quantitative polymerase chain reaction (RT-qPCR) analyses showed up-regulation of the PfCYP51 and PfABCG25 genes in the HN-R population. Expression of PfCYP51 in yeast cells significantly enhanced glyphosate tolerance, whereas expression of PfABCG25 did not. Molecular docking experiments suggest that PfCYP51 may catalyze glyphosate metabolism. This is the first report of glyphosate resistance in P. fugax, with evidence suggesting P450 involvement in this resistance. This study enhances our understanding of herbicide-resistant weeds and provides valuable insights into the management of glyphosate-resistant weeds in agriculture. © 2025 Society of Chemical Industry.

Herbicide resistance in Leptochloa chinensis (L.) Nees populations from different regions of Jiangsu Province, China: sensitivity differences and underlying mechanisms

Leptochloa chinensis (L.) Nees, a noxious weed species commonly found in rice fields, has become a significant challenge in Jiangsu Province, China, as it has developed resistance to multiple herbicides due to extensive and continuous herbicide use in recent years. Therefore, this study was conducted to elucidate sensitivity differences and the mechanisms underlying the resistance of L. chinensis (L.) Nees populations to commonly used herbicides across different regions of Jiangsu Province, China. A whole-plant bioassay was used to assess the sensitivity of 46 L. chinensis populations collected from various areas within Jiangsu to several herbicides frequently applied in paddy fields, including: cyhalofop-butyl, fenoxaprop-P-ethyl, pyraclonil, benzobicyclon, anilofos, and oxaziclomefone. After treatment with cyhalofop-butyl, 38 out of 46 populations showed relative resistance-index values that were over four times that of the controls, indicating significant resistance to cyhalofop-butyl. All 41 cyhalofop-butyl-resistant populations showed cross-resistance to fenoxaprop-P-ethyl but remained susceptible to pyraclonil, benzobicyclon, anilofos, and oxaziclomefone. The proportion of populations resistant to acetyl-CoA carboxylase (ACCase)-inhibiting herbicides increased progressively from the south to the north of Jiangsu. Cross-resistance was evident between cyhalofop-butyl and fenoxaprop-P-ethyl; however, all resistant populations were susceptible to pyraclonil, benzobicyclon, anilofos, and oxaziclomefone. Furthermore, mutations in the ACCase gene were identified as a crucial mechanism for cyhalofop-butyl resistance. Specifically, we found ACCase mutations I1781L, W1999C, W2027C/L/S, I2041N, and D2078G in cyhalofop-butyl-resistant L. chinensis populations, among which, W1999C and W2027C accounted for a relatively high proportion, while I1781L, W2027L/S, I2041N, and D2078G were found in one population each. ACCase gene mutations are seemingly a key mechanism for the development of resistance to cyhalofop-butyl, thus, our study provides useful information for developing effective weed-management strategies for controlling this noxious weed species, while ensuring sustainable agricultural practices.

Versatile waste wood-chitosan composites for 2,4-D and paraquat adsorption: Isotherm modelling and thermodynamic evaluation.

2,4-dichlorophenoxy acetic acid (2,4-D) and 1,1-dimethyl-4,4-bipiridinium chloride (paraquat) are among the most widely used herbicides and are known to be toxic. Fabrication of green adsorbents which are capable of removing both herbicides remains a challenge. Here, we fabricate a novel adsorbent from tropical waste wood and use a facile, chitosan-mediated N-heteroatom functionalization technique to augment surface nitrogen and improve specific surface area. The addition of 20wt% chitosan to the waste wood feedstock prior to activation, increased specific surface area by 300m2/g (∼25%) and nitrogen content by 7-fold. This functionalized material removed 69% of 2,4-D and 82% of paraquat at initial concentrations of 4ppm and 40ppm from model solutions at pH 7. It also removed 39% 2,4-D and 93% paraquat from binary mixtures demonstrating its versatility. 2,4-D adsorption increased with chitosan addition suggesting synergistic effects between protonated amine functions and the anionic herbicide form. Paraquat adsorption was negatively correlated with chitosan addition, implying antagonistic interaction between protonated amine functions and quaternary nitrogen atoms on herbicide molecules. Adsorption of both herbicides was spontaneous, entropically favored and exothermic with ΔG °: 19.2kJ/mol and -28.8kJ/mol; ΔS °: 7.42 and 28.6J/Kmol and ΔH °: 17.0kJ/mol and -20.1kJ/mol for 2,4-D and paraquat respectively. Chitosan addition therefore provides a facile and green alternative for N-heteroatom functionalization, and these nitrogen-doped materials are promising candidates for the removal of multiple herbicides from aqueous systems.

Inferring herbicide non-target-site cross-resistance from dose response and mixture treatments.

Herbicide cross-resistance is of increasing concern because it compromises the effectiveness of both existing and new chemical options. However, a common misconception is that if a weed population shows dose-response shifts to two herbicides, it is cross-resistant to both. The possibility that individual plants may possess different resistance mechanisms is often overlooked. To better characterise non-target-site cross-resistance, we propose that the accession be treated with mixtures of the two herbicides of interest. A population model could be used to simulate the expected dose responses to the mixtures, assuming different cross-resistance levels in the population, as well as synergistic or antagonistic effects between the two herbicides. The simulated responses can then be compared with the actual responses, and the cross-resistance level approximated. We demonstrated this approach from a mathematical standpoint and validated it with experimental data from glasshouse tests on four well-characterised biotypes of Lolium multiflorum and two commercial herbicides, clodinafop and iodosulfuron. Results also showed that understanding chemical interactions such as synergy and antagonism is crucial to a better estimate of cross-resistance levels. Because this method utilises standard dose-response tests, it is potentially easier and less time-consuming than those involving plant cloning or divergent recurrent selection. Quick characterisation of the degree of cross-resistance provides important insights as to whether chemical rotations or mixtures can still effectively control the weed population displaying resistances to multiple herbicides. This approach could be applied more broadly in cross-resistance studies with other xenobiotics. © 2025 Society of Chemical Industry.

Investigating non-target site resistance to pyroxsulam in a glyphosate-resistant Lolium rigidum population.

Resistance to multiple herbicides is common in Lolium rigidum. Here, resistance to acetolactate synthase (ALS)- and susceptibilityto acetyl-CoA carboxylase (ACCase)-inhibiting herbicides was confirmed in a glyphosate-resistant L. rigidum population (NLR70) from Australia and the mechanisms of pyroxsulam resistance were examined. No ALS target-site mutations nor gene overexpression were detected. Cytochrome P450 monooxygenase (P450) and glutathione S-transferase (GST) inhibitors (indicators of some certain P450s or GSTs) did not significantly affect the resistance to pyroxsulam. Nevertheless, HPLC analysis showed that plants of the NLR70 population metabolized pyroxsulam faster than plants of the herbicide-susceptible population (SVLR1). RNA sequencing analysis and RT-qPCR validation confirmed that four P450s (CYP709B2, CYP72A14, CYP89A2, CYP94B3), one GT (UGT79), and one ABC transporter (ABCG41) genes were constitutively upregulated in NLR70 plants. This study demonstrates that the glyphosate-resistant L. rigidum population (NLR70) also exhibits resistance to pyroxsulam and identifies six candidate genes associated with non-target site resistance to pyroxsulam. © 2025 Society of Chemical Industry.

Herbicide Strategies for Weed Control in Wisconsin Conventional Tillage Corn Production Systems

Abstract Selection of effective herbicide strategies (i.e., one- versus two-pass and timing [preemergence (PRE) versus postemergence (POST)]) is of great importance to corn growers. Field studies were conducted at Arlington (2018 and 2019), Brooklyn (2019), Lancaster (2019), and Janesville (2018 and 2019), Wisconsin (six site-years) to evaluate overall end-of-season weed control efficacy of multiple herbicide strategies in conventional tillage corn production systems. Herbicide strategy treatments included one-pass PRE, one-pass POST, two-pass PRE followed by (fb) POST, and two-pass PRE fb POST with layered residual herbicides (LRPOST). The weed species present at the experimental site-years were common lambsquarters, giant foxtail, giant ragweed, velvetleaf, and/or waterhemp. Except Arlington-2019, the herbicide strategy was not as influential for the site-years infested with common lambsquarters, giant foxtail, velvetleaf, and/or waterhemp species (e.g., Arlington 2018, Brooklyn 2019, Lancaster 2019), as effective overall end-of-season control (>90%) was achieved regardless of the herbicide strategy, and no significant differences were observed in the combined weed biomass across strategies. A two-pass strategy (e.g., PRE followed by POST, or PRE followed by LRPOST) was necessary for effective overall end-of-season control at the site-years infested with giant ragweed (Janesville 2018 and 2019). Weed interference reduced corn yield by 11 to 75% across site-years. Although certain weed communities can be effectively controlled by a one-pass herbicide strategy, two-pass strategies provided the greatest and most consistent overall end-of-season weed control and corn yield across all site-years, regardless of weed species composition and environmental conditions. Hence, a two-pass herbicide strategy is recommended for Wisconsin conventional tillage corn production to ensure effective end-of-season weed control while protecting yield potential of the crop, particularly in fields infested with moderate to high density of troublesome weeds such as giant ragweed.

Selection and validation of reference genes for qRT-PCR normalization in dayflower (Commelina communis) based on the transcriptome profiling

BackgroundDayflower (Commelina communis), a widely invasive weed, thrives well under a variety of abiotic stresses, including drought and herbicides, and harms the growth of crops such as maize and soybean. Gene expression in dayflower is an important but understudied area due to the lack of reliable reference genes.ResultsFifteen candidate reference genes, which are common reference genes and homologous to those used in other plants, were selected through RNA-seq datasets of dayflower. The expression stability of these screened reference genes was evaluated under three abiotic stresses (drought, herbicide and copper) and in five organs (roots, stems, leaves, flowers and seeds) using five commonly used software programs (geNorm, NormFinder, BestKeeper, ΔCt and RefFinder). The results showed that API5 and SAND had the highest stability in stems, while SAND, EF1A and API5 had the highest stability in roots. Moreover, SAND, ALDH113 and API5 were most stably expressed under copper stress, and EF1A, SAND and API5 were most stable under drought stress. SAND was consistently the most stably expressed gene in both the organs and all samples. Notably, The SAND gene ranked among the top three in terms of stability in all abiotic treatments and in various organs. This result indicates that the SAND gene is suitable for qRT-PCR experimentation in diverse tissues and under multiple (drought, herbicide and copper) abiotic stress conditions in dayflower.ConclusionThis study identified the most stably expressed reference genes under three abiotic stresses and in five organs of dayflower, and SAND showed high expression stability under various experimental conditions, making it a reliable reference gene for gene expression analysis experiments under different conditions in dayflower. This study will enhance the precision of the qRT-PCR quantification of candidate genes related to the adaptation significance of dayflower.

Characterization of acetolactate synthase genes and resistance mechanisms of multiple herbicide resistant Lolium multiflorum

Combining imidazolinone-tolerant wheat with imazamox presents an effective solution to combat weed resistance. However, Lolium multiflorum, a troublesome resistant weed infesting wheat fields, may have developed resistance to imazamox, and the potential resistance mechanisms are intriguing. In this study, we explored the susceptibility of L. multiflorum to imazamox and investigated the resistance mechanisms, including the contribution of the target enzyme acetolactate synthase (ALS) to resistance and the presence of non-target-site resistance (NTSR). Eight L. multiflorum populations suspected of being resistant to imazamox were collected, and six populations exhibited resistance, ranging from 2.45-fold to 16.32-fold. The LmALS1 gene from susceptible population D3 plants and multiple copies of the LmALS gene (LmALS1, LmALS2, LmALS2α, LmALS3, LmALS3α, LmALS3β) from resistant populations D5 and D8 plants were separately amplified. Two mutations (Pro/Gln197 to Thr, Trp574 to Leu) were found in LmALS1 in the resistant populations. Compared to D3, LmALS1 was overexpressed in D5 but not in D8. The presence of LmALS1 mutants (LmALS1-Thr197 and LmALS1- Leu574), along with LmALS2, LmALS3, and their subunits, contribute to the resistance phenotype by increasing bonding energies, weakening hydrogen bonds, or decreasing protein binding pocket volumes and surface area. Additionally, D5 and D8 populations exhibited multiple resistance (>40-fold) to three other ALS inhibitors: pyroxsulam, flucarbazone-sodium, and mesosulfuron-methyl. Pre-treatment with malathion and 4-chloro-7-nitrobenzoxadiazole (cytochrome P450 monooxygenase and glutathione S-transferase inhibitors respectively) reversed the resistance of the D8 population and partially reversed the resistance of the D5 population to imazamox. This study characterizes ALS genes and extends our knowledge into the ALS resistance mechanisms involved in L. multiflorum. It also deepens our understanding of the complex diversification resistance mechanisms, thereby facilitating advances in weed resistance management strategies in wheat fields.

Using a seed impact mill to limit waterhemp (Amaranthus tuberculatus) seed inputs in Iowa soybean

AbstractMounting cases of herbicide-resistant waterhemp [Amaranthus tuberculatus (Moq.) Sauer] in the U.S. Midwest have renewed the interest in nonchemical weed management strategies. Field experiments were conducted in 2021 and 2022 to quantify the effectiveness of a commercial combine equipped with a seed impact mill in preventing A. tuberculatus seed return to the soil seedbank in soybean [Glycine max (L.) Merr.]. Amaranthus tuberculatus seed shattering before crop harvest was quantified. Amaranthus tuberculatus started shattering seeds during the last week of August in both years. Overall, 51% of A. tuberculatus seeds were retained on the plant at harvest on October 23, 2021, compared with 61% at harvest on October 7, 2022. Viability of shattered A. tuberculatus seeds ranged from 84% to 94%. Additional seed shattering occurred when plants were disturbed by the combine header during soybean harvest, which caused 15% and 9% shattering in 2021 and 2022, respectively. Amaranthus tuberculatus seeds passed through the impact mill were grouped in three categories: no damage, moderate damage, and severe damage. In 2021, A. tuberculatus seeds with moderate damage had 26% lower germination and viability than seeds with no visible damage. In 2022, seed germination and viability of no-damage seeds did not differ from seeds with a moderate level of damage. No severely damaged seed germinated or tested viable in either year. Altogether, impact mill treatment reduced the number of germinable seeds by 87% compared with the no–impact mill treatment. These results indicate that seed impact mills can be a useful tool in Iowa soybean production to help manage multiple herbicide–resistant A. tuberculatus populations. However, A. tuberculatus seed shattering before crop harvest reduces the overall effectiveness of seed impact mills in preventing seedbank replenishments.

Synergistic effect of pyridate-based herbicide mixtures for controlling multiple herbicide-resistant kochia

Abstract Multiple herbicide classes resistant (MHCR) kochia poses a serious concern for producers in the Central Great Plains, including western Kansas. Greenhouse and field experiments were conducted at Kansas State University Research and Extension Centers near Hays and Garden City, KS to evaluate pyridate-based postemergence (POST) herbicide mixtures for controlling MHCR kochia. One previously confirmed MHCR population (resistant to atrazine, glyphosate, dicamba, and fluroxypyr) and a susceptible (SUS) kochia population were tested in a greenhouse study. The kochia population at Hays field site was resistant to atrazine, dicamba, and glyphosate while kochia population at Garden City site was resistant to atrazine and glyphosate. Colby’s analysis revealed synergistic interactions when pyridate was mixed with atrazine, dicamba, dichlorprop-p, fluroxypyr, glyphosate, or halauxifen/fluroxypyr and resulted in ≥94% control and shoot dry biomass reduction of MHCR kochia in a greenhouse study. Similarly, synergistic interactions were observed for MHCR kochia control in fallow field studies at both sites when pyridate was mixed with glyphosate or atrazine. Kochia control was increased from 26% to 90% with the application of glyphosate + pyridate and from 28% to 95% with atrazine + pyridate at both sites as compared to separate applications of glyphosate or atrazine. This is the first report for such a strong synergistic effect for both glyphosate and atrazine mixtures with pyridate on a weed resistant to both. All other pyridate-based herbicide mixtures showed an additive interaction and resulted in better control of MHCR kochia (87 to 100%) as compared to their individual applications (23 to 92%) across both sites except 2,4-D. These results suggest that pyridate can play a crucial role in various POST herbicide mixtures for effective control of MHCR kochia.

Cereal Rye Cover Crop Termination Management for Palmer Amaranth (Amaranthus palmeri) Suppression in Soybean

Abstract Palmer amaranth is a troublesome weed species displaying the ability to adapt and evolve resistance to multiple herbicide modes of action, and there is a need for additional weed suppression tactics. Growing interest in the use of cover crops (CC) has led to questions regarding the most appropriate forms of CC management prior to cash crop planting in order to maximize weed suppression benefits. Experiments were conducted between 2021 to 2023 to test: 1) cover crop termination timing (i.e., green or brown), 2) CC biomass amount and 3) CC termination method (i.e., rolled or left standing) on Palmer amaranth suppression. Treatments included “planting brown” (cereal rye terminated two weeks before soybean planting), “planting green” (cereal rye terminated at soybean planting), and a no CC (winter fallow) check. Palmer amaranth emergence was evaluated at 4 and 6 weeks after soybean planting, and yield was calculated at harvest. CC reduced Palmer amaranth emergence when compared to the no CC check, and more suppression was observed as CC biomass increased. This decrease in emergence is potentially due to a decrease in light penetration reaching the soil surface and physical suppression as CC biomass increased. Yield, however, was unaffected by any CC management practice, indicating that growers can tailor CC termination practices for weed suppression. This information will provide better recommendations for farmers interested in using cover crops for weed suppression. Overall, the importance of CC biomass accumulation to achieve weed suppression is highlighted in our findings. Additionally, we add to the growing body of documentation that soybean yield may be variable from year to year as a result of CC presence.

Changing dominance of invasive common reed (Phragmites australis) and native plant colonization with variation in management, wildfires, and soils in a desert wetland

Abstract Among the most widely distributed species globally, common reed [Phragmites australis (Cav.) Trin. ex Steud.] has generated extensive interest in invasive plant science and management because its introduced strains are highly invasive and often form monocultures that alter ecosystem properties. In desert wetlands in Las Vegas, NV, USA, where management goals included reducing hazardous P. australis fuels and increasing native plant diversity, we assessed variation in P. australis cover, the degree of native plant colonization, and soil seedbanks after P. australis management treatments (cutting, glyphosate–imazapyr herbicide) and wildfires across gradients in soil properties. Based on change in P. australis cover during six measurement events over 24 mo, 24 study sites formed three groups: (1) decreasing cover, where initially high P. australis cover (60% to 85%) decreased to <5% following multiple cutting or herbicide treatments; (2) sustaining low cover, where wildfire or clearing was associated with initially low P. australis cover which remained low (<30%) after multiple herbicide applications; and (3) sustaining high cover (45% to 100% initially and remaining at 30% to 100%), including sites unmanaged or treated/burned only once. High soil salinity correlated with low postmanagement P. australis cover. No native plants were detected in the sustaining high P. australis cover group, despite natives occurring in the seedbank. Where management reduced P. australis cover, minimal native plant colonization did occur. Secondary invasion by other non-native plants was nearly absent. Our results suggest that if P. australis can be initially cleared, multiple herbicide applications can persistently keep cover low, especially on drier, saline soils. Slow native plant colonization suggests that a phased approach may be useful to initially reduce P. australis cover, keep it low via repeated treatments, and actively revegetate sites with native species tailored to the moisture–salinity gradient across P. australis–invaded habitats.

Multiple Herbicide Resistance in Annual Ryegrass (Lolium rigidum Gaudin) in the Southeastern Cropping Region of Australia

Annual ryegrass (Lolium rigidum) is a problematic weed in winter crops and fallows in the southeastern cropping region (SCR) of Australia. This weed has evolved resistance to multiple herbicide groups, globally. In Australia, L. rigidum is more prevalent in the western and southern regions than in SCR. To assess the herbicide resistance status of L. rigidum, the response of five L. rigidum populations (collected from the SCR) to glyphosate, glufosinate, paraquat, haloxyfop-P-ethyl, and clethodim is determined using dose–response curves. Three parametric logistic models are used to determine the herbicide dose required to achieve 50% survival (LD50) and 50% growth reduction (GR50). The LD50 values for 50% survival at 28 days after treatment range from 1702 g a.e. ha−1 to 8225 g a.e. ha−1 for glyphosate, 1637 g a.i. ha−1 to 1828 g a.i. ha−1 for glufosinate, 141 g a.i. ha−1 to 307 g a.i. ha−1 for paraquat, 11 g a.i. ha−1 to 107 g a.i. ha−1 for haloxyfop-P-ethyl, and 17 g a.i. ha−1 to 48 g a.i. ha−1 for clethodim. The resistance factor, based on GR50 value, is highest in the S7 population (2.2 times) for glyphosate, the S11 population (2.3 times) for glufosinate, the S11 population (2.0 time) for paraquat, the S7 population (3.9 times) for haloxyfop-P-ethyl, and the S3 population (3.1 times) for clethodim, compared with the susceptible or less tolerant population. The S11 population is found to be resistant to five tested herbicides, based on resistance factors. Similarly, the S3 population is highly resistant to glyphosate, haloxyfop-P-ethyl, and clethodim compared with the W4 population. These results suggest that L. rigidum populations in the SCR exhibit resistance to multiple herbicide groups at labelled field rates. The findings highlight the necessity of adopting an integrated management approach, including the use of residual herbicides, tank mixing herbicides with different modes of action, and rotating herbicides in conjunction with cultural and mechanical control methods.

Degradation of Three Herbicides and Effect on Bacterial Communities under Combined Pollution.

Pesticide residues in soil, especially multiple herbicide residues, cause a series of adverse effects on soil properties and microorganisms. In this work, the degradation of three herbicides and the effect on bacterial communities under combined pollution was investigated. The experimental results showed that the half-lives of acetochlor and prometryn significantly altered under combined exposure (5.02-11.17 d) as compared with those of individual exposure (4.70-6.87 d) in soil, suggesting that there was an antagonistic effect between the degradation of acetochlor and prometryn in soil. No remarkable variation in the degradation rate of atrazine with half-lives of 6.21-6.85 d was observed in different treatments, indicating that the degradation of atrazine was stable. 16S rRNA high-throughput sequencing results showed that the antagonistic effect of acetochlor and prometryn on the degradation rate under combined pollution was related to variation of the Sphingomonas and Nocardioide. Furthermore, the potential metabolic pathways of the three herbicides in soil were proposed and a new metabolite of acetochlor was preliminarily identified. The results of this work provide a guideline for the risk evaluation of combined pollution of the three herbicides with respect to their ecological effects in soil.

Efficient Biodegradation of Multiple Aryloxyphenoxypropionate Herbicides by Corynebacterium sp. Z-1 and the Proposed Degradation Mechanism.

Esterases are crucial for aryloxyphenoxypropionate herbicide (AOPP) biodegradation. However, the underlying molecular mechanisms of AOPP biodegradation by esterases are poorly understood. In the current work, Corynebacterium sp. Z-1 was isolated and found to degrade multiple AOPPs, including quizalofop-p-ethyl (QPE), haloxyfop-p-methyl (HPM), fenoxaprop-p-ethyl (FPE), cyhalofop-butyl (CYB), and clodinafop-propargyl (CFP). A novel esterase, QfeH, which catalyzes the cleavage of ester bonds in AOPPs to form AOPP acids, was identified from strain Z-1. The catalytic activities of QfeH toward AOPPs decreased in the following order: CFP > FPE > CYB > QPE > HPM. Molecular docking, computational analyses, and site-directed mutagenesis indicated the catalytic mechanisms of QfeH-mediated degradation of different AOPPs. Notably, the key residue S159 is essential for the activity of QfeH. Moreover, V222Y, T227M, T227A, A271R, and M275K mutants, exhibiting 2.9-5.0 times greater activity than QfeH, were constructed. This study facilitates the mechanistic understanding of AOPPs bioremediation by esterases.

Multiple antibiotic resistance and herbicide catabolic profiles of bacteria isolated from Lake Villarrica surface sediments (Chile)

Antibiotics and herbicides are contaminants of emerging concern in aquatic environments. Lake Villarrica is a relevant freshwater body in Chile and was recently designated a 'saturated nutrient zone'. Here, we investigated the occurrence of multiple antibiotic resistance (MAR) and herbicide catabolic profiles among bacteria present in the surface sediments of Lake Villarrica. The occurrence of antibiotic-resistant genes (ARGs; blaTEM, catA and tetM) and herbicide-catabolic genes (HCGs; phnJ and atzA) was investigated by qPCR. Subsequently, the presence of culturable bacteria with multiple resistance to amoxicillin (AMX), chloramphenicol (CHL) and oxytetracycline (OXT) was studied. Forty-six culturable MAR (AMX+CHL+OXT) strains were isolated and characterized with respect to their resistance to 11 antibiotics by using a disc diffusion assay and testing their ability to use herbicides as a nutrient source. qPCR analyses revealed that ARGs and HCGs were present in all sediment samples (101 to 103 gene copies g-1), with significant (P≤0.05) higher values in sites near Villarrica city and cattle pastures. The plate method was used to recover MAR isolates from sediment (103-106CFUg-1), and most of the 46 isolates also showed resistance to oxacillin (100%), cefotaxime (83%), erythromycin (96%) and vancomycin (93%). Additionally, 54 and 57% of the MAR isolates were able to grow on agar supplemented (50mgL-1) with atrazine and glyphosate as nutrient sources, respectively. Most of the MAR isolates were taxonomically close to Pseudomonas (76.1%) and Pantoea (17.4%), particularly those isolated from urbanized sites (Pucón city). This study shows the presence of MAR bacteria with herbicide catabolic activity in sediments, which is valuable for conservation strategies and risk assessments of Lake Villarrica. However, major integrative studies on sediments as reservoirs or on the fate of MAR strains and traces of antibiotics and herbicides as a result of anthropic pressure are still needed.

Early detection and management of herbicide-resistant weeds

Small weed patches may be noticed in fields after herbicide application, but they typically do not have a significant impact on the season’s crop yield. As a result, they are usually not treated as a threat to future yields. However, if these patches harbour weed biotypes resistant to one or multiple herbicides, resistance alleles can spread both spatially (via various dispersal pathways, including seed transport by machinery and commodity contamination) and temporally (through seed persistence). This poses a significant threat to herbicide-based weed management. Once these populations spread and cover a large enough area, eradication becomes improbable despite all the resistance management efforts. Therefore, a proactive and collaborative endeavour is needed to detect and manage small and patchy resistant weed populations. In this paper, we review the current potential of weed resistance detection using imagery and molecular markers as well as possible weed management approaches. Finally, we advocate for the use of a combination of these techniques to manage herbicide-resistant weeds when populations are small. This multifaceted approach is presently not applicable to all resistance mechanisms, and all weed species located in any crop, but could initially focus on biotypes and species that are easy to detect and represent the greatest threat.

Two Cytochrome P450s, CYP709B1 and CYP704C1, Play Essential Roles in Metabolism-Based Multiple Herbicide Resistance in American Sloughgrass (Beckmannia syzigachne (Steud.) Fernald).

This study confirmed a field population of American sloughgrass (Beckmannia syzigachne (Steud.) Fernald) that developed simultaneously high levels of resistance (resistance index >10) to three divergent modes of action herbicides: fenoxaprop-P-ethyl, mesosulfuron-methyl, and isoproturon. The resistance phenotype observed in this population was not attributed to target-site alterations; rather, the resistant plants exhibited a significant increase in the activity of cytochrome P450s (P450s) and enhanced metabolism rates for all three herbicides. RNA sequencing revealed significant upregulation of two P450s, CYP709B1 and CYP704C1, in the resistant plants both before and after herbicide treatments. Molecular docking predicted that the homology models of these P450s should exhibit a binding affinity for a range of herbicides. The heterologous expression of the identified P450s in yeast cells indicated improved growth in the presence of all three of the aforementioned herbicides. Collectively, the increased expression of CYP709B1 and CYP704C1 likely contributed to the P450s-mediated enhanced metabolism, thereby conferring multiple herbicide resistance in B. syzigachne.

Current Status of Auxin‐Mimic Herbicides

The challenge of managing pesticide resistance in modern agriculture has become increasingly daunting. Herbicide resistance presents a complex scenario due to the continuous selection pressure from synthetic herbicides in global agricultural systems. Auxin‐mimic herbicides (AMHs) have traditionally served as a dependable tool for managing resistant weed populations in crop systems. However, overreliance on AMHs because of the lower incidence of resistance compared to other herbicide classes has intensified selection pressure on weed populations. Resistance to AMHs primarily arises from generalist mechanisms such as non-target site resistance (NTSR), involving reduced translocation, absorption, and enhanced metabolism. Nevertheless, specific mechanisms like target site resistance have also emerged in certain problematic weed species. These resistance mechanisms are complex to manage because some of them can confer resistance to single, cross, or multiple herbicide groups with diverse chemical and action mechanisms.

Metabolism-Based Nontarget-Site Mechanism Is the Main Cause of a Four-Way Resistance in Shortawn Foxtail (Alopecurus aequalis Sobol.).

Alopecurus aequalis Sobol. is a predominant grass weed in Chinese winter wheat fields, posing a substantial threat to crop production owing to its escalating herbicide resistance. This study documented the initial instance of an A. aequalis population (AHFT-3) manifesting resistance to multiple herbicides targeting four distinct sites: acetyl-CoA carboxylase (ACCase), acetolactate synthase, photosystem II, and 1-deoxy-d-xylulose-5-phosphate synthase. AHFT-3 carried an Asp-to-Gly mutation at codon 2078 of ACCase, with no mutations in the remaining three herbicide target genes, and exhibited no overexpression of any target gene. Compared with the susceptible population AHFY-3, AHFT-3 metabolized mesosulfuron-methyl, isoproturon, and bixlozone faster. The inhibition and comparison of herbicide-detoxifying enzyme activities indicated the participation of cytochrome P450s in the resistance to all four herbicides, with glutathione S-transferases specifically linked to mesosulfuron-methyl. Three CYP72As and a Tau class glutathione S-transferase, markedly upregulated in resistant plants, potentially played pivotal roles in the multiple-herbicide-resistance phenotype.