Acetolactate Synthase-inhibiting Herbicides Research Articles

Growth Suppression of Hybrid Bermudagrass with Imazapic

Imazapic is an acetolactate synthase-inhibiting herbicide labeled for weed control in pastures, rangeland, and noncrop areas. Field research was conducted in Knoxville, TN, USA, during 2020 and 2021 to evaluate the tolerance of four hybrid bermudagrass (Cynodon dactylon × Cynodon transvaalensis Burtt Davy) cultivars to applications of imazapic for growth suppression. Separate experiments were conducted on ‘TifTuf’, ‘Tifway’, ‘Tahoma 31’, and ‘Latitude 36’ hybrid bermudagrass. Experiments included plots (1.5 m2) arranged in a randomized complete block design with four replications and were repeated. Treatments were applied 14 Aug 2020 and 6 Aug 2021, and were mixed with methylated seed oil. Imazapic rates were 0, 35, 52.5, 70, or 105 g⋅ha–1. Cultivar tolerance was assessed via visual ratings of turfgrass injury relative to untreated check plots. Normalized differential vegetation index data were collected on each date turfgrass injury was evaluated. Growth suppression was quantified via reductions in dry clipping weight after mowing. Hybrid bermudagrass injury increased with imazapic rate for all cultivars, and peak injury (> 30%) following all imazapic treatments occurred within 14 days. At the lowest imazapic rate (35 g⋅ha–1), injury was transient, with all hybrid bermudagrass cultivars fully recovered by 28 days. All rates of imazapic reduced hybrid bermudagrass dry clipping weight for 21 days for all cultivars. Further research is warranted to explore lower application rates than those tested in our study, in addition to determining tolerance and growth suppression of other turfgrass species commonly managed on golf courses.

Distinctive physiological and molecular responses of foxtail millet and maize to nicosulfuron.

Nicosulfuron is the leading acetolactate synthase inhibitor herbicide product, and widely used to control gramineous weeds. Here, we investigated the metabolic process of nicosulfuron into foxtail millet and maize, in order to clarify the mechanism of the difference in sensitivity of foxtail millet and maize to nicosulfuron from the perspective of physiological metabolism and provide a theoretical basis for the breeding of nicosulfuron-resistant foxtail millet varieties. We treated foxtail millet (Zhangzagu 10, Jingu 21) and maize (Nongda 108, Ditian 8) with various doses of nicosulfuron in both pot and field experiments. The malonaldehyde (MDA) content, target enzymes, detoxification enzymes, and antioxidant enzymes, as well as related gene expression levels in the leaf tissues of foxtail millet and maize were measured, and the yield was determined after maturity. The results showed that the recommended dose of nicosulfuron caused Zhangzagu 10 and Jingu 21 to fail to harvest; the yield of the sensitive maize variety (Ditian 8) decreased by 37.09%, whereas that of the resistant maize variety (Nongda 108) did not decrease. Nicosulfuron stress increased the CYP450 enzyme activity, MDA content, and antioxidant enzyme activity of foxtail millet and maize, reduced the acetolactate synthase (ALS) activity and ALS gene expression of foxtail millet and Ditian 8, and reduced the glutathione S-transferase (GST) activity and GST gene expression of foxtail millet. In conclusion, target enzymes, detoxification enzymes, and antioxidant enzymes were involved in the detoxification metabolism of nicosulfuron in plants. ALS and GST are the main factors responsible for the metabolic differences among foxtail millet, sensitive maize varieties, and resistant maize varieties. These findings offer valuable insights for exploring the target resistance (TSR) and non-target resistance (NTSR) mechanisms in foxtail millet under herbicide stress and provides theoretical basis for future research of develop foxtail millet germplasm with diverse herbicide resistance traits.

Study on the toxicity prediction model ofacetolactate synthase inhibitor herbicides based on human serum albumin and superoxide dismutase binding information

Toxicity significantly influences the successful development of drugs. Based on the toxicity prediction method (carrier protein binding information-toxicity relationship) previously established by the our group, this paper introduces information on the interaction between pesticides and environmental markers (SOD) into the model for the first time, so that the toxicity prediction model can not only predict the toxicity of pesticides to humans and animals, but also predict the toxicity of pesticides to the environment. Firstly, the interaction of acetolactate synthase inhibitor herbicides (ALS inhibitor herbicides) with human serum albumin (HSA) and superoxide dismutase (SOD) was investigated systematically from theory combined with experiments by spectroscopy methods and molecular docking, and important fluorescence parameters were obtained. Then, the fluorescence parameters, pesticides acute toxicity LD50 and structural splitting information were used to construct predictive modeling of ALS inhibitor herbicides based on the carrier protein binding information (R2 = 0.977) and the predictive modeling of drug acute toxicity based on carrier protein binding information and conformational relationship (R2 = 0.991), which had effectively predicted pesticides toxicity in humans and animals. To predict potential environmental toxicity, the predictive modeling of drug acute toxicity based on superoxide dismutase binding information was established (R2 = 0.883) by ALS inhibitor herbicides-SOD binding information, which has a good predictive ability in the potential toxicity of pesticides to the environment. This study lays the foundation for developing low toxicity pesticides.

Discovery and Mode-of-Action Characterization of a New Class of Acetolactate Synthase-Inhibiting Herbicides.

Herbicides are effective tools to manage weeds and enable food production and sustainable agriculture. Corteva Agriscience R&D has recently discovered new diphenyl-ether compounds displaying excellent postemergent efficacy on important weed species along with corn safety. Here, we describe the chemistry, biology, biochemistry, and computational modeling research that led to the discovery and elucidation of the primary mode of action for these compounds. The target protein was found to be acetolactate synthase (ALS), a key enzyme in the biosynthesis of branched chain amino acids (valine, leucine, and isoleucine). While weed resistance evolution to ALS herbicides is widespread, the molecular interaction of the diphenyl-ether compounds at the active site of the ALS enzyme differs significantly from that of some commercial ALS inhibitors. The unique biochemical profile of these molecules along with their excellent herbicidal activity and corn selectivity make them a noteworthy development in the pursuit of novel, safe, and sustainable weed control solutions.

Target-site and non–target site mechanisms of pronamide resistance in annual bluegrass (Poa annua) populations from Mississippi golf courses

AbstractThe mitotic-inhibiting herbicide pronamide controls susceptible annual bluegrass (Poa annua L.) pre- and postemergence, but in some resistant populations, postemergence activity is compromised, hypothetically due to a target-site mutation, lack of root uptake, or an unknown resistance mechanism. Three suspected pronamide-resistant (LH-R, SC-R, and SL-R) and two pronamide-susceptible (BS-S and HH-S) populations were collected from Mississippi golf courses. Dose–response experiments were conducted to confirm and quantify pronamide resistance, as well as resistance to flazasulfuron and simazine. Target sites known to confer resistance to mitotic-inhibiting herbicides were sequenced, as were target sites for herbicides inhibiting acetolactate synthase (ALS) and photosystem II (PSII). Pronamide absorption and translocation were investigated following foliar and soil applications. Dose–response experiments confirmed pronamide resistance of LH-R, SC-R, and SL-R populations, as well as instances of multiple resistance to ALS- and PSII-inhibiting herbicides. Sequencing of the α-tubulin gene confirmed the presence of a mutation that substituted isoleucine for threonine at position 239 (Thr-239-Ile) in LH-R, SC-R, SL-R, and BS-S populations. Foliar application experiments failed to identify differences in pronamide absorption and translocation between the five populations, regardless of harvest time. All populations had limited basipetal translocation—only 3% to 13% of the absorbed pronamide—across harvest times. Soil application experiments revealed that pronamide translocation was similar between SC-R, SL-R, and both susceptible populations across harvest times. The LH-R population translocated less soil-applied pronamide than susceptible populations at 24, 72, and 168 h after treatment, suggesting that reduced acropetal translocation may contribute to pronamide resistance. This study reports three new pronamide-resistant populations, two of which are resistant to two modes of action (MOAs), and one of which is resistant to three MOAs. Results suggest that both target site– and translocation-based mechanisms may be associated with pronamide resistance. Further research is needed to confirm the link between pronamide resistance and the Thr-239-Ile mutation of the α-tubulin gene.

Comparative proteomic and physiological analyses reveal tribenuron-methyl phytotoxicity and nontarget-site resistance mechanisms in Brassicanapus.

The application of herbicides is the most effective strategy for weed control and the development of herbicide-resistant crops will facilitate the weed management. The acetolactate synthase-inhibiting herbicide, tribenuron-methyl (TBM), is broadly used for weed control. However, its application in rapeseed field is restricted since rapeseed is sensitive to TBM. Herein, an integrated study of cytological, physiological and proteomic analysis of the TBM-resistant rapeseed mutant M342 and its wild-type (WT) plants was conducted. After TBM spraying, M342 showed improved tolerance to TBM, and proteins implicated in non-target-site resistance (NTSR) to herbicides had a significantly higher level in M342 as compared with the WT. Differentially accumulated proteins (DAPs) between these two genotypes were enriched in glutathione metabolism and oxidoreduction coenzyme metabolic process, which protected the mutant from oxidative stress triggered by TBM. Important DAPs related to stress or defence response were up-accumulated in M342regardless of the TBM treatment, which might serve as the constitutive part of NTSR to TBM. These results provide new clues for further exploration of the NTSR mechanism in plants and establish a theoretical basis for the development of herbicide-resistant crops.

Metabolic resistance to acetolactate synthase inhibitors in Beckmannia syzigachne: identification of CYP81Q32 and its transcription regulation.

Frequent herbicide use selects for herbicide resistance in weeds. Cytochrome P450s are important detoxification enzymes responsible for herbicide resistance in plants. We identified and characterized a candidate P450 gene (BsCYP81Q32) from the problematic weed Beckmannia syzigachne to test whether it conferred metabolic resistance to the acetolactate synthase-inhibiting herbicides mesosulfuron-methyl, bispyribac-sodium, and pyriminobac-methyl. Transgenic rice overexpressing BsCYP81Q32 was resistant to the three herbicides. Equally, rice overexpressing the rice ortholog gene OsCYP81Q32 was more resistant to mesosulfuron-methyl. Conversely, an OsCYP81Q32 gene knockout generated using CRISPR/Cas9 enhanced mesosulfuron-methyl sensitivity in rice. Overexpression of the BsCYP81Q32 gene resulted in enhanced mesosulfuron-methyl metabolism in transgenic rice seedlings via O-demethylation. The major metabolite, demethylated mesosulfuron-methyl, was chemically synthesized and displayed reduced herbicidal effect in plants. Moreover, a transcription factor (BsTGAL6) was identified and shown to bind a key region in the BsCYP81Q32 promoter for gene activation. Inhibition of BsTGAL6 expression by salicylic acid treatment in B. syzigachne plants reduced BsCYP81Q32 expression and consequently changed the whole plant response to mesosulfuron-methyl. Sequence polymorphisms in an important region of the BsTGAL6 promoter may explain the higher expression of BsTGAL6 in resistant versus susceptible B. syzigachne plants. Collectively, the present study reveals the evolution of an herbicide-metabolizing and resistance-endowing P450 and its transcription regulation in an economically important weedy plant species.

Effects of nozzle type and adjuvant selection on common lambsquarters (Chenopodium album) and johnsongrass (Sorghum halepense) control using nicosulfuron in corn

AbstractWeed control in corn is a major challenge due to increasing problems with highly dominant weed species and herbicide resistance evolution. Common lambsquarters and johnsongrass constitute up to 80% to 90% of the weed population in many spring crops, such as soybean [Glycine max (L.) Merr.], sunflower (Helianthus annuus L.), and corn, in Serbia. Currently, acetolactate synthase–inhibiting herbicides, such as the systemic selective sulfonylurea nicosulfuron, are most commonly used for chemical weed control of those species. A better understanding of the impact of nozzle type and adjuvant use on nicosulfuron efficacy can help to improve control of common lambsquarters and johnsongrass and minimize herbicide resistance development. Field trials were conducted in Serbia from 2020 to 2022 to evaluate the impact of two adjuvants (a non-ionic surfactant [NIS] and a mineral fertilizer ammonium sulfate [AMS]) and two nozzle types (drift-reducing nozzles and flat-fan nozzles) on common lambsquarters and johnsongrass control using nicosulfuron. Satisfactory biomass reduction of common lambsquarters (83% to 87%) and johnsongrass (83% to 97%) was achieved after nicosulfuron application. Adding a NIS adjuvant increased the biomass reduction for common lambsquarters (94% to 98%) and johnsongrass (90% to 100%) independently of the nozzle type used. Selection of nozzle type did not show consistent effects on common lambsquarters and johnsongrass control. Nicosulfuron efficacy was increased with NIS adjuvant for both nozzle types compared to nicosulfuron solo for both species, and Extended Range (XR) TeeJet® nozzles on average resulted in a higher efficacy for common lambsquarters compared to Turbo TeeJet® induction. Adding a mineral AMS adjuvant resulted in lower biomass reduction for both nozzle types and weed species (65% to 78% and 61% to 91% for common lambsquarters and johnsongrass, respectively). Corn grain yield was predominantly influenced by annual meteorological conditions and adjuvant type added to nicosulfuron. This research suggests that addition of the non-ionic adjuvant is an essential factor for successful control of common lambsquarters and johnsongrass in corn and enables use of drift-reducing nozzles.

Diversity and Spread of Acetolactate Synthase Allelic Variants at Position 574 Endowing Resistance in Amaranthus hybridus in Italy

Poor control of Amaranthus spp. with herbicides inhibiting acetolactate synthase (ALS) has been observed for several years in soybean fields in north-eastern Italy, but to date only a few ALS-resistant populations have been confirmed. An extensive sampling of putatively resistant Amaranthus accessions was completed in the Friuli Venezia Giulia region, across an arable land area of about 3000 km2. In total, 58 accessions were tested to confirm their resistance status, recognize the Amaranthus species, identify the mutant ALS alleles endowing the resistance and determine the efficacy of 3 pre-emergence herbicides. Most accessions resulted in cross-resistance to thifensulfuron-methyl and imazamox. Genomic DNA were extracted from single seeds with a newly developed protocol; an allele-specific PCR assay revealed the presence of the 574-leucine in 20 accessions, of the 574-methionine in 22, and of both alleles in 9 accessions. The two variants showed a different spatial distribution. All resistant populations were ascribed to A. hybridus. A. hybridus resistant to ALS herbicides is well-established in this Italian region and its resistance is due to two ALS mutant alleles. Metribuzin, clomazone and metobromuron can be used as alternative herbicides to be applied in pre-emergence and they should be integrated into the management strategies to limit the spread of resistance.

Role of oxidative stress in the physiology of sensitive and resistant Amaranthus palmeri populations treated with herbicides inhibiting acetolactate synthase.

The aim of the present study was to elucidate the role of oxidative stress in the mode of action of acetolactate synthase (ALS) inhibiting herbicides. Two populations of Amaranthus palmeri S. Watson from Spain (sensitive and resistant to nicosulfuron, due to mutated ALS) were grown hydroponically and treated with different rates of the ALS inhibitor nicosulfuron (one time and three times the field recommended rate). Seven days later, various oxidative stress markers were measured in the leaves: H2O2, MDA, ascorbate and glutathione contents, antioxidant enzyme activities and gene expression levels. Under control conditions, most of the analysed parameters were very similar between sensitive and resistant plants, meaning that resistance is not accompanied by a different basal oxidative metabolism. Nicosulfuron-treated sensitive plants died after a few weeks, while the resistant ones survived, independently of the rate. Seven days after herbicide application, the sensitive plants that had received the highest nicosulfuron rate showed an increase in H2O2 content, lipid peroxidation and antioxidant enzymatic activities, while resistant plants did not show these responses, meaning that oxidative stress is linked to ALS inhibition. A supralethal nicosulfuron rate was needed to induce a significant oxidative stress response in the sensitive population, providing evidence that the lethality elicited by ALS inhibitors is not entirely dependent on oxidative stress.

Performance of different herbicides on pondweed (Potamogeton nodosus) in rice

AbstractPondweed is a rhizomatous perennial weed of aquatic habitats that recently adapted to rice ecosystems in northern Iran. Two field experiments were conducted at the Rice Research Institute of Iran to determine the impact of pondweed on rice yield and identify effective herbicides for pondweed control. The focus of the first study was to evaluate the herbicides commonly used in Iranian rice, including butachlor, pretilachlor, oxadiargyl, pendimethalin, thiobencarb, and bensulfuron-methyl. None of these herbicides effectively controlled pondweed, except bensulfuron, which reduced pondweed biomass by ≥95% and produced 26% higher rough rice grain yield than the nontreated plots. The second experiment evaluated the performance of acetolactate synthase–inhibiting herbicides on pondweed control, rough rice yield, and pondweed regrowth. Herbicide efficacy on pondweed varied from 36% to 100%. Five preemergence herbicides, bensulfuron at 45 g ai ha−1, flucetosulfuron at 30 g ai ha−1, triafamone plus ethoxysulfuron at 40 g ai ha−1, and metsulfuron-methyl at 15 g ai ha−1, provided ≥98% control of pondweed. Use of postemergence herbicides penoxsulam at 35 g ai ha−1, bispyribac-sodium at 30 g ai ha−1, and pyribenzoxim at 35 g ai ha−1provided 36%, 89%, and 93% pondweed control, respectively. Rough rice yields ranged from 107% to 124% in herbicide-treated plots compared with the nontreated plots. Soil-applied herbicide treatments produced higher (≥119%) yield than the hand-weeded control or foliar-applied herbicides. Pondweed regrowth was affected by herbicides and was variable. Soil-applied residual herbicides metazosulfuron, flucetosulfuron, and metsulfuron provided complete control of pondweed and prevented regrowth. In contrast, pondweed regrowth in other soil- and foliar-applied herbicide treatments occurred, indicating their lesser translocation to underground vegetative rhizomes. This study shows that although most sulfonylurea herbicides can control pondweed effectively to achieve high rough rice yield, only a few soil-applied herbicides were able to prevent pondweed regrowth.

Miscanthus×giganteusgrowth and control in simulated upland and wetland habitats

AbstractGlobally, giant miscanthus (Miscanthus × giganteusJ.M. Greef & Deuter ex Hodkinson & Renvoize [sacchariflorus × sinensis]) is used as a biofuel crop due to its ability to persist in a wide range of climates. However, little work has assessed this plant’s ability to invade and persist in wetland habitats. In outdoor mesocosms, we examinedM.×giganteus’s ability to grow in simulated wetland versus upland habitats and examined chemical control strategies for both habitats using aquatic-labeled herbicides.Miscanthus×giganteusgrowth was consistently greater in simulated wetland habitats, with wetland plants 2.4 to 3 times taller than upland plants at 6 wk after treatment (WAT) and 2.8 to 3.3 times taller than upland plants at 12 WAT.Miscanthus×giganteusaboveground biomass was 12.7 to 17.7 times greater in wetland- versus upland-grown plants at 6 WAT and 9.6 to 12.5 times greater at 12 WAT. Belowground biomass was 4.5 to 10.7 times greater in wetland versus upland grown plants at 6 WAT and 4.0 to 6.1 times greater at 12 WAT.Miscanthus×giganteusbelowground biomass was always greater than aboveground in both habitats at 6 (6.0 times greater in wetlands and 2.9 times greater in uplands) and 12 WAT (3.8 times greater in wetlands and 1.3 times greater in uplands). Generally, all herbicide treatments reducedM.×giganteusheight (66% to 100% reduction) and biomass (84% to 100%) compared with nontreated plants at 12 WAT; however, glyphosate (5,716.3 g ai ha−1) and imazapyr (1,120.8 g ai ha−1) performed better than imazamox (560.4 g ai ha−1) and penoxsulam (98.6 g ai ha−1). This is the first work to provide evidence thatM.×giganteuscan be chemically controlled in wetland habitats. Furthermore, this is the first work to show that penoxsulam (an acetolactate synthase–inhibiting herbicide) can reduceM.×giganteusgrowth in upland or wetland habitats.

Tolerance of Pima and Upland cotton to trifloxysulfuron (Envoke) herbicide under field conditions

Trifloxysulfuron (Envoke) is an acetolactate synthase-inhibitor herbicide and can be used to control many broadleaf weeds and nutsedges in cotton production. However, there is a lack of information on genotypic variation in response to the herbicide. In this field study, 60 Pima (Gossypium barbadense L.) lines, 122 Upland (G. hirsutum L.) lines, and 9 Upland × Pima segregating populations were divided into five tests (18A, 18B, 18G, 18RB, and 18HQ) to evaluate trifloxysulfuron tolerance at the 7-true leaf stage (42 days after planting) under the same field conditions in 2018. Across the five tests, Pima cotton genotypes tested in this study did not show any visual crop injury based on percentage of plants with chlorosis at 6 days after treatment (DAT), indicating consistent and high levels of trifloxysulfuron tolerance. However, the response to trifloxysulfuron within Upland cotton is highly variable. While Upland cotton is overall more sensitive to trifloxysulfuron with crop injury up to 80% than Pima cotton, 19 lines had injury below 5% including one line with no visual injury, and 19 lines had injury between 5% and 10%. In test 18HQ with 15 transgenic Upland cultivars and 17 non-transgenic Upland lines, the analysis of variance detected a significant genotypic difference. The broad-sense heritability estimates for trifloxysulfuron tolerance based on crop injury at 6 DAT was 0.555, suggesting that trifloxysulfuron tolerance in Upland cotton is moderately heritable. This study represents the first report that Pima cotton and many Upland cotton lines are highly tolerant to trifloxysulfuron with no or little crop injury.

Apera spica‐venti in the Czech Republic develops resistance to three herbicide modes of action

AbstractInvestigation of molecular mechanisms of herbicide resistance to three modes of action was carried out in a population of Apera spica‐venti with resistance to herbicides inhibiting acetolactate synthase (ALS), acetyl‐CoA‐carboxylase (ACCase) and inhibitors of photosystem‐II (PSII). Greenhouse experiments were conducted to detect and characterize resistance pattern to pyroxsulam (ALS inhibitor), pinoxaden (ACCase) and chlorotoluron (PSII) using the recommended rate of each herbicide alone and in tank‐mix, sequential application and dose–response tests. Metabolic detoxification and/or reduced herbicide uptake and translocation studies were conducted using dose–response tests with the herbicides in combination with malathion. After treatment, leaves from surviving plants were collected for pyrosequencing to identify target‐site mutations in specific regions of ALS, ACCase and psbA genes. Among 32 analysed plants, target‐site mutations in specific regions were detected for ALS and ACCase gene, but no psbA mutations were found. Dose–response assay showed high resistance factors for pyroxsulam (RF = 269.4), pinoxaden (RF = 66.3), but lower for chlorotoluron (RF = 14.3). Testing for metabolic detoxification by pre‐treatment with malathion resulted in an increased susceptibility to pinoxaden in all doses, some increased susceptibility to chlorotoluron at the highest doses and no difference for pyroxsulam. Our results indicate that while target‐site mutations were present for ALS and ACCase sites of action, metabolic detoxification does play a role for pinoxaden resistance. This research provided key insights into the resistance mechanisms in Apera spica‐venti and will be important for developing control strategies for this weed in the Czech Republic.

Performance of grain sorghum hybrids resistant to acetolactate synthase and acetyl coenzyme‐A carboxylase inhibitor herbicides

AbstractAcreage under grain sorghum [Sorghum bicolor (L.) Moench] in the United States has sharply declined over the past several decades. Among the major causes are the lack of better postemergence weed control options. Farmers opt for crops and tools that allow better management of weeds, and sorghum is not one of them. The discovery of sources of resistance to acetolactate synthase (ALS) and acetyl‐coenzyme A carboxylase (ACCase) inhibitor herbicides in feral relatives of sorghum opened a new horizon for development of a resistance‐based weed control option for the crop. The objective of this study is to demonstrate the agronomic potential of sorghum hybrids resistant to ALS and ACCase inhibitor herbicides and shed light on concerns that deployment of resistance traits may cause yield drags. A total of 186 hybrids involving homozygous ALS resistant, homozygous ACCase resistant, heterozygous ALS and ACCase resistance, and conventional hybrids plus commercial checks were grown in three sets consisting of 68, 62 and 56 entries for Set I, Set II, and Set III, respectively. The experiments were conducted during the 2014 and 2015 season in three replications at Kansas State University Agronomy Research Farm near Manhattan, KS. Data were collected on plant height, maturity, yield, and yield components, as well as grain nutritional traits. The analysis of the data revealed that the resistance technology has no negative effect on agronomic adaptability, yield potential, and nutritional traits of grain sorghum.

Occurrence, resistance factors and cross-resistance patterns to herbicides inhibiting acetolactate synthase (ALS) of Echinochloa crus-galli (L.) Pal. Beauv. in Central Europe

Echinochloa crus-galli (L.) Pal. Beauv. is one of the most noxious weeds in maize cultivation and has evolved target-site resistance to ALS-inhibiting herbicides. Due to the hexaploid nature of E. crus-galli, resistance inducing mutations can be harbored by multiple gene copies, but up to now, studies did not include an analysis of the ALS gene copies conferring resistance. Investigations on E. crus-galli populations from different countries in Central Europe revealed the presence of several point mutations conferring resistance and occurring independently at the positions Ala-122, Pro-197 and Trp-574. Moreover, an Asp-376-Glu mutation in the ALS gene in E. crus-galli and a Ser-653-Tyr amino acid change in the ALS protein of a weed were detected for the first time. Additionally, the study revealed the first case of an ALS-resistant population (Trp-574-Leu) in the Czech Republic. The detection of mutations is relevant to the evolution and management of herbicide resistance. Dose–response experiments showed different effects depending on the population and the individual active ingredients and mixtures. Due to the genetic variability of E. crus-galli, results were highly variable. Overall, the populations showed a high level of resistance. For the prevailing mutated positions, i.e., Ala-122, Pro-197 and Trp-574, gene copies were examined separately using molecular genetic methods. A single mutation in at least one out of three ALS gene copies was sufficient to confer resistance at the positions Pro-197 and Trp-574. At Ala-122, point mutations co-occurring in the ALS 1, ALS 2 and ALS 3 gene copy were identified. The study provides a starting point for differentiation of the resistance mechanisms in polyploid E. crus-galli.

Response of Alopecurus myosuroides Huds. to varying intensities of acetolactate synthase-inhibiting herbicides in a crop rotation including imidazolinone-tolerant oilseed rape

Herbicide-tolerant winter oilseed rape (OSR) varieties offer the opportunity of using imazamox for weed control, an active ingredient belonging to the chemical group of acetolactate synthase (ALS-) inhibitors. However, ALS inhibitors are used in many different crops and are the most resistance-prone herbicide mode of action. Their frequent application in a crop rotation increases the selection pressure for ALS herbicide resistance in weeds, which has to be considered when designing resistance management strategies. Alopecurus myosuroides Huds. is a frequent and economically important grass weed in Northwestern Europe, which has evolved resistance to several herbicide modes of action. For the sustainable use of herbicide-tolerant OSR varieties, studies on the effects of the different intensities of ALS inhibitor use on A. myosuroides population dynamics and resistance development are required. Two field trials were conducted including susceptible and multiple resistant A. myosuroides individuals and four weed control strategies varying in their intensity of ALS inhibitor use over a three-year trial period. A. myosuroides head numbers, the presence of target-site mutations in surviving plants, and crop yields were assessed annually, and the amount of A. myosuroides seeds in the soil seed bank was determined at the end of the trial period. The results show that the intensity of ALS inhibitor use significantly influenced the density of A. myosuroides and the development of resistance. Under weed control strategy IV (no ALS inhibitors), an increase in A. myosuroides head number was observed due to multiple resistance in the A. myosuroides population employed in the field trial. None of the four weed control strategies was able to control A. myosuroides infestation to an acceptable level. The results on A. myosuroides densities in the soil seed bank were highly variable and inconsistent. Molecular analysis of surviving plants showed a selection of ALS-resistant biotypes depending on the ALS inhibitor selection pressure. This study did not reveal any specific deterioration of A. myosuroides infestation associated with the use of imidazolinone-tolerant OSR in a short-term crop rotation. However, this OSR production system should not be employed if ALS-resistant A. myosuroides plants are present in the field.

Enhanced metabolism and target gene overexpression confer resistance against acetolactate synthase-inhibiting herbicides in Bromus sterilis.

Intensive application of acetolactate synthase (ALS)-inhibiting herbicides has resulted in herbicide-resistance in many weeds, including Bromus sterilis. The present study was conducted to identify the mechanisms conferring resistance to ALS-inhibiting herbicides in a Bromus sterilis biotype. Dose-response studies revealed the resistant biotype to be 288 times less sensitive to pyroxsulam than the susceptible biotype. Furthermore, experiment with a single-dose, proved this biotype was also cross-resistant to propoxycarbazone, iodosulfuron plus mesosulfuron and sulfosulfuron. Prior treatment with malathion, a known inhibitor of cytochrome P450s, reduced the level of resistance to pyroxsulam. No mutations were detected from the partial ALS gene sequencing. Flow cytometry and chromosome counting rejected ploidy level variation between the susceptible and resistant biotypes. Relative copy number variation ruled out gene amplification. Quantitative real-time polymerase chain reaction (PCR) detected a significant difference in ALS gene expression between the susceptible and resistant biotypes. Target gene overexpression and enhanced metabolism by cytochrome P450s are likely mechanisms of resistance to pyroxsulam in Bromus sterilis. The current findings highlight the need to monitor additional brome populations for herbicide resistance in Europe and endorse the need for alternate herbicides in integrated weed management to delay the possible evolution of herbicide resistance in these species. © 2020 Society of Chemical Industry.

Ambrosia trifida L. - trolisna ambrozija

Ambrosia trifida L. (AMBTR, fam. Asteraceae/Compositae) is native to North America. It was introduced accidentally to Europe at the end of the 19th century, with contaminated animal feed and seeds for planting. Today A. trifida is present in ruderal and agricultural habitats of many European countries (France, Italy, Germany, Russia, Spain, Romania, Slovakia, Czech Republic, Poland, Serbia, Bulgaria, etc.). Giant ragweed was detected for the first time in 1981 in Serbia (site Čoka). Over the following period it disappeared from this site, but was recorded again in 2006 in another site (central Bačka: Despotovo, Kucura, Savino Selo, Ravno Selo, Ruski Krstur). Currently in Serbia it has the status of an alien naturalized weed species. This summer annual plant can grow up to 6 m in height and exhibits a high degree of morphological and reproductive plasticity in response to encroachment by neighboring plants. It is present in disturbed habitats, such as agriculture fields, where it plays the role of the dominant species throughout the entire growing season. In most cases, leaves are opposite and always simple and generally have 3 distinct lobes but can also have as many as 5. It is a diploid (2n = 24), meso-hygrophilic species, preferring wet habitatse and can tolerate a wide variety of soil types. Also, this is a monoecious plant, where male and female flowers are separated on the same individual. A. trifida can hybridise with A. artemisiifolia (A. x helenae Rouleau, with 2n= 27 and 2n= 33), but this hybrid has been described as sterile. Compared to other summer annual species, A. trifida is among the first to emerge in early spring, at optimal temperatures from 10-24°C. Under optimal environmental conditions, giant ragweed produces around 1,800 (max 5,100) seeds plant-1. It flowers and bears fruit from July to September (October).The pollen of this species has allergenic potential. Additionally, in the USA and Canada giant ragweed populations have developed resistance to acetolactate synthase inhibitor herbicides and glyphosate. Giant ragweed can be a problematic weed in row crops (corn, soybean, sunflower, sugerbeet) and vegetables. In A. trifida the control measures should prevent further spread, and existing populations should be controlled by integrated weed management practices. Furthermore, A. trifida has a relatively low fecundity, a transient soil seedbank and a high percentage of non-viable or low-survivorship seeds, which are features that may have constrained its establishment and spread in the current environmental conditions in Serbia.

Cytochrome P450 CYP81A10v7 in Lolium rigidum confers metabolic resistance to herbicides across at least five modes of action.

Rapid and widespread evolution of multiple herbicide resistance in global weed species endowed by increased capacity to metabolize (degrade) herbicides (metabolic resistance) is a great threat to herbicide sustainability and global food production. Metabolic resistance in the economically damaging crop weed species Lolium rigidum is well known but a molecular understanding has been lacking. We purified a metabolic resistant (R) subset from a field evolved R L. rigidum population. The R, the herbicide susceptible (S) and derived F2 populations were used for candidate herbicide resistance gene discovery by RNA sequencing. A P450 gene CYP81A10v7 was identified with higher expression in R vs. S plants. Transgenic rice overexpressing this Lolium CYP81A10v7 gene became highly resistant to acetyl-coenzyme A carboxylase- and acetolactate synthase-inhibiting herbicides (diclofop-methyl, tralkoxydim, chlorsulfuron) and moderately resistant to hydroxyphenylpyruvate dioxygenase-inhibiting herbicide (mesotrione), photosystem II-inhibiting herbicides (atrazine and chlorotoluron) and the tubulin-inhibiting herbicide trifluralin. This wide cross-resistance profile to many dissimilar herbicides in CYP81A10v7 transgenic rice generally reflects what is evident in the R L. rigidum. This report clearly showed that a single P450 gene in a cross-pollinated weed species L. rigidum confers resistance to herbicides of at least five modes of action across seven herbicide chemistries.