Pinoxaden resistance in Lolium perenne L. is due to both target-site and non-target-site mechanisms

An oral developmental toxicity study of generic pesticide pinoxaden in rabbits

The safety assessment of pinoxaden by the Joint Meeting on Pesticide Residues (JMPR) established a NOAEL of 30 mg/kg bw/day for maternal and embryo/fetal toxicity from a rabbit developmental toxicity study. However, the Pesticide Peer Review Expert meeting (EFSA) lowered the NOAEL to 10 mg/kg bw/day due to observed diaphragm malformations in one developmental toxicity study in rabbits, proposing a classification for developmental effects as Category 2 R63 or H361d. Both JMPR and EFSA set the Acceptable Daily Intake (ADI) at 0.1 mg/kg bw/day, derived from a 2-year rat study NOAEL with a safety factor of 100, but EFSA also supported ADI by teratology study in rabbits. The current prenatal developmental toxicity study on pinoxaden aimed to elucidate and clarify the potential teratogenic effects and could provide supplementary data for determining the ADI for pinoxaden. The study design exceeded the OECD TG 414 by including an assessment of internal organs. The test item was orally administered by gavage daily from day 6 to day 28 of gestation to three groups of animals, each composed of 21 females, in dose levels of 0, 10 and 30 mg/kg/bw/day. One female from the 30 mg/kg/bw/day dose group was euthanized in extremis on Day 27 post-coitum due to premature delivery, likely induced by poor general condition and was therefore considered to be an indirect effect of the test item. One female at 30 mg/kg/bw/day had entirely dead litters except for one live male pup (9 non-live implants vs 1 live fetus). Since the incidence of post-implantation loss or mean number of the dead pups within the remaining dams at 30 mg/kg/ bw/day that survived to necropsy was not significantly increased, we assume that the toxic effect was on the dam, rather than on the conceptus. No pinoxaden-related skeletal or visceral variations or malformations were observed. No evidence of developmental toxicity was observed. Under the conditions of the study, the pinoxaden produced maternal toxicity at a high dose tested; thus, NOAEL for maternal toxicity was determined to be 10 mg/kg bw/day. NOAEL for developmental toxicity was established at 30 mg/kg bw/day. The obtained results may supplement the overall safety and toxicity profile of pinoxaden. Nevertheless, the NOAEL determined in this study does not affect the previously established ADI.

Investigation of acetyl-CoA carboxylase-inhibiting herbicides that exhibit soybean crop selectivity

The sustainable control of weed populations, particularly resistant species, is a significant challenge in agriculture around the world. The α-aryl-keto-enol (aryl-KTE) class of acetyl-CoA carboxylase (ACCase)-inhibiting herbicides represent a possible solution for the control of resistant grasses even though achieving crop selectivity remains a challenge. Herein, we present some of our investigations into identifying the most promising structural features within the aryl-KTE class that give the highest chance of achieving soybean crop selectivity, whilst also maintaining strong and broad efficacy against problematic weed species. We further examined our results by preparing new aryl-KTE molecules which were evaluated in glasshouse screening assays for their herbicidal efficacy as well as their soybean selectivity. We consider that uniting this approach with other optimization criteria, such as toxicological and environmental safety profiles, will enable the streamlining of crop protection optimizations programmes, ultimately delivering safer and more sustainable solutions to farmers and consumers. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Target-site mutations Ile1781Leu and Ile2041Asn in the ACCase2 gene confer resistance to fluazifop-p-butyl and pinoxaden herbicides in a johnsongrass accession from Arkansas, USA

Johnsongrass [Sorghum halepense (L.) Pers.] is a troublesome weed species in different agricultural and non-agricultural areas. Because of its biology, reproductive system, and seed production, effective management is challenging. An accession with low susceptibility to the acetyl-CoA carboxylase (ACCase)-inhibiting herbicides fluazifop-p-butyl (fluazifop) and pinoxaden was collected in eastern Arkansas. In this research, the molecular mechanisms responsible for ACCase resistance were investigated. Dose-response experiments showed a resistance factor of 181 and 133 for fluazifop and pinoxaden, respectively. Molecular analysis of both ACCase1 and ACCase2 genes was researched. Nucleotide comparison of ACCase1 between resistant and susceptible accessions showed no single nucleotide polymorphisms. Nonetheless, analysis of ACCase2 in fluazifop-resistant johnsongrass plants revealed the Ile1781Leu target-site mutation was dominant (nearly 75%), whereas the majority of pinoxaden-resistant johnsongrass plants had the Ile2041Asn (60%). Not all sequenced johnsongrass plants displayed a target-site mutation, suggesting the presence of additional resistance mechanisms. Amplification of ACCase1 and ACCase2 was not responsible for resistance because of the similar values obtained in both resistant and susceptible accessions. Experiments with malathion and NBD-Cl suggest the presence of herbicide metabolism. Outcomes of this research demonstrated that fluazifop- and pinoxaden-resistant johnsongrass plants displayed a target-site mutation in ACCase2, but also that non-target-site resistance mechanisms would be involved and require a detailed study.

Repeated evolution of herbicide resistance in Lolium multiflorum revealed by haplotype-resolved analysis of acetyl-CoA carboxylase

Herbicide resistance in weeds is one of the greatest challenges in modern food production. The grass species Lolium multiflorum is an excellent model species to investigate evolution under similar selection pressure because populations have repeatedly evolved resistance to many herbicides, utilizing a multitude of mechanisms to neutralize herbicide damage. In this work, we investigated the gene that encodes acetyl-CoA carboxylase (ACCase), the target site of the most successful herbicide group available for grass weed control. We sampled L. multiflorum populations from agricultural fields with history of intense herbicide use, and studied their response to three ACCase-inhibiting herbicides. To elucidate the mechanisms of herbicide resistance and the genetic relationship among populations, we resolved the haplotypes of 97 resistant and susceptible individuals by sequencing ACCase amplicons using long-read DNA sequencing technologies. Our dose-response data indicated the existence of many, often unpredictable, resistance patterns to ACCase-inhibiting herbicides, where populations exhibited as much as 37-fold reduction in herbicide response. The majority of the populations exhibited resistance to all three herbicides studied. Phylogenetic and molecular genetic analyses revealed multiple evolutionary origins of resistance-endowing ACCase haplotypes, as well as widespread admixture in the region regardless of cropping system. The amplicons generated were diverse, with haplotypes exhibiting 26-110 polymorphisms. Polymorphisms included insertions and deletions 1-31 bp in length, none of which were associated with the resistance phenotype based on an association analysis. We also found evidence that some populations have multiple mechanisms of resistance. Our results highlight the astounding genetic diversity in L. multiflorum populations, and the potential for repeated evolution of herbicide resistance across the landscape that challenges weed management approaches and jeopardizes sustainable weed control practices. We provide an in-depth discussion of the evolutionary and practical implications of our results.

Sequence and structural similarities of ACCase protein of Phalaris minor and wheat: An insight to explain herbicide selectivity

Uncontrolled growth of Phalaris minor in the wheat (Triticum aestivum) crop has remained a problem, leading to a massive reduction in wheat grain production. Herbicides have been used to control the weed, which leads to the development of frequent resistance in P. minor and mutant biotypes were also reported (Trp2027Cys and Ile2041Asn). Development of resistance enforced agro researchers to analyses the action of herbicide on P. minor. In this study, the sequence and structure of P. minor and T. aestivum Acetyl CoA Carboxylase (ACCase) have been analysed to locate the differences in their sequence and structure and to formulate a plausible explanation of the selectivity of herbicides which may help in the rationale discovery of noble herbicides. The sequence and 3D structure analysis of weed and wheat ACCase indicate minute differences in the distantly located amino acid residues. However, proteins are conserved at the binding site of herbicides with no mutation at the catalytic site. Analysis indicates that herbicides selectively target P. minor ACCase might be due to unknown other reasons, but not due to differences in their protein sequence and structure.

First Asp-2078-Gly Mutation Conferring Resistance to Different ACCase Inhibitors in a Polypogon fugax Population from China

Asia minor bluegrass (Polypogon fugax) is a common and problematic weed throughout China. P. fugax that is often controlled by acetyl-CoA carboxylase (ACCase) inhibitors in canola fields. Herein, we confirmed a P. fugax population (R) showing resistance to all ACCase inhibitors tested with resistance indexes ranging from 5.4-18.4. We further investigated the resistance mechanisms of this R population. Molecular analyses revealed that an amino acid mutation (Asp-2078-Gly) was present in the R population by comparing ACCase gene sequences of the sensitive population (S). In addition, differences in susceptibility between the R and S population were unlikely to be related to herbicide metabolism. Furthermore, a new derived cleaved amplified polymorphic sequence (dCAPS) method was developed for detecting the Asp-2078-Gly mutation in P. fugax efficiently. We found that 93.75% of plants in the R population carried the Asp-2078-Gly mutation, and all the herbicide-resistant phenotype of this R population is inseparable from this mutation. This is the first report of cross resistance to ACCase inhibitors conferred by the Asp-2078-Gly target-site mutation in P. fugax. The research suggested the urgent need to improve the diversity of weed management practices to prevent the widespread evolution of herbicide resistance in P. fugax in China.