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BACKGROUND: Giant ragweed (Ambrosia trifida L.) is one of the most troublesome weed species in corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] cropping systems. Following numerous reports in 2018 of suspected herbicide resistance in several Ambrosia trifida populations from Wisconsin, our objective was to characterize the response of these accessions to acetolactate synthase (ALS), enolpyruvyl shikimate phosphate synthase (EPSPS), and protoporphyrinogen oxidase (PPO) inhibitors applied POST. RESULTS: Four accessions (AT1, AT4, AT6, and AT10) exhibited ≥ 50% plant survival after exposure to the cloransulam 3× rate. Two accessions (AT8 and AT10) and one accession (AT2) exhibited ≥ 50% plant survival after exposure to glyphosate and fomesafen 1× rates, respectively. The AT10 accession exhibited multiple resistance to cloransulam and glyphosate. The AT12 accession was 28.8‐fold resistant to fomesafen and 3.7‐fold resistant to lactofen. A codon change in PPX2 conferring a R98L substitution was identified as the most likely mechanism conferring PPO‐inhibitor resistance. CONCLUSION: To our knowledge, this is the first confirmed case of PPO‐inhibitor resistance in Ambrosia trifida globally and we identified the genetic mutation likely conferring resistance. Proactive and diversified integrated weed management strategies are of paramount importance for sustainable long‐term Ambrosia trifida management. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry. [ABSTRACT FROM AUTHOR]

Introduction: Effective weed management tools are crucial for maintaining the profitable production of snap bean (Phaseolus vulgaris L.). Preemergence herbicides help the crop to gain a size advantage over the weeds, but the few preemergence herbicides registered in snap bean have poor waterhemp (Amaranthus tuberculatus) control, a major pest in snap bean production. Waterhemp and other difficult-to-control weeds can be managed by flumioxazin, an herbicide that inhibits protoporphyrinogen oxidase (PPO). However, there is limited knowledge about crop tolerance to this herbicide. We aimed to quantify the degree of snap bean tolerance to flumioxazin and explore the underlying mechanisms. Methods: We investigated the genetic basis of herbicide tolerance using genome-wide association mapping approach utilizing field-collected data from a snap bean diversity panel, combined with gene expression data of cultivars with contrasting response. The response to a preemergence application of flumioxazin was measured by assessing plant population density and shoot biomass variables. Results: Snap bean tolerance to flumioxazin is associated with a single genomic location in chromosome 02. Tolerance is influenced by several factors, including those that are indirectly affected by seed size/weight and those that directly impact the herbicide's metabolism and protect the cell from reactive oxygen species-induced damage. Transcriptional profiling and co-expression network analysis identified biological pathways likely involved in flumioxazin tolerance, including oxidoreductase processes and programmed cell death. Transcriptional regulation of genes involved in those processes is possibly orchestrated by a transcription factor located in the region identified in the GWAS analysis. Several entries belonging to the Romano class, including Bush Romano 350, Roma II, and Romano Purpiat presented high levels of tolerance in this study. The alleles identified in the diversity panel that condition snap bean tolerance to flumioxazin shed light on a novel mechanism of herbicide tolerance and can be used in crop improvement. [ABSTRACT FROM AUTHOR]

Management of herbicide‐resistant weeds can be improved by understanding the biology of resistant biotypes. While the majority of research has focused on female plants and seed production of Palmer amaranth (Amaranthus palmeri S. Watson) that are resistant to glyphosate, growth of male plants that are resistant to this herbicide has not been studied in detail. Additionally, interference of male versus female Palmer amaranth plants on cotton (Gossypium hirsutum) yield has not been reported. Plant height and biomass of male and female plants from a mixed population of glyphosate‐resistant (GR) and glyphosate‐susceptible (GS) plants was studied in North Carolina when grown season‐long with cotton. Palmer amaranth height was less for GR male plants compared with GS males and both GR and GS females. Biomass of Palmer amaranth female plants was twice that of male plants irrespective of glyphosate resistance. Cotton yield was affected similarly by Palmer amaranth regardless of either gender or glyphosate resistance status. The implications of shorter GR male plants on pollen dispersal and ramifications on management of glyphosate resistance are not known. Results from these trials did not address implications of the height of male plants on fitness of GR resistance. Nonetheless, the finding that GR male plants were shorter in the field than GS male plants warrants a new look at this topic. Similar reductions for cotton yield in presence of both GR and GS biotypes and genders suggest that current yield loss assessments and management decisions do not need to consider these variables in Palmer amaranth populations. Plain Language Summary: Understanding the biology of Palmer amaranth (Amaranthus palmeri S. Watson) plants that are glyphosate‐resistant and glyphosate‐susceptible can facilitate improved management of this weed in crops. Research in North Carolina found that glyphosate‐resistant male plants were shorter than glyphosate‐susceptible male plants. Cotton (Gossypium hirsutum L.) yield was affected the same regardless of gender or susceptibility to glyphosate even though female plants exhibited twice the biomass of male plants. Further research is needed to address the impact of shorter male plants that are glyphosate resistant on pollen movement and possible fitness of this weed associated with herbicide resistance. These results also indicate that management decisions for Palmer amaranth do not need adjustment based on gender or glyphosate resistance relative to interference with cotton yield. [ABSTRACT FROM AUTHOR]

The emergence of herbicide‐resistant weeds is a significant threat to modern agriculture. Cross resistance, a phenomenon where resistance to one herbicide confers resistance to another, is a particular concern owing to its unpredictability. Nontarget‐site (NTS) cross resistance is especially challenging to predict, as it arises from genes that encode enzymes that do not directly involve the herbicide target site and can affect multiple herbicides. Recent advancements in genomic and structural biology techniques could provide new venues for predicting NTS resistance in weed species. In this review, we present an overview of the latest approaches that could be used. We discuss the use of genomic and epigenomics techniques such as ATAC‐seq and DAP‐seq to identify transcription factors and cis‐regulatory elements associated with resistance traits. Enzyme/protein structure prediction and docking analysis are discussed as an initial step for predicting herbicide binding affinities with key enzymes to identify candidates for subsequent in vitro validation. We also provide example analyses that can be deployed toward elucidating cross resistance and its regulatory patterns. Ultimately, our review provides important insights into the latest scientific advancements and potential directions for predicting and managing herbicide cross resistance in weeds. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]