BACKGROUND:A nearly complete genome assembly consisting of 14 scaffolds, a total length of 969.6 Mb, and an N50 scaffold length of 99.88 Mb, was generated to better understand how transposable element activity has led to adaptive evolution in Bassia scoparia (kochia), an agronomically important weed. RESULTS:The nine largest scaffolds correspond to the nine chromosomes of the close relative, Beta vulgaris. From this assembly, 54 387 protein-coding gene loci were annotated. We determined that genes containing Far-Red Elongated Hypocotyl 3 (FHY3) or Far-Red Impaired Response 1 (FAR1) functional domains have undergone a large, kochia-specific gene family expansion. We discovered that putative Mutator Don-Robertson (MuDR) transposable elements with detectable FHY3/FAR1 domains were tightly associated with segmental duplications of 5-enolpyruvylshikimate-3-phosphate synthase subsequently conferring resistance to the herbicide glyphosate. Further, we characterized a new MuDR subtype, named here as 'Muntjac', which contributes to the evolution of herbicide resistance in kochia through the process of transduplication. CONCLUSION:Collectively, our study provides insights into the role FHY3/FAR1 genes as active transposable elements and contributes new perspectives on the interaction between transposons and herbicide resistance evolution. © 2025 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

This report presents two phased chromosome-scale genome assemblies of allotetraploid Salsola tragus (2n = 4x = 36) and fills the current genomics resource gap for this species. Flow cytometry estimated 1C genome size was 1.319 Gb. PacBio HiFi reads were assembled and scaffolded with Hi-C chromatin contact mapping and Bionano optical mapping data. For annotation, a PacBio Iso-Seq library was generated from root, stem, leaf, and floral tissues followed by annotation using a modified Maker pipeline. The assembled haploid S. tragus genomes contained 18 chromosomes each, with 9 chromosomes assigned to subgenome A and 9 chromosomes to subgenome B. Each haplome assembly represented 95% of the total flow cytometry estimated genome size. Haplome 1 and haplome 2 contained 43,354 and 42,221 annotated genes, respectively. The availability of high-quality reference genomes for this economically important weed will facilitate future omics analysis of S. tragus and a better understanding of chenopod plants.

Abstract Auxin‐mimic herbicides chemically mimic the phytohormone indole‐3‐acetic‐acid (IAA). Within the auxin‐mimic herbicide class, the herbicide fluroxypyr has been extensively used to control kochia (Bassia scoparia). A 2014 field survey for herbicide resistance in kochia populations across Colorado identified a putative fluroxypyr‐resistant (Flur‐R) population that was assessed for response to fluroxypyr and dicamba (auxin‐mimics), atrazine (photosystem II inhibitor), glyphosate (EPSPS inhibitor), and chlorsulfuron (acetolactate synthase inhibitor). This population was resistant to fluroxypyr and chlorsulfuron but sensitive to glyphosate, atrazine, and dicamba. Subsequent dose‐response studies determined that Flur‐R was 40 times more resistant to fluroxypyr than a susceptible population (J01‐S) collected from the same field survey (LD50 720 and 20 g ae ha−1, respectively). Auxin‐responsive gene expression increased following fluroxypyr treatment in Flur‐R, J01‐S, and in a dicamba‐resistant, fluroxypyr‐susceptible line 9,425 in an RNA‐sequencing experiment. In Flur‐R, several transcripts with molecular functions for conjugation and transport were constitutively higher expressed, such as glutathione S‐transferases (GSTs), UDP‐glucosyl transferase (GT), and ATP binding cassette transporters (ABC transporters). After analyzing metabolic profiles over time, both Flur‐R and J01‐S rapidly converted [14C]‐fluroxypyr ester, the herbicide formulation applied to plants, to [14C]‐fluroxypyr acid, the biologically active form of the herbicide, and three unknown metabolites. The formation and flux of these metabolites were faster in Flur‐R than J01‐S, reducing the concentration of phytotoxic fluroxypyr acid. One unique metabolite was present in Flur‐R that was not present in the J01‐S metabolic profile. Gene sequence variant analysis specifically for auxin receptor and signaling proteins revealed the absence of non‐synonymous mutations affecting auxin signaling and binding in candidate auxin target site genes, further supporting our hypothesis that non‐target site metabolic degradation is contributing to fluroxypyr resistance in Flur‐R. Significance Statement: Herbicide resistance is an ever‐present issue in weeds of cropping and rangeland systems. Every characterized herbicide resistance mechanism contributes to a working database used to address herbicide resistance in an agricultural or open‐space setting. Knowing the exact resistance mechanism helps public sector and industry researchers understand why herbicide applications are failing, and if resistant plants can still be controlled with other herbicide modes of action. In the kochia line Flur‐R, there is strong evidence to support a non‐target site resistance mechanism, specifically herbicide degradation via increased enzymatic activity. Increased fluroxypyr degradation represents a novel resistance mechanism to fluroxypyr in Bassia scoparia.

A prelaunch survey of broadleaf weeds was conducted to predict the weed management efficacy of a novel genetically engineered sugar beet with resistance traits for glyphosate, dicamba, and glufosinate. We targeted problematic broadleaf weed species prevalent in sugar beet fields, including kochia, common lambsquarters, Palmer amaranth, and redroot pigweed in Colorado, Nebraska, and Wyoming. The results revealed that a significant percentage of kochia populations in Colorado, Nebraska, and Wyoming exhibited resistance to glyphosate (94%, 98%, and 75%, respectively) and dicamba (30%, 42%, and 17%, respectively). Palmer amaranth populations had resistance frequencies for glyphosate and dicamba of 80% and 20% in Colorado and 20% and 3% in Nebraska, respectively. No resistance to the tested herbicides was identified in common lambsquarters or redroot pigweed. Glufosinate resistance was not identified for any species. Kochia and Palmer amaranth populations from Colorado and Nebraska exhibited glyphosate resistance primarily through 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene amplification. However, one glyphosate-resistant kochia population from Wyoming lacked EPSPS gene amplification, indicating the presence of an alternative resistance mechanism. We identified the previously characterized IAA16 G(73)N substitution in a dicamba-resistant kochia population from Nebraska. However, dicamba-resistant kochia populations from Colorado did not possess this substitution, suggesting an alternative, yet-to-be-determined resistance mechanism. The widespread prevalence of glyphosate and dicamba resistance, coupled with the emergence of novel resistance mechanisms, poses a significant challenge to the long-term efficacy of this novel genetically engineered sugar beet technology. These findings underscore the urgent need for integrated weed management strategies that diversify effective herbicide sites-of-action and incorporate alternative weed management practices within cropping systems.

Transposable element activity has greatly impacted the evolution of Kochia scoparia syn. Bassia scoparia (kochia), an important agricultural weed. In this study, an improved kochia genome assembly of the glyphosate-susceptible line 7710 was analyzed in comparison to a glyphosate-resistant line, M32. A near complete genome assembly consisting of 14 super-scaffolds with a total length of 969.6 Mb, and N50 scaffold length of 99.88 Mb, was generated with hybrid short-read (Illumina), and singlemolecule (PacBio/Bionano genomics) data, in addition to chromatin interaction mapping (Hi-C). The 9 largest scaffolds corresponded to the 9 chromosomes of a close relative, Beta vulgaris . From this assembly, 54,387 protein-coding gene loci were annotated. After gene annotation it was revealed that 58% of the whole-genome consisted of transposable elements. We identified that gene families containing FHY3 (FAR-Red Elongated Hypocotyl 3) and FAR1 (FAR-Red Impaired Response 1) functional domains have undergone a massive Bassia -lineage specific expansion. FHY3 and FAR1 domains were first described as a “domesticated” (non-active) sub-type of Mutator Don-Robertson transposase (MuDR) that function as transcription factors; however, the domains may still function as transposases in the correct genomic context, but this has yet to be observed. Mutator elements are associated with large DNA segmental transposition within the genome including gene duplication events, which can greatly influence plant phenotypes and evolution. We discovered in our work that putative MuDR elements with detectable FHY3 / FAR1 domains were tightly associated with segmental duplications of 5-enolpyruvylshikimate-3-phosphate synthase ( EPSPS ) that confer resistance to the herbicide glyphosate. Further, in a case study of MuDR activity, we characterized a new MuDR subtype, named here as “Muntjac”, which contributes to the evolution of herbicide resistance in kochia through the process of transduplication. Collectively, our study provides insights into the de-domestication of a FHY3/FAR1 transposon and provides new perspectives on the evolution of glyphosate resistance in kochia.

AbstractGlyphosate’s efficacy is influenced by the amount absorbed and translocated throughout the plant to inhibit 5-enolpyruvyl shikimate-3-phosphate synthase (EPSPS). Glyphosate resistance can be due to target-site (TS) or non–target site (NTS) resistance mechanisms. TS resistance includes an altered target site and gene overexpression, while NTS resistance includes reduced absorption, reduced translocation, enhanced metabolism, and exclusion/sequestration. The goal of this research was to elucidate the mechanism(s) of glyphosate resistance in common ragweed (Ambrosia artemisiifolia L.) from Ontario, Canada. The resistance factor for this glyphosate-resistant (GR) A. artemisiifolia biotype is 5.1. No amino acid substitutions were found at positions 102 or 106 of the EPSPS enzyme in this A. artemisiifolia biotype. Based on [14C]glyphosate studies, there was no difference in glyphosate absorption or translocation between glyphosate-susceptible (GS) and GR A. artemisiifolia biotypes. Radio-labeled glyphosate metabolites were similar for GS and GR A. artemisiifolia 96 h after application. Glyphosate resistance in this A. artemisiifolia biotype is not due to an altered target site due to amino acid substitutions at positions 102 and 106 in the EPSPS and is not due to the NTS mechanisms of reduced absorption, reduced translocation, or enhanced metabolism.

Abstract Background Early detection of herbicide resistance in weeds is crucial for successful implementation of integrated weed management. We conducted a herbicide resistance survey of the winter annual grasses feral rye (Secale cereale), downy brome (Bromus tectorum), and jointed goatgrass (Aegilops cylindrica) from Colorado winter wheat production areas for resistance to imazamox and quizalofop. Results All samples were susceptible to quizalofop. All downy brome and jointed goatgrass samples were susceptible to imazamox. Out of 314 field collected samples, we identified three feral rye populations (named A, B, and C) that were imazamox resistant. Populations B and C had a target‐site mechanism with mutations in the Ser653 residue of the acetolactate synthase (ALS) gene to Asn in B and to Thr in C. Both populations B and C had greatly reduced ALS in vitro enzyme inhibition by imazamox. ALS feral rye protein modeling showed that steric interactions induced by the amino acid substitutions at Ser653 impaired imazamox binding. Individuals from population A had no mutations in the ALS gene. The ALS enzyme from population A was equally sensitive to imazamox as to known susceptible feral rye populations. Imazamox was degraded two times faster in population A compared with a susceptible control. An oxidized imazamox metabolite formed faster in population A and this detoxification reaction was inhibited by malathion. Conclusion Population A has a nontarget‐site mechanism of enhanced imazamox metabolism that may be conferred by cytochrome P450 enzymes. This is the first report of both target‐site and metabolism‐based imazamox resistance in feral rye. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Winter annual grass species such as jointed goatgrass (Aegilops cylindrica Host), downy brome (Bromus tectorum L.), and feral rye (Secale cereale L.) negatively affect winter wheat (Triticum aestivum L.) yields. To generate a novel in-crop selective herbicide trait for winter annual grass control, mutagenesis was used to generate multiple winter wheat lines resistant to the acetyl co-A carboxylase (ACCase) inhibitor quizalofop p-ethyl (QPE). Field trials were performed to assess crop safety for wheat lines with one or two homoeologous mutations and to assess winter annual grass control with QPE treatments. Crop safety parameters including visual injury rating, plant height, grain yield, and kernel weight were evaluated following QPE treatment at multiple growth stages. The susceptible winter wheat cultivar Hatcher displayed 100% crop injury when treated at tillering in early spring or at jointing in late spring. Single-gene lines generally had crop injury and yield loss, especially at the jointing application timing. The two-mutation cultivars Incline AX and LCS Fusion AX had low to no injury or yield loss from QPE applications at all timings and no injury from high rate QPE applications (93 and 185 g ha(-1)) in either the fall or the spring. Control of downy brome and feral rye was highest (92-99%), while jointed goatgrass control ranged from a low of 73% at 93 g a.i. ha(-1) to a high of 98% at 109 a.i. ha(-1) QPE. Our data indicate that the targeted winter annual grass weeds are controlled by QPE at rates that have acceptable crop safety for two-mutation QPE-resistant wheat cultivars.

BACKGROUND Lack of fitness costs has been reported for multiple herbicide resistance traits, but the underlying evolutionary mechanisms are not well understood. Compensatory evolution that ameliorates resistance costs, has been documented in bacteria and insects but rarely studied in weeds. Dicamba resistant IAA16 (G73N) mutated kochia was previously found to have high fecundity in the absence of competition, regardless of significant vegetative growth defects. To understand if costs of dicamba resistance can be compensated through traits promoting reproductive success in kochia, we thoroughly characterized the reproductive growth and development of different G73N kochia biotypes. Flowering phenology, seed production and reproductive allocation were quantified through greenhouse studies, floral (stigma-anthers distance) and seed morphology, as well as resulting mating and seed dispersal systems were studied through time-course microcopy images. RESULTS G73N covaried with multiple phenological, morphological and ecological traits that improve reproductive fitness: (i) 16-60% higher reproductive allocation; (ii) longer reproduction phase through early flowering (2-7 days); (iii) smaller stigma-anthers separation (up to 60% reduction of herkogamy and dichogamy) that can potentially promote selfing and reproductive assurance; (iv) 'winged' seeds with 30-70% longer sepals that facilitate long-distance seed dispersal. CONCLUSION The current study demonstrates that costs of herbicide resistance can be ameliorated through coevolution of other fitness penalty alleviating traits. As illustrated in a hypothetical model, the evolution of herbicide resistance is an ongoing fitness maximization process, which poses challenges to contain the spread of resistance.

BACKGROUND: Amaranthus palmeri is an aggressive and prolific weed species with major impact on agricultural yield and is a prohibited noxious weed across the Midwest. Morphological identification of A. palmeri from other Amaranthus species is extremely difficult in seeds, which has led to genetic testing for seed identification in commercial seed lots. RESULTS: We created an inexpensive and reliable genetic test based on novel, species-specific, single nucleotide polymorphisms (SNPs) from GBS (Genotyping by Sequencing) data. We report three SNP-based genetic tests for identifying A. palmeri alone or in a mixed pool of Amaranthus spp. Sensitivity ranged from 99.8 to 100%, specificity from 99.59 to 100%. Accuracy for all three tests is > 99.7%. All three are capable of reliably detecting one A. palmeri seed in a pool of 200 Amaranthus spp. seeds. The test was validated across 20 populations of A. palmeri, along with eight other Amaranthus species, the largest and most genetically diverse panel of Amaranthus samples to date. CONCLUSION: Our work represents a marked improvement over existing commercial assays resulting in an identification assay that is (i) accurate, (ii) robust, (iii) easy to interpret and (iv) applicable to both leaf tissue and pools of up to 200 seeds. Included is a data transformation method for calling of closely grouped competitive fluorescence assays. We also present a comprehensive GBS dataset from the largest geographic panel of Amaranthus populations sequenced. Our approach serves as a model for developing markers for other difficult to identify species. (c) 2021 Society of Chemical Industry

The authors discuss the importance of wheat as a global food source and describe a novel multi-institutional, public-private partnership between Colorado State University, the Colorado Wheat Research Foundation, and private chemical and seed companies that resulted in the development of a new herbicide-resistant wheat production system.

Field trials were conducted to assess the impact of aminocyclopyrachlor on green ash (Fraxinus pennsylvanica Marshall) and honey locust (Gleditsia triacanthos L.) trees in an urban environment. Aminocyclopyrachlor is a relatively new, selective, plant-growth-regulator herbicide in the pyrimidine carboxylic acid family. Treatments were applied to Kentucky bluegrass (Poa pratensis L.) sod growing with and without trees present. Evaluations included determination of a safe spraying distance from target trees and the effect of application timing on tree response. This multi-year study showed that green ash was highly tolerant to aminocyclopyrachlor while honey locust developed severe injury in trees closest to applications. Honey locust trees up to 7 m (23 ft) from the tree trunk to the edge of the application displayed moderate to severe injury symptoms and fall treatment in October and November had the lowest tree injury compared to all other application timings. Honey locust trees exhibiting moderate to severe cosmetic injury would not be acceptable to landowners; recovery over time was minimal. Trees located 13 m (43 ft) away displayed no injury for any treatment timing. Soil analysis demonstrated that aminocyclopyrachlor dissipation was the same underneath green ash and honey locust trees, and that dissipation was faster in the presence of growing trees. Taken together, these results provide a basic groundwork necessary for improving aminocyclopyrachlor labels, and a better understanding of this herbicide's effect on certain woody species. Index words: Herbicide injury, aminocyclopyrachlor, herbicide fate, tree safety. Species used in this study: Green ash, Fraxinus pennsylvanica Marshall, honey locust, Gleditsia triacanthos L. Chemicals used in this study: Aminocyclopyrachlor.

Genomic-based epidemiology can provide insight into the origins and spread of herbicide resistance mechanisms in weeds. We used kochia (Bassia scoparia) populations resistant to the herbicide glyphosate from across western North America to test the alternative hypotheses that (i) a single EPSPS gene duplication event occurred initially in the Central Great Plains and then subsequently spread to all other geographical areas now exhibiting glyphosate-resistant kochia populations or that (ii) gene duplication occurred multiple times in independent events in a case of parallel evolution. We used qPCR markers previously developed for measuring the structure of the EPSPS tandem duplication to investigate whether all glyphosate-resistant individuals had the same EPSPS repeat structure. We also investigated population structure using simple sequence repeat markers to determine the relatedness of kochia populations from across the Central Great Plains, Northern Plains and the Pacific Northwest. We found that the original EPSPS duplication genotype was predominant in the Central Great Plains where glyphosate resistance was first reported. We identified two additional EPSPS duplication genotypes, one having geographical associations with the Northern Plains and the other with the Pacific Northwest. The EPSPS duplication genotype from the Pacific Northwest seems likely to represent a second, independent evolutionary origin of a resistance allele. We found evidence of gene flow across populations and a general lack of population structure. The results support at least two independent evolutionary origins of glyphosate resistance in kochia, followed by substantial and mostly geographically localized gene flow to spread the resistance alleles into diverse genetic backgrounds.

Feral rye (Secale cereale L.) is one of the most troublesome winter annual grass weed species in semi-arid winter wheat (Triticum aestivum L.) production systems across the central Great Plains (CGP). Lack of effective herbicide options further complicates the selective control of feral rye in winter wheat. The main objectives of this research were: (a) determine the sensitivity response of 10 feral rye populations collected from Kansas wheat fields to quizalofop-p-ethyl in a dose-response assay, and (b) to determine the effectiveness of various rates and application timings (fall or spring) of quizalofop-p-ethyl for feral rye control in quizalofop-resistant winter wheat in the CGP. Dose-response assays indicated that all tested feral rye populations were highly sensitive to quizalofop-p-ethyl with GR(90) values (doses of quizalofop-p-ethyl needed for 90% reductions in shoot biomass at 3 wk after treatment) ranging from 33 to 72 g a.i. ha(-1). A field study conducted in Colorado, Kansas, and Oklahoma for a total of 5 site-years indicated that quizalofop-p-ethyl applied at >= 77 g a.i. ha(-1) in the fall or spring provided season-long feral rye control at 92-100% in quizalofop-resistant winter wheat. Irrespective of tested rates and application timings, a quizalofop-p-ethyl treatment protected winter wheat yield loss of 17-27% due to reduced feral rye competition compared to the nontreated weedy check in Colorado and Oklahoma. Results suggested that quizalofop-p-ethyl at labelled use rates (77-92 g a.i. ha(-1)) applied in fall or spring can provide effective feral rye control in quizalofop-resistant winter wheat.

AbstractBACKGROUNDPrecise quantification of the fitness cost of synthetic auxin resistance has been impeded by lack of knowledge about the genetic basis of resistance in weeds. Recent elucidation of a resistance‐endowing IAA16 mutation (G73N) in the key weed species kochia (Bassia scoparia), allows detailed characterization of the contribution of resistance alleles to weed fitness, both in the presence and absence of herbicides. Different G73N genotypes from a segregating resistant parental line (9425) were characterized for cross‐resistance to dicamba, 2,4‐d and fluroxypyr, and changes on stem/leaf morphology and plant architecture. Plant competitiveness and dominance of the fitness effects was quantified through measuring biomass and seed production of three F2 lines in two runs of glasshouse replacement series studies.RESULTSG73N confers robust resistance to dicamba but only moderate to weak resistance to 2,4‐D and fluroxypyr. G73N mutant plants displayed significant vegetative growth defects: (i) they were 30–50% shorter, with a more tumbling style plant architecture, and (ii) they had thicker and more ovate (versus lanceolate and linear) leaf blades with lower photosynthesis efficiency, and 40–60% smaller stems with less‐developed vascular bundle systems. F2 mutant plants had impaired plant competitiveness, which can lead to 80‐90% less biomass and seed production in the replacement series study. The pleiotropic effects of G73N were mostly semidominant (0.5) and fluctuated with the environments and traits measured.CONCLUSIONG73N is associated with significant vegetative growth defects and reduced competitiveness in synthetic auxin‐resistant kochia. Management practices should target resistant kochia's high vulnerability to competition in order to effectively contain the spread of resistance.

Downy brome, feral rye, and jointed goatgrass are problematic winter annual grasses in central Great Plains winter wheat production. Integrated control strategies are needed to manage winter annual grasses and reduce selection pressure exerted on these weed populations by the limited herbicide options currently available. Harvest weed-seed control (HWSC) methods aim to remove or destroy weed seeds, thereby reducing seed-bank enrichment at crop harvest. An added advantage is the potential to reduce herbicide-resistant weed seeds that are more likely to be present at harvest, thereby providing a nonchemical resistance-management strategy. Our objective was to assess the potential for HWSC of winter annual grass weeds in winter wheat by measuring seed retention at harvest and destruction percentage in an impact mill. During 2015 and 2016, 40 wheat fields in eastern Colorado were sampled. Seed retention was quantified and compared per weed species by counting seed retained above the harvested fraction of the wheat upper canopy (15 cm and above), seed retained below 15 cm, and shattered seed on the soil surface at wheat harvest. A stand-mounted impact mill device was used to determine the percent seed destruction of grass weed species in processed wheat chaff. Averaged across both years, seed retention (+/- SE) was 75% +/- 2.9%, 90% +/- 1.7%, and 76% +/- 4.3% for downy brome, feral rye, and jointed goatgrass, respectively. Seed retention was most variable for downy brome, because 59% of the samples had at least 75% seed retention, whereas the proportions for feral rye and jointed goatgrass samples with at least 75% seed retention were 93% and 70%, respectively. Weed seed destruction percentages were at least 98% for all three species. These results suggest HWSC could be implemented as an integrated strategy for winter annual grass management in central Great Plains winter wheat cropping systems.

Glufosinate is considered a contact herbicide because of its fast activity and limited translocation in plants. We used Palmer amaranth (Amaranthus palmeri S. Watson) as a model species to study plant-related factors affecting glufosinate uptake and translocation. Glufosinate uptake increased rapidly during the initial 24 h, achieving maximum uptake from this time on. The rate of uptake saturated with doses higher than 250 μM glufosinate, suggesting the involvement of a membrane transporter. When glufosinate concentrations were higher (>1 mM), uptake was a simple diffusion process in favor of a concentration gradient between the inside and the outside of the cells. Glufosinate uptake was inhibited by the presence of glutamine. The fast action of glufosinate did not limit its own translocation. Because glufosinate is highly water soluble, it translocates mostly through the apoplast or the xylem system. Consequently, old leaves tend to accumulate more herbicide than young meristematic leaves.

Glufosinate targets glutamine synthetase (GS), but its fast herbicidal action is triggered by reactive oxygen species (ROS). The relationship between GS inhibition and ROS accumulation was investigated in Amaranthus palmeri. Glufosinate’s fast action is light-dependent with no visual symptoms or ROS formation in the dark. Inhibition of GS leads to accumulation of ammonia and metabolites of the photorespiration pathway, such as glycolate and glyoxylate, as well as depletion of other intermediates such as glycine, serine, hydroxypyruvate, and glycerate. Exogenous supply of glycolate to glufosinate-treated plants enhanced herbicidal activity and dramatically increased hydrogen peroxide accumulation (possibly from peroxisomal glycolate oxidase activity). Glufosinate affected the balance between ROS generation and scavenging. The activity of superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase increased after glufosinate treatment in an attempt to quench the nascent ROS burst. Low doses of atrazine and dinoseb were used to investigate the sources of ROS by manipulating photosynthetic electron transport and oxygen (O2) evolution. ROS formation depended on electron flow and O2 evolution in photosystem II (PSII). Inhibition of GS disrupted photorespiration, carbon assimilation, and linear electron flow in the light reactions. Consequently, the antioxidant machinery and the water–water cycle are overwhelmed in the presence of light and glufosinate. The O2 generated by the splitting of water in PSII becomes the acceptor of electrons, generating ROS. The cascade of events leads to lipid peroxidation and forms the basis for the fast action of glufosinate.

BACKGROUND: We previously identified a glyphosate-resistant A. trifida phenotype from Wisconsin USA that showed a non-rapid response to glyphosate. The mechanism of glyphosate resistance in this phenotype has yet to be elucidated. We conducted experiments to investigate non-target-site resistance and target-site resistance mechanisms. The roles of glyphosate absorption, translocation, and metabolism in resistance of this phenotype have not been reported previously, nor have EPSPS protein abundance or mutations to the full-length sequence of EPSPS. RESULTS: Whole-plant dose- response results confirmed a 6.5-level of glyphosate resistance for the resistant (R) phenotype compared to a susceptible (S) phenotype. Absorption and translocation of C-14-glyphosate were similar between R and S phenotypes over 72 h. Glyphosate and AMPA concentrations in leaf tissue did not differ between R and S phenotypes over 96 h. In vivo shikimate leaf disc assays confirmed that glyphosate EC50 valueswere 4.6- to 5.4-fold greater for the R than S phenotype. Shikimate accumulation was similar between phenotypes at high glyphosate concentrations (>1000 mu M), suggesting that glyphosate entered chloroplasts and inhibited EPSPS. This finding was supported by results showing that EPSPS copy number and EPSPS protein abundance did not differ between R and S phenotypes, nor did EPSPS sequence at Gly101, Thr102, and Pro106 positions. Comparison of full-length EPSPS sequences found five nonsynonymous polymorphisms that differed between R and S phenotypes. However, their locations were distant from the glyphosate target site and, therefore, not likely to affect enzyme-glyphosate interaction. CONCLUSION: The results suggest that a novel mechanism confers glyphosate resistance in this A. trifida phenotype. (c) 2019 Society of Chemical Industry

Glufosinate inhibits glutamine synthetase (GS), a key enzyme for amino acid metabolism and photorespiration. Protoporphyrinogen oxidase (PPO) inhibitors block chlorophyll biosynthesis and cause protoporphyrin accumulation, a highly photodynamic intermediate. Both herbicides ultimately lead to plant death by a massive accumulation of reactive oxygen species (ROS) through different mechanisms. We investigated a potential synergistic effect by the mixture of the two herbicide mechanisms of action (MoAs). The tank mix between a low rate of glufosinate (280 g ai ha(-1)) with an ultra-low dose of saflufenacil (1 g ha(-1)) provided enhanced herbicidal activity compared with the products applied individually on Palmer amaranth (Amaranthus palmeriS. Watson). The synergism between the two herbicides was also confirmed by isobole analysis and field trials. The herbicide combination provided high levels of efficacy when applied at low temperature and low humidity. Mechanistically, glufosinate caused a transient accumulation of glutamate, the building block for chlorophyll biosynthesis. Consequently, inhibition of both GS and PPO resulted in greater accumulation of protoporphyrin and ROS, forming the physiological basis for the synergism between glufosinate and PPO inhibitors. While the synergy between the two herbicide MoAs provided excellent efficacy on weeds, it caused low injury to PPO-resistant waterhemp [Amaranthus tuberculatus(Moq.) Sauer] and high injury to both glufosinate-resistant and glufosinate-susceptible soybean [Glycine max(L.) Merr.]. Glufosinate enhances the activity of PPO inhibitors through glutamate and protoporphyrin accumulation, leading to increased levels of ROS and lipid peroxidation. The synergism between the two herbicide MoAs can help to overcome environmental effects limiting the efficacy of glufosinate. Future research is needed to optimize the uses for this herbicidal composition across different cropping systems.