Enhanced herbicide metabolism induced by 2,4-D in herbicide susceptible Lolium rigidum provides protection against diclofop-methyl

Intergenerational consequences of an auxin-like herbicide on plant sensitivity to a graminicide mediated by a fungal endophyte

In agroecosystems, herbicides are the predominant anthropogenic selection pressure for agriculture weed species. While weeds are the primary target, herbicides can have adverse impacts on non-target plant beneficial microorganisms. We aimed to investigate the influence of a foliar endophytic fungus (Epichloë occultans) on the sensitivity of Lolium multiflorum to a graminicide herbicide (diclofop-methyl) during both plant ontogeny and progeny. Susceptible individuals to diclofop-methyl with and without endophyte were pre-exposed to the auxin 2,4-D herbicide. This herbicide is known to stimulate the metabolic detoxification mechanism (CYP-450) of diclofop-methyl. Regardless of the endophyte, 2,4-D pre-treatment increased mother plant survival to nearly 100 % under diclofop treatment but not in the progeny. Furthermore, maternal plant exposure to 2,4-D reduced endophyte transmission to the seeds and from seed-to-seedlings. Our findings suggest that, despite a reduction in diclofop-methyl sensitivity during the ontogeny of mother plants, 2,4-D-mediated induction of likely CYP-450 metabolism is not intergenerationally transmitted and shows detrimental effects on the symbiotic endophyte persistence.

Investigation of resistance mechanisms to fomesafen in Ipomoea nil from China

Recently, the herbicide fomesafen has frequently failed to control the troublesome weed Ipomoea nil in soybean fields in Liaoning Province, China. Hence, we collected 10 suspected resistant populations and evaluated their sensitivity to fomesafen. The results revealed various degrees of Ipomoea nil resistance to fomesafen, with a resistance index of 2.88 to 22.43; the highest value occurred in the LN3 population. Therefore, the mechanisms of the resistance in LN3 to fomesafen were explored. After fomesafen treatment, the expression levels of InPPX1 and InPPX2 genes were 4.19- and 9.29-fold higher, respectively, in LN3 than those in the susceptible (LN1) population. However, mutations and copy number variations were not detected between the two populations. Additionally, malathion pretreatment reduced the dose necessary to halve the growth rate of LN3 by 58%. Liquid chromatography with tandem mass spectrometry demonstrated that metabolism of fomesafen was significantly suppressed by malathion. Moreover, LN3 displayed increased reactive oxygen species scavenging capacity, which was represented by higher superoxide dismutase and peroxidase activities after fomesafen application than those in LN1. An orthogonal partial least squares-discriminant analysis revealed that the high resistance in LN3 could be attributed mainly to enhanced metabolism. Fortunately, the fomesafen-resistant I. nil remained sensitive to 2,4-D-ethylhexylester and bentazon, providing methods for its control.

Aldo-keto reductase may contribute to glyphosate resistance in Lolium rigidum

BACKGROUND

We have previously demonstrated that an aldo-keto reductase (AKR) from Echinochloa colona (EcAKR4-1) can metabolize glyphosate and confers glyphosate resistance. This study aims to investigate if the EcAKR4-1 orthologs from Lolium rigidum also play a role in glyphosate resistance in non-target-site based, glyphosate-resistant (R) L. rigidum populations from Western Australia.

RESULTS

The full-length L. rigidum AKR gene (LrAKR4C10) orthologous to EcAKR4-1, together with a distinct LrAKR1, were cloned from plants of a glyphosate-susceptible (S) (VLR1) and three glyphosate R L. rigidum populations (WALR50, WALR60 and WALR70). Reverse transcription quantitative polymerase chain reaction (RT-qPCR) results showed that basal expression levels of the two LrAKR genes did not differ between the R and S populations, but their expression was significantly induced by glyphosate (up to 4.3-fold) or 2,4-D treatment (up to 3.4-fold) in R populations. Escherichia coli cells transformed respectively with LrAKR4C10 and LrAKR1 were more tolerant to glyphosate. Rice (Oryza sativa) seedlings overexpressing each of the two LrAKR gene survived glyphosate rates that were lethal to the green fluorescence protein (GFP) control plants. Structural modeling predicts a similar way of glyphosate binding and detoxification by LrAKR4C10 and EcAKR4-1, but an alternative way of glyphosate binding by LrAKR1. Relatively lower capacity of the two LrAKRs in conferring glyphosate resistance than the known EcAKR4-1 was discussed in relation to structural interaction.

CONCLUSION

Glyphosate-induced higher expression of the two LrAKR genes in L. rigidum populations contributes to a moderate level of glyphosate resistance likely through enhanced glyphosate metabolism. The herbicide 2,4-D can also induce the LrAKR expression, indicating the potential antagonistic effect of 2,4-D to glyphosate. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Mechanistic basis for synergism of 2,4-D amine and metribuzin in Avena sterilis

The combination of herbicides with different modes of action has been adopted not only to improve weed control but also to increase the environmental sustainability of plant-protection products. In this study, we showed a synergistic effect of the auxin herbicide 2,4-D amine with the PSII-inhibiting herbicide metribuzin to control the global grass weed wild oat (Avena sterilis) population and investigated the underlying mechanisms. Pretreatment with 2,4-D amine did not change the foliar absorption of metribuzin but did increase metribuzin translocation to the roots and new leaves, although enhancement of the metribuzin metabolism was also observed. Considering that the expression level of the target site psbA gene is significantly higher in leaves than in roots, increased metribuzin translocation to new leaves is likely the major cause of the observed synergism, even though enhanced metribuzin metabolism may offset the metribuzin efficacy. This is the first report on the synergistic mechanism between 2,4-D amine and metribuzin in weed control.

2,4-D antagonizes glyphosate in glyphosate-resistant barnyard grass Echinochloa colona

Glyphosate is often tank-mixed with auxinic herbicide 2,4-D for grass and broadleaf weed control. Here we examined the possible interaction of 2,4-D and glyphosate in barnyard grass, Echinochloa colona (L.) Link. The results showed that 2,4-D antagonizes glyphosate remarkably in glyphosate-resistant populations but only marginally in susceptible populations. This antagonism is related to reduced glyphosate uptake and (to a lesser extent) translocation. As 2,4-D has multiple, unpredictable effects on other herbicides, care must be taken when tank-mixing herbicides with 2,4-D.

Non-Target-Site Resistance to Herbicides: Recent Developments

Non-target-site resistance (NTSR) to herbicides in weeds can be conferred as a result of the alteration of one or more physiological processes, including herbicide absorption, translocation, sequestration, and metabolism. The mechanisms of NTSR are generally more complex to decipher than target-site resistance (TSR) and can impart cross-resistance to herbicides with different modes of action. Metabolism-based NTSR has been reported in many agriculturally important weeds, although reduced translocation and sequestration of herbicides has also been found in some weeds. This review focuses on summarizing the recent advances in our understanding of the physiological, biochemical, and molecular basis of NTSR mechanisms found in weed species. Further, the importance of examining the co-existence of TSR and NTSR for the same herbicide in the same weed species and influence of environmental conditions in the altering and selection of NTSR is also discussed. Knowledge of the prevalence of NTSR mechanisms and co-existing TSR and NTSR in weeds is crucial for designing sustainable weed management strategies to discourage the further evolution and selection of herbicide resistance in weeds.

Metabolism of 2,4-dichlorophenoxyacetic acid contributes to resistance in a common waterhemp (Amaranthus tuberculatus) population

BACKGROUND

Synthetic auxins such as 2,4-dichlorophenoxyacetic acid (2,4-D) have been widely used for selective control of broadleaf weeds since the mid-1940s. In 2009, an Amaranthus tuberculatus (common waterhemp) population with 10-fold resistance to 2,4-D was found in Nebraska, USA. The 2,4-D resistance mechanism was examined by conducting [¹⁴ C] 2,4-D absorption, translocation and metabolism experiments.

RESULTS

No differences were found in 2,4-D absorption or translocation between resistant and susceptible A. tuberculatus plants. Resistant plants metabolized [¹⁴ C] 2,4-D more rapidly than did susceptible plants. The half-life of [¹⁴ C] 2,4-D in susceptible plants was 105 h, compared with 22 h in resistant plants. Pretreatment with the cytochrome P450 inhibitor malathion inhibited [¹⁴ C] 2,4-D metabolism in resistant plants and reduced the 2,4-D dose required for 50% growth inhibition (GR₅₀ ) of resistant plants by 7-fold to 27 g ha^-1 , similar to the GR₅₀ for susceptible plants in the absence of malathion.

CONCLUSION

Our results demonstrate that rapid 2,4-D metabolism is a contributing factor to resistance in A. tuberculatus, potentially mediated by cytochrome P450. Metabolism-based resistance to 2,4-D could pose a serious challenge for A. tuberculatus control because of the potential for cross-resistance to other herbicides. © 2017 Society of Chemical Industry.

Characterisation of glufosinate resistance mechanisms in Eleusine indica

BACKGROUND

An Eleusine indica population has evolved resistance to glufosinate, a major post-emergence herbicide of global agriculture. This population was analysed for target-site (glutamine synthetase) and non-target-site (glufosinate uptake, translocation and metabolism) resistance mechanisms.

RESULTS

Glutamine synthetase (GS) activity extracted from susceptible (S) and resistant (R*) plants was equally sensitive to glufosinate inhibition, with IC₅₀ values of 0.85 mm and 0.99 mm, respectively. The extractable GS activity was also similar in S and R* samples. Foliar uptake of [¹⁴ C]-glufosinate did not differ in S and R* plants, nor did glufosinate net uptake in leaf discs. Translocation of [¹⁴ C]-glufosinate into untreated shoots and roots was also similar in both populations, with 44% to 47% of the herbicide translocated out from the treated leaf 24 h after treatment. The HPLC and LC-MS analysis of glufosinate metabolism revealed no major metabolites in S or R* leaf tissue.

CONCLUSIONS

Glufosinate resistance in this resistant population is not due to an insensitive GS, or increased activity, or altered glufosinate uptake and translocation, or enhanced glufosinate metabolism. Thus, target-site resistance is likely excluded and the exact resistance mechanism(s) remain to be determined. © 2017 Society of Chemical Industry.

Evidence, Mechanism and Alternative Chemical Seedbank-Level Control of Glyphosate Resistance of a Rigid Ryegrass (Lolium rigidum) Biotype from Southern Spain

Rigid ryegrass (Lolium rigidum) is one of the most troublesome weeds in different crops in the Mediterranean region. A rigid ryegrass biotype from an olive grove in Jaén province (Andalusía, southern Spain), potentially resistant to glyphosate (RG), was tested for its resistance level through dose-response assays using a susceptible biotype (SG). To test the hypothesis of a non-target-site-based resistance, as point mutations are far less common mechanisms of glyphosate resistance, studies were also conducted to elucidate whether resistance was associated with biochemical, metabolism, molecular and/or physiological mechanisms. Alternative herbicide-based control options, including single-herbicide or herbicide mixtures with glyphosate, applied at seedling, tillering or full heading stages, were tested in field experiments for 2 years for their efficacy against rigid ryegrass plants and their effects on the soil seed bank. Resistance levels of the RG biotype were 23- (LD₅₀) and 7-fold (GR₅₀) higher compared to the SG biotype. The SG biotype exhibited a significantly greater shikimic acid accumulation than the RG one. At 96 HAT, 58 and 89% of applied ¹⁴C-glyphosate was up taken by leaves of RG and SG biotype plants, respectively, and, at this time, a significantly higher proportion of the glyphosate taken up by the treated leaf remained in its tissue in RG plants compared to the SG ones. The RG biotype did not reveal any point mutation in the glyphosate target site EPSP synthase. Overall, results confirmed reduced glyphosate uptake and translocation as being the mechanism involved in glyphosate resistance in the RG biotype. RG biotype responses to the alternative treatments tested in situ indicated that herbicide applications at the later growth stage tended to be less effective in terms of immediate effects on population size than earlier applications, and that only in some cases, the removal of at least 85% of the RG biotype was achieved. However, with few exceptions, the alternative treatments tested appeared to be highly effective in reducing the seed bank irrespective of the growth stage. The frequency of the resistant phenotype in the progeny of surviving plants of the RG biotype was dependent on treatment. Results suggest that a potential exists for effective management of glyphosate-resistant rigid ryegrass in olive groves in southern Spain.

Enhanced 2,4-D Metabolism in Two Resistant Papaver rhoeas Populations from Spain

Corn poppy (Papaver rhoeas), the most problematic broadleaf weed in winter cereals in Southern Europe, has developed resistance to the widely-used herbicide, 2,4-D. The first reported resistance mechanism in this species to 2,4-D was reduced translocation from treated leaves to the rest of the plant. However, the presence of other non-target site resistance (NTSR) mechanisms has not been investigated up to date. Therefore, the main objective of this research was to reveal if enhanced 2,4-D metabolism is also present in two Spanish resistant (R) populations to synthetic auxins. With this aim, HPLC experiments at two 2,4-D rates (600 and 2,400 g ai ha^-1) were conducted to identify and quantify the metabolites produced and evaluate possible differences in 2,4-D degradation between resistant (R) and susceptible (S) plants. Secondarily, to determine the role of cytochrome P450 in the resistance response, dose-response experiments were performed using malathion as its inhibitor. Three populations were used: S, only 2,4-D R (R-703) and multiple R to 2,4-D and ALS inhibitors (R-213). HPLC studies indicated the presence of two hydroxy metabolites in these R populations in shoots and roots, which were not detected in S plants, at both rates. Therefore, enhanced metabolism becomes a new NTSR mechanism in these two P. rhoeas populations from Spain. Results from the dose-response experiments also showed that pre-treatment of R plants with the cytochrome P450 (P450) inhibitor malathion reversed the phenotype to 2,4-D from resistant to susceptible in both R populations. Therefore, it could be hypothesized that a malathion inhibited P450 is responsible of the formation of the hydroxy metabolites detected in the metabolism studies. This and previous research indicate that two resistant mechanisms to 2,4-D could be present in populations R-703 and R-213: reduced translocation and enhanced metabolism. Future experiments are required to confirm these hypotheses, understand the role of P450, and the relationship between both NTSR mechanisms. On this basis, selection pressure with synthetic auxins bears the risk of promoting the evolution enhanced metabolism in Papaver rhoeas.

Climate change increases the risk of herbicide-resistant weeds due to enhanced detoxification

Global warming will increase the incidence of metabolism-based reduced herbicide efficacy on weeds and, therefore, the risk for evolution of non-target site herbicide resistance. Climate changes affect food security both directly and indirectly. Weeds are the major biotic factor limiting crop production worldwide, and herbicides are the most cost-effective way for weed management. Processes associated with climatic changes, such as elevated temperatures, can strongly affect weed control efficiency. Responses of several grass weed populations to herbicides that inhibit acetyl-CoA carboxylase (ACCase) were examined under different temperature regimes. We characterized the mechanism of temperature-dependent sensitivity and the kinetics of pinoxaden detoxification. The products of pinoxaden detoxification were quantified. Decreased sensitivity to ACCase inhibitors was observed under elevated temperatures. Pre-treatment with the cytochrome-P450 inhibitor malathion supports a non-target site metabolism-based mechanism of herbicide resistance. The first 48 h after herbicide application were crucial for pinoxaden detoxification. The levels of the inactive glucose-conjugated pinoxaden product (M5) were found significantly higher under high- than low-temperature regime. Under high temperature, a rapid elevation in the level of the intermediate metabolite (M4) was found only in pinoxaden-resistant plants. Our results highlight the quantitative nature of non-target-site resistance. To the best of our knowledge, this is the first experimental evidence for temperature-dependent herbicide sensitivity based on metabolic detoxification. These findings suggest an increased risk for the evolution of herbicide-resistant weeds under predicted climatic conditions.

High-throughput sequencing reveals differential regulation of miRNAs in fenoxaprop-P-ethyl-resistant Beckmannia syzigachne

Non-target site resistance (NTSR) to herbicides is an increasing concern for weed control. The majority of previous studies have focused on metabolic resistance mechanisms of NTSR, but no research exists on gene regulation mechanisms behind herbicide resistance, such as microRNA (miRNA). Here, we identified 3 American sloughgrass (Beckmannia syzigachne Steud.) populations containing fenoxaprop-P-ethyl-resistant plants. We then constructed small RNA libraries and subjected them to deep sequencing and bioinformatics analyses. Forty known and 36 potentially novel, predicted miRNAs were successfully identified. Of these, we identified 3 conserved, predicted candidate NTSR-determinant miRNAs and their potential corresponding target genes, as well as 4 novel potential miRNAs with high count. Target gene prediction and annotation indicated that these 7 differentially expressed miRNAs potentially play a role in regulating specific stress-responsive genes, very likely related to herbicide resistance. Expression profiles were determined with quantitative real-time PCR. The present study is a novel, large-scale characterization of weed miRNAs. The results should further our understanding of miRNA expression profiles associated with herbicide resistance, allowing for the development of more effective weed management strategies.

Widespread occurrence of both metabolic and target-site herbicide resistance mechanisms in Lolium rigidum populations

BACKGROUND

Lolium rigidum populations in Australia and globally have demonstrated rapid and widespread evolution of resistance to acetyl coenzyme A carboxylase (ACCase)-inhibiting and acetolactate synthase (ALS)-inhibiting herbicides. Thirty-three resistant L. rigidum populations, randomly collected from crop fields in a most recent resistance survey, were analysed for non-target-site diclofop metabolism and all known target-site ACCase gene resistance-endowing mutations.

RESULTS

The HPLC profile of [(14) C]-diclofop-methyl in vivo metabolism revealed that 79% of these resistant L. rigidum populations showed enhanced capacity for diclofop acid metabolism (metabolic resistance). ACCase gene sequencing identified that 91% of the populations contain plants with ACCase resistance mutation(s). Importantly, 70% of the populations exhibit both non-target-site metabolic resistance and target-site ACCase mutations.

CONCLUSIONS

This work demonstrates that metabolic herbicide resistance is commonly occurring in L. rigidum, and coevolution of both metabolic resistance and target-site resistance is an evolutionary reality. Metabolic herbicide resistance can potentially endow resistance to many herbicides and poses a threat to herbicide sustainability and thus crop production, calling for major research and management efforts.

Cross-resistance patterns to ACCase-inhibitors in American sloughgrass (Beckmannia syzigachne Steud.) homozygous for specific ACCase mutations

American sloughgrass is a troublesome annual grass weed in winter wheat field rotated with rice in China. The overreliance on acetyl-coenzyme A carboxylase (ACCase) inhibiting herbicides has resulted in resistance evolution in this weed. In this study, the cross-resistance patterns to fenoxaprop-p-ethyl, clodinafop-propargyl, fluazifop-p-butyl, haloxyfop-p-methyl, sethoxydim, clethodim and pinoxaden were established using purified plants individually homozygous for specific mutant ACCase alleles. Results indicated that 1781Leu allele endows high-level resistance to APPs, CHDs and pinoxaden while confers moderate resistance to haloxyfop-p-methyl. The 2027Cys and 2041Asn alleles endow high-level resistance to APPs and pinoxaden and lower level resistance to CHDs. The 2078Gly allele confers high-level resistance to all herbicides tested in this study, however, moderate resistance to sethoxydim. The 2096Ala very likely endows high-level resistance to fluazifop-p-butyl, haloxyfop-p-methyl and moderate resistance to sethoxydim. In addition, one undefined resistance mechanism was involved in population SD-04.

Metabolism of agrochemicals and related environmental chemicals based on cytochrome P450s in mammals and plants

A yeast gene expression system originally established for mammalian cytochrome P450 monooxygenase cDNAs was applied to functional analysis of a number of mammalian and plant P450 species, including 11 human P450 species (CYP1A1, CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C18, CYP2C19, CYP2D6, CYP2E1 and CYP3A4). The human P450 species CYP1A1, CYP1A2, CYP2B6, CYP2C18 and CYP2C19 were identified as P450 species metabolising various agrochemicals and environmental chemicals. CYP2C9 and CYP2E1 specifically metabolised sulfonylurea herbicides and halogenated hydrocarbons respectively. Plant P450 species metabolising phenylurea and sulfonylurea herbicides were also identified mainly as the CYP71 family, although CYP76B1, CYP81B1 and CYP81B2 metabolised phenylurea herbicides. The transgenic plants expressing these mammalian and plant P450 species were applied to herbicide tolerance as well as phytoremediation of agrochemical and environmental chemical residues. The combined use of CYP1A1, CYP2B6 and CYP2C19 belonging to two families and three subfamilies covered a wide variety of herbicide tolerance and phytoremediation of these residues. The use of 2,4-D-and bromoxynil-induced CYP71AH11 in tobacco seemed to enhance herbicide tolerance and selectivity.

Evolution of herbicide resistance mechanisms in grass weeds

Herbicide resistant weeds are becoming increasingly common, threatening global food security. Here, we present BrIFAR: a new model system for the functional study of mechanisms of herbicide resistance in grass weeds. We have developed a large collection of Brachypodium accessions, the BrI collection, representing a wide range of habitats. Wide screening of the responses of the accessions to four major herbicide groups (PSII, ACCase, ALS/AHAS and EPSPS inhibitors) identified 28 herbicide-resistance candidate accessions. Target-site resistance to PSII inhibitors was found in accessions collected from habitats with a known history of herbicide applications. An amino acid substitution in the psbA gene (serine264 to glycine) conferred resistance and also significantly affected the flowering and shoot dry weight of the resistant accession, as compared to the sensitive accession. Non-target site resistance to ACCase inhibitors was found in accessions collected from habitats with a history of herbicide application and from a nature reserve. In-vitro enzyme activity tests and responses following pre-treatment with malathion (a cytochrome-P450 inhibitor) indicated sensitivity at the enzyme level, and give strong support to diclofop-methyl and pinoxaden enhanced detoxification as NTS resistance mechanism. BrIFAR can promote better understanding of the evolution of mechanisms of herbicide resistance and aid the implementation of integrative management approaches for sustainable agriculture.

Metabolism-based herbicide resistance and cross-resistance in crop weeds: a threat to herbicide sustainability and global crop production

Weedy plant species that have evolved resistance to herbicides due to enhanced metabolic capacity to detoxify herbicides (metabolic resistance) are a major issue. Metabolic herbicide resistance in weedy plant species first became evident in the 1980s in Australia (in Lolium rigidum) and the United Kingdom (in Alopecurus myosuroides) and is now increasingly recognized in several crop-weed species as a looming threat to herbicide sustainability and thus world crop production. Metabolic resistance often confers resistance to herbicides of different chemical groups and sites of action and can extend to new herbicide(s). Cytochrome P450 monooxygenase, glycosyl transferase, and glutathione S-transferase are often implicated in herbicide metabolic resistance. However, precise biochemical and molecular genetic elucidation of metabolic resistance had been stalled until recently. Complex cytochrome P450 superfamilies, high genetic diversity in metabolic resistant weedy plant species (especially cross-pollinated species), and the complexity of genetic control of metabolic resistance have all been barriers to advances in understanding metabolic herbicide resistance. However, next-generation sequencing technologies and transcriptome-wide gene expression profiling are now revealing the genes endowing metabolic herbicide resistance in plants. This Update presents an historical review to current understanding of metabolic herbicide resistance evolution in weedy plant species.

RNA-Seq transcriptome analysis to identify genes involved in metabolism-based diclofop resistance in Lolium rigidum

Weed control failures due to herbicide resistance are an increasing and worldwide problem that significantly affect crop yields. Metabolism-based herbicide resistance (referred to as metabolic resistance) in weeds is not well characterized at the genetic level. An RNA-Seq transcriptome analysis was used to find candidate genes that conferred metabolic resistance to the herbicide diclofop in a diclofop-resistant population (R) of the major global weed Lolium rigidum. A reference cDNA transcriptome (19 623 contigs) was assembled and assigned putative annotations. Global gene expression was measured using Illumina reads from untreated control, adjuvant-only control, and diclofop treatment of R and susceptible (S). Contigs that showed constitutive expression differences between untreated R and untreated S were selected for further validation analysis, including 11 contigs putatively annotated as cytochrome P450 (CytP450), glutathione transferase (GST), or glucosyltransferase (GT), and 17 additional contigs with annotations related to metabolism or signal transduction. In a forward genetics validation experiment, nine contigs had constitutive up-regulation in R individuals from a segregating F2 population, including three CytP450, one nitronate monooxygenase (NMO), three GST, and one GT. Principal component analysis using these nine contigs differentiated F2 -R from F2 -S individuals. In a physiological validation experiment in which 2,4-D pre-treatment induced diclofop protection in S individuals due to increased metabolism, seven of the nine genetically validated contigs were induced significantly. Four contigs (two CytP450, NMO, and GT) were consistently highly expressed in nine field-evolved metabolic resistant L. rigidum populations. These four contigs were strongly associated with the resistance phenotype and are major candidates for contributing to metabolic diclofop resistance.