2,4-D resistance in wild radish: reduced herbicide translocation via inhibition of cellular transport

A novel mutation in IAA16 is associated with dicamba resistance in Chenopodium album

BACKGROUND

Resistance to dicamba in Chenopodium album was first documented over a decade ago, however, the molecular basis of dicamba resistance in this species has not been elucidated. In this research, the resistance mechanism in a dicamba-resistant C. album phenotype was investigated using a transcriptomics (RNA-sequence) approach.

RESULTS

The dose-response assay showed that the resistant (R) phenotype was nearly 25-fold more resistant to dicamba than a susceptible (S) phenotype of C. album. Also, dicamba treatment significantly induced transcription of the known auxin-responsive genes, Gretchen Hagen 3 (GH3), small auxin-up RNAs (SAURs), and 1-aminocyclopropane-1-carboxylate synthase (ACS) genes in the susceptible phenotype. Comparing the transcripts of auxin TIR/AFB receptors and auxin/indole-3-acetic acid (AUX/IAA) proteins identified from C. album transcriptomic analysis revealed that the R phenotype contained a novel mutation at the first codon of the GWPPV degron motif of IAA16, resulting in an amino acid substitution of glycine (G) with aspartic acid (D). Sequencing the IAA16 gene in other R and S individuals further confirmed that all the R individuals contained the mutation.

CONCLUSION

In this research, we describe the dicamba resistance mechanism in the only case of dicamba-resistant C. album reported to date. Prior work has shown that the dicamba resistance allele confers significant growth defects to the R phenotype investigated here, suggesting that dicamba-resistant C. album carrying this novel mutation in the IAA16 gene may not persist at high frequencies upon removal of dicamba application. © 2024 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Nontarget-site resistance due to rapid physiological response in 2,4-D resistant Conyza sumatrensis: reduced 2,4-D translocation and auxin-induced gene expression

BACKGROUND

Resistance to 2,4-Dichlorophenoxyacetic acid (2,4-D) has been reported in several weed species since the 1950s; however, a biotype of Conyza sumatrensis showing a novel physiology of the rapid response minutes after herbicide application was reported in 2017. The objective of this research was to investigate the mechanisms of resistance and identify transcripts associated with the rapid physiological response of C. sumatrensis to 2,4-D herbicide.

RESULTS

Differences were found in 2,4-D absorption between the resistant and susceptible biotypes. Herbicide translocation was reduced in the resistant biotype compared to the susceptible. In resistant plants 98.8% of [14 C] 2,4-D was found in the treated leaf, whereas ≈13% translocated to other plant parts in the susceptible biotype at 96 h after treatment. Resistant plants did not metabolize [14 C] 2,4-D and had only intact [14 C] 2,4-D at 96 h after application, whereas susceptible plants metabolized [14 C] 2,4-D into four detected metabolites, consistent with reversible conjugation metabolites found in other 2,4-D sensitive plant species. Pre-treatment with the cytochrome P450 inhibitor malathion did not enhance 2,4-D sensitivity in either biotype. Following treatment with 2,4-D, resistant plants showed increased expression of transcripts within plant defense response and hypersensitivity pathways, whereas both sensitive and resistant plants showed increased expression of auxin-response transcripts.

CONCLUSION

Our results demonstrate that reduced 2,4-D translocation contributes to resistance in the C. sumatrensis biotype. The reduction in 2,4-D transport is likely to be a consequence of the rapid physiological response to 2,4-D in resistant C. sumatrensis. Resistant plants had increased expression of auxin-responsive transcripts, indicating that a target-site mechanism is unlikely. © 2023 Society of Chemical Industry.

Target-site and non-target-site resistance mechanisms confer multiple resistance to glyphosate and 2,4-D in Carduus acanthoides

Carduus acanthoides L. is mainly a range-land weed, but in the 2010s has begun to invade GM crop production systems in Córdoba (Argentina), where glyphosate and 2,4-D have been commonly applied. In 2020, C. acanthoides was found with multiple resistance to these two herbicides. In this study, the mechanisms that confer multiple resistance to glyphosate and 2,4-D, were characterized in one resistant (R) population of C. acanthoides in comparison to a susceptible (S) population. No differences in ¹⁴C-herbicide absorption and translocation were observed between R and S populations. In addition, ¹⁴C-glyphosate was well translocated to the shoots (∼30%) and roots (∼16%) in both R and S plants, while most of ¹⁴C-2,4-D remained restricted in the treated leaf. Glyphosate metabolism did not contribute to resistance of the R population; however, as corroborated by malathion pretreatment, the mechanism of resistance to 2,4-D was enhanced metabolism (63% of the herbicide) mediated by cytochrome P450 (Cyt-P450). No differences were found in baseline EPSPS activity, copy number, and/or gene expression between the R and S populations, but a Pro-106-Ser mutation in EPSPS was present in the R population. Multiple resistances in the R population of C. acanthoides from Argentina were governed by target site resistance (a Pro-106 mutation for glyphosate) and non-target site resistance (Cyt-P450-based metabolic resistance for 2,4-D) mechanisms. This is the first case of resistance to glyphosate and 2,4-D confirmed for this weed in the world.

High aquaporin expression correlates with increased translocation of quinclorac from shoots to roots in resistant Echinochloa crus-galli var. zelayensis

BACKGROUND

Echinochloa crus-galli var. zelayensis is a troublesome weed in rice fields and can be controlled by using quinclorac. However, over-reliance on quinclorac has resulted in resistant (R) barnyardgrass, which differs significantly in its ability to transport quinclorac compared to susceptible (S) barnyardgrass. This study aimed to investigate the underlying mechanisms for this different translocation between R and S barnyardgrass.

RESULTS

Larger amount of quinclorac was transferred from shoots to roots in R compared to S barnyardgrass. After 1 day of quinclorac [300 g active ingredient (a.i.) ha^-1 ] foliar treatment, its content in shoots of R was 81.92% of that in S barnyardgrass; correspondingly, in roots of R was 1.17 fold of that in S barnyardgrass. RNA-sequencing and quantitative real-time polymerase chain reaction (qRT-PCR) confirmed the expression levels of PIPs belonging to aquaporins (AQPs) in R were higher than in S barnyardgrass, with or without quinclorac treatment. With co-application of quinclorac and AQPs inhibitors [mercury(II) chloride (HgCl₂ )] treatment, even though the expression levels of PIPs and the transport rates of quinclorac were both suppressed in R and S barnyardgrass, this process was less pronounced in R than in S barnyardgrass.

CONCLUSION

This report provides clear evidence that higher PIPs expression results in rapid quinclorac translocation from shoots to roots and reduces the quinclorac accumulation in the shoot meristems in R barnyardgrass, thus reducing the control efficacy of quinclorac. © 2022 Society of Chemical Industry.

Interspecific variation in resistance and tolerance to herbicide drift reveals potential consequences for plant community co-flowering interactions and structure at the agro-eco interface

BACKGROUND AND AIMS

When plant communities are exposed to herbicide 'drift', wherein particles containing the active ingredient travel off-target, interspecific variation in resistance or tolerance may scale up to affect community dynamics. In turn, these alterations could threaten the diversity and stability of agro-ecosystems. We investigated the effects of herbicide drift on the growth and reproduction of 25 wild plant species to make predictions about the consequences of drift exposure on plant-plant interactions and the broader ecological community.

METHODS

We exposed potted plants from species that commonly occur in agricultural areas to a drift-level dose of the widely used herbicide dicamba or a control solution in the glasshouse. We evaluated species-level variation in resistance and tolerance for vegetative and floral traits. We assessed community-level impacts of drift by comparing the species evenness and flowering networks of glasshouse synthetic communities comprised of drift-exposed and control plants.

KEY RESULTS

Species varied significantly in resistance and tolerance to dicamba drift: some were negatively impacted while others showed overcompensatory responses. Species also differed in the way they deployed flowers over time following drift exposure. While drift had negligible effects on community evenness based on vegetative biomass, it caused salient differences in the structure of co-flowering networks within communities. Drift reduced the degree and intensity of flowering overlap among species, altered the composition of groups of species that were more likely to co-flower with each other than with others and shifted species roles (e.g. from dominant to inferior floral producers, and vice versa).

CONCLUSIONS

These results demonstrate that even low levels of herbicide exposure can significantly alter plant growth and reproduction, particularly flowering phenology. If field-grown plants respond similarly, then these changes would probably impact plant-plant competitive dynamics and potentially plant-pollinator interactions occurring within plant communities at the agro-ecological interface.

Identification of a Novel 2,4-D Metabolic Detoxification Pathway in 2,4-D-Resistant Waterhemp (Amaranthus tuberculatus)

A 2,4-dichlorophenoxyactic acid (2,4-D)-resistant population of Amaranthus tuberculatus (common waterhemp) from Nebraska, USA, was previously found to have rapid metabolic detoxification of the synthetic auxin herbicide 2,4-D. We purified the main 2,4-D metabolites from resistant and susceptible plants, solved their structures by nuclear magnetic resonance (NMR) and high-resolution mass spectrometry (HRMS), and synthesized the metabolites to determine their in planta toxicity. Susceptible plants conjugated 2,4-D to aspartate to form 2,4-D-aspartic acid (2,4-D-Asp), while resistant plants had a unique metabolic profile where 2,4-D was hydroxylated into 5-OH-2,4-D, followed by conjugation into a sugar metabolite (2,4-D-5-O-d-glucopyranoside) and subsequent malonylation into 2,4-D-(6'-O-malonyl)-5-O-d-glucopyranoside. Toxicological studies on waterhemp and Arabidopsis thaliana confirmed that the hydroxylated metabolite lost its auxinic action and toxicity. In contrast, the 2,4-D-Asp metabolite found in susceptible plants retained some auxinic action and toxicity. These results demonstrate that 2,4-D-resistant A. tuberculatus evolved novel detoxification reactions not present in susceptible plants to rapidly metabolize 2,4-D, potentially mediated by cytochrome P450 enzymes that perform the initial 5-hydroxylation reaction. This novel mechanism is more efficient to detoxify 2,4-D and produces metabolites with lower toxicity compared to the aspartic acid conjugation found in susceptible waterhemp.

Transcriptome analysis of the 2,4-dichlorophenoxyacetic acid (2,4-D)-tolerant cotton chromosome substitution line CS-B15sh and its susceptible parental lines G. hirsutum L. cv. Texas Marker-1 and G. barbadense L. cv. Pima 379

The cotton chromosome substitution line, CS-B15sh, exhibits 41% lower injury from 2,4-D when applied at the field recommended rate of 1.12 kg ae ha-1 (1×) than does Texas Marker-1 (TM-1). CS-B15sh was developed in the genetic background of Gossypium hirsutum L. cv TM-1 and has chromosome introgression on the short arm of chromosome 15 from Gossypium barbadense L. cv. Pima 379. In a previous experiment, we observed reduced translocation of [14C]2,4-D outside the treated leaf tissue in CS-B15sh, which contrasted with an increased translocation of the herbicide in the tissues above and below the treated leaf in TM-1. Our results indicate a potential 2,4-D tolerance mechanism in CS-B15sh involving altered movement of 2,4-D. Here, we used RNA sequencing (RNA-seq) to determine the differential expression of genes between 2,4-D-challenged and control plants of the tolerant (CS-B15sh) and susceptible lines (TM-1 and Pima 379). Several components of the 2,4-D/auxin-response pathway-including ubiquitin E3 ligase, PB1|AUX/IAA, ARF transcription factors, and F-box proteins of the SCFTIR1/AFB complex-were upregulated with at least threefold higher expression in TM-1 compared with CS-B15sh, while both Pima 379 and TM-1 showed the same fold change expression for PB1|AUX/IAA mRNA. Some genes associated with herbicide metabolism, including flavin monooxygenase (Gohir.A01G174100) and FAD-linked oxidase (Gohir.D06G002600), exhibited at least a twofold increase in CS-B15sh than in TM-1 (the gene was not expressed in Pima 379), suggesting a potential relationship between the gene's expression and 2,4-D tolerance. It is interesting to note that glutathione S-transferase was differentially expressed in both CS-B15sh and Pima 379 but not in TM-1, while cytochrome P450 and other genes involved in the oxidation-reduction process were significantly expressed only in CS-B15sh in response to 2,4-D. Gene set enrichment analysis on the union DEGs of the three cotton genotypes revealed the depletion of transcripts involved in photosynthesis and enrichment of transcripts involved in ABA response and signaling.

Cross-resistance of barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] to aryloxyphenoxypropionate herbicides

Managing emerged weeds that have evolved resistance to acetyl CoA carboxylase (ACCase)-inhibiting herbicides is a challenging task. A dose-response experiment was conducted on barnyardgrass biotypes resistant (R) and susceptible (S) to three aryloxyphenoxypropionate herbicides cyhalofop-butyl (CyB), fenoxaprop-ethyl (FeE), and quizalofop-ethyl (QuE) along with investigations into the potential resistance mechanism of these biotypes. The tested R barnyardgrass biotypes had strong resistance to CyB and FeE (resistant/susceptible ratio: 7.9-14.4) but weak resistance to QuE (resistant/susceptible ratio: 2.4-3.1). Absorption, translocation, and total metabolism of CyB and QuE were not associated with differences among S and R barnyardgrass biotypes. However, differences between S and R barnyardgrass were observed in production of active acid forms of each herbicide (cyhalofop-acid and quizalofop-acid). Production of cyhalofop-acid was >1.6-fold less in R barnyardgrass (3-8%) for 24 h after herbicide application than in the S barnyardgrass (8-16%). Meanwhile, production of quizalofop-acid was less in R barnyardgrass (< 14%) throughout the study period than in the S barnyardgrass (< 22%). Sequencing results of ACCase gene showed no difference between S and R barnyardgrass. Overall results show that a non-target-site resistance mechanism altering metabolism of CyB and QuE likely contributes to resistance of the barnyardgrass biotypes to these herbicides.

An in-frame deletion mutation in the degron tail of auxin coreceptor IAA2 confers resistance to the herbicide 2,4-D in Sisymbrium orientale

Synthetic auxin herbicides intersect basic plant developmental biology and applied weed management. We investigated resistance to 2,4-D in the Australian weed Sisymbrium orientale (Indian hedge mustard). We identified a mechanism involving an in-frame 27-bp deletion in the degron tail of auxin coreceptor IAA2, one member of the gene family of Aux/IAA auxin co-receptors. We show that this deletion in IAA2 is a gain-of-function mutation that confers synthetic auxin resistance. This field-evolved mechanism of resistance to synthetic auxin herbicides confirms previous biochemical studies showing the role of the Aux/IAA degron tail in regulating Aux/IAA protein degradation upon auxin perception. The deletion mutation could be generated in crops using gene-editing approaches for cross-resistance to multiple synthetic auxin herbicides.

The natural auxin indole-3-acetic acid (IAA) is a key regulator of many aspects of plant growth and development. Synthetic auxin herbicides such as 2,4-D mimic the effects of IAA by inducing strong auxinic-signaling responses in plants. To determine the mechanism of 2,4-D resistance in a Sisymbrium orientale (Indian hedge mustard) weed population, we performed a transcriptome analysis of 2,4-D-resistant (R) and -susceptible (S) genotypes that revealed an in-frame 27-nucleotide deletion removing nine amino acids in the degron tail (DT) of the auxin coreceptor Aux/IAA2 (SoIAA2). The deletion allele cosegregated with 2,4-D resistance in recombinant inbred lines. Further, this deletion was also detected in several 2,4-D-resistant field populations of this species. Arabidopsis transgenic lines expressing the SoIAA2 mutant allele were resistant to 2,4-D and dicamba. The IAA2-DT deletion reduced binding to TIR1 in vitro with both natural and synthetic auxins, causing reduced association and increased dissociation rates. This mechanism of synthetic auxin herbicide resistance assigns an in planta function to the DT region of this Aux/IAA coreceptor for its role in synthetic auxin binding kinetics and reveals a potential biotechnological approach to produce synthetic auxin-resistant crops using gene-editing.

2,4-D and 2,4-D butoxyethyl ester behavior in Eurasian and hybrid watermilfoil (Myriophyllum spp.)

BACKGROUND

Hybrid watermilfoil is becoming more prevalent in many lakes where the invasive Eurasian (Myriophyllum spicatum, EWM) and native northern watermilfoil (M. sibiricum) co-occur. These Eurasian and northern watermilfoil hybrids (HWM) grow 30% faster and in many cases are less sensitive to 2,4-dichlorophenoxy acetic acid (2,4-D) than either parent. The mechanism(s) impacting 2,4-D tolerance in these hybrids was investigated by comparing the absorption, translocation, metabolism, and desorption of two 2,4-D formulations in EWM and HWM.

RESULTS

2,4-D absorption in EWM and HWM was 5.7 and 7.9 times the external herbicide concentration determined by the plant concentration factor, a metric used to determine herbicide bioaccumulation, and 2,4-D butoxyethyl ester absorption was 35.6 and 52.1 times the external concentration in EWM and HWM, respectively. Herbicide bioaccumulation was greater in HWM than in EWM. Herbicide translocation to HWM roots was limited at 192 HAT and herbicide desorption in HWM was slightly lower than EWM. No differences were found in herbicide metabolism between the two plant species.

CONCLUSION

2,4-D resistance in HWM is not due to non-target-site resistance as no differences in herbicide absorption, translocation, desorption and/or metabolism were identified; therefore, target-site resistance is the most likely resistance mechanism. More research is needed to identify the molecular basis for the 2,4-D-resistant trait in HWM. © 2021 Society of Chemical Industry.

Absorption, translocation, and metabolism of florpyrauxifen-benzyl and cyhalofop-butyl in cyhalofop-butyl-resistant barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.]

Dose-response experiments were conducted to assess the sensitivity of one susceptible and three putative resistant (R1, R2, and R3) barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] biotypes to florpyrauxifen-benzyl and cyhalofop-butyl alone and as a formulated premix. Subsequently, potential resistance mechanisms of the barnyardgrass were evaluated. Based on biomass reduction results, resistant/susceptible ratios were calculated for R1 (7.0-50), R2 (7.0-150), and R3 (18-214) biotypes. Absorption and translocation of [¹⁴C]-florpyrauxifen-benzyl decreased in R1 and R3 biotypes, but not for [¹⁴C]-cyhalofop-butyl. The metabolism of [¹⁴C]-florpyrauxifen-benzyl to [¹⁴C]-florpyrauxifen-acid was >2-fold less in resistant biotypes (9-11%) than in the susceptible biotype (23%). Moreover, the production of [¹⁴C]-florpyrauxifen-acid in susceptible barnyardgrass (not in the R biotypes) increased 3-fold when florpyrauxifen-benzyl and cyhalofop-butyl were applied in mixture compared to florpyrauxifen-benzyl applied alone. The tested barnyardgrass biotypes had no mutation in the Transport Inhibitor Response1, auxin-signaling F-box, and acetyl coenzyme A carboxylase genes. Although further studies on cyhalofop-butyl resistance with respect to analysis of specific metabolites are needed, our findings in this study demonstrates that the evolution of florpyrauxifen-benzyl resistance in multiple resistant barnyardgrass can be related to non-target-site resistance mechanisms reducing absorption and translocation of the herbicide and causing reduced conversion or rapid degradation of florpyrauxifen-acid.

Biotechnological Road Map for Innovative Weed Management

In most agriculture farmlands, weed management is predominantly reliant on integrated weed management (IWM) strategies, such as herbicide application. However, the overuse and misuse of herbicides, coupled with the lack of novel active ingredients, has resulted in the uptrend of herbicide-resistant weeds globally. Moreover, weedy traits that contribute to weed seed bank persistence further exacerbate the challenges in weed management. Despite ongoing efforts in identifying and improving current weed management processes, the pressing need for novel control techniques in agricultural weed management should not be overlooked. The advent of CRISPR/Cas9 gene-editing systems, coupled with the recent advances in "omics" and cheaper sequencing technologies, has brought into focus the potential of managing weeds in farmlands through direct genetic control approaches, but could be achieved stably or transiently. These approaches encompass a range of technologies that could potentially manipulate expression of key genes in weeds to reduce its fitness and competitiveness, or, by altering the crop to improve its competitiveness or herbicide tolerance. The push for reducing or circumventing the use of chemicals in farmlands has provided an added incentive to develop practical and feasible molecular approaches for weed management, although there are significant technical, practical, and regulatory challenges for utilizing these prospective molecular technologies in weed management.

Non-Target-Site Resistance Mechanisms Endow Multiple Herbicide Resistance to Five Mechanisms of Action in Conyza bonariensis

The repeated use of herbicides can lead to the selection of multiple resistance weeds. Some populations of Conyza bonariensis occurring in olive groves from southern Spain have developed resistance to various herbicides. This study determined the resistance levels to 2,4-D, glyphosate, diflufenican, paraquat, and tribenuron-methyl in a putative resistant (R) C. bonariensis population, and the possible non-target-site resistance (NTSR) mechanisms involved were characterized. Resistance factors varied as follows: glyphosate (8.9), 2,4-D (4.8), diflufenican (5.0), tribenuron-methyl (19.6), and paraquat (85.5). Absorption of ¹⁴C-glyphosate was up to 25% higher in the susceptible (S) population compared to the R one, but ¹⁴C-paraquat absorption was similar (up to 70%) in both populations. S plants translocated more than 60% of both ¹⁴C-glyphosate and ¹⁴C-paraquat toward shoots and roots, while R plants translocated less than 10%. The R population was able to metabolize 57% of the 2,4-D into nontoxic metabolites and 68% of the tribenuron-methyl into metsulfuron-methyl (10%), metsulfuron-methyl-hydroxylate (18%), and conjugate-metsulfuron-methyl (40%). Among the NTSR mechanisms investigated, absorption and translocation could be involved in glyphosate resistance, but only translocation for paraquat. Proofs of the presence of enhanced metabolism as a resistance mechanism were found for tribenuron-methyl and 2,4-D, but not for diflufenican. This research informs the first occurrence of multiple resistance to five herbicide classes (acetolactate synthase inhibitors, 5-enolpyruvylshikimate-3-phosphate synthase inhibitors, photosystem I electron diverters, photosystem II inhibitors, and synthetic auxin herbicides) in C. bonariensis.

Metabolism and detoxification of pesticides in plants

Pesticides make indispensable contributions to agricultural productivity. However, the residues after their excessive use may be harmful to crop production, food safety and human health. Although the ability of plants (especially crops) to accumulate and metabolize pesticides has been intensively investigated, data describing the chemical and metabolic processes in plants are limited. Understanding how pesticides are metabolized is a key step toward developing cleaner crops with minimal pesticides in crops, creating new green pesticides (or safeners), and building up the engineered plants for environmental remediation. In this review, we describe the recently discovered mechanistic insights into pesticide metabolic pathways, and development of improved plant genotypes that break down pesticides more effectively. We highlight the identification of biological features and functions of major pesticide-metabolized enzymes such as laccases, glycosyltransferases, methyltransferases and ATP binding cassette (ABC) transporters, and discuss their chemical reactions involved in diverse pathways including the formation of pesticide S-conjugates. The recent findings for some signal molecules (phytohomormes) like salicylic acid, jasmonic acid and brassinosteroids involved in metabolism and detoxification of pesticides are summarized. In particular, the emerging research on the epigenetic mechanisms such DNA methylation and histone modification for pesticide metabolism is emphasized. The review would broaden our understanding of the regulatory networks of the pesticide metabolic pathways in higher plants.

Nanosensor Detection of Synthetic Auxins In Planta using Corona Phase Molecular Recognition

Synthetic auxins such as 1-naphthalene acetic acid (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D) have been extensively used in plant tissue cultures and as herbicides because they are chemically more stable and potent than most endogenous auxins. A tool for rapid in planta detection of these compounds will enhance our knowledge about hormone distribution and signaling and facilitate more efficient usage of synthetic auxins in agriculture. In this work, we show the development of real-time and nondestructive in planta NAA and 2,4-D nanosensors based on the concept of corona phase molecular recognition (CoPhMoRe), to replace the current state-of-the-art sensing methods that are destructive and laborious. By designing a library of cationic polymers wrapped around single-walled carbon nanotubes with general affinity for chemical moieties displayed on auxins and its derivatives, we developed selective sensors for these synthetic auxins, with a particularly large quenching response to NAA (46%) and a turn-on response to 2,4-D (51%). The NAA and 2,4-D nanosensors are demonstrated in planta across several plant species including spinach, Arabidopsis thaliana (A. thaliana), Brassica rapa subsp. chinensis (pak choi), and Oryza sativa (rice) grown in various media, including soil, hydroponic, and plant tissue culture media. After 5 h of 2,4-D supplementation to the hydroponic medium, 2,4-D is seen to accumulate in susceptible dicotyledon pak choi leaves, while no uptake is observed in tolerant monocotyledon rice leaves. As such, the 2,4-D nanosensor had demonstrated its capability for rapid testing of herbicide susceptibility and could help elucidate the mechanisms of 2,4-D transport and the basis for herbicide resistance in crops. The success of the CoPhMoRe technique for measuring these challenging plant hormones holds tremendous potential to advance the plant biology study.

Investigating the presence of compensatory evolution in dicamba resistant IAA16 mutated kochia (Bassia scoparia)†

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. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Advance in Methodology and Strategies To Unveil Metabolic Mechanisms of Pesticide Residues in Food Crops

Pesticide residues are a food safety concern. A good detection method is critical for rapid and accurate determination of pesticide metabolites in crops and studying metabolism. The pretreatment methods have mainly been ultrasonic extraction-solid-phase extraction and QuEChERS, while detection methods have been radio-chromatography, nuclear magnetic resonance, and mass spectrometry. This perspective briefed the progress of analytical methods used for studying pesticide transformation in crops over the past decade. With the combination of the characteristics of the pesticide molecular structure and the transformation principles of pesticides in crops, we presented specific methods for elucidating new metabolites and the approaches to identify metabolites using multi-high-resolution mass spectrometry.

Synthetic auxin herbicides: finding the lock and key to weed resistance

Synthetic auxin herbicides are designed to mimic indole-3-acetic acid (IAA), an integral plant hormone affecting cell growth, development, and tropism. In this review, we explore target site genes in the auxin signaling pathway including SCF^TIR1/AFB, Aux/IAA, and ARFs that are confirmed or proposed mechanisms for weed resistance to synthetic auxin herbicides. Resistance to auxin herbicides by metabolism, either by enhanced cytochrome P450 detoxification or by loss of pro-herbicide activation, is a major non-target-site resistance pathway. We speculate about potential fitness costs of resistance due to effects of resistance-conferring mutations, provide insight into the role of polyploidy in synthetic auxin resistance evolution, and address the genetic resources available for weeds. This knowledge will be the key to unlock the long-standing questions as to which components of the auxin signaling pathway are most likely to have a role in resistance evolution. We propose that an ambitious research effort into synthetic auxin herbicide/target site interactions is needed to 1) explain why some synthetic auxin chemical families have activity on certain dicot plant families but not others and 2) fully elucidate target-site cross-resistance patterns among synthetic auxin chemical families to guide best practices for resistance management.

Coexpression Clusters and Allele-Specific Expression in Metabolism-Based Herbicide Resistance

In the last decade, Amaranthus tuberculatus has evolved resistance to 2,4-dichlorophenoxyacetic acid (2,4-D) and 4-hydroxyphenylpyruvate dioxygenase inhibitors in multiple states across the midwestern United States. Two populations resistant to both mode-of-action groups, one from Nebraska (NEB) and one from Illinois (CHR), were studied using an RNA-seq approach on F2 mapping populations to identify the genes responsible for resistance. Using both an A. tuberculatus transcriptome assembly and a high-quality grain amaranth (A. hypochondriacus) genome as references, differential transcript and gene expression analyses were conducted to identify genes that were significantly over- or underexpressed in resistant plants. When these differentially expressed genes (DEGs) were mapped on the A. hypochondriacus genome, physical clustering of the DEGs was apparent along several of the 16 A. hypochondriacus scaffolds. Furthermore, single-nucleotide polymorphism calling to look for resistant-specific (R) variants, and subsequent mapping of these variants, also found similar patterns of clustering. Specifically, regions biased toward R alleles overlapped with the DEG clusters. Within one of these clusters, allele-specific expression of cytochrome  P450  81E8 was observed for 2,4-D resistance in both the CHR and NEB populations, and phylogenetic analysis indicated a common evolutionary origin of this R allele in the two populations.

Investigation of physiological and molecular mechanisms conferring diurnal variation in auxinic herbicide efficacy

The efficacy of auxinic herbicides, a valuable weed control tool for growers worldwide, has been shown to vary with the time of day in which applications are made. However, little is known about the mechanisms causing this phenomenon. Investigating the differential in planta behavior of these herbicides across different times of application may grant an ability to advise which properties of auxinic herbicides are desirable when applications must be made around the clock. Radiolabeled herbicide experiments demonstrated a likely increase in ATP-binding cassette subfamily B (ABCB)-mediated 2,4-D and dicamba transport in Palmer amaranth (Amaranthus palmeri S. Watson) at simulated dawn compared to mid-day, as dose response models indicated that many orders of magnitude higher concentrations of N-1-naphthylphthalamic acid (NPA) and verapamil, respectively, are required to inhibit translocation by 50% at simulated sunrise compared to mid-day. Gas chromatographic analysis displayed that ethylene evolution in A. palmeri was higher when dicamba was applied during mid-day compared to sunrise. Furthermore, it was found that inhibition of translocation via 2,3,5-triiodobenzoic acid (TIBA) resulted in an increased amount of 2,4-D-induced ethylene evolution at sunrise, and the inhibition of dicamba translocation via NPA reversed the difference in ethylene evolution across time of application. Dawn applications of these herbicides were associated with increased expression of a putative 9-cis-epoxycarotenoid dioxygenase biosynthesis gene NCED1, while there was a notable lack of trends observed across times of day and across herbicides with ACS1, encoding 1-aminocyclopropane-1-carboxylic acid synthase. Overall, this research indicates that translocation is differentially regulated via specific protein-level mechanisms across times of application, and that ethylene release, a chief phytotoxic process involved in the response to auxinic herbicides, is related to translocation. Furthermore, transcriptional regulation of abscisic acid involvement in phytotoxicity and/or translocation are suggested.

Mechanisms of evolved herbicide resistance

The widely successful use of synthetic herbicides over the past 70 years has imposed strong and widespread selection pressure, leading to the evolution of herbicide resistance in hundreds of weed species. Both target-site resistance (TSR) and nontarget-site resistance (NTSR) mechanisms have evolved to most herbicide classes. TSR often involves mutations in genes encoding the protein targets of herbicides, affecting the binding of the herbicide either at or near catalytic domains or in regions affecting access to them. Most of these mutations are nonsynonymous SNPs, but polymorphisms in more than one codon or entire codon deletions have also evolved. Some herbicides bind multiple proteins, making the evolution of TSR mechanisms more difficult. Increased amounts of protein target, by increased gene expression or by gene duplication, are an important, albeit less common, TSR mechanism. NTSR mechanisms include reduced absorption or translocation and increased sequestration or metabolic degradation. The mechanisms that can contribute to NTSR are complex and often involve genes that are members of large gene families. For example, enzymes involved in herbicide metabolism-based resistances include cytochromes P450, GSH S-transferases, glucosyl and other transferases, aryl acylamidase, and others. Both TSR and NTSR mechanisms can combine at the individual level to produce higher resistance levels. The vast array of herbicide-resistance mechanisms for generalist (NTSR) and specialist (TSR and some NTSR) adaptations that have evolved over a few decades illustrate the evolutionary resilience of weed populations to extreme selection pressures. These evolutionary processes drive herbicide and herbicide-resistant crop development and resistance management strategies.

Non-target-site resistance to PDS-inhibiting herbicides in a wild radish (Raphanus raphanistrum) population

BACKGROUND

Diflufenican resistance has been reported in wild radish populations since 1998, but the resistance mechanisms have not been investigated. Recently, we identified a wild radish population (H2/10) from the Western Australian grain belt that is resistant (R) to the phytoene desaturase (PDS)-inhibiting herbicide diflufenican.

RESULTS

Dose-response results showed this R population is 4.9-fold more resistant than the susceptible (S) population based on the LD₅₀ R/S ratio. In addition, the R population also exhibits cross-resistance to the PDS-inhibiting herbicide fluridone. The cytochrome P450 inhibitor malathion reversed diflufenican resistance and partially reversed fluridone resistance in the R population. The full coding sequences of the PDS gene were cloned from the S and R plants and there are natural variations in the PDS gene transcripts/alleles with no correlation to resistance. In addition, the R plants had a level of PDS gene expression that is not significantly different from the S plants.

CONCLUSION

These results demonstrated that diflufenican resistance in this R wild radish population is likely due to non-target-site based enhanced herbicide metabolism involving cytochrome P450s. © 2019 Society of Chemical Industry.

Evolution of resistance to HPPD-inhibiting herbicides in a wild radish population via enhanced herbicide metabolism

BACKGROUND

Relatively new herbicides that target 4-hydroxyphenylpyruvate dioxygenase (HPPD) are now available for use on the world's great grain crops (rice, wheat, corn and soybean) and for other uses. With widespread and persistent use of HPPD-inhibiting herbicides, the evolution of HPPD-inhibiting herbicide resistant weeds is inevitable. Currently, resistance to HPPD-inhibiting herbicides is known in two weed species, waterhemp and Palmer amaranth. Here, we report a HPPD-inhibiting herbicide resistant wild radish population from the Western Australia grain belt. This population was not selected with HPPD-inhibiting herbicides, rather it evolved resistance to earlier used herbicides with different modes of action and exhibits cross-resistance to HPPD-inhibiting herbicides.

RESULTS

Dose-response experiments showed the resistant (R) population exhibits 4 to 6.5-fold resistance to the HPPD-inhibiting herbicides mesotrione, tembotrione and isoxaflutole, compared to the susceptible (S) population. This resistance is not target-site based as cloning of full coding sequences of the HPPD genes from S and R plants did not reveal resistance-endowing single nucleotide polymorphisms. The HPPD gene expression levels are similar in S and R plants. In addition, no differences in [¹⁴ C]-mesotrione uptake and translocation were observed in the S and R plants. However, the time required for R plants to metabolise 50% [¹⁴ C]-mesotrione is 7.7-fold faster than for the S plants.

CONCLUSION

We confirm resistance to HPPD-inhibiting herbicides exists in a population of the economically damaging global weed wild radish. The resistance in this population is due to a non-target-site based enhanced rate of herbicide metabolism. © 2019 Society of Chemical Industry.

Plasma membrane receptor-like kinases and transporters are associated with 2,4-D resistance in wild radish

BACKGROUND AND AIMS

Resistance to the synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) in wild radish (Raphanus raphanistrum) appears to be due to a complex, multifaceted mechanism possibly involving enhanced constitutive plant defence and alterations in auxin signalling. Based on a previous gene expression analysis highlighting the plasma membrane as being important for 2,4-D resistance, this study aimed to identify the components of the leaf plasma membrane proteome that contribute to resistance.

METHODS

Isobaric tagging of peptides was used to compare the plasma membrane proteomes of a 2,4-D-susceptible and a 2,4-D-resistant wild radish population under control and 2,4-D-treated conditions. Eight differentially abundant proteins were then targeted for quantification in the plasma membranes of 13 wild radish populations (two susceptible, 11 resistant) using multiple reaction monitoring.

KEY RESULTS

Two receptor-like kinases of unknown function (L-type lectin domain-containing receptor kinase IV.1-like and At1g51820-like) and the ATP-binding cassette transporter ABCB19, an auxin efflux transporter, were identified as being associated with auxinic herbicide resistance. The variability between wild radish populations suggests that the relative contributions of these candidates are different in the different populations.

CONCLUSIONS

To date, no receptor-like kinases have been reported to play a role in 2,4-D resistance. The lectin-domain-containing kinase may be involved in perception of 2,4-D at the plasma membrane, but its ability to bind 2,4-D and the identity of its signalling partner(s) need to be confirmed experimentally. ABCB19 is known to export auxinic compounds, but its role in 2,4-D resistance in wild radish appears to be relatively minor.

Resistance Mechanisms to 2,4-D in Six Different Dicotyledonous Weeds Around the World

2,4-D resistance is increasing around the world due to both transgenic crops and resistance to other herbicides. The objective of the this study was to characterize the currently unknown mechanisms of 2,4-D resistance in five weed species from around the globe: Amaranthus hybridus (Argentina), Conyza canadensis (Hungary), Conyza sumatrensis (France), Hirschfeldia incana (Argentina) and Parthenium hysterophorus (Dominican Republic), using Papaver rhoeas (Spain) as a standard resistant (R) species. Dose-response trials using malathion and absorption, translocation and metabolism experiments were performed to unravel the resistance mechanisms. R plants produced at least 3-folds less ethylene than susceptible plants, confirming the resistance to 2,4-D, together with resistance factors >4. A. hybridus, P. hysterophorus and P. rhoeas showed both reduced translocation and enhanced metabolism. In the two Conyza sps., the only resistance mechanism found was enhanced metabolism. Malathion synergized with 2,4-D in all these species, indicating the role of cytochrome P450 in the herbicide degradation. In H. incana, reduced translocation was the only contributing mechanism to resistance. Among the six dicotyledonous weed species investigated, there was a differential contribution to 2,4-D resistance of enhanced metabolism and reduced translocation. Thus, extrapolating 2,4-D resistance mechanisms from one weed species to another is very risky, if even related.

Herbicide resistant weeds: A call to integrate conventional agricultural practices, molecular biology knowledge and new technologies

Herbicide resistant (HR) weeds are of major concern in modern agriculture. This situation is exacerbated by the massive adoption of herbicide-based technologies along with the overuse of a few active ingredients to control weeds over vast areas year after year. Also, many other anthropological, biological, and environmental factors have defined a higher rate of herbicide resistance evolution in numerous weed species around the world. This review focuses on two central points: 1) how these factors have affected the resistance evolution process; and 2) which cultural practices and new approaches would help to achieve an effective integrated weed management. We claim that global climate change is an unnoticed factor that may be acting on the selection of HR weeds, especially those evolving into non-target-site resistance mechanisms. And we present several new tools -such as Gene Drive and RNAi technologies- that may be adopted to cope with herbicide resistance spread, as well as discuss their potential application at field level. This is the first review that integrates agronomic and molecular knowledge of herbicide resistance. It covers not only the genetic basis of the most relevant resistance mechanisms but also the strengths and weaknesses of traditional and forthcoming agricultural practices.

Cost-effective detection of genome-wide signatures for 2,4-D herbicide resistance adaptation in red clover

Herbicide resistance is a recurrent evolutionary event that has been reported across many species and for all major herbicide modes of action. The synthetic auxinic herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) has been widely used since the 1940s, however the genetic variation underlying naturally evolving resistance remains largely unknown. In this study, we used populations of the forage legume crop red clover (Trifolium pratense L.) that were recurrently selected for 2,4-D resistance to detect genome-wide signatures of adaptation. Four susceptible and six derived resistant populations were sequenced using a less costly approach by combining targeted sequencing (Capture-Seq) with pooled individuals (Pool-Seq). Genomic signatures of selection were identified using: (i) pairwise allele frequency differences; (ii) genome scan for overly differentiated loci; and (iii) genome‐wide association. Fifty significant SNPs were consistently detected, most located in a single chromosome, which can be useful for marker assisted selection. Additionally, we searched for candidate genes at these genomic regions to gain insights into potential molecular mechanisms underlying 2,4-D resistance. Among the predicted functions of candidate genes, we found some related to the auxin metabolism, response to oxidative stress, and detoxification, which are also promising for further functional validation studies.

Rapid metabolism increases the level of 2,4-D resistance at high temperature in common waterhemp (Amaranthus tuberculatus)

Common waterhemp emerges throughout the crop growing season in the Midwestern United States, and as a result, the seedlings are exposed to a wide range of temperature regimes. Typically, 2,4-D is used in the Midwest to control winter annual broad-leaf weeds before planting soybean and in an early post-emergence application in corn and sorghum; however, the evolution of 2,4-D-resistant common waterhemp in several Midwestern states may limit the use of 2.4-D for controlling this problem weed. Moreover, temperature is one of the crucial factors affecting weed control efficacy of 2,4-D. This research investigated the effect of temperature on efficacy of 2,4-D to control 2,4-D susceptible (WHS) and -resistant (WHR) common waterhemp. Do se-response of WHS and WHR to 2,4-D was assessed at two temperature regimes, high (HT; 34/20 °C, d/n) and low (LT; 24/10 °C, d/n). Whole plant dose response study indicated an increased level of 2,4-D resistance in WHR at HT compared to LT. Additional investigation of the physiological mechanism of this response indicated that both WHS and WHR common waterhemp plants rapidly metabolized ¹⁴C 2,4-D at HT compared to LT. In conclusion, a rapid metabolism of 2,4-D conferred increased level of resistance to 2,4-D in WHR at HT. Therefore, application of 2,4-D when temperatures are cooler can improve control of 2,4-D resistant common waterhemp.

Target site as the main mechanism of resistance to imazamox in a Euphorbia heterophylla biotype

Euphorbia heterophylla is a weed species that invades extensive crop areas in subtropical regions of Brazil. This species was previously controlled by imazamox, but the continuous use of this herbicide has selected for resistant biotypes. Two biotypes of E. heterophylla from southern Brazil, one resistant (R) and one susceptible (S) to imazamox, were compared. The resistance of the R biotype was confirmed by dose-response assays since it required 1250.2 g ai ha^-1 to reduce the fresh weight by 50% versus 7.4 g ai ha^-1 for the S biotype. The acetolactate synthase (ALS) enzyme activity was studied using ALS-inhibiting herbicides from five different chemical families. The R biotype required the highest concentrations to reduce this enzyme activity by 50%. A Ser653Asn mutation was found in the ALS gene of the R biotype. The experiments carried out showed that imazamox absorption and metabolism were not involved in resistance. However, greater ¹⁴C-imazamox root exudation was found in the R biotype (~70% of the total absorbed imazamox). Target site mutation in the ALS gene is the principal mechanism that explains the imazamox resistance of the R biotype, but root exudation seems to also contribute to the resistance of this biotype.

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.

Multiple Resistance to Synthetic Auxin Herbicides and Glyphosate in Parthenium hysterophorus Occurring in Citrus Orchards

Dominican farmers have started to apply synthetic auxin herbicides (SAHs) as the main alternative to mitigate the impacts of the occurrence of glyphosate-resistant (GR) Parthenium hysterophorus populations in citrus orchards. A GR P. hysterophorus population survived field labeled rates of glyphosate, 2,4-dichlorophenoxyacetic acid (2,4-D), dicamba, and picloram, which showed poor control (<50%). In in vivo assays, resistance levels were high for glyphosate and moderate for picloram, dicamba, and 2,4-D. Sequencing the 5-enolpyruvylshikimate-3-phosphate synthase gene revealed the double Thr-102-Ile and Pro-106-Ser amino acid substitution, conferring resistance to glyphosate. Additionally, reduced absorption and impaired translocation contributed to this resistance. Regarding SAH, impaired 2,4-D transport and enhanced metabolism were confirmed in resistant plants. The application of malathion improved the efficacy of SAHs (control >50%), showing that metabolism of these herbicides was mediated by cytochrome P450 enzymes. This study reports, for the first time, multiple resistance to SAHs and glyphosate in P. hysterophorus.

Do plants pay a fitness cost to be resistant to glyphosate?

We reviewed the literature to understand the effects of glyphosate resistance on plant fitness at the molecular, biochemical and physiological levels. A number of correlations between enzyme characteristics and glyphosate resistance imply the existence of a plant fitness cost associated with resistance-conferring mutations in the glyphosate target enzyme, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). These biochemical changes result in a tradeoff between the glyphosate resistance of the EPSPS enzyme and its catalytic activity. Mutations that endow the highest resistance are more likely to decrease catalytic activity by reducing the affinity of EPSPS for its natural substrate, and/or slowing the velocity of the enzyme reaction, and are thus very likely to endow a substantial plant fitness cost. Prediction of fitness costs associated with EPSPS gene amplification and overexpression can be more problematic. The validity of cost prediction based on the theory of evolution of gene expression and resource allocation has been cast into doubt by contradictory experimental evidence. Further research providing insights into the role of the EPSPS cassette in weed adaptation, and estimations of the energy budget involved in EPSPS amplification and overexpression are required to understand and predict the biochemical and physiological bases of the fitness cost of glyphosate resistance.

Metribuzin Resistance in a Wild Radish ( Raphanus raphanistrum) Population via Both psbA Gene Mutation and Enhanced Metabolism

There have been many studies on target-site resistance (TSR) to PSII-inhibiting herbicides, but only a few on the non-target-site resistance (NTSR). Here, we reported both TSR and NTSR to metribuzin in a wild radish population. Dose-response studies revealed a higher level of resistance to metribuzin in the resistant (R) compared to the susceptible (S) population. Sequencing of the target psbA gene revealed the known Ser-264-Gly mutation in R plants. In addition, a higher level of [¹⁴C]-metribuzin metabolism and, consequently, a lower level of [¹⁴C] translocation were also detected in the R plants. These results demonstrated that both psbA gene mutation and enhanced metabolism contribute to metribuzin resistance in this wild radish population. Furthermore, this resistant population showed resistance to ALS-inhibiting herbicides due to multiple ALS gene mutations. This is the first report in wild radish of metabolic herbicide resistance, in addition to the target-site psbA gene mutation.

Identity and Activity of 2,4-Dichlorophenoxyacetic Acid Metabolites in Wild Radish ( Raphanus raphanistrum)

Synthetic auxin herbicides, such as 2,4-dichlorophenoxyacetic acid (2,4-D), are widely used for selective control of broadleaf weeds in cereals and transgenic crops. Although the troublesome weed wild radish ( Raphanus raphanistrum) has developed resistance to 2,4-D, no populations have yet displayed an enhanced capacity for metabolic detoxification of the herbicide, with both susceptible and resistant wild radish plants readily metabolizing 2,4-D. Using mass spectrometry and nuclear magnetic resonance, the major 2,4-D metabolite was identified as the glucose ester, and its structure was confirmed by synthesis. As expected, both the endogenous and synthetic compounds retained auxin activity in a bioassay. The lack of detectable 2,4-D hydroxylation in wild radish and the lability of the glucose ester suggest that metabolic 2,4-D resistance is unlikely to develop in this species.

Molecular and physiological characterization of six-way resistance in an Amaranthus tuberculatus var. rudis biotype from Missouri

BACKGROUND

Previous research reported the first case of six-way herbicide resistance in a common waterhemp (Amaranthus tuberculatus var. rudis) biotype from Missouri, USA designated MO-Ren. This study investigated the mechanisms of multiple-resistance in the MO-Ren biotype to herbicides from six site-of-action (SOA) groups, i.e. synthetic auxins, 5-enolypyruvyl-shikimate-3-phosphate synthase (EPSPS)-, protoporphyrinogen oxidase (PPO)-, acetolactate synthase (ALS)-, photosystem II (PSII)-, and 4-hydroxyphenyl-pyruvate-dioxygenase (HPPD)-inhibitors.

RESULTS

Genomic DNA sequencing confirmed the presence of known mutations associated with ALS- or PPO-inhibiting herbicide resistance: the Trp-574-Leu amino acid substitution in the ALS enzyme and the codon deletion corresponding to the ΔG210 in the PPX2 enzyme. No target-site point mutations associated with resistance to PSII- and EPSPS-inhibitors were detected. Quantitative polymerase chain reaction (qPCR) indicated that MO-Ren plants contained five-fold more copies of the EPSPS gene than susceptible plants. Malathion in combination with 2,4-D (2,4-dichlorophenoxyacetic acid), mesotrione, and chlorimuron POST enhanced the activity of these herbicides indicating that metabolism due to cytochrome P450 monooxygenase activity was involved in herbicide resistance. 4-Chloro-7-nitrobenzofurazan (NBD-Cl), a glutathione-S-transferase (GST)-inhibitor, in combination with atrazine did not reduce the biomass accumulation. Reduced absorption or translocation of 2,4-D did not contribute to resistance. However, the resistant biotype metabolized 2,4-D, seven- to nine-fold faster than the susceptible.

CONCLUSION

Target-site point mutations, gene amplification, and elevated rates of metabolism contribute to six-way resistance in the MO-Ren biotype, suggesting both target site and non-target site mechanisms contribute to multiple herbicide resistance in this Amaranthus tuberculatus biotype. © 2018 Society of Chemical Industry.

Increased chalcone synthase (CHS) expression is associated with dicamba resistance in Kochia scoparia

BACKGROUND

Resistance to the synthetic auxin herbicide dicamba is increasingly problematic in Kochia scoparia. The resistance mechanism in an inbred dicamba-resistant K. scoparia line (9425R) was investigated using physiological and transcriptomics (RNA-Seq) approaches.

RESULTS

No differences were found in dicamba absorption or metabolism between 9425R and a dicamba-susceptible line, but 9425R was found to have significantly reduced dicamba translocation. Known auxin-responsive genes ACC synthase (ACS) and indole-3-acetic acid amino synthetase (GH3) were transcriptionally induced following dicamba treatment in dicamba-susceptible K. scoparia but not in 9425R. Chalcone synthase (CHS), the gene regulating synthesis of the flavonols quertecin and kaemperfol, was found to have twofold higher transcription in 9425R both without and 12 h after dicamba treatment. Increased CHS transcription co-segregated with dicamba resistance in a forward genetics screen using an F₂ population.

CONCLUSION

Prior work has shown that the flavonols quertecin and kaemperfol compete with auxin for intercellular movement and vascular loading via ATP-binding cassette subfamily B (ABCB) membrane transporters. The results of this study support a model in which constitutively increased CHS expression in the meristem produces more flavonols that would compete with dicamba for intercellular transport by ABCB transporters, resulting in reduced dicamba translocation. © 2017 Society of Chemical Industry.

Weed resistance to synthetic auxin herbicides

Herbicides classified as synthetic auxins have been most commonly used to control broadleaf weeds in a variety of crops and in non-cropland areas since the first synthetic auxin herbicide (SAH), 2,4-D, was introduced to the market in the mid-1940s. The incidence of weed species resistant to SAHs is relatively low considering their long-term global application with 30 broadleaf, 5 grass, and 1 grass-like weed species confirmed resistant to date. An understanding of the context and mechanisms of SAH resistance evolution can inform management practices to sustain the longevity and utility of this important class of herbicides. A symposium was convened during the 2nd Global Herbicide Resistance Challenge (May 2017; Denver, CO, USA) to provide an overview of the current state of knowledge of SAH resistance mechanisms including case studies of weed species resistant to SAHs and perspectives on mitigating resistance development in SAH-tolerant crops. © 2017 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

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.

2,4-D and dicamba resistance mechanisms in wild radish: subtle, complex and population specific?

BACKGROUND AND AIMS

Resistance to synthetic auxin herbicides such as 2,4-dichlorophenoxyacetic acid (2,4-D) is increasing in weed populations worldwide, which is of concern given the recent introduction of synthetic auxin-resistant transgenic crops. Due to the complex mode of action of the auxinic herbicides, the mechanisms of evolved resistance remain largely uncharacterized. The aims of this study were to assess the level of diversity in resistance mechanisms in 11 populations of the problem weed Raphanus raphanistrum, and to use a high-throughput, whole-genome transcriptomic analysis on one resistant and one susceptible population to identify important changes in gene expression in response to 2,4-D.

METHODS

Levels of 2,4-D and dicamba (3,6-dichloro-2-methoxybenzoic acid) resistance were quantified in a dose-response study and the populations were further screened for auxin selectivity, 2,4-D translocation and metabolism, expression of key 2,4-D-responsive genes and activation of the mitogen-activated proein kinase (MAPK) pathway. Potential links between resistance levels and mechanisms were assessed using correlation analysis.

KEY RESULTS

The transcriptomic study revealed early deployment of the plant defence response in the 2,4-D-treated resistant population, and there was a corresponding positive relationship between auxinic herbicide resistance and constitutive MAPK phosphorylation across all populations. Populations with shoot-wide translocation of 2,4-D had similar resistance levels to those with restricted translocation, suggesting that reduced translocation may not be as strong a resistance mechanism as originally thought. Differences in auxin selectivity between populations point to the likelihood of different resistance-conferring alterations in auxin signalling and/or perception in the different populations.

CONCLUSIONS

2,4-D resistance in wild radish appears to result from subtly different auxin signalling alterations in different populations, supplemented by an enhanced defence response and, in some cases, reduced 2,4-D translocation. This study highlights the dangers of applying knowledge generated from a few populations of a weed species to the species as a whole.

First Report of Amaranthus hybridus with Multiple Resistance to 2,4-D, Dicamba, and Glyphosate

In many countries, Amaranthus hybridus is a widespread weed in agricultural systems. The high prolificacy and invasive capacity as well as the resistance of some biotypes to herbicides are among the complications of handling this weed. This paper reports on the first A. hybridus biotypes with resistance to auxinic herbicides and multiple resistance to auxinic herbicides and the EPSPs inhibitor, glyphosate. Several dose response assays were carried out to determine and compare sensitivity of six population of A. hybridus to glyphosate, 2,4-D, and dicamba. In addition, shikimic acid accumulation and piperonil butoxide effects on 2,4-D and dicamba metabolism were tested in the same populations. The results showed four populations were resistant to dicamba and three of these were also resistant to 2,4-D, while only one population was resistant to glyphosate. The glyphosate-resistant population also showed multiple resistance to auxinic herbicides. Pretreatment with piperonil butoxide (PBO) followed by 2,4-D or dicamba resulted in the death of all individual weeds independent of herbicide or population.

Reduced translocation in 2,4-D-resistant oriental mustard populations (Sisymbrium orientale L.) from Australia

BACKGROUND

Two oriental mustard populations (P2 and P13) collected from Port Broughton, South Australia were identified as resistant to 2,4-D. The level of resistance, mechanism and the mode of inheritance for 2,4-D resistance in these populations were investigated.

RESULTS

Populations P2 and P13 were confirmed to be resistant to 2,4-D at the field rate (600 g a.e. ha^-1 ). P2 and P13 were 81- and 67-fold more resistant than the susceptible populations (S1 and S2) at the dose required for 50% mortality (LD₅₀ ), respectively. No predicted amino acid modification was detected in sequences of potential target-site genes (ABP, TIR1 and AFB5). Resistant populations had reduced 2,4-D translocation compared with the susceptible populations, with 77% of [¹⁴ C]2,4-D retained in the treated leaf versus 32% at 72 h after treatment. Resistance to 2,4-D is encoded on the nuclear genome and is dominant, as the response to 2,4-D of all F₂ individuals were similar to the resistant biotypes. The segregation of F₂ phenotypes fitted a 3: 1 (R: S) inheritance model.

CONCLUSION

Resistance to 2,4-D in oriental mustard is likely due to reduced translocation of 2,4-D out of the treated leaf. Inheritance of 2,4-D resistance is conferred by a single gene with a high level of dominance. © 2017 Society of Chemical Industry.

Cross-resistance to dicamba, 2,4-D, and fluroxypyr in Kochia scoparia is endowed by a mutation in an AUX/IAA gene

The understanding and mitigation of the appearance of herbicide-resistant weeds have come to the forefront of study in the past decade, as the number of weed species that are resistant to one or more herbicide modes of action is on the increase. Historically, weed resistance to auxin herbicides has been rare, but examples, such as Kochia scoparia L. Schrad (kochia), have appeared, posing a challenge to conventional agricultural practices. Reports of dicamba-resistant kochia populations began in the early 1990s in areas where auxin herbicides were heavily utilized for weed control in corn and wheat cropping systems, and some biotypes are resistant to other auxin herbicides as well. We have further characterized the auxin responses of one previously reported dicamba-resistant biotype isolated from western Nebraska and found that it is additionally cross-resistant to other auxin herbicides, including 2,4-dichlorophenoxyacetic acid (2,4-D) and fluroxypyr. We have utilized transcriptome sequencing and comparison to identify a 2-nt base change in this biotype, which results in a glycine to asparagine amino acid change within a highly conserved region of an AUX/indole-3-acetic acid (IAA) protein, KsIAA16. Through yeast two-hybrid analysis, characterization of F2 segregation, and heterologous expression and characterization of the gene in Arabidopsis thaliana, we show that that the single dominant KsIAA16R resistance allele is the causal basis for dicamba resistance in this population. Furthermore, we report the development of a molecular marker to identify this allele in populations and facilitate inheritance studies. We also report that the resistance allele confers a fitness penalty in greenhouse studies.

Auxin molecular field maps define AUX1 selectivity: many auxin herbicides are not substrates

Developmental responses to auxin are regulated by facilitated uptake and efflux, but detailed molecular understanding of the carrier proteins is incomplete. We have used pharmacological tools to explore the chemical space that defines substrate preferences for the auxin uptake carrier AUX1. Total and partial loss-of-function aux1 mutants were assessed against wild-type for dose-dependent resistance to a range of auxins and analogues. We then developed an auxin accumulation assay with associated mathematical modelling to enumerate accurate IC50 values for a small library of auxin analogues. The structure activity relationship data were analysed using molecular field analyses to create a pharmacophoric atlas of AUX1 substrates. The uptake carrier exhibits a very high level of selectivity towards small substrates including the natural indole-3-acetic acid, and the synthetic auxin 2,4-dichlorophenoxyacetic acid. No AUX1 activity was observed for herbicides based on benzoic acid (dicamba), pyridinyloxyacetic acid (triclopyr) or the 6-arylpicolinates (halauxifen), and very low affinity was found for picolinic acid-based auxins (picloram) and quinolinecarboxylic acids (quinclorac). The atlas demonstrates why some widely used auxin herbicides are not, or are very poor substrates. We list molecular descriptors for AUX1 substrates and discuss our findings in terms of herbicide resistance management.

Potential impact of the herbicide 2,4-dichlorophenoxyacetic acid on human and ecosystems

The herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) is applied directly to aquatic and conventional farming systems to control weeds, and is among the most widely distributed pollutants in the environment. Non-target organisms are exposed to 2,4-D via several ways, which could produce toxic effects depending on the dose, frequency of exposure, and the host factors that influence susceptibility and sensitivity. An increasing number of experimental evidences have shown concerns about its presence/detection in the environment, because several investigations have pointed out its potential lethal effects on non-target organisms. In this review, we critically evaluated the environmental fate and behavior of 2,4-D along with its eco-toxicological effects on aquatic, plants and human life to provide concise assessment in the light of recently published reports. The findings demonstrate that 2,4-D is present in a low concentration in surface water of regions where its usage is high. The highest concentrations of 2,4-D were detected in soil, air and surface water surrounded by crop fields, which suggest that mitigation strategies must be implanted locally to prevent the entry of 2,4-D into the environment. A general public may have frequent exposure to 2,4-D due to its wide applications at home lawns and public parks, etc. Various in vivo and in vitro investigations suggest that several species (or their organs) at different trophic levels are extremely sensitive to the 2,4-D exposure, which may explain variation in outcomes of reported investigations. However, implications for the prenatal exposure to 2,4-D remain unknown because 2,4-D-induced toxicity thresholds in organism have only been derived from juveniles or adults. In near future, introduction of 2,4-D resistant crops will increase its use in agriculture, which may cause relatively high and potentially unsafe residue levels in the environment. The recent findings indicate the urgent need to further explore fate, accumulation and its continuous low level exposure impacts on the environment to generate reliable database which is key in drafting new regulation and policies to protect the population from further exposure.

Determination of 2,4-Dichlorophenoxyacetic acid (2,4-D) in rat serum for pharmacokinetic studies with a simple HPLC method

2,4-Dichlorophenoxyacetic acid (2,4-D) is a chlorophenoxy herbicide used worldwide. We describe a high-performance liquid chromatography (HPLC) method with UV detection for the determination of 2,4-D in female and male rat serum. This allows to observe the change of serum 2,4-D concentration in rats with time and its pharmacokinetics characteristics with a simple, rapid, optimized and validated method. The serum samples are pretreated and introduced into the HPLC system. The analytes are separated in a XDB-C18 column with a mobile phase of acetonitrile (solvent A) and 0.02 M ammonium acetate (containing 0.1% formic acid) (solvent B) using a gradient elution at a flow rate of 1.0 mL/min. The wavelength for UV detection was set at 230 nm. Calibration curve for 2,4-D was constructed over a range of 0.1-400 mg/L. The method was successfully applied to study the pharmacokinetics of 2,4-D in rats in this study. After oral administration of 300 mg/kg and 60 mg/kg 2,4-D, the mean Cmax values were 601.9 and 218.4 mg/L, the AUC0→∞ values were 23,722 and 4,127 mg×h/L and the clearance (Cl) were 1.10 and 0.02 L/(h×kg), respectively. The developed method was found to be specific, precise, reproducible and rapid.

A cheminformatics review of auxins as herbicides

Herbicides are an important asset in ensuring food security, especially when faced with an ever-increasing demand on food production to feed the global population. The current selection of herbicides is increasingly encountering resistance in agricultural weeds they once targeted effectively. It is imperative that new compounds or more effective modes of action are discovered in order to overcome this resistance. This cheminformatics review looks at current herbicides and evaluates their physiochemical properties on a class-by-class basis. We focus in particular on the synthetic auxin herbicides, Herbicide Resistance Action Committee class O, analyzing these against herbicides more generally and for class-specific features such as mobility in plant vasculature. We summarise the physiochemical properties of all 24 compounds used commercially as auxins and relate these results to ongoing approaches to novel auxin discovery. We introduce an interactive, open source cheminformatics tool known as DataWarrior for herbicide discovery, complete with records for over 300 herbicidal compounds. We hope this tool helps researchers as part of a rational approach to not only auxin discovery but agrochemical discovery in general.

Suitable reference genes for accurate gene expression analysis in Papaver rhoeas under 2,4-D herbicide stress

Resistance to 2,4-D (2,4-diclorophenoxyacetic acid) herbicide is increasing in various dicotyledonous weed species, including Papaver rhoeas, a weed infesting Southern European wheat crops. Non-target-site resistance to this herbicide is governed by a range of genes involved in herbicide stress response. To enable reliable measurement of gene expression levels in herbicide-resistant and susceptible plants it is necessary to normalize qPCR data using internal control genes with stable expression. In an attempt to find the best reference genes, the stability of seven candidate reference genes was assessed in plants resistant and susceptible to 2,4-D, subjected or not to herbicide stress. Using three statistical algorithms (geNorm, BestKeeper and NormFinder), the overall results revealed that glyceraldehyde-3-phosphate dehydrogenase, actin and ubiquitin were the most stable reference genes. The normalization expression levels of GH3 (indole-3-acetic acid amido synthetase) and GST3 (glutathione S-transferase) which are two genes up-regulated following 2,4-D treatment, were determined to verify the stability of these selected reference genes. A sudden increase in GH3 and GST3 expression was already detected 5h after herbicide application, confirming their involvement in plant response to 2,4-D. The validation results confirmed the applicability and accuracy of these reference genes. This study identified and validated reference genes in the non-model weed species P. rhoeas and these will facilitate gene expression analysis studies aimed at identifying functional genes associated with non-target-site resistance.

Biokinetic Analysis and Metabolic Fate of 2,4-D in 2,4-D-Resistant Soybean (Glycine max)

The Enlist weed control system allows the use of 2,4-D in soybean but slight necrosis in treated leaves may be observed in the field. The objectives of this research were to measure and compare uptake, translocation, and metabolism of 2,4-D in Enlist (E, resistant) and non-AAD-12 transformed (NT, sensitive) soybeans. The adjuvant from the Enlist Duo herbicide formulation (ADJ) increased 2,4-D uptake (36%) and displayed the fastest rate of uptake (U50= 0.2 h) among treatments. E soybean demonstrated a faster rate of 2,4-D metabolism (M50= 0.2 h) compared to NT soybean, but glyphosate did not affect 2,4-D metabolism. Metabolites of 2,4-D in E soybean were qualitatively different than NT. Applying 2,4-D-ethylhexyl ester instead of 2,4-D choline (a quaternary ammonium salt) eliminated visual injury to E soybean, likely due to the time required for initial de-esterification and bioactivation. Excessive 2,4-D acid concentrations in E soybean resulting from ADJ-increased uptake may significantly contribute to foliar injury.

Mechanism of resistance to mesotrione in an Amaranthus tuberculatus population from Nebraska, USA

Amaranthus tuberculatus is a troublesome weed in corn and soybean production systems in Midwestern USA, due in part to its ability to evolve multiple resistance to key herbicides including 4-hydroxyphenylpyruvate dioxygenase (HPPD). Here we have investigated the mechanism of resistance to mesotrione, an important chemical for managing broadleaf weeds in corn, in a multiple herbicide resistant population (NEB) from Nebraska. NEB showed a 2.4-fold and 45-fold resistance increase to mesotrione compared to a standard sensitive population (SEN) in pre-emergence and post-emergence dose-response pot tests, respectively. Sequencing of the whole HPPD gene from 12 each of sensitive and resistant plants did not detect any target-site mutations that could be associated with post-emergence resistance to mesotrione in NEB. Resistance was not due to HPPD gene duplication or over-expression before or after herbicide treatment, as revealed by qPCR. Additionally, no difference in mesotrione uptake was detected between NEB and SEN. In contrast, higher levels of mesotrione metabolism via 4-hydroxylation of the dione ring were observed in NEB compared to the sensitive population. Overall, the NEB population was characterised by lower levels of parent mesotrione exported to other parts of the plant, either as a consequence of metabolism in the treated leaves and/or impaired translocation of the herbicide. This study demonstrates another case of non-target-site based resistance to an important class of herbicides in an A. tuberculatus population. The knowledge generated here will help design strategies for managing multiple herbicide resistance in this problematic weed species.