MCPA (4-Chloro-2-ethylphenoxyacetate) resistance in hemp-nettle (Galeopsis tetrahit L.)

Metabolism of 2,4-D in plants: comparative analysis of metabolic detoxification pathways in tolerant crops and resistant weeds

The commercialization of 2,4-D (2,4-dichlorophenoxyacetic acid) latifolicide in 1945 marked the beginning of the selective herbicide market, with this active ingredient playing a pivotal role among commercial herbicides due to the natural tolerance of monocots compared with dicots. Due to its intricate mode of action, involving interactions within endogenous auxin signaling networks, 2,4-D was initially considered a low-risk herbicide to evolve weed resistance. However, the intensification of 2,4-D use has contributed to the emergence of 2,4-D-resistant broadleaf weeds, challenging earlier beliefs. This review explores 2,4-D tolerance in crops and evolved resistance in weeds, emphasizing an in-depth understanding of 2,4-D metabolic detoxification. Nine confirmed 2,4-D-resistant weed species, driven by rapid metabolism, highlight cytochrome P450 monooxygenases in Phase I and glycosyltransferases in Phase II as key enzymes. Resistance to 2,4-D may also involve impaired translocation associated with mutations in auxin/indole-3-acetic acid (Aux/IAA) co-receptor genes. Moreover, temperature variations affect 2,4-D efficacy, with high temperatures increasing herbicide metabolism rates and reducing weed control, while drought stress did not affect 2,4-D efficacy. Research on 2,4-D resistance has primarily focused on non-target-site resistance (NTSR) mechanisms, including 2,4-D metabolic detoxification, with limited exploration of the inheritance and genetic basis underlying these traits. Resistance to 2,4-D in weeds is typically governed by a single gene, either dominant or incompletely dominant, raising questions about gain-of-function or loss-of-function mutations that confer resistance. Future research should unravel the physiological and molecular-genetic basis of 2,4-D NTSR, exploring potential cross-resistance patterns and assessing fitness costs that may affect future evolution of auxin-resistant weeds. © 2024 Society of Chemical Industry.

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.

Inheritance of 2,4-dichlorophenoxyacetic acid (2,4-D) resistance in Amaranthus palmeri

In this study, the inheritance of 2,4-D resistance in a multiple herbicide-resistant Palmer amaranth (KCTR) was investigated. Direct and reciprocal crosses were performed using 2,4-D-resistant KCTR and susceptible KSS plants to generate F₁ progenies. 2,4-D dose-response assays were conducted to evaluate the response of progenies from each F₁ family along with KCTR and KSS plants in controlled environmental growth chambers. Additionally, 2,4-D-resistant male and female plants from each of the F₁ families were used in pairwise crosses to generate pseudo-F₂ families. Segregation (resistance or susceptibility) of progenies from the F₂ families in response to a discriminatory rate of 2,4-D (i.e., 560 g ae ha^-1) was evaluated. Dose-response analysis of F₁ progenies derived from direct and reciprocal crosses suggested that the 2,4-D resistance in KCTR is a nuclear trait. Chi-square analyses of F₂ segregation data implied that 2,4-D resistance in KCTR is controlled by multiple gene(s). Overall, our data suggest that the 2,4-D resistance in KCTR Palmer amaranth is a nuclear inherited trait controlled by multiple genes. Such resistance can spread both via pollen or seed-mediated gene flow. In future, efforts will be directed towards identifying genes mediating 2,4-D resistance in KCTR population.

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.

Inheritance of dicamba-resistance in allotetraploid Chenopodium album

BACKGROUND

Chenopodium album L. is a troublesome weed in spring-planted crops, and different levels of ploidy have been documented for this weed species. A population of C. album has evolved resistance to dicamba. The level of ploidy and inheritance of dicamba resistance was studied in this population.

RESULTS

The resistant and susceptible individuals of C. album were confirmed as tetraploid by flow cytometry. Pair-crosses were made between ten resistant and susceptible individuals. Eight F₁ individuals from five crosses were confirmed resistant after treating with dicamba at 400 g a.e. ha^-1 . These individuals were selfed, and the response of their progenies to dicamba was assessed in dose-response experiments, and the results confirmed the resistance trait was dominant. Furthermore, an analysis of the segregation patterns revealed that the segregation response of all F₂ progenies fitted a 3:1 (resistant/susceptible) ratio when treated with dicamba at 200, 400 and 800 g a.e. ha^-1 , suggesting a single gene was responsible for dicamba resistance.

CONCLUSIONS

Dicamba resistance in the studied tetraploid population of C. album is governed by a single dominant gene. This type of inheritance suggests that selection for dicamba resistance can occur readily. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Acetolactate Synthase-Inhibitor Resistance in Monochoria vaginalis (Burm. f.) C. Presl from Indonesia

Monochoria vaginalis (Burm. f.) C. Presl, belonging to the family Pontederiaceae, is an aquatic herbaceous plant, native to temperate and tropical Asia. The species often occurs in paddy fields as a noxious weed in East Asia, and in the USA, and causes a significant reduction in rice production. The objective of the present research was the evaluation of the resistance levels of M. vaginalis against three chemical groups of acetolactate synthase (ALS)-inhibitor herbicides and other two different groups of herbicides, and the investigation of the mutations in the ALS gene of the resistant biotype of M. vaginalis. Herbicide dose-response experiments showed that the resistant biotype of M. vaginalis was highly resistant to bensulfuron-methyl, moderately resistant to bispyribac-sodium, had low resistance to penoxsulam and 2,4-D dimethyl ammonium, and was susceptible to sulfentrazone. The nucleotide sequences of the ALS gene of resistant and susceptible biotypes showed 14 base substitutions, which caused two amino acid substitutions: Val-143-Ile and Val-148-Ile. It is the first report of the substitutions of amino acids Val-143-Ile and Val-148-Ile in ALS protein. Those mutations may give different resistance spectra against three ALS-inhibitor herbicides: bensulfuron-methyl, bispyribac-sodium, and penoxsulam. Further research is needed to elucidate the molecular basis of target-site resistance mechanisms such as the transformation of the ALS gene of M. vaginalis. It is also necessary to evaluate herbicide mixtures and/or the rotation of herbicide sites of action to control the resistant biotype of M. vaginalis.

Exploring quinclorac resistance mechanisms in Echinochloa crus-pavonis from China

BACKGROUND

Barnyardgrass (Echinochloa spp.) is a global weed in rice fields. Quinclorac is commonly used to control barnyardgrass. However, due to persistent use, quinclorac resistance has evolved. We obtained quinclorac-susceptible (QS) and -resistant (QR1, QR2) lines from the progeny of a single resistant E. crus-pavonis for a resistance mechanism study.

RESULTS

Line QR1 exhibited resistance to high quinclorac rates (up to 6400 g ha^-1 ), whereas line QR2 exhibited a resistance/susceptibility segregation ratio of 3:1 at the field or lower rates (400, 100 g ha^-1 ). Intriguingly, a lower level of ¹⁴ C-quinclorac metabolism and hence a higher level of ¹⁴ C-quinclorac translocation was observed in QR1 than QS plants. The basal expression levels of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) and ACC oxidase 2 (ACO2) genes did not differ significantly between the QR1 and QS lines. However, more expression of ACS and ACO genes was induced by quinclorac treatment in QS than in QR1. Basal levels of β-cyanoalanine synthase (β-CAS) gene expression were similar in QS and QR1 plants, but a greater level of down-regulation was detected in QS than in QR1 plants after quinclorac treatment.

CONCLUSION

These results indicate QR plants are less responsive to quinclorac than QS plants in terms of up-regulating quinclorac metabolism and ethylene synthesis. Resistance in this E. crus-pavonis line is likely controlled by a single major gene, involving possibly an alteration in auxin signal perception/transduction to the ethylene biosynthesis pathway. The β-CAS is unlikely to play a major role in quinclorac resistance in this particular population.

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.

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.

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.

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.

Synthesis and Structure-Property Relationships in Herbicidal Ionic Liquids and their Double Salts

In this study, two homologous series of novel herbicidal ionic liquids (HILs) were synthesized in a simple metathesis reaction between alkyl[2-(2-hydroxyethoxy)ethyl]dimethylammonium bromides and alkali metal salts of 4-chloro-2-methylphenoxyacetic acid (MCPA) or 3,6-dichloro-2-methoxybenzoic acid (dicamba), known as popular herbicides from the class of growth regulators. These HILs were subsequently mixed to prepare double-salt herbicidal ionic liquids (DSHILs). The DSHILs were characterized by substantially altered parameters of viscosity, refractive index, glass transition temperatures and surface activity compared to the average values expected for ideal mixtures of their individual components (HILs). Interestingly, DSHILs possessed superior physicochemical properties such as relatively low viscosity or facilitated formation of micelles, which emphasizes the complex nature of multi-ion interactions in the microstructures of ionic liquid mixtures. The biological tests showed improved efficiency of DSHILs against tested weeds compared to the reference herbicides and parent HILs.

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.

Inheritance of 2,4-D resistance traits in multiple herbicide- resistant Raphanus raphanistrum populations

A relatively low number of weed species have evolved resistance to auxinic herbicides despite their use for almost 70 years. This inheritance study with two Raphanus raphanistrum populations multiple-resistant 2,4-D and the ALS-inhibiting herbicide chlorsulfuron determined the number of genes and genetic dominance of 2,4-D resistance and investigated the association between traits conferring resistance to the two herbicide modes of action. Levels of 2,4-D phenotypic resistance and resistance segregation patterns were assessed in parental populations, F₁ and F₂ families.

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.

Unravelling the resistance mechanisms to 2,4-D (2,4-dichlorophenoxyacetic acid) in corn poppy (Papaver rhoeas)

In southern Europe, the intensive use of 2,4-D (2,4-dichlorophenoxyacetic acid) and tribenuron-methyl in cereal crop systems has resulted in the evolution of resistant (R) corn poppy (Papaver rhoeas L.) biotypes. Experiments were conducted to elucidate (1) the resistance response to these two herbicides, (2) the cross-resistant pattern to other synthetic auxins and (3) the physiological basis of the auxin resistance in two R (F-R213 and D-R703) populations. R plants were resistant to both 2,4-D and tribenuron-methyl (F-R213) or just to 2,4-D (D-R703) and both R populations were also resistant to dicamba and aminopyralid. Results from absorption and translocation experiment revealed that R plants translocated less [14C]-2,4-D than S plants at all evaluation times. There was between four and eight-fold greater ethylene production in S plants treated with 2,4-D, than in R plants. Overall, these results suggest that reduced 2,4-D translocation is the resistance mechanism in synthetic auxins R corn poppy populations and this likely leads to less ethylene production and greater survival in R plants.

Differences in Germination, Growth, and Fecundity Characteristics of Dicamba-Fluroxypyr-Resistant and Susceptible Kochia scoparia

The widespread occurrence of herbicide-resistant (HR) Kochia scoparia is an increasing concern for growers in the US Great Plains and Canada. K. scoparia populations resistant to dicamba have been reported in six US states. Populations cross-resistant to dicamba and fluroxypyr have been reported from wheat fields in Montana, USA. It is unclear whether resistance to the auxinic herbicides (dicamba and/or fluroxypyr), can alter the fitness traits of K. scoparia. The objectives of this research were to compare the germination dynamics in response to thermal environment, vegetative growth and fecundity characteristics, and the relative competitive ability of dicamba-fluroxypyr–susceptible (S) vs.–resistant (R) K. scoparia selected from within a single segregating population (collected from wheat-fallow field in MT). S and R selected lines were developed after three generations of recurrent group selection. Compared to the S selected line, the R selected line had lower cumulative germination at all constant temperatures except 25°C, and at all alternating temperatures except 30/35°C. Also, the R selected line had delayed germination relative to the S selected line. The R had lower plant height, plant width, primary branches, total leaf area, stem diameter, and shoot dry weight compared with the S plants in the absence of competition. The reduction in seed production per plant resulted in a 39% fitness cost. The 1000-seed weight of R (1.6 g) was also less than that of S (2.6 g). When grown in an intraspecific competition at different mixture proportions, replacement series indices for the growth parameters further indicated that the R was less competitive than the S. Evident from this research, the dicamba-fluroxypyr–resistant R selected line is less likely to persist in a field population in the absence of the auxinic herbicides.

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

Resistance to auxinic herbicides is increasing in a range of dicotyledonous weed species, but in most cases the biochemical mechanism of resistance is unknown. Using (14)C-labelled herbicide, the mechanism of resistance to 2,4-dichlorophenoxyacetic acid (2,4-D) in two wild radish (Raphanus raphanistrum L.) populations was identified as an inability to translocate 2,4-D out of the treated leaf. Although 2,4-D was metabolized in wild radish, and in a different manner to the well-characterized crop species wheat and bean, there was no difference in metabolism between the susceptible and resistant populations. Reduced translocation of 2,4-D in the latter was also not due to sequestration of the herbicide, or to reduced uptake by the leaf epidermis or mesophyll cells. Application of auxin efflux or ABCB transporter inhibitors to 2,4-D-susceptible plants caused a mimicking of the reduced-translocation resistance phenotype, suggesting that 2,4-D resistance in the populations under investigation could be due to an alteration in the activity of a plasma membrane ABCB-type auxin transporter responsible for facilitating long-distance transport of 2,4-D.

Inheritance of resistance to 2,4-D and chlorsulfuron in a multiple-resistant population of Sisymbrium orientale

BACKGROUND

A population of Sisymbrium orientale from South Australia has multiple resistance to auxinic herbicides and inhibitors of acetohydroxyacid synthase (AHAS). Inheritance of resistance to 2,4-D and chlorsulfuron was studied in this population.

RESULTS

Crosses were made between seven resistant individuals as pollen donors to seven susceptible individuals. Sixteen F1 individuals from three crosses were identified by their lack of strong epinasty when treated with 200 g 2,4-D ha(-1). These individuals were selfed, and segregation analysis of strong epinasty in the resulting progeny fitted a 3:1 ratio for resistant:susceptible individuals when treated with 200 g 2,4-D ha(-1), as predicted by a single major gene. A detailed dose-response analysis of the F2 populations to 2,4-D confirmed single-gene inheritance. Analysis of segregation to 1 g chlorsulfuron ha(-1), a concentration that kills all susceptible individuals, was unable to determine the mode of inheritance. A detailed dose-response analysis indicated that two genes contributed to chlorsulfuron resistance: a dominant target-site mutation of Pro 197 to Ser and a second gene with dose-dependent dominance.

CONCLUSIONS

This population has a single dominant allele conferring 2,4-D resistance, whereas two genes contribute to chlorsulfuron resistance. Single dominant gene inheritance demonstrates that 2,4-D resistance can be readily selected.

Investigation of MCPA (4-Chloro-2-ethylphenoxyacetate) resistance in wild radish (Raphanus raphanistrum L.)

The phenoxy herbicides (e.g., 2,4-D and MCPA) are used widely in agriculture for the selective control of broadleaf weeds. In Western Australia, the reliance on phenoxy herbicides has resulted in the widespread evolution of phenoxy resistance in wild radish (Raphanus raphanistrum) populations. In this research the inheritance and mechanism of MCPA resistance in wild radish were determined. Following classical breeding procedures, F1, F2, and backcross progeny were generated. The F1 progeny showed an intermediate response to MCPA, compared to parents, suggesting that MCPA resistance in wild radish is inherited as an incompletely dominant trait. Segregation ratios observed in F2 (3:1; resistant:susceptible) and backcross progeny (1:1; resistant to susceptible) indicated that the MCPA resistance is controlled by a single gene in wild radish. Radiolabeled MCPA studies suggested no difference in MCPA uptake or metabolism between resistant and susceptible wild radish; however, resistant plants rapidly translocated more (14)C-MCPA to roots than susceptible plants, which may have been exuded from the plant. Understanding the genetic basis and mechanism of phenoxy resistance in wild radish will help formulate prudent weed management strategies to reduce the incidence of phenoxy resistance.

Inheritance and physiological basis for 2,4-D resistance in prickly lettuce (Lactuca serriola L.)

Experiments were conducted to determine the inheritance and physiological basis for resistance to the synthetic auxinic herbicide (2,4-dichlorophenoxy)acetic acid (2,4-D) in a prickly lettuce biotype. Inheritance of 2,4-D resistance in prickly lettuce is governed by a single codominant gene. Absorption and translocation were conducted using (14)C-2,4-D applied to 2,4-D-resistant and -susceptible biotypes. At 96 h after treatment (HAT), the resistant biotype absorbed less applied 2,4-D and retained more 2,4-D in the treated portion of the leaf compared to the susceptible biotype. The resistant biotype translocated less applied 2,4-D to leaves above the treated leaf and crown at 96 HAT compared to the susceptible biotype. No difference in the rate of metabolism of 2,4-D was observed between the two biotypes. Resistance to 2,4-D appears to originate from a reduced growth deregulatory and overstimulation response compared to the susceptible biotype, resulting in lower translocation of 2,4-D in the resistant prickly lettuce biotype.

Evolution in action: plants resistant to herbicides

Modern herbicides make major contributions to global food production by easily removing weeds and substituting for destructive soil cultivation. However, persistent herbicide selection of huge weed numbers across vast areas can result in the rapid evolution of herbicide resistance. Herbicides target specific enzymes, and mutations are selected that confer resistance-endowing amino acid substitutions, decreasing herbicide binding. Where herbicides bind within an enzyme catalytic site very few mutations give resistance while conserving enzyme functionality. Where herbicides bind away from a catalytic site many resistance-endowing mutations may evolve. Increasingly, resistance evolves due to mechanisms limiting herbicide reaching target sites. Especially threatening are herbicide-degrading cytochrome P450 enzymes able to detoxify existing, new, and even herbicides yet to be discovered. Global weed species are accumulating resistance mechanisms, displaying multiple resistance across many herbicides and posing a great challenge to herbicide sustainability in world agriculture. Fascinating genetic issues associated with resistance evolution remain to be investigated, especially the possibility of herbicide stress unleashing epigenetic gene expression. Understanding resistance and building sustainable solutions to herbicide resistance evolution are necessary and worthy challenges.