Bearded sprangletop is a problematic native grass weed in California’s rice fields. The widespread and extensive use of acetyl-CoA carboxylase (ACCase)–inhibiting herbicides, such as cyhalofop-p-butyl (cyhalofop), has led to speculation that biotypes of bearded sprangletop have developed herbicide resistance to ACCase. The aim of this study was to evaluate suspected resistant bearded sprangletop biotypes, R1, R2, R3, and the susceptible biotype, S1, in terms of their levels of resistance to three ACCase-inhibiting herbicides and to characterize the molecular mechanisms of resistance. Dose–response experiments suggested that the biotype R1, R2, and R3 had high-level resistance to cyhalofop and to quizalofop-p-ethyl (quizalofop), but not clethodim. The study determined that the resistance to ACCase inhibitors was a target-site mechanism resulting from nucleotide substitution. The carboxyl transferase (CT) domain of the ACCase gene’s sequence analysis revealed the substitutions Trp-2027-Cys for R1 and R2 biotypes and Ile-2041-Asn for the R3 biotype. This study revealed the presence of target-site resistance to cyhalofop and quizalofop in at least two mutation points in representative biotypes of bearded sprangletop in California. This research highlights the significance of careful herbicide selection due to weed species responding quite rapidly to selection pressure, so as to aid in managing bearded sprangletop in rice fields.

Common lambsquarters (Chenopodium album L.) is one of the most troublesome weeds in soybean [Glycine max (L.) Merr.] and corn (Zea mays L.) fields in northeast China. In 2017, a C. album population that survived imazethapyr at the recommended field rate was collected from a soybean field in Heilongjiang Province in China. Experiments were conducted to determine the basis of resistance to imazethapyr and investigate the herbicide-resistance pattern in C. album. Dose–response tests showed that the resistant population (R) displayed high resistance to imazethapyr (20-fold) compared with the susceptible population (S). An in vitro acetolactate synthase (ALS) activity assay indicated that the ALS of the R population was resistant to imazethapyr compared with the ALS of the S population. Sequence analysis of the ALS gene revealed that the GCA was replaced by ACA at amino acid position 122, which resulted in an alanine to threonine substitution (Ala-122-Thr) in the R population. The R population displayed cross-resistance to thifensulfuron-methyl and flumetsulam but susceptibility to bispyribac-sodium, flucarbazone, glyphosate, mesotrione, and fomesafen. These results confirmed that the basis of imazethapyr resistance in C. album was conferred by the Ala-122-Thr substitution in the ALS enzyme. This is the first report of the target-site basis of ALS-inhibiting herbicide resistance in C. album.

Resistance to atrazine (a photosystem II [PSII] inhibitor) is prevalent in waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer] across the U.S. Midwest. Previous research suggests that target-site mutation or rapid metabolism of atrazine mediated by glutathione S-transferase (GST) conjugation confers resistance in A. tuberculatus from Illinois. The distribution and mechanism of resistance to atrazine in A. tuberculatus populations from Nebraska (NE) are unknown. In this research we (1) evaluated the response and frequency of resistance in NE A. tuberculatus to soil-applied PSII (metribuzin and atrazine) and protoporphyrinogen oxidase (sulfentrazone) inhibitors, as well as POST-applied atrazine; and (2) determined the mechanism of atrazine resistance in NE A. tuberculatus. The chloroplastic psbA gene, coding for a D1 protein (the target site of atrazine) was sequenced in 85 plants representing 27 populations of A. tuberculatus. Furthermore, 24 plants selected randomly from four atrazine-resistant (AR) populations were used to determine the metabolism of atrazine via GST conjugation. Results from the soil-applied herbicide evaluation suggest that metribuzin (0.56 kg ai ha−1) and sulfentrazone (0.28 kg ai ha−1) were effective on A. tuberculatus management. PRE and POST screenings against atrazine in the greenhouse indicate that atrazine (1.345 kg ai ha−1) was not effective on 39% and 73% of the A. tuberculatus populations evaluated (total of 109 and 85 populations, respectively), suggesting the prevalence of atrazine resistance in A. tuberculatus in NE. Sequence analysis of the psbA gene found no known point mutations conferring atrazine resistance. However, the AR plants conjugated atrazine via GST activity faster than the known atrazine-susceptible A. tuberculatus. Overall, the outcome of this study demonstrates the predominance of metabolism-based resistance to atrazine in A. tuberculatus from NE, which may predispose this species to evolve resistance to other herbicides. The use of integrated management strategies for A. tuberculatus is crucial for the control of this troublesome species.

Resistance to acetolactate synthase (ALS)-inhibitor herbicides due to continuous and repeated selection is widespread in many troublesome weed species, including Palmer amaranth, throughout the United States. The objective of this research was to investigate the physiological and molecular basis of resistance to ALS inhibitors in a chlorsulfuron-resistant Palmer amaranth population (KSR). Our results indicate that the KSR population exhibits a high level of resistance to chlorsulfuron compared with two known susceptible populations, MSS and KSS from Mississippi and Kansas, respectively. MSS is highly susceptible to chlorsulfuron, whereas KSS is moderately sensitive. Dose–response analysis revealed that KSR was more than 275-fold more resistant compared with KSS. Nucleotide sequence analysis of the ALS gene from the plants that survived chlorsulfuron treatment revealed the possibility of evolution of both target site–based and non–target site based resistance to ALS inhibitors in the KSR population. The most common mutation (Pro-197-Ser) in the ALS gene associated with resistance to the sulfonylureas in many weed species was found only in 30% of the KSR population. A preliminary malathion study showed that the remaining 70% of resistant plants might have cytochrome P450–mediated non–target site resistance. This is the first report elucidating the mechanism of resistance to ALS inhibitors in Palmer amaranth from Kansas. Presence of both target site– and non–target site based mechanisms of resistance limits the herbicide options to manage Palmer amaranth in cropping systems.

Methiozolin is an isoxazoline herbicide being investigated for selective POST annual bluegrass control in managed turfgrass. Research was conducted to evaluate methiozolin efficacy for controlling two annual bluegrass phenotypes with target-site resistance to photosystem II (PSII) or enolpyruvylshikimate-3-phosphate synthase (EPSPS)-inhibiting herbicides (i.e., glyphosate), as well as phenotypes with multiple resistance to microtubule and EPSPS or PSII and acetolactate synthase (ALS)-inhibiting herbicides. All resistant phenotypes were established in glasshouse culture along with a known herbicide-susceptible control and treated with methiozolin at 0, 125, 250, 500, 1000, 2000, 4000, or 8000 g ai ha−1. Methiozolin effectively controlled annual bluegrass with target-site resistance to inhibitors of EPSPS, PSII, as well as multiple resistance to EPSPS and microtubule inhibitors. Methiozolin rates required to reduce aboveground biomass of these resistant phenotypes 50% (GR50 values) were not significantly different from the susceptible control, ranging from 159 to 421 g ha−1. A phenotype with target-site resistance to PSII and ALS inhibitors was less sensitive to methiozolin (GR50=862 g ha−1) than a susceptible phenotype (GR50=423 g ha−1). Our findings indicate that methiozolin is an effective option for controlling select annual bluegrass phenotypes with target-site resistance to several herbicides.

Herbicides inhibit biochemical and physiological processes or both with lethal consequences. The target sites of these small molecules are usually enzymes involved in primary metabolic pathways or proteins carrying out essential physiological functions. Herbicides tend to be highly specific for their respective target sites and have served as tools to study these physiological and biochemical processes in plants (Dayan et al. 2010b).

A Powell amaranth population suspected to be resistant (R) to linuron was discovered in a carrot field in Keswick, Ontario, Canada, in 1999. Dose–response analysis with different herbicides and DNA sequencing of the psbA gene encoding the D1 protein of photosystem II were done to confirm the resistance and identify its basis. A calculated resistance factor indicated a 12-fold increased resistance when linuron was applied to an R population compared with a susceptible (S) population. Moreover, the R population showed 6.4- and 3.1-fold greater resistance to two other phenylurea herbicides (diuron and monolinuron), 1.8- and 1.4-fold greater resistance to two triazine herbicides (metribuzin and prometryn), and 2.6-fold greater resistance to the triazinone metribuzin. R population was also cross-resistant to bentazon and bromoxynil when compared with S population, with a calculated resistance factor of 1.4 and 2.2, respectively. The partial nucleotide sequence of the psbA gene of R populations differed at two locations when compared with S populations. The first mutation coded for a Val219Ile substitution in the deduced amino acid sequence of the D1 protein, and the second mutation was silent and encoded for a proline at position 279 in both R and S populations. The Val219Ile substitution in the psbA gene is most likely the cause of this Powell amaranth population resistance to linuron and other PSII inhibitors. This is the first recorded instance of a Val219Ile substitution in an Amaranthus species.

Herbicides targeting grass plastidic acetyl coenzyme A carboxylase (ACC) are effective selective graminicides. Their intensive use worldwide has selected for resistance genes in a number of grass weed species. Biochemistry and molecular biology have been the means of determining the herbicidal activity and selectivity toward crop plants of ACC-inhibiting herbicides. In recent years, elucidation of the tridimensional structure of ACC and identification of five amino acid residues within the ACC carboxyl transferase domain that are critical determinants for herbicide sensitivity shed light on the basis of ACC-based resistance to herbicides. However, metabolism-based resistance to ACC-inhibiting herbicides is much less well known, although this type of resistance seems to be widespread. A number of genes thus endow resistance to ACC-inhibiting herbicides, with the possibility for various resistance genes that confer dominant resistance at the herbicide field rate to accumulate within a single weed population or plant. This, together with a poor knowledge of the genetic parameters driving resistance, renders the evolution of resistance to ACC-inhibiting herbicides unpredictable. Future research should consider developing tactics to slow the spread of resistance. For this purpose, it is crucial that our understanding of metabolism-based resistance improves rapidly because this mechanism is complex and can confer resistance to herbicides with different target sites.

Selection by herbicides has resulted in widespread evolution of herbicide resistance in agricultural weeds. In California, resistance to glyphosate was first confirmed in rigid ryegrass in 1998. Objectives of this study were to determine the current distribution and level of glyphosate resistance in Italian ryegrass, and to assess whether resistance could be due to an altered target site. Seeds were sampled from 118 populations and seedlings were treated with glyphosate at 866 g ae ha−1. Percentage of survivors ranged from 5 to 95% in 54 populations. All plants from 64 populations died. One susceptible (S) population, four putatively resistant (R) populations, and one S accession from Oregon were used for pot dose–response experiments, shikimic acid analyses, and DNA sequencing. Seedlings were treated with glyphosate at eight rates, ranging from 108 to 13,856 g ae ha−1. Shoot biomass was evaluated 3 wk after treatment and fit to a log-logistic regression equation. On the basis of GR50 (herbicide rate required to reduce growth by 50%) values, seedlings from putatively R populations were roughly two to 15 times more resistant to glyphosate than S plants. Shikimic acid accumulation was similar in all plants before glyphosate treatment, but at 4 and 7 DAT, S plants from California and Oregon accumulated approximately two and three times more shikimic acid, respectively, than R plants. Sequencing of a cDNA fragment of the EPSPS coding region revealed two different codons, both of which encode proline at amino acid position 106 in S individuals. In contrast, all R plants sequenced exhibited missense mutations at site 106. Plants from one population revealed a mutation resulting in a proline to serine substitution. Plants from three R populations exhibited a mutation corresponding to replacement of proline with alanine. Our results indicate that glyphosate resistance is widespread in Italian ryegrass populations of California, and that resistance is likely due to an altered target enzyme.

The biochemical and molecular basis of resistance to acetolactate synthase (ALS)–inhibiting herbicides was investigated in eight resistant (R) and three susceptible (S) wild radish populations. In vitro enzyme assays revealed an ALS herbicide–resistant ALS enzyme in all R populations. ALS enzyme extracted from the shoots of all eight R populations was highly resistant to the ALS-inhibiting sulfonylurea herbicide chlorsulfuron (20- to 160-fold) and the triazolopyrimidine herbicide metosulam (10- to 46-fold) and moderately resistant to metsulfuron (three to eightfold). There was little or no cross-resistance to the imidazolinone herbicides imazapyr and imazethapyr. The ALS gene fragment covering potential mutation sites in these populations was amplified, sequenced, and compared. All eight R populations had point mutations in the codon for the proline residue in Domain A. However, the point mutations varied and encoded four different amino acid substitutions: histidine, threonine, alanine, and serine. No nucleotide difference in the DNA sequence of Domains C and D resulting in amino acid substitutions was observed between the R and S populations examined. In addition, a three- to fivefold higher ALS-specific activity was consistently observed in all R populations compared with S populations, whereas Northern blot analysis detected a similar level of ALS mRNA, suggesting a possible translational–posttranslational regulation of the enzyme. It is concluded that selection pressure from chlorsulfuron on eight separate wild radish populations has resulted in target site mutation at the same proline residue in the ALS gene. Higher ALS activity also may play a role in the resistance level.

Multiple-herbicide resistance represents an added weed management challenge to growers as it can considerably reduce their options for weed control. The widespread nature of triazine resistance in Ontario coupled with the more recent appearance of resistance to ALS inhibitors in Amaranthus species warranted documenting biotypes with multiple resistance. A collection of Powell amaranth and redroot pigweed biotypes that had previously been characterized for resistance to ALS inhibitors was therefore screened with atrazine. Dose–response analysis with atrazine and imazethapyr was also conducted. High-level resistance to imazethapyr and atrazine was determined in a Powell amaranth biotype from Perth County, Ontario. This biotype had a > 1,860-fold and 109-fold resistance to atrazine and imazethapyr, respectively. Sequence analysis was conducted for the psbA and ALS genes that code for the target sites of the triazines and imidazolinones, respectively. A mutation in the psbA gene was identified that coded for an amino acid substitution of glycine for serine at residue 264 of the D1 protein. This mutation is the most likely cause for triazine resistance in this biotype. Similarly, a nucleotide substitution was identified that codes for threonine in place of serine at position 652 of the ALS protein. This mutation in the ALS gene has only been observed previously in laboratory-selected mutants of arabidopsis and tobacco and is known to endow resistance to imidazolinones in plants. It is concluded that multiple resistance in this Powell amaranth biotype is due to the presence of altered target sites for triazine and imidazolinone herbicides.

Acetyl coenzyme A carboxylase (ACCase)-inhibiting herbicides affect fatty acid biosynthesis in plants and are widely used to control smooth and hare barley in dicot crops in Australia. Recently, growers have experienced difficulty in controlling smooth and hare barley with herbicides from this mode of action. Dose–response experiments conducted on five suspected resistant populations confirmed varying levels of resistance to quizalofop and haloxyfop. The level of resistance in these populations was greater than 27-fold to quizalofop and greater than 15-fold to haloxyfop. The quizalofop dose required to reduce shoot biomass by 50% (GR50) for the resistant populations varied from 52.6 to 111.9 g ha−1, and for haloxyfop from 26.5 to 71.3 g ha−1. Sequencing the CT domain of the ACCase gene from resistant plants of different populations confirmed the presence of previously known mutations Ile1781Leu and Gly2096Ala. Amino acid substitution at the 2096 position conferred a greater level of resistance to haloxyfop than the substitution at the 1781 position. This study documents the first known case of field-evolved target-site resistance to ACCase-inhibiting herbicides in Australian populations of smooth barley.