Acetolactate Synthase Alleles Research Articles

Prevalence, spatial structure and evolution of resistance to acetolactate-synthase (ALS) inhibitors and 2,4-D in the major weed Papaver rhoeas (L.) assessed using a massive, country-wide sampling.

Corn poppy (Papaver rhoeas) is the most damaging broadleaf weed in France. Massively parallel amplicon sequencing was used to investigate the prevalence, mode of evolution and spread of resistance-endowing ALS alleles in 422 populations randomly sampled throughout poppy's range in France. Bioassays were used to detect resistance to the synthetic auxin 2,4-D in 43 of these populations. A total of 21 100 plants were analysed and 24 mutant ALS alleles carrying an amino-acid substitution involved or potentially involved in resistance were identified. The vast majority (97.6%) of the substitutions occurred at codon Pro197, where all six possible single-nucleotide non-synonymous substitutions plus four double-nucleotide substitutions were identified. Changes observed in the enzymatic properties of the mutant ALS isoforms could not explain the differences in prevalence among the corresponding alleles. Sequence read analysis showed that mutant ALS alleles had multiple, independent evolutionary origins, and could have evolved several times independently within an area of a few kilometres. Finally, 2,4-D resistance was associated with mutant ALS alleles in individual plants in one third of the populations assayed. The intricate geographical mosaic of mutant ALS alleles observed is the likely result of the combination of huge population sizes, multiple independent mutation events and human-mediated spread of resistance. Our work highlights the ability of poppy populations and individual plants to accumulate different ALS alleles and as yet unknown mechanisms conferring resistance to synthetic auxins. This does not bode well for the continued use of chemical herbicides to control poppy. © 2023 Society of Chemical Industry.

Diversity and Spread of Acetolactate Synthase Allelic Variants at Position 574 Endowing Resistance in Amaranthus hybridus in Italy.

Poor control of Amaranthus spp. with herbicides inhibiting acetolactate synthase (ALS) has been observed for several years in soybean fields in north-eastern Italy, but to date only a few ALS-resistant populations have been confirmed. An extensive sampling of putatively resistant Amaranthus accessions was completed in the Friuli Venezia Giulia region, across an arable land area of about 3000 km2. In total, 58 accessions were tested to confirm their resistance status, recognize the Amaranthus species, identify the mutant ALS alleles endowing the resistance and determine the efficacy of 3 pre-emergence herbicides. Most accessions resulted in cross-resistance to thifensulfuron-methyl and imazamox. Genomic DNA were extracted from single seeds with a newly developed protocol; an allele-specific PCR assay revealed the presence of the 574-leucine in 20 accessions, of the 574-methionine in 22, and of both alleles in 9 accessions. The two variants showed a different spatial distribution. All resistant populations were ascribed to A. hybridus. A. hybridus resistant to ALS herbicides is well-established in this Italian region and its resistance is due to two ALS mutant alleles. Metribuzin, clomazone and metobromuron can be used as alternative herbicides to be applied in pre-emergence and they should be integrated into the management strategies to limit the spread of resistance.

Lab meets field: Accelerated selection and field monitoring concur that non-target-site-based resistance evolves first in the dicotyledonous, allergenic weed Ambrosia artemisiifolia

Assessing weed capacity to evolve herbicide resistance before resistance occurs in the field is of major interest for chemical weed control. We used herbicide selection followed by controlled crosses to provoke accelerated evolution of resistance to imazamox (imidazolinones) and tribenuron (sulfonyurea), two acetolactate-synthase (ALS) inhibitors targeting Ambrosia artemisiifolia. In natural populations with no herbicide application records, some plants were initially resistant to metsulfuron (sulfonylurea), a cereal herbicide. Non-target-site-based resistance (NTSR) to metsulfuron was substantially increased from these plants within two generations. NTSR to imazamox and/or tribenuron emerged in metsulfuron-selected G1 progenies and was strongly reinforced in G2 progenies selected by imazamox or tribenuron. NTSR to the herbicides assayed was endowed by partly overlapping and partly specific pathways. Herbicide sensitivity bioassays conducted over 62 ALS-inhibitor-sprayed fields identified emerging resistance to imazamox and/or tribenuron in 14 A. artemisiifolia populations. Only NTSR was detected in 13 of these populations. In the last population, NTSR was present together with a mutant, herbicide-resistant ALS allele bearing an Ala-205-Thr substitution. NTSR was thus by far the predominant type of resistance to ALS inhibitors in France. This confirmed accelerated selection results and demonstrated the relevance of this approach to anticipate resistance evolution in a dicotyledonous weed.

Diversified Resistance Mechanisms in Multi-Resistant Lolium spp. in Three European Countries.

Annual ryegrass species (Lolium spp.) infest cereal crops worldwide. Ryegrass populations with multiple resistance to the acetyl coenzyme A carboxylase (ACCase) and acetolactate synthase (ALS) inhibitors are an increasing problem in several European countries. We investigated the resistance pattern and level of resistance in ryegrass populations collected in Denmark, Greece and Italy and studied the diversity of mechanisms endowing resistance, both target-site and metabolism based. All populations showed high resistance indexes (RI) to the ALS inhibitors, iodosufuron-methyl-sodium + mesosulfuron-methyl (RI from 8 to 70), whereas the responses to the two ACCase inhibitors, clodinafop-propargyl and pinoxaden, differed. The Greek and Italian populations were moderately to highly resistant to clodinafop (RI > 8) and showed low to moderate resistance to pinoxaden (RI ranged from 3 to 13) except for one Italian population. In contrast, the Danish Lolium populations showed low to moderate resistance to clodinafop (RI ranged from 2 to 7) and only one population was resistant to pinoxaden. Different mutant ACCase alleles (Leu1781, Cys2027, Asn2041, Val2041, Gly2078, Arg2088, Ala2096) and ALS alleles (Gly122, Ala197, Gln197, Leu197, Ser197, Thr197, Val205, Asn376, Glu376, Leu574) endowing resistance were detected in the Greek and Italian populations. In several plants, no mutated ALS and ACCase alleles were found showing a great heterogeneity within and among the Greek and Italian populations. Conversely, no mutant ACCase alleles were identified in the four Danish populations and only one mutant ALS allele (Leu574) was detected in two Danish populations. The expression level of nitronate monooxygenase (NMO), glutathione S-transferase (GST) and cytochrome P450s (CYP72A1 and CYP72A2) varied broadly among populations and individual plants within the populations. Constitutive up-regulation of GST, CYP72A1 and CYP72A2 was detected in resistant plants respect to susceptible plants in one Danish and one Italian population. It appears that the mechanisms underlying resistance are rather complex and diversified among Lolium spp. populations from the three countries, coevolution of both target-site resistance and metabolic based herbicide resistance appears to be a common feature in Denmark and Italy. This must be considered and carefully evaluated in adopting resistance management strategies to control Lolium spp. in cereal crops.

Multiple resistance of Papaver rhoeas L. to 2,4‐D and acetolactate synthase inhibitors in four European countries

SummaryThe issue of cross‐ or multiple resistance to acetolactate synthase (ALS) inhibitors and the auxinic herbicide 2,4‐D was investigated in Papaver rhoeas L., a common and troublesome weed in winter cereals, in a broad‐scale study across four European countries. A combination of herbicide sensitivity bioassays and molecular assays targeting mutations involved in resistance was conducted on 27 populations of P. rhoeas originating from Greece (9), Italy (5), France (10) and Spain (3). Plants resistant to the field rate of 2,4‐D were observed in 25 of the 27 populations assayed, in frequencies ranging from 5% to 85%. Plants resistant to ALS‐inhibiting herbicides (sulfonylureas) were present in 24 of the 27 populations, in frequencies ranging from 4% to 100%. Plants resistant to 2,4‐D co‐occurred with plants resistant to sulfonylureas in 23 populations. In four of these, the probability of presence of plants with cross‐ or multiple resistance to 2,4‐D and sulfonylureas was higher than 0.5. ALS genotyping of plants from the field populations or of their progenies, identified ALS alleles carrying a mutation at codon Pro197 or Trp574 in 2,4‐D‐sensitive and in 2,4‐D‐resistant plants. The latter case confirmed multiple resistance to 2,4‐D and ALS inhibitors at the level of individual plants in all four countries investigated. This study is the first to identify individual plants with multiple resistance in P. rhoeas, an attribute rarely assessed in other weed species, but one with significant implications in designing chemical control strategies.

Occurrence of Different Resistance Mechanisms to Acetolactate Synthase Inhibitors in European Sorghum halepense

Four Hungarian and two Italian Sorghum halepense populations harvested in maize fields were investigated to elucidate the levels and mechanisms underlying acetolactate synthase (ALS) inhibitors resistance. The two Italian populations were highly cross-resistant to all ALS inhibitors tested, and the variant ALS allele Leu574 was identified in most of the plants; no differences were observed when the plants were treated with herbicide plus malathion. This suggests that the main resistance mechanism is target-site mediated. The Hungarian populations proved to be controlled by imazamox, while they were resistant to sulfonylureas and bispyribac-Na. All Hungarian populations, but not all plants of population 12-49H, presented the variant allele Glu376. This is the first documented occurrence of the Asp-376-Glu substitution in S. halepense. ALS enzyme bioassay and treatment with malathion confirmed that at least in plants of two populations the resistance is very likely due to both target-site and enhanced metabolism of P450 enzymes.

Genetic basis, evolutionary origin and spread of resistance to herbicides inhibiting acetolactate synthase in common groundsel (Senecio vulgaris).

Following control failure by herbicides inhibiting acetolactate synthase (ALS) in French wheat fields and vineyards, we aimed to confirm resistance evolution and investigate the evolutionary origin and spread of resistance in the tetraploid species Senecio vulgaris (common groundsel), a widespread, highly mobile weed. Sequencing of two ALS homeologues in S. vulgaris enabled the first identification and characterisation of ALS-based resistance in this species. Cross-resistance patterns associated with Leu-197 and Ser-197 ALS1 were established using eight herbicides. Sequencing and genotyping showed that ALS-based resistance evolved by multiple, independent appearances of mutant ALS1 and ALS2 alleles followed by spread. Spread of a mutant ALS1 allele issued from one particular appearance event was observed over 60 km. Independent resistance appearance events and easy seed dispersion are the most likely reasons for populations of S. vulgaris containing different mutant ALS alleles. Accumulation of different alleles probably due to sexual reproduction was observed in the same plant. Mutant ALS alleles and possibly other mechanisms cause resistance to ALS inhibitors in S. vulgaris. Management strategies should aim at limiting S. vulgaris establishment and seed set. Considering the mobility of this species, control coordination at a regional level is clearly necessary if resistance spread is to be contained.

Occurrence, genetic control and evolution of non-target-site based resistance to herbicides inhibiting acetolactate synthase (ALS) in the dicot weed Papaver rhoeas

Non-target-site resistance (NTSR) to herbicides is a major issue for the chemical control of weeds. Whilst predominant in grass weeds, NTSR remains largely uninvestigated in dicot weeds. We investigated the occurrence, inheritance and genetic control of NTSR to acetolactate synthase (ALS) inhibitors in Papaver rhoeas (corn poppy) using progenies from plants with potential NTSR to the imidazolinone herbicide imazamox. NTSR to imazamox was inherited from parents over two successive generations. NTSR to tritosulfuron (a sulfonylurea) was observed in F1 generations and inherited in F2 generations. NTSR to florasulam (a triazolopyrimidine) emerged in F2 generations. Our findings suggest NTSR was polygenic and gradually built-up by accumulation over generations of loci with moderate individual effects in single plants. We also demonstrated that ALS alleles conferring herbicide resistance can co-exist with NTSR loci in P. rhoeas plants. Previous research focussed on TSR in P. rhoeas, which most likely caused underestimation of NTSR significance in this species. This may also apply to other dicot species. From our data, resistance to ALS inhibitors in P. rhoeas appears complex, and involves well-known mutant ALS alleles and a set of unknown NTSR loci that confer resistance to ALS inhibitors from different chemical families.

Gene flow increases the initial frequency of herbicide resistance alleles in unselected Lolium rigidum populations

In two different locations of the Western Australian “wheatbelt”, Lolium rigidum (rigid ryegrass) seeds were collected from organic fields (no herbicide use) and neighbouring conventional fields (persistent herbicide use), the latter infested with herbicide-resistant plants, to investigate the occurrence of gene flow among field populations as revealed by herbicide resistance gene transfer. Herbicides targeting acetyl-CoA carboxylase (ACCase) or acetolactate-synthase (ALS) were used to detect herbicide-resistant plants. Overall, the frequency of plants resistant to ACCase- or ALS-inhibiting herbicides was, respectively, 21% and 74% in the conventional fields and 2% and 37% in neighbouring organic fields. Mutant, herbicide-resistant ACCase and ALS alleles were detected in 16% and 38% of plants from conventional fields and in 0.53% and 3.7% of plants from organic fields. Identical mutant, herbicide-resistant ALS haplotypes were detected both in conventional and organic fields, supporting the occurrence of gene flow between L. rigidum populations in different fields. Gene flow can thus substantially increase the frequency of herbicide-resistant plants in unselected L. rigidum populations. Although gene flow cannot be prevented, it can be limited or managed. Hygiene tactics such as clean crop seed, weed seed removal at harvest and seed destruction post-harvest should be considered in order to minimize gene transfer among farms.

Evolution and diversity of the mechanisms endowing resistance to herbicides inhibiting acetolactate-synthase (ALS) in corn poppy ( Papaver rhoeas L.)

We investigated the diversity of mechanisms conferring resistance to herbicides inhibiting acetolactate synthase (ALS) in corn poppy ( Papaver rhoeas L.) and the processes underlying the selection for resistance. Six mutant ALS alleles, Arg 197, His 197, Leu 197, Ser 197, Thr 197 and Leu 574 were identified in five Italian populations. Different alleles were found in a same population or a same plant. Comparison of individual plant phenotype (herbicide sensitivity) and genotype (amino-acid substitution(s) at codon 197) showed that all mutant ALS alleles conferred dominant resistance to the field rate of the sulfonylurea tribenuron and moderate or no resistance to the field rate of the triazolopyrimidine florasulam. Depending on the allele, dominant or partially dominant resistance to the field rate of the imidazolinone imazamox was observed. Putative non-target-site resistance mechanisms were also likely present in the populations investigated. The derived Cleaved Amplified Polymorphic Sequence assays targeting ALS codons crucial for herbicide sensitivity developed in this work will facilitate the detection of resistance due to mutant ALS alleles. Nucleotide variation around codon 197 indicated that mutant ALS alleles evolved by multiple, independent appearances. Resistance to ALS inhibitors in P. rhoeas clearly evolved by redundant evolution of a set of mutant ALS alleles and likely of non-target-site mechanisms.

Variation in the gene encoding acetolactate‐synthase in Lolium species and proactive detection of mutant, herbicide‐resistant alleles

Summary Lolium species (ryegrasses) are genetically highly variable plants that are both forage crops and major weeds across the globe. As weeds, they rapidly evolve resistance under the selective pressure of acetolactate‐synthase (ALS) inhibitors, the most resistance‐prone herbicide group. Quick and accurate diagnosis is therefore of importance to prevent resistance spread in ryegrass. To develop proactive molecular tools for the detection of mutant, resistant ALS alleles, we assessed variation in the ryegrass ALS gene. Sequencing the full 1929‐bp ALS coding sequence in 59 plants from six distant locations revealed a total of 208 polymorphic nucleotide positions (one every 9.3 nucleotides). The heterogeneous distribution of synonymous and non‐synonymous substitutions along the ALS coding sequence suggested that nucleotide variation of ALS is shaped by purifying and background selection. Using regions of the ALS coding sequence with a low number of polymorphic nucleotide sites, five derived cleaved amplified polymorphic sequence (dCAPS) assays were developed targeting codons crucial for herbicide sensitivity. These enabled the first detection in ryegrass of a Pro‐197‐Thr substitution that confers herbicide resistance. Most assays could also be used to genotype Festuca and Vulpia plants. These dCAPS assays should prove powerful tools for both resistance diagnosis and population genetics studies.

Identification of a Chemically Induced Point Mutation Mediating Herbicide Tolerance in Annual Medics (Medicago spp.)

Sulfonylurea (SU) herbicides are used extensively in cereal-livestock farming zones as effective and cheap herbicides with useful levels of residual activity. These residues can persist beyond the cropping year, severely affecting legumes in general, and annual medics in particular, resulting in reduced dry matter production, lower seed yields and decreased nitrogen fixation. A strand medic cultivar, Medicago littoralis 'Angel', has been developed via chemical mutagenesis with tolerance to SU soil residues. Identifying the molecular basis of the observed tolerance was the aim of this study. Two F(2) populations were generated from crosses between 'Angel' and varieties of intolerant M. truncatula, the male-sterile mutant tap and the cultivar 'Caliph'. Genetic mapping with SSR (single sequence repeat) and gene-based markers allowed identification of the trait-defining gene. Quantitative gene expression studies showed the activity of the respective alleles. Segregation ratios indicated the control of SU-herbicide tolerance by a single dominant gene. SU herbicides inhibit the biosynthesis of the branched-chain amino acids by targeting the acetolactate synthase enzyme, allowing the choice of a mapping approach using acetolactate synthase (ALS) gene homologues as candidates. SSR-marker analysis suggested the ALS-gene homologue on chromosome 3 in M. truncatula. The ALS-gene sequences from 'Angel' and intolerant genotypes were sequenced. In 'Angel', a single point mutation from C to T translating into an amino acid change from proline to leucine was identified. The polymorphism was used to develop a diagnostic marker for the tolerance trait. Expression of the mutant ALS allele was confirmed by quantitative RT-PCR and showed no differences at various seedling stages and treatments to the corresponding wild-type allele. The identification of the trait-defining gene and the development of a diagnostic marker enable efficient introgression of this economically important trait in annual medic improvement programs.

Development and application of allele-specific PCR assays for imazethapyr resistance in rice (Oryza sativa)

Outcrossing or cross hybridization is a potential concern in herbicide-resistant crop management strategies such as in the Clearfield™ rice system. Recent studies have shown that the mutated acetolactate synthase (ALS) gene that confers resistance to imazethapyr (Newpath) herbicide can be transferred from Clearfield rice cultivars via cross pollination under field conditions to weedy red rice. Resistance of commercial Clearfield rice cultivars to imazethapyr is due to the presence of two point mutations in the ALS gene that result in amino acid substitutions from serine to asparagine (S to D) and glycine to glutamic acid (G to E). We report here development of a DNA-based method that involves application of allele-specific PCR assays to distinguish herbicide-susceptible and resistant ALS alleles in either homozygous or heterozygous genotypes produced from natural outcrosses between Clearfield varieties CL121, CL141, CL161 and weedy red rice. PCR assays that can distinguish between the homozygous and heterozygous imazethapyr-resistant S653D and G654E SNP alleles of the rice ALS gene were developed and evaluated. A total of 483 individual red rice plants were successfully screened for the presence of S653D SNP and another 145 F2 individuals from natural red rice × CL121 hybridizations were screened for the presence of the G654E SNP. The PCR-based assays produced during this study are simple, rapid, inexpensive, reproducible and require only standard PCR and electrophoretic instruments that can be applied toward outcrossing evaluation and effective weed management strategies for the Clearfield crop system.

Jointed Goatgrass (Aegilops Cylindrica) by Imidazolinone-Resistant Wheat Hybridization under Field Conditions

Gene flow between jointed goatgrass and winter wheat is a concern because transfer of herbicide-resistance genes from imidazolinone-resistant (IR) winter wheat cultivars to jointed goatgrass could restrict weed-management options for this serious weed of winter wheat cropping systems. The objectives of this study were (1) to investigate the frequency of interspecific hybridization between IR wheat and jointed goatgrass in eastern Colorado, and (2) to determine the gene action of the IR acetolactate synthase (ALS) allele in IR wheat by jointed goatgrass and in IR wheat by imidazolinone-susceptible (IS) wheat backgrounds. Jointed goatgrass was sampled side-by-side with IR wheat and at distances up to 53 m away in both experimental plots and at commercial field study sites in 2003, 2004, and 2005. A greenhouse-screening method was used to identify IR hybrids in collected jointed goatgrass seed. The average percentage of hybridization across sites and years when IR wheat and jointed goatgrass were grown side-by-side was 0.1%, and the maximum was 1.6%. The greatest distance over which hybridization was documented was 16 m. The IR ALS allele contributed 25% of untreated ALS activity in jointed goatgrass by IR wheat F1 plants, as measured by an in vitro ALS assay. The hybridization rate between wheat and jointed goatgrass and the expression of the IR wheat ALS allele in hybrid plants will both influence trait introgression into jointed goatgrass.

Association of the W574L ALS substitution with resistance to cloransulam and imazamox in common ragweed (Ambrosia artemisiifolia)

Previous research revealed that resistance to cloransulam in at least one population of common ragweed was conferred by an altered herbicide target site, specifically, by a tryptophan-to-leucine amino acid substitution at position 574 (W574L) of acetolactate synthase (ALS). In this study, 22 common ragweed populations, several of which were suspected cloransulam resistant, were assayed to determine if the W574L ALS substitution was correlated with resistance to ALS inhibitors. From each population, 16 greenhouse-grown plants were treated with cloransulam, and another 16 were treated with imazamox. Plant dry weights were recorded 20 d after treatment and individual plants were considered resistant if their dry weight exceeded 50% of that of nonherbicide-treated controls. For each herbicide-treated plant, allele-specific primers were used in polymerase chain reactions to determine whether the ALS alleles contained leucine or tryptophan codons at position 574. Of the 352 plants treated with cloransulam, 70 were determined to be resistant, and all but two contained one or more Leu574alleles. The frequency of imazamox resistance was much higher than that of cloransulam in the populations, with 149 of 352 plants identified as imazamox resistant. However, only about half (80) of the imazamox-resistant plants contained one or more Leu574alleles. Correlation of imazamox resistance and Leu574alleles was population dependent. ALS activity assays confirmed that imazamox resistance in plants from at least one population was due to an altered target site, even though plants from that population did not have a W574L substitution. These results lead to the conclusion that a Leu574allele is the predominant basis for cloransulam resistance in common ragweed; however, other mechanisms of resistance to ALS inhibitors exist in some populations.

Promiscuity in weedy amaranths: high frequency of female tall waterhemp (Amaranthus tuberculatus) × smooth pigweed (A. hybridus) hybridization under field conditions

Several populations of different Amaranthus species have been reported resistant to single or multiple herbicides. Interspecific hybridization among amaranths is hypothesized to contribute to the evolution of herbicide resistance. Although other studies have shown the occurrence of interspecific Amaranthus hybrids, little has been done to establish the likelihood of hybridization under field conditions. The main objective of this study was to determine potential field frequencies of hybridization between tall waterhemp females and smooth pigweed. Field hybridization plots were established during each of two growing seasons. Individuals of the two species were transplanted to field plots and arranged at varying distances from each other. Hybrid progeny were detected using the acetolactate synthase (ALS) gene as a marker. Smooth pigweed parents were homozygous for a herbicide-resistance ALS allele, whereas maternal tall waterhemps were homozygous for a herbicide-sensitive ALS form. Heterozygous interspecific progeny were thus detected by means of herbicide selection. Molecular and cytogenetic tools were used to verify the validity of the data obtained. Averaged among female waterhemp plants and across the two field seasons, hybridization occurred at a frequency of 33%. A single tall waterhemp plant was capable of producing more than 200,000 hybrids, suggesting little if any gametic incompatibility. All flowering hybrids obtained from tall waterhemp females were of dioecious condition and female sex. Observed sexual segregation was consistent with previously postulated chromosomal XY-type system in tall waterhemp sex determination, where males are the heterogametic sex.

Transmission of herbicide resistance from a monoecious to a dioecious weedy Amaranthus species.

The genus Amaranthus includes several important monoecious and dioecious weed species, and several populations of these species have developed resistance to herbicides. These species are closely related and two or more species often coexist in agricultural settings. Collectively, these attributes raise the concern that herbicide resistance might transfer from one weedy Amaranthus species to another. We performed research to determine if a dominant allele encoding a herbicide-insensitive form of acetolactate synthase (ALS) could be transferred from a monoecious species, A. hybridus, to a dioecious species, A. rudis. Numerous F(1) hybrids were obtained from controlled crosses in a greenhouse between A. rudis and herbicide-resistant A. hybridus, and most (85%) of these hybrids were herbicide-resistant. Molecular analysis of the ALS gene was used to verify that herbicide-resistant hybrids contained both an A. rudis and an A. hybridus ALS allele. Although hybrids had greatly reduced fertility, 42 BC(1) plants were obtained by backcrossing 33 hybrids with male A. rudis. Fertility was greatly restored in BC(1) progeny, and numerous BC(2) progeny were obtained from a second backcross to A. rudis. The herbicide-resistance allele from A. hybridus was transmitted to 50% of the BC(1) progeny. The resistance allele was subsequently transmitted to and conferred herbicide resistance in 39 of 110 plants analyzed from four BC(2) families. Parental species, hybrids, and BC(2) progeny were compared for 2C nuclear DNA contents. The mean hybrid 2C nuclear DNA content, 1.27 pg, was equal to the average between A. rudis and A. hybridus, which had 2C DNA contents of 1.42 and 1.12 pg, respectively. The mean 2C DNA content of BC(2) plants, 1.40 pg, was significantly (alpha < 0.01) less than that of the recurring A. rudis parent and indicated that BC(2) plants were not polyploid. This report demonstrates that herbicide resistance can be acquired by A. rudis through a hybridization event with A. hybridus.

A common ragweed population resistant to cloransulam-methyl

Abstract A population of common ragweed not controlled by an acetolactate synthase (ALS)-inhibiting herbicide, cloransulam-methyl, was sampled near Dunkirk, IN, the first year of the herbicide's commercialization in 1998. Resistance in the Dunkirk population was confirmed by treating greenhouse-grown seedlings with cloransulam-methyl. ALS activity assays and DNA sequencing were used to identify the resistance mechanism. ALS isolated from plants of the Dunkirk population exhibited an R/S ratio for cloransulam-methyl of >5,000 when compared to ALS activity of populations from Claire City, SD, and V & J Seed Farms. R/S ratios of 4,100 and 110 were observed for two other ALS-inhibiting herbicides, chlorimuron and imazaquin, respectively. DNA sequencing revealed that an inferred leucine for tryptophan substitution at amino acid position 574 in ALS was responsible for the observed herbicide resistance. Additionally, DNA sequencing revealed significant variability among common ragweed ALS alleles. Two fragments ...