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

Fenoxaprop-P-ethyl, mesosulfuron-ethyl, and isoproturon resistance status in Beckmannia syzigachne from wheat fields across Anhui Province, China

Severe infestations of American sloughgrass (Beckmannia syzigachne (Steud.) Fernald) in wheat fields throughout Anhui Province, China, pose a significant threat to local agricultural production. This study aims to evaluate the susceptibility of 37 B. syzigachne populations collected from diverse wheat fields in Anhui Province to three commonly used herbicides: fenoxaprop-P-ethyl, mesosulfuron-ethyl, and isoproturon. Single-dose testing revealed that out of the 37 populations, 31, 26, and 11 populations had either evolved or were evolving resistance to fenoxaprop-P-ethyl, mesosulfuron-ethyl, and isoproturon, respectively. Among them, 25 populations displayed concurrent resistance to both fenoxaprop-P-ethyl and mesosulfuron-ethyl, while eight exhibited resistance to all three tested herbicides. Whole-plant bioassays confirmed that approximately 84% of the fenoxaprop-P-ethyl-resistant populations manifested high-level resistance (resistance index (RI) ≥10); 62% of the mesosulfuron-ethyl-resistant populations and 82% of the isoproturon-resistant populations exhibited low- to moderate-level resistance (2 ≤ RI <10). Three distinct target-site mutations were identified in 27% of fenoxaprop-P-ethyl-resistant populations, with no known resistance mutations detected in the remaining herbicide-resistant populations. The inhibition of cytochrome P450s (P450s) and/or glutathione S-transferases (GSTs) substantially increased susceptibility in the majority of resistant populations lacking mutations at the herbicide target site. In conclusion, resistance to fenoxaprop-P-ethyl and mesosulfuron-ethyl was widespread in B. syzigachne within Anhui Province's wheat fields, while resistance to isoproturon was rapidly evolving due to its escalating usage. Target-site mutations were present in approximately one-third of fenoxaprop-P-ethyl-resistant populations, and alternative mechanisms involving P450s and/or GSTs could explain the resistance observed in most of the remaining populations.

Target-site and non-target-site based resistance to clodinafop-propargyl in wild oats (Avena fatua L.)

Wild oat (Avena fatua L.) is a common and problematic weed in wheat fields in China. In recent years, farmers found it increasingly difficult to control A. fatua using acetyl-CoA carboxylase (ACCase)-inhibiting herbicides. The purpose of this study was to identify the molecular basis of clodinafop-propargyl resistance in A. fatua. In comparison to the S1496 population, whole dose response studies revealed that the R1623 and R1625 populations were 71.71- and 67.76-fold resistant to clodinafop-propargyl, respectively. The two resistant A. fatua populations displayed high resistance to fenoxaprop-p-ethyl (APP) and low resistance to clethodim (CHD) and pinoxaden (PPZ), but they were still sensitive to the ALS inhibitors mesosulfuron-methyl and pyroxsulam. An Ile-2041-Asn mutation was identified in both resistant individual plants. The copy number and relative expression of the ACCase gene in the resistant population were not significantly different from those in the S1496 population. Under the application of 2160 g ai ha -1 of clodinafop-propargyl, the fresh weight of the R1623 population was reduced to 74.9%; however, pretreatment with the application of the cytochrome P450 inhibitor malathion and the GST inhibitor NBD-Cl reduced the fresh weight to 50.91% and 47.16%, respectively, which proved the presence of metabolic resistance. This is the first report of an Ile-2041-Asn mutation and probable metabolic resistance in A. fatua, resulting in resistance to clodinafop-propargyl.

Investigating resistance levels to cyhalofop-butyl and mechanisms involved in Chinese sprangletop (Leptochloa chinensis L.) from Anhui Province, China

Chinese sprangletop (Leptochloa chinensis (L.) Nees) is a common grass species that severely threatens rice (Oryza sativa L.) cropping systems globally. Cyhalofop-butyl is a highly efficient acetyl-CoA carboxylase (ACCase)-inhibiting herbicide widely used for control of this species in China. However, some L. chinensis populations have gradually evolved resistance to this herbicide in recent years. To better understand the cyhalofop-butyl resistance status of L. chinensis in the major rice planting area of the middle-lower Yangtze River basin, 73 populations collected from the rice fields across Anhui Province were investigated for cyhalofop-butyl susceptibility and potential herbicide resistance-conferring mutations. Single-dose testing indicated that of the 73 populations, 25 had evolved resistance to cyhalofop-butyl and were separately classified as "RRR" and "RR" populations according to their fresh weight reductions, 8 had a high risk of evolving cyhalofop-butyl resistance and were classified as "R?" populations, and 40 were susceptible and classified as "S" populations. Whole-plant dose-response experiments showed that the resistance index (RI) of these R?, RR, and RRR populations to cyhalofop-butyl ranged from 2.47 to 36.94. Target gene sequencing identified seven ACCase resistance mutations (I1781L, W1999C, W2027S, W2027L, W2027C, I2041N, and D2078G), with W1999C and W2027C the two most common detected in about three quarters of all the resistant populations. Seven populations including LASC3, BBHY1, AQQS1, HFFD3, HFFD4, AQWJ1, and HFLJ6 each carrying a specific ACCase mutation were tested for their cross- and multiple-resistance patterns. Compared with a standard susceptible population HFLY1, the seven resistant populations showed distinct cross-resistance. All had low- to high-level cross-resistance to metamifop (RIs ranging from 6.16 to 17.65), fenoxaprop-P-ethyl (RIs ranging from 6.39 to 24.08), and quizalofop-P-ethyl (RIs ranging from 2.20 to 10.25), but responded differently to clodinafop-propargyl and clethodim. Multiple-resistance testing suggested that the seven resistant populations were all susceptible to the 4-hydroxyphenylpyruvate dioxygenase inhibitor tripyrasulfone, the protoporphyrinogen oxidase inhibitor oxyfluorfen, and the auxin mimic herbicide florpyrauxifen. In conclusion, this study has shown that cyhalofop-butyl resistance was prevalent in L. chinensis in Anhui Province, China, and target site mutation was one of the most common resistance mechanisms.

Multiple resistance to ACCase- and ALS-inhibiting herbicides in black-grass (Alopecurus myosuroides Huds.) in China

Two black-grass (Alopecurus myosuroides Huds.) populations (R2105 and R1027) that were suspected to be resistant to clodinafop-propargyl, an acetyl-CoAcarboxylase (ACCase) inhibitor, were found in winter wheat fields in China. Research was carried out to investigate whether resistance to clodinafop-propargyl was present and the molecular mechanism of herbicide resistance in these two populations. Dose-response assays confirmed high level resistance to clodinafop-propargyl in both R2105 and R1027 populations, with resistance indexes 25.1 and 22.1. ACCase gene sequence comparison revealed three amino acid mutations (Trp-1999-Leu, Ile-2041-Asn, or Asp-2078-Gly) in R2105 population and Ile-2041-Asn mutation in R1027 population. Sensitivity to other herbicides assays indicated that R2105 and R1027 populations were cross resistant to fenoxaprop-P-ethyl and multiple resistant to pyroxsulam and mesosulfuron-methyl. The ALS gene sequence analysis revealed that all resistant individuals in R2105 and R1027 populations had the Trp-574-Leu mutation. Applying malathion, significantly decreased the rate of metabolism of clodinafop-propargyl in both R2105 and R1027 populations. This is the first report of multiple resistance to ACCase- and ALS-inhibiting herbicides conferred by target-site mutations and enhanced metabolism in black-grass in China.

Impact of a Novel W2027L Mutation and Non-Target Site Resistance on Acetyl-CoA Carboxylase-Inhibiting Herbicides in a French Lolium multiflorum Population

Herbicides that inhibit acetyl-CoA carboxylase (ACCase) are among the few remaining options for the post-emergence control of Lolium species in small grain cereal crops. Here, we determined the mechanism of resistance to ACCase herbicides in a Lolium multiflorum population (HGR) from France. A combined biological and molecular approach detected a novel W2027L ACCase mutation that affects aryloxyphenoxypropionate (FOP) but not cyclohexanedione (DIM) or phenylpyraxoline (DEN) subclasses of ACCase herbicides. Both the wild-type tryptophan and mutant leucine 2027-ACCase alleles could be positively detected in a single DNA-based-derived polymorphic amplified cleaved sequence (dPACS) assay that contained the targeted PCR product and a cocktail of two discriminating restriction enzymes. Additionally, we identified three well-characterised I1781L, I2041T, and D2078G ACCase target site resistance mutations as well as non-target site resistance in HGR. The non-target site component endowed high levels of resistance to FOP herbicides whilst partially impacting on the efficacy of pinoxaden and cycloxydim. This study adequately assessed the contribution of the W2027L mutation and non-target site mechanism in conferring resistance to ACCase herbicides in HGR. It also highlights the versatility and robustness of the dPACS method to simultaneously identify different resistance-causing alleles at a single ACCase codon.

Point Mutations and Cytochrome P450 Can Contribute to Resistance to ACCase-Inhibiting Herbicides in Three Phalaris Species

Species of Phalaris have historically been controlled by acetyl-coenzyme A carboxylase (ACCase)-inhibiting herbicides; however, overreliance on herbicides with this mechanism of action has resulted in the selection of resistant biotypes. The resistance to ACCase-inhibiting herbicides was characterized in Phalaris brachystachys, Phalaris minor, and Phalaris paradoxa samples collected from winter wheat fields in northern Iran. Three resistant (R) biotypes, one of each Phalaris species, presented high cross-resistance levels to diclofop-methyl, cycloxydim, and pinoxaden, which belong to the chemical families of aryloxyphenoxypropionates (FOPs), cyclohexanediones (DIMs), and phenylpyrazolines (DENs), respectively. The metabolism of ¹⁴C-diclofop-methyl contributed to the resistance of the P. brachystachys R biotype, while no evidence of herbicide metabolism was found in P. minor or P. paradoxa. ACCase in vitro assays showed that the target sites were very sensitive to FOP, DIM, and DEN herbicides in the S biotypes of the three species, while the R Phalaris spp. biotypes presented different levels of resistance to these herbicides. ACCase gene sequencing confirmed that cross-resistance in Phalaris species was conferred by specific point mutations. Resistance in the P. brachystachys R biotype was due to target site and non-target-site resistance mechanisms, while in P. minor and P. paradoxa, only an altered target site was found.

The complete chloroplast genome sequence of Beckmannia syzigachne

The first complete chloroplast genome (cpDNA) sequence of Beckmannia syzigachne was determined from Illumina HiSeq pair-end sequencing data in this study. The cpDNA is 136,181 bp in length, contains a large single-copy region (LSC) of 80,345 bp and a small single-copy region (SSC) of 12,810 bp, which were separated by a pair of inverted repeats (IR) regions of 21,513 bp. The genome contains 132 genes, including 85 protein-coding genes, 8 ribosomal RNA genes, and 39 transfer RNA genes. Further phylogenomic analysis showed that B. syzigachne clustered in a unique clade in the Pooideae subfamily.

Pro-197-Ser Mutation in ALS and High-Level GST Activities: Multiple Resistance to ALS and ACCase Inhibitors in Beckmannia syzigachne

American sloughgrass (Beckmannia syzigachne Steud.) is one of the most troublesome weeds infesting wheat and canola fields in China. Some biotypes cannot be controlled, either by acetolactate synthase (ALS) or acetyl coenzyme A carboxylase (ACCase) inhibitors, which are the main herbicides for controlling this weed. However, very few studies have investigated multiple resistance mechanism in B. syzigachne. In this study, a B. syzigachne biotype with a high resistance to ALS inhibitors we have reported was also showed relatively lower resistance to ACCase inhibitors, with a resistance index around 7. RNA-seq analysis was used to investigate the factors responsible for multiple resistance, and 60,108 unigenes were assembled by de novo transcriptome assembly and then annotated across eight databases. A Pro-197-Ser mutation was identified in the ALS gene by SNPs analysis and validated by PCR, while no mutation was identified in the ACCase gene. Nineteen candidate metabolic genes were screened and their overexpression was confirmed by qPCR. The expression of GST-T3 and GST-U6 in resistant plants ranged from 7.5- to 109.4-folds than that in susceptible ones at different times after two kinds of herbicide treatment. In addition, GST activities in resistant plants were 3.0-5.0 times higher than that in susceptible plants. Other novel resistance factors also showed high correlation with multiple resistance which included four genes encoding disease resistance proteins, a transcription factor (MYC3), and one gene conferring blight resistance. In this research, a B. syzigachne biotype was confirmed to have evolved multiple resistance to ACCase and ALS inhibitors. The Pro-197-Ser mutation in ALS gene and high-level GST activities were confirmed responsible for the multiple resistance. Characterized disease-resistance proteins, transcription factor, and blight-resistance proteins may play an essential role in these multiple herbicide resistance.

Identification and expression of main genes involved in non-target site resistance mechanisms to fenoxaprop-p-ethyl in Beckmannia syzigachne

BACKGROUND

Non-target-site resistance (NTSR) to herbicides is a serious threat to global agriculture. Although metabolic resistance is the dominant mechanism of NTSR, the molecular mechanisms are not yet well-characterized. This study aimed to uncover the likely metabolism-related genes in Beckmannia syzigachne (American sloughgrass) resistant to fenoxaprop-p-ethyl.

RESULTS

Ultra-performance liquid chromatography - tandem mass spectrometry experiments showed that the resistant American sloughgrass biotype (R, SD-04-SS) showed enhanced degradation of this herbicide compared to the susceptible biotype (S, SD-12). R and S biotype were harvested at 24 h after fenoxaprop-p-ethyl treatment to conduct RNA sequencing (RNA-Seq) analysis to investigate the likely fenoxaprop-p-ethyl metabolic genes. The RNA-Seq libraries yield 417 969 980 clean reads. The de novo assembly generated 115 112 unigenes, of which 57 906 unigenes were annotated. Finally, we identified 273 cytochrome P450s, 178 oxidases, 47 glutathione S-transferases (GSTs), 166 glucosyltransferases (GTs) and 180 ABC transporter genes to determine the likely fenoxaprop-p-ethyl metabolism-related genes in R biotype. Twelve overlapping up-regulated genes in the R biotype (fenoxaprop-p-ethyl-treated R/non-treated R, fenoxaprop-p-ethyl-treated R/fenoxaprop-p-ethyl-treated S) were identified by RNA-Seq and the results were validated using qRT-PCR. Ten were identified as fenoxaprop-p-ethyl metabolism-related genes, including three P450s (homologous to CYP71D7, CYP99A2 and CYP71D10), one GST (homologous to GSTF1), two GTs (homologous to UGT90A1 and UGT83A1) and four oxidase genes.

CONCLUSION

This work demonstrates that the NTSR mechanism by means of enhanced detoxification of fenoxaprop-p-ethyl in American sloughgrass is very likely driven by herbicide metabolism related genes. The RNA-Seq data presented here provide a valuable resource for understanding the molecular mechanism of NTSR in American sloughgrass. © 2020 Society of Chemical Industry.

Cross-resistance mechanisms to ACCase-inhibiting herbicides in short-spike canarygrass (Phalaris brachystachys)

Herbicides that inhibit acetyl-coenzyme A carboxylase (ACCase) are commonly used to control weedy grasses such as short-spike canarygrass (Phalaris brachystachys). Two resistant biotypes of P. brachystachys (R1 and R2) were found in different winter wheat fields in Iran. This study was done to confirm the suspected resistance observed in the field and to elucidate the resistance mechanisms involved. The results indicated that the both resistant biotypes showed cross-resistance to diclofop-methyl (DM), pinoxaden (PN) and cycloxydim (CD) herbicides. Based on the herbicide dose that inhibited 50% of the ACCase activity (I₅₀), the ACCase activity of the resistant biotypes was less sensitive than the S biotype to DM, CD, and PN. No differences in translocation were detected between biotypes; most of the herbicide remained in the treated leaves. The ¹⁴C-DM metabolites were identified using thin-layer chromatography. Pre-treatment with the cytochrome P450 inhibitor ABT inhibited ¹⁴C-DM metabolism in the R1 biotype, indicating that metabolism is involved in the DM resistance in the R1 biotype. DNA sequencing studies found an Ile-1781-Thr change in both resistant biotypes, conferring cross-resistance to ACCase inhibitors. In general, in the R1 biotype which showed a higher level of resistance than that of the R2 biotype, cross-resistance was observed because of mutation and DM metabolism, while in the R2 biotype, the mutation confers resistance to ACCase-inhibiting herbicides. This is the first reported evidence of the mechanisms responsible for the resistance to ACCase herbicides in P. brachystachys. These results could be useful for improved management of resistant biotypes carrying similar mutations.

Cyhalofop-butyl and Glyphosate Multiple-Herbicide Resistance Evolved in an Eleusine indica Population Collected in Chinese Direct-Seeding Rice

Eleusine indica is a typical xerophytic weed species with a cosmopolitan distribution. It is invasive and highly adaptable to diverse habitats and crops. Due to rice cropping-pattern changes, E indica has become one of the main dominant grass weeds infecting direct-seeding paddy fields. A Chinese E. indica population has evolved multiple-herbicide resistance to cyhalofop-butyl and glyphosate. In this study, the multiple-resistance profile of E. indica to these two different types of herbicides and their resistance mechanisms were investigated. Whole-plant dose-response assays indicated that the multiple-herbicide-resistant (MHR) population exhibited 10.8-fold resistance to cyhalofop-butyl and 3.1-fold resistance to glyphosate compared with the susceptible (S) population. ACCase sequencing revealed that the Asp-2078-Gly mutation was strongly associated with E. indica resistance to cyhalofop-butyl. The MHR plants accumulated less shikimic acid than S plants at 4, 6, and 8 days after glyphosate treatment. In addition, no amino acid substitution in the EPSPS gene was found in MHR plants. Further analysis revealed that the relative expression level of EPSPS in MHR plants was 6-10-fold higher than that in S plants following glyphosate treatment, indicating that EPSPS overexpression may contribute to the glyphosate resistance. Furthermore, the effectiveness of nine post-emergence herbicides against E. indica were evaluated, and one PPO inhibitor pyraclonil was identified as highly effective in controlling the S and MHR E. indica populations.

Cross-resistance pattern to ACCase-inhibiting herbicides in a novel Trp1999Leu mutation American sloughgrass (Beckmannia syzigachne) population

The plastid acetyl coenzyme carboxylase (ACCase) Trp₁₉₉₉Leu mutation was identified in a Beckmannia syzigachne population resistant to fenoxaprop-p-ethyl. The pattern of cross-resistance for the Trp₁₉₉₉Leu mutation is still ambiguous. In this paper, mutant homozygote (1999Leu/Leu, RR) and wild type (1999Trp/Trp, SS) B. syzigachne plants with the same genetic background were purified from the JS-26 population using the dCAPS method. The activity of ACCase in RR and SS was determined. Then, the cross-resistance pattern to ACCase inhibiting herbicides of the Trp₁₉₉₉Leu mutation was determined using the whole-plant method. ACCase activity showed that the Trp₁₉₉₉Leu mutation decreased ACCase sensitivity to fenoxaprop-p-ethyl by 2.73-fold. A dose-response experiment indicated that the Trp₁₉₉₉Leu mutation conferred high resistance to quizalofop-p-ethyl (20.29-fold), metamifop (12.22-fold) and pinoxaden (18.60-fold), moderate resistance to fenoxaprop-p-ethyl (8.20-fold) and sethoxydim (6.38-fold), low resistance to cyhalofop-butyl (2.73-fold) and no resistance to clodinafop-propargyl (1.42 fold) and clethodim (1.59-fold). This is the first report of the role of Trp₁₉₉₉Leu in fenoxaprop-p-ethyl resistance and of the patterns of cross-resistance to ACCase-inhibiting herbicides in B. syzigachne.

Molecular basis of resistance to ACCase-inhibiting herbicide cyhalofop-butyl in Chinese sprangletop (Leptochloa chinensis (L.) Nees) from China

Chinese sprangletop (Leptochloa chinensis (L.) Nees) is one of the most troublesome grass weeds in rice in China. Seven suspected cyhalofop-butyl-resistant L. chinensis populations were collected from different rice fields with a history of cyhalofop-butyl use. The level of resistance and resistance mechanisms in seven populations were studied. Dose-response tests indicated that five populations (JS3, JS4, JS6, JS7 and JS8) had evolved high-level resistance (26.9 to 123.0-fold) to cyhalofop-butyl compared with the susceptible (S) population, and other two populations (JS2 and JS5) were still sensitive to the herbicide. Two acetyl-coenzyme A carboxylase (ACCase) genes were cloned from each population, and three different ACCase mutations (Ile-1781-Leu, Trp-1999-Cys, and Trp-2027-Cys) in ACCase2 gene were determined in different resistant (R) populations. In addition, no resistance-conferring mutations was detected in the R population (JS7), and ACCase gene expression was similar between the S and R populations. Thus, non-target-site resistance mechanisms may be involved in the JS7 population. Moreover, the patterns of cross-resistance of JS6 (Ile-1781-Leu), JS4 (Trp-1999-Cys), JS8 (Trp-2027-Cys), and JS7 (unknown resistance mechanisms) populations to other ACCase-inhibiting herbicides were determined. The JS6 and JS8 populations showed resistance to fenoxaprop-P-ethyl, metamifop, clethodim and pinoxaden, the JS4 population was resistant to fenoxaprop-P-ethyl, metamifop and pinoxaden, and the JS7 population had resistance only to fenoxaprop-P-ethyl and metamifop. These results indicated the diversity of the target-site mutations in ACCase gene of L. chinensis, and provide a better understanding of cross-resistance in L. chinensis, which would be helpful for the management of cyhalofop-butyl-resistant L. chinensis.

Fitness costs associated with acetyl-coenzyme A carboxylase mutations endowing herbicide resistance in American sloughgrass (Beckmannia syzigachne Steud.)

Weed resistance to herbicide can be conferred by gene mutations, and some mutations can cause pleiotropic effects in some cases. We investigated the pleiotropic effects associated with five specific ACCase mutations (Ile1781Leu, Trp2027Cys, Ile2041Asn, Asp2078Gly, and Gly2096Ala) on the plant growth, seed production, and resource competitiveness in American sloughgrass.Resistant plants (M/M) homozygous for specific ACCase mutation and susceptible wild-type plants (W/W) were derived from single heterozygous mother plant (M/W) by genotyping. Plant growth assay and neighborhood experiments were performed to quantify variation between M/M plants and W/W plants.The Ile1781Leu mutation resulted in slight increases in plant growth in pure stands and improved resource competitiveness under low-competition conditions in pot experiments, but no clear variation was observed under high competitive pressure or field conditions. During competition with wheat plants under field conditions, American sloughgrass plants containing Ile2041Asn ACCase exhibited a significantly lower (12.5%) aboveground biomass but no distinct differences in seed production or resource competitiveness. No significant detrimental pleiotropic effects associated with Gly2096Ala were detected in American sloughgrass.The Trp2027Cys mutation distinctly reduced seed production, especially under high competitive pressure, but did not significantly alter plant growth. The Asp2078Gly mutation consistently reduced not only plant growth and seed production but also resource competitiveness. Synthesis. The Trp2027Cys and Asp2078Gly mutations led to significant fitness costs, which may reduce the frequency of resistance alleles and reduce the propagation speed of resistant weeds in the absence of ACCase inhibitor herbicides. The Ile1781Leu, Ile2041Asn, and Gly2096Ala mutations displayed no obvious fitness costs or displayed very small fitness penalties, which would likely have no effect on the establishment of resistant weeds in the field.

Mechanism of resistance to cyhalofop-butyl in Chinese sprangletop (Leptochloa chinensis (L.) Nees)

Chinese sprangletop (Leptochloa chinensis (L.) Nees) is a serious grass weed in rice paddies. In some areas, L. chinensis has become resistant to the herbicide cyhalofop-butyl because of its frequent and extensive use over the past five years. In this study, whole-plant dose-response assays were conducted, and a L. chinensis population (ZHYH) had a 75.8-fold resistance index to cyhalofop-butyl. Molecular analyses revealed that this resistance was attributed to a tryptophan (Trp)-2027-to-cysteine (Cys) substitution in the CT domain of the ACCase gene. To our knowledge, this is the first report revealing the mechanism underlying cyhalofop-butyl resistance in L. chinensis. Furthermore, a derived cleaved amplified polymorphic (dCAPS) assay was developed to rapidly detect the Trp-2027-Cys mutation. Of the 100 ZHYH plants analyzed, 52 were heterozygous mutants and 48 were susceptible homozygous plants. In addition, the cyhalofop-butyl-resistant L. chinensis was cross-resistant to aryloxyphenoxypropionate and phenylpyrazoline herbicides, but not to cyclohexanedione, acetolactate synthase-inhibiting, protoporphyrinogen oxidase, and urea herbicides, and had only slight resistance to the hormonal herbicide quinclorac.

Underlying Resistance Mechanisms in the Cynosurus echinatus Biotype to Acetyl CoA Carboxylase-Inhibiting Herbicides

Hedgehog dogtail (Cynosurus echinatus) is an annual grass, native to Europe, but also widely distributed in North and South America, South Africa, and Australia. Two hedgehog dogtail biotypes, one diclofop-methyl (DM)-resistant and one DM-susceptible were studied in detail for experimental dose-response resistance mechanisms. Herbicide rates that inhibited shoot growth by 50% (GR50) were determined for DM, being the resistance factor (GR50R/GR50S) of 43.81. When amitrole (Cyt. P450 inhibitor) was applied before treatment with DM, the R biotype growth was significantly inhibited (GR50 of 1019.9 g ai ha(-1)) compared with the GR50 (1484.6 g ai ha(-1)) found for the R biotype without pretreatment with amitrole. However, GR50 values for S biotype do not vary with or without amitrole pretreatment. Dose-response experiments carried out to evaluate cross-resistance, showed resistance to aryloxyphenoxypropionate (APP), cyclohexanedione (CHD) and phenylpyrazoline (PPZ) inhibiting herbicides. Both R and S biotypes had a similar (14)C-DM uptake and translocation. The herbicide was poorly distributed among leaves, the rest of the shoot and roots with unappreciable acropetal and/or basipetal DM translocation at 96 h after treatment (HAT). The metabolism of (14)C-DM, D-acid and D-conjugate metabolites were identified by thin-layer chromatography. The results showed that DM resistance in C. echinatus is likely due to enhanced herbicide metabolism, involving Cyt. P450 as was demonstrated by indirect assays (amitrole pretreatment). The ACCase in vitro assays showed that the target site was very sensitive to APP, CHD and PPZ herbicides in the C. echinatus S biotype, while the R biotype was insensitive to the previously mentioned herbicides. DNA sequencing studies confirmed that C. echinatus cross-resistance to ACCase inhibitors has been conferred by specific ACCase double point mutations Ile-2041-Asn and Cys-2088-Arg.