A novel Phe-206-Leu mutation in acetolactate synthase confers resistance to penoxsulam in barnyardgrass (Echinochloa crus-galli (L.) P. Beauv)

A natural glutathione S-transferase gene GSTU23 confers metabolic resistance to metamifop in Echinochloa crus-galli

Herbicides are essential for farmers to control weed. However, prolonged use of herbicides has caused the development of herbicide resistance in weeds. Here, the resistant Echinochloa crus-galli (RL5) was obtained by continuous treatment with metamifop for five generations in paddy fields. RL5 plants showed a 13.7-fold higher resistance to metamifop compared to susceptible E. crus-galli (SL5) plants. Pre-treatment with GST inhibitor (NBD-Cl) significantly increased the susceptibility of RL5 plants to metamifop. Faster metamifop metabolism and higher GST activity in RL5 plants than in SL5 plants were also confirmed, highlighting the role of GST in metabolic resistance. RNA-Seq analysis identified EcGSTU23 as a candidate gene, and this gene was up-regulated in RL5 and field-resistant E. crus-galli plants. Furthermore, the EcGSTU23 gene was overexpressed in the transgenic EcGSTU23-Maize, and the EcGSTU23-Maize showed resistance to metamifop. In vitro metabolic studies also revealed that the purified EcGSTU23 displayed catalytic activity in glutathione (GSH) conjugation, and metamifop was rapidly metabolized in the co-incubation system containing EcGSTU23 protein. These results provide direct experimental evidence of EcGSTU23's involvement in the metabolic resistance of E. crus-galli to metamifop. Understanding the resistance mechanism can help in devising effective strategies to combat herbicide resistance and breeding of genetically modified herbicide resistant crops.

A novel mutation Trp-2027-Gly in acetyl-CoA carboxylase confers resistance to cyhalofop-butyl in Chinese sprangletop (Leptochloa chinensis)

BACKGROUND

Chinese sprangletop [Leptochloa chinensis (L.) Nees] control is threatened by resistance to acetyl-CoA carboxylase (ACCase)-inhibiting herbicides. In this study, a L. chinensis population, HFLJ18, that survived cyhalofop-butyl [aryloxyphenoxypropionate (APP) herbicide, CyB] treatment was collected from a rice field in Lujiang County, Anhui Province, China. This study aimed to evaluate the susceptibility of HFLJ18 to herbicides with different modes-of-action and investigate the potential mechanisms of resistance to CyB.

RESULTS

The HFLJ18 population exhibited high levels of resistance to CyB (10.92-fold) and showed resistance to the ACCase inhibitors metamifop (4.63-fold) and fenoxaprop-P-ethyl (8.39-fold), but was susceptible to clethodim, pinoxaden, florpyrauxifen-benzyl, oxadiazon and pretilachlor. Target gene sequencing revealed a novel Trp-to-Gly substitution at codon position 2027 of ACCase in the resistant plants. Molecular docking revealed that the spatial structure of ACCase changed significantly following the substitution, as indicated by reduced H-bonds. A newly derived cleaved amplified polymorphic sequence (dCAPS) marker was subsequently developed to detect the Trp-2027-Gly mutation in the ACCase of L. chinensis. Additionally, pretreatment with the cytochrome P450 (P450) inhibitor piperonyl butoxide (PBO) and the glutathione S-transferase (GST) inhibitor 4-chloro-7-nitrobenzoxadiazole (NBD-Cl) did not reverse resistance to CyB, suggesting that nontarget-site resistance mechanisms were not involved in CyB resistance in the HFLJ18 population.

CONCLUSION

Overall, the resistance to CyB in the HFLJ18 population derived from the mutation of ACCase gene, and to the best of our knowledge, this is the first report of the ACCase Trp-2027-Gly mutation conferring resistance to ACCase-inhibiting herbicides in grass species. © 2024 Society of Chemical Industry.

Mechanism of multiple resistance to fenoxaprop-P-ethyl, mesosulfuron-methyl, and isoproturon in Avena fatua L. from China

Avena fatua L. is one of the most damaging and malignant weeds in wheat fields in China. Fenoxaprop-P-ethyl, mesosulfuron-methyl, and isoproturon, which belong to Acetyl-CoA carboxylase- (ACCase), acetolactate synthase- (ALS), and photosystem II- (PS II) inhibitors, respectively, are commonly used in wheat fields and have a long history of use on A. fatua. An A. fatua population (R) resistant to fenoxaprop-P-ethyl, mesosulfuron-methyl, and isoproturon was collected from a wheat field in 2020. This study explored the mechanisms of target site resistance (TSR) and non-target site resistance (NTSR) in the multi-resistant A. fatua. Whole-plant bioassays showed that the R population had evolved high resistance to fenoxaprop-P-ethyl and moderate resistance to mesosulfuron-methyl and isoproturon. However, no mutations were detected in the ACCase, ALS, or psbA genes in the R population. In addition, the ACCase and ALS gene expression levels in the R group were significantly higher than those in the susceptible population (S) after treatment with fenoxaprop-P-ethyl or mesosulfuron-methyl. In vitro ACCase and ALS activity assays showed that ACCase and ALS from the R population were insensitive to fenoxaprop and mesosulfuron-methyl, respectively, with resistance indices 6.12-fold and 17.46-fold higher than those of the S population. Furthermore, pretreatment with P450 inhibitors significantly (P < 0.05) reversed the multi-resistant A. fatua's resistance to fenoxaprop-P-ethyl, mesosulfuron-methyl, and isoproturon. Sethoxydim, flucarbazone‑sodium, chlortoluron, and cypyrafluone were effective in controlling multi-resistance A. fatua. Therefore, the overexpression of ACCase and ALS to synthesize sufficient herbicide-targeting proteins, along with P450-mediated metabolism, conferred resistance to fenoxaprop-P-ethyl, mesosulfuron-methyl, and isoproturon in the R population.

An Asp376Glu substitution and P450s-involved metabolism endow resistance to ALS inhibitors in an Ammannia auriculata Willd. Population

Ammannia auriculata Willd. is a noxious broadleaf weed, commonly infesting rice ecosystems across southern China. A putative resistant A. auriculata population (AHSC-5) was sampled from a rice field of Anhui Province, where bensulfuron-methyl (BM) was unable to control its occurrence. This study aimed to determine the sensitivities of the AHSC-5 population to common-use herbicides, and to investigate the underlying resistance mechanisms. The bioassays showed that the AHSC-5 population was 138.1-fold resistant to BM, compared with the susceptible population (JSGL-1). Pretreatment of malathion reduced the resistance index to 19.5. ALS sequencing revealed an Asp376Glu substitution in the AHSC-5 population, and in vitro ALS activity assays found that 50% activity inhibition (I50) of BM in AHSC-5 was 75.4 times higher than that of JSGL-1. Moreover, the AHSC-5 population displayed cross-resistance to pyrazosulfuron-ethyl (10.6-fold), bispyribac‑sodium (3.6-fold), and imazethapyr (2.2-fold), and was in the process of evolving multiple resistance to synthetic auxin herbicides fluroxypyr (2.3-fold) and florpyrauxifen-benzyl (3.1-fold). This study proved the BM resistance in A. auriculata caused by the Asp376Glu mutation and P450-regulated metabolism. This multi-resistant population can still be controlled by penoxsulam, MCPA, bentazone, and carfentrazone-ethyl, which aids in developing targeted and effective weed management strategies.

Target-site and non-target-site resistance mechanisms confer mesosulfuron-methyl resistance in Alopecurus aequalis

BACKGROUND

Shortawn foxtail (Alopecurus aequalis Sobol.) is a noxious weed in China. The resistance of A. aequalis developed rapidly due to the long-term application of acetolactate synthase (ALS)-inhibiting herbicides. Here, a suspected mesosulfuron-methyl-resistant A. aequalis population, Aa-R, was collected from a wheat field in China.

RESULTS

A dose‒response test showed that the Aa-R population has evolved a high level of resistance to mesosulfuron-methyl, and its growth was suppressed by imazamox, pyroxsulam and bispyribac-sodium. ALS gene sequence analysis revealed that a known resistance-related mutation (Pro-197-Thr) was present in the Aa-R population. Moreover, ALS gene overexpression was detected in the Aa-R population. The mesosulfuron-methyl resistance could be reversed by cytochrome P450 monooxygenase (CYP450) and glutathione S-transferase (GST) inhibitors. In addition, enhanced metabolism of mesosulfuron-methyl was detected in the Aa-R population compared with the susceptible population. NADPH-cytochrome P450 reductase and GST activities were strongly inducible in the Aa-R population. One CYP450 gene, CYP74A2, and one GST gene, GST4, were constitutively upregulated in the Aa-R population. Molecular docking results showed the binding affinity of CYP74A2 and GST4 for the tested ALS-inhibiting herbicides, respectively.

CONCLUSION

This study confirmed that target-site resistance and non-target-site resistance involving CYP450 and GST were the main mechanisms involved in resistance in the mesosulfuron-methyl-resistant A. aequalis population.

Distribution and Mechanism of Japanese Brome (Bromus japonicus) Resistance to ALS-Inhibiting Herbicides in China

Bromus japonicus is a common monocot weed that occurs in major winter wheat fields in the Huang-Huai-Hai region of China. Pyroxsulam is a highly efficient and safe acetolactate synthase (ALS)-inhibiting herbicide that is widely used to control common weeds in wheat fields. However, B. japonicus populations in China have evolved resistance to pyroxsulam by different mutations in the ALS gene. To understand the resistance distribution, target-site resistance mechanisms, and cross-resistance patterns, 208 B. japonicus populations were collected from eight provinces. In the resistant population screening experiment, 59 populations from six provinces showed different resistance levels to pyroxsulam compared with the susceptible population, of which 17 B. japonicus populations with moderate or high levels of resistance to pyroxsulam were mainly from the Hebei (4), Shandong (4) and Shanxi (9) Provinces. Some resistant populations were selected to investigate the target site-resistance mechanism to the ALS-inhibiting herbicide pyroxsulam. Three pairs of primers were designed to amplify the ALS sequence, which was assembled into the complete ALS sequence with a length of 1932 bp. DNA sequencing of ALS revealed that four different ALS mutations (Pro-197-Ser, Pro-197-Thr, Pro-197-Phe and Asp-376-Glu) were found in 17 moderately or highly resistant populations. Subsequently, five resistant populations, QM21-41 with Pro-197-Ser, QM20-8 with Pro-197-Thr and Pro-197-Phe, and QM21-72, QM21-76 and QM21-79 with Asp-376-Glu mutations in ALS genes, were selected to characterize their cross-resistance patterns to ALS inhibitors. The QM21-41, QM20-8, QM21-72, QM21-76 and QM21-79 populations showed broad-spectrum cross-resistance to pyroxsulam, mesosulfuron-methyl and flucarbazone-sodium. This study is the first to report evolving cross-resistance to ALS-inhibiting herbicides due to Pro-197-Phe mutations in B. japonicus.

The Pro-197-Thr mutation in the ALS gene confers novel resistance patterns to ALS-inhibiting herbicides in Bromus japonicus in China

Introduction

Bromus japonicus is one of the most notorious agricultural weeds in China. The long-term use of ALS-inhibiting herbicides has led to rapid evolution of herbicide resistance in B. japonicus. B. japonicus population (BJ-R) surviving mesosulfuron-methyl treatment was collected from wheatland. Here, we aimed to confirm the resistance mechanisms in this putative resistant population.

Methods

The dose-reponse tests were used to test the resistance level of the B. japonicus to ALS-inhibiting herbicides. Pretreatment with P450 and GST inhibitors and GST activity assays were used to determine whether P450 or GST was involved in the resistance of the BJ-R population. Sanger sequencing was used to analyse the ALS mutation of the BJ-R population. RT-qPCR was used to confirm the the expression levels of the ALS gene in mesosulfuron-methyl -resistant (BJ-R) and-susceptible (BJ-S) B. japonicus. An in vitro ALS activity assay was used to determine the ALS activity of the BJ-R and BJ-S populations. Homology modelling and docking were used to determine the binding energy of the BJ-R and BJ-S populations with ALS-inhibiting herbicides.

Results

B. japonicus population (BJ-R) was confirmed to be 454- and 2.7-fold resistant to the SU herbicides mesosulfuron-methyl and nicosulfuron, and 7.3-, 2.3-, 1.1- and 10.8-fold resistant to the IMI herbicide imazamox, the TP herbicide penoxsulam, the PTB herbicide pyribenzoxim and the SCT herbicide flucarbazone-sodium, respectively, compared with its susceptible counterpart (BJ-S). Neither a P450 inhibitor nor a GST inhibitor could reverse the level of resistance to mesosulfuron-methyl in BJ-R. In addition, no significant differences in GST activity were found between the BJ-R and BJ-S. ALS gene sequencing revealed a Pro-197-Thr mutation in BJ-R, and the gene expression had no significant differences between the BJ-R and BJ-S. The ALS activity of BJ-R was 106-fold more tolerant to mesosulfuron-methyl than that of BJ-S. Molecular docking showed that the binding energy of the ALS active site and mesosulfuron-methyl was changed from -6.67 to -4.57 kcal mol-1 due to the mutation at position 197.

Discussion

These results suggested that the Pro-197-Thr mutation was the main reason for the high resistance level of BJ-R to mesosulfuron-methyl. Unlike previous reports of the cross-resistance pattern conferred by this mutation, we firstly documented that the Pro-197-Thr mutation confers broad cross-resistance spectrums to ALS-inhibiting herbicides in B. japonicus.

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

BACKGROUND

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.

RESULTS

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.

CONCLUSION

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.

Genetic basis and origin of resistance to acetolactate synthase inhibitors in Amaranthus palmeri from Spain and Italy

BACKGROUND

Amaranthus palmeri is an aggressive annual weed native to the United States, which has become invasive in some European countries. Populations resistant to acetolactate synthase (ALS) inhibitors have been recorded in Spain and Italy, but the evolutionary origin of the resistance traits remains unknown. Bioassays were conducted to identify cross-resistance to ALS inhibitors and a haplotype-based genetic approach was used to elucidate the origin and distribution of resistance in both countries.

RESULTS

Amaranthus palmeri populations were resistant to thifensulfuron-methyl and imazamox, and the 574-Leu mutant ALS allele was found to be the main cause of resistance among them. In two Spanish populations, 376-Glu and 197-Thr mutant ALS alleles were also found. The haplotype analyses revealed the presence of two and four distinct 574-Leu mutant haplotypes in the Italian and Spanish populations, respectively. None was common to both countries, but some mutant haplotypes were shared between geographically close populations or between populations more than 100 km apart. Wide genetic diversity was found in two very close Spanish populations.

CONCLUSION

ALS-resistant A. palmeri populations were introduced to Italy and Spain from outside Europe. Populations from both countries have different evolutionary histories and originate from independent introduction events. ALS resistance then spread over short and long distances by seed dispersal. The higher number and genetic diversity among mutant haplotypes from the Spanish populations indicated recurrent invasions. The implementation of control tactics to limit seed dispersal and the establishment of A. palmeri is recommended in both countries. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Point mutations including a novel Pro-197-Phe mutation confer cross-resistance to acetolactate synthase (ALS) inhibiting herbicides in Lactuca serriola in Australia

BACKGROUND

Control of prickly lettuce has become increasingly difficult for lentil growers in southern Australia because of widespread resistance to common herbicides, a lack of alternative herbicide options and the prolific production of highly mobile seed. This study aimed to quantify acetolactate synthase (ALS)-inhibiting herbicide resistance in the Mid North (MN) and Yorke Peninsula (YP) of South Australia, characterize the resistance mutations present and investigate population structure and gene flow in this species.

RESULTS

Resistance was identified in all populations tested, with average survival of 92% to chlorsulfuron and 95% to imazamox + imazapyr. Five different amino acid substitutions were identified at proline 197 of the ALS gene. There was no significant difference in the median lethal dose (LD50 ) between plants with these five different substitutions when treated with metsulfuron-methyl; however, the imidazolinone resistance level was higher in plants with a phenylalanine substitution and lower in plants with a serine. Population structure based on 701 single nucleotide polymorphisms and 271 individuals provided evidence for both independent evolution of the same mutation in different populations, as well as frequent short- to medium-distance dispersal accompanied by occasional long-distance dispersal events. The overall inbreeding coefficient (FIS ) was calculated at 0.5174, indicating an intermediate level of outcrossing despite the cross-pollination experiment showing only low outcrossing. In the structure analyses, most individuals from YP were assigned to a single cluster, whereas most individuals from MN were assigned 50% to each of two clusters, indicating some genetic differences between these two regions, but also evidence for dispersal between them.

CONCLUSIONS

Use of imidazolinone herbicides has selected for mutations conferring higher levels of resistance, such as the Pro-197-Phe mutation, and resulted in further spread of resistance in this species. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Target gene mutations endowed cross-resistance to acetolactate synthase-inhibiting herbicides in wild Brassica juncea

Wild Brassica juncea is a troublesome weed that infests wheat fields in China. Two suspected wild B. juncea populations (19-5 and 19-6) resistant to acetolactate synthase (ALS) inhibitors were collected from wheat fields in China. To clarify their resistance profiles and resistance mechanism, the resistance levels of populations 19-5 and 19-6 to ALS-inhibiting herbicides and their underlying target-site resistance mechanism were investigated. The results showed that the 19-5 population exhibited resistance to tribenuron-methyl, pyrithiobac‑sodium and florasulam, while the 19-6 population was resistant to tribenuron-methyl, pyrithiobac‑sodium, imazethapyr and florasulam. Using the homologous cloning method, two ALS genes were identified in wild B. juncea, with one gene (ALS1) encoding 652 amino acids and the other (ALS2) encoding 655 amino acids. Pro-197-Arg mutation on ALS2 and Trp-574-Leu mutation on ALS1, together with the combination of these two mutations in a single plant, were observed in both 19-5 and 19-6 populations. ALS2 enzymes carrying the Pro-197-Arg mutation were cross-resistant to tribenuron-methyl, pyrithiobac‑sodium, imazerthapyr and florasulam, with resistance index (RI) values of 6.23, 32.81, 7.97 and 1162.50, respectively. Similarly, ALS1 enzymes with Trp-574-leu substitutions also displayed high resistance to these four herbicides (RI values ranging from 132.61 to 3375.00). In addition, the combination of Pro-197-Arg (ALS2) and Trp-574-Leu (ALS1) mutations increased the resistance level of the ALS enzyme to ALS inhibitors, with its RI values 3.83-214.19, 6.88-37.34, 1.91-31.82 and 2.03-5.90-fold higher than a single mutation for tribenuron-methyl, pyrithiobac‑sodium, imazerthapyr and florasulam, respectively. Collectively, Pro-197-Arg mutation on ALS2, Trp-574-Leu mutation on ALS1 and the combination of Pro-197-Arg (ALS2) and Trp-574-Leu (ALS1) mutations in wild B. juncea could endow broad-spectrum resistance to ALS inhibitors, which might provide guides for establishing effective strategies to prevent or delay such resistance evolution in this weed.

Target-Site and Metabolic Resistance Mechanisms to Penoxsulam in Late Watergrass (Echinochloa phyllopogon) in China

Echinochloa phyllopogon, a malignant weed in Northeast China's paddy fields, is currently presenting escalating resistance concerns. Our study centered on the HJHL-715 E. phyllopogon population, which showed heightened resistance to penoxsulam, through a whole-plant bioassay. Pretreatment with a P450 inhibitor malathion significantly increased penoxsulam sensitivity in resistant plants. In order to determine the resistance mechanism of the resistant population, we purified the resistant population from individual plants and isolated target-site resistance (TSR) and nontarget-site resistance (NTSR) materials. Pro-197-Thr and Trp-574-Leu mutations in acetolactate synthase (ALS) 1 and ALS2 of the resistant population drove reduced sensitivity of penoxsulam to the target-site ALS, the primary resistance mechanisms. To fully understand the NTSR mechanism, NTSR materials were investigated by using RNA-sequencing (RNA-seq) combined with a reference genome. High-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) analysis further supported the enhanced penoxsulam metabolism in NTSR materials. Gene expression data and quantitative reverse transcription polymerase chain reaction (qRT-PCR) validation confirmed 29 overexpressed genes under penoxsulam treatment, with 16 genes concurrently upregulated with quinclorac and metamifop treatment. Overall, our study confirmed coexisting TSR and NTSR mechanisms in E. phyllopogon's resistance to ALS inhibitors.

ALS gene overexpression and enhanced metabolism conferring Digitaria sanguinalis resistance to nicosulfuron in China

Crabgrass (Digitaria sanguinalis) is a common malignant weed in corn fields in China. Recently, the acetolactate synthase (ALS) inhibitor, nicosulfuron, has shown decreasing efficacy against crabgrass. In order to elucidate the molecular basis of resistance to nicosulfuron in crabgrass, we conducted bioassays, combined with gene sequence analysis, relative expression and relative copy number analysis, to characterize resistance in crabgrass populations collected from Beijing, Heilongjiang, Jilin and Anhui provinces. Whole-plant dose-response results indicated that only population collected in Heilongjiang province (HLJ) had developed low level of resistance to nicosulfuron compared with the sensitive population (SD22). No known resistant mutation of ALS gene was found in HLJ population. The real-time fluorescence quantitative PCR results showed that the ALS gene copy number did not differ significantly between the HLJ and SD22 populations. However, the ALS gene expression in the HLJ was 2.07-fold higher than that of the SD22 population at 24 h after treatment with nicosulfuron. Pretreatment with the cytochrome P450 (CYP450) inhibitor malathion, piperonyl butoxide (PBO), and the glutathione S-transferase (GST) inhibitor 4-Chloro-7-nitro-1,2,3-benzoxadiazole (NBD-Cl) all partially reversed HLJ resistance. Among them, the synergistic effect of PBO and nicosulfuron is the most significant. This is the first report of resistance to nicosulfuron in crabgrass through ALS gene overexpression and possible metabolic resistance.

Multiple herbicide resistance in a Cyperus difformis population in rice field from China

Herbicide resistance is rapidly emerging in Cyperus difformis in rice fields across China. The response of a C. difformis population GX-35 was tested against five acetolactate synthase (ALS)-inhibiting herbicides, auxin herbicide MCPA and photosynthesis II (PSII)-inhibitor bentazone. Population GX-35 evolved multiple resistance to ALS-inhibiting herbicides (penoxsulam, bispyribac‑sodium, pyrazosulfuron-ethyl, halosulfuron-methly and imazapic) and auxin herbicide MCPA, with resistance levels of 140-, 1253-, 578-, 18-, 13-, and 21-fold, respectively, compared to the susceptible population. In this population, ALS gene expression was similar to that of the susceptible population. However, an Asp376Glu mutation in ALS gene was observed, leading to reduced inhibition of in-vitro ALS activities by five ALS-inhibiting herbicides. Furthermore, CYP71D8, CYP77A3, CYP78A5 and three ABC transporter genes (cluster-14412.23067, cluster-14412.25321, and cluster-14412.24716) over-expressed in absence of penoxsulam. On the other hand, an UGT73C1 and an ABC transporter (cluster-14412.25038) were induced by penoxsulam. Additionally, both over-expression and induction were observed for CYP74, CYP71A1, UGT88A1 and an ABC transporter (cluster-14412.21723). The GX-35 population has indeed evolved multiple herbicide resistance in China. Therefore, a diverse range of weed control tactics should be implemented in rice field.

Cytochrome P450s-Involved Enhanced Metabolism Contributes to the High Level of Nicosulfuron Resistance in Digitaria sanguinalis from China

Large crabgrass (Digitaria sanguinalis (L.) Scop.) is one of the major malignant grass weeds in Chinese maize (Zea mays L.) fields, and it has recently developed resistance to the acetolactate synthase (ALS)-inhibiting herbicide nicosulfuron. This study focused on a suspected nicosulfuron-resistant (R) population (LJ-01) of D. sanguinalis, collected from Lujiang County in Anhui Province, China, to explore the resistance level and potential resistance mechanism. Whole-plant dose-response testing confirmed that the LJ-01 population evolved a high level of resistance to nicosulfuron (11.5-fold) compared to the susceptible (S) population, DY-02. The ALS gene sequencing and relative expression assay of the R plants indicated that target gene mutation and overexpression were not responsible for the resistance phenotype. However, pretreatment with malathion, a known cytochrome P450 monooxygenase (P450) inhibitor, alleviated the resistance of the R population to nicosulfuron by approximately 36%. High-performance liquid chromatography (HPLC) analysis revealed that the R plants metabolized nicosulfuron faster than the S plants. Moreover, cross-resistance testing suggested that the R population exhibited low levels of resistance to thifensulfuron-methyl and pyrazosulfuron-ethyl, but it remained susceptible to rimsulfuron. Multiple resistance patterns showed that the R population evolved low resistance to the photosystem inhibitors bromoxynil octanoate and atrazine and sensitivity to the acetyl-CoA carboxylase (ACCase) inhibitor cyhalofop-butyl and the 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors tembotrione, mesotrione, and topramezone. This study reports, for the first time, the simultaneous resistance to ALS and different photosystem inhibitors in D. sanguinalis. The nicosulfuron resistance observed in the R population could primarily be attributed to an enhanced metabolism involving P450 enzymes.

Computational molecular explanation of Soybean AHAS resistance from P197S mutation

The first enzyme in the pathway involving branched-chain amino is acetohydroxyacid synthase (AHAS, E.C. 2.2.1.6), which is inhibited by five commercial herbicide families. In this work a computational study of a point mutation of Proline-197-Serine of the Soybean AHAS enzyme, which was obtained by mutagenesis, explains the latter's S197 resistance to the commonly used Chlorsulfuron. Using protein-ligand docking and large-scale sampling and distributions from AlphaFold-generated the resistant and susceptible soybean AHAS protein structure. The computational approach here is scaled to screen for mutation probabilities of protein binding sites, similar to screening compounds for potential hits in therapeutic design using the docking software. P197 and S197 AHAS structures were found to be different even if only one amino acid was changed. The non-specific distribution of bindings in the S197 cavity after the P197S change has been rigorously calculated by RMSD analysis that it would require x20 more concentrations to fill the P197 site by the same amount. There is no previously performed detailed chlorsulfuron soybean P197S AHAS binding calculation. In the herbicide site of AHAS, several amino acids interact - a computational study could elucidate the optimal choice of point mutations for herbicidal resistance either individually or collectively by mutations one at a time and analyzing the effects with a set of herbicides individually. With a computational approach, enzymes involved in crop research and development could be analyzed more quickly, enabling faster discovery and development of herbicides.

Mutation at the 197 site and P450-mediated metabolic resistance are involved in bensulfuron-methyl resistance in Sagittaria trifolia

Sagittaria trifolia control is threatened by the emergence of resistance to acetolactate synthase (ALS)-inhibiting herbicides. Hence, we systematically uncovered the molecular mechanism of resistance to the main herbicide (bensulfuron-methyl) in Liaoning Province from target-site and non-target-site resistance perspectives. The suspected resistant population (TR-1) exhibited high-level resistance. A new amino acid substitution (Pro-197-Ala) in resistant Sagittaria trifolia for ALS was detected, and the molecular docking results showed that the spatial structure of ALS changed significantly after the substitution, manifested by an increase in the number of contacted amino acid residues and the disappearance of hydrogen bonds. Dose-response test of transgenic Arabidopsis thaliana further demonstrated that the Pro-197-Ala substitution conferred bensulfuron-methyl resistance. The assays found that the sensitivity of the ALS enzyme in TR-1 to this herbicide was decreased in vitro; and this population had developed resistance to other types of ALS-inhibiting herbicides. Furthermore, the resistance of TR-1 to bensulfuron-methyl was significantly alleviated after co-treatment with a P450-inhibitor (malathion). TR-1 metabolized bensulfuron-methyl significantly faster than sensitive population (TS-1) did, but this gap was narrowed after malathion treatment. Overall, the resistance of Sagittaria trifolia to bensulfuron-methyl was derived from the mutation of the target-site gene and the enhancement of the P450s-mediated detoxification metabolism.

Population structure of ALS-inhibiting herbicide-resistant Sonchus oleraceus in South Australia

BACKGROUND

Annual sowthistle is a weed that is difficult to control in lentil crops in southern Australia due to a lack of herbicide options, widespread herbicide resistance and prolific production of highly mobile seed. This study investigates herbicide resistance in annual sowthistle in the Mid-North (MN) and Yorke Peninsula (YP) regions of South Australia, identifies and characterizes the mechanisms of acetolactate-synthase (ALS)-inhibitor resistance in this amphidiploid species, and combines this with analyses of population structure and gene flow.

RESULTS

ALS-inhibitor-resistant annual sowthistle is widespread across the YP and MN of South Australia and is associated with a variety of Proline-197 mutations of the ALS gene, including leucine, alanine, arginine, serine, threonine and histidine. These mutations were found in different combinations on either of the two copies of the ALS gene. An additional 200 tissue samples were collected from across a single field on the YP and the ALS gene was sequenced for all these individuals. Different ALS-inhibitor resistance profiles were evident between mutation combinations and within mutation combinations, possibly mediated by differing subgenome assortment of the mutations, or altered gene experession of the two ALS homeologs. Population genetics analysis showed evidence of long-distance dispersal, resulting in highly mobile resistance genes, and multiple instances of resistance mutation evolution.

CONCLUSIONS

Continuing selection of Sonchus oleraceus populations with ALS-inhibiting herbicides has resulted in the accumulation of additional mutations within the ALS gene. New practices to control herbicide-resistant S. oleraceus should be examined, and control should focus on reducing seed set and dispersal to prevent the spread of emerging cases of resistance. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Occurrence and mechanism of target-site resistance to bensulfuron-methyl in Monochoria korsakowii from China

Monochoria korsakowii is an increasingly significant threat to rice production across China, particularly in Liaoning province. Few studies have reported herbicide resistance in M. korsakowii, and resistance status and mechanisms are poorly understood. Here, thirty field populations of M. korsakowii were collected from 11 rice-growing regions of Liaoning, and 97% of populations had evolved resistance to bensulfuron-methyl (BM), with majority (24 of 28) showing high resistance levels (RI > 10). The first in-depth analysis of molecular features of AHAS1 and AHAS2 in BM-resistant populations showed that four Pro197 mutations (Pro197 to His, Ala, Leu or Ser) in AHAS1 and one mutation (Pro197Ser) in AHAS2 were identified. Notably, novel double Pro197Ser mutations co-occurred in both AHAS1 and AHAS2 in the most resistant line LN-20. Furthermore, resistant mutants were used to investigate the effect of Pro197 mutations on AHAS functionality, binding modes, gene expression and cross-resistance in M. korsakowii. All the detected Pro197 mutations considerably reduced in vitro AHAS sensitivity to BM by weakening hydrogen bonds and hydrophobic interactions in the predicted BM-AHAS complexes, especially the double Pro197Ser mutations. This novel resistance mutation combination slightly impacted the extractable AHAS activity, and increased the affinity and catalytic rate of pyruvate. Also, the AHAS expression level was significantly up-regulated. Moreover, all mutations provided resistance only to other sulfonylureas herbicides but not triazolopyrimidine or pyrimidinyl-benzoates herbicides. In conclusion, bensulfuron-methyl resistance in M. korsakowii was grim in Liaoning, China, and amino acid mutations on AHAS isozymes were the primary resistance mechanism. Double Pro197Ser mutations in both AHAS1 and AHAS2 confer higher herbicide resistance than single mutations in AHAS1. Thus, this work deepens our understanding of resistance status and mechanisms of M. korsakowii.

Penoxsulam-resistant barnyardgrass-mediated rhizosphere microbial communities affect the growth of rice

BACKGROUND

The incidence of herbicide-resistant barnyardgrass is escalating in paddy fields, yet the interactions between resistant weeds and rice are largely unknown. The microbiota of herbicide-resistant barnyardgrass rhizosphere soil is critical for both barnyardgrass and rice fitness.

RESULTS

Rice has different biomass allocation and root traits in the presence of penoxsulam-resistant versus penoxsulam-susceptible barnyardgrass or in their conditioned soil. Compared to susceptible barnyardgrass, resistant barnyardgrass led to an allelopathic increase in rice root, shoot, and whole-plant biomasses. Resistant barnyardgrass recruited distinct core and unique microbes in rhizosphere soil compared to susceptible barnyardgrass. In particular, resistant barnyardgrass assembled more Proteobacteria and Ascomycota to enhance plant stress tolerance. Furthermore, the root exudates from resistant and susceptible barnyardgrass were responsible for the assembly and establishment of the root microbial structure. Importantly, (-)-loliolide and jasmonic acid in root exudates were correlated with the core microbes in the rhizosphere soil.

CONCLUSION

The interference of barnyardgrass with rice can be mediated by rhizosphere microbial communities. Biotype-specific variation in the ability to generate soil microbial communities appears to ameliorate the negative consequences for rice growth, providing an intriguing possibility for modulation of the rhizosphere microbiota to increase crop productivity and sustainability. © 2023 Society of Chemical Industry.

Metabolism Difference Is Involved in Mesosulfuron-Methyl Selectivity between Aegilops tauschii and Triticum aestivum

The acetolactate synthase (ALS) inhibitor mesosulfuron-methyl is currently the only selective herbicide to control Aegilops tauschii in wheat fields; however, the mechanism underlying this selectivity remains unclear. Results showed that the tolerance of Triticum aestivum to mesosulfuron-methyl was much higher than that of A. tauschii. Mesosulfuron-methyl inhibited the in vitro ALS activity of A. tauschii and T. aestivum similarly, but the predicted structural interactions of ALS with mesosulfuron-methyl and induced expression of als were different in the two species. Compared with T. aestivum, A. tauschii was found to absorb more mesosulfuron-methyl and metabolize much less mesosulfuron-methyl. The cytochrome P450 monooxygenase (CYP450) inhibitor, malathion, greatly increased the sensitivity of T. aestivum to mesosulfuron-methyl, while its synergistic effect was smaller in A. tauschii. Finally, 19 P450 genes were selected as candidate genes related with metabolism-based mesosulfuron-methyl selectivity. Collectively, different sensitivities to mesosulfuron-methyl in the two species were likely to be attributed to metabolism variances.

An Asp376Glu substitution in ALS gene and enhanced metabolism confers high tribenuron-methyl resistance in Sinapis alba

Acetolactate synthase (ALS) inhibiting herbicides (group 2) have been widely applied for the last 20 years to control Sinapis alba in cereal crops from southern Spain. In 2008, a tribenuron-methyl (TM) resistant (R) S. alba population was first reported in a cereal field in Malaga (southern Spain). In 2018, three suspected R S. alba populations (R1, R2 and R3) to TM were collected from three different fields in Granada (southern Spain, 100 km away from Malaga). The present work aims to confirm the putative resistance of these populations to TM and explore their resistance mechanisms. Dose-response assays showed that the R1, R2 and R3 populations ranging between 57.4, 44.4 and 57.1 times more resistance to TM than the susceptible population (S). A mutation in the ALS gene (Asp376Glu) was detected in the Rs S. alba populations. 14C-metabolism studies show that metabolites and TM were changing significantly faster in the R than in the S plants. Alternative chemical control trials showed that 2,4-D and MCPA (auxin mimics), glyphosate (enolpyruvyl shikimate phosphate synthase,EPSPS, inhibitor-group 9), metribuzin (PSII inhibitors/Serine 264 Binders, -group 5) and mesotrione (hydroxyphenyl pyruvate dioxygenase, HPPD, inhibitor-group 27) presented a high control of the four populations of S. alba tested, both S and R. Based on these results, it is the first case described where the Asp376Glu mutation and P450-mediated metabolism participates in resistance to TM in S. alba. Comparing these results with those found in the S. alba population in Malaga in 2008, where the resistance was TSR type (Pro197Ser), we can suggest that despite the geographical proximity (over 100 km), the resistance in these cases was due to different evolutionary events.

First Report of the Molecular Mechanism of Resistance to Tribenuron-Methyl in Silene conoidea L

Silene conoidea L. is an annual troublesome broadleaf weed in winter wheat fields in China. In recent years, field applications of tribenuron-methyl have been ineffective in controlling S. conoidea in Hebei Province, China. The aim of this study was to determine the molecular basis of tribenuron-methyl resistance in S. conoidea. Whole-plant response assays revealed that the resistant population (R) exhibited a higher level of resistance (382.3-fold) to tribenuron-methyl. The R population also showed high cross-resistance to other acetolactate synthase (ALS) inhibitors, including imazethapyr, bispyribac-sodium and florasulam. However, the R population could be controlled by the field-recommended rates of bentazone, MCPA, fluroxypyr, carfentrazone-ethyl and bromoxynil. In vitro ALS activity assays indicated that the tribenuron-methyl I₅₀ value for the R population was 18.5 times higher than those for the susceptible population (S). ALS gene sequencing revealed an amino acid mutation, Trp-574-Leu, in the R population. Pretreatment with the P450 inhibitor malathion indicated that the R population might have cytochrome P450-mediated metabolic resistance. These results suggest that the Trp-574-Leu mutation and P450-mediated enhanced metabolism coexist in S. conoidea to generate tribenuron-methyl resistance. This is the first time that target-site and non-target-site resistance to tribenuron-methyl has been reported in S. conoidea.

Two mechanisms provide tolerance to cyhalofop-butyl in pond lovegrass [Eragrostis japonica (Thunb.) Trin.]

Pond lovegrass [Eragrostis japonica (Thunb.) Trin.] is an annual grass weed of rice fields worldwide. Cyhalofop-butyl has been widely used for controlling annual grass weeds in rice fields. However, E. japonica is tolerant to cyhalofop-butyl. The effective dose values of cyhalofop-butyl for 29 E. japonica populations causing 50% inhibition of fresh weight (GR₅₀: 130.15 to 187.61 g a.i. ha^-1) were much higher than the recommended dose of cyhalofop-butyl (75 g a.i. ha^-1) in the field. The mechanisms of tolerance to cyhalofop-butyl in E. japonica were identified. In vitro activity assays revealed that the cyhalofop-butyl concentration required to inhibit 50% of the acetyl-coenzyme A carboxylase (ACCase) activity (IC₅₀) was 6.22-fold higher in E. japonica than that in the cyhalofop-butyl-susceptible Chinese sprangletop [Leptochloa chinensis (L.) Nees]. However, mutations in the ACCase gene, previously found to endow target-site resistance in weeds, were not detected in the sequences obtained. Additionally, the expression level of genes encoding ACCase in E. japonica was found to be as similar to L. chinensis. Tolerance was reduced by two cytochrome P450 monooxygenases (Cyt P450s) inhibitors (1-aminobenzotriazole and piperonyl butoxide) and the activity of NADPH-dependent cytochrome P450 reductase in E. japonica was approximately 4.46-fold higher than that of L. chinensis after cyhalofop-butyl treatment. Taken together, it is concluded that two co-existing mechanisms, an insensitive target ACCase and an enhanced metabolism mediated by Cyt P450s, endow tolerance to cyhalofop-butyl in E. japonica.

A novel naturally Phe206Tyr mutation confers tolerance to ALS-inhibiting herbicides in Alopecurus myosuroides

Herbicide-resistant weeds pose a serious threat to world food production. The rapid and widespread development of target-site based resistance limits the application of herbicides. Alopecurus myosuroides Huds. (blackgrass) has spread rapidly in winter wheat regions in China, and the field recommended dose of ALS herbicides no longer controls blackgrass populations in recent years. A highly resistant population TW18(R) was collected in 2018 from Shandong Province. Dose-response assays showed that the TW18 was resistant to mesosulfuron-methyl, flucarbazone-sodium, and imazethapyr, with resistance index values of 5.96, 6.1, and 4.09, respectively. DNA sequencing of the TW18 population revealed a Phe206Tyr (F206Y) mutation in the ALS, which was not yet reported. Blackgrass ALS gene with the F206Y mutation (R gene) was expressed in Arabidopsis and rice. Transgenic studies have shown that both Arabidopsis and rice expressing this R gene have resistance to imazethapyr. However, it did not confer resistance to tribenuron methyl and florasulam in transgenic Arabidopsis. This study showed that the F206Y substitution caused herbicide resistance in blackgrass. To our knowledge, this is the first-reported F206Y mutation of a weed species in the natural environment. Transgenic plants showed this functional site could be utilized to generate imazethapyr-resistant rice to control herbicide-resistant weed damage.

Comparison between the mechanisms of Clearfield ® wheat and Lolium rigidum multiple resistant to acetyl CoA carboxylase and acetolactate synthase inhibitors

Clearfield® wheat (Triticum aestivum) have helped eliminate the toughest grasses and broadleaf weeds in Spain since 2005. This crop production system includes other tolerant cultivars to the application of imidazolinone (IMI) herbicides. However, the continuous use and off-label rates of IMI herbicides can contribute to the development of resistance in Lolium rigidum and other weed species. In this research, the main objectives were to study the resistance mechanisms to acetolactate synthase (ALS) and acetyl coenzyme A carboxylase (ACCase) inhibitors in a L. rigidum accession (LrR) from a Clearfield® wheat field, with a long history rotating these IMI-tolerant crops and compare them with those present in the IMI-tolerant wheat. The resistance to ACCase inhibitors in LrR was due to point mutations (Ile1781Leu plus Asp2078Gly) of the target site gene plus an enhanced herbicide metabolism (EHM), on the other hand, in wheat accessions was due only by EHM. Mechanisms involved in the resistance to ALS inhibitors were both point mutations of the target gene and EHM in the IMI-tolerant wheat, while only evidence of mutation (Trp574Leu) was found in the multiple herbicide resistant L. rigidum accession. This research demonstrates that if crop rotation is not accompanied by the use of alternative sites of action in herbicide-tolerant crops, resistant weeds to herbicide to which crops are tolerant, can easily be selected. Moreover, repeated and inappropriate use of Clearfield® crops and herbicide rotations can lead to the evolution of multiple resistant weeds, as shown in this study, and have also inestimable environmental impacts.

Occurrence of Bensulfuron-Methyl Resistance and Target-Site Resistance Mechanisms in Ammannia auriculata Biotypes from Paddy Fields

Ammanniaauriculata is a troublesome broadleaf weed, widely distributed in the paddy fields of southern China. In this study, 10 biotypes of A. auriculata were sampled from Yangzhou City, China, where the paddy fields were seriously infested with A. auriculata, and their resistance levels to acetolactate synthase (ALS) inhibitor bensulfuron-methyl were determined. The whole-plant response assays showed that nine A. auriculata biotypes were highly resistant (from 16.4- to 183.1-fold) to bensulfuron-methyl in comparison with a susceptible YZ-S biotype, and only one YZ-6 biotype was susceptible. ALS gene sequencing revealed that three ALS gene copies existed in A. auriculata, and four different amino acid substitutions (Pro197-Leu, -Ala, -Ser, and -His) at site 197 in the AaALS1 or 2 genes were found in eight resistant biotypes. In addition, no amino acid mutations in three ALS genes were found in the YZ-3 biotype. These results suggested that target-site mutations or non-target-site resistance mechanisms were involved in tested resistant A. auriculata biotypes. Finally, a cleaved amplified polymorphic sequence (CAPS) marker was identified to rapidly detect the Pro197 mutations in A. auriculata.