Target-Site and Metabolic Resistance Mechanisms to Penoxsulam in Barnyardgrass (Echinochloa crus-galli (L.) P. Beauv)

Epigenetic Regulation of CYP72A385-Mediated Metabolic Resistance to Novel Auxin Herbicide Florpyrauxifen-benzyl in Echinochloa crus-galli (L.) P. Beauv

Weed's metabolic resistance to herbicides has undermined the sustainability of herbicides and global food security. Notably, we identified an Echinochloa crus-galli (L.) P. Beauv population (R) that evolved resistance to the never-used florpyrauxifen-benzyl, in which florpyrauxifen-benzyl was metabolized faster than the susceptible E. crus-galli population (S). RNA-seq identified potential metabolism-related genes, EcCYP72A385 and EcCYP85A1, whose expression in yeast exhibited the capacity to degrade florpyrauxifen-benzyl. Region-2 in the EcCYP72A385 promoter showed significant demethylation after florpyrauxifen-benzyl treatment in the R population. DNA methyltransferase inhibitors induce EcCYP72A385 overexpression in the S population and endow it with tolerance to florpyrauxifen-benzyl. Moreover, methyltransferase-like 7A (EcMETTL7A) was overexpressed in the S population and specifically bound to the EcCYP72A385 promoter. Transgenic EcCYP72A385 in Arabidopsis and Oryza sativa L. exhibited resistance to florpyrauxifen-benzyl, whereas EcMETTL7A transgenic plants were sensitive. Overall, EcCYP72A385 is the principal functional gene for conferring resistance to florpyrauxifen-benzyl and is regulated by EcMETTL7A in E. crus-galli.

Weed biology and management in the multi-omics era: Progress and perspectives

Weeds pose a significant threat to crop production, resulting in substantial yield reduction. In addition, they possess robust weedy traits that enable them to survive in extreme environments and evade human control. In recent years, the application of multi-omics biotechnologies has helped to reveal the molecular mechanisms underlying these weedy traits. In this review, we systematically describe diverse applications of multi-omics platforms for characterizing key aspects of weed biology, including the origins of weed species, weed classification, and the underlying genetic and molecular bases of important weedy traits such as crop-weed interactions, adaptability to different environments, photoperiodic flowering responses, and herbicide resistance. In addition, we discuss limitations to the application of multi-omics techniques in weed science, particularly compared with their extensive use in model plants and crops. In this regard, we provide a forward-looking perspective on the future application of multi-omics technologies to weed science research. These powerful tools hold great promise for comprehensively and efficiently unraveling the intricate molecular genetic mechanisms that underlie weedy traits. The resulting advances will facilitate the development of sustainable and highly effective weed management strategies, promoting greener practices in agriculture.

Role of glutathione S-transferases in the mode of action of herbicides that inhibit amino acid synthesis in Amaranthus palmeri

Acetolactate synthase inhibitors (ALS inhibitors) and glyphosate are two classes of herbicides that act by inhibiting an enzyme in the biosynthetic pathway of branched-chain or aromatic amino acids, respectively. Besides amino acid synthesis inhibition, both herbicides trigger similar physiological effects in plants. The main aim of this study was to evaluate the role of glutathione metabolism, with special emphasis on glutathione S-transferases (GSTs), in the mode of action of glyphosate and ALS inhibitors in Amaranthus palmeri. For that purpose, plants belonging to a glyphosate-sensitive (GLS) and a glyphosate-resistant (GLR) population were treated with different doses of glyphosate, and plants belonging to an ALS-inhibitor sensitive (AIS) and an ALS-inhibitor resistant (AIR) population were treated with different doses of the ALS inhibitor nicosulfuron. Glutathione-related contents, GST activity, and related gene expressions (glutamate-cysteine ligase, glutathione reductase, Phi GST and Tau GST) were analysed in leaves. According to the results of the analytical determinations, there were virtually no basal differences between GLS and GLR plants or between AIS and AIR plants. Glutathione synthesis and turnover did not follow a clear pattern in response to herbicides, but GST activity and gene expression (especially Phi GSTs) increased with both herbicides in treated sensitive plants, possibly related to the rocketing H2O2 accumulation. As GSTs offered the clearest results, these were further investigated with a multiple resistant (MR) population, compressing target-site resistance to both glyphosate and the ALS inhibitor pyrithiobac. As in single-resistant plants, measured parameters in the MR population were unaffected by herbicides, meaning that the increase in GST activity and expression occurs due to herbicide interactions with the target enzymes.

Exploiting N-Centered Umpolung Reactivity of α-Iminomalonates for the Synthesis of N-Sulfenylimines and Sulfonamides

An efficient and interesting N-centered umpolung method has been disclosed to construct beneficial S-N bonds, furnishing N-sulfenylimines, which can readily be converted into the corresponding sulfonamide derivatives in a one-pot sequential operation. N-Sulfenylimines are potent intermediates in organic synthesis, whereas sulfonamides are of major molecular interest due to their rich biological activities and wide applicability in medicinal chemistry. Owing to the simple reaction conditions and setup, this protocol displays a broad and versatile substrate scope, resulting in excellent functional group tolerability toward the synthesis of both N-sulfenylimines and sulfonamides. A density functional theory (DFT) computed and experimentally supported convenient mechanism has been proposed for this unique method.

Overexpression of Acetyl CoA Carboxylase 1 and 3 (ACCase1 and ACCase3), and CYP81A21 were related to cyhalofop resistance in a barnyardgrass accession from Arkansas

Barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] is the most difficult-to-control weed species of rice production systems worldwide. It has evolved resistance to different herbicide sites of action, including the acetyl-CoA carboxylase (ACCase)-inhibiting herbicides. Target-site mutations conferring resistance to ACCase-inhibiting herbicides are well documented; however, the role of the different ACCase genes in conferring resistance to cyhalofop-p-butyl (cyhalofop), an ACCase-inhibiting herbicide, remains poorly understood. This research assessed the contribution of gene amplification and expression of ACCase genes in a cyhalofop-resistant barnyardgrass accession. Additionally, the expression of glutathione-S-transferases (GSTs) and cytochrome P450 monooxygenases (P450s) genes as possible contributors to resistance to cyhalofop were investigated. Results demonstrated that ACCase gene amplification does not contribute to cyhalofop resistance. However, ACCase1 and ACCase3 were found to be overexpressed in the cyhalofop-resistant barnyardgrass accession. At 24 h after cyhalofop treatment, an overexpression of 2.0- and 2.8-fold was detected in ACCase1 and ACCase3, respectively. In addition, CYP81A21 (a P450 gene) was found to be 2.5-fold overexpressed compared to the susceptible accession in the same time period. These results suggest that ACCase1, ACCase3, and CYP81A21 are crucial genes in contributing cyhalofop resistance in this barnyardgrass accession.

A glutathione S-transferase and a cytochrome P450 may confer cyhalofop-butyl resistance in Leptochloa chinensis (L.) Nees

BACKGROUND

Leptochloa chinensis (L.) Nees is a troublesome weed across China in rice fields, and a suspected L. chinensis resistant population (R) that has survived the recommended field dose of cyhalofop-butyl was collected in a rice field of Hunan Province, China. In this study, we aimed to determine the acetyl-CoA carboxylase-inhibiting herbicide resistance profile of this R population and to investigate its mechanisms of resistance to cyhalofop-butyl.

RESULTS

Compared with the susceptible population (S), the R population was confirmed to be 18.9-, 3.2-, 4.1-, 3.6- and 5.8- fold resistant to the APP herbicides cyhalofop-butyl, haloxyfop-P-methyl, clodinafop-propargyl, metamifop and fenoxaprop-P-ethyl, respectively. ACCase gene sequencing analysis revealed no known resistance mutations for TSR in the R population. Pretreatment with the glutathione S-transferase (GST) inhibitor 4-chloro-7-nitrobenzoxadiazole (NBD-Cl) and cytochrome P450 (CYP450) inhibitor malathion reversed resistance to cyhalofop-butyl. The GST gene GSTU1 and CYP450 gene CYP707A5 were constitutively upregulated in the R population according to RNA-seq analysis and RT-qPCR verification. The molecular docking results indicated a good affinity of the active site for five APP herbicides with GSTU1 and CYP707A5.

CONCLUSION

This study shows that the GSTU1 and CYP707A5 genes expressed highly in the R population may be responsible for cyhalofop-butyl resistance in L. chinensis.

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.

Milletdb: a multi-omics database to accelerate the research of functional genomics and molecular breeding of millets

Millets are a class of nutrient-rich coarse cereals with high resistance to abiotic stress; thus, they guarantee food security for people living in areas with extreme climatic conditions and provide stress-related genetic resources for other crops. However, no platform is available to provide a comprehensive and systematic multi-omics analysis for millets, which seriously hinders the mining of stress-related genes and the molecular breeding of millets. Here, a free, web-accessible, user-friendly millets multi-omics database platform (Milletdb, http://milletdb.novogene.com) has been developed. The Milletdb contains six millets and their one related species genomes, graph-based pan-genomics of pearl millet, and stress-related multi-omics data, which enable Milletdb to be the most complete millets multi-omics database available. We stored GWAS (genome-wide association study) results of 20 yield-related trait data obtained under three environmental conditions [field (no stress), early drought and late drought] for 2 years in the database, allowing users to identify stress-related genes that support yield improvement. Milletdb can simplify the functional genomics analysis of millets by providing users with 20 different tools (e.g., 'Gene mapping', 'Co-expression', 'KEGG/GO Enrichment' analysis, etc.). On the Milletdb platform, a gene PMA1G03779.1 was identified through 'GWAS', which has the potential to modulate yield and respond to different environmental stresses. Using the tools provided by Milletdb, we found that the stress-related PLATZs TFs (transcription factors) family expands in 87.5% of millet accessions and contributes to vegetative growth and abiotic stress responses. Milletdb can effectively serve researchers in the mining of key genes, genome editing and molecular breeding of millets.

Investigation of resistance mechanisms to fomesafen in Ipomoea nil from China

Recently, the herbicide fomesafen has frequently failed to control the troublesome weed Ipomoea nil in soybean fields in Liaoning Province, China. Hence, we collected 10 suspected resistant populations and evaluated their sensitivity to fomesafen. The results revealed various degrees of Ipomoea nil resistance to fomesafen, with a resistance index of 2.88 to 22.43; the highest value occurred in the LN3 population. Therefore, the mechanisms of the resistance in LN3 to fomesafen were explored. After fomesafen treatment, the expression levels of InPPX1 and InPPX2 genes were 4.19- and 9.29-fold higher, respectively, in LN3 than those in the susceptible (LN1) population. However, mutations and copy number variations were not detected between the two populations. Additionally, malathion pretreatment reduced the dose necessary to halve the growth rate of LN3 by 58%. Liquid chromatography with tandem mass spectrometry demonstrated that metabolism of fomesafen was significantly suppressed by malathion. Moreover, LN3 displayed increased reactive oxygen species scavenging capacity, which was represented by higher superoxide dismutase and peroxidase activities after fomesafen application than those in LN1. An orthogonal partial least squares-discriminant analysis revealed that the high resistance in LN3 could be attributed mainly to enhanced metabolism. Fortunately, the fomesafen-resistant I. nil remained sensitive to 2,4-D-ethylhexylester and bentazon, providing methods for its control.

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.

Metabolic Exploration for Cyhalofop-Butyl Antagonism in Barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] Following Pretreatment of Malathion

The present study investigated the effects of broad-spectrum metabolic inhibitors malathion (cytochrome P450 inhibitor) and/or 4-chloro-7-nitrobenzofurazan (NBD-Cl; glutathione S-transferase inhibitor) on the metabolism of cyhalofop-butyl (CyB) in barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] biotypes confirmed previously with multiple resistance to two herbicides CyB and florpyrauxifen-benzyl. The metabolic inhibitors were not effective at recovering the sensitivity of resistant barnyardgrass biotypes to CyB treated at the labeled rate (313 g ai ha^-1). Rather, treatment with malathion followed by CyB caused antagonism, reducing the efficacy of CyB and promoting the growth of resistant biotypes. Pretreatment with malathion did not influence absorption/translocation of the applied form CyB and its conversion to the active herbicide form cyhalofop-acid (CyA), in both susceptible and resistant biotypes. In contrast, metabolism of the applied form (CyB) decreased 1.5 to 10.5 times by the malathion pretreatment. Taken together, the maintained CyA production against the reduced CyB metabolism could be the mechanism to account for the cause of CyB antagonism observed in barnyardgrass following malathion pretreatment. Additionally, the evolution of CyB resistance in barnyardgrass might be associated with reduced production of CyA in resistant biotypes, independent of activities of cytochrome P450 or GST enzymes.

Enhanced Herbicide Metabolism and Target-Site Mutations Confer Multiple Resistance to Fomesafen and Nicosulfuron in Amaranthus retroflexus L

Amaranthus retroflexus L. is a highly competitive broadleaf weed of corn-soybean rotation in northeastern China. In recent years, the herbicide(s) resistance evolution has been threatening its effective management in crop fields. One resistant A. retroflexus (HW-01) population that survived the protoporphyrinogen oxidase (PPO) inhibitor fomesafen and acetolactate synthase (ALS) inhibitor nicosulfuron applied at their field-recommended rate was collected from a soybean field in Wudalianchi City, Heilongjiang Province. This study aimed to investigate the resistance mechanisms of fomesafen and nicosulfuron and determine the resistance profile of HW-01 to other herbicides. Whole plant dose-response bioassays revealed that HW-01 had evolved resistance to fomesafen (50.7-fold) and nicosulfuron (5.2-fold). Gene sequencing showed that the HW-01 population has a mutation in PPX2 (Arg-128-Gly) and a rare mutation in ALS (Ala-205-Val, eight/twenty mutations/total plants). In vitro enzyme activity assays showed that ALS extracted from the HW-01 plants was less sensitive to nicosulfuron (3.2-fold) than ST-1 plants. Pre-treatment with the cytochrome P450 inhibitors malathion, piperonyl butoxide (PBO), 3-amino-1,2,4-triazole (amitrole), and the GSTs inhibitor 4-chloro-7-nitrobenzofurazan (NBD-Cl) significantly increased fomesafen and nicosulfuron sensitivity in the HW-01 population compared with that of the sensitive (S) population ST-1. Moreover, the rapid fomesafen and nicosulfuron metabolism in the HW-01 plants was also confirmed via HPLC-MS/MS analysis. Furthermore, the HW-01 population showed multiple resistance (MR) to PPO, ALS, and PSII inhibitors, with resistance index (RI) values ranging from 3.8 to 9.6. This study confirmed MR to PPO-, ALS-, and PSII-inhibiting herbicides in the A. retroflexus population HW-01, as well as confirming that the cytochrome P450- and GST-based herbicide metabolic along with TSR mechanisms contribute to their multiple resistance to fomesafen and nicosulfuron.

Exploratory Analysis on Herbicide Metabolism and Very-Long-Chain Fatty Acid Production in Metolachlor-Resistant Palmer Amaranth (Amaranthus palmeri S. Wats.)

A Palmer amaranth (Amaranthus palmeri S. Wats.) biotype resistant to S-metolachlor was confirmed from crop fields in Arkansas, USA. This study investigated the metabolic effects of malathion (cytochrome P450 inhibitor) and 4-chloro-7-nitrobenzofurazan [NBD-Cl; glutathione S-transferase inhibitor] on the S-metolachlor-resistant A. palmeri biotype. Root elongation of the resistant biotype was 20% more inhibited by treatment of NBD-Cl (50 nM) and S-metolachlor (2 μM) in mixture than by treatment of S-metolachlor alone. Metabolites of S-metolachlor were 1.4-12.1 times greater produced in the resistant biotype for 7 d than in the susceptible standard. Production of cerotic acid, one of the very-long-chain fatty acids containing 26 carbons, was more reduced in the susceptible standard (3.8-fold) than in the resistant biotype (1.8-fold) by S-metolachlor treatment. Conclusively, evolution of S-metolachlor resistance observed in this study was likely associated with improved activity of glutathione S-transferases. Further studies are needed to genetically evaluate plant endogenous enzymes involving cerotic acid production.

Calcium signalling in weeds under herbicide stress: An outlook

The continuous use of herbicides for controlling weeds has led to the evolution of resistance to all major herbicidal modes of action globally. Every year, new cases of herbicide resistance are reported. Resistance is still in progress in many species, which must be stopped before it becomes a worldwide concern. Several herbicides are known to cause stressful conditions that resemble plant abiotic stresses. Variation in intracellular calcium (Ca²⁺) concentration is a primary event in a wide range of biological processes in plants, including adaptation to various biotic and abiotic stresses. Ca²⁺ acts as a secondary messenger, connecting various environmental stimuli to different biological processes, especially during stress rejoindering in plants. Even though many studies involving Ca²⁺ signalling in plants have been published, there have been no studies on the roles of Ca²⁺ signalling in herbicide stress response. Hence, this mini-review will highlight the possible sensing and molecular communication via Ca²⁺ signals in weeds under herbicide stress. It will also discuss some critical points regarding integrating the sensing mechanisms of multiple stress conditions and subsequent molecular communication. These signalling responses must be addressed in the future, enabling researchers to discover new herbicidal targets.

Metamifop resistance in Echinochloa glabrescens via glutathione S-transferases-involved enhanced metabolism

BACKGROUND

Echinochloa glabrescens Munro ex Hook. f. is one of the main Echinochloa spp. seriously invading Chinese rice fields and has evolved resistance to commonly used herbicides. Previously, an E. glabrescens population (LJ-02) with suspected resistance to the acetyl-CoA carboxylase (ACCase)-inhibiting herbicide metamifop was collected. This study aimed to determine its resistance status to metamifop and investigate the internal molecular mechanisms of resistance.

RESULTS

Single-dose testing confirmed that the LJ-02 population had evolved resistance to metamifop. Gene sequencing and a relative expression assay of ACCase ruled out target-site based resistance to metamifop in LJ-02. Whole-plant bioassays revealed that, compared with the susceptible population XZ-01, LJ-02 was highly resistant to metamifop and exhibited cross-resistance to fenoxaprop-P-ethyl. Pretreatment with the known glutathione S-transferase (GST) inhibitor, 4-chloro-7-nitrobenzoxadiazole (NBD-Cl), largely reversed the resistance to metamifop by approximately 81%. Liquid chromatography-tandem mass spectrometry analysis indicated that the metabolic rates of one of the major metabolites of metamifop, N-(2-fluorophenyl)-2-hydroxy-N-methylpropionamide (HPFMA), were up to 383-fold faster in LJ-02 plants than in XZ-01 plants. There were higher basal and metamifop-inducible GST activities toward 1-chloro-2,4-dinitrobenzene (CDNB) in LJ-02 than in XZ-01. Six GST genes were metamifop-induced and overexpressed in the resistant LJ-02 population.

CONCLUSION

This study reports, for the first time, the occurrence of metabolic metamifop resistance in E. glabrescens worldwide. The high-level metamifop resistance in the LJ-02 population may mainly involve specific isoforms of GSTs that endow high catalytic activity and strong substrate specificity. © 2023 Society of Chemical Industry.

The blackgrass genome reveals patterns of non-parallel evolution of polygenic herbicide resistance

Globally, weedy plants are a major constraint to sustainable crop production. Much of the success of weeds rests with their ability to rapidly adapt in the face of human-mediated management of agroecosystems. Alopecurus myosuroides (blackgrass) is a widespread and impactful weed affecting agriculture in Europe. Here we report a chromosome-scale genome assembly of blackgrass and use this reference genome to explore the genomic/genetic basis of non-target site herbicide resistance (NTSR). Based on our analysis of F2 seed families derived from two distinct blackgrass populations with the same NTSR phenotype, we demonstrate that the trait is polygenic and evolves from standing genetic variation. We present evidence that selection for NTSR has signatures of both parallel and non-parallel evolution. There are parallel and non-parallel changes at the transcriptional level of several stress- and defence-responsive gene families. At the genomic level, however, the genetic loci underpinning NTSR are different (non-parallel) between seed families. We speculate that variation in the number, regulation and function of stress- and defence-related gene families enable weedy species to rapidly evolve NTSR via exaptation of genes within large multi-functional gene families. These results provide novel insights into the potential for, and nature of plant adaptation in rapidly changing environments.

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.

Direct Reaction of Nitroarenes and Thiols via Photodriven Oxygen Atom Transfer for Access to Sulfonamides

Sulfonamide is a common motif in medicines and agrochemicals. Typically, this class of functional groups is prepared by reacting amines with sulfonyl chlorides that are presynthesized from nitro compounds and thiols, respectively. Here, we report a novel strategy that directly couples nitro compounds and thiols to form sulfonamides atom- and redox-economically. Mechanistic studies suggest our reaction proceeds via direct photoexcitation of nitroarenes that eventually transfers the oxygen atoms from the nitro group to the thiol unit.

Involvement of miRNAs in Metabolic Herbicide Resistance to Bispyribac-Sodium in Echinochloa crus-galli (L.) P. Beauv

Several mechanisms involved in weed herbicide resistance are unknown, particularly those acting at the epigenetic level, such as the capacity of small-non-coding RNAs (sncRNAs) to target messenger RNAs of genes involved in herbicide detoxification. The transcription of these sncRNAs is stimulated by epigenetic factors, thereby affecting gene expression. This study was carried out in order to evaluate, for the first time in Echinochloa crus-galli (L.) P. Beauv. (barnyardgrass), the capacity of miRNAs to regulate the expression of genes associated with bispyribac-sodium detoxification. The expression profiles of eight miRNAs with a high degree of complementarity (≥80%) with mRNAs of genes involved in herbicide detoxification (CYP450, GST and eIF4B) were determined by qRT-PCR before and after herbicide spraying. Five of the miRNAs studied (gra-miR7487c, gma-miR396f, gra-miR8759, osa-miR395f, ath-miR847) showed an increased expression after herbicide application in both susceptible and resistant biotypes. All the miRNAs, except gra-miR8759, were more highly expressed in the herbicide-resistant biotypes. In specimens with increased expression of miRNAs, we observed reduced expression of the target genes. The remaining three miRNAs (ata-miR166c-5p, ath-miR396b-5p and osa-miR5538) showed no over-expression after herbicide treatment, and no difference in expression was recorded between susceptible and resistant biotypes. Our results represent a first overview of the capacity of miRNAs to regulate the expression of genes involved in bispyribac-sodium detoxification in the genus Echinochloa. Further research is required to identify novel miRNAs and target genes to develop more focused and sustainable strategies of weed control.

Comparative Analysis of Whole Chloroplast Genomes of Three Common Species of Echinochloa (Gramineae) in Paddy Fields

Echinochloa crus-galli var. crus-galli, E. crus-galli var. zelayensis, and E. glabrescens, morphologically similar at the seedling stage, are the most pernicious barnyard grass species in paddy fields worldwide. Chloroplast (cp) genomes could be conducive to their identification. In this study, we assembled the complete cp genome sequences of Echinochloa crus-galli var. crus-galli (139,856 bp), E. crus-galli var. zelayensis (139,874 bp), and E. glabrescens (139,874 bp), which exhibited a typical circular tetramerous structure, large and small single-copy regions, and a pair of inverted repeats. In Echinochloa crus-galli var. crus-galli, there were 136 simple sequence (SSRs) and 62 long (LRs) repeats, and in the other two species, 139 SSRs and 68 LRs. Each cp genome contains 92 protein-encoding genes. In Echinochloa crus-galli var. crus-galli and E. glabrescens, 321 and 1 single-nucleotide polymorphisms were detected compared to Echinochloa crus-galli var. zelayensis. IR expansion and contraction revealed small differences between the three species. The phylogenetic tree based on cp genomes demonstrated the phylogenetic relationship between ten barnyard grass species and other common Gramineae plants, showing new genetic relationships of the genus Echinochloa. This study provides valuable information on cp genomes, useful for identifying and classifying the genus Echinochloa and studying its phylogenetic relationships and evolution.

Up-regulation of bZIP88 transcription factor is involved in resistance to three different herbicides in both Echinochloa crus-galli and E. glabrescens

The resistance of weeds to herbicides poses a major threat to agricultural production, and non-target-site resistance (NTSR) is often a serious problem as its mechanisms can in some cases confer resistance to herbicides with different modes of action. In this study, we hypothesized that bZIP transcription factors (TFs), which regulate abiotic stress responses in many plants, play a regulatory role in NTSR. Whole-plant assays indicated that the wild grasses Echinochloa crus-galli and E. glabrescens are resistant to the herbicides penoxsulam, cyhalofop-butyl, and quintrione. Transcriptome sequencing then identified 101 and 49 bZIP TFs with differential expression following penoxsulam treatment in E. crus-galli and E. glabrescens, respectively. Twelve of these genes had >60% homology with rice genes. The expression of bZIP88 was considerably up-regulated 6 h after treatment with the three different herbicides, and it was similar between resistant and susceptible populations; however, the relative expression levels before herbicide treatment and 24 h after were the same. We used rice (Oryza sativa ssp. japonica cv Nipponbare) as a model system for functional validation and found that CRISPR-Cas9-knockout of the rice bZIP88 ortholog increased the sensitivity to herbicide, whereas overexpression reduced it. The OsbZIP88 protein was localized to the nucleus. Using ChIP coupled with high-throughput sequencing, OsbZIP88 was found to form a network regulatory center with other TFs such as bZIP20/52/59 to regulate OsKS1, OsCOE1, and OsIM1, which are related to auxin, abscisic acid, brassinosteroids, and gibberellic acid. Based on these results, we have established a database of bZIP TFs corresponding to herbicide stress, and resolved the mechanisms of the positive regulation of herbicide resistance by bZIP88, thereby providing new insights for NTSR.

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.

PsbA gene over-expression and enhanced metabolism conferring resistance to atrazine in Commelina communis

Commelina communis L. is a troublesome weed in agronomic fields and increasingly threatens the yield security of corn in north-eastern China. Previously, we found that a C. communis population (JL-1) has evolved resistance to atrazine. Although the potential genetic and enzymic differences contributing to atrazine resistance in this population have been investigated, the specific molecular mechanisms underlying C. communis resistance are still poorly understood. Here, the expression level of the target gene PsbA and the non-target-site resistance (NTSR) mechanism for this population were studied. The results showed that the decline in chlorophyll content in JL-1 leaves was less than in the susceptible JS-10 population following atrazine treatment. JL-1 exhibited an enhanced expression of the PsbA gene compared with JS-10 of 7.28- and 14.28-fold higher at 0 and 24 h after treatment with atrazine, respectively. The cytochrome P450 monooxygenase (P450) inhibitor piperonyl butoxide (PBO) increased the phytotoxicity of atrazine in both populations of C. communis. Seven candidate genes associated with NTSR of Jl-1 were identified through RNA-seq and validated by quantitative real-time PCR, including 5 upregulated genes involved in herbicide metabolism. In addition, the activities of glutathione S-transferases and P450s in JL-1 were increased compared with JS-10. Collectively, PsbA gene overexpression and enhanced metabolism are likely to be responsible for JL-1 resistance to atrazine.

Multiple resistance mechanisms to penoxsulam in Echinochloa crus-galli from China

Penoxsulam is an important herbicide for the control of Echinochloa crus-galli (L.) P. Beauv. Two resistant populations 17GA (R1) and 16NXB (R2) showed 17- and 3-fold resistance to penoxsulam, respectively. A known resistance mutation of Trp-574-Leu in ALS gene and enhanced rates of penoxsulam metabolism likely involving GST contribute to penoxsulam resistance in R1 population. This population had resistance to the ALS-inhibitors pyribenzoxim and bispyribac‑sodium and the auxin herbicide quinclorac, but was susceptible to ACCase-inhibitors quizalofop-p-ethyl and cyhalofop-butyl. No known mutations in the ALS gene conferring target site resistance to ALS-inhibiting herbicides were presented in R2 population. However, penoxsulam metabolism in R2 plants was about 4-fold greater than in susceptible population 14YC (S0) plants. The enzyme inhibitors piperonyl butoxide, malathion and 4-chloro-7-nitrobenzoxadiazole reversed penoxsulam resistance in this population. GST and P450 enzyme activities and the genes of GST1-1, GST1-2, GST1-3, CYP81A18, CYP81A12, CYP81A21 were increased significantly in R2 population. These results indicate that multiple resistance mechanisms had occurred in E. crus-galli populations in central China and resistance needs to be managed effectively by diverse chemical and non-chemical methods.

Investigating the Metabolic Mesosulfuron-Methyl Resistance in Aegilops tauschii Coss. By Transcriptome Sequencing Combined with the Reference Genome

Aegilops tauschii Coss. is a malignant weed in wheat fields in China, its herbicide resistance has been threatening crop production. This study identified one mesosulfuron-methyl-resistant(R) population, JJMHN2018-05 (R), without target resistance mutations. To fully understand the resistance mechanism, non-target site resistance was investigated by using transcriptome sequencing combined with a reference genome. Results showed that the cytochrome P450 monooxygenase (P450) inhibitor malathion significantly increased the mesosulfuron-methyl sensitivity in R plants, and greater herbicide-induced glutathione S-transferase (GST) activity was also confirmed. Liquid chromatography with tandem mass spectrometry analysis further supported the enhanced mesosulfuron-methyl metabolism in R plants. Gene expression data analysis and qRT-PCR validation indicated that eight P450s, six GSTs, two glycosyltransferases (GTs), four peroxidases, and one aldo-keto reductase (AKRs) stably upregulated in R plants. This research demonstrates that the P450s and GSTs involved in enhanced mesosulfuron-methyl metabolism contribute to mesosulfuron-methyl resistance in A. tauschii and identifies potential contributors from metabolic enzyme families.

Crystal structure of Arabidopsis thaliana HPPK/DHPS, a bifunctional enzyme and target of the herbicide asulam

Herbicides are vital for modern agriculture, but their utility is threatened by genetic or metabolic resistance in weeds, as well as regulatory barriers. Of the known herbicide modes of action, 7,8-dihydropterin synthase (DHPS), which is involved in folate biosynthesis, is targeted by just one commercial herbicide, asulam. A mimic of the substrate para-aminobenzoic acid, asulam is chemically similar to sulfonamide antibiotics, and although it is still in widespread use, asulam has faced regulatory scrutiny. With an entire mode of action represented by just one commercial agrochemical, we sought to improve the understanding of its plant target. Here we solve a 2.3 Å resolution crystal structure for Arabidopsis thaliana DHPS that is conjoined to 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase (HPPK), and we reveal a strong structural conservation with bacterial counterparts at the sulfonamide-binding pocket of DHPS. We demonstrate that asulam and the antibiotic sulfamethoxazole have herbicidal as well as antibacterial activity, and we explore the structural basis of their potency by modeling these compounds in mitochondrial HPPK/DHPS. Our findings suggest limited opportunity for the rational design of plant selectivity from asulam and indicate that pharmacokinetic or delivery differences between plants and microbes might be the best ways to safeguard this mode of action.

Cross-resistance of barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] to aryloxyphenoxypropionate herbicides

Managing emerged weeds that have evolved resistance to acetyl CoA carboxylase (ACCase)-inhibiting herbicides is a challenging task. A dose-response experiment was conducted on barnyardgrass biotypes resistant (R) and susceptible (S) to three aryloxyphenoxypropionate herbicides cyhalofop-butyl (CyB), fenoxaprop-ethyl (FeE), and quizalofop-ethyl (QuE) along with investigations into the potential resistance mechanism of these biotypes. The tested R barnyardgrass biotypes had strong resistance to CyB and FeE (resistant/susceptible ratio: 7.9-14.4) but weak resistance to QuE (resistant/susceptible ratio: 2.4-3.1). Absorption, translocation, and total metabolism of CyB and QuE were not associated with differences among S and R barnyardgrass biotypes. However, differences between S and R barnyardgrass were observed in production of active acid forms of each herbicide (cyhalofop-acid and quizalofop-acid). Production of cyhalofop-acid was >1.6-fold less in R barnyardgrass (3-8%) for 24 h after herbicide application than in the S barnyardgrass (8-16%). Meanwhile, production of quizalofop-acid was less in R barnyardgrass (< 14%) throughout the study period than in the S barnyardgrass (< 22%). Sequencing results of ACCase gene showed no difference between S and R barnyardgrass. Overall results show that a non-target-site resistance mechanism altering metabolism of CyB and QuE likely contributes to resistance of the barnyardgrass biotypes to these herbicides.

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

BACKGROUND

Barnyardgrass (Echinochloa crus-galli (L.) P. Beauv) has evolved resistance to the acetolactate synthase (ALS) inhibitor penoxsulam which is used to control weeds in rice fields in China. The present study is conducted to identify the target-site resistance (TSR) mechanisms conferring resistance in a penoxsulam-resistant population.

RESULTS

The ALS sensitivity in vitro of the resistant population was sixfold lower to penoxsulam than that of the sensitive population. ALS sequencing revealed that no known mutation conferring ALS herbicide resistance was detected. However, a novel mutation Phe-206-Leu was identified in the ALS gene. Additionally, ALS gene expression level of the resistant population was lower than that of the sensitive population. Therefore, the penoxsulam resistance was not due to the overexpression of ALS gene. Molecular docking revealed that this mutation may change the interaction of the penoxsulam-ALS binding and weaken its mutual affinity by approximately 10%. Arabidopsis thaliana transformed with mutant ALS had fourfold greater resistance to penoxsulam and varied cross-resistance to other ALS herbicides than those transformed with sensitive ALS. Mutant and sensitive ALS proteins expressed by the baculovirus system exhibited different in vitro penoxsulam sensitivity levels. Mutant ALS had eightfold lower sensitivity to penoxsulam than sensitive ALS.

CONCLUSION

This report provides clear evidence that the ALS mutation at position 206 (Phe-206-Leu) confers penoxsulam resistance in barnyardgrass. Phe-206 was confirmed to be the ninth amino acid residue related to ALS herbicide resistance in weeds. © 2022 Society of Chemical Industry.

P450s mediated enhanced herbicide metabolism involved in the thifensulfuron-methyl resistance in Ipomoea purpurea L

Ipomea purpurea (L.) Roth. reduces dry land crop yield and quality in Northeast China, especially in Liaoning Province. Frequent use of thifensulfuron-methyl in recent years has resulted in herbicide resistance in I. purpurea. We evaluated resistance levels of I. purpurea to thifensulfuron-methyl, an acetolactate synthase (ALS) inhibitor, in Liaoning Province and further investigated the resistance mechanisms. The results showed that 15 populations of I. purpurea have evolved up to 5.81-34.44-fold resistance to thifensulfuron-methyl, compared to the susceptible population (S), among which LN3 was the most resistant. DNA sequencing of the ALS gene in susceptible and resistant populations did not reveal any target site mutations that could be associated with resistance to thifensulfuron-methyl in I. purpurea. Additionally, no significant difference was detected between the in vitro ALS activity of LN3 and S. The GR₅₀ of LN3 decreased sharply by 47% when malathion (a P450 inhibitor) was applied with thifensulfuron-methyl. Absorption of thifensulfuron-methyl by LN3 was equal to that of S; however, LN3 metabolized the herbicide significantly faster. This was repressed after the inhibition of P450s activity. Collectively, our results confirmed that I. purpurea in Liaoning Province has developed resistance to thifensulfuron-methyl and implied that the resistance was conferred by the increase in detoxification mediated by P450s. Furthermore, LN3 was sensitive to fluroxypyr, which can be used as an alternative to control I. purpurea.

CYP81A68 confers metabolic resistance to ALS and ACCase-inhibiting herbicides and its epigenetic regulation in Echinochloa crus-galli

Long-term and excessive herbicide use has led to some environmental concerns and especially, herbicide resistance evolution in weeds. Here, we confirmed acetolactate synthase (ALS) inhibiting herbicide penoxsulam resistance and cross resistance to acetyl-coenzyme carboxylase (ACCase) inhibiting herbicides (cyhalofop-butyl and metamifop) in a global weed Echinochloa crus-galli population resistant to these herbicides (R). Penoxsulam metabolism study indicated that degradation rate was significantly higher in R than susceptible E. crus-galli population (S). RNA-sequencing revealed that a cytochrome P450 (P450) gene, CYP81A68, expressed higher in R versus S. Rice seedlings overexpressing this CYP81A68 gene are resistant to penoxsulam, cyhalofop-butyl and metamifop, and penoxsulam resistance is due to enhanced metabolism via O-demethylation. Deletion analysis of the CYP81A68 gene promoter identified an efficient region, in which differential methylation of CpG islands occurred between R and S. Collectively, these results demonstrate that upregulation of E. crus-galli CYP81A68 gene endows generalist metabolic resistance to commonly used ALS- and ACCase-inhibiting herbicides in rice fields and epigenetic regulation may play a role in the resistance evolution. This research could contribute to strategies reducing herbicide environmental impacts by judicious selection of alternative herbicide and non-chemical control tactics.

Absorption, translocation, and metabolism of florpyrauxifen-benzyl and cyhalofop-butyl in cyhalofop-butyl-resistant barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.]

Dose-response experiments were conducted to assess the sensitivity of one susceptible and three putative resistant (R1, R2, and R3) barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] biotypes to florpyrauxifen-benzyl and cyhalofop-butyl alone and as a formulated premix. Subsequently, potential resistance mechanisms of the barnyardgrass were evaluated. Based on biomass reduction results, resistant/susceptible ratios were calculated for R1 (7.0-50), R2 (7.0-150), and R3 (18-214) biotypes. Absorption and translocation of [¹⁴C]-florpyrauxifen-benzyl decreased in R1 and R3 biotypes, but not for [¹⁴C]-cyhalofop-butyl. The metabolism of [¹⁴C]-florpyrauxifen-benzyl to [¹⁴C]-florpyrauxifen-acid was >2-fold less in resistant biotypes (9-11%) than in the susceptible biotype (23%). Moreover, the production of [¹⁴C]-florpyrauxifen-acid in susceptible barnyardgrass (not in the R biotypes) increased 3-fold when florpyrauxifen-benzyl and cyhalofop-butyl were applied in mixture compared to florpyrauxifen-benzyl applied alone. The tested barnyardgrass biotypes had no mutation in the Transport Inhibitor Response1, auxin-signaling F-box, and acetyl coenzyme A carboxylase genes. Although further studies on cyhalofop-butyl resistance with respect to analysis of specific metabolites are needed, our findings in this study demonstrates that the evolution of florpyrauxifen-benzyl resistance in multiple resistant barnyardgrass can be related to non-target-site resistance mechanisms reducing absorption and translocation of the herbicide and causing reduced conversion or rapid degradation of florpyrauxifen-acid.

The phytotoxicity mechanism of florpyrauxifen-benzyl to Echinochloa crus-galli (L.) P. Beauv and weed control effect

Weeds infest rice causing high yield losses, leading to the increasing use of herbicides for weed control. However, many weeds have evolved resistance to common commercial herbicides, including penoxsulam, metamifop and quinclorac. This study investigated the weed control effect and the phytotoxicity mechanism of florpyrauxifen-benzyl, a novel synthetic auxin herbicide registered for weed management in rice fields in China. The greenhouse study showed that florpyrauxifen-benzyl was highly efficient (GR₅₀ < 6 and GR₉₀ < 15 g a.i ha^-1) at controlling 10 weed species commonly found in rice fields, including penoxsulam- and quinclorac- resistant(R) biotypes of Echinochloa Beauv. and bensulfuron-methyl-R biotype of Ammannia arenaria. The typical plant hormone content showed that following florpyrauxifen-benzyl treatment, indole-3-acetic acid (IAA) production changed only slightly at 12 h, while abscisic acid (ABA) production increased with time in the treated group, whose content was significantly higher than that of the control. Besides, ethylene biosynthesis was stimulated by florpyrauxifen-benzyl, ethylene production, 1-aminocyclopropane-1-carboxylic acid (ACC) content, and 1-aminocyclopropane-1-carboxylate synthase (ACS) and 1-aminocyclopropane-1-carboxylate oxidase (ACO) activities, which evidently increased in the treated group, and ethylene peaked at 36 h. For the antioxidant enzyme activities and malondialdehyde (MDA) content in the treated group, results showed that MDA content continuously increased with time and was greater than that in the untreated group at 48 h and 72 h, superoxide dismutase (SOD) activity changed with exposure time and was significantly higher in the treatment group than the control at 48 h. A similar phenomenon was observed in peroxidase (POD) activity, which reached a peak at 48 h, and no distinct difference in catalase (CAT) activity was observed among groups except for the higher activity in the treated groups than control at 36 h and 48 h. Our results showed that that the stimulation ethylene biosynthesis and accumulation of ABA and reactive oxygen species (ROS) play important roles in the phytotoxicity mechanism of florpyrauxifen-benzyl in plants. Our findings demonstrate the potential of florpyrauxifen-benzyl to provide an alternative weed management strategy for rice fields.

Herbicide-Resistant Invasive Plant Species Ludwigia decurrens Walter

Ludwigia decurrens Walter is a dicotyledonous plant belonging to the family Onagraceae. It is native to Central Eastern USA but has been spreading quickly and has naturalized in aquatic and riparian ecosystems (including rice paddy fields) in many countries; therefore, it is now considered an invasive noxious weed. L. decurrens is highly competitive with rice and causes a significant reduction in rice production. The objective of the present study was to evaluate the efficacy of the herbicide penoxsulam for the control of L. decurrens in rice fields. The seeds of L. decurrens were collected from four villages in Indonesia, and penoxsulam was applied to L. decurrens in seven dosages (0, 2.5, 5, 10, 20, 40, and 80 g a.i. ha^-1) 3 weeks after seed sowing. The plant populations from Hegarmanah, Jatisari, and Joho showed complete mortality at the recommended dosage of penoxsulam (10 g a.i. ha^-1). However, the plants from Demakan grew, flowered, and produced seeds 56 days after treatment with 40 g a.i. ha^-1 of penoxsulam. The resistance index value of the population was 36.06. This is the first report of a penoxsulam-resistant weed from a dicotyledonous plant species and also the first report of a herbicide-resistant population of L. decurrens. The appearance of herbicide-resistant L. decurrens is a serious issue from both an environmental and an economic perspective, especially since protected forest and freshwater ecosystems are located at a short distance from the study area. Further research is needed to evaluate herbicide mixtures and/or the rotation of herbicide action sites. The identification of the penoxsulam-resistance mechanism in L. decurrens is also necessary to develop a herbicide resistance management strategy.

Fate and adaptive plasticity of heterogeneous resistant population of Echinochloa colona in response to glyphosate

Understanding the fate of heterogenous herbicide resistant weed populations in response to management practices can help towards overcoming the resistance issues. We selected one pair of susceptible (S) and resistant (R) phenotypes (2B21-R vs 2B21-S and 2B37-R vs 2B37-S) separately from two glyphosate resistant heterogeneous populations (2B21 and 2B37) of Echinochloa colona and their fate and adaptive plasticity were evaluated after glyphosate application. Our study revealed the glyphosate concentration required to cause a 50% plant mortality (LD50) was 1187, 200, 3064, and 192 g a. e. ha-1 for the four phenotypes 2B21-R, 2B21-S, 2B37-R, and 2B37-S respectively. Both S phenotypes accumulated more biomass than the R phenotypes at the lower application rates (34 and 67.5 g a. e. ha-1) of glyphosate. However, the R phenotypes generally produced more biomass at rates of glyphosate higher than 100 g a. e. ha-1 throughout the growth period. Plants from the R phenotypes of 2B21 and 2B37 generated 32% and 38% fewer spikesplant-1 than their respective S counterparts in the absence of glyphosate respectively. The spike and seed numbersplant-1 significantly higher in R than S phenotypes at increased rates of glyphosate and these relationships were significant. Our research suggests that glyphosate-resistant E. colona plants will be less fit than susceptible plants (from the same population) in the absence of glyphosate. But in the presence of glyphosate, the R plants may eventually dominate in the field. The use of glyphosate is widespread in field, would favour the selection towards resistant individuals.

Distribution of Glyphosate-Resistance in Echinochloa crus-galli Across Agriculture Areas in the Iberian Peninsula

The levels of resistance to glyphosate of 13 barnyard grass (Echinochloa crus-galli) populations harvested across different agriculture areas in the Southern Iberian Peninsula were determined in greenhouse and laboratory experiments. Shikimate accumulation fast screening separated the populations regarding resistance to glyphosate: susceptible (S) E2, E3, E4, and E6 and resistant (R) E1, E5, E7, E8, E9, E10, E11, E12, and E13. However, resistance factor (GR₅₀ E1-E13/GR₅₀ E6) values separated these populations into three groups: (S) E2, E3, E4, and E6, (R) E1, E5, E7, E8, and E9, and very resistant (VR) E10, E11, E12, and E13. ¹⁴C-glyphosate assays performed on two S populations (E2 and E6) showed greater absorption and translocation than those found for R (E7 and E9) and VR (E10 and E12) populations. No previous population metabolized glyphosate to amino methyl phosphonic acid (AMPA) and glyoxylate, except for the E10 population that metabolized 51% to non-toxic products. The VR populations showed two times more 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) activity without herbicide than the rest, while the inhibition of the EPSPS activity by 50% (I₅₀) required much higher glyphosate in R and VR populations than in S populations. These results indicated that different target-site and non-target-site resistance mechanisms were implicated in the resistance to glyphosate in E. crus-galli. Our results conclude that resistance is independent of climate, type of crop, and geographic region and that the level of glyphosate resistance was mainly due to the selection pressure made by the herbicide on the different populations of E. crus-galli studied.

Accumulation of Target Gene Mutations Confers Multiple Resistance to ALS, ACCase, and EPSPS Inhibitors in Lolium Species in Chile

Different Lolium species, common weeds in cereal fields and fruit orchards in Chile, were reported showing isolated resistance to the acetyl CoA carboxylase (ACCase), acetolactate synthase (ALS) and 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) inhibiting herbicides in the late 1990s. The first case of multiple resistance to these herbicides was Lolium multiflorum found in spring barley in 2007. We hypothesized that other Lolium species may have evolved multiple resistance. In this study, we characterized the multiple resistance to glyphosate, diclofop-methyl and iodosulfuron-methyl-sodium in Lolium rigidum, Lolium perenne and Lolium multiflorum resistant (R) populations from Chile collected in cereal fields. Lolium spp. populations were confirmed by AFLP analysis to be L. rigidum, L. perenne and L. multiflorum. Dose-response assays confirmed multiple resistance to glyphosate, diclofop-methyl and iodosulfuron methyl-sodium in the three species. Enzyme activity assays (ACCase, ALS and EPSPS) suggested that the multiple resistance of the three Lolium spp. was caused by target site mechanisms, except the resistance to iodosulfuron in the R L. perenne population. The target site genes sequencing revealed that the R L. multiflorum population presented the Pro-106-Ser/Ala (EPSPS), Ile-2041-Asn++Asp-2078-Gly (ACCase), and Trp-574-Leu (ALS) mutations; and the R L. rigidum population had the Pro-106-Ser (EPSPS), Ile-1781-Leu+Asp-2078-Gly (ACCase) and Pro-197-Ser/Gln+Trp-574-Leu (ALS) mutations. Alternatively, the R L. perenne population showed only the Asp-2078-Gly (ACCase) mutation, while glyphosate resistance could be due to EPSPS gene amplification (no mutations but high basal enzyme activity), whereas iodosulfuron resistance presumably could involve non-target site resistance (NTSR) mechanisms. These results support that the accumulation of target site mutations confers multiple resistance to the ACCase, ALS and EPSPS inhibitors in L. multiflorum and L. rigidum from Chile, while in L. perenne, both target and NTSR could be present. Multiple resistance to three herbicide groups in three different species of the genus Lolium in South America represents a significant management challenge.

Non-Target-Site Resistance to Herbicides: Recent Developments

Non-target-site resistance (NTSR) to herbicides in weeds can be conferred as a result of the alteration of one or more physiological processes, including herbicide absorption, translocation, sequestration, and metabolism. The mechanisms of NTSR are generally more complex to decipher than target-site resistance (TSR) and can impart cross-resistance to herbicides with different modes of action. Metabolism-based NTSR has been reported in many agriculturally important weeds, although reduced translocation and sequestration of herbicides has also been found in some weeds. This review focuses on summarizing the recent advances in our understanding of the physiological, biochemical, and molecular basis of NTSR mechanisms found in weed species. Further, the importance of examining the co-existence of TSR and NTSR for the same herbicide in the same weed species and influence of environmental conditions in the altering and selection of NTSR is also discussed. Knowledge of the prevalence of NTSR mechanisms and co-existing TSR and NTSR in weeds is crucial for designing sustainable weed management strategies to discourage the further evolution and selection of herbicide resistance in weeds.