The basis of resistance mechanism to mesosulfuron-methyl in Tausch's goatgrass (Aegilops tauschii Coss.)

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.

A Double Mutation in the ALS Gene Confers a High Level of Resistance to Mesosulfuron-Methyl in Shepherd's-Purse

Shepherd's-purse (Capsella bursa-pastoris), a globally distributed noxious weed species often found in wheat, has evolved resistance to ALS-inhibiting herbicides mainly due to single mutations in the ALS gene. In the present study, dose-response bioassays showed that a shepherd's-purse population (R), collected from Xinghua, Jiangsu Province, China, had high level of resistance to the ALS-inhibiting herbicide, mesosulfuron-methyl (800-fold), and even much higher resistance levels to other reported ALS-inhibiting herbicides, tribenuron-methyl (1313-fold), bensulfuron-methyl (969-fold) and penoxsulam (613-fold). Sequencing of the open reading frame of the ALS gene revealed a double ALS gene mutation (Pro197-Ser plus Trp574-Leu) conferring the high resistance in the R plants. Docking analysis of the ALS protein and mesosulfuron-methyl predicts that the two amino acid substitutions in the R samples reduces the binding energy to the herbicide by decreasing the hydrogen bonds (H-bonds) and other interactions, thus endowing resistance to ALS-inhibiting herbicides. These results demonstrate that the double ALS mutation confers high resistance levels to ALS-inhibiting herbicides. To our knowledge, this is the first evidence of the double ALS mutation in shepherd's-purse endowing ALS-inhibiting herbicide resistance.

Metabolic resistance to acetolactate synthase inhibitors in Beckmannia syzigachne: identification of CYP81Q32 and its transcription regulation

Frequent herbicide use selects for herbicide resistance in weeds. Cytochrome P450s are important detoxification enzymes responsible for herbicide resistance in plants. We identified and characterized a candidate P450 gene (BsCYP81Q32) from the problematic weed Beckmannia syzigachne to test whether it conferred metabolic resistance to the acetolactate synthase-inhibiting herbicides mesosulfuron-methyl, bispyribac-sodium, and pyriminobac-methyl. Transgenic rice overexpressing BsCYP81Q32 was resistant to the three herbicides. Equally, rice overexpressing the rice ortholog gene OsCYP81Q32 was more resistant to mesosulfuron-methyl. Conversely, an OsCYP81Q32 gene knockout generated using CRISPR/Cas9 enhanced mesosulfuron-methyl sensitivity in rice. Overexpression of the BsCYP81Q32 gene resulted in enhanced mesosulfuron-methyl metabolism in transgenic rice seedlings via O-demethylation. The major metabolite, demethylated mesosulfuron-methyl, was chemically synthesized and displayed reduced herbicidal effect in plants. Moreover, a transcription factor (BsTGAL6) was identified and shown to bind a key region in the BsCYP81Q32 promoter for gene activation. Inhibition of BsTGAL6 expression by salicylic acid treatment in B. syzigachne plants reduced BsCYP81Q32 expression and consequently changed the whole plant response to mesosulfuron-methyl. Sequence polymorphisms in an important region of the BsTGAL6 promoter may explain the higher expression of BsTGAL6 in resistant versus susceptible B. syzigachne plants. Collectively, the present study reveals the evolution of an herbicide-metabolizing and resistance-endowing P450 and its transcription regulation in an economically important weedy plant species.

Advances in the Mining of Disease Resistance Genes from Aegilops tauschii and the Utilization in Wheat

Aegilops tauschii is one of the malignant weeds that affect wheat production and is also the wild species ancestor of the D genome of hexaploid wheat (Triticum aestivum, AABBDD). It contains many disease resistance genes that have been lost in the long-term evolution of wheat and is an important genetic resource for the mining and utilization of wheat disease resistance genes. In recent years, the genome sequence of Aegilops tauschii has been preliminarily completed, which has laid a good foundation for the further exploration of wheat disease resistance genes in Aegilops tauschii. There are many studies on disease resistance genes in Aegilops tauschii; in order to provide better help for the disease resistance breeding of wheat, this paper analyzes and reviews the relationship between Aegilops tauschii and wheat, the research progress of Aegilops tauschii, the discovery of disease resistance genes from Aegilops tauschii, and the application of disease resistance genes from Aegilops tauschii to modern wheat breeding, providing a reference for the further exploration and utilization of Aegilops tauschii in wheat disease resistance breeding.

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.

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

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

Amino acid substitution (Gly-654-Tyr) in acetolactate synthase (ALS) confers broad spectrum resistance to ALS-inhibiting herbicides

BACKGROUND

Amaranthus retroflexus L. is a problematic weed in agricultural fields. In China, the repeated use of acetolactate synthase (ALS) inhibiting herbicides has led to the evolution of many A. retroflexus resistant populations. The objective of this research was to investigate the physiological and molecular basis for resistance to ALS-inhibiting herbicides in A. retroflexus.

RESULTS

Sequence analysis of the ALS revealed a Trp to Leu substitution at amino acid position 574 (Trp-574-Leu), and a previously unreported substitution of Gly to Tyr substitution at 654 (Gly-654-Tyr) in the resistant A. retroflexus population. A purified R-Tyr654 subpopulation homozygous for the Gly-654-Tyr mutation was generated and characterized in response to five different classes of ALS-inhibiting herbicides. Dose-response analysis revealed that the R-Tyr654 population was highly resistant to two imidazolinones (imazethapyr, >42-fold; imazamox, 48.3-fold), one triazolopyrimidine (flumetsulam, 57.4-fold), and one sulfonylamino-carbonyltriazolinone (flucarbazone, 17.3-fold). Moreover, it was moderately resistant to two sulfonylureas (thifensulfuron-methyl, 4.9-fold; nicosulfuron, 9.1-fold), and one pyrimidinylthio-benzoate (bispyribac-sodium, 3.1-fold). An in vitro ALS activity assay showed that ALS with the Gly-654-Tyr substitution was resistant to all the tested ALS-inhibiting herbicides. However, the field rate of herbicides binding other sites of action demonstrated effective control of the R-Tyr654 population.

CONCLUSION

These results indicate that the molecular basis of ALS-inhibiting herbicide resistance in A. retroflexus was caused by the Trp-574-Leu and Gly-654-Tyr substitutions in the ALS. This is the first report of Gly-654-Tyr substitution in ALS, and this substitution confers broad spectrum resistance to ALS-inhibiting herbicides. © 2021 Society of Chemical Industry.

The Basis of Tolerance Mechanism to Metsulfuron-Methyl in Roegneria kamoji (Triticeae: Poaceae)

Roegneria kamoji, a perennial monocot weed that belongs to the tribe Triticeae (family: Poaceae), is an emerging problematic weed in winter wheat (Triticum aestivum) fields in China. We have previously confirmed four R. kamoji populations tolerant to acetyl-CoA carboxylase (ACCase) inhibitors, and failed control of these populations by metsulfuron-methyl was observed. The objective of this study was to characterize the level of tolerance to metsulfuron-methyl, the basis of tolerance mechanism, and cross-tolerance to acetolactate synthase (ALS) inhibitors in R. kamoji. A whole-plant dose–response assay showed that plants of all R. kamoji populations (both from wheat fields and uncultivated areas) exhibited high tolerance to metsulfuron-methyl, based on their 100% survival at 6-fold recommended field dose (RFD) and ED₅₀ values >6.84-fold RFD, no susceptible population was found. Gene sequencing indicated that no reported amino acid substitutions associated with resistance to ALS inhibitor were found in the ALS gene among the R. kamoji populations. Pretreatment with the known cytochrome P450 monooxygenases (CytP450) inhibitor malathion reduced the ED₅₀ values of metsulfuron-methyl in two R. kamoji populations. These populations also exhibited cross-tolerance to RFD of mesosulfuron-methyl and bispyribac-sodium. The activities of glutathione-S-transferase (GST) and CytP450 could be induced by metsulfuron-methyl in R. kamoji, which is similar to the known tolerant crop wheat. This is the first report elucidating metsulfuron-methyl tolerance in R. kamoji. The reversal of tolerance by malathion and the GST and/or CytP450 enhanced herbicide metabolism suggests that non-target-site mechanisms confer tolerance to metsulfuron-methyl in R. kamoji.

Seed dressing with mefenpyr-diethyl as a safener for mesosulfuron-methyl application in wheat: The evaluation and mechanisms

Mesosulfuron-methyl is always applied by foliar spraying in combination with the safener mefenpyr-diethyl to avoid phytotoxicity on wheat (Triticum aestivum L.) cultivars. However, it was observed that the tolerance of Tausch’s goatgrass (Aegilops tauschii Coss.) to mesosulfuron-methyl significantly increased in the presence of mefenpyr-diethyl by performing bioassay. This confirmed phenomenon may lead to overuse of mesosulfuron-methyl and weed resistance evolution in field conditions. Therefore, we tested the effect of wheat seed dressing with mefenpyr-diethyl as a possible alternative and disclosed the underlying mechanisms by herbicide dissipation study, enzymatic analysis and transcriptome profiling. The results suggest that increase of ALS activity, enhancement of metabolic processes, and other stress responses are crucial for the regulation of herbicide detoxification induced by mefenpyr-diethyl. Additionally, transcription factors such as AP2/ERF-ERF, bHLH, NAC, and MYB, and protein kinase such as RLK-Pelle_DLSV might play vital regulatory roles. The current study has important implications for mesosulfuron-methyl application in wheat field to control Tausch’s goatgrass and provides a comprehensive understanding of the protective effect of mefenpyr-diethyl.

Different leaf-mediated deposition, absorbed and metabolism behaviors of 2,4-D isooctyl ester between Triticum aestivum and Aegilops tauschii Coss

Tausch's goatgrass (Aegilops tauschii Coss.), is a major weed species, infesting wheat (Triticum aestivum) fields in China. 2,4-D isooctyl ester is widely used for broadleaf weed control and selected as a tool to study the differences between, A. tauschii and T. aestivum. In this study, we measured the growth responses of these species to 2,4-D isooctyl ester and found that T. aestivum was more sensitive to the herbicide than A. tauschii. To clarify the reasons for this difference, we measured the leaf-mediated deposition, absorption and metabolism of 2,4-D isooctyl ester and the expression of auxin receptor transport inhibitor response (TIR1) gene in T. aestivum and A. tauschii. The results indicated that the deposition of 2,4-D isooctyl ester droplets may be lower on A. tauschii than on T. aestivum, because of the increased contact angle and greater density of trichomes on the leaves of the former. A distinct increase in 2,4-D isooctyl ester uptake was detected in T. aestivum during the entire experimental period, and the rate was 2.2-fold greater than that in A. tauschii at 6 h after treatment. Compared with A. tauschii, T. aestivum exhibited a greater accumulation of primary metabolite 2,4-D in plants, which may be responsible for the different responses of the two species. Additionally, the absolute expression level of TIR1 was clearly greater in T. aestivum than that in A. tauschii. These data will be helpful to further understand the differences between T. aestivum and A. tauschii, which may provide a unique perspective for the development and identification of new target compounds that are effective against this weed species.

Effects of plant density on tillering in the weed grass Aegilops tauschii Coss. and its phytohormonal regulation

Aegilops tauschii Coss, a notorious wheat field weed, poses a serious threat to wheat in China. Tillers are an important agronomic tool for yield. In this study, a total of 12 Ae. tauschii populations were collected from China to investigate the effect of plant density on tiller occurrence and its phytohormonal regulation. We assayed the growth parameters of Ae. tauschii and the levels of endogenous hormones at different plant densities. The results showed that plant density had a significant effect on the quantity and quality of Ae. tauschii seeds produced per plant. In particular, the tiller and spike numbers per plant were negatively affected by plant density (P < 0.0001). The contents of 13 endogenous hormones in the tiller nodes changed in response to plant density. Among them, indole-3-acetic acid (IAA) and gibberellin (GA) positively responded to plant density. However, the reverse result was found for cytokinin (CTK). Interestingly, phylogenetic tree analysis of auxin (AeYUCCA), CK (AeIPT) and GA (AeCPS) biosynthesis related genes found that phylogenies in the Gramineae for the three different genes were various, hinting at horizontal gene transfer. Moreover, the dynamics of the expression of AeYUCCA, AeIPT and AeCPS were roughly consistent with their phytohormone contents during tillering stage. When externally sprayed on plants of Ae. tauschii, 2,4-D isooctyl ester and GA₃ markedly reduced its tillering while 6-BA had no significant effect.

Naphthalic anhydride decreases persistence of alachlor and atrazine and elevates tolerance of maize

The present study aimed at alleviating the impacts of alachlor (Ala) or atrazine (Atr) on maize growth by seed-dressing with naphthalic anhydride (NA, 0.4% w/w by seed weight). The dressed and undressed seeds were germinated for 10 days and treated with Ala or Atr at 3.2 or 1.8 kg ha^-1, respectively, then the herbicide residues were determined in shoots one day after treatment. Atr residues were higher than Ala and remained in the same level for the 2^nd day then retracted consistently up to 12 days. Meanwhile, GSH and GST were significantly induced while growth parameters were reduced; the effect of Atr was higher than Ala. Nonetheless, ABA, phenolics and anthocyanins as well as PAL, TAL and CI were increased but IAA was decreased coincidently with enhanced IAA-O and peroxidase. The immediate detection of the herbicide residues could conclude that growth reduction as elucidated from the decreased IAA concomitant with elevating ABA, phenolics and anthocyanin contents and enzyme activities are consequences of the herbicide persistence. The drop of IAA was preceded by the stimulation of IAA-O and peroxidase while the increased phenolics and anthocyanins followed PAL, TAL and CI stimulation confirming the regulatory roles of these enzymes. The application of NA greatly lowered the herbicide residues concurrently with ameliorations in growth parameters, GSH, GST, and maintained the balancing of secondary metabolites and plant growth regulators. Lowering Ala and Atr residues by NA in synchronization with enhanced GSH and GST could conclude that NA encouraged the detoxification of the herbicide. Moreover, the balances of IAA, ABA, phenolics and anthocyanins were mostly maintained in normal levels concomitantly with growth ameliorations suggesting that phytohormones and secondary metabolites are involved in the elevation of maize tolerance to Ala and Atr.

Investigation of resistance mechanism to fomesafen in Amaranthus retroflexus L

Amaranthus retroflexus L. is one of the most troublesome weeds in autumn-crop fields in Northeast China. In recent years, field applications of fomesafen have failed to control an A. retroflexus population in Heilongjiang Province, China. Therefore, in this study, experiments were conducted to determine the resistance of A. retroflexus to fomesafen and investigate the molecular basis of herbicide resistance. Whole-plant dose-response experiments showed that the resistant (R) population exhibited 41.8-fold resistance to fomesafen compared with the susceptible (S) population. Target-gene sequence analysis revealed an Arg-128-Gly substitution in the protoporphyrinogen oxidase (PPO) in the R population. The response of PPO2 transgenic Arabidopsis thaliana to fomesafen demonstrated that the Arg-128-Gly substitution conferred high resistance to fomesafen. Cross- and multiple-resistance analyses indicated that the R population was cross-resistant to lactofen and carfentrazone-ethyl but was sensitive to imazethapyr, thifensulfuron-methyl, atrazine, and glyphosate. This study indicated that the Arg-128-Gly substitution is the main reason for A. retroflexus resistance to fomesafen. To our knowledge, this is the first report of a target-site based mechanism for the resistance to a PPO-inhibiting herbicide in A. retroflexus.