A novel Pro197Glu substitution in acetolactate synthase (ALS) confers broad-spectrum resistance across ALS inhibitors

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.

Transcriptome changes in chlorsulfuron-treated plants are caused by acetolactate synthase inhibition and not induction of a herbicide detoxification system in Marchantia polymorpha

A sensing mechanism in mammals perceives xenobiotics and induces the transcription of genes encoding proteins that detoxify these molecules. However, it is unclear if plants sense xenobiotics, and activate an analogous signalling system leading to their detoxification. Using the liverwort Marchantia polymorpha, we tested the hypothesis that there is a sensing system in plants that perceives herbicides resulting in the increased transcription of genes encoding proteins that detoxify these herbicides. Consistent with the hypothesis, we show that chlorsulfuron-treatment induces changes in the M. polymorpha transcriptome. However, these transcriptome changes do not occur in chlorsulfuron (CS)-treated target site resistant mutants, where the gene encoding the target carries a mutation that confers resistance to chlorsulfuron. Instead, we show that inactivation of the chlorsulfuron target, acetolactate synthase (ALS) (also known as acetohydroxyacid synthase (AHAS)), is required for the transcriptome response. These data demonstrate that the transcriptome changes in chlorsulfuron-treated plants are caused by disrupted amino acid synthesis and metabolism resulting from acetolactate synthase inhibition, and indicate that the transcriptome changes are not caused by a herbicide sensing mechanism.

Resistance to ALS inhibitors conferred by non-target-site resistance mechanisms in Myosoton aquaticum L

Myosoton aquaticum L. is a competitive broadleaf weed commonly found in wheat fields in China and has become challenging due to its evolving herbicide resistance. In this study, one subpopulation, RF1 (derived from the tribenuron-methyl-resistant population HN10), with none of the known acetolactate synthase (ALS) resistance mutations was confirmed to exhibit resistance to tribenuron-methyl (SU), pyrithiobac‑sodium (PTB), florasulam (TP), flucarbazone-Na (SCT), and diflufenican (PDS). In vitro ALS activity assays showed that the total ALS activity of RF1 was lower than that of the susceptible (S) population. However, there was no difference in ALS gene expression induced by tribenuron-methyl between the two populations. The combination of the cytochrome P450 monooxygenase (P450) inhibitor malathion and tribenuron-methyl resulted in the RF1 population behaving like the S population. The rapid P450-mediated tribenuron-methyl metabolism in RF1 plants was also confirmed by liquid chromatography tandem mass spectrometry (LC-MS/MS) analysis. In addition, approximately equal glutathione S-transferase (GST) activity was observed in RF1 and S plants of untreated and tribenuron-methyl treated groups. This study reported one M. aquaticum L. population without ALS resistance mutations exhibiting resistance to ALS inhibitors and the PDS inhibitor diflufenican, and the non-target-site resistance mechanism played a vital role in herbicide resistance.

The Ile-2041-Val mutation in the ACCase gene confers resistance to clodinafop-propargyl in American sloughgrass (Beckmannia syzigachne Steud)

BACKGROUND

Exploring the mechanisms of herbicide resistance in weeds is an important part of designing resistance management strategies and rationalizing herbicide use. Beckmannia syzigachne is one of the most important agricultural weeds in China. Long-term use of acetyl-CoA carboxylase (ACCase)-inhibiting herbicides has led to the evolution of herbicide resistance in B. syzigachne. ACCase-inhibiting herbicides comprise three chemical families: aryloxyphenoxypropionates (APPs), cyclohexanediones (CHDs) and phenylpyraxoline (DENs).

RESULTS

Based on whole-plant dose-response experiments, a B. syzigachne population (BS-R) was confirmed to be 12- and 20-fold resistant to the APP herbicides quizalofop-P-ethyl and clodinafop-propargyl, and 2.2-, 2.8- and 2.8-fold resistant to fenoxaprop-P-ethyl, the CHD herbicide sethoxydim and the PPZ herbicide pinoxaden, respectively, compared with its susceptible counterpart (BS-S). Resistance to clodinafop-propargyl in the BS-R population could not be reversed by the known cytochrome P450 (CYP450) inhibitor malathion and the glutathione S-transferase (GST) inhibitor 4-chloro-7-nitrobenzoxadiazole. In addition, no difference in CYP450 and GST activity was confirmed between the BS-R and BS-S populations. ACCase gene sequencing revealed an Ile-2041-Val mutation in the BS-R population. A derived cleaved amplified polymorphic sequence marker was developed for rapid detection of the specific Ile-2041-Val mutation. Correlation quantification of resistance in homo- and hetero-resistant versus wild-type plants showed that resistance to clodinafop-propargyl in this population is conferred by the Ile-2041-Val mutation.

CONCLUSION

Unlike previous reports on the unique cross-resistance pattern conferred by the 2041 mutation, this study demonstrates that the Ile-2041-Val mutation in BS-R population confers resistance to certain ACCase-inhibiting APP, CHD and PPZ herbicides in B. syzigachne. © 2021 Society of Chemical Industry.

Point Mutations as Main Resistance Mechanism Together With P450-Based Metabolism Confer Broad Resistance to Different ALS-Inhibiting Herbicides in Glebionis coronaria From Tunisia

Resistance to acetolactate synthase (ALS) inhibiting herbicides has recently been reported in Glebionis coronaria from wheat fields in northern Tunisia, where the weed is widespread. However, potential resistance mechanisms conferring resistance in these populations are unknown. The aim of this research was to study target-site resistance (TSR) and non-target-site resistance (NTSR) mechanisms present in two putative resistant (R) populations. Dose-response experiments, ALS enzyme activity assays, ALS gene sequencing, absorption and translocation experiments with radiolabeled herbicides, and metabolism experiments were carried out for this purpose. Whole plant trials confirmed high resistance levels to tribenuron and cross-resistance to florasulam and imazamox. ALS enzyme activity further confirmed cross-resistance to these three herbicides and also to bispyribac, but not to flucarbazone. Sequence analysis revealed the presence of amino acid substitutions in positions 197, 376, and 574 of the target enzyme. Among the NTSR mechanisms investigated, absorption or translocation did not contribute to resistance, while evidences of the presence of enhanced metabolism were provided. A pretreatment with the cytochrome P450 monooxygenase (P450) inhibitor malathion partially synergized with imazamox in post-emergence but not with tribenuron in dose-response experiments. Additionally, an imazamox hydroxyl metabolite was detected in both R populations in metabolism experiments, which disappeared with the pretreatment with malathion. This study confirms the evolution of cross-resistance to ALS inhibiting herbicides in G. coronaria from Tunisia through TSR and NTSR mechanisms. The presence of enhanced metabolism involving P450 is threatening the chemical management of this weed in Tunisian wheat fields, since it might confer cross-resistance to other sites of action.

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

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

Target-Site Mutations Conferring Herbicide Resistance

Mutations conferring evolved herbicide resistance in weeds are known in nine different herbicide sites of action. This review summarizes recently reported resistance-conferring mutations for each of these nine target sites. One emerging trend is an increase in reports of multiple mutations, including multiple amino acid changes at the glyphosate target site, as well as mutations involving two nucleotide changes at a single amino acid codon. Standard reference sequences are suggested for target sites for which standards do not already exist. We also discuss experimental approaches for investigating cross-resistance patterns and for investigating fitness costs of specific target-site mutations.

A Trp-574-Leu mutation in the acetolactate synthase (ALS) gene of Lithospermum arvense L. confers broad-spectrum resistance to ALS inhibitors

Lithospermum arvense is a troublesome dicotyledonous winter annual weed of wheat in China. A L. arvense population (HN01) suspected of being resistant to acetolactate synthase (ALS) inhibitors was found in Henan Province, China. This study aimed to testify the sensitivity of this HN01 population to eight herbicides from 3 different modes of action, and to explore the potential target-site-resistance mechanism to tribenuron-methyl. The whole-plant bioassays indicated that the population was highly resistant to tribenuron-methyl (SU, 350-fold), pyrithiobac sodium (PTB, 151-fold), pyroxsulam (TP, 62.7-fold), florasulam (TP, 80.6-fold), and imazethapyr (IMI, 136-fold), but was sensitive to carfentrazone-ethyl and fluroxypyr-meptyl. ALS gene sequencing revealed that the Trp (TGG) was substituted by Leu (TTG) at codon 574 in resistant plants. In in vitro ALS assays, the concentration of tribenuron-methyl required to inhibit 50% ALS activity (I₅₀) for HN01 was 117-fold greater than that required to inhibit a susceptible population (HN05), indicating that resistance was due to reduced sensitivity of the ALS enzyme to tribenuron-methyl. To the best of our knowledge, this is the first report of ALS gene Trp-574-Leu amino acid mutation confer resistance to tribenuron-methyl in L. arvense.

Resistance of Rapistrum rugosum to tribenuron and imazamox due to Trp574 or Pro197 substitution in the acetolactate synthase

Ten putative resistant and two susceptible Rapistrum rugosum populations originating from Greece were studied for resistance to acetolactate synthase (ALS)-inhibiting herbicides, using dose-response assays, sequencing of als gene and in vitro ALS activity assays. The dose-response assays showed that one (P1) out of ten putative resistant populations was cross-resistant to tribenuron and imazamox, while another population (P4) was resistant to tribenuron only. All populations were susceptible to MCPA at the recommended rate. Gene sequencing of als revealed that the P4 population had a point mutation at Pro197 by His providing resistance to tribenuron, whereas the P1 had a Trp574 by Leu point mutation conferring cross-resistance to tribenuron and imazamox. The in vitro activity of the ALS enzyme indicated I₅₀ values (tribenuron concentration required for 50% reduction of the ALS activity) ranging from 66.68 to 137.01 μM, whereas the respective value for the S populations ranged from 0.29 to 0.54 μM. These results strongly support that two R. rugosum populations evolved resistance to ALS-inhibiting herbicides due different point mutations in the als gene.

Target site mutations and cytochrome P450s confer resistance to fenoxaprop-P-ethyl and mesosulfuron-methyl in Alopecurus aequalis

BACKGROUND

Shortawn foxtail (Alopecurus aequalis Sobol.) is a competitive grass weed infesting winter wheat- and canola-growing fields in China. In May 2016, a suspected A. aequalis resistant population AHTC-06 that survived fenoxaprop-P-ethyl and mesosulfuron-methyl applied at their field-recommended rates was collected from a wheat field in Jinji County, Anhui Province, China. This study aimed to determine the resistance profile of this AHTC-06 population to ACCase- and ALS-inhibitors, and to investigate its mechanisms of resistance to fenoxaprop-P-ethyl and mesosulfuron-methyl.

RESULTS

Two mutations, a common Ile-2041-Asn (ACCase gene) and a very rare Pro-197-Tyr (ALS1 gene), were both identified in resistant individual plants. The homozygous subpopulation AHTC-06F1 for the two mutations was generated, and it showed broad-spectrum resistance to APPs, DENs, and ALS-inhibiting herbicides of all five chemical families tested, with resistance index (RI) values that ranged from 2.2 to 36.5. In vitro ALS activity assays showed the ALS from the resistant population was insensitive to all the tested ALS inhibitors, with RI values ranging from 3.10 to 22.51. Pre-treatment with piperonyl butoxide (PBO) and malathion significantly (P < 0.05) reversed the weed's resistance to fenoxaprop-P-ethyl and mesosulfuron-methyl, respectively. Two P450 genes, c21190_g1 and c43350_g3, were constitutively overexpressed and mesosulfuron-methyl-induced upregulated in resistant plants, for which c43350_g3 was also fenoxaprop-P-ethyl-induced upregulated.

CONCLUSION

This study confirms the first case of a grass weed featuring broad-spectrum resistance to ALS-inhibiting herbicides due to a Pro-197-Tyr mutation in the ALS gene. Fenoxaprop-P-ethyl and mesosulfuron-methyl resistances in AHTC-06 plants were conferred by target site mutations and P450s-based metabolism. © 2018 Society of Chemical Industry.

Non-target site-based resistance to tribenuron-methyl and essential involved genes in Myosoton aquaticum (L.)

BACKGROUND

Water chickweed (Myosoton aquaticum (L.)) is a dicot broadleaf weed that is widespread in winter fields in China, and has evolved serious resistance to acetolactate synthase (ALS) inhibiting herbicides.

RESULTS

We identified a M. aquaticum population exhibiting moderate (6.15-fold) resistance to tribenuron-methyl (TM). Target-site ALS gene sequencing revealed no known resistance mutations in these plants, and the in vitro ALS activity assays showed no differences in enzyme sensitivity between susceptible and resistant populations; however, resistance was reversed by pretreatment with the cytochrome P450 (CYP) monooxygenase inhibitor malathion. An RNA sequencing transcriptome analysis was performed to identify candidate genes involved in metabolic resistance, and the unigenes obtained by de novo transcriptome assembly were annotated across seven databases. In total, 34 differentially expressed genes selected by digital gene expression analysis were validated by quantitative real-time (qRT)-PCR. Ten consistently overexpressed contigs, including four for CYP, four for ATP-binding cassette (ABC) transporter, and two for peroxidase were further validated by qRT-PCR using additional plants from resistant and susceptible populations. Three CYP genes (with homology to CYP734A1, CYP76C1, and CYP86B1) and one ABC transporter gene (with homology to ABCC10) were highly expressed in all resistant plants.

CONCLUSION

The mechanism of TM resistance in M. aquaticum is controlled by NTSR rather than TSR. Four genes, CYP734A1, CYP76C1, CYP86B1, and ABCC10 could play essential role in metabolic resistance to TM and justify further functional studies. To our knowledge, this is the first large-scale transcriptome analysis of genes associated with NTSR in M. aquaticum using the Illumina platform. Our data provide resource for M. aquaticum biology, and will facilitate the study of herbicide resistance mechanism at the molecular level in this species as well as in other weeds.

Comparison of ALS functionality and plant growth in ALS-inhibitor susceptible and resistant Myosoton aquaticum L

Herbicide target-site resistance mutations may cause pleiotropic effects on plant ecology and physiology. The effect of several known (Pro197Ser, Pro197Leu Pro197Ala, and Pro197Glu) target-site resistance mutations of the ALS gene on both ALS functionality and plant vegetative growth of weed Myosoton aquaticum L. (water chickweed) have been investigated here. The enzyme kinetics of ALS from four purified water chickweed populations that each homozygous for the specific target-site resistance-endowing mutations were characterized and the effect of these mutations on plant growth was assessed via relative growth rate (RGR) analysis. Plants homozygous for Pro197Ser and Pro197Leu exhibited higher extractable ALS activity than susceptible (S) plants, while all ALS mutations with no negative change in ALS kinetics. The Pro197Leu mutation increased ALS sensitivity to isoleucine and valine, and Pro197Glu mutation slightly increased ALS sensitivity to isoleucine. RGR results indicated that none of these ALS resistance mutations impose negative pleiotropic effects on relative growth rate. However, resistant (R) seeds had a lowed germination rate than S seeds. This study provides baseline information on ALS functionality and plant growth characteristics associated with ALS inhibitor resistance-endowing mutations in water chickweed.

Trp574 substitution in the acetolactate synthase of Sinapis arvensis confers cross-resistance to tribenuron and imazamox

Rate-response experiments with nine putative resistant wild mustard (Sinapis arvensis) populations from Greece showed cross-resistance to tribenuron and imazamox. The calculated GR₅₀ values [herbicide rate (gaiha^-1) required for 50% reduction of fresh weight] of the nine resistant (R) populations ranged from 51.8 to 555.6gaitribenuronha^-1 and from 66.3 to 900.4gaiimazamoxha^-1. Regarding the susceptible population, GR₅₀ value was not estimated for tribenuron as its lower treatment reduced fresh weight by >95%, whereas the respective value for imazamox was 0.5gaiha^-1. Gene sequencing of als revealed that a point mutation at Trp574 position, leading to amino acid substitution by Leu in the ALS enzyme was present and the likely cause of resistance. The in vitro activity of the ALS enzyme indicated I₅₀ values (herbicide concentration required for 50% reduction of the ALS activity) ranging from 19.11 to 217.45μM for tribenuron, whereas the respective value for the S population was 1.17μM. All populations were susceptible to MCPA at the recommended rate. These results strongly support that cross-resistance of 9 S. arvensis populations was due a point mutation of the als gene, which resulted in a less sensitive ALS enzyme.