Structure and mechanism of inhibition of plant acetohydroxyacid synthase

First case of evolved herbicide resistance in the holoparasite sunflower broomrape, Orobanche cumana Wallr

The development and commercialisation of sunflower varieties tolerant to acetolactate synthase (ALS)-inhibiting herbicides some 20 years ago provided farmers with an alternative method for the cost-effective control of Orobanche cumana. In 2020, however, two independent sunflower broomrape populations from Drama (GR-DRA) and Orestiada (GR-ORE), Greece, were reported to be heavily infested with O. cumana after application of the ALS-inhibiting herbicide imazamox. Here we have investigated the race of GR-DRA and GR-ORE and determined the basis of resistance to imazamox in the two Greek O. cumana samples. Using a set of five diagnostic sunflower varieties characterised by different resistant genes with respect to O. cumana infestation, we have clearly established that the GR-ORE and GR-DRA populations belong to the invasive broomrape races G and G+, respectively. Live underground tubercles and emerged shoots were identified at the recommended field rate of imazamox for GR-DRA and GR-ORE but not for two other standard sensitive populations in a whole plant dose response test using two different herbicide-tolerant sunflower hybrids as hosts. Sequencing of the ALS gene identified an alanine 205 to aspartate mutation in all GR-ORE samples. Most GR-DRA tubercles were characterised by a second serine 653 to asparagine ALS mutation whilst a few GR-DRA individuals contained the A205D mutation. Mutations at ALS codons 205 and 653 are known to impact on the binding and efficacy of imazamox and other imidazolinone herbicides. The knowledge generated here will be important for tracking and managing broomrape resistance to ALS-inhibiting herbicides in sunflower growing regions.

Pro197Ser and the new Trp574Leu mutations together with enhanced metabolism contribute to cross-resistance to ALS inhibiting herbicides in Sinapis alba

White mustard, (Sinapis alba), a problematic broadleaf weed in many Mediterranean countries in arable fields has been detected as resistant to tribenuron-methyl in Tunisia. Greenhouse and laboratory studies were conducted to characterize Target-Site Resistance (TSR) and the Non-Target Site Resistance (NTSR) mechanisms in two suspected white mustard biotypes. Herbicide dose-response experiments confirmed that the two S. alba biotypes were resistant to four dissimilar acetolactate synthase (ALS)-pinhibiting herbicide chemistries indicating the presence of cross-resistance mechanisms. The highest resistance factor (>144) was attributed to tribenuron-methyl herbicide and both R populations survived up to 64-fold the recommended field dose (18.7 g ai ha-1). In this study, the metabolism experiments with malathion (a cytochrome P450 inhibitor) showed that malathion reduced resistance to tribenuron-methyl and imazamox in both populations, indicating that P450 may be involved in the resistance. Sequence analysis of the ALS gene detected target site mutations in the two R biotypes, with amino acid substitutions Trp574Leu, the first report for the species, and Pro197Ser. Molecular docking analysis showed that ALSPro197Ser enzyme cannot properly bind to tribenuron-methyl's aromatic ring due to a reduction in the number of hydrogen bonds, while imazamox can still bind. However, Trp574Leu can weaken the binding affinity between the mutated ALS enzyme and both herbicides with the loss of crucial interactions. This investigation provides substantial evidence for the risk of evolving multiple resistance in S. alba to auxin herbicides while deciphering the TSR and NTSR mechanisms conferring cross resistance to ALS inhibitors.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

Host-induced gene silencing in wild apple germplasm Malus hupehensis confers resistance to the fungal pathogen Botryosphaeria dothidea

Host-induced gene silencing (HIGS) is an inherent mechanism of plant resistance to fungal pathogens, resulting from cross-kingdom RNA interference (RNAi) mediated by small RNAs (sRNAs) delivered from plants into invading fungi. Introducing artificial sRNA precursors into crops can trigger HIGS of selected fungal genes, and thus has potential applications in agricultural disease control. To investigate the HIGS of apple (Malus sp.) during the interaction with Botryosphaeria dothidea, the pathogenic fungus causing apple ring rot disease, we evaluated whether apple miRNAs can be transported into and target genes in B. dothidea. Indeed, miR159a from Malus hupehensis, a wild apple germplasm with B. dothidea resistance, silenced the fungal sugar transporter gene BdSTP. The accumulation of miR159a in extracellular vesicles (EVs) of both infected M. hupehensis and invading B. dothidea suggests that this miRNA of the host is transported into the fungus via the EV pathway. Knockout of BdSTP caused defects in fungal growth and proliferation, whereas knockin of a miR159a-insensitive version of BdSTP resulted in increased pathogenicity. Inhibition of miR159a in M. hupehensis substantially enhanced plant sensitivity to B. dothidea, indicating miR159a-mediated HIGS against BdSTP being integral to apple immunity. Introducing artificial sRNA precursors targeting BdSTP and BdALS, an acetolactate synthase gene, into M. hupehensis revealed that double-stranded RNAs were more potent than engineered MIRNAs in triggering HIGS alternative to those natural of apple and inhibiting infection. These results provide preliminary evidence for cross-kingdom RNAi in the apple-B. dothidea interaction and establish HIGS as a potential disease control strategy in apple.

Generation and characterisation of an Arabidopsis thaliana f3h/fls1/ans triple mutant that accumulates eriodictyol derivatives

BACKGROUND

Flavonoids are plant specialised metabolites, which derive from phenylalanine and acetate metabolism. They possess a variety of beneficial characteristics for plants and humans. Several modification steps in the synthesis of tricyclic flavonoids cause for the amazing diversity of flavonoids in plants. The 2-oxoglutarate-dependent dioxygenases (2-ODDs) flavanone 3-hydroxylase (F3H, synonym FHT), flavonol synthase (FLS) and anthocyanidin synthase (ANS, synonym leucoanthocyanidin dioxygenase (LDOX)), catalyse oxidative modifications to the central C ring. They are highly similar and have been shown to catalyse, at least in part, each other's reactions. FLS and ANS have been identified as bifunctional enzymes in many species, including Arabidopsis thaliana, stressing the capability of plants to bypass missing or mutated reaction steps on the way to flavonoid production. However, little is known about such bypass reactions and the flavonoid composition of plants lacking all three central flavonoid 2-ODDs.

RESULTS

To address this issue, we generated a f3h/fls1/ans mutant, as well as the corresponding double mutants and investigated the flavonoid composition of this mutant collection. The f3h/fls1/ans mutant was further characterised at the genomic level by analysis of a nanopore DNA sequencing generated genome sequence assembly and at the transcriptomic level by RNA-Seq analysis. The mutant collection established, including the novel double mutants f3h/fls1 and f3h/ans, was used to validate and analyse the multifunctionalities of F3H, FLS1, and ANS in planta. Metabolite analyses revealed the accumulation of eriodictyol and additional glycosylated derivatives in mutants carrying the f3h mutant allele, resulting from the conversion of naringenin to eriodictyol by flavonoid 3'-hydroxylase (F3'H) activity.

CONCLUSIONS

We describe the in planta multifunctionality of the three central flavonoid 2-ODDs from A. thaliana and identify a bypass in the f3h/fls1/ans triple mutant that leads to the formation of eriodictyol derivatives. As (homo-)eriodictyols are known as bitter taste maskers, the annotated eriodictyol (derivatives) and in particular the observations made on their in planta production, could provide valuable insights for the creation of novel food supplements.

Prevalence, spatial structure and evolution of resistance to acetolactate-synthase (ALS) inhibitors and 2,4-D in the major weed Papaver rhoeas (L.) assessed using a massive, country-wide sampling

BACKGROUND

Corn poppy (Papaver rhoeas) is the most damaging broadleaf weed in France. Massively parallel amplicon sequencing was used to investigate the prevalence, mode of evolution and spread of resistance-endowing ALS alleles in 422 populations randomly sampled throughout poppy's range in France. Bioassays were used to detect resistance to the synthetic auxin 2,4-D in 43 of these populations.

RESULTS

A total of 21 100 plants were analysed and 24 mutant ALS alleles carrying an amino-acid substitution involved or potentially involved in resistance were identified. The vast majority (97.6%) of the substitutions occurred at codon Pro197, where all six possible single-nucleotide non-synonymous substitutions plus four double-nucleotide substitutions were identified. Changes observed in the enzymatic properties of the mutant ALS isoforms could not explain the differences in prevalence among the corresponding alleles. Sequence read analysis showed that mutant ALS alleles had multiple, independent evolutionary origins, and could have evolved several times independently within an area of a few kilometres. Finally, 2,4-D resistance was associated with mutant ALS alleles in individual plants in one third of the populations assayed.

CONCLUSION

The intricate geographical mosaic of mutant ALS alleles observed is the likely result of the combination of huge population sizes, multiple independent mutation events and human-mediated spread of resistance. Our work highlights the ability of poppy populations and individual plants to accumulate different ALS alleles and as yet unknown mechanisms conferring resistance to synthetic auxins. This does not bode well for the continued use of chemical herbicides to control poppy. © 2023 Society of Chemical Industry.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

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

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

Discovery and Mode-of-Action Characterization of a New Class of Acetolactate Synthase-Inhibiting Herbicides

Herbicides are effective tools to manage weeds and enable food production and sustainable agriculture. Corteva Agriscience R&D has recently discovered new diphenyl-ether compounds displaying excellent postemergent efficacy on important weed species along with corn safety. Here, we describe the chemistry, biology, biochemistry, and computational modeling research that led to the discovery and elucidation of the primary mode of action for these compounds. The target protein was found to be acetolactate synthase (ALS), a key enzyme in the biosynthesis of branched chain amino acids (valine, leucine, and isoleucine). While weed resistance evolution to ALS herbicides is widespread, the molecular interaction of the diphenyl-ether compounds at the active site of the ALS enzyme differs significantly from that of some commercial ALS inhibitors. The unique biochemical profile of these molecules along with their excellent herbicidal activity and corn selectivity make them a noteworthy development in the pursuit of novel, safe, and sustainable weed control solutions.

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.

A high diversity of non-target site resistance mechanisms to acetolactate-synthase (ALS) inhibiting herbicides has evolved within and among field populations of common ragweed (Ambrosia artemisiifolia L.)

BACKGROUND

Non-target site resistance (NTSR) to herbicides is a polygenic trait that threatens the chemical control of agricultural weeds. NTSR involves differential regulation of plant secondary metabolism pathways, but its precise genetic determinisms remain fairly unclear. Full-transcriptome sequencing had previously been implemented to identify NTSR genes. However, this approach had generally been applied to a single weed population, limiting our insight into the diversity of NTSR mechanisms. Here, we sought to explore the diversity of NTSR mechanisms in common ragweed (Ambrosia artemisiifolia L.) by investigating six field populations from different French regions where NTSR to acetolactate-synthase-inhibiting herbicides had evolved.

RESULTS

A de novo transcriptome assembly (51,242 contigs, 80.2% completeness) was generated as a reference to seek genes differentially expressed between sensitive and resistant plants from the six populations. Overall, 4,609 constitutively differentially expressed genes were identified, of which none were common to all populations, and only 197 were shared by several populations. Similarly, population-specific transcriptomic response was observed when investigating early herbicide response. Gene ontology enrichment analysis highlighted the involvement of stress response and regulatory pathways, before and after treatment. The expression of 121 candidate constitutive NTSR genes including CYP71, CYP72, CYP94, oxidoreductase, ABC transporters, gluco and glycosyltransferases was measured in 220 phenotyped plants. Differential expression was validated in at least one ragweed population for 28 candidate genes. We investigated whether expression patterns at some combinations of candidate genes could predict phenotype. Within populations, prediction accuracy decreased when applied to an additional, independent plant sampling. Overall, a wide variety of genes linked to NTSR was identified within and among ragweed populations, of which only a subset was captured in our experiments.

CONCLUSION

Our results highlight the complexity and the diversity of NTSR mechanisms that can evolve in a weed species in response to herbicide selective pressure. They strongly point to a non-redundant, population-specific evolution of NTSR to ALS inhibitors in ragweed. It also alerts on the potential of common ragweed for rapid adaptation to drastic environmental or human-driven selective pressures.

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.

Enhanced metabolic ability enabled wild panicgrass (Panicum miliaceum L. var. ruderale kit.) resistance to ALS inhibitor herbicide

Wild panicgrass (Panicum miliaceum L. var. ruderale kit.) is an annual grass weed that primarily occurs in maize fields. Nicosulfuron is a widely used selective herbicide that effectively controls gramineous weeds in maize fields. However, owing to its long-term and extensive application, the control of P. miliaceum has been substantially reduced. The objective of this study was to determine the resistance pattern to ALS inhibitors in P. miliaceum and investigate the underlying resistance mechanisms. These are important for guiding the prevention and eradication of resistant weeds. Whole plant bioassays showed P. miliaceum had evolved high levels of resistance to nicosulfuron and multiple resistance to atrazine and mesotrione. The ALS gene sequence results indicated the absence of mutations in the resistant population. Additionally, there was no significant difference found in the inhibition rate of the ALS enzyme activity (I50) between the resistant and sensitive populations. Following the application of malathion the resistant P. miliaceum population became more sensitive to nicosulfuron. At 96 h after application of nicosulfuron, glutathione-S-transferase activity in the resistant population was significantly higher than that in the susceptible population. The study reveals that the main cause of resistance to ALS inhibitor herbicide in P. miliaceum is likely increased metabolism of herbicides. These findings may assist in devising effective strategies for preventing and eliminating resistant P. miliaceum.

Computational molecular explanation of Soybean AHAS resistance from P197S mutation

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

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.

Comparative proteomic and physiological analyses reveal tribenuron-methyl phytotoxicity and nontarget-site resistance mechanisms in Brassica napus

The application of herbicides is the most effective strategy for weed control and the development of herbicide-resistant crops will facilitate the weed management. The acetolactate synthase-inhibiting herbicide, tribenuron-methyl (TBM), is broadly used for weed control. However, its application in rapeseed field is restricted since rapeseed is sensitive to TBM. Herein, an integrated study of cytological, physiological and proteomic analysis of the TBM-resistant rapeseed mutant M342 and its wild-type (WT) plants was conducted. After TBM spraying, M342 showed improved tolerance to TBM, and proteins implicated in non-target-site resistance (NTSR) to herbicides had a significantly higher level in M342 as compared with the WT. Differentially accumulated proteins (DAPs) between these two genotypes were enriched in glutathione metabolism and oxidoreduction coenzyme metabolic process, which protected the mutant from oxidative stress triggered by TBM. Important DAPs related to stress or defence response were up-accumulated in M342 regardless of the TBM treatment, which might serve as the constitutive part of NTSR to TBM. These results provide new clues for further exploration of the NTSR mechanism in plants and establish a theoretical basis for the development of herbicide-resistant crops.

Target and nontarget mechanisms of AHAS inhibitor cross-resistance patterns in Cyperus difformis

Weed resistance to acetohydroxyacid synthase (AHAS) inhibiting herbicides has been a critical issue for rice growers worldwide since the early 1990's. In California, resistance to bensulfuron-methyl was first detected in Cyperus difformis in 1993. Since then, populations of most major weeds of rice in California have been reported to show resistance to at least one AHAS inhibitor. We sought to describe the magnitude and mechanisms of AHAS inhibitor cross-resistance in California populations of C. difformis. Sixty-two populations were collected and screened for cross-resistance to bensulfuron-methyl (BEN), halosulfuron-methyl (HAL), bispyribac‑sodium (BIS), and penoxsulam (PEN), revealing six major patterns of cross-resistance. Representative C. difformis populations from each cross-resistance pattern were then subjected to dose-response, cytochrome P450 inhibition, AHAS gene sequencing, and metabolic studies with the same herbicides as in the screening. Dose-response confirmed the detected resistances in the representative populations, and suggested that the majority of observed resistance was dose-dependent. Cytochrome P450 inhibition via malathion revealed evidence of increased metabolic activity in resistant populations to BEN, BIS, and PEN. AHAS gene sequencing revealed amino acid substitutions in five of six populations: R3 (Pro197-Ser), R4 (Pro97-His), R10 (Asp376), R41 (Ala122-Asn), and R18 (Trp574-Leu). Metabolic studies confirmed evidence of increased activity of cytochrome P450s in all populations. Metabolic BEN and HAL analysis did not yield similar results to malathion inhibition, suggesting different P450's or other pathways. Taken together, the results of the studies confirm the complexity of AHAS inhibitor cross-resistance in C. difformis, and the presence of both target-site and metabolic resistance in most of the representative populations underscores the importance of proper herbicide selection, rotation, and scouting in fields.

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.

Multiple resistance of Echinochloa phyllopogon to synthetic auxin, ALS-, and ACCase-inhibiting herbicides in Northeast China

Echinochloa phyllopogon is a self-pollinating allotetraploid weed and a serious threat to global rice production. One sensitive and three multiple-resistant populations collected from two provinces of Northeast China were used to analyze the mechanism of multiple resistance of E. phyllopogon to penoxsulam, metamifop, and quinclorac. Compared with the sensitive population LN12, LN1 showed higher resistance to these three herbicides; LN24 showed medium resistance to penoxsulam and metamifop and higher resistance to quinclorac (274-fold); HLJ4 showed low resistance to penoxsulam and high resistance to metamifop and quinclorac. Target sequence analysis showed no mutations in acetolactate synthase or acetyl-CoA carboxylase genes. In-vitro enzyme activity analysis showed that the activity of the target enzyme of multiple herbicide-resistant populations was similar to that of the sensitive population. The P450 inhibitor, malathion, noticeably increased the sensitivity of LN1, LN24, and HLJ4 to penoxsulam, LN1 to metamifop, and HLJ4 to quinclorac. Under all four treatments, the GSTs activities of resistant and sensitive populations showed an increasing trend from day 1 to day 5, but the sensitivity and activity of GSTs were higher in the multiple-resistant population than that in the sensitive population LN12. This study identified the development of multiple-resistant E. phyllopogon populations that pose a serious threat to rice production in rice fields in Northeast China, preliminarily confirming that multiple-resistance was likely due to non-target-site resistance mechanisms. These populations of E. phyllopogon are likely to be more difficult to control.

Functional and comparative analysis of THI1 gene in grasses with a focus on sugarcane

De novo synthesis of thiamine (vitamin B1) in plants depends on the action of thiamine thiazole synthase, which synthesizes the thiazole ring, and is encoded by the THI1 gene. Here, we investigated the evolution and diversity of THI1 in Poaceae, where C4 and C3 photosynthetic plants co-evolved. An ancestral duplication of THI1 is observed in Panicoideae that remains in many modern monocots, including sugarcane. In addition to the two sugarcane copies (ScTHI1-1 and ScTHI1-2), we identified ScTHI1-2 alleles showing differences in their sequence, indicating divergence between ScTHI1-2a and ScTHI1-2b. Such variations are observed only in the Saccharum complex, corroborating the phylogeny. At least five THI1 genomic environments were found in Poaceae, two in sugarcane, M. sinensis, and S. bicolor. The THI1 promoter in Poaceae is highly conserved at 300 bp upstream of the start codon ATG and has cis-regulatory elements that putatively bind to transcription factors associated with development, growth, development and biological rhythms. An experiment set to compare gene expression levels in different tissues across the sugarcane R570 life cycle showed that ScTHI1-1 was expressed mainly in leaves regardless of age. Furthermore, ScTHI1 displayed relatively high expression levels in meristem and culm, which varied with the plant age. Finally, yeast complementation studies with THI4-defective strain demonstrate that only ScTHI1-1 and ScTHI1-2b isoforms can partially restore thiamine auxotrophy, albeit at a low frequency. Taken together, the present work supports the existence of multiple origins of THI1 harboring genomic regions in Poaceae with predicted functional redundancy. In addition, it questions the contribution of the levels of the thiazole ring in C4 photosynthetic plant tissues or potentially the relevance of the THI1 protein activity.

Crystal Structure of the Commercial Herbicide, Amidosulfuron, in Complex with Arabidopsis thaliana Acetohydroxyacid Synthase

Amidosulfuron (AS) is from the commercial sulfonylurea herbicide family. It is highly effective against dicot broad-leaf weeds. This herbicide targets acetohydroxyacid synthase (AHAS), the first enzyme in the branched chain amino acid biosynthesis pathway. Here, we have determined the crystal structure of AS in complex with wildtype Arabidopsis thaliana AHAS (AtAHAS) and with the resistance mutant, S653T. In both structures, the cofactor, ThDP, is modified to a peracetate adduct, consistent with time-dependent accumulative inhibition. Compared to other AHAS-inhibiting herbicides of the sulfonylurea family, AS lacks a second aromatic ring. The replacement is an aryl sulfonyl group with a reduced number of interactions with the enzyme and relatively low affinity (K_i = 4.2 μM vs low nM when two heteroaromatic rings are present). This study shows that effective herbicides can have a relatively high K_i for plant AHAS but can still be a potent herbicide provided accumulative inhibition also occurs.

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

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

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

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

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

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

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

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

Thermostable and O2-Insensitive Pyruvate Decarboxylases from Thermoacidophilic Archaea Catalyzing the Production of Acetaldehyde

Simple Summary

Pyruvate decarboxylase (PDC) is a key enzyme involved in ethanol fermentation, a process for the production of biofuels. Thermostable and oxygen-stable PDC activity is highly desirable for biotechnological applications at high temperatures. The enzymes from the thermoacidophiles Saccharolobus (formerly Sulfolobus) solfataricus (Ss, T_opt = 80 °C) and Sulfolobus acidocaldarius (Sa, T_opt = 80 °C) were purified and characterized, and their biophysical and biochemical properties were determined comparatively. The purified enzymes were CoA-dependent and thermostable. There was no loss of activity in the presence of oxygen. In conclusion, both thermostable SsPDC and SaPDC catalyze the CoA-dependent production of acetaldehyde from pyruvate in the presence of oxygen.

Abstract

Pyruvate decarboxylase (PDC) is a key enzyme involved in ethanol fermentation, and it catalyzes the decarboxylation of pyruvate to acetaldehyde and CO₂. Bifunctional PORs/PDCs that also have additional pyruvate:ferredoxin oxidoreductase (POR) activity are found in hyperthermophiles, and they are mostly oxygen-sensitive and CoA-dependent. Thermostable and oxygen-stable PDC activity is highly desirable for biotechnological applications. The enzymes from the thermoacidophiles Saccharolobus (formerly Sulfolobus) solfataricus (Ss, T_opt = 80 °C) and Sulfolobus acidocaldarius (Sa, T_opt = 80 °C) were purified and characterized, and their biophysical and biochemical properties were determined comparatively. Both enzymes were shown to be heterodimeric, and their two subunits were determined by SDS-PAGE to be 37 ± 3 kDa and 65 ± 2 kDa, respectively. The purified enzymes from S. solfataricus and S. acidocaldarius showed both PDC and POR activities which were CoA-dependent, and they were thermostable with half-life times of 2.9 ± 1 and 1.1 ± 1 h at 80 °C, respectively. There was no loss of activity in the presence of oxygen. Optimal pH values for their PDC and POR activity were determined to be 7.9 and 8.6, respectively. In conclusion, both thermostable SsPOR/PDC and SaPOR/PDC catalyze the CoA-dependent production of acetaldehyde from pyruvate in the presence of oxygen.

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

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

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

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

Identification of acetolactate synthase resistant Amaranthus retroflexus in Ukraine

The problem of weed resistance to herbicides has become very important in the last decade and threatens to dramatically reduce the productivity and profitability of modern crop production. Herbicides – ALS inhibitors dominate among current herbicides and are used annually on large areas of sunflower, wheat, corn, soybean, and rapeseed. Also, in recent years, Clearfield seeds of sunflower, corn, canola, soybean and wheat have been sown in large areas. In recent years, there has been a sharp decrease in Amaranthus retroflexus L. control levels by imidazolinone class herbicides. Thus, the effects of herbicides with different modes of action on the development of A. retroflexus on sunflower after imidazolinone application were investigated in field research. In the conditions of the Cherkasy region of Ukraine, the biotype A. retroflexus was identified, which is resistant to the post-emergence application of herbicides - acetolactate synthase (ALS) inhibitors of the imidazolinone class – imazapyr and imazamox. Weed plants treated with imidazolinone derivatives in the maximum doses registered in Ukraine did not differ from untreated control plants. Also, in the conditions of field experiments, cross resistance of the weed biotype to herbicides – ALS inhibitors of the sulfonylurea class – foramsulfuron and iodosulfuron-methyl-sodium, thifensulfuron-methyl, tribenuron-methyl, nicosulfuron was established; and also, to the triazolinone derivative – thiencarbazone-methyl; to triazolpyrimidine derivatives – florasulam and flumetsulam. Multiple resistance of the A. retroflexus biotype to herbicides of the classes of glycine derivatives – glyphosate, phenoxycarboxylates – 2,4-D, benzoic acid – dicamba has not been established; compositions of dicamba with triketone – topramesone; diphenyl ethers – aclonifen; pyridine carboxylates – clopyralid, picloram and aminopyralid. It was shown for the first time that herbicide compositions with selected nutrients (ammonium pool) can increase the level of effectiveness of controlling resistant weed biotypes. Thus, the addition of ammonium sulfate increases the effectiveness of controlling ALS-resistant A. retroflexus with herbicides – a derivative of benzoic acid (dianate) and a derivative of benzoic acid with a 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor (stellar – dicamba + topramesone). Thus, the A. retroflexus biotype resistant to ALS-herbicides of the imidazolinone class was identified for the first time in Ukraine, which is cross-resistant to other ALS-inhibitors of the sulfonylureas, triazolinones, and triazolpyrimidine classes. Multiple resistance of A. retroflexus to herbicides of the classes of glycine derivatives – glyphosate; phenoxycarboxylates – 2,4-D; benzoic acid – dicamba, triketones – topramesone; diphenyl ethers – aclonifen; pyridine carboxylates – clopyralid, picloram and aminopyralid has not been established. The identification of a highly harmful weed species resistant to widely used herbicides – ALS inhibitors in the central part of the "grain belt" of Ukraine requires a significant revision of the principles of crop rotation formation and ways of controlling weeds in the country in order to maintain high levels of profitability and productivity of agrophytocenoses.

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.

Tribenuron-methyl-resistant Descurainia sophia L. exhibits negative cross-resistance to imazethapyr conferred by a Pro197Ser mutation in acetolactate synthase and reduced metabolism

BACKGROUND

Descurainia sophia L. is one of the most notorious weeds infesting winter wheat in China. Mutations at Pro197 in acetolactate synthase (ALS) results in resistance of D. sophia to tribenuron-methyl and cross-resistance to many ALS inhibitors. Negative cross-resistance to imazethapyr was observed in tribenuron-methyl-resistant (TR) D. sophia with the Pro197Ser mutation in a previous study. In the present research, another TR D. sophia with the Pro197Ser mutation was obtained. To explore the mechanisms of negative cross-resistance, the ALS sensitivity, the absorption and metabolism of imazethapyr in tribenuron-methyl-susceptible (TS) and TR D. sophia were studied.

RESULTS

The TR D. sophia population with the Pro197Ser mutation (pHB23) displayed negative cross-resistance to imazethapyr and no cross-resistance to imazamox and imazapic. In contrast, TR D. sophia populations with other Pro197 mutations had no or low resistance to imazethapyr. The ALS in the pHB23 population was more susceptible to imazethapyr than that in the TS population. There was no difference in the absorption of imazethapyr, imazamox, and imazapic between TS and pHB23 plants. However, the metabolism of imazethapyr in TS D. sophia was faster than that in pHB23 plants up to 1 week after treatment. There was no significant difference in the metabolism of imazamox and imazapic between TS and pHB23 plants.

CONCLUSION

The TR D. sophia population with the Pro197Ser mutation exhibited negative cross-resistance to imazethapyr, which was likely due to reduced metabolism and increased sensitivity of ALS to imazethapyr. © 2021 Society of Chemical Industry.

Lab meets field: Accelerated selection and field monitoring concur that non-target-site-based resistance evolves first in the dicotyledonous, allergenic weed Ambrosia artemisiifolia

Assessing weed capacity to evolve herbicide resistance before resistance occurs in the field is of major interest for chemical weed control. We used herbicide selection followed by controlled crosses to provoke accelerated evolution of resistance to imazamox (imidazolinones) and tribenuron (sulfonyurea), two acetolactate-synthase (ALS) inhibitors targeting Ambrosia artemisiifolia. In natural populations with no herbicide application records, some plants were initially resistant to metsulfuron (sulfonylurea), a cereal herbicide. Non-target-site-based resistance (NTSR) to metsulfuron was substantially increased from these plants within two generations. NTSR to imazamox and/or tribenuron emerged in metsulfuron-selected G1 progenies and was strongly reinforced in G2 progenies selected by imazamox or tribenuron. NTSR to the herbicides assayed was endowed by partly overlapping and partly specific pathways. Herbicide sensitivity bioassays conducted over 62 ALS-inhibitor-sprayed fields identified emerging resistance to imazamox and/or tribenuron in 14 A. artemisiifolia populations. Only NTSR was detected in 13 of these populations. In the last population, NTSR was present together with a mutant, herbicide-resistant ALS allele bearing an Ala-205-Thr substitution. NTSR was thus by far the predominant type of resistance to ALS inhibitors in France. This confirmed accelerated selection results and demonstrated the relevance of this approach to anticipate resistance evolution in a dicotyledonous weed.

Computer-Assisted Improvement of Sulfonylureas with Antifungal Properties and Limited Herbicidal Activity: Potential Application in Forage Conservation

This work reports studies at the molecular level of a series of modified sulfonylureas to determine the chemophoric sites responsible for their antifungal and herbicidal activities. For forage conservation, high antifungal potency and low phytotoxicity are required. A molecular modeling study based on multivariate image analysis applied to quantitative structure-activity relationship (MIA-QSAR) was performed to model these properties, as well as to guide the design of new agrochemical candidates. As a result, the MIA-QSAR models were reliable, robust, and predictive; for antifungal activity, the averages of the main validation parameters were r² = 0.936, q² = 0.741, and r²_pred = 0.720, and for herbicidal activity, the model was very predictive (r²_pred = 0.981 and r²_m = 0.944). From the interpretation of the MIA-plots, 46 novel sulfonylureas with likely improved performance were proposed, from which 9 presented promising calculated selectivity indexes. Docking studies were performed to validate the QSAR predictions and to understand the interaction mode of the proposed ligands with the acetohydroxyacid synthase enzyme.

A122S, A205V, D376E, W574L and S653N substitutions in acetolactate synthase (ALS) from Amaranthus palmeri show different functional impacts on herbicide resistance

BACKGROUND

Amaranthus palmeri S. Watson, a problematic weed infesting summer crops in Argentina, has developed multiple herbicide resistance. Resistance to acetolactate synthase (ALS)-inhibiting herbicides is particularly common, with high-level resistance mostly caused by different mutations in the ALS enzyme. Six versions of the enzyme were identified from a resistant A. palmeri population, carrying substitutions D376E, A205V, A122S, A282D, W574L and S653N. This work aims to provide a comparative analysis of these mutants and the wild-type (WT) enzyme to fully understand the herbicide resistance. Thus, all the versions of the ALS gene from A. palmeri were heterologously expressed and purified to evaluate their kinetics and inhibitory response against imazethapyr, diclosulam, chlorimuron-ethyl, flucarbazone-sodium and bispyribac-sodium.

RESULTS

A decrease in catalytic efficiency was detected in the A205V, A122S-A282D, W574L and S653N ApALS enzymes, whereas only A205V and W574L substitutions also produced a decrease in the substrate affinity. In vitro ALS inhibition assays confirmed cross-resistance to almost all the herbicides tested, with the exception of A282D ApALS, which was as susceptible as WT ApALS. Moreover, the results confirmed that the novel substitution A122S provides cross-resistance to at least one herbicide within each of the five families of ALS inhibitors, and this property could be explained by a lower number of hydrophobic interactions between the herbicides and the mutant enzyme.

CONCLUSION

This is the first report to compare various mutations in vitro from A. palmeri ALS. Our data contribute to understanding the impacts of herbicide resistance in this species. © 2021 Society of Chemical Industry.

Acetolactate synthases regulatory subunit and catalytic subunit genes VdILVs are involved in BCAA biosynthesis, microscletotial and conidial formation and virulence in Verticillium dahliae

Acetolactate synthase (AHAS) catalyses the first common step in the biosynthesis pathways of three branched-chain amino acids (BCAAs) of valine, isoleucine and leucine. Here, we characterized one regulatory subunit (VdILV6) and three catalytic subunits (VdILV2A, VdILV2B and VdILV2C) of AHAS from the important cotton Verticillium wilt fungus Verticillium dahliae. Phenotypic analysis showed that VdILV6 knockout mutants were auxotrophic for valine and isoleucine and were defective in conidial morphogenesis, hypha penetration and virulence to cotton, and lost ability of microscletotial formation. The growth of single catalytic subunit gene knockout mutants were significantly inhibited by leucine at higher concentration and single catalytic subunit gene knockout mutants showed significantly reduced virulence to cotton. VdILV2B knockout also led to obviously reduced microscletotial formation and conidial production, VdILV2C knockout led to reduced conidial production. Further studies suggested that both feedback inhibition by leucine and the inhibition by AHAS inhibiting herbicides of tribenuron and bispyribac resulted in significantly down-regulated expression of the four subunit VdILVs genes (VdILV2A, VdILV2B, VdILV2C and VdILV6). Any single catalytic subunit gene knockout led to reduced expression of the other three subunit genes, whereas VdILV6 knckout induced increased expression of the three catalytic subunit genes. VdILV2B, VdILV2C and VdILV6 knockout resulted in increased expression of VdCPC1 regulator gene of the cross-pathway control of amino acid biosynthesis. Taken together, these results indicate multiple roles of four VdILVs genes in the biosynthesis of BCAAs, virulence, fungal growth and development in the filamentous fungi V. dahliae.

Increased circulating butyrate and ursodeoxycholate during probiotic intervention in humans with type 2 diabetes

Background

An increasing body of evidence implicates the resident gut microbiota as playing a critical role in type 2 diabetes (T2D) pathogenesis. We previously reported significant improvement in postprandial glucose control in human participants with T2D following 12-week administration of a 5-strain novel probiotic formulation (‘WBF-011’) in a double-blind, randomized, placebo controlled setting (NCT03893422). While the clinical endpoints were encouraging, additional exploratory measurements were needed in order to link the motivating mechanistic hypothesis - increased short-chain fatty acids - with markers of disease.

Results

Here we report targeted and untargeted metabolomic measurements on fasting plasma (n = 104) collected at baseline and end of intervention. Butyrate and ursodeoxycholate increased among participants randomized to WBF-011, along with compelling trends between butyrate and glycated haemoglobin (HbA1c). In vitro monoculture experiments demonstrated that the formulation’s C. butyricum strain efficiently synthesizes ursodeoxycholate from the primary bile acid chenodeoxycholate during butyrogenic growth. Untargeted metabolomics also revealed coordinated decreases in intermediates of fatty acid oxidation and bilirubin, potential secondary signatures for metabolic improvement. Finally, improvement in HbA1c was limited almost entirely to participants not using sulfonylurea drugs. We show that these drugs can inhibit growth of formulation strains in vitro.

Conclusion

To our knowledge, this is the first description of an increase in circulating butyrate or ursodeoxycholate following a probiotic intervention in humans with T2D, adding support for the possibility of a targeted microbiome-based approach to assist in the management of T2D. The efficient synthesis of UDCA by C. butyricum is also likely of interest to investigators of its use as a probiotic in other disease settings. The potential for inhibitory interaction between sulfonylurea drugs and gut microbiota should be considered carefully in the design of future studies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-021-02415-8.

Novel Mutation in the Acetohydroxyacid Synthase (AHAS), Gene Confers Imidazolinone Resistance in Chickpea Cicer arietinum L. Plants

Chickpea (Cicer arietinum L.) is an important crop in crop-rotation management in Israel. Imidazolinone herbicides have a wide spectrum of weed control, but chickpea plants are sensitive to acetohydroxyacid synthase (AHAS; also known as acetolactate synthase [ALS]) inhibitors. Using the chemical mutagen ethyl methanesulfonate (EMS), we developed a chickpea line (M2033) that is resistant to imidazolinone herbicides. A point mutation was detected in one of the two genes encoding the AHAS catalytic subunit of M2033. The transition of threonine to isoleucine at position 192 (203 according to Arabidopsis) conferred resistance of M2033 to imidazolinones, but not to other groups of AHAS inhibitors. The role of this substitution in the resistance of line M2033 was proven by genetic transformation of tobacco plants. This resistance showed a single-gene semidominant inheritance pattern. Conclusion: A novel mutation, T192I (T203I according to Arabidopsis), providing resistance to IMI herbicides but not to other groups of AHAS inhibitors, is described in the AHAS1 protein of EMS-mutagenized chickpea line M2033.

Evidence for the Application of Emerging Technologies to Accelerate Crop Improvement - A Collaborative Pipeline to Introgress Herbicide Tolerance Into Chickpea

Accelerating genetic gain in crop improvement is required to ensure improved yield and yield stability under increasingly challenging climatic conditions. This case study demonstrates the effective confluence of innovative breeding technologies within a collaborative breeding framework to develop and rapidly introgress imidazolinone Group 2 herbicide tolerance into an adapted Australian chickpea genetic background. A well-adapted, high-yielding desi cultivar PBA HatTrick was treated with ethyl methanesulfonate to generate mutations in the ACETOHYDROXYACID SYNTHASE 1 (CaAHAS1) gene. After 2 years of field screening with imidazolinone herbicide across >20 ha and controlled environment progeny screening, two selections were identified which exhibited putative herbicide tolerance. Both selections contained the same single amino acid substitution, from alanine to valine at position 205 (A205V) in the AHAS1 protein, and KASP™ markers were developed to discriminate between tolerant and intolerant genotypes. A pipeline combining conventional crossing and F2 production with accelerated single seed descent from F2:4 and marker-assisted selection at F2 rapidly introgressed the herbicide tolerance trait from one of the mutant selections, D15PAHI002, into PBA Seamer, a desi cultivar adapted to Australian cropping areas. Field evaluation of the derivatives of the D15PAHI002 × PBA Seamer cross was analyzed using a factor analytic mixed model statistical approach designed to accommodate low seed numbers resulting from accelerated single seed descent. To further accelerate trait introgression, field evaluation trials were undertaken concurrent with crop safety testing trials. In 2020, 4 years after the initial cross, an advanced line selection CBA2061, bearing acetohydroxyacid synthase (AHAS) inhibitor tolerance and agronomic and disease resistance traits comparable to parent PBA Seamer, was entered into Australian National Variety Trials as a precursor to cultivar registration. The combination of cross-institutional collaboration and the application of novel pre-breeding platforms and statistical technologies facilitated a 3-year saving compared to a traditional breeding approach. This breeding pipeline can be used as a model to accelerate genetic gain in other self-pollinating species, particularly food legumes.

Characterization of tribenuron-methyl-induced male sterility in Brassica juncea L

Significant heterosis has been documented in Brassica juncea L. that are grown as agriculturally important oilseeds, vegetables and condiments crops. Male sterility induced by chemical hybridizing agents is an important pollination control system in hybrid crop breeding. Herein, we show that tribenuron-methyl (TBM), a sulfonylurea herbicide, is an effective male gametocide in B. juncea when used at a very low dosage. In the present study, foliar application of various rates of TBM induced a significant increase in pollen sterility in B. juncea (90.57-100%). TBM-treated plants exhibited reductions in size of floral organ and yield components; however, lower dose of TBM (0.075 g a.i. ha^-1) did not cause a significant reduction in seed yield per plant. Tapetum cells of TBM-treated plants were hypertrophied and degenerated earlier, and abnormal meiosis was observed at the meiotic stage. A significant decrease of acetohydroxyacid synthase (AHAS) activities was detected in buds of plants treated with 0.10 g a.i. ha^-1 TBM, and RT-qPCR analysis showed that TBM exposure perturbed AHAS expression in small buds, which support that TBM induces male sterility in B. juncea by targeting AHAS expression. Our results suggest that TBM could be used as an efficient chemical hybridization agent in B. juncea, which has practical implications for the application of hybrid breeding in B. juncea.

A high diversity of mechanisms endows ALS-inhibiting herbicide resistance in the invasive common ragweed (Ambrosia artemisiifolia L.)

Ambrosia artemisiifolia L. (common ragweed) is a globally invasive, allergenic, troublesome arable weed. ALS-inhibiting herbicides are broadly used in Europe to control ragweed in agricultural fields. Recently, ineffective treatments were reported in France. Target site resistance (TSR), the only resistance mechanism described so far for ragweed, was sought using high-throughput genotyping-by-sequencing in 213 field populations randomly sampled based on ragweed presence. Additionally, non-target site resistance (NTSR) was sought and its prevalence compared with that of TSR in 43 additional field populations where ALS inhibitor failure was reported, using herbicide sensitivity bioassay coupled with ALS gene Sanger sequencing. Resistance was identified in 46 populations and multiple, independent resistance evolution demonstrated across France. We revealed an unsuspected diversity of ALS alleles underlying resistance (9 amino-acid substitutions involved in TSR detected across 24 populations). Remarkably, NTSR was ragweed major type of resistance to ALS inhibitors. NTSR was present in 70.5% of the resistant plants and 74.1% of the fields harbouring resistance. A variety of NTSR mechanisms endowing different resistance patterns evolved across populations. Our study provides novel data on ragweed resistance to herbicides, and emphasises that local resistance management is as important as mitigating gene flow from populations where resistance has arisen.

Functional Analysis of Keto-Acid Reductoisomerase ILVC in the Entomopathogenic Fungus Metarhizium robertsii

Ketol-acid reductoisomerase (ILVC) is the second enzyme in the branched-chain amino acid (BCAA) biosynthesis, which regulates many physiological activities in a variety of organisms from bacteria to fungi and plants. In this work, function mechanisms of ILVC in Metarhizium robertsii Metchnikoff (Hypocreales: Clavicipitaceae) were explored with site-directed mutagenesis, reductase activity assays and transcriptomics analysis. The reductase activity assays showed that ILVC from phytopathogenic fungi exhibited significantly higher activities than those from entomopathogenic fungi but lower than those from yeast. Site-directed mutagenesis and enzymatic activities of MrILVC with different active-site mutants (Arg-113, Ser-118, Asp-152, Asp-260, and Glu-264) confirmed that active sites of MrILVC are conserved with plant and bacterial ILVCs. Deleting MrilvC causes the complete failures of vegetative growth and conidial germination, feeding with branched-chain amino acids (BCAAs) recovers the fungal growth but not conidial germination, while both characteristics are restored when supplemented with yeast extract. Compared to ΔMrilvC cultured in czapek agar (CZA), plenty of genes involved in the biosynthesis of antibiotics and amino acids were up- or down-regulated in the wild type or ΔMrilvC feeding with either BCAAs or yeast extract. Further analysis showed some genes, such as catalase A, participate in mycelial growth and conidial germination was down-regulated in ΔMrilvC from CZA, revealing that MrILVC might control the fungal development by gene regulation and BCAAs or yeast extract could play partial roles of MrILVC. This study will advance our understanding of ILVC function mechanisms in fungi.

The scope of flavin-dependent reactions and processes in the model plant Arabidopsis thaliana

Flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) are utilized as coenzymes in many biochemical reduction-oxidation reactions owing to the ability of the tricyclic isoalloxazine ring system to employ the oxidized, radical and reduced state. We have analyzed the genome of Arabidopsis thaliana to establish an inventory of genes encoding flavin-dependent enzymes (flavoenzymes) as a basis to explore the range of flavin-dependent biochemical reactions that occur in this model plant. Expectedly, flavoenzymes catalyze many pivotal reactions in primary catabolism, which are connected to the degradation of basic metabolites, such as fatty and amino acids as well as carbohydrates and purines. On the other hand, flavoenzymes play diverse roles in anabolic reactions most notably the biosynthesis of amino acids as well as the biosynthesis of pyrimidines and sterols. Importantly, the role of flavoenzymes goes much beyond these basic reactions and extends into pathways that are equally crucial for plant life, for example the production of natural products. In this context, we outline the participation of flavoenzymes in the biosynthesis and maintenance of cofactors, coenzymes and accessory plant pigments (e. g. carotenoids) as well as phytohormones. Moreover, several multigene families have emerged as important components of plant immunity, for example the family of berberine bridge enzyme-like enzymes, flavin-dependent monooxygenases and NADPH oxidases. Furthermore, the versatility of flavoenzymes is highlighted by their role in reactions leading to tRNA-modifications, chromatin regulation and cellular redox homeostasis. The favorable photochemical properties of the flavin chromophore are exploited by photoreceptors to govern crucial processes of plant adaptation and development. Finally, a sequence- and structure-based approach was undertaken to gain insight into the catalytic role of uncharacterized flavoenzymes indicating their involvement in unknown biochemical reactions and pathways in A. thaliana.

CRISPR/Cas9-Mediated Multi-Allelic Gene Targeting in Sugarcane Confers Herbicide Tolerance

Sugarcane is the source of 80% of the sugar and 26% of the bioethanol produced globally. However, its complex, highly polyploid genome (2n = 100 - 120) impedes crop improvement. Here, we report efficient and reproducible gene targeting (GT) in sugarcane, enabling precise co-editing of multiple alleles via template-mediated and homology-directed repair (HDR) of DNA double strand breaks induced by the programmable nuclease CRISPR/Cas9. The evaluation of 146 independently transformed plants from five independent experiments revealed a targeted nucleotide replacement that resulted in both targeted amino acid substitutions W574L and S653I in the acetolactate synthase (ALS) in 11 lines in addition to single, targeted amino acid substitutions W574L or S653I in 25 or 18 lines, respectively. Co-editing of up to three ALS copies/alleles that confer herbicide tolerance was confirmed by Sanger sequencing of cloned long polymerase chain reaction (PCR) amplicons. This work will enable crop improvement by conversion of inferior alleles to superior alleles through targeted nucleotide substitutions.

Harzianic Acid from Trichoderma afroharzianum Is a Natural Product Inhibitor of Acetohydroxyacid Synthase

Acetohydroxyacid synthase (AHAS) is the first enzyme in the branched-chain amino acid biosynthetic pathway and is a validated target for herbicide and fungicide development. Here we report harzianic acid (HA, 1) produced by the biocontrol fungus Trichoderma afroharzianum t-22 (Tht22) as a natural product inhibitor of AHAS. The biosynthetic pathway of HA was elucidated with heterologous reconstitution. Guided by a putative self-resistance enzyme in the genome, HA was biochemically demonstrated to be a selective inhibitor of fungal AHAS, including those from phytopathogenic fungi. In addition, HA can inhibit a common resistant variant of AHAS in which the active site proline is mutated. Structural analysis of AHAS complexed with HA revealed the molecular basis of competitive inhibition, which differs from all known commercial AHAS inhibitors. The alternative binding mode also rationalizes the selectivity of HA, as well as effectiveness toward resistant mutants. A proposed role of HA biosynthesis by Tht22 in the rhizosphere is discussed based on the data.

Population structure and evolution of resistance to acetolactate synthase (ALS)-inhibitors in Amaranthus tuberculatus in Italy

BACKGROUND

Before 2010, Amaranthus tuberculatus (Moq.) J. D. Sauer was barely known to farmers and stakeholders in Italy. Since then, several populations resistant to acetolactate synthase (ALS)-inhibiting herbicides have been collected. In most populations, a known target site resistance-endowing mutation was found, a Trp to Leu substitution at position 574 of the ALS gene, but it was unclear whether they had evolved resistance independently or not. The aims of the work were (i) to elucidate the population structure of Italian ALS-resistant A. tuberculatus populations, and (ii) to analyze the ALS haplotypes of the various populations to determine whether resistance arose multiple times independently.

RESULTS

In order to determine the population structure of eight A. tuberculatus populations, eight previously described microsatellite loci were used. Two ancestors were found: three populations derived from one, and five from the other. In the 4-kb ALS region of the genome, including the 2-kb coding region, 389 single nucleotide polymorphisms were found. In silico haplotype estimation was used to reconstruct the sequence of three distinct haplotypes carrying the Trp574Leu mutation. In addition, no mutation was found in 83% of plants of a single population.

CONCLUSIONS

(i) Resistance must have arisen independently at least three times; (ii) at least one population was already resistant to ALS inhibitors when introduced in Italy; (iii) a single haplotype with a Trp574Leu mutation was shared among six populations, probably because of broad seed dispersal; and (iv) one population likely evolved nontarget site ALS inhibitors resistance. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Electrophoretic banding patterns of protein induced by pinoxaden, tribenuron-methyl, and pyroxsulam herbicides in wheat leaves (Triticum aestivum L.)

Herbicides are the most effective tool against weed flora in cereal crops that help to maintain and increase crop yields. This investigation was conducted in the winter season of 2018 to study the stress effect of three post-emergence herbicides including pinoxaden, tribenuron-methyl, and pyroxsulam on the biochemical changes at the molecular cell level of wheat. These herbicides were applied either lonely with a rate of 0.45 L.ha^-1, 22.5 gm.ha^-1, and 0.16 Ib a.i/A, respectively, or in combinations together on three Egyptian varieties of bread wheat known as Misr 1, Giza17 1, and Gemmiza 11. Firstly, the abovementioned herbicides were used at the recommended and half recommended doses with their combinations for these varieties to investigate DNA-protein linkage as a signal effect of herbicides at the molecular cell level.Our data showed that the treatment of wheat varieties with the tested herbicides induced new bands with low and high molecular weights of 37.49, 40.08, 146.55, and 147.23 KDa with relative mobility of 0.1574, 0.1603, 0.2166, and 0.2168, respectively. These bands were not presented in the control treatment, suggesting that it might be used as a biochemical marker for plant defense genes. Meanwhile, the control treatment exhibited only five or six bands in the three varieties. However, the tested varieties showed that the same number of bands, the molecular weights of bands, and their relative mobility were significantly varied between the single and the combinations treatment of herbicides. The best treatment was achieved by the combination between pinoxaden and tribenuron-methyl at a recommended dose which induced a large number of protein bands compared to the control treatment on the wheat variety cv. Misr 1, which gave one band with low molecular weight 71.44 KDa at R_f 0.1854 and other with the highest molecular weight 147.23 KDa at R_f 0.2168, compared to the control treatment.

Selectivity of metsulfuron applied to soybean before sowing in different intervals and soils

This work aimed to evaluate the selectivity of the herbicide metsulfuron applied at different times on the development of soybeans grown in soils with different characteristics. The experiment was conducted in a randomized block design, in a factorial scheme (4 x 4), with four replicates. Factor A was application time (0, 15, 30, and 45 days before sowing, DBS) and factor B was soil type (Erechim, Itaqui, Piratini, and Santa Maria). Soybean plants cultivated in the Erechim soil showed moderate phytotoxicity, with greater damage to the leaf area and plant dry matter, mainly after application at 30 DBS. Those cultivated in Itaqui soil showed gradual phytotoxicity between 14 and 28 days after emergence (DAE). Soybean plants grown in the Piratini and Santa Maria soils showed the highest phytotoxicity and photosynthetic reduction, mainly at 15 and 0 DBS. Metsulfuron application at 45 DBS caused reduced plant growth by up to 40%, and reduced shoot development (30%) in soybean plants grown in Piratini and Santa Maria soils, respectively. There were gradual changes in phytotoxicity and the morphophysiological traits of soybean plants exposed to the residual effect of metsulfuron in different soils, which indicates that soybeans should be sown more than 45 days after the application of metsulfuron, regardless of soil characteristics.