Differential induction of cytochrome P450-mediated triasulfuron metabolism by naphthalic anhydride and triasulfuron

Seed pretreatment with cloquintocet-mexyl protects wheat seedlings from fomesafen injury by promoting photosynthesis and decreasing oxidative stress

BACKGROUND

Fomesafen is a selective herbicide widely used to control post-emergent broad-leaf weeds in soybean and peanut fields. Because of its persistent nature in soil, it can suppress subsequent crops, including wheat. There is limited information focusing on methods of protecting wheat from fomesafen injury by soil residue.

RESULTS

Bioassay results showed slight variations in tolerance to fomesafen among 31 wheat cultivars. Soil-applied biochar (200 g m-2) could alleviate fomesafen injury at 2 mg L-1, and partially alleviate fomesafen injury at 4 mg L-1. Seed soaking in safeners cloquintocet-mexyl and mefenpyr-diethyl was more effective in protecting seedlings from fomesafen injury at 4 mg L-1 compared with 1,8-naphthalic anhydride, fenclorim or dichlormid. Indoor tests suggested that the combined application of biochar and cloquintocet-mexyl or mefenpyr-diethyl had a synergistic alleviating effect on wheat injury caused by fomesafen, which was further confirmed in a field trial. Wheat seeds treated with cloquintocet-mexyl increased expression of the fomesafen target protoporphyrinogen IX oxidase and light-harvesting chlorophyll a/b binding protein, leading to an increase in chlorophyll content and a decrease in oxidative stress of wheat exposed to fomesafen. Cloquintocet-mexyl treatment had no influence on the expression of P450 genes, toxic transport genes, most glutathione S-transferase (GST) genes, the enzyme activity of P450 or GSTs, and the metabolism rate of fomesafen.

CONCLUSION

These results suggested that cloquintocet-mexyl acted by increasing photosynthesis and decreasing oxidative stress to alleviate injury to wheat exposed to fomesafen stress, showing no influence on fomesafen metabolism in wheat. This study provided valuable information for fomesafen injury management in wheat. © 2025 Society of Chemical Industry.

Crystal Structures of Arabidopsis thaliana Acetohydroxyacid Synthase in Complex with the Herbicide Triasulfuron and Two Analogues with Herbicidal Activity in Field Trials

Triasulfuron is a commercial herbicide of the sulfonylurea family. This compound targets acetohydroxyacid synthase (AHAS, E.C. 2.2.1.6), the first enzyme in the branched chain amino acid biosynthesis pathway. Here, we have determined crystal structures of Arabidopsis thaliana AHAS (AtAHAS) in complex with triasulfuron and two newly designed herbicidal compounds, identified as FMO and CMO, showing that their binding modes are subtly different. Kinetic studies showed all three compounds exhibit varying Ki values, 0.192 ± 0.013 μM for triasulfuron, 0.086 ± 0.013 μM for FMO, and 1.448 ± 0.058 μM for CMO, but all are strong time-dependent accumulative inhibitors of AtAHAS. Apart from triasulfuron being a powerful herbicide with application rates of 10-15 g/ha in wheat fields, CMO and FMO are also herbicidal at 7.5-30 g/ha for barnyard grass. This study emphasizes that accumulative inhibition is an important factor that contributes to herbicidal activity.

Toxicity of the herbicides used on herbicide-tolerant crops, and societal consequences of their use in France

In France, the implementation of mutant herbicide-tolerant crops and the use of the related herbicides - sulfonylureas and imidazolinones - have triggered a strong societal reaction illustrated by the intervening actions of environmentalist groups illegally mowing such crops. Trials are in progress, and therefore should be addressed the questions of the environmental risks and the toxicity of these herbicides for the animals and humans consuming the products derived from these plants. Regulatory authorities have allowed these mutant and herbicide-tolerant plants arguing that the herbicides against which they resist only target an enzyme found in 'weeds' (the acetolactate synthase, ALS), and that therefore all organisms lacking this enzyme would be endowed with immunity to these herbicides. The toxicological literature does not match with this argument: 1) Even in organisms displaying the enzyme ALS, these herbicides impact other molecular targets than ALS; 2) These herbicides are toxic for animals, organisms that do not possess the enzyme ALS, and especially invertebrates, amphibians and fish. In humans, epidemiological studies have shown that the use and handling of these toxins are associated with a significantly increased risk of colon and bladder cancers, and miscarriages. In agricultural soils, these herbicides have a persistence of up to several months, and water samples have concentrations of some of these herbicides above the limit value in drinking water.

The P450 gene CYP749A16 is required for tolerance to the sulfonylurea herbicide trifloxysulfuron sodium in cotton (Gossypium hirsutum L.)

BACKGROUND

Weed management is critical to global crop production and is complicated by rapidly evolving herbicide resistance in weeds. New sources of herbicide resistance are needed for crop plants so that applied herbicides can be rotated or combined to thwart the evolution of resistant weeds. The diverse family of cytochrome P450 proteins has been suggested to be a source of detoxifying herbicide metabolism in both weed and crop plants, and greater understanding of these genes will offer avenues for crop improvement and novel weed management practices.

RESULTS

Here, we report the identification of CYP749A16 (Gh_D10G1401) which is responsible for the natural tolerance exhibited by most cotton, Gossypium hirsutum L., cultivars to the herbicide trifloxysulfuron sodium (TFS, CGA 362622, commercial formulation Envoke). A 1-bp frameshift insertion in the third exon of CYP749A16 results in the loss of tolerance to TFS. The DNA marker designed from this insertion perfectly co-segregated with the phenotype in 2145 F₂ progeny of a cross between the sensitive cultivar Paymaster HS26 and tolerant cultivar Stoneville 474, and in 550 recombinant inbred lines of a multi-parent advanced generation inter-cross population. Marker analysis of 382 additional cotton cultivars identified twelve cultivars containing the 1-bp frameshift insertion. The marker genotypes matched perfectly with phenotypes in 188 plants from the selected twelve cultivars. Virus-induced gene silencing of CYP749A16 generated sensitivity in the tolerant cotton cultivar Stoneville 474.

CONCLUSIONS

CYP749A16 located on chromosome D10 is required for TFS herbicide tolerance in cotton. This finding should add to the repertoire of tools available to farmers and breeders for the advancement of agricultural productivity.

The expression of detoxification genes in two maize cultivars by interaction of isoxadifen-ethyl and nicosulfuron

Herbicide safeners protect crop plants from herbicide phytotoxicity, but an understanding of their molecular mechanisms is still lacking. We investigated the effects of the safener isoxadifen-ethyl and/or nicosulfuron on the expression of 10 genes, 8 glutathione transferases (GSTs), 1 glutathione transporter and 1 multidrug resistance protein gene in two maize cultivars. Nicosulfuron and isoxadifen-ethyl induce different detoxification enzyme genes. The expression analyses of the 10 genes revealed that most were expressed much higher in 'Zhengdan958' than those in 'Zhenghuangnuo No.2', both in control and in isoxadifen-ethyl- and/or nicosulfuron-treated plants. The expression levels of ZmGSTIV, ZmGST6, ZmGST31 and ZmMRP1 in two maize cultivars were up-regulated by isoxadifen-ethyl only, or in combination with nicosulfuron, whereas nicosulfuron down-regulated the expression of eight genes. Thus, ZmGSTIV, ZmGST6, ZmGST31 and ZmMRP1 could be considered safener-responsive and may be the core genes responsible for isoxadifen-ethyl increasing the tolerance of maize to nicosulfuron.

Mechanism of resistance to penoxsulam in late watergrass [ Echinochloa phyllopogon (Stapf) Koss.]

Late watergrass [ Echinochloa phyllopogon (Stapf.) Koss.] is a major weed of California rice that has evolved P450-mediated metabolic resistance to multiple herbicides. Resistant (R) populations are also poorly controlled by the recently introduced herbicide penoxsulam. Ratios (R/S) of the R to susceptible (S) GR(50) (herbicide rate for 50% growth reduction) ranged from 5 to 9. Although specific acetolactate synthase (ALS) activity was 1.7 higher in R than in S plants, the enzyme in R plants was about 6 times more susceptible to the herbicide. R plants exhibited faster (2.8 times) oxidative [(14)C]-penoxsulam metabolism than S plants 24 h after treatment. Addition of malathion (P450 inhibitor) enhanced herbicide phytotoxicity and reduced penoxsulam metabolism in R plants. Tank mixtures with thiobencarb (can induce P450) antagonized penoxsulam toxicity in R plants, suggesting penoxsulam may be broken down by a thiobencarb-inducible enzyme. These results suggest E. phyllopogon resistance to penoxsulam is mostly due to enhanced herbicide metabolism, possibly via P450 monooxidation.

Engineering herbicide metabolism in tobacco and Arabidopsis with CYP76B1, a cytochrome P450 enzyme from Jerusalem artichoke

The Jerusalem artichoke (Helianthus tuberosus) xenobiotic inducible cytochrome P450, CYP76B1, catalyzes rapid oxidative dealkylation of various phenylurea herbicides to yield nonphytotoxic metabolites. We have found that increased herbicide metabolism and tolerance can be achieved by ectopic constitutive expression of CYP76B1 in tobacco (Nicotiana tabacum) and Arabidopsis. Transformation with CYP76B1 conferred on tobacco and Arabidopsis a 20-fold increase in tolerance to linuron, a compound detoxified by a single dealkylation, and a 10-fold increase in tolerance to isoproturon or chlortoluron, which need successive catalytic steps for detoxification. Two constructs for expression of translational fusions of CYP76B1 with P450 reductase were prepared to test if they would yield even greater herbicide tolerance. Plants expressing these constructs had lower herbicide tolerance than CYP76B1 alone, which is apparently a consequence of reduced stability of the fusion proteins. In all cases, increased herbicide tolerance results from more extensive metabolism, as demonstrated with exogenously fed phenylurea. Beside increased herbicide tolerance, expression of CYP76B1 has no other visible phenotype in the transgenic plants. Our data indicate that CYP76B1 can function as a selectable marker for plant transformation, allowing efficient selection in vitro and in soil-grown plants. Plants expressing CYP76B1 may also be a potential tool for phytoremediation of contaminated sites.

Involvement of cytochrome P-450 enzyme activity in the selectivity and safening action of pyrazosulfuron-ethyl

To investigate the selectivity and safening action of the sulfonylurea herbicide pyrazosulfuron-ethyl (PSE), pyrazosulfuron-ethyl O-demethylase (PSEOD) activity involving oxidative metabolism by cytochrome P-450 was studied in rice (Oryza sativa L cv Nipponbare) and Cyperus serotinus Rottb. Cytochrome P-450-dependent activity was demonstrated by the use of the inducers 1,8-naphthalic anhydride and ethanol, the herbicides PSE, bensulfuron-methyl, dimepiperate and dymron, or the inhibitor piperonyl butoxide (PBO). Growth inhibition in C serotinus seedlings was more severe than that in rice seedlings. O-Dealkylation activities of PSE were induced differently in rice and in C serotinus, with distinctly higher activity in rice seedlings. The induced PSEOD activities were slightly inhibited by PBO in rice seedlings, whereas they were strongly inhibited in C serotinus seedlings. Dimepiperate and dymron were effective safeners of rice against PSE treatment. Treatments with herbicide alone resulted in less induction of PSEOD activity compared with combined treatments of the herbicide and safener. PSEOD activity in rice seedlings induced with herbicide alone was strongly inhibited by PBO, whereas it was weakly inhibited in rice seedlings induced with combinations of PSE and two safeners. These results suggest that O-demethylation by cytochrome P-450 enzymes may be involved in the metabolism of PSE and may contribute to its selectivity and safening action. Furthermore, these results suggest the existence of a multiple form of cytochrome P-450 in plants.

Characterization of maize cytochrome P450 monooxygenases induced in response to safeners and bacterial pathogens

Plants use a diverse array of cytochrome P450 monooxygenases in their biosynthetic and detoxification pathways. To determine the extent to which various maize P450s are induced in response to chemical inducers, such as naphthalic anhydride (NA), triasulfuron (T), phenobarbital, and bacterial pathogens (Erwinia stuartii, Acidovorax avenae), we have analyzed the response patterns of seven P450 transcripts after treatment of seedlings with these inducers. Each of these P450 transcripts has distinct developmental, tissue-specific, and chemical cues regulating their expression even when they encode P450s within the same biosynthetic pathway. Most notably, the CYP71C1 and CYP71C3 transcripts, encoding P450s in the DIMBOA biosynthetic pathway, are induced to the same level in response to wounding and NA treatment of younger seedlings and differentially in response to NA/T treatment of younger seedlings and NA and NA/T treatment of older seedlings. NA and T induce expression of both CYP92A1 and CYP72A5 transcripts in older seedling shoots, whereas phenobarbital induces CYP92A1 expression in older seedling shoots and highly induces CYP72A5 expression in young and older seedling roots. Expressed sequence tag (EST) 6c06b11 transcripts, encoding an undefined P450 activity, are highly induced in seedling shoots infected with bacterial pathogens.

Cytochromes P450 for engineering herbicide tolerance

In recent years, genome sequencing has revealed that cytochromes P450 (P450s) constitute the largest family of enzymatic proteins in higher plants. P450s are mono-oxygenases that insert one atom of oxygen into inert hydrophobic molecules to make them more reactive and hydrosoluble. Besides their physiological functions in the biosynthesis of hormones, lipids and secondary metabolites, P450s help plants to cope with harmful exogenous chemicals including pesticides and industrial pollutants, making them less phytotoxic. The recovery of an increasing number of plant P450 genes in recombinant form has enabled their use in experimentation, which has revealed their extraordinary potential for engineering herbicide tolerance, biosafening, bioremediation and green chemistry.

The chemically inducible plant cytochrome P450 CYP76B1 actively metabolizes phenylureas and other xenobiotics

Cytochrome P450s (P450s) constitute one of the major classes of enzymes that are responsible for detoxification of exogenous molecules both in animals and plants. On the basis of its inducibility by exogenous chemicals, we recently isolated a new plant P450, CYP76B1, from Jerusalem artichoke (Helianthus tuberosus) and showed that it was capable of dealkylating a model xenobiotic compound, 7-ethoxycoumarin. In the present paper we show that CYP76B1 is more strongly induced by foreign compounds than other P450s isolated from the same plant, and metabolizes with high efficiency a wide range of xenobiotics, including alkoxycoumarins, alkoxyresorufins, and several herbicides of the class of phenylureas. CYP76B1 catalyzes the double N-dealkylation of phenylureas with turnover rates comparable to those reported for physiological substrates and produces nonphytotoxic compounds. Potential uses for CYP76B1 thus include control of herbicide tolerance and selectivity, as well as soil and groundwater bioremediation.

Molecular cloning and functional expression in yeast of CYP76B1, a xenobiotic-inducible 7-ethoxycoumarin O-de-ethylase from Helianthus tuberosus

In order to obtain plant markers of chemical stress and possible tools for the bio-monitoring of pollution, a protein purification/PCR approach was used to isolate cDNAs of xenobiotic-inducible P450 oxygenases. O-dealkylation of 7-ethoxycoumarin is catalysed in Helianthus tuberosus by cytochromes P450 strongly inducible by a wide range of xenobiotics. Therefore, a 7-ethoxycoumarin O-de-ethylase (ECOD) was purified from induced tuber tissues (Batard et al., 1995). A primer designed from an internal peptide sequence, but also corresponding to a conserved P450 haem-binding region, led to the generation of a gene-specific probe corresponding to a P450 strongly inducible by aminopyrine. Two partial and 98% identical coding sequences were isolated from a cDNA library prepared from aminopyrine-induced tuber. A full-length cDNA was reconstituted by 5'-RACE elongation. The protein deduced from this full-length sequence, with 41.1% amino acid identity to CYP76A1 and high phylogenetic relationship to other CYP76s, was termed CYP76B1. CYP76B1 was expressed in yeast. Microsomes from the transformed yeast catalysed the NADPH-dependent O-dealkylation of 7-ethoxycoumarin. However, protein sequence as well as enzymological data indicated that CYP76B1 does not correspond to the purified ECOD protein. These results confirm previous data and demonstrate that several P450s in H. tuberosus are capable of actively catalysing the O-de-ethylation of ethoxycoumarin. Determination of the steady-state level of CYP76B1 transcripts after slicing tuber tissues and ageing them in water, alone or in the presence of various chemicals, showed that the expression of this P450 was not responsive to mechanical stress, but was strongly induced by chemical treatments. CYP76B1 thus appears to be a good potential marker of chemical stress and of environmental pollution.

Induction of ABA 8'-hydroxylase by (+)-S-, (-)-R- and 8'-8'-8'-trifluoro-S-abscisic acid in suspension cultures of potato and Arabidopsis

Suspension cultures of potato and Arabidopsis were incubated with 50 microM of (+)-ABA and (-)-ABA for 3 hr. These pretreatments were found to increase the rate, by two- to seven-fold, of formation of [2H6] phaseic acid (PA) from [2H6] ABA, applied in a subsequent incubation. Pretreatment with trifluoro-ABA had a higher efficacy, increasing the rate of conversion 15-fold. Suspension cell cultures that had been dehydrated and then rehydrated in the presence of [2H6] ABA displayed a much lower enhancement of PA formation. We conclude that ABA induces its own oxidative catabolism in suspension cultures.

Regulation of the Cinnamate 4-Hydroxylase (CYP73A1) in Jerusalem Artichoke Tubers in Response to Wounding and Chemical Treatments

trans-Cinnamate 4-hydroxylase (C4H) is a plant-specific cytochrome (P450) that is encoded by the gene CYP73A and catalyzes the second step of the multibranched phenylpropanoid pathway. Increases in C4H activity in response to physical and chemical stresses have been well documented, but the mechanism of these increases has never been studied in detail. This paper reports on the regulatory mechanism controlling C4H activity in Jerusalem artichoke (Helianthus tuberosus) tubers in response to wounding and chemical treatments. We compared induction of C4H and other P450-catalyzed activities. C4H was moderately induced by chemicals relative to other P450s. Increases in enzyme activity, C4H protein, and transcripts were quantified and compared in tuber tissue 48 h after wounding and chemical treatments. Our data suggest that induction of the enzyme activity results primarily from gene activation. Time-course experiments were performed after wounding and aminopyrine treatment. Compared with wounded tissues, aminopyrine triggered an additional and delayed peak of transcript accumulation. The timing of the induced changes in activity, protein, and transcripts confirms that C4H induction results primarily from an increase in CYP73A1 mRNA, in both wounded and aminopyrine-treated tissues. However, posttranscriptional mechanisms might also contribute to the regulation of C4H activity.