Imazethapyr, an inhibitor of the branched-chain amino acid biosynthesis, induces aerobic fermentation in pea plants

Current Research Status, Opportunities, and Future Challenges of Nine Representative Persistent Herbicides

Pesticides are extensively utilized in contemporary agriculture to manage pests, enhance crop yields, and sustain productivity. Nevertheless, the persistent herbicide represents a dual-edged weapon. On one hand, their prolonged efficacy enables reduced application frequency during crop growth seasons, resulting in cost savings on labor. However, the presence of these residues within fields poses safety risks to soil quality, sensitive crops in subsequent rotations, agricultural product quality, and the ecological environment. This review presents a comprehensive review on the mechanisms of action, application risks, ecotoxicology, and residue analysis methods of nine representative persistent herbicides (namely, atrazine, imazethapyr, imazapic, mesosulfuron-methyl, halosulfuron-methyl, fomesafen, diflufenican, quinclorac, and pyroxasulfone). The objective is to guide their scientific and rational utilization in agricultural practices while minimizing phytotoxicity risks and effectively monitoring and controlling soil pollution. These can not only provide practical recommendations for mitigating potential plant toxicity and ecological environmental risks but also contribute valuable technical insights for efficient soil pollution monitoring and prevention. Additionally, unaddressed research objectives were also anticipated.

Cysteine proteases are activated in sensitive Amaranthus palmeri populations upon treatment with herbicides inhibiting amino acid biosynthesis

The herbicides glyphosate and pyrithiobac inhibit the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) in the aromatic amino acid biosynthetic pathway and acetolactate synthase (ALS) in the branched-chain amino acid biosynthetic pathway, respectively. Here we characterise the protease activity profiles of a sensitive (S), a glyphosate-resistant (GR) and a multiple-resistant (MR) population of Amaranthus palmeri in response to glyphosate and pyrithiobac. Amino acid accumulation and cysteine protease activities were induced with both herbicides in the S population and with pyrithiobac in the GR population, suggesting that the increase in cysteine proteases is responsible for the increased degradation of the available proteins and the observed increase in free amino acids. Herbicides did not induce any changes in the proteolytic activities in the populations with target-site resistance, indicating that this effect was only induced in sensitive plants.

Effect of herbicide stress on synchronization of carbon and nitrogen metabolism in lentil (Lens culinaris Medik.)

Weed invasion causes significant yield losses in lentil. Imazethapyr (IM), a broad-spectrum herbicide inhibits the biosynthesis of branched chain amino acids necessary for plant growth. Plant growth depends upon translocation of photo-assimilates and their partitioning regulated by carbon and nitrogen metabolism. This study aimed to investigate the impact of imazethapyr spray on carbon and nitrogen metabolism in tolerant (LL1397 and LL1612) and susceptible (FLIP2004-7L and PL07) lentil genotypes during vegetative and reproductive development. Significantly higher activities of invertases and sucrose synthase (cleavage) in leaves and in podwall and seeds during early phase of development in tolerant genotypes were observed as compared to susceptible genotypes under herbicide stress that might be responsible for providing hexoses required for their growth. Activities of sucrose synthesizing enzymes, sucrose phosphate synthase and sucrose synthase (synthesis) increased significantly in podwalls and seeds of LL1397 and LL1612 genotypes during later phase of development towards maturity while the activities decreased in FLIP2004-7L and PL07 genotypes under herbicide stress. Activities of nitrate and nitrite reductase, glutamine 2-oxoglutarate aminotransferase, glutamine synthetase and glutamate dehydrogenase were increased in leaves, podwalls and seeds of LL1397 and LL1612 under herbicide stress. A proper synchronization of carbon and nitrogen metabolism in tolerant lentil genotypes during vegetative and reproductive phase might be one of the mechanisms for their recovery from herbicide stress. This first ever comprehensive information will provide a basis for future studies on the molecular mechanism of source sink relationship in lentil under herbicide stress and will be utilized in breeding programmes.

Role of oxidative stress in the physiology of sensitive and resistant Amaranthus palmeri populations treated with herbicides inhibiting acetolactate synthase

The aim of the present study was to elucidate the role of oxidative stress in the mode of action of acetolactate synthase (ALS) inhibiting herbicides. Two populations of Amaranthus palmeri S. Watson from Spain (sensitive and resistant to nicosulfuron, due to mutated ALS) were grown hydroponically and treated with different rates of the ALS inhibitor nicosulfuron (one time and three times the field recommended rate). Seven days later, various oxidative stress markers were measured in the leaves: H₂O₂, MDA, ascorbate and glutathione contents, antioxidant enzyme activities and gene expression levels. Under control conditions, most of the analysed parameters were very similar between sensitive and resistant plants, meaning that resistance is not accompanied by a different basal oxidative metabolism. Nicosulfuron-treated sensitive plants died after a few weeks, while the resistant ones survived, independently of the rate. Seven days after herbicide application, the sensitive plants that had received the highest nicosulfuron rate showed an increase in H₂O₂ content, lipid peroxidation and antioxidant enzymatic activities, while resistant plants did not show these responses, meaning that oxidative stress is linked to ALS inhibition. A supralethal nicosulfuron rate was needed to induce a significant oxidative stress response in the sensitive population, providing evidence that the lethality elicited by ALS inhibitors is not entirely dependent on oxidative stress.

Adaptability and Stability of Faba Bean (Vicia faba L.) Accessions under Diverse Environments and Herbicide Treatments

The adaptability and stability of 37 faba bean (Vicia faba L.) accessions with different levels of tolerance to metribuzin or imazethapyr was assessed across 12 season–location–herbicide experiments. Significant Genotype x environment (GE) interaction was found for the days to flowering (DFLR), plant height (PLHT) and grain yield (GY). Performance and stability of the accessions regarding PLHT and GY were assessed using four different stability parameters: cultivar superiority, static stability, Wricke’s eco-valence and Finlay and Wilkinson’s regression model. The stability parameters ranked these genotypes differently suggesting that PLHT and GY stability should be assessed not only on a single or a few stability parameters but on a combination of them. GGE biplot analysis indicated that the environments representing metribuzin treatment at Marchouch 2014–2015 and the non-treated treatment at Terbol 2018–2019 are the ideal environments for evaluating faba bean genotypes. GGE biplots showed herbicide tolerant accession IG12983 with simultaneous average PLHT, GY and stability across the environments. The performance of other tolerant accessions, namely IG13945, IG13906, IG106453, FB2648, and FB1216 was less stable but superior under specific mega environments. Therefore, utilizing these accessions in faba bean breeding programs would help broaden the adaptability to diverse locations–season–herbicide treatments.

Hypoxic Treatment Decreases the Physiological Action of the Herbicide Imazamox on Pisum sativum Roots

The inhibition of acetolactate synthase (ALS; EC 2.2.1.6), an enzyme located in the biosynthetic pathway of branched-chain amino acids, is the target site of the herbicide imazamox. One of the physiological effects triggered after ALS inhibition is the induction of aerobic ethanol fermentation. The objective of this study was to unravel if fermentation induction is related to the toxicity of the herbicide or if it is a plant defense mechanism. Pea plants were exposed to two different times of hypoxia before herbicide application in order to induce the ethanol fermentation pathway, and the physiological response after herbicide application was evaluated at the level of carbohydrates and amino acid profile. The effects of the herbicide on total soluble sugars and starch accumulation, and changes in specific amino acids (branched-chain, amide, and acidic) were attenuated if plants were subjected to hypoxia before herbicide application. These results suggest that fermentation is a plant defense mechanism that decreases the herbicidal effect.

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.

ERF-VII transcription factors induce ethanol fermentation in response to amino acid biosynthesis-inhibiting herbicides

Herbicides inhibiting either aromatic or branched-chain amino acid biosynthesis trigger similar physiological responses in plants, despite their different mechanism of action. Both types of herbicides are known to activate ethanol fermentation by inducing the expression of fermentative genes; however, the mechanism of such transcriptional regulation has not been investigated so far. In plants exposed to low-oxygen conditions, ethanol fermentation is transcriptionally controlled by the ethylene response factors-VII (ERF-VIIs), whose stability is controlled in an oxygen-dependent manner by the Cys-Arg branch of the N-degron pathway. In this study, we investigated the role of ERF-VIIs in the regulation of the ethanol fermentation pathway in herbicide-treated Arabidopsis plants grown under aerobic conditions. Our results demonstrate that these transcriptional regulators are stabilized in response to herbicide treatment and are required for ethanol fermentation in these conditions. We also observed that mutants with reduced fermentative potential exhibit higher sensitivity to herbicide treatments, thus revealing the existence of a mechanism that mimics oxygen deprivation to activate metabolic pathways that enhance herbicide tolerance. We speculate that this signaling pathway may represent a potential target in agriculture to affect tolerance to herbicides that inhibit amino acid biosynthesis.

An imazamox-based herbicide causes apoptotic changes in rat liver and pancreas

We studied the acute toxicity of an imazamox-based herbicide at 12, 24 and 36 mg/kg body (bw) weight imazamox equivalent dose on the liver and pancreatic tissue in Sprague Dawley rats. Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities, glucose, calcium as well as creatinine, were determined in blood samples, which were collected after 24, 48 and 72 h exposure. Caspase 3 and anti-insulin expression and immunopositivity were evaluated using in situ hybridization and immunohistochemistry, respectively. The imazamox-based herbicide evaluated in this study induced toxic effects even from the lowest dose tested (12 mg/kg bw). The two highest doses caused a statistically significant cytotoxicity on the Langerhans islet cells. Necrotic and degenerative changes were detected in hepatocytes at the two highest doses. Imazamox is considered to be poorly toxic to the liver. Nevertheless, the imazamox-based herbicide formulation tested here reduced the size of the β-islet cells, induced an elevation in serum glucose and calcium. Our data shows that commercial formulations of imazamox containing various co-formulants can have hepatic and pancreatic toxic effects.

An aerated axenic hydroponic system for the application of root treatments: exogenous pyruvate as a practical case

BACKGROUND

Hydroponic systems are a convenient platform for plant cultivation when treatments are applied to the roots because they provide precise control of the composition of the growth medium, ensuring the availability of different compounds. A problem arises when axenic conditions are needed but the treatment of choice (exogenous organic acids or sugars) promote the growth of unwanted microorganisms. Moreover, axenic conditions are usually applied in liquid and semi-liquid growing systems, where oxygen availability can be compromised, if no aeration is provided.

RESULTS

The driver for the development of this hydroponic system was the application of the organic acid pyruvate to the roots of plants grown under aerated axenic conditions. No contamination was detected in the nutrient solution, even after the addition of pyruvate. The system was validated in pea plants treated with either pyruvate or herbicides inhibiting amino acid biosynthesis. The effects on ethanol fermentation were compared by analysing the enzymatic activity, protein content and transcriptional levels in plants treated with either pyruvate or herbicides.

CONCLUSIONS

The developed system enables the study of the exogenous application of organic acids in the nutrient solution under axenic conditions and without oxygen limitation. This system allows the study of the effect of any type of treatments applied to roots under aerated axenic hydroponic systems at physiological and molecular levels. The role of pyruvate in the induction of fermentation by herbicides cannot be simply explained by an increase in substrate availability.

Induction of the PDH bypass and upregulation of the ALDH7B4 in plants treated with herbicides inhibiting amino acid biosynthesis

Imazamox and glyphosate represent two classes of herbicides that inhibit the activity of acetohydroxyacid synthase in the branched-chain amino acid biosynthesis pathway and the activity of 5-enolpyruvylshikimate-3-phosphate synthase in the aromatic amino acid biosynthesis pathway, respectively. However, it is still unclear how imazamox and glyphosate lead to plant death. Both herbicides inhibit amino-acid biosynthesis and were found to induce ethanol fermentation in plants, but an Arabidopsis mutant deficient in alcohol dehydrogenase 1 was neither more susceptible nor more resistant than the wild-type to the herbicides. In this study, we investigated the effects of the amino acid biosynthesis inhibitors, imazamox and glyphosate, on the pyruvate dehydrogenase bypass reaction and fatty acid metabolism in A. thaliana. We found that the pyruvate dehydrogenase bypass was upregulated following the treatment by the two herbicides. Our results suggest that the Arabidopsis aldehyde dehydrogenase 7B4 gene might be participating in the pyruvate dehydrogenase bypass reaction. We evaluated the potential role of the aldehyde dehydrogenase 7B4 upon herbicide treatment in the plant defence mechanism. Plants that overexpressed the ALDH7B4 gene accumulated less soluble sugars, starch, and fatty acids and grew better than the wild-type after herbicide treatment. We discuss how the upregulation of the ALDH7B4 alleviates the effects of the herbicides, potentially through the detoxification of the metabolites produced in the pyruvate dehydrogenase bypass.

Photosynthetic Performance of the Imidazolinone Resistant Sunflower Exposed to Single and Combined Treatment by the Herbicide Imazamox and an Amino Acid Extract

The herbicide imazamox may provoke temporary yellowing and growth retardation in IMI-R sunflower hybrids, more often under stressful environmental conditions. Although, photosynthetic processes are not the primary sites of imazamox action, they might be influenced; therefore, more information about the photosynthetic performance of the herbicide-treated plants could be valuable for a further improvement of the Clearfield technology. Plant biostimulants have been shown to ameliorate damages caused by different stress factors on plants, but very limited information exists about their effects on herbicide-stressed plants. In order to characterize photosynthetic performance of imazamox-treated sunflower IMI-R plants, we carried out experiments including both single and combined treatments by imazamox and a plant biostimulants containing amino acid extract. We found that imazamox application in a rate of 132 μg per plant (equivalent of 40 g active ingredient ha^-1) induced negative effects on both light-light dependent photosynthetic redox reactions and leaf gas exchange processes, which was much less pronounced after the combined application of imazamox and amino acid extract.

Both foliar and residual applications of herbicides that inhibit amino acid biosynthesis induce alternative respiration and aerobic fermentation in pea roots

The objective of this work was to ascertain whether there is a general pattern of carbon allocation and utilisation in plants following herbicide supply, independent of the site of application: sprayed on leaves or supplied to nutrient solution. The herbicides studied were the amino acid biosynthesis-inhibiting herbicides (ABIH): glyphosate, an inhibitor of aromatic amino acid biosynthesis, and imazamox, an inhibitor of branched-chain amino acid biosynthesis. All treated plants showed impaired carbon metabolism; carbohydrate accumulation was detected in both leaves and roots of the treated plants. The accumulation in roots was due to lack of use of available sugars as growth was arrested, which elicited soluble carbohydrate accumulation in the leaves due to a decrease in sink strength. Under aerobic conditions, ethanol fermentative metabolism was enhanced in roots of the treated plants. This fermentative response was not related to a change in total respiration rates or cytochrome respiratory capacity, but an increase in alternative oxidase capacity was detected. Pyruvate accumulation was detected after most of the herbicide treatments. These results demonstrate that both ABIH induce the less-efficient, ATP-producing pathways, namely fermentation and alternative respiration, by increasing the key metabolite, pyruvate. The plant response was similar not only for the two ABIH but also after foliar or residual application.

A toxicokinetic and toxicodynamic modeling approach using Myriophyllum spicatum to predict effects caused by short-term exposure to a sulfonylurea

Toxicokinetic and toxicodynamic models are a promising tool to address the effects of time-variable chemical exposure. Standard toxicity tests usually rely on static concentrations, but these chemical exposure patterns are unlikely to appear in the field, where time-variable exposure of chemicals is typical. In the present study, toxicodynamic processes were integrated into an existing model that includes the toxicokinetics and growth of the aquatic plant Myriophyllum spicatum, to predict the impact on plant growth of 2 iofensulfuron short-term exposure patterns. To establish a method that can be used with standard data from risk assessments, the toxicodynamics of iofensulfuron were based on effect data from a 14-d standard toxicity test using static concentrations. Modeling showed that the toxicokinetic and toxicodynamic growth model of M. spicatum can be successfully used to predict effects of short-term iofensulfuron exposure by using effect data from a standard toxicity test. A general approach is presented, in which time-variable chemical exposures can be evaluated more realistically without conducting additional toxicity studies.

Characterization of the Amaranthus palmeri Physiological Response to Glyphosate in Susceptible and Resistant Populations

The herbicide glyphosate inhibits the plant enzyme 5-enolpyruvylshikimate3-phosphate synthase (EPSPS) in the aromatic amino acid (AAA) biosynthetic pathway. The physiologies of an Amaranthus palmeri population exhibiting resistance to glyphosate by EPSPS gene amplification (NC-R) and a susceptible population (NC-S) were compared. The EPSPS copy number of NC-R plants was 47.5-fold the copy number of NC-S plants. Although the amounts of EPSPS protein and activity were higher in NC-R plants than in NC-S plants, the AAA concentrations were similar. The increases in total free amino acid and in AAA contents induced by glyphosate were more evident in NC-S plants. In both populations, the EPSPS protein increased after glyphosate exposure, suggesting regulation of gene expression. EPSPS activity seems tightly controlled in vivo. Carbohydrate accumulation and a slight induction of ethanol fermentation were detected in both populations.

Grain Yield and Quality of Foxtail Millet (Setaria italica L.) in Response to Tribenuron-Methyl

Foxtail millet (Setaria italica L.) is cultivated around the world for human and animal consumption. There is no suitable herbicide available for weed control in foxtail millet fields during the post-emergence stage. In this study, we investigated the effect and safety of the post-emergence herbicide tribenuron-methyl (TBM) on foxtail millet in terms of grain yield and quality using a split-plot field design. Field experiments were conducted using two varieties in 2013 and 2014, i.e., high-yielding hybrid Zhangzagu 10 and high-quality conventional Jingu 21. TBM treatments at 11.25 to 90 g ai ha⁻¹ reduced root and shoot biomass and grain yield to varying degrees. In each of the two years, grain yield declined by 50.2% in Zhangzagu 10 with a herbicide dosage of 45 g ai ha⁻¹ and by 45.2% in Jingu 21 with a herbicide dosage of 22.5 g ai ha⁻¹ (recommended dosage). Yield reduction was due to lower grains per panicle, 1000-grain weight, panicle length, and panicle diameter. Grain yield was positively correlated with grains per panicle and 1000-grain weight, but not with panicles ha⁻¹. With respect to grain protein content at 22.5 g ai ha⁻¹, Zhangzagu 10 was similar to the control, whereas Jingu 21 was markedly lower. An increase in TBM dosage led to a decrease in grain Mn, Cu, Fe, and Zn concentrations. In conclusion, the recommended dosage of TBM was relatively safe for Zhangzagu 10, but not for Jingu 21. Additionally, the hybrid variety Zhangzagu 10 had a greater tolerance to TBM than the conventional variety Jingu 21.

Analyzing Arabidopsis thaliana root proteome provides insights into the molecular bases of enantioselective imazethapyr toxicity

Imazethapyr (IM) is a widely used chiral herbicide that inhibits the synthesis of branched-chain amino acids (BCAAs). IM is thought to exert its toxic effects on amino acid synthesis mainly through inhibition of acetolactate synthase activity, but little is known about the potential effects of IM on other key biochemical pathways. Here, we exposed the model plant Arabidospsis thaliana to trace S- and R-IM enantiomer concentrations and examined IM toxicity effects on the root proteome using iTRAQ. Conventional analyses of root carbohydrates, organic acids, and enzyme activities were also performed. We discovered several previously unknown key biochemical pathways targeted by IM in Arabidospsis. 1,322 and 987 proteins were differentially expressed in response to R- and S-IM treatments, respectively. Bioinformatics and physiological analyses suggested that IM reduced the BCAA tissue content not only by strongly suppressing BCAA synthesis but also by increasing BCAA catabolism. IM also affected sugar and starch metabolism, changed the composition of root cell walls, increased citrate production and exudation, and affected the microbial community structure of the rhizosphere. The present study shed new light on the multiple toxicity mechanisms of a selective herbicide on a model plant.

Comparative transcriptome analysis reveals carbohydrate and lipid metabolism blocks in Brassica napus L. male sterility induced by the chemical hybridization agent monosulfuron ester sodium

BACKGROUND

Chemical hybridization agents (CHAs) are often used to induce male sterility for the production of hybrid seeds. We previously discovered that monosulfuron ester sodium (MES), an acetolactate synthase (ALS) inhibitor of the herbicide sulfonylurea family, can induce rapeseed (Brassica napus L.) male sterility at approximately 1% concentration required for its herbicidal activity. To find some clues to the mechanism of MES inducing male sterility, the ultrastructural cytology observations, comparative transcriptome analysis, and physiological analysis on carbohydrate content were carried out in leaves and anthers at different developmental stages between the MES-treated and mock-treated rapeseed plants.

RESULTS

Cytological analysis revealed that the plastid ultrastructure was abnormal in pollen mother cells and tapetal cells in male sterility anthers induced by MES treatment, with less material accumulation in it. However, starch granules were observed in chloroplastids of the epidermis cells in male sterility anthers. Comparative transcriptome analysis identified 1501 differentially expressed transcripts (DETs) in leaves and anthers at different developmental stages, most of these DETs being localized in plastid and mitochondrion. Transcripts involved in metabolism, especially in carbohydrate and lipid metabolism, and cellular transport were differentially expressed. Pathway visualization showed that the tightly regulated gene network for metabolism was reprogrammed to respond to MES treatment. The results of cytological observation and transcriptome analysis in the MES-treated rapeseed plants were mirrored by carbohydrate content analysis. MES treatment led to decrease in soluble sugars content in leaves and early stage buds, but increase in soluble sugars content and decrease in starch content in middle stage buds.

CONCLUSIONS

Our integrative results suggested that carbohydrate and lipid metabolism were influenced by CHA-MES treatment during rapeseed anther development, which might responsible for low concentration MES specifically inducing male sterility. A simple action model of CHA-MES inducing male sterility in B. napus was proposed. These results will help us to understand the mechanism of MES inducing male sterility at low concentration, and might provide some potential targets for developing new male sterility inducing CHAs and for genetic manipulation in rapeseed breeding.

Fermentation and alternative oxidase contribute to the action of amino acid biosynthesis-inhibiting herbicides

Acetolactate synthase inhibitors (ALS-inhibitors) and glyphosate (GLP) are two classes of herbicide that act by the specific inhibition of an enzyme in the biosynthetic pathway of branched-chain or aromatic amino acids, respectively. The physiological effects that are detected after application of these two classes of herbicides are not fully understood in relation to the primary biochemical target inhibition, although they have been well documented. Interestingly, the two herbicides' toxicity includes some common physiological effects suggesting that they kill the treated plants by a similar pattern despite targeting different enzymes. The induction of aerobic ethanol fermentation and alternative oxidase (AOX) are two examples of these common effects. The objective of this work was to gain further insight into the role of fermentation and AOX induction in the toxic consequences of ALS-inhibitors and GLP. For this, Arabidopsis T-DNA knockout mutants of alcohol dehydrogenase (ADH) 1 and AOX1a were used. The results found in wild-type indicate that both GLP and ALS-inhibitors reduce ATP production by inducing fermentation and alternative respiration. The main physiological effects in the process of herbicide activity upon treated plants were accumulation of carbohydrates and total free amino acids. The effects of the herbicides on these parameters were less pronounced in mutants compared to wild-type plants. The role of fermentation and AOX regarding pyruvate availability is also discussed.

Physiological and biochemical responses of common vetch to the imazamox accumulation

Common vetch (Vicia sativa L.) is a forage and grain legume, widely distributed throughout the world. Alterations induced by the herbicide imazamox on plant growth, acetohydroxyacid synthase activity, total free amino acids, as well as concentrations of valine, leucine, isoleucine and imazamox in young and mature leaves were investigated at 2 and 7 days after the herbicide application. Plant growth decreased significantly after 7 days of imazamox treatment. The herbicide was detected in both young and mature leaves inducing an inhibition of acetohydroxyacid synthase activity in the former and consequently decreasing valine and leucine contents in this organ. At the same time, the treatment caused an increase of total free amino acids in young leaves, presumably as result of proteolysis stimulation in such conditions. Given that these effects were not observed in mature leaves, we suggest a different sensitivity of the acetohydroxyacid synthase activity to imazamox depending on leaf age. Common vetch seems not to degrade imazamox since the herbicide was accumulated in shoot with increasing treatment time. To our knowledge, no physiological and biochemical studies of common vetch responses to imazamox have been previously reported.

Cytological and comparative proteomic analyses on male sterility in Brassica napus L. induced by the chemical hybridization agent monosulphuron ester sodium

Male sterility induced by a chemical hybridization agent (CHA) is an important tool for utilizing crop heterosis. Monosulphuron ester sodium (MES), a new acetolactate synthase-inhibitor herbicide belonging to the sulphonylurea family, has been developed as an effective CHA to induce male sterility in rapeseed (Brassica napus L.). To understand MES-induced male sterility in rapeseed better, comparative cytological and proteomic analyses were conducted in this study. Cytological analysis indicated that defective tapetal cells and abnormal microspores were gradually generated in the developing anthers of MES-treated plants at various development stages, resulting in unviable microspores and male sterility. A total of 141 differentially expressed proteins between the MES-treated and control plants were revealed, and 131 of them were further identified by MALDI-TOF/TOF MS. Most of these proteins decreased in abundance in tissues of MES-treated rapeseed plants, and only a few increased. Notably, some proteins were absent or induced in developing anthers after MES treatment. These proteins were involved in several processes that may be crucial for tapetum and microspore development. Down-regulation of these proteins may disrupt the coordination of developmental and metabolic processes, resulting in defective tapetum and abnormal microspores that lead to male sterility in MES-treated plants. Accordingly, a simple model of CHA-MES-induced male sterility in rapeseed was established. This study is the first cytological and dynamic proteomic investigation on CHA-MES-induced male sterility in rapeseed, and the results provide new insights into the molecular events of male sterility.

Proteolytic pathways induced by herbicides that inhibit amino acid biosynthesis

BACKGROUND

The herbicides glyphosate (Gly) and imazamox (Imx) inhibit the biosynthesis of aromatic and branched-chain amino acids, respectively. Although these herbicides inhibit different pathways, they have been reported to show several common physiological effects in their modes of action, such as increasing free amino acid contents and decreasing soluble protein contents. To investigate proteolytic activities upon treatment with Gly and Imx, pea plants grown in hydroponic culture were treated with Imx or Gly, and the proteolytic profile of the roots was evaluated through fluorogenic kinetic assays and activity-based protein profiling.

RESULTS

Several common changes in proteolytic activity were detected following Gly and Imx treatment. Both herbicides induced the ubiquitin-26 S proteasome system and papain-like cysteine proteases. In contrast, the activities of vacuolar processing enzymes, cysteine proteases and metacaspase 9 were reduced following treatment with both herbicides. Moreover, the activities of several putative serine protease were similarly increased or decreased following treatment with both herbicides. In contrast, an increase in YVADase activity was observed under Imx treatment versus a decrease under Gly treatment.

CONCLUSION

These results suggest that several proteolytic pathways are responsible for protein degradation upon herbicide treatment, although the specific role of each proteolytic activity remains to be determined.

Branched-chain amino acid biosynthesis inhibitors: herbicide efficacy is associated with an induced carbon-nitrogen imbalance

Acetolactate synthase (ALS; EC 4.1.3.18) and ketol-acid reductoisomerase (KARI; EC 1.1.1.86) are two consecutive enzymes in the biosynthesis of branched-chain amino acids. Several commercial herbicides inhibit ALS as their primary site of action. KARI has also attracted attention as a potential target for herbicides. Although potent and selective inhibitors of KARI have been discovered, these inhibitors display less herbicidal activity than ALS-inhibiting herbicides. To obtain a better understanding of these findings, we have compared the physiological effects induced in pea plants after KARI or ALS inhibition. Although, both types of inhibitors induce growth arrest and photosynthesis inhibition, plant death occurs more rapidly under ALS inhibition than KARI inhibition. Carbohydrates accumulated in the leaves and roots following treatments with both inhibitors. The carbohydrate accumulation in the leaves occurred as a consequence of a decrease in sink strength. In contrast, the free amino acid content was only affected through ALS inhibition. These results indicate that although KARI and ALS inhibition block the same biosynthetic pathway and exert common effects on carbon metabolism, nitrogen metabolism is more affected via ALS than KARI inhibition. Thus, metabolic alterations in nitrogen metabolism induced through ALS inhibitors might contribute to the increased efficacy of these chemicals as herbicides.

Impairment of carbon metabolism induced by the herbicide glyphosate

The herbicide glyphosate reduces plant growth and causes plant death by inhibiting the biosynthesis of aromatic amino acids. The objective of this work was to determine whether glyphosate-treated plants show a carbon metabolism pattern comparable to that of plants treated with herbicides that inhibit branched-chain amino acid biosynthesis. Glyphosate-treated plants showed impaired carbon metabolism with an accumulation of carbohydrates in the leaves and roots. The growth inhibition detected after glyphosate treatment suggested impaired metabolism that impedes the utilization of available carbohydrates or energy at the expected rate. These effects were common to both types of amino acid biosynthesis inhibitors. Under aerobic conditions, ethanolic fermentative metabolism was enhanced in the roots of glyphosate-treated plants. This fermentative response was not related to changes in the respiratory rate or to a limitation of the energy charge. This response, which was similar for both types of herbicides, might be considered a general response to stress conditions.

Role of exogenously supplied ferulic and p-coumaric acids in mimicking the mode of action of acetolactate synthase inhibiting herbicides

Chlorsulfuron and imazethapyr (herbicides that inhibit acetolactate synthase; ALS, EC 4.1.3.18) produced a strong accumulation of hydroxycinnamic acids that was related to the induction of the first enzyme of the shikimate pathway, 3-deoxy-d-arabino-heptulosonate 7-phosphate synthase (EC 2.5.2.54). The exogenous application of two hydroxycinnamic acids, ferulic and p-coumaric acids, to pea plants resulted in their internal accumulation, arrested growth, carbohydrate and quinate accumulation in the leaves, and the induction of ethanolic fermentation. These effects resemble some of the physiological effects detected after acetolactate synthase inhibition and suggest important roles for ferulic and p-coumaric acids in the mode of action of herbicides inhibiting the biosynthesis of branched chain amino acids.

Unraveling the role of fermentation in the mode of action of acetolactate synthase inhibitors by metabolic profiling

Herbicides that inhibit branched chain amino acid biosynthesis induce aerobic fermentation. The role of fermentation in the mode of action of these herbicides is not known, nor is the importance of this physiological response in the growth inhibition and the lethality caused by them. Metabolic profiling was used to compare the effects of the herbicide imazethapyr (IM) on pea plants with two other treatments that also induce fermentation: hypoxia and the exogenous supply pyruvate for seven days. While hypoxic roots did not show internal anoxia, feeding pyruvate or applying IM to the roots led to internal anoxia, probably related to the respiratory burst detected. The three treatments induced ethanol fermentation, but fermentation induced following herbicide treatment was earlier than that following pyruvate supply and was not associated with a decrease in the energy status. No striking changes were detected in the metabolic profiling of hypoxic roots, indicating that metabolism was only slightly impaired. Feeding pyruvate resulted in marked succinate accumulation and a general amino acid accumulation. IM-treated roots showed a general accumulation of glycolytic metabolites upstream of pyruvate, a decrease in some TCA intermediates and an increase in the free amino acid pool sizes. All treatments caused GABA and putrescine accumulation. Our results indicate that IM supply impairs carbon/nitrogen metabolism and this impaired metabolism is likely to be related to the growth arrest detected. As growth is arrested, carbohydrates and glycolytic intermediates accumulate and energy becomes more available.

Enantioselective phytotoxicity of the herbicide imazethapyr and its effect on rice physiology and gene transcription

Imazethapyr (IM) is a chiral herbicide and a widely used racemic mixture. This report investigated the enantioselectivity between R- and S-IM in rice and explored its causative mechanism at the physiological and molecular levels. The results suggested that R-IM inhibited acetolactate synthase (ALS) activity to a greater extent than S-IM, which reduced the synthesis of branched-chain amino acids (BCAAs). Additionally, most other amino acids showed enantioselectivity. On the cellular level, R-IM showed stronger toxicity against protoplasts than S-IM. A gene transcription profile analysis showed that gene transcripts in many metabolic pathways, including amino acid metabolism, photosynthesis, starch and sugar metabolism, and the tricarboxylic acid cycle displayed enantioselectivity between the IM enantiomers. R-IM regulated more genes more strongly than S-IM. This study suggested that R-IM has stronger toxicity against plants than S-IM; this toxicity is caused not only by changing targeted enzyme activity and amino acid synthesis, but also by affecting gene transcription in other metabolic pathways directly or indirectly in an enantioselective manner.

Enantioselective phytotoxicity of the herbicide imazethapyr on the response of the antioxidant system and starch metabolism in Arabidopsis thaliana

BACKGROUND

The enantiomers of a chiral compound possess different biological activities, and one of the enantiomers usually shows a higher level of toxicity. Therefore, the exploration of the causative mechanism of enantioselective toxicity is regarded as one of primary goals of biological chemistry. Imazethapyr (IM) is an acetolactate synthase (ALS)-inhibiting chiral herbicide that has been widely used in recent years with racemate. We investigated the enantioselectivity between R- and S-IM to form reactive oxygen species (ROS) and to regulate antioxidant gene transcription and enzyme activity.

RESULTS

Dramatic differences between the enantiomers were observed: the enantiomer of R-IM powerfully induced ROS formation, yet drastically reduced antioxidant gene transcription and enzyme activity, which led to an oxidative stress. The mechanism by which IM affects carbohydrate metabolism in chloroplasts has long remained a mystery. Here we report evidence that enantioselectivity also exists in starch metabolism. The enantiomer of R-IM resulted in the accumulation of glucose, maltose and sucrose in the cytoplasm or the chloroplast and disturbed carbohydrates utilization.

CONCLUSION

The study suggests that R-IM more strongly retarded plant growth than S-IM not only by acting on ALS, but also by causing an imbalance in the antioxidant system and the disturbance of carbohydrate metabolism with enantioselective manner.

The possible role of quinate in the mode of action of glyphosate and acetolactate synthase inhibitors

BACKGROUND

The herbicide glyphosate inhibits the biosynthesis of aromatic amino acids by blocking the shikimate pathway. Imazethapyr and chlorsulfuron are two herbicides that act by inhibiting branched-chain amino acid biosynthesis. These herbicides stimulate secondary metabolism derived from the aromatic amino acids. The aim of this study was to test if they cause any cross-effect in the amino acid content and if they have similar effects on the shikimate pathway.

RESULTS

The herbicides inhibiting two different amino acid biosynthesis pathways showed a common pattern in general content of free amino acids. There was a general increase in total free amino acid content, with a transient decrease in the proportion of amino acids whose pathways were specifically inhibited. Afterwards, an increase in these inhibited amino acids was detected; this was probably related to proteolysis. All herbicides caused quinate accumulation. Exogenous application of quinate arrested growth, decreased net photosynthesis and stomatal conductance and was ultimately lethal, similarly to glyphosate and imazethapyr.

CONCLUSIONS

Quinate accumulation was a common effect of the two different classes of herbicide. Moreover, exogenous quinate application had phytotoxic effects, showing that this plant metabolite can trigger the toxic effects of the herbicides. This ability to mimic the herbicide effects suggests a possible link between the mode of action of these herbicides and the potential role of quinate as a natural herbicide.

Comparative genotoxicity evaluation of imidazolinone herbicides in somatic cells of Drosophila melanogaster

In the present study, five analogous herbicides, namely Imazapyr (IMZR), Imazapic (IMZC), Imazethapyr (IMZT), Imazamox (IMZX) and Imazaquin (IMZQ), were evaluated for genotoxicity (mutagenic and recombinagenic activity) in the wing somatic mutation and recombination test (SMART) of Drosophila melanogaster. They are classified as imidazolinone (IMI) herbicides and their mode of action is to inhibit acetohydroxyacid synthase (AHAS), an enzyme involved in the biosynthesis of the amino acids leucine, isoleucine and valine. Two crosses were used: the standard (ST) cross and the high bioactivation (HB) cross. The latter is characterized by high levels of cytochrome P450 conferring increased sensitivity to promutagens and procarcinogens. Three-day-old larvae were exposed by chronic feeding (48 h) to four different concentrations of these herbicides (2.5, 5.0, 10.0 or 20.0 mM). For the evaluation of genotoxic effects, the frequencies of spots per individual in the treated series were compared to the concurrent negative control series (ultrapure water). Imazapyr, Imazapic and Imazethapyr gave negative results with both crosses of the wing spot test. In the ST cross, Imazamox showed positive results only for large single spots (20.0 mM IMZX) and weak positive results for total spots (10.0 and 20.0 mM IMZX), while Imazaquin showed positive results only for large single spots (5.0 and 20.0mM IMZQ) and a weak positive result for total spots (20.0 mM IMZQ). These positive results are mainly due to induced recombination and to a minor extent to mutations. In the HB cross, only Imazamox (5.0 mM IMZX) showed a weak positive result for small single spots. The positive control urethane, a promutagen, caused an increase in the number of all types of spots in both crosses. In conclusion, the results of chronic treatments performed at high doses (toxicity was observed at higher doses) shows the existence of a genotoxic risk for IMZX and IMZQ exposure under these experimental conditions, and indicate the need for further research to delineate the exact mechanisms involved.

Proteomic analysis of root meristems and the effects of acetohydroxyacid synthase-inhibiting herbicides in the root of Medicago truncatula

Quantitative proteome analyses of meristematic and nonmeristematic tissues from Medicago truncatula primary and lateral roots and meristem tissues from plants treated with acetohydroxyacid synthase-inhibiting herbicides were made. The accumulation of 81 protein spots changed in meristematic and nonmeristematic tissues and 51 protein spots showed significant changes in accumulation in herbicide-treated meristems. Identified proteins indicate two trends, (i) increased accumulation of cell division and redox-mediating proteins in meristems compared to nonmeristematic tissues and (ii) increased accumulation of pathogenesis-related and decreased accumulation of metabolic proteins in herbicide-treated roots.

Nitrogen assimilation studies using 15N in soybean plants treated with imazethapyr, an inhibitor of branched-chain amino acid biosynthesis

The pattern of nitrogen assimilation in soybean plants treated with a herbicide that inhibits branched-chain amino acid biosynthesis was evaluated by (15)N isotopic analysis. The herbicide imazethapyr caused a strong decrease in nitrate uptake by roots, partly due to a reduced stomatal conductance. The inhibition of (15)N uptake was accompanied by a decrease in the (15)N content in the plant and, concomitantly, an inhibition of translocation to the shoot. Imazethapyr inhibited nitrate reductase activity in leaves and roots. Among all parameters studied, "de novo" synthesis of proteins was the first parameter of the N assimilation metabolism affected by the herbicide. These results show that this class of herbicides totally damages N metabolism and indicates a regulatory effect on N uptake and translocation that would be mediated by the increase in free amino acid pool provoked by the inhibition of branched-chain amino acid biosynthesis.

Fermentative metabolism is induced by inhibiting different enzymes of the branched-chain amino acid biosynthesis pathway in pea plants

The inhibition of branched-chain amino acid (BCAA) biosynthesis was evaluated in pea plants in relation to the ability for induction of fermentative metabolism under aerobic conditions. Chlorsulfuron and imazethapyr (inhibitors of acetolactate synthase, ALS, EC 4.1.3.18) produced a strong induction of pyruvate decarboxylase (PDC, EC 4.1.1.1) and alcohol dehydrogenase (ADH, EC 1.1.1.1) activities and a lesser induction of lactate dehydrogenase (LDH, EC 1.1.1.27) and alanine aminotransferase (AlaAT, EC 2.6.1.2) activities in roots. Inhibition of the second enzyme of the BCAA biosynthesis (ketol-acid reductoisomerase, KARI, EC 1.1.1.86) by Hoe 704 (2-dimethylphosphinoyl-2-hydroxyacetic acid) and CPCA (1,1-cyclopropanedicarboxylic acid) enhanced fermentative enzyme activities including PDC, ADH, and AlaAT. Fermentative metabolism induction occurring with ALS- and KARI-inhibitors was related to a higher expression of PDC. In the case of KARI inhibition, it is proposed that fermentation induction is due to an inhibition of ALS activity resulted from an increase in acetolactate concentration. Fermentative metabolism induction in roots, or at least ethanolic fermentation, appeared to be a general physiological response to the BCAA biosynthesis inhibition.

Carbohydrate accumulation in leaves of plants treated with the herbicide chlorsulfuron or imazethapyr is due to a decrease in sink strength

Herbicides that inhibit branched chain amino acid biosynthesis produce a rapid carbohydrate increase in leaves of treated plants. The relationship between these processes is not known nor is the importance of carbohydrate accumulation in the growth inhibition caused by these herbicides. This work analyzes carbohydrate concentration in sources and sinks after herbicide treatments in pea (Pisum sativum L.), as well as photosynthetic carbon assimilation, using two classes of chemicals, chlorsulfuron and imazethapyr, applied to roots or leaves. The most remarkable result was that, in addition to carbohydrate accumulation in leaves, accumulation of sucrose and/or starch in roots was detected. This pattern of carbohydrate accumulation was similar for both herbicides and independent of whether the herbicides were applied to leaves or roots. This indicates that root growth inhibition was not caused by sugar starvation in sinks. Nevertheless, the results are consistent with a decrease in sink strength, leading to the inhibition of photoassimilate translocation.

Influence of chlorsulfuron on rhizobial growth, nodule formation, and nitrogen fixation with chickpea

Sulfonylurea residues have been found to inhibit the growth of some legume crops and pastures in seasons following application. Negative effects of these herbicides on symbiotic nitrogen fixation by legume crops and pastures have been demonstrated. Reductions in nitrogen fixation may result from a direct effect of the herbicide on rhizobial growth and/or an indirect effect on plant growth. In this study the influence of chlorsulfuron on the growth of chickpea rhizobia [Mesorhizobium ciceri (CC1192)], the growth of chickpea plants, and the extent of nodulation and nitrogen fixation by the chickpea/rhizobia symbiosis were examined. In vitro studies (in yeast mannitol broth and a defined medium) showed that chlorsulfuron applied at double the recommended field application rate did not influence the growth of chickpea rhizobia. An experiment using 14C-labelled chlorsulfuron was conducted to determine if rhizobial cells exposed to chlorsulfuron could deliver the herbicide to the point of root infection and nodule formation. Approximately 1% of the herbicide present in the rhizobial growth medium remained with the cell/inoculum material after rinsing with 1/4 strength Ringer’s solution. This was considered unlikely to affect chickpea growth, nodulation, or nitrogen fixation. A pot experiment was used to define the influence of chlorsulfuron on the growth, nodulation, and nitrogen fixation of chickpeas. The presence of chlorsulfuron in the soil reduced the nodulation and nitrogen fixation of the chickpea plants. Pre-exposing rhizobia to chlorsulfuron before inoculating them into pots with germinating chickpea seeds, reduced the number of nodules formed by 51%. Exposure of chickpeas and chickpea rhizobia to chlorsulfuron can adversely affect the formation and activity of symbiotic nitrogen-fixing nodules, even when only the rhizobial inoculant is exposed briefly to the herbicide.

Changes in mitochondrial electron partitioning in response to herbicides inhibiting branched-chain amino acid biosynthesis in soybean

The adaptation of the respiratory metabolism in roots of soybean (Glycine max L. Merr. cv Ransom) treated with herbicides that inhibit the enzyme acetolactate synthase (ALS) was analyzed. A new gas phase dual-inlet mass spectrometry system for simultaneous measurement of 34O2 to 32O2 and O2 to N2 ratios has been developed. This system is more accurate than previously described systems, allows measurements of much smaller oxygen gradients, and, as a consequence, works with tissues that have lower respiration rates. ALS inhibition caused an increase of the alternative oxidase (AOX) protein and an accumulation of pyruvate. The combination of these two effects is likely to induce the activation of the alternative pathway and its participation in the total respiration. Moreover, the start of the alternative pathway activation and the increase of AOX protein were before the decline in the activity of cytochrome pathway. The possible role of AOX under ALS inhibition is discussed.