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

Unravelling the Phytotoxic Effects of Glyphosate on Sensitive and Resistant Amaranthus palmeri Populations by GC-MS and LC-MS Metabolic Profiling

Glyphosate, the most successful herbicide in history, specifically inhibits the activity of the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS; EC 2.5.1.19), one of the key enzymes in the shikimate pathway. Amaranthus palmeri is a driver weed in agriculture today that has evolved glyphosate-resistance through increased EPSPS gene copy number and other mechanisms. Non-targeted GC-MS and LC-MS metabolomic profiling was conducted to examine the innate physiology and the glyphosate-induced perturbations in one sensitive and one resistant (by EPSPS amplification) population of A. palmeri. In the absence of glyphosate treatment, the metabolic profile of both populations was very similar. The comparison between the effects of sublethal and lethal doses on sensitive and resistant populations suggests that lethality of the herbicide is associated with an amino acid pool imbalance and accumulation of the metabolites of the shikimate pathway upstream from EPSPS. Ferulic acid and its derivatives were accumulated in treated plants of both populations, while quercetin and its derivative contents were only lower in the resistant plants treated with glyphosate.

Searching for biomarkers of early detection of 2,4-D effects in a native tree species from the Brazilian Cerrado biome

Biodiversity in the Brazilian Cerrado biome has been declining sharply with the continued expansion of agriculture and the excessive use of herbicides. Thus, the aim of this study was to evaluate the morphophysiological and biochemical responses in Dipteryx alata plants to various doses of the herbicide 2,4-D. Specific biomarkers that characterize the phytoindicator potential of this species were determined. Gas exchange, chlorophyll a fluorescence, photosynthetic pigments, and the activities of antioxidant enzymes and cellulase were performed after 24, 96 and/or 396 hours after 2,4-D application (HAA). The herbicide caused higher antioxidant enzymatic activity 24 HAA and damage to the photosynthetic machinery after 96 HAA. Reduction in gas exchange, chlorophyll content, and photochemical traits were observed. Increased respiratory rates, non-photochemical quenching, and carotenoid concentrations in 2,4-D-treated plants were important mechanisms in the defense against the excess energy absorbed. Furthermore, the absence of leaf symptoms suggested tolerance of D. alata to 2,4-D. Nevertheless, changes in the photosynthetic and biochemical metabolism of D. alata are useful as early indicators of herbicide contamination, especially in the absence of visual symptoms. These results are important for early monitoring of plants in conserved areas and for preventing damage to sensitive species.

Local-scale dynamics of plant-pesticide interactions in a northern Brittany agricultural landscape

Soil pollution by anthropogenic chemicals is a major concern for sustainability of crop production and of ecosystem functions mediated by natural plant biodiversity. Understanding the complex effects of soil pollution requires multi-level and multi-scale approaches. Non-target and agri-environmental plant communities of field margins and vegetative filter strips are confronted with agricultural xenobiotics through soil contamination, drift, run-off and leaching events that result from chemical applications. Plant-pesticide dynamics in vegetative filter strips was studied at field scale in the agricultural landscape of a long-term ecological research network in northern Brittany (France). Vegetative filter strips effected significant pesticide abatement between the field and riparian compartments. However, comparison of pesticide usage modalities and soil chemical analysis revealed the extent and complexity of pesticide persistence in fields and vegetative filter strips, and suggested the contribution of multiple sources (yearly carry-over, interannual persistence, landscape-scale contamination). In order to determine the impact of such persistence, plant dynamics was followed in experimentally-designed vegetative filter strips of identical initial composition (Agrostis stolonifera, Anthemis tinctoria/Cota tinctoria, Centaurea cyanus, Fagopyrum esculentum, Festuca rubra, Lolium perenne, Lotus corniculatus, Phleum pratense, Trifolium pratense). After homogeneous vegetation establishment, experimental vegetative filter strips underwent rapid changes within the following two years, with Agrostis stolonifera, Festuca rubra, Lolium perenne and Phleum pratense becoming dominant and with the establishment of spontaneous vegetation. Co-inertia analysis showed that plant dynamics and soil residual pesticides could be significantly correlated, with the triazole fungicide epoxiconazole, the imidazole fungicide prochloraz and the neonicotinoid insecticide thiamethoxam as strong drivers of the correlation. However, the correlation was vegetative-filter-strip-specific, thus showing that correlation between plant dynamics and soil pesticides likely involved additional factors, such as threshold levels of residual pesticides. This situation of complex interactions between plants and soil contamination is further discussed in terms of agronomical, environmental and health issues.

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.

Dipteryx alata, a tree native to the Brazilian Cerrado, is sensitive to the herbicide nicosulfuron

The expansion of land use for agricultural interests and the excessive use of herbicides are among the causes of biodiversity losses in the Brazilian Cerrado biome. Therefore, we aimed to test the hypothesis that Dipteryx alata Vogel, a common species in this biome, is sensitive to nicosulfuron because of its high phytotoxicity. We evaluated physiological, biochemical and morphological responses in D. alata plants exposed to increasing doses of the herbicide. Young plants were transplanted to 10 L pots containing substrate composed of soil and sand (2:1) after fertilization. After an acclimation period, the following doses of nicosulfuron were applied: 0 (control), 6, 12, 24, 48, and 60 g a.e. ha^-1. The experiment was conducted in a randomized block design factorial scheme with six doses of nicosulfuron, three evaluation times, and five replicates per treatment. The effects of the herbicide were assessed by measuring gas exchange, chlorophyll a fluorescence, photosynthetic pigments, membrane permeability, antioxidant enzymes and acetolactate synthase. Nicosulfuron altered the photosynthetic machinery and enzymatic metabolism of D. alata. Reductions in physiological traits, increased catalase and ascorbate peroxidase activities, enhanced malondialdehyde concentrations rate of electrolyte leakage and decreased acetolactate synthase activity in response to nicosulfuron all suggest that D. alata is sensitive to this herbicide.

Species- and organ-specific responses of agri-environmental plants to residual agricultural pollutants

Soil pollution by anthropogenic chemicals is a major concern for sustainability of crop production and of ecosystem functions mediated by natural plant biodiversity. The complex effects on plants are however difficult to apprehend. Plant communities of field margins, vegetative filter strips or rotational fallows are confronted with agricultural pollutants through residual soil contamination and/or through drift, run-off and leaching events that result from chemical applications. Exposure to xenobiotics and heavy metals causes biochemical, physiological and developmental effects. However, the range of doses, modalities of exposure, metabolization of contaminants into derived xenobiotics, and combinations of contaminants result in variable and multi-level effects. Understanding these complex plant-pollutant interactions cannot directly rely on toxicological or agronomical approaches that focus on the effects of field-rate pesticide applications. It must take into account exposure at root level, sublethal concentrations of bioactive compounds and functional biodiversity of the plant species that are affected. The present study deals with agri-environmental plant species of field margins, vegetative filter strips or rotational fallows in European agricultural landscapes. Root and shoot physiological and growth responses were compared under controlled conditions that were optimally adjusted for each plant species. Contrasted responses of growth inhibition, no adverse effect or growth enhancement depended on species, organ and nature of contaminant. However, all of the agricultural contaminants under study (pesticides, pesticide metabolites, heavy metals, polycyclic aromatic hydrocarbons) had significant effects under conditions of sublethal exposure on at least some of the plant species. The fungicide tebuconazole and polycyclic aromatic hydrocarbon fluoranthene, which gave highest levels of responses, induced both activation or inhibition effects, in different plant species or in different organs of the same plant species. These complex effects are discussed in terms of dynamics of agri-environmental plants and of ecological consequences of differential root-shoot growth under conditions of soil contamination.

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.

Foliar Roundup application has minor effects on the compositional and functional diversity of soil microorganisms in a short-term greenhouse experiment

The herbicide Roundup (and glyphosate, its active ingredient) is extensively used for weed control on a worldwide scale. It is absorbed after foliar application and quickly translocated inside the plant. In this study, we investigated the effects of Roundup speed, a commercial glyphosate formulation, on the structural composition (dominance of microbial groups, phospholipid fatty acid analysis - PLFA) and functional diversity (use of carbon sources, Multiple Substrate Induced Respiration - MSIR) of soil microorganisms. We specifically aimed at understanding the potential impact of biotic interactions on herbicide effects and included plants, earthworms, and endomycorrhizal fungi in the experimental setup. For this, we grew clover (Trifolium repens) in the greenhouse and added mycorrhizal inoculum (Glomus mosseae) and earthworms (Lumbricus terrestris) to the pots. Two weeks after foliar Roundup application and subsequent plant death, the pots were destructively sampled. The application resulted in a significant increase of microbial respiration (SIR) by approximately 30%. A multivariate analysis of the MSIR data exhibited small but significant differences between the microbial communities of treated and untreated pots, while no significant difference was apparent for the PLFA data. Bacterial PLFAs generally decreased following herbicide application, while mycorrhizal and fungal PLFAs were not affected. We did not find a consistent difference between the fatty acid markers of gram negative and gram positive bacteria. For all investigated parameters, there were highly significant differences between the upper (0-5 cm depth) and lower (5-10 cm) soil layers. The fact that rooting density differed by a factor of 3.5 between the two layers indicated that herbicide effects were especially pronounced in the clover rhizosphere and were likely due to changes in root exudate composition. We found significant, though very small, interactions between Roundup and other experimental factors (especially mycorrhizal inoculum).

Separate and combined effects of glyphosate and copper on growth and antioxidative enzymes in Salvinia natans (L.) All

The coexistence of glyphosate and copper is widely found in bodies of water and terrestrial ecosystems due to widespread application of herbicides and heavy metal. However, their joint ecotoxicological risks in aquatic environments remain unknown. The experiment investigated the individual and combined effects of glyphosate and copper on the growth and physiological response in Salvinia natans (L.) All. The results showed that their joint toxicity is related to concentration. Antagonistic effects were induced when plants were exposed to low concentrations of glyphosate and copper (≤1 + 0.2 mg l-1). Synergistic effects were elicited at higher doses (≥5 + 1 mg l-1). In addition, increased hydrogen peroxide levels indicated the occurrence of oxidative stress at individual or combined exposures. To cope with oxidative stress, S. natans can activate the antioxidant defense systems, including increased superoxide dismutase and changes in peroxidase, ascorbate peroxidase and catalase. High concentrations of combined pollution exceed the oxidative defense capabilities of plants, and therefore, malondialdehyde content increased significantly. Our results indicated that the ecotoxicity of glyphosate or copper may be exacerbated in aquatic environments and caused obvious damage to S. natans.

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.

Responses of Hydrilla verticillata (L.f.) Royle and Vallisneria natans (Lour.) Hara to glyphosate exposure

Glyphosate is a broad-spectrum herbicide that is frequently detected in water bodies and is harmful to aquatic systems. We conducted an experiment to explore the ecological sensitivity of Hydrilla verticillata (L.f.) Royle and Vallisneria natans (Lour.) Hara to glyphosate. Our research focused on the physiological responses of H. verticillata and V. natans after exposure to various concentrations of glyphosate (0, 1, 10, 20, 30, 40, 50 and 80 mg/L) in hydroponic culture after one day (1D) and seven days (7D). The results show that after 1D, the soluble protein content of H. verticillata was significantly stimulated under low herbicide concentrations. Other indices for H. verticillata and V. natans had no remarkable changes at 1D. After 7D of treatment, the soluble protein content of H. verticillata showed no significant differences, while the malondialdehyde (MDA), pigment contents and catalase (CAT) activity significantly increased at low glyphosate concentrations. Guaiacol peroxidase (POD) activity in H. verticillata significantly increased with increasing herbicide concentrations. The chlorophyll a/b ratio of H. verticillata sharply decreased above 10 mg/L. For V. natans, soluble protein, chlorophyll a, and carotenoid content; and CAT activity declined significantly after glyphosate application, while other indicators showed no significant changes. Our results indicate that glyphosate concentrations from 0 to 80 mg/L can induce oxidative stress in H. verticillate and may impede metabolism processes for protein and pigments without causing oxidative stress in V. natans. Taken together, our results suggest that the sensitivity of H. verticillata to glyphosate exposure is higher than that of V. natans.

An integrated multi-omics analysis of the NK603 Roundup-tolerant GM maize reveals metabolism disturbances caused by the transformation process

Glyphosate tolerant genetically modified (GM) maize NK603 was assessed as 'substantially equivalent' to its isogenic counterpart by a nutrient composition analysis in order to be granted market approval. We have applied contemporary in depth molecular profiling methods of NK603 maize kernels (sprayed or unsprayed with Roundup) and the isogenic corn to reassess its substantial equivalence status. Proteome profiles of the maize kernels revealed alterations in the levels of enzymes of glycolysis and TCA cycle pathways, which were reflective of an imbalance in energy metabolism. Changes in proteins and metabolites of glutathione metabolism were indicative of increased oxidative stress. The most pronounced metabolome differences between NK603 and its isogenic counterpart consisted of an increase in polyamines including N-acetyl-cadaverine (2.9-fold), N-acetylputrescine (1.8-fold), putrescine (2.7-fold) and cadaverine (28-fold), which depending on context can be either protective or a cause of toxicity. Our molecular profiling results show that NK603 and its isogenic control are not substantially equivalent.

First Resistance Mechanisms Characterization in Glyphosate-Resistant Leptochloa virgata

Leptochloa virgata (L.) P. Beauv. is an annual weed common in citrus groves in the states of Puebla and Veracruz, Mexico limiting their production. Since 2010, several L. virgata populations were identified as being resistant to glyphosate, but studies of their resistance mechanisms developed by this species have been conducted. In this work, three glyphosate-resistant populations (R8, R14, and R15) collected in citrus orchards from Mexico, were used to study their resistance mechanisms comparing them to one susceptible population (S). Dose-response and shikimic acid accumulation assays confirmed the glyphosate resistance of the three resistant populations. Higher doses of up to 720 g ae ha^-1 (field dose) were needed to control by 50% plants of resistant populations. The S population absorbed between 7 and 13% more ¹⁴C-glyphosate than resistant ones, and translocated up to 32.2% of ¹⁴C-glyphosate to the roots at 96 h after treatment (HAT). The R8, R14, and R15 populations translocated only 24.5, 26.5, and 21.9%, respectively. The enzyme activity of 5-enolpyruvyl shikimate-3-phosphate synthase (EPSPS) was not different in the S, R8 and R14 populations. The R15 Population exhibited 165.9 times greater EPSPS activity. Additionally, this population showed a higher EPSPS basal activity and a substitution in the codon 106 from Proline to Serine in the EPSPS protein sequence. EPSPS gene expression in the R15 population was similar to that of S population. In conclusion, the three resistant L. virgata populations show reduced absorption and translocation of ¹⁴C-glyphosate. Moreover, a mutation and an enhanced EPSPS basal activity at target-site level confers higher resistance to glyphosate. These results describe for the first time the glyphosate resistance mechanisms developed by resistant L. virgata populations of citrus orchards from Mexico.

Stable Isotope Resolved Metabolomics Reveals the Role of Anabolic and Catabolic Processes in Glyphosate-Induced Amino Acid Accumulation in Amaranthus palmeri Biotypes

Biotic and abiotic stressors often result in the buildup of amino acid pools in plants, which serve as potential stress mitigators. However, the role of anabolic (de novo amino acid synthesis) versus catabolic (proteolytic) processes in contributing to free amino acid pools is less understood. Using stable isotope-resolved metabolomics (SIRM), we measured the de novo amino acid synthesis in glyphosate susceptible (S-) and resistant (R-) Amaranthus palmeri biotypes. In the S-biotype, glyphosate treatment at 0.4 kg ae/ha resulted in an increase in total amino acids, a proportional increase in both (14)N and (15)N amino acids, and a decrease in soluble proteins. This indicates a potential increase in de novo amino acid synthesis, coupled with a lower protein synthesis and a higher protein catabolism following glyphosate treatment in the S-biotype. Furthermore, the ratio of glutamine/glutamic acid (Gln/Glu) in the glyphosate-treated S- and R-biotypes indicated that the initial assimilation of inorganic nitrogen to organic forms is less affected by glyphosate. However, amino acid biosynthesis downstream of glutamine is disproportionately disrupted in the glyphosate treated S-biotype. It is thus concluded that the herbicide-induced amino acid abundance in the S-biotype is contributed by both protein catabolism and de novo synthesis of amino acids such as glutamine and asparagine.

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.