Browsing by Subject "Glyphosate"

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Adaptation of model organisms and environmental bacilli to glyphosate gives insight to species-specific peculiarities of the shikimate pathway
(2024) Schwedt, Inge; Commichau, Fabian M.
Glyphosate (GS), the active ingredient of the popular herbicide Roundup, inhibits the 5-enolpyruvyl shikimate-3-phosphate (EPSP) synthase of the shikimate pathway, which is present in archaea, bacteria, Apicomplexa, algae, fungi, and plants. In these organisms, the shikimate pathway is essential for de novo synthesis of aromatic amino acids, folates, quinones and other metabolites. Therefore, the GS-dependent inhibition of the EPSP synthase results in cell death. Previously, it has been observed that isolates of the soil bacteria Burkholderia anthina and Burkholderia cenocepacia are resistant to high amounts of GS. In the framework of this PhD thesis, it could be demonstrated that B. anthina isolates are not intrinsically resistant to GS. However, B. anthina rapidly adapts to the herbicide at the genome level and the characterization of GS-resistant suppressor mutants led to the discovery of a novel GS resistance mechanism. In B. anthina, the acquisition of loss-of-function mutations in the ppsR gene increases GS resistance. The ppsR gene encodes a regulator of the phosphoenolpyruvate (PEP) synthetase PpsA. In the absence of a functional PpsR protein, the bacteria synthesize more PEP, which competes with GS for binding in the active site of the EPSP synthase, increasing GS resistance. The EPSP synthase in B. anthina probably does not allow changes in the amino acid sequence as it is the case in other organisms. Indeed, the Gram-negative model organism Escherichia coli evolves GS resistance by the acquisition of mutations that either reduce the sensitivity of the EPSP synthase or increase the cellular concentration of the enzyme. Unlike E. coli, the EPSP synthase is also critical for the viability of Gram-positive model bacterium Bacillus subtilis. This observation is surprising because the enzyme belongs to the class of GS-insensitive EPSP synthases. In fact, the EPSP synthase is essential for growth of B. subtilis. The determination of the nutritional requirements allowing the growth of B. subtilis and E. coli mutants lacking EPSP synthase activity revealed that the demand for shikimate pathway intermediates is higher in the former organism. This finding explains why laboratory as well as environmental Bacilli exclusively adapt to GS by the mutational inactivation of glutamate transporter genes. Here, it was also shown that a B. subtilis mutant lacking EPSP synthase activity grows in minimal medium only when additional mutations accumulate in genes involved in the regulation of aerobic/anaerobic metabolism and central carbon metabolism. The characterization of these additional mutants will help to elucidate the peculiarities of the shikimate pathway in B. subtilis. Moreover, the mutants could be useful to identify the aromatic amino acid transporters that still await their discovery.
Exploring the effects of different stubble tillage practices and glyphosate application combined with the new soil residual herbicide cinmethylin against Alopecurus myosuroides Huds. in winter wheat
(2022) Messelhäuser, Miriam Hannah; Saile, Marcus; Sievernich, Bernd; Gerhards, Roland
Effective control of Alopecurus myosuroides Huds. (blackgrass) solely with a chemical treatment is not guaranteed anymore because populations exhibit resistance to almost all herbicide modes of action. Integrated weed management (IWM) against blackgrass is necessary to maintain high weed control efficacies in winter cereals. Four field experiments were conducted in Southwest Germany from 2018 to 2020 to control A. myosuroides with a combination of cultural and chemical methods. Stubble treatments, including flat, deep and inversion soil tillage; false seedbed preparation and glyphosate use, were combined with the application of the new pre-emergence herbicide cinmethylin in two rates in winter wheat. Average densities of A. myosuroides in the untreated control plots were up to 505 plants m−2. The combination of different stubble management strategies and the pre-emergence herbicide cinmethylin controlled 86–97% of A. myosuroides plants at the low rate and 95–100% at the high rate until 120 days after sowing. The different stubble tillage practices varied in their efficacy between trials and years. Most effective and consistent were pre-sowing glyphosate application on the stubble and stale seedbed preparation with a disc harrow. Stubble treatments increased winter wheat density in the first year but had no effect on crop density in the second year. Pre-emergence application of cinmethylin did not reduce winter wheat densities. Multiple tactics of weed control, including stubble treatments and pre-emergence application of cinmethylin, provided higher and more consistent control of A. myosuroides. Integration of cultural weed management could prevent the herbicide resistance development.
Integrating perennial biomass crops into crop rotations: How to remove miscanthus and switchgrass without glyphosate
(2023) Lewin, Eva; Kiesel, Andreas; Magenau, Elena; Lewandowski, Iris
Perennial energy grasses have gained attention in recent years as a promising resource for the bioeconomy because of their benign environmental profile, high stress tolerance, high biomass yields and low input requirements. Currently, strong breeding efforts are being made to extend the range of commercially available miscanthus and switchgrass genotypes. In order to foster farmers' acceptance of these crops, and especially of novel hybrids, more information is required about how they can be efficiently integrated into cropping rotations, how they can be removed at the end of their productive lifespan, and what effect they have on subsequently grown crops. Farmers in Europe are meanwhile increasingly constrained in the methods available to them to remove these crops, and there is a risk that the herbicide glyphosate, which has been used in many studies to remove them, will be banned in coming years. This study looks at the removal of seven‐year‐old stands of miscanthus and switchgrass over 1 year at an experimental site in Southern‐Germany. Three novel miscanthus genotypes were studied, alongside one variety of switchgrass, and the impact of each crop's removal on the yield of maize grown as a follow‐on crop was examined. A combination of soil tillage and grass herbicides for maize cultivation was successful in controlling miscanthus regrowth, such that yields of maize grown after miscanthus did not differ significantly from yields of maize grown in monoculture rotation (18.1 t dry biomass ha−1). Yields of maize grown after switchgrass (14.4 t dry biomass ha−1) were significantly lower than maize in monoculture rotation caused by insufficient control of switchgrass regrowth by the applied maize herbicide. Although some regrowth of miscanthus and switchgrass was observed in the follow‐on crop maize, complete eradication of both crops was achieved by subsequent winter wheat cultivation.
Re-plant problems in long-term no-tillage cropping systems : causal analysis and mitigation strategies
(2016) Afzal; Neumann, Günter
No-tillage is considered as a promising alternative for tillage-based conventional farming, by saving energy-input and time, reducing groundwater pollution and counteracting soil erosion and losses of the soil-organic matter. However, in the recent past, no-tillage farmers in Southwest Germany repeatedly reported problems particularly in winter wheat production, characterized by stunted plant growth in early spring, chlorosis, impaired fine root development and increased disease susceptibility. These symptoms were particularly apparent on field sites with long-term (≥ 10 years) no-tillage history (LT) but not on adjacent short-term (≤ 2 years) no-tillage plots (ST). The effects could be reproduced in pot experiments under controlled conditions, with soils collected from the respective field sites in five different locations, providing a basis for causal analysis. The expression of damage symptoms in pot experiments with sieved soils, excluded differences in soil compaction, induced by long-term no-tillage farming as a potential cause. Soil analysis revealed higher levels of soil organic matter in the topsoil, as expected for LT field sites and no apparent mineral nutrient deficiencies, both, on LT and ST soils. However, phosphate (P) deficiency was characteristic for plants grown on LT soils. Obviously, this was caused by the limited acquisition of sparingly soluble soil P, due to impaired root development but not by low P availability on LT soils. In four out of five cases, gamma-ray soil sterilization did not affect the expression of plant damage symptoms on LT soils, excluding pathogen effects as a major cause. Soil application of biochar, at a rate of 5% (v/v), rapidly restored plant growth on LT soils, detectable already during the first week after sowing. This finding points to the presence of a phytotoxic compound since binding of soil xenobiotics by biochar is well documented. Accumulation of allelopathic compounds, originating from crop residues and root exudates remaining in the topsoil, is a problem related to no-tillage farming, particularly in cases of limited crop rotations or in monocultures, which also applied to the investigated field sites. However, a specific wheat auto-allelopathic effect is unlikely, since similar crop damage was also observed in soybean, sunflower, oilseed rape and various cover crops. Typical for allelopathic effects, in the pot experiments, plant damage symptoms in winter wheat appeared rapidly during emergence and early seedling development. However, under field conditions, germination and early growth were usually not affected, and symptoms were first detectable during re-growth in early spring. Moreover, damage symptoms disappeared when soil sampling was performed in summer instead of early spring, suggesting degradation of the toxic compound, which is also not compatible with the hypothesis of long-term accumulation of allelopathic compounds. The observed temporal pattern of plant damage rather resembled residual effects, occasionally observed after application of certain herbicides with soil activity (e.g., sulfonylureas, propyzamide). Therefore, a systematic survey of herbicide residues was conducted for topsoils on six pairs of LT and ST-field sites. Characteristic for no-tillage farming, glyphosate was the only herbicide, commonly and regularly used on all investigated field sites. The soil analysis revealed higher levels of glyphosate residues on all investigated LT, soils as compared with directly neighboured ST plots. Particularly on LT plots with strong expression of plant damage symptoms, high concentrations of glyphosate (2-4 mg kg-1 soil), and of its metabolite AMPA were detected in the 10 cm topsoil layer. This concentration range is characteristic for residual levels, usually observed several days after glyphosate applications but was still detectable in early spring, six months after the last glyphosate treatment, while only trace concentrations below the detection limit (0.05 mg kg-1 soil) were found in ST soils. Coinciding with the declining plant damage potential, residual glyphosate and AMPA concentrations on LT plots declined during the vegetation period until early summer. No comparable pattern was detectable for residues of other herbicides, such as pendimethalin and propyzamide. Degradation of glyphosate residues in soils correlates with microbial activity. Accordingly, reduced soil respiration as an indicator for microbial activity was detected in four out of five cases in soil samples collected from LT field sites, suggesting delayed glyphosate degradation as compared with ST plots. Due to rapid adsorption, glyphosate usually exhibits extremely limited soil activity. However, at least trace concentrations of glyphosate and AMPA (1.5-3.5 µg L-1) were detectable also in the potentially plant-available, water-soluble phase in spring samples, collected from LT field plots with high potential for plant damage. Nutrient solution experiments, with 3-6 weeks exposure of winter wheat to the residual herbicide concentrations detected in the LT soil solution, revealed the development of chlorosis and similar to soil experiments, a 30%-50% reduction in fine root production, which surprisingly was mainly induced by AMPA and to a lesser extent by glyphosate itself. Accordingly, both, in hydroponics and LT soil experiments, the plant damage symptoms were not associated with shikimate accumulation in the root tissue as a physiological indicator for glyphosate but not for AMPA toxicity. The dominant role of AMPA toxicity also became apparent by the fact that, both, glyphosate resistant (GR) and non-resistant (NR) soybean plants were affected on LT no-tillage soils since transgenic GR plants are not resistant to AMPA. A preliminary RNAseq gene expression analysis of the root tissue just prior to the appearance of visible plant damage symptoms, revealed down-regulation of genes involved in general stress responses, down-regulation of aquaporin genes (PIPs and TIPs) with functions in water uptake and root elongation, down-regulation of ethylene-related genes but up-regulation of cytokinin-related gene expression indicating interferences with hormonal balances. These changes in gene expression patterns relative to the untreated control were detected in plants treated with AMPA and glyphosate+AMPA but not with glyphosate alone. The findings suggest that long-term exposure to subtoxic levels of AMPA, as major glyphosate metabolite temporally accumulated in LT no-tillage soils, can finally interfere with metabolic processes essential for normal root development. A series of pot and field experiments were initiated to test the potential of selected commercial formulations of plant growth-promoting microorganisms, based on strains of Pseudomonas sp., Bacillus amyloliquefaciens, and Trichoderma harzianum, for mitigation of plant stress symptoms, expressed on LT no-tillage field sites in spring. For members of the selected microbial genera, root growth-promoting effects, pathogen suppression, and glyphosate degradation potential have been reported. Unfortunately, plant growth promotion was detectable only on ST soils but was not successful on LT plots, both, in pot and field experiments, probably related to limited root development for microbial colonization and early summer drought under field conditions. As an alternative approach, incorporation of pyrolysis biochar from woody substrates at a rate of 5 % (v/v) to the top 10 cm soil layer of LT soils, equivalent to approx. 35 t ha-1, were able to restore plant growth completely in pot experiments and protected wheat plants from glyphosate overdose applications (up to 8 L Roundup Ultramax® ha-1), even on artificial substrates with low potential for glyphosate adsorption. As a short-term mitigation strategy, field-testing with different biochar concentrations is recommended. During the last two years, farmers also modified their no-tillage management strategies on the investigated field sites by introducing more variable crop rotations including, winter wheat, winter rape, maize and soybean and using mustard, pea, and Crotalaria as cover crops. Despite further annual applications of glyphosate (3 L ha-1 of a 360 g ai L-1 formulation), plant performance on the respective field sites was significantly improved. These observations suggest that limited crop rotation favored the development of a soil microflora with low degradation potential for glyphosate, leading to a decline in degradation rates of glyphosate soil residues and underline the importance of crop diversity management.
Rhizosphere processes as determinants for glyphosate damage of non-target plants
(2010) Bott, Sebastian; Neumann, Günter
Due to low production costs and high herbicidal efficiency, glyphosate is the most widely used wide-spectrum herbicide. Glyphosate acts as a non-selective, total herbicide by inhibiting the biosynthesis of aromatic amino acids. Apart from glyphosate drift contamination, risks of glyphosate toxicity to crop plants and other non-target organisms are generally considered as marginal, because glyphosate is almost instantaneously inactivated by adsorption to the soil matrix and rapid microbial/chemical degradation in the soil solution. However, in the recent past, an increasing number of yet unexplained observations on significant damage of crop plants have been reported in the literature and by farmers, suggesting gaps in the risk assessment, with respect to the fate glyphosate in the rhizosphere and the interaction with rhizosphere processes. According to these observations, the aim of present study was a systematic evaluation of potential rhizosphere effects of glyphosate, including direct toxicity, risks of re-mobilisation by fertiliser application, potential role of pathogens and allelopathic compounds, and interactions with micronutrients, both in glyphosate-sensitive and transgenic glyphosate-resistant crops. A series of field trials in reduced soil tillage cropping systems as well as green-house experiments on soils with contrasting properties with sunflower, winter wheat and soybean, consistently revealed a close clausal relationship between crop damage and (a) short waiting times between glyphosate application on target weeds and subsequent sowing of crops and (b) the density and speed of decay of glyphosate-treated weeds. The results suggested that damage of crop plants is induced by a rhizosphere transfer of glyphosate from weeds to subsequently sown crops. This transfer might take place by contact contamination due to exudation of glyphosate from living roots of treated weeds and/or release during decomposition of the root residues. A comparison between phytotoxic effects of glyphosate and aminomethylphosphonic acid (AMPA) as major metabolite of glyphosate in soils, revealed high toxicity in case of root exposure to glyphosate, but not to AMPA. By contrast, a significant decline of germination was induced by seed exposure to AMPA, while germination was not affected by glyphosate treatments. The observed differences in sensitivity to glyphosate and AMPA in different stages of plant development may explain variable symptoms of crop damage under field conditions, ranging from growth depressions and chlorosis to reduced field emergence. The results of the present study further suggest that risks for crop damage associated with rhizosphere transfer of glyphosate are additionally influenced by a range of environmental factors, such as growth season (spring or fall application), temperature, soil moisture, redox potential of soils and soil microbial activity. These factors might shorten or prolongate the time window for crop damage of glyphosate contact contamination in the rhizosphere under field conditions. Model experiments investigating the sensitivity of different plant species to glyphosate root exposure, revealed significant differences between winter wheat, maize and soybean in terms of glyphosate-induced plant damage but also in their ability for recovery from glyphosate damage suggesting marked genotypic differences in the expression of damage symptoms also under field conditions. In agreement with previous investigations, results of the present study indicated a rapid inactivation of glyphosate by adsorption to the soil matrix. Glyphosate adsorption in soils seem to be mainly mediated by the phosphonate group of the molecule in a way similar to the adsorption of inorganic phosphate. Accordingly glyphosate re-mobilisation is possible via ligand exchange by phosphate application. The results of the present study have demonstrated for the first time that depending on soil properties also the application of fertiliser phosphate is able to re-mobilise glyphosate in sufficient quantities to mediate crop damage in pot experiments. This finding suggest, that re-mobilisation of glyphosate potentially by fertiliser P or root-induced chemical modifications for P and Fe mobilisation needs to be considered as additional potential rhizosphere pathway for glyphosate damage to non-target plants. Field trials and model experiments under soil and hydroponic conditions consistently revealed a significantly impaired nutritional status of glyphosate-sensitive but also glyphosate-resistant crops. However, depending on the culture conditions different mineral nutrients were affected by the glyphosate treatments and plant damage was not related with a certain nutrient deficiency. These findings suggest that damaged root growth, induced by glyphosate toxicity, rather than specific interactions with certain mineral nutrients are responsible for the observed impairment of nutrient acquisition. In conclusion, results of the present study highlight that risks for crop damage associated with glyphosate toxicity in the rhizosphere can be substantial and is influenced by factors such as waiting time after herbicide application, weed density, cropping systems, fertilizer management, genotypic differences, and probably also environmental factors including temperature, soil moisture, and soil microbial activity. The independency between these factors is so far not entirely clear but should be investigated in future studies. Nevertheless, results of present study suggest that risks could be minimized by simple management tools such as the consideration of waiting times between application of glyphosate and sowing of crops particularly in case of high weed densities and alternation of herbicides to reduce not only risk for remobilization of glyphosate but also problems associated to the selection of glyphosate-resistant weeds.