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Turrini et. al, 2015

Alessandra Turrini, Cristiana Sbrana, Manuela Giovannetti, “Belowground environmental effects of transgenic crops: a soil microbial perspective,” Research in Microbiology, 2015, 166, 121-131, DOI: 10.1016/j.resmic.2015.02.006.

ABSTRACT:

Experimental studies investigated the effects of transgenic crops on the structure, function and diversity of soil and rhizosphere microbial communities playing key roles in belowground environments. Here we review available data on direct, indirect and pleiotropic effects of engineered plants on soil microbiota, considering both the technology and the genetic construct utilized. Plants modified to express phytopathogen/phytoparasite resistance, or traits beneficial to food industries and consumers, differentially affected soil microorganisms depending on transformation events, experimental conditions and taxa analyzed. Future studies should address the development of harmonized methodologies by taking into account the complex interactions governing soil life. FULL TEXT

Newman et. al, 2016

Molli M. Newman, Nicola Lorenz, Nigel Hoilett, Nathan R. Lee, Richard P. Dick, Mark R. Liles, Cliff Ramsier, Joseph W. Kloepper, “Changes in rhizosphere bacterial gene expression following glyphosate treatment,” Science of the Total Environment, 2016, 553, 32-41, DOI: 10.1016/j.scitotenv.2016.02.078.

ABSTRACT:

In commercial agriculture, populations and interactions of rhizosphere microflora are potentially affected by the use of specific agrichemicals, possibly by affecting gene expression in these organisms. To investigate this, we examined changes in bacterial gene expression within the rhizosphere of glyphosate-tolerant corn (Zea mays) and soybean (Glycine max) in response to long-term glyphosate (PowerMAX™, Monsanto Company, MO, USA) treatment. A long-term glyphosate application study was carried out using rhizoboxes under greenhouse conditions with soil previously having no history of glyphosate exposure. Rhizosphere soil was collected from the
rhizoboxes after four growing periods. Soil microbial community composition was analyzed using microbial phospholipid fatty acid (PLFA) analysis. Total RNA was extracted from rhizosphere soil, and samples were analyzed using RNA-Seq analysis. A total of 20–28 million bacterial sequences were obtained for each sample. Transcript abundance was compared between control and glyphosate-treated samples using edgeR. Overall rhizosphere bacterial metatranscriptomes were dominated by transcripts related to RNA and carbohydrate metabolism. We identified 67 differentially expressed bacterial transcripts from the rhizosphere. Transcripts downregulated following glyphosate treatment involved carbohydrate and amino acid metabolism, and upregulated transcripts involved protein metabolism and respiration. Additionally, bacterial transcripts involving nutrients, including iron, nitrogen, phosphorus, and potassium, were also affected by long-term glyphosate application. Overall, most bacterial and all fungal PLFA biomarkers decreased after glyphosate treatment compared to the control. These results demonstrate that long-term glyphosate use can affect rhizosphere bacterial activities and potentially shift bacterial community composition favoring more glyphosate-tolerant bacteria. FULL TEXT

Aristilde et. al, 2017

Ludmilla Aristilde, Michael L. Reed, Rebecca A. Wilkes, Tracy Youngster, Matthew A. Kukurugya, Valerie Katz, and Clayton R. S. Sasaki, “Glyphosate-Induced Specific and Widespread Perturbations in the Metabolome of Soil Pseudomonas Species,” Frontiers in Environmental Science, 2017, 5:34, DOI: 10.3389/fenvs.2017.00034.

ABSTRACT:

Previous studies have reported adverse effects of glyphosate on crop-beneficial soil bacterial species, including several soil Pseudomonas species. Of particular interest is the elucidation of the metabolic consequences of glyphosate toxicity in these species. Here we investigated the growth and metabolic responses of soil Pseudomonas species grown on succinate, a common root exudate, and glyphosate at different concentrations. We conducted our experiments with one agricultural soil isolate, P. fluorescens RA12, and three model species, P. putida KT2440, P. putida S12, and P. protegens Pf-5. Our results demonstrated both species- and strain-dependent growth responses to glyphosate. Following exposure to a range of glyphosate concentrations (up to 5 mM), the growth rate of both P. protegens Pf-5 and P. fluorescens RA12 remained unchanged whereas the two P. putida strains exhibited from 0 to 100% growth inhibition. We employed a 13C-assisted metabolomics approach using liquid chromatography-mass spectrometry to monitor disruptions in metabolic homeostasis and fluxes. Profiling of the whole-cell metabolome captured deviations in metabolite levels involved in the tricarboxylic acid cycle, ribonucleotide biosynthesis, and protein biosynthesis. Altered metabolite levels specifically in the biosynthetic pathway of aromatic amino acids (AAs), the target of toxicity for glyphosate in plants, implied the same toxicity target in the soil bacterium. Kinetic flux experiments with 13C-labeled succinate revealed that biosynthetic fluxes of the aromatic AAs were not inhibited in P. fluorescens Pf-5 in the presence of low and high glyphosate doses but these fluxes were inhibited by up to 60% in P. putida KT2440, even at sub-lethal glyphosate exposure. Notably, the greatest inhibition was found for the aromatic AA tryptophan, an important precursor to secondary metabolites. When the growth medium was supplemented with aromatic AAs, P. putida S12 exposed to a
lethal dose of glyphosate completely recovered in terms of both growth rate and selected metabolite levels. Collectively, our findings led us to conclude that the glyphosateinduced specific disruption of de novo biosynthesis of aromatic AAs accompanied by widespread metabolic disruptions was responsible for dose-dependent adverse effects of glyphosate on sensitive soil Pseudomonas species. FULL TEXT

Kremer, 2014

Robert J. Kremer, “Environmental Implications of Herbicide Resistance: Soil Biology and Ecology,” Weed Science, 2014, 62.

ABSTRACT:

Soil microbial community structure and activity are linked to plant communities. Weeds may alter their soil environment, selecting for specific rhizosphere microbial communities. Rhizosphere modification occurs for many crop and horticultural plants. However, impacts of weeds in agroecosystems on soil biology and ecology have received less attention because effective weed management practices were developed to minimize their impacts on crop production. The recent development of herbicide resistance (HR) in several economically important weeds leading to widespread infestations in crop fields treated with a single herbicide has prompted a re-evaluation of the effects of weed growth on soil biology and ecology. The objective of this article is to review the potential impacts of herbicide-resistant weeds on soil biological and ecological properties based on reports for crops, weeds, and invasive plants. Persistent weed infestations likely establish extensive root systems and release various plant metabolites through root exudation. Many exudates are selective for specific soil microbial groups mediating biochemical and nutrient acquisition processes. Exudates may stimulate development of microbial groups beneficial to weed but detrimental to crop growth or beneficial to both. Changes in symbiotic and associative microbial interactions occur, especially for arbuscular mycorrhizal fungi (AMF) that are important in plant uptake of nutrients and water, and protecting from phytopathogens. Mechanisms used by weeds to disrupt symbioses in crops are not clearly described. Many herbicide-resistant weeds including Amaranthus and Chenopodium do not support AMF symbioses, potentially reducing AMF propagule density and establishment with crop plants. Herbicides applied to control HR weeds may compound effects of weeds on soil microorganisms. Systemic herbicides released through weed roots may select microbial groups that mediate detrimental processes such as nutrient immobilization or serve as opportunistic pathogens. Understanding complex interactions of weeds with soil microorganisms under extensive infestations is important in developing effective management of herbicide-resistant weeds. FULL TEXT

Tesfamariam et. al, 2009

Tsehaye Tesfamariam, S. Bott, I. Cakmak, V. Römheld, G. Neumann, “Glyphosate in the rhizosphere—Role of waiting times and different glyphosate binding forms in soils for phytotoxicity to non-target plants,” European Journal of Agronomy, 2009, 31, 126-132, DOI: 10.1016/j.eja.2009.03.007.

ABSTRACT:

Glyphosate is the most widely used non-selective, systemic herbicide. It is easily translocated from shoot to roots and released into the rhizosphere, where it is immobilized at the soil matrix or microbially degraded. However, contradictory results are reported in the literature concerning the bio-availability of glyphosate residues in soils and the potential risks for intoxication of non-target organisms. This study addresses the question whether plant residues of glyphosate-treated weeds (model plant perennial rye grass, Lolium perenne L.) or direct soil application of glyphosate bears an intoxication risk for subsequently cultivated sunflower (Helianthus annuus L.) seedlings. The experiments were conducted as greenhouse studies on two soils with contrasting properties (acidic, sandy Arenosol, calcareous loess subsoil). Also the potential role of different waiting times between glyphosate application and sunflower cultivation was considered.On both soils, sunflower seedling growth and biomass production was strongly impaired by glyphosate pre-sowing treatments in the variants with 0 d waiting time and recovered within a waiting time of 7–21 d. Generally, the detrimental effects were more pronounced after glyphosate weed application (90% biomass reduction) compared with direct soil application (55–70% biomass reduction) at waiting time 0 d. The inhibitory effects on seedling growth were associated with a corresponding increase in shikimate accumulation in the root tissue as physiological indicator for glyphosate toxicity. Glyphosate intoxication of sunflower seedlings was also associated with an impairment of the manganese-nutritional status, which was still detectable after a waiting time of up to 21 d, particularly on the Arenosol in the variants with glyphosate weed application. These findings indicate an important and yet uninvestigated role of glyphosate in plant residues in determining the risk of non-target plant intoxication. FULL TEXT

Kremer and Means, 2009

Robert J. Kremer and Nathan E. Means, “Glyphosate and glyphosate-resistant crop interactions with rhizosphere microorganisms,” European Journal of Agronomy, 2009, 31:3, 153-161, DOI: 10.1016/j.eja.2009.06.004.

ABSTRACT:

Current crop production relies heavily on transgenic, glyphosate-resistant (GR) cultivars. Widespread cultivation of transgenic crops has received considerable attention. Impacts of glyphosate on rhizosphere microorganisms and activities are reviewed based on published and new data from long-term field projects documenting effects of glyphosate applied to GR soybean and maize. Field studies conducted in Missouri, U.S.A. during 1997–2007 assessed effects of glyphosate applied to GR soybean and maize on root colonization and soil populations of Fusarium and selected rhizosphere bacteria. Frequency of root-colonizing Fusarium increased significantly after glyphosate application during growing seasons in each year at all sites. Roots of GR soybean and maize treated with glyphosate were heavily colonized by Fusarium compared to non-GR or GR cultivars not treated with glyphosate. Microbial groups and functions affected by glyphosate included Mn transformation and plant availability; phytopathogen–antagonistic bacterial interactions; and reduction in nodulation. Root-exuded glyphosate may serve as a nutrient source for fungi and stimulate propagule germination. The specific microbial indicator groups and processes were sensitive to impacts of GR crops and are part of an evolving framework in developing polyphasic microbial analyses for complete assessment of GR technology that is more reliable than single techniques or general microbial assays. FULL TEXT

Kremer et. al, 2005

Robert J. Kremer, Nathan E. Means, and Sujung Kim, “Glyphosate Affects Soybean Root Exudation and Rhizosphere Microorganisms,” International Journal of Environmental Analytical Chemistry, 2005, 85:15, 1155-1174, DOI: 10.1080/03067310500273146.

ABSTRACT:

Glyphosate is a non-selective, broad-spectrum herbicide that kills plants by inhibiting the enzyme 5-enolpyruvylshikimic acid-3-phosphate synthase (EPSPS), which is necessary for synthesis of aromatic amino acids. A secondary mode of action involves infection of roots of glyphosate-susceptible plants by soil-borne micro-organisms due to decreased production of plant protection compounds known as phytoalexins. Varieties of several crops, including glyphosate-resistant (GR) or Roundup Ready soybean, are genetically modified to resist the herbicidal effects of glyphosate and provide farmers with an effective weed-management tool. After glyphosate is applied to GR soybean, glyphosate that is not bound to glyphosate-resistant EPSPS is translocated throughout the plant and accumulates primarily in meristematic tissues. We previously reported that fungal colonization of GR soybean roots increased significantly after application of glyphosate but not after conventional postemergence herbicides. Because glyphosate may be released into soil from GR roots, we characterized the response of rhizosphere fungi and bacteria to root exudates from GR and non-GR (Williams 82; W82) cultivars treated with and without glyphosate at field application rates. Using an immunoassay technique, glyphosate at concentrations >1000 ng plant-1 were detected in exudates of hydroponically grown GR soybean at 16 days post-glyphosate application. Glyphosate also increased carbohydrate and amino acid contents in root exudates in both soybean cultivars. However, GR soybean released higher carbohydrate and amino acid contents in root exudates than W82 soybean without glyphosate treatment. In vitro bioassays showed that glyphosate in the exudates stimulated growth of selected rhizosphere fungi, possibly by providing a selective C and N source combined with the high levels of soluble carbohydrates and amino acids associated with glyphosate treatment of the soybean plants. Increased fungal populations that develop under glyphosate treatment of GR soybean may adversely affect plant growth and biological processes in the soil and rhizosphere. FULL TEXT

Saska et.al, 2016

Pavel Saska, Jiří Skuhrovec, Jan Lukáš, HsinChi, Shu-JenTuan & Alois Honěk, “Treatment by glyphosate-based herbicide alters life history parameters of the rose-grain aphid Metopolophium dirhodum,” Nature: Scientific Reports, 2016, 6:27801, DOI: 10.1038/srep27801.

ABSTRACT:

Glyphosate is the number one herbicide in the world. We investigated the sub-lethal effects of this herbicide on the aphid Metopolophium dirhodum (Walker), using an age-stage, two-sex life table approach. Three concentrations of the herbicide (low – 33.5, medium – 66.9 and high – 133.8mmol dm−3 of active ingredient) and distilled water as the control were used. The LC50 of the IPA salt of glyphosate on M. dirhodum was equivalent to 174.9mmol dm−3 of the active ingredient (CI95: 153.0, 199.0). The population parameters were significantly negatively affected by herbicide application, and this negative effect was progressive with the increasing concentration of the herbicide. A difference of two orders of magnitude existed in the predicted population development of M. dirhodum between the high concentration of the herbicide and the control. This is the first study that comprehensively documents such a negative effect on the population of an herbivorous insect. FULL TEXT

Hansen and Rosley, 2016

Lone Rykær Hansen, Peter Roslev, “Behavioral responses of juvenile Daphnia magna after exposure to glyphosate and glyphosate-copper complexes,” Aquatic Toxicology, 2016, 179: 36-43, DOI: 10.1016/j.aquatox.2016.08.010.

ABSTRACT:

Glyphosate (N-(phosphonomethyl)glycine) is the active ingredient in a range of popular broad-spectrum herbicide formulations. Glyphosate is a chelating agent that can form stable complexes with divalent metal ions including Cu(II). Little is known about the bioavailability and ecotoxicity of glyphosateCu(II) complexes to aquatic organisms. In this study, we used video tracking and behavior analysis to investigate sublethal effects of binary mixtures of glyphosate and Cu(II) to juvenile D. magna. Behavioral responses were quantified for individual D. magna after 24 h and 48 h exposure to glyphosate and glyhosate-Cu(II) mixtures. Sublethal concentrations resulted in decreases in swimming velocity, acceleration speed, and distance moved whereas inactive time of D. magna increased. Distance moved and inactive time were the most responsive parameters to glyphosate and glyphosate-Cu(II) exposure. On a molar basis, glyphosate-Cu(II) complexes appeared more toxic to D. magna than glyphosate alone. The 48 h EC50 for glyphosate and glyphosate-Cu(II) determined from swimming distance were 75.2 M and 8.4 M, respectively. In comparison, traditional visual observation of mobility resulted in 48 h EC50 values of 52.8 M and 25.5 M for glyphosate and glyphosate-Cu(II), respectively. The behavioral responses indicated that exposure of D. magna to mixtures of glyphosate and Cu(II) attenuated acute metal toxicity but increased apparent glyphosate toxicity due to complexation with Cu(II). The study suggests that glyphosate is a likely mediator of aquatic metal toxicity, and that video tracking provides an opportunity for quantitative studies of sublethal effects of pesticide complexes. FULL TEXT

Sol Balbuena et. al, 2015

María Sol Balbuena, Léa Tison, Marie-Luise Hahn, Uwe Greggers, Randolf Menzel, and Walter M. Farina, “Effects of sub-lethal doses of glyphosate on honeybee navigation,” Journal of Experimental Biology, 2015, 218: 2799-2805; doi: 10.1242/jeb.117291.

ABSTRACT:

Glyphosate (GLY) is a herbicide that is widely used in agriculture for weed control. Although reports about the impact of GLY in snails, crustaceans and amphibians exist, few studies have investigated its sublethal effects in non-target organisms such as the honeybee Apis mellifera, the main pollen vector in commercial crops. Here, we tested whether exposure to three sublethal concentrations of GLY (2.5, 5 and 10 mg l⁻¹: corresponding to 0.125, 0.250 and 0.500 μg per animal) affects the homeward flight path of honeybees in an open field. We performed an experiment in which forager honeybees were trained to an artificial feeder, and then captured, fed with sugar solution containing traces of GLY and released from a novel site either once or twice. Their homeward trajectories were tracked using harmonic radar technology. We found that honeybees that had been fed with solution containing 10 mg l⁻¹ GLY spent more time performing homeward flights than control bees or bees treated with lower concentrations. They also performed more indirect homing flights. Moreover, the proportion of direct homeward flights performed after a second release from the same site increased in control bees but not in treated bees. These results suggest that, in honeybees, exposure to levels of GLY commonly found in agricultural settings impairs the cognitive capacities needed to retrieve and integrate spatial information for a successful return to the hive. Therefore, honeybee navigation is affected by ingesting traces of the most widely used herbicide worldwide, with potential long-term negative consequences for colony foraging success. FULL TEXT

Bibliography Index