Fate of Glyphosate during Production and Processing of Glyphosate-Resistant Sugar Beet ( Beta vulgaris)

Glyphosate, glufosinate ammonium, and AMPA occurrences and sources in groundwater of hilly vineyards

Glyphosate [N-(phosphonomethyl) glycine] and glufosinate ammonium [ammonium dl-homoalanin-4-(methyl) phosphinate] are broad-spectrum, nonselective, post-emergence herbicides extensively used in various applications for weed control in both agricultural and non-crop areas. Aminomethylphosphonic acid (AMPA) is the major degradation product of glyphosate found in plants, water, and soil. Due to glyphosate's presumed low mobility, its monitoring in European water was limited. Recently both glyphosate and AMPA have been detected in several groundwater samples in Europe, U.S, Canada, Argentina, and China. Understanding the sources of these substances in water, especially in groundwater used for drinking, becomes a priority. In the present work the occurrences and the main drives of glyphosate, AMPA, and glufosinate ammonium in the groundwater of hilly vineyards located in the North-West of Italy were evaluated. Groundwater monitoring results showed frequent detection and concentrations above EQS_GW for glyphosate and AMPA, while glufosinate ammonium was never detected. More frequent occurrences and higher concentrations were detected in the samples collected from wells located in the farmyards, most of them being used for irrigation and/or preparation of PPPs mixtures. Indeed, AMPA was the only compound detected in one groundwater well used for drinking, at values bellow EQS_Gw/DWQS. Such monitoring results were not expected as the modelling estimations under local pedoclimatic conditions indicated no risk of leaching to groundwater. However, the modelling performance and output may have been influenced by the non-consideration of important specific processes. Integrating monitoring and modelling results with information concerning the agricultural practices adopted and the wells use and location, possible contamination drivers were identified. These include the non-agricultural use of glyphosate in the farmyard, the point source contamination of wells and the possible transport with the subsurface lateral inflow of water from up-hill vineyard. This study strengthens the position of SETAC EMAG-Pest GW group concerning the necessity of spatial and temporal contextualisation of groundwater monitoring for a better understanding of its contamination drivers by PPPs.

Rapeseed Transformation with aroA Bacterial Gene Containing P101S Mutation Confers Glyphosate Resistance

Field weed infestations can cause serious problems and require regular and planned programs to control them. Glyphosate is a broad-spectrum herbicide that inactivates the 5-enolpyruvylshikimate 3-phosphate synthase (EPSPS) enzyme and causes plant death. It has been reported that the mutation of proline 101 to serine in EPSPS is one of the effective mutations to reduce the affinity of glyphosate to EPSPS enzyme. In this study, we investigated the effect of the bacterial P101S mutant aromatic acid (aroA) gene on glyphosate resistance in transgenic rapeseeds. For this purpose, the mutant gene was synthesized and cloned into the pUC18 and pBI121 vectors. The gene was transferred to rapeseed by the Agrobacterium-mediated method. In this experiment, three generations of transgenic plants (T0, T1, and T2) were studied under in vitro and in vivo conditions. After the treatment of 75 putative transgenic plants with 10 mM glyphosate in T0 generation, resistant plants were identified and their seeds were harvested. In the T1 generation, out of 200 cultivated plants, 141 showed resistance. After the plants were treated with herbicides and resistance was determined, the seeds were harvested when they mature. In the T2 generation, most plants (162 plants of 200) were resistant to glyphosate. Therefore, the inheritance of resistance followed Mendel's first law, which is a sign of the monogenic character of resistance. Purification and increasing the percentage of resistant plants will be carried out in the next generations. It is concluded that P101S mutation guarantees glyphosate resistance of rapeseed and is recommended to study it in other plants.

Root exudation of prometryn and its metabolites from Chinese celery (Apium graveolens)

Root exudates from Chinese celery (Apium graveolens) and Chinese cabbage (pak choi, Brassica chinensis) plants treated by prometryn, an herbicide, were qualitatively and quantitatively investigated and compared under hydroponic cultivation. Prometryn and its metabolites released into the nutrient solution were analyzed by ultra-performance liquid chromatograph coupled with orbitrap mass spectrometer to investigate whether this xylem-mobile herbicide is exuded from the roots. The results showed that celery and pak choi had different root exudation profiles. Celery metabolized prometryn to prometryn sulfoxide and released both compounds from the roots. In contrast, pak choi barely metabolized or actively released prometryn from the roots. The concentration of prometryn sulfoxide released from celery after 96 hr was 21 µg/L, which was nearly one-third that of released prometryn. Our results indicate that the root exudation and translocation of xylem-mobile herbicides could be significant in plants and are highly species dependent compared with phloem-mobile herbicides.

The Influence of Herbicides to Marine Organisms Aliivibrio fischeri and Artemia salina

The aim of this work was to determine the toxic effect of the most used herbicides on marine organisms, the bacterium Aliivibrio fischeri, and the crustacean Artemia salina. The effect of these substances was evaluated using a luminescent bacterial test and an ecotoxicity test. The results showed that half maximal inhibitory concentration for A. fischeri is as follows: 15minIC50 (Roundup® Classic Pro) = 236 μg·L-1, 15minIC50 (Kaput® Premium) = 2475 μg·L-1, 15minIC50 (Banvel® 480 S) = 2637 μg·L-1, 15minIC50 (Lontrel 300) = 7596 μg·L-1, 15minIC50 (Finalsan®) = 64 μg·L-1, 15minIC50 (glyphosate) = 7934 μg·L-1, 15minIC50 (dicamba) = 15,937 μg·L-1, 15minIC50 (clopyralid) = 10,417 μg·L-1, 15minIC50 (nonanoic acid) = 16,040 μg·L-1. Median lethal concentrations for A. salina were determined as follows: LC50 (Roundup® Classic Pro) = 18 μg·L-1, LC50 (Kaput® Premium) = 19 μg·L-1, LC50 (Banvel® 480 S) = 2519 μg·L-1, LC50 (Lontrel 300) = 1796 μg·L-1, LC50 (Finalsan®) = 100 μg·L-1, LC50 (glyphosate) = 811 μg·L-1, LC50 (dicamba) = 3705 μg·L-1, LC50 (clopyralid) = 2800 μg·L-1, LC50 (nonanoic acid) = 7493 μg·L-1. These findings indicate the need to monitor the herbicides used for all environmental compartments.

Residues of glyphosate in food and dietary exposure

Glyphosate is the active ingredient in Roundup® brand nonselective herbicides, and residue testing for food has been conducted as part of the normal regulatory processes. Additional testing has been conducted by university researchers and nongovernmental agencies. Presence of residues needs to be put into the context of safety standards. Furthermore, to appropriately interpret residue data, analytical assays must be validated for each food sample matrix. Regulatory agency surveys indicate that 99% of glyphosate residues in food are below the European maximum residue limits (MRLs) or U.S. Environmental Protection Agency tolerances. These data support the conclusion that overall residues are not elevated above MRLs/tolerances due to agricultural practices or usage on genetically modified (GM) crops. However, it is important to understand that MRLs and tolerances are limits for legal pesticide usage. MRLs only provide health information when the sum of MRLs of all foods is compared to limits established by toxicology studies, such as the acceptable daily intake (ADI). Conclusions from dietary modeling that use actual food residues, or MRLs themselves, combined with consumption data indicate that dietary exposures to glyphosate are within established safe limits. Measurements of glyphosate in urine can also be used to estimate ingested glyphosate exposure, and studies indicate that exposure is <3% of the current European ADI for glyphosate, which is 0.5 mg glyphosate/kg body weight. Conclusions of risk assessments, based on dietary modeling or urine data, are that exposures to glyphosate from food are well below the amount that can be ingested daily over a lifetime with a reasonable certainty of no harm.

Glyphosate use in urban landscape soils: Fate, distribution, and potential human and environmental health risks

This novel study investigated the fate and distribution in soils, and potential exposure risk of glyphosate, an extensively used herbicide in urban landscapes. The rate-determining step of glyphosate sorption in urban soils involved chemisorption processes through exchange or sharing of electrons that followed the pseudo-second-order kinetics model. As evidenced by the Freundlich isotherm model, glyphosate gets partitioned into heterogeneous surfaces of soil organic matter (OM) and clay minerals, and then diffused into soil micropores. The principal component analysis revealed that soil OM (R2 = 0.873), oxides of Al (R2 = 0.361) and Fe (R2 = 0.126), and contents of clay (R2 = 0.061) and silt (R2 = 0.432) were positively correlated with the distribution coefficient (Kd) of glyphosate, while alkaline pH (R2 = -0.389) and sand content (R2 = -0.343) negatively correlated with the Kd values. Well-decomposed soil OM, consisting of C-H and CO functional groups, enhanced glyphosate sorption, whereas partially decomposed/undecomposed OM facilitated desorption process. Desorption of glyphosate was favoured in seven of nine selected soils due to adverse hysteresis effects (HI = 0.74-1.0). The higher values of leachability index (0.31-1.0) and groundwater ubiquity score (1.60-3.44) calculated for the urban soils indicated the great leaching potential of glyphosate from soil surface to waterbodies. Use of glyphosate on impermeable surfaces might directly contaminate water sources and affect potability of water, non-target biota, and food safety. The calculated values of cancer risk (10-8‒10-12) and hazard quotient (1.47 × 10-6‒4.12 × 10-6) suggested that the human exposure to glyphosate-contaminated soils through dermal, ingestion and inhalation pathways might cause negligible or no carcinogenic and non-carcinogenic risks to humans. Therefore, glyphosate should be applied judiciously at recommended concentrations in the urban landscapes, mainly on impervious surfaces, to minimize its health impacts in humans and environment.

Glyphosate: Uses Other Than in Glyphosate-Resistant Crops, Mode of Action, Degradation in Plants, and Effects on Non-target Plants and Agricultural Microbes

Glyphosate is the most used herbicide globally. It is a unique non-selective herbicide with a mode of action that is ideal for vegetation management in both agricultural and non-agricultural settings. Its use was more than doubled by the introduction of transgenic, glyphosate-resistant (GR) crops. All of its phytotoxic effects are the result of inhibition of only 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), but inhibition of this single enzyme of the shikimate pathway results in multiple phytotoxicity effects, both upstream and downstream from EPSPS, including loss of plant defenses against pathogens. Degradation of glyphosate in plants and microbes is predominantly by a glyphosate oxidoreductase to produce aminomethylphosphonic acid and glyoxylate and to a lesser extent by a C-P lyase to produce sarcosine and phosphate. Its effects on non-target plant species are generally less than that of many other herbicides, as it is not volatile and is generally sprayed in larger droplet sizes with a relatively low propensity to drift and is inactivated by tight binding to most soils. Some microbes, including fungal plant pathogens, have glyphosate-sensitive EPSPS. Thus, glyphosate can benefit GR crops by its activity on some plant pathogens. On the other hand, glyphosate can adversely affect some microbes that are beneficial to agriculture, such as Bradyrhizobium species, although GR crop yield data indicate that such an effect has been minor. Effects of glyphosate on microbes of agricultural soils are generally minor and transient, with other agricultural practices having much stronger effects.

Perspective: root exudation of herbicides as a novel mode of herbicide resistance in weeds

The evolution of resistance to herbicides in weeds has become a great challenge for global agricultural production. Weeds have evolved resistance to herbicides through many different physiological mechanisms. Some weed species are known to secrete herbicide molecules from roots into the rhizosphere upon being treated. However, root exudation of herbicides as a mechanism of resistance has only recently been identified in two weed species. Root exudation pathways have been investigated in Arabidopsis, and this work suggested that ATP-binding cassette (ABC) and multidrug and toxic compound extrusion (MATE) transporters play a role in the secretion of primary and secondary plant products from roots. We hypothesize that the mechanisms involved in root exudation of herbicides that result in resistance are mediated by overactive or overexpressed transporters, probably similar to those found for the exudation of primary and secondary compounds from roots. Elucidating the molecular and physiological basis of root exudation in herbicide-resistant weeds would improve our understanding of the pathways involved in herbicide root secretion mediated by transporters in plants. © 2020 Society of Chemical Industry.

Physiological Factors Affecting Uptake and Translocation of Glufosinate

Glufosinate is considered a contact herbicide because of its fast activity and limited translocation in plants. We used Palmer amaranth (Amaranthus palmeri S. Watson) as a model species to study plant-related factors affecting glufosinate uptake and translocation. Glufosinate uptake increased rapidly during the initial 24 h, achieving maximum uptake from this time on. The rate of uptake saturated with doses higher than 250 μM glufosinate, suggesting the involvement of a membrane transporter. When glufosinate concentrations were higher (>1 mM), uptake was a simple diffusion process in favor of a concentration gradient between the inside and the outside of the cells. Glufosinate uptake was inhibited by the presence of glutamine. The fast action of glufosinate did not limit its own translocation. Because glufosinate is highly water soluble, it translocates mostly through the apoplast or the xylem system. Consequently, old leaves tend to accumulate more herbicide than young meristematic leaves.

A novel TIPT double mutation in EPSPS conferring glyphosate resistance in tetraploid Bidens subalternans

BACKGROUND

Bidens subalternans (greater beggarticks) is a tetraploid and troublesome weed infesting annual crops in most tropical regions of the world. A glyphosate-resistant (GR) B. subalternans biotype was detected in a soybean field from Paraguay. A series of physiological and molecular analyses were conducted to elucidate its resistance mechanisms.

RESULTS

The GR biotype had a high level of resistance (> 15-fold LD50 ), relative to a glyphosate-susceptible (GS) biotype. Shikimate accumulation was up to ten-fold greater for GS compared with GR. We found no differences in sensitivity when plants were treated and kept under lower (10/4 °C) or higher temperatures (25/20 °C). GS and GR had the same relative EPSPS gene copy number, and similar glyphosate absorption and translocation rates. Neither biotype metabolized glyphosate. A double amino acid substitution (TIPT - Thr102Ile and Pro106Thr) was found in only one EPSPS allele from one of the two EPSPS homoeologs present in tetraploid GR B. subalternans.

CONCLUSION

This is the first report of a TIPT double mutation conferring high levels of glyphosate resistance in a weed species. The presence of both wild-type and TIPT mutant EPSPS on the polyploid genome of GR B. subalternans may offset a potential fitness cost, requiring additional research to confirm the absence of deleterious effects. © 2019 Society of Chemical Industry.