From tolerance to resistance: mechanisms governing the differential response to glyphosate in Chloris barbata

GLYPHOSATE IMPACT on human health and the environment: Sustainable alternatives to replace it in Mexico

Glyphosate is a broad-spectrum, non-selective herbicide used to control weeds and protect agricultural crops, and it is classified as potentially carcinogenic by the International Agency for Research on Cancer. In Mexico, the use of pesticides is a common practice, including glyphosate. However, on December 31st, 2020, the Mexican government decreed the prohibition of this herbicide as of January 2024. In this review, we investigate the association between glyphosate and cancer risk and found that most of the studies focused using animals showing negative effects such as genotoxicity, cytotoxicity and neurotoxicity, some studies used cancer cell lines showing proliferative effects due to glyphosate exposure. To our knowledge, in Mexico, there are no scientific reports on the association of glyphosate with any type of cancer. In addition, we reviewed the toxicological effects of the herbicide glyphosate, and the specific case of the current situation of the use and environmental damage of this herbicide in Mexico. We found that few studies have been published on glyphosate, and that the largest number of publications are from the International Agency for Research on Cancer classification to date. Additionally, we provide data on glyphosate stimulation at low doses as a biostimulant in crops and analytical monitoring techniques for the detection of glyphosates in different matrices. Finally, we have tried to summarize the actions of the Mexican government to seek sustainable alternatives and replace the use of glyphosate, to obtain food free of this herbicide and take care of the health of the population and the environment.

Antioxidant Enzyme and Cytochrome P450 Activities Are Involved in Horseweed (Conyza Sumatrensis) Resistance to Glyphosate

The intensive global use of glyphosate has led to the evolution of glyphosate resistant (GR) weed species, including the economically damaging horseweed (Conyza sumatrensis). We evaluated the glyphosate resistance mechanisms of C. sumatrensis. While 5-enolpyruvylshikimate-3-phosphate synthase activity was similar between the glyphosate resistant (GR) and nonresistant biotypes, plants from the GR population accumulated lower shikimate levels than susceptible ones, suggesting the absence of target-site resistance mechanisms. Decreases over time in glyphosate concentrations in GR leaves were not accompanied by increases in glyphosate concentrations in their stem and roots, indicating lower glyphosate distribution rates in GR plants. The early appearance of aminomethylphosphonic acid (the main glyphosate metabolite) in leaves, as well as its presence only in the stems and roots of GR plants, suggests faster glyphosate metabolism in GR plants than in susceptible ones. GR plants treated with glyphosate also showed greater antioxidant (ascorbate peroxidase [APX] and catalase [CAT]) and cytochrome P450-enzyme activities, indicating their great capacity to avoid glyphosate-induced oxidative stress. Three non-target mechanisms (reduced glyphosate translocation, increased metabolism, and increased antioxidant activity) therefore confer glyphosate resistance in C. sumatrensis plants. This is the first time that APX, CAT and P450-enzyme activities are related to GR in C. sumatrensis.

Glyphosate resistance in Chloris radiata from colombian rice fields involves one target-site mechanism

At present, appearance of herbicide resistant weeds is not new because repeated herbicide treatments per agricultural year/cycle are usual in both perennial and annual crops worldwide. Characterizing resistance mechanisms implied in each herbicide resistant weed is the best tool and the basis to develop integrated weed management (IWM) strategies. The main resistance mechanisms which confer low sensibility to glyphosate in a previously confirmed glyphosate-resistant Chloris radiata population (ChrR), occurring in Colombian rice fields, were characterized. Pure line selection by clone plants showed high resistance levels in ChrR. Comparing with GR50 and LD50 values, ChrR was 9.6 and 10.8 times more resistant with respect to a representative susceptible population (ChrS). The nontarget site mechanisms reduced glyphosate absorption and translocation did not contribute to the glyphosate resistance of the ChrR population. However, enzyme activity assays and DNA sequencing demonstrated that at least one target-site resistance mechanism is involved in such resistance. All ten ChrR plants tested had the amino acid substitution Pro-106-Ser. The results may be crucial to decrease the resistance distribution of C. radiata in Colombia by implementing IWM programs. The change in weed control strategies in rice fields from Colombia must include herbicides with different mode of action from glyphosate and non chemical methods to preserve the useful life of glyphosate longer for weed control in the country.

Non-Target Site Mechanisms Endow Resistance to Glyphosate in Saltmarsh Aster (Aster squamatus)

Of the six-glyphosate resistant weed species reported in Mexico, five were found in citrus groves. Here, the glyphosate susceptibility level and resistance mechanisms were evaluated in saltmarsh aster (Aster squamatus), a weed that also occurs in Mexican citrus groves. The R population accumulated 4.5-fold less shikimic acid than S population. S plants hardly survived at 125 g ae ha⁻¹ while most of the R plants that were treated with 1000 g ae ha⁻¹, which suffered a strong growth arrest, showed a vigorous regrowth from the third week after treatment. Further, 5-enolpyruvylshikimate-3-phosphate basal and enzymatic activities did not diverge between populations, suggesting the absence of target-site resistance mechanisms. At 96 h after treatment, R plants absorbed ~18% less glyphosate and maintained 63% of the ¹⁴C-glyphsoate absorbed in the treated leaf in comparison to S plants. R plants metabolized twice as much (72%) glyphosate to amino methyl phosphonic acid and glyoxylate as the S plants. Three non-target mechanisms, reduced absorption and translocation and increased metabolism, confer glyphosate resistance saltmarsh aster. This is the first case of glyphosate resistance recorded for A. squamatus in the world.

Glyphosate ban in Mexico: potential impacts on agriculture and weed management

Since glyphosate was classified as potentially carcinogenic by the International Agency for Research on Cancer, public debate regarding the environmental impact and health risks from its use has intensified. Almost all regulatory agencies throughout the world have concluded that the judicious use of glyphosate does not pose risks to the environment and human health. However, on the last day of 2020 the Mexican government decreed a ban of this herbicide beginning January, 2024. In current Mexican agriculture there are no safer chemical and/or other weed management technologies that allow for the economical substitution of glyphosate for weed control. Many Mexican weed scientists agree that glyphosate use should be reduced, but not banned outright. This decree could have more negative economic and social consequences as well as environmental and human health risks than benefits, which could compromise the country's food and public security. Crop yields are projected by some to decline by up to 40% with this ban, increasing food prices, making food less accessible to low-income consumers. In addition, a black market for the smuggling and illegal sale of glyphosate is possible. The possible environmental, economic and social impacts caused by the glyphosate ban in Mexico are discussed, emphasizing the impact on weed management. © 2021 Society of Chemical Industry.

Comparison of premix glyphosate and 2,4-D formulation and direct tank mixture for control of Conyza canadensis and Epilobium ciliatum

Premix or tank mix of glyphosate and 2,4-D are a good alternative to control glyphosate-resistant and -tolerant weeds; however, the combination of herbicides may increase the environmental impacts, since mixtures often have higher toxicity than a single herbicide. In addition, antagonism between these herbicides has also been reported. We compared the efficacy of a premix glyphosate+2,4-D formulation with respect to the tank mix of both herbicides on glyphosate-resistant Conyza canadensis and -tolerant Epilobium ciliatum populations in laboratory and field experiments. 2,4-D suppressed the glyphosate-resistance/tolerance of both species, whose populations presented similar responses to their susceptible counterparts (LD50 ≥ 480+320 g ha-1 glyphosate + 2,4-D, respectively). Plants of both species treated with the premix formulations retained ∼100-μL more herbicide solution, accumulated 20-25% and 28-38% more shikimate and ethylene, respectively, and presented greater 14C-glyphosate absorption and translocation, depending on the species, compared to plants treated with the tank mix treatment. Although doubling the field dose (720 + 480 g ha-1) improved (5-22%) the control of these weeds in the field, split applications of both premix and tank mix provided the best control levels (≤70%), but premix treatments maintained control levels above 85% for longer (120-d). No antagonism between glyphosate and 2,4-D was found. The addition of 2,4-D controlled both broadleaf species. For all parameters evaluated on the C. canadensis and E. ciliatum populations in the laboratory and in the field, the premix treatments showed better performance than the tank mix treatments. Premix formulations could reduce the environmental impact of herbicides used to control glyphosate resistant/tolerant weeds by decreasing the herbicide amount needed to achieve an acceptable weed control level.

Confirmation of Multiple Resistant Chloris radiata Population, Harvested in Colombian Rice Fields

This paper reports the first C. radiata population with resistance to glyphosate and multiple resistance to the acetolactate synthase (ALS) inhibitor, imazamox. Two populations, one putative resistant (R) and one susceptible (S), were used in the studies. Dose–response experiments were performed to evaluate the resistance factor (RF). Shikimic acid accumulation, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) and ALS enzyme activities were studied together with chemical integrated weed management (adjuvants and alternative herbicides). The resistance to glyphosate and imazamox was confirmed based on the dry weight reduction, visual evaluation and survival. The results of dose–response curve assays showed for the R population intermedium RF for glyphosate (5.1 and 9.7 for amount of herbicide needed to reduce the dry weight by 50% GR50 and lethal dose of 50% LD50, respectively) and high RF for imazamox (34.9 and 37.4, respectively). The low shikimic acid accumulation in R population confirmed the glyphosate resistance. The glyphosate concentration which inhibited the EPSPS enzyme in 50% (I50) was approximately 20 times higher for R population than the S population, while the imazamox I50 in ALS enzyme for the R plants was 89 times greater than the S plants. In the chemical integrated weed management, the foliar retention and effectivity assays showed that the use of adjuvants improves the retention of glyphosate and imazamox, and the reduction in dry weight of weeds. The alternative herbicides study showed that the acetyl-CoA carboxylase (ACCase) inhibitors, paraquat and glufosinate, had better results for control in this species. However, poor control was observed with bispyribac-sodium, metsulfuron-methyl and quinclorac, indicating possible cross-resistance for ALS-inhibitors and also multiple resistance for auxinic herbicides (quinclorac). Nevertheless dose–response experiments are required to confirm this assumption.

Distribution of Glyphosate-Resistance in Echinochloa crus-galli Across Agriculture Areas in the Iberian Peninsula

The levels of resistance to glyphosate of 13 barnyard grass (Echinochloa crus-galli) populations harvested across different agriculture areas in the Southern Iberian Peninsula were determined in greenhouse and laboratory experiments. Shikimate accumulation fast screening separated the populations regarding resistance to glyphosate: susceptible (S) E2, E3, E4, and E6 and resistant (R) E1, E5, E7, E8, E9, E10, E11, E12, and E13. However, resistance factor (GR₅₀ E1-E13/GR₅₀ E6) values separated these populations into three groups: (S) E2, E3, E4, and E6, (R) E1, E5, E7, E8, and E9, and very resistant (VR) E10, E11, E12, and E13. ¹⁴C-glyphosate assays performed on two S populations (E2 and E6) showed greater absorption and translocation than those found for R (E7 and E9) and VR (E10 and E12) populations. No previous population metabolized glyphosate to amino methyl phosphonic acid (AMPA) and glyoxylate, except for the E10 population that metabolized 51% to non-toxic products. The VR populations showed two times more 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) activity without herbicide than the rest, while the inhibition of the EPSPS activity by 50% (I₅₀) required much higher glyphosate in R and VR populations than in S populations. These results indicated that different target-site and non-target-site resistance mechanisms were implicated in the resistance to glyphosate in E. crus-galli. Our results conclude that resistance is independent of climate, type of crop, and geographic region and that the level of glyphosate resistance was mainly due to the selection pressure made by the herbicide on the different populations of E. crus-galli studied.

Resistance to Fomesafen, Imazamox and Glyphosate in Euphorbia heterophylla from Brazil

Euphorbia heterophylla is a species of weed that was previously controlled by fomesafen, imazamox and glyphosate, but continued use of these herbicides has selected resistant populations from the Rio Grande do Sul (Brazil). One resistant (R) strain and one susceptible (S) strain to fomesafen, imazamox and glyphosate were compared, the latter by recurrent selection. Dose-response tests showed multiple resistance to these herbicides. The required imazamox concentration to inhibit ALS by 50% was approximately 16 times greater in the R population than in the S population. Based on the EPSPS activity results, the R population was 10 fold less sensitive to glyphosate than the S counterpart. In addition, basal EPSPS activity from R plants was 3.3 fold higher than the level detected on S plants. The Proto IX assays showed high resistance to fomesafen in the R population that accumulated less Proto IX than the S population. Malathion assays showed the participation of CytP450 in fomesafen resistance, but a molecular mechanism could also be involved. To our knowledge, this is the first characterisation of multiple resistance to these three groups of herbicides in E. heterophylla in the world.

Resistance Evolution to EPSPS Inhibiting Herbicides in False Barley (Hordeum murinum) Harvested in Southern Spain

A failure of the EPSPS-inhibiting herbicide glyphosate to control several populations of Hordeum murinum subsp. leporinum (or H. murinum) occurred in southern Spain after more than fifteen applications in both crop (olive, orchards, and citrus) and non-crop (dry areas, roadsides and ditches) areas. Eight out of 18 populations studied were resistant (R) to glyphosate with R factors higher than four based on GR50. These populations also had the highest values of LD50 and the lowest levels of shikimic acid accumulation. Two adjuvants tested increased glyphosate efficacy in both susceptible (S) and R populations thanks to better spray foliar retention. Moreover, PS I-, PS II-, and ACCase-inhibiting herbicides, in pre- or post-emergence, proved to be the best chemical alternatives with different sites of action (SoA) to control both S and glyphosate-R populations. This study represents the first report worldwide of glyphosate resistance in H. murinum found in very different crop and non-crop areas from southern Spain. To design chemical strategies to implement integrated weed management programs for glyphosate-R H. murinum, both adjuvants and herbicides with alternative SoA as well as application timings should be considered.

New Case of False-Star-Grass (Chloris distichophylla) Population Evolving Glyphosate Resistance

Chloris distichophylla, suspected of glyphosate resistance (GR), was collected from areas of soybean cultivation in Rio Grande do Sul, Brazil. A comparison was made with a susceptible population (GS) to evaluate the resistance level, mechanisms involved, and control alternatives. Glyphosate doses required to reduce the dry weight (GR50) or cause a mortality rate of 50% (LD50) were around 5.1–3 times greater in the GR population than in the GS population. The shikimic acid accumulation was around 6.2-fold greater in GS plants than in GR plants. No metabolized glyphosate was found in either GR or GS plants. Both populations did not differ in the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) basal activity or in vitro inhibition of EPSPS activity by glyphosate (I50). The maximum glyphosate absorption was observed at 96 hours after treatment (HAT), which was twofold higher in the GS plants than in the GR plants. This confirms the first case of glyphosate resistance in C. distichophylla. In addition, at 96 HAT, the GS plants translocated more 14C-glyphosate than the GR ones. The best options for the chemical control of both C. distichophylla populations were clethodim, quizalofop, paraquat, glufosinate, tembotrione, diuron, and atrazine. The first case of glyphosate resistance in C. distichophylla was due to impaired uptake and translocation. Chemical control using multiple herbicides with different modes of action (MOA) could be a tool used for integrated weed management (IWM) programs.

Physiological and molecular analysis of glyphosate resistance in non-rapid response Ambrosia trifida from Wisconsin

BACKGROUND

We previously identified a glyphosate-resistant A. trifida phenotype from Wisconsin USA that showed a non-rapid response to glyphosate. The mechanism of glyphosate resistance in this phenotype has yet to be elucidated. We conducted experiments to investigate non-target-site resistance and target-site resistance mechanisms. The roles of glyphosate absorption, translocation, and metabolism in resistance of this phenotype have not been reported previously, nor have EPSPS protein abundance or mutations to the full-length sequence of EPSPS.

RESULTS

Whole-plant dose-response results confirmed a 6.5-level of glyphosate resistance for the resistant (R) phenotype compared to a susceptible (S) phenotype. Absorption and translocation of 14 C-glyphosate were similar between R and S phenotypes over 72 h. Glyphosate and AMPA concentrations in leaf tissue did not differ between R and S phenotypes over 96 h. In vivo shikimate leaf disc assays confirmed that glyphosate EC50 values were 4.6- to 5.4-fold greater for the R than S phenotype. Shikimate accumulation was similar between phenotypes at high glyphosate concentrations (>1000 μM), suggesting that glyphosate entered chloroplasts and inhibited EPSPS. This finding was supported by results showing that EPSPS copy number and EPSPS protein abundance did not differ between R and S phenotypes, nor did EPSPS sequence at Gly101, Thr102, and Pro106 positions. Comparison of full-length EPSPS sequences found five nonsynonymous polymorphisms that differed between R and S phenotypes. However, their locations were distant from the glyphosate target site and, therefore, not likely to affect enzyme-glyphosate interaction.

CONCLUSION

The results suggest that a novel mechanism confers glyphosate resistance in this A. trifida phenotype. © 2019 Society of Chemical Industry.

Management of Glyphosate-Resistant Weeds in Mexican Citrus Groves: Chemical Alternatives and Economic Viability

Glyphosate is a cheap herbicide that has been used to control a wide range of weeds (4-6 times/year) in citrus groves of the Gulf of Mexico; however, its excessive use has selected for glyphosate-resistant weeds. We evaluated the efficacy and economic viability of 13 herbicide treatments (glyphosate combined with PRE- and/or POST-emergence herbicides and other alternative treatments), applied in tank-mixture or sequence, to control glyphosate-resistant weeds in two Persian lime groves (referred to as SM-I and SM-II) of the municipality of Acateno, Puebla, during two years (2014 and 2015). The SM-I and SM-II fields had 243 and 346 weeds/m², respectively, composed mainly of Bidens pilosa and Leptochloa virgata. Echinochloa colona was also frequent in SM-II. The glyphosate alone treatments (1080, 1440, or 1800 g ae ha^-1) presented control levels of the total weed population ranging from 64% to 85% at 15, 30, and 45 d after treatment (DAT) in both fields. Mixtures of glyphosate with grass herbicides such as fluazifop-p-butyl, sethoxydim, and clethodim efficiently controlled E. colona and L. virgata, but favored the regrowth of B. pilosa. The sequential applications of glyphosate + (bromacil + diuron) and glufosinate + oxyfluorfen controlled more than 85% the total weed community for more than 75 days. However, these treatments were between 360% and 390% more expensive (1.79 and 1.89 $/day ha^-1 of satisfactory weed control, respectively), compared to the representative treatment (glyphosate 1080 g ae ha^-1 = USD $29.0 ha^-1). In practical and economic terms, glufosinate alone was the best treatment controlling glyphosate resistant weeds maintaining control levels >80% for at least 60 DAT ($1.35/day ha^-1). The rest of the treatments, applied in tank-mix or in sequence with glyphosate, had similar or lower control levels (~70%) than glyphosate at 1080 g ae ha^-1. The adoption of glufosiante alone, glufosinate + oxyfluorfen or glyphosate + (bromacil + diuron) must consider the cost of satisfactory weed control per day, the period of weed control, as well as other factors associated with production costs to obtain an integrated weed management in the short and long term.

Low temperatures enhance the absorption and translocation of 14C-glyphosate in glyphosate-resistant Conyza sumatrensis

Influence of low temperatures on the glyphosate efficacy was studied in glyphosate-resistant (R) and -susceptible (S) Conyza sumatrensis biotypes. For this purpose, the physiological and enzymatic aspects involved were characterized under two growing temperature regimes [high (30/20 °C) and low 15/5 °C temperatures day/night]. The R biotype was 5.5 times more resistant than the S biotype at high temperatures; however, this R-to-S ratio decreased to 1.6 at low temperatures. At 96 h after treatment (HAT), the shikimic acid accumulation was higher in the S biotype in both temperature regimes (4.6 and 1.9 more shikimic acid at high and low temperatures, respectively), but the accumulation of the R biotype increased 2.6 times at low temperatures compared to high ones. From 24 to 96 HAT, the 14C-glyphosate absorption ranged from 28 to 65% (percentage reached from 48 HAT) at low temperatures, and from 20 to 50% at high temperatures (gradual increase), but there were no differences between C. sumatrensis biotypes within each temperature regime. At high temperatures, the 14C-glyphosate translocation was different between biotypes, where the R one retained at least 10% more herbicide in the treated leaves than the S biotype at 96 HAT. So, the S biotype translocated 40% of 14C-glyphosate absorbed to roots, and the R biotype translocated only 28% of herbicide at the same period. At low temperatures, there were no differences between biotypes, and at 96 HAT, the 14C-glyphosate found in treated leaves was ˜47% and up to ˜42% reached the roots, i.e., the resistance mechanism was suppressed. The basal and enzymatic activities of the 5-enolpyruvyishikimate 3-phosphate synthase were different between temperature regimes, but there was no differences between biotypes within each temperature regime, showing that target-site resistance mechanisms did not contribute in the glyphosate resistance of the R biotype. Low temperatures enhanced the absorption and translocation of glyphosate by suppressing the resistance mechanisms improving its efficacy on resistant plants. This is the first characterization about the role of temperatures in the glyphosate efficacy on C. sumatrensis.

Response of Chloris truncata to moisture stress, elevated carbon dioxide and herbicide application

Herbicide resistance has been observed in Chloris truncata, an Australian native C4 grass and a summer-fallow weed, which is common in no-till agriculture situations where herbicides are involved in crop management. To investigate the role of drought and increased atmospheric carbon dioxide (CO2) in determining weed growth, three trials were conducted using a 'glyphosate-resistant' and a 'glyphosate-susceptible' biotype. The first two trials tested the effect of herbicide (glyphosate) application on plant survival and growth under moisture stress and elevated CO2 respectively. A third trial investigated the effect on plant growth and reproduction under conditions of moisture stress and elevated CO2 in the absence of herbicide. In the first trial, water was withheld from half of the plants prior to application of glyphosate to all plants, and in the second trial plants were grown in either ambient (450 ppm) or elevated CO2 levels (750 ppm) prior to, and following, herbicide application. In both biotypes, herbicide effectiveness was reduced when plants were subjected to moisture stress or if grown in elevated CO2. Plant productivity, as measured by dry biomass per plant, was reduced with moisture stress, but increased with elevated CO2. In the third trial, growth rate, biomass and seed production were higher in the susceptible biotype compared to the resistant biotype. This suggests that a superior ability to resist herbicides may come at a cost to overall plant fitness. The results indicate that control of this weed may become difficult in the future as climatic conditions change.

The Triple Amino Acid Substitution TAP-IVS in the EPSPS Gene Confers High Glyphosate Resistance to the Superweed Amaranthus hybridus

The introduction of glyphosate-resistant (GR) crops revolutionized weed management; however, the improper use of this technology has selected for a wide range of weeds resistant to glyphosate, referred to as superweeds. We characterized the high glyphosate resistance level of an Amaranthus hybridus population (GRH)-a superweed collected in a GR-soybean field from Cordoba, Argentina-as well as the resistance mechanisms that govern it in comparison to a susceptible population (GSH). The GRH population was 100.6 times more resistant than the GSH population. Reduced absorption and metabolism of glyphosate, as well as gene duplication of 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) or its overexpression did not contribute to this resistance. However, GSH plants translocated at least 10% more 14C-glyphosate to the rest of the plant and roots than GRH plants at 9 h after treatment. In addition, a novel triple amino acid substitution from TAP (wild type, GSH) to IVS (triple mutant, GRH) was identified in the EPSPS gene of the GRH. The nucleotide substitutions consisted of ATA102, GTC103 and TCA106 instead of ACA102, GCG103, and CCA106, respectively. The hydrogen bond distances between Gly-101 and Arg-105 positions increased from 2.89 Å (wild type) to 2.93 Å (triple-mutant) according to the EPSPS structural modeling. These results support that the high level of glyphosate resistance of the GRH A. hybridus population was mainly governed by the triple mutation TAP-IVS found of the EPSPS target site, but the impaired translocation of herbicide also contributed in this resistance.

Characterization of three glyphosate resistant Parthenium hysterophorus populations collected in citrus groves from Mexico

Continuous use of glyphosate in citrus groves in the Gulf of Mexico region has selected for resistant Parthenium hysterophorus L. populations. In this study, the target-site and non-target-site resistance mechanisms were characterized in three putative glyphosate-resistant (GR) P. hysterophorus populations, collected in citrus groves from Acateno, Puebla (GR1 and GR2) and Martínez de la Torre, Veracruz (GR3), and compared with a susceptible population (GS). Based on plant mortality, the GR populations were 9.2-17.3 times more resistant to glyphosate than the GS population. The low shikimate accumulation in the GR population confirmed this resistance. Based on plant mortality and shikimate accumulation, the GR3 population showed intermediate resistance to glyphosate. The GR populations absorbed 15-28% less 14C-glyphosate than the GS population (78.7% absorbed from the applied) and retained 48.7-70.7% of 14C-glyphosate in the treated leaf, while the GS population translocated ~68% of absorbed herbicide to shoots and roots. The GR3 population showed the lowest translocation and absorption rates, but was found to be susceptible at the target site level. The 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene sequence of the GR1 and GR2 populations showed the Pro106-Ser mutation, conferring 19- and 25-times more resistance in comparison to the GS population, respectively. Reduced absorption and impaired translocation conferred glyphosate resistance on the GR3 population, and contributed partially to the resistance of the GR1 and GR2 populations. Additionally, the Pro-106-Ser mutation increased the glyphosate resistance of the last two P. hysterophorus populations.

Sequencing the Plastid Genome of Giant Ragweed (Ambrosia trifida, Asteraceae) From a Herbarium Specimen

We report the first plastome sequence of giant ragweed (Ambrosia trifida); with this new genome information, we assessed the phylogeny of Asteraceae and the transcriptional profiling against glyphosate resistance in giant ragweed. Assembly and genic features show a normal angiosperm quadripartite plastome structure with no signatures of deviation in gene directionality. Comparative analysis revealed large inversions across the plastome of giant ragweed and the previously sequenced members of the plant family. Asteraceae plastid genomes contain two inversions of 22.8 and 3.3 kb; the former is located between trnS-GCU and trnG-UCC genes, and the latter between trnE-UUC and trnT-GGU genes. The plastid genome sequences of A. trifida and the related species, Ambrosia artemisiifolia, are identical in gene content and arrangement, but they differ in length. The phylogeny is well-resolved and congruent with previous hypotheses about the phylogenetic relationship of Asteraceae. Transcriptomic analysis revealed divergence in the relative expressions at the exonic and intronic levels, providing hints toward the ecological adaptation of the genus. Giant ragweed shows various levels of glyphosate resistance, with introns displaying higher expression patterns at resistant time points after the assumed herbicide treatment.

Multiple Resistance Evolution in Bipyridylium-Resistant Epilobium ciliatum After Recurrent Selection

The use of herbicides with different modes of action is the primary strategy used to control weeds possessing resistance to a single mechanism of action (MOA). However, this practice can lead to selection for generalist resistance mechanisms and may cause resistance to all MOAs. In this research, we characterized the resistance to diquat/paraquat (bipyridiliums) in an Epilobium ciliatum biotype (R1) collected in an olive orchard from Chile, where alternatives herbicides (2,4-D, glyphosate, glufosinate, flazasulfuron and pyraflufen-ethyl) with different MOAs were used, but they have also showed failure in controlling this species. Because the resistance/susceptibility patterns of the R1 biotype to glufosinate, 2,4-D and pyraflufen-ethyl were not clear, a recurrent resistance selection was carried out in field and greenhouse using these herbicides on R1 plants for three generations (R2 biotype). One biotype that was never treated with herbicides (S) was included as control. Results indicated that the S biotype was controlled at the field dose of all herbicides tested. The biotype R1 exhibited resistance to diquat, paraquat and flazasulfuron and natural tolerance to glyphosate. The R2 biotype displayed resistance to glufosinate, 2,4-D and pyraflufen-ethyl with LD₅₀ (herbicide dose to kill 50% of plants) values higher than field doses in all assays. Physiological and biochemical studies determined the resistance to diquat of the R1 biotype, which was due to impaired translocation. The resistance to flazasulfuron in the R1 and R2 biotypes was confirmed by the low sensitivity of the acetolactate synthase (ALS) activity compared to the S biotype. The similar accumulation of shikimate in treated S, R1, and R2 plants with glyphosate supported the existence of innate tolerance to this herbicide in E. ciliatum. Resistance to glufosinate, 2,4-D and pyraflufen-ethyl in the R2 biotype, acquired after recurrent selection, was determined by low sensitivity of the glutamine synthetase, low accumulation of ethylene and protoporphyrinogen IX oxidase, respectively, in comparison to the S biotype. Epilobium ciliatum from Chilean olive orchards had resistance to only two MAOs (photosystem I and ALS inhibitors), but resistance to five MOAs could occur in the next cropping seasons, if alternatives to weed management, other than herbicides, are not included.