Glyphosate-resistant Populations Research Articles

Role of glutathione S-transferases in the mode of action of herbicides that inhibit amino acid synthesis in Amaranthus palmeri

Acetolactate synthase inhibitors (ALS inhibitors) and glyphosate are two classes of herbicides that act by inhibiting an enzyme in the biosynthetic pathway of branched-chain or aromatic amino acids, respectively. Besides amino acid synthesis inhibition, both herbicides trigger similar physiological effects in plants. The main aim of this study was to evaluate the role of glutathione metabolism, with special emphasis on glutathione S-transferases (GSTs), in the mode of action of glyphosate and ALS inhibitors in Amaranthus palmeri. For that purpose, plants belonging to a glyphosate-sensitive (GLS) and a glyphosate-resistant (GLR) population were treated with different doses of glyphosate, and plants belonging to an ALS-inhibitor sensitive (AIS) and an ALS-inhibitor resistant (AIR) population were treated with different doses of the ALS inhibitor nicosulfuron. Glutathione-related contents, GST activity, and related gene expressions (glutamate-cysteine ligase, glutathione reductase, Phi GST and Tau GST) were analysed in leaves. According to the results of the analytical determinations, there were virtually no basal differences between GLS and GLR plants or between AIS and AIR plants. Glutathione synthesis and turnover did not follow a clear pattern in response to herbicides, but GST activity and gene expression (especially Phi GSTs) increased with both herbicides in treated sensitive plants, possibly related to the rocketing H2O2 accumulation. As GSTs offered the clearest results, these were further investigated with a multiple resistant (MR) population, compressing target-site resistance to both glyphosate and the ALS inhibitor pyrithiobac. As in single-resistant plants, measured parameters in the MR population were unaffected by herbicides, meaning that the increase in GST activity and expression occurs due to herbicide interactions with the target enzymes.

Glyphosate-resistant and susceptible downy brome (Bromus tectorum) management with soil-applied residual herbicides

Abstract Downy brome is a cleistogamous facultative winter-annual grass weed that invades cropland, pastureland, and ruderal areas in western North America. Glyphosate-resistant downy brome, the first known glyphosate-resistant grass weed in Canada, was confirmed in a glyphosate-resistant canola field in southern Alberta in 2021. A controlled-environment study was conducted to determine the impact of preemergence soil-applied residual herbicides on glyphosate-resistant and susceptible downy brome in two field soils. Flumioxazin/pyroxasulfone (70/89 g ai ha−1), carfentrazone/pyroxasulfone (18/150 g ai ha−1), sulfentrazone/pyroxasulfone (100/100 or 150/150 g ai ha−1), and saflufenacil/pyroxasulfone (36/120 g ai ha−1) resulted in excellent (≥90%) visible control and downy brome biomass reduction 8 wk after treatment (WAT). The low rate of carfentrazone/pyroxasulfone (12/100 g ai ha−1) resulted in good (≥80%) visible control and biomass reduction 8 WAT, while the low and medium rates of saflufenacil/pyroxasulfone (18/60 or 25/84 g ai ha−1) resulted in ≥80% biomass reduction but suppression only (66% to 75%) based on visible control. Flumioxazin alone (105 g ai ha−1) resulted in good visible control (81%) 8 WAT in a sandy loam soil, but poor (13%) control in a clay loam soil. Soil type affected plant growth as evidenced by reduced growth in the untreated sandy loam soil compared to clay loam soil. The glyphosate-resistant population emerged and grew more vigorously than the glyphosate-susceptible population resulting in greater plant densities in the untreated control and some less-effective herbicide treatments. These results suggest that mixtures of a protoporphyrinogen oxidase-inhibiting herbicide with the very-long-chain fatty acid elongase inhibitor pyroxasulfone applied preemergence at ≥89 g ai ha−1 could be effective components of an herbicide layering strategy targeting glyphosate-resistant and glyphosate-susceptible downy brome.

Response characterization and target site mechanism study in glyphosate-resistant populations of Lolium multiflorum L. from Brazil

Italian ryegrass (Lolium multiflorum L.) is an invasive species widely spread in croplands worldwide. The intensive use of glyphosate has resulted in the selection of resistance to this herbicide in Italian ryegrass. This work characterized the response to glyphosate of Italian ryegrass populations from the South and Southwest regions of Paraná, Brazil. A total of 44 Italian ryegrass populations were collected in farming areas, and were classified for glyphosate resistance with 75% of populations resistant to gloyphosate. Of these, 3 resistant (VT05AR, MR20AR and RN01AR) and three susceptible (VT07AS, MR05AS and RN01AS) of these populations were selected to determine the resistance level and the involvement of the target site mechanisms for glyphosate resistance. Susceptible populations GR50 ranged from 165.66 to 218.17 g.e.a. ha−1 and resistant populations from 569.37 to 925.94, providing RI ranging from 2.88 and 4.70. No mutation in EPSPS was observed in the populations, however, in two (MR20AR and RN02AR) of the three resistant populations, an increase in the number of copies of the EPSPs gene (11 to 57×) was detected. The number of copies showed a positive correlation with the gene expression (R2 = 0.86) and with the GR50 of the populations (R2 = 0.81). The increase in EPSPS gene copies contributes to glyphosate resistance in Italian ryegrass populations from Brazil.

Cysteine proteases are activated in sensitive Amaranthus palmeri populations upon treatment with herbicides inhibiting amino acid biosynthesis.

The herbicides glyphosate and pyrithiobac inhibit the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) in the aromatic amino acid biosynthetic pathway and acetolactate synthase (ALS) in the branched-chain amino acid biosynthetic pathway, respectively. Here we characterise the protease activity profiles of a sensitive (S), a glyphosate-resistant (GR) and a multiple-resistant (MR) population of Amaranthus palmeri in response to glyphosate and pyrithiobac. Amino acid accumulation and cysteine protease activities were induced with both herbicides in the S population and with pyrithiobac in the GR population, suggesting that the increase in cysteine proteases is responsible for the increased degradation of the available proteins and the observed increase in free amino acids. Herbicides did not induce any changes in the proteolytic activities in the populations with target-site resistance, indicating that this effect was only induced in sensitive plants.

Glyphosate-resistant downy brome (Bromus tectorum) control using alternative herbicides applied postemergence

AbstractDowny brome is a troublesome facultative winter-annual grass weed that invades agricultural and nonagricultural lands in western North America and can cause substantial crop yield losses particularly in no-till winter wheat. Glyphosate-resistant (GR) downy brome was identified in southern Alberta in 2021, representing the first confirmation of a GR grass weed in Canada. This study was designed to evaluate alternative herbicides and herbicide mixtures applied postemergence (POST) for control of GR and glyphosate-susceptible (GS) downy brome populations at the seedling stage under a controlled environment. The GR downy brome did not exhibit cross-resistance to other herbicides applied POST. Quizalofop alone or in combination with imazamox, imazamox + bentazon, or imazamox/imazethapyr, and glufosinate mixed with either clethodim or tiafenacil resulted in ≥80% visible control, plant mortality, and reduction in biomass of both GR and GS downy brome populations 21 d after treatment. Diligent stewardship of these remaining herbicide options is warranted since downy brome populations with resistance to herbicides that inhibit acetyl-CoA carboxylase or acetolactate synthase have been reported in neighboring states.

Non-Target Site Resistance in Summer-Emerging Lolium rigidum and the Effect of Alternative Herbicides

Herbicide resistance is an important weed management issue. Glyphosate is the most dominant herbicide, which controls a broad spectrum of weeds, including grasses such as Lolium rigidum. Lolium rigidum is a major weed of winter crops in Australia that is developing glyphosate resistance in increasing numbers of populations and has been observed growing throughout summer in recent years. Three L. rigidum populations, one summer-emerging glyphosate-resistant (GR), one winter-emerging glyphosate-susceptible (GS), and one winter-emerging population with unknown resistance status (CC04), were analyzed for target-site resistance to glyphosate after confirming their resistance or susceptibility to glyphosate in a dose–response experiment. Population GR was obtained from plants that emerged in summer and contained plants that survived all rates of glyphosate applied (0 to 4560 g a.e. ha−1). It was found to be 6.1 and 4.4 times more resistant than population GS across two experiments. Population CC04 was identified as susceptible. Plants from each population were analyzed for the presence of target-site mutations in the conserved region of the EPSPS gene; however, no mutations were identified that could cause resistance, suggesting non-target-site resistance in population GR. The effectiveness of alternative herbicides was also analyzed for each population. Paraquat was the most effective herbicide, with 0% survival across all populations. The ACCase-inhibiting herbicide clethodim was also highly effective (0 to 8% survival across populations). Other ACCase-inhibiting herbicides, propaquizafop, haloxyfop, and pinoxaden, were effective at controlling the two susceptible populations, CC04 and GS (0 to 36% and 0 to 20% survival, respectively), but were only moderately effective for controlling GR (28 to 51% survival). Paraquat and clethodim may be alternative options for controlling GR populations of L. rigidum and could be effective for use in management programs to slow the development of GR populations.

Widespread Occurrence of Glyphosate-Resistant Hairy Fleabane (Erigeron bonariensis L.) in Colombia and Weed Control Alternatives

Glyphosate, the most applied herbicide globally, offers effective non-selective and post-emergent weed control. Evolution of herbicide-resistant weeds is commonly associated with the recurrent application of herbicides with the same mode of action. Native to South America, hairy fleabane (Erigeron bonariensis L.) is the most problematic weed in this sub-continent and has previously been confirmed glyphosate resistant. This research aimed at characterizing glyphosate-resistant populations, thus estimating the frequency of resistance, resistance levels and identifying effective herbicide alternatives to control glyphosate-resistant populations. Glyphosate resistance characterization was initially conducted on ten suspected populations collected in plantain, banana, cassava, passionfruit, papaya, and drybean crops. Two resistant populations were selected and further characterized through dose-response tests; in addition, response to alternative herbicides (paraquat, glufosinate, 2,4-D, pyraflufen-ethyl, and mesotrione) was determined. All surveyed hairy fleabane populations survived (≥80% of individuals) the recommended glyphosate rate of 1080 g ae ha−1; conversely, five populations collected from non-cropping areas were effectively controlled at this same rate. Dose-response tests estimated resistance factors of 3.15- to 22.3-fold versus the most susceptible population. Interestingly, resistance profile and dose-response tests detected hormesis responses at field-recommended rates. The most effective herbicide alternatives to control glyphosate-resistant hairy fleabane were pyraflufen-ethyl and mesotrione.

First report of a wide distribution of glyphosate-resistant compact brome (Bromus madritensis) in the Iberian Peninsula: confirmation and field management

AbstractGlyphosate resistance is spreading in Spain and Portugal due to excessive herbicide use, in both annual and perennial crops. Compact brome (Bromus madritensis L.) is increasing in frequency in these different cropping systems when under conservation agriculture, particularly when glyphosate fails to control it. Fourteen populations from different areas in the Iberian Peninsula were confirmed as being B. madritensis using simple sequence repeat markers and clearly separated from the closely related species red brome (Bromus rubens L.) and ripgut brome (Bromus diandrus Roth). Six B. madritensis populations were classified as resistant, according to both their shikimic acid accumulation levels and their resistance factors based on LD50 or GR50 (values between 4 and 8). Populations with higher resistance factors also showed lower shikimic acid concentrations. Moreover, these resistant populations were able to survive the minimum registered dose for glyphosate in Spain in perennial crops (1,080 g ae ha−1, five populations) or in arable crops before seeding for annual weeds (540 g ae ha−1, one population), under both greenhouse and field conditions. The trials carried out in a glyphosate-resistant field during 2 consecutive years showed that acceptable control (≥90%) was only consistently achieved 90 d after application for preemergence treatment with flazasulfuron in a tank mix with glyphosate, while control with postemergence treatments, such as propaquizafop plus glyphosate, was below 80%. This research describes the first herbicide-resistance report for the weed species B. madritensis, confirming the presence of glyphosate-resistant populations mainly in perennial cropping systems but also in winter cereals from Spain. Due to the limited chemical tools to manage these populations, there is an urgent need for farmers to implement integrated weed management strategies.

Effects of elevated CO2 on the response of glyphosate resistant and susceptible Palmer amaranth (Amaranthus palmeri S. Wats.) to varying rates of glyphosate

ABSTRACT Repeated application of herbicides has led to herbicide-resistant weed populations. Elevated CO2 often increases weed growth, exacerbating problems including impacts on crop yield and weed control. How rising CO2 affects herbicide-resistant weeds remains largely unknown. Palmer amaranth (Amaranthus palmeri) populations resistant to or susceptible to glyphosate were exposed to ambient or elevated (ambient +200 μmol mol−1) concentrations of CO2 in open-top chambers. Two weeks after CO2 exposure, each population was sprayed with glyphosate at 0, 0.5×, 1.0×, or 1.5× the label rate. Afterwards, CO2 exposure continued for an additional two weeks when visual herbicide injury and leaf gas exchange were assessed. Plants were harvested for determination of treatment effects on growth. Elevated CO2 had little effect on most variables except for decreasing stomatal conductance (~25.5%) and increasing water use efficiency (~49.2%). Glyphosate resistant plants showed decreased height (9.8%) and damage (~56.4%) and increased root dry weight (12.1%), photosynthesis (~41.2%), stomatal conductance (~63.3%), and water use efficiency (~32.0%). In general, increasing rates of glyphosate decreased growth and gas exchange variables, which was frequently only seen in the susceptible ecotype. Data suggest that rising atmospheric CO2 is unlikely to affect Palmer amaranth, including control of glyphosate-resistant populations.

Investigations into differential glyphosate sensitivity between two horseweed (Conyza canadensis) growth types

AbstractHorseweed [Conyza canadensis (L.) Cronquist] grows in one of two distinct growth phenotypes, “rosette” and “upright” growth types, and they have recently been observed co-occurring in Michigan fields. Previous research found that upright plants from two glyphosate-resistant populations were 3- and 4-fold less sensitive to glyphosate than their rosette siblings. Further experiments were conducted to investigate whether differential glyphosate sensitivity of the growth types was due to glyphosate retention, absorption, or translocation. The total amount of glyphosate retained on the C. canadensis leaf surface was similar for both growth types; however, on a per-weight and per-area bases, the upright growth type retained 21% and 18% less glyphosate, respectively. Glyphosate absorption was up to 85% at 168 h after treatment (HAT), and was not different between the rosette and upright growth types or between the susceptible (S) and resistant (R) biotypes. Additionally, there was no difference in translocation between the two growth types within each biotype at any time point. Interestingly, at 168 HAT, [14C]glyphosate translocation was higher in the S rosette compared with the two growth types from the R biotype; however, the S upright type was similar to both R growth types. Thus, glyphosate resistance in the R biotype may be due to an alternative mechanism rather than impaired translocation, which has been cited as the primary mechanism of glyphosate resistance in C. canadensis. These results suggest that reduced glyphosate retention on a per-weight and per-area bases of the upright growth type may contribute to increased glyphosate tolerance due to a diluted concentration of glyphosate in the plant. However, another factor is likely related to the mechanism of resistance within the R biotype, which is contributing to a 3-fold difference in glyphosate sensitivity between the two growth types, such as alterations in EPSPS gene expression or changes in undescribed metabolism genes.

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.

Australia’s first case of glyphosate-resistant tall fleabane (Conyza sumatrensis) and alternative herbicide options for its control

AbstractTall fleabane is emerging as a problematic weed species in the eastern cropping region of Australia. Recently, growers indicated poor control of tall fleabane to the field rate of glyphosate in fallow fields. Pot studies were conducted in an open field at the Gatton farm of the University of Queensland, Queensland, Australia, to confirm the level of glyphosate resistance in a putative glyphosate-resistant (GR) tall fleabane population and to evaluate the performance of alternative postemergence herbicides to control GR tall fleabane. Compared with a glyphosate-susceptible (GS) population, the level of resistance in the GR population was 4-fold and 3.5-fold greater based on plant survival and biomass, respectively. The target-site resistance mechanism was not present because both the GR and GS populations had the same gene sequence. There were several effective alternative herbicides for the control of small (4-leaf stage) plants of tall fleabane, but to control large (12- to 14-leaf stage) plants, the sole application of saflufenacil + trifludimoxazin or its mixtures with glyphosate, glufosinate, or paraquat were the best herbicide treatments. This is the first published report on the occurrence of GR tall fleabane in Australia. Growers need to use integrated management strategies to mitigate the further spread of GR tall fleabane in fallow fields as well as glyphosate-resistant crops.

Constitutive overexpression of EPSPS by gene duplication is involved in glyphosate resistance in Salsola tragus.

Glyphosate-resistant Salsola tragus accessions have been identified in the USA and Argentina; however, the mechanisms of glyphosate resistance have not been elucidated. The goal of this study was to determine the mechanism/s of glyphosate resistance involved in two S.tragus populations (R1 and R2) from Argentina. Both glyphosate-resistant populations had a six-fold lower sensitivity to glyphosate than the S population (i.e. resistance index). No evidence of differential absorption, translocation or metabolism of glyphosate was found in the R1 and R2 populations compared to a susceptible population (S). No 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) mutations were detected, but S.tragus R1 and R2 plants had ≈14-fold higher EPSPS gene relative copy number compared to the S counterpart. In R1 and R2, EPSPS duplication entailed a greater constitutive EPSPS transcript abundance by approximately seven-fold and a basal EPSPS activity approximately three-fold higher than the S population. The current study reports EPSPS gene duplication for the first time as a mechanism of glyphosate resistance in S.tragus populations. The increase of glyphosate dose needed to kill R1 and R2 plants was linked to the EPSPS transcript abundance and level of EPSPS activity. This evidence supports the convergent evolution of the overexpression of the EPSPS gene in several Chenopodiaceae/Amaranthaceae species adapted to drought environments and the role of gene duplication as an adaptive advantage for plants to withstand stress. © 2022 Society of Chemical Industry.

The Resistance Levels and Target-Site Based Resistance Mechanisms to Glyphosate in Eleusine indica from China

The Dongting and Poyang Lakes are the important rice growing areas, and the Bohai Rim and Loess Plateau are the main producing areas of apples in China, where glyphosate has been used continuously to control weeds including Eleusine. indica for many years. In this study, the resistance levels and target-site based resistance (TSR) mechanisms to glyphosate in E. indica populations, which were collected from above areas were investigated. A total of 35 out of 50 (70%) E. indica populations have evolved resistance to glyphosate with resistance index (RI) of 2.01~10.43. The glyphosate-resistant (GR) E. indica accumulated less shikimic acid than glyphosate-susceptible (GS) populations, when treated by 1.0 mg/L, 10 mg/L or 100 mg/L glyphosate. There was no mutation at Thr102 and Pro106 in 5-enolpyruvate shikimate-3-phosphate synthase (EPSPS), which endowed glyphosate resistance in E. indica and other weed species. A Pro-381-Leu was found in EPSPS in GR populations. In contrast, the expression level of EPSPS gene was highly correlated with glyphosate resistance in E. indica with a determination coefficient of 0.88. These indicate that the glyphosate resistance in aforementioned E. indica populations was mainly caused by the overexpression of EPSPS, not by amino acid mutation in EPSPS.

First report of glyphosate-resistant downy brome (Bromus tectorum L.) in Canada

Glyphosate is the most used herbicide worldwide, and is an important source of economical weed control in glyphosate-resistant crops, and conservation tillage systems, among other uses. Downy brome (Bromus tectorum L.), otherwise known as cheatgrass, is a highly invasive winter-annual grass weed in cropping systems, pastureland, and naturalized or ruderal areas in western North America. In 2021, a downy brome population remained uncontrolled following four applications of glyphosate in a glyphosate-resistant canola (Brassica napus L.) field located in Taber County, Alberta, Canada. All individuals from the subsequent generation of the population survived glyphosate treatment at the typical field rate (900 g ae ha−1). In dose-response bioassays, the putative glyphosate-resistant population exhibited 10.6- to 11.9-fold, 7.7- to 8.7-fold, 7.8- to 8.8-fold, and 8.3- to 9.5-fold resistance to glyphosate based on plant survival, visible control, and biomass fresh weight and dry weight, respectively, compared with two susceptible populations 21 days after treatment. Estimated glyphosate rates for 80% control of this population ranged from 2795 to 4511 g ae ha−1; well above common usage rates. This downy brome population represents the first glyphosate-resistant grass weed confirmed in Canada, and the second weed species exhibiting glyphosate resistance in the Canadian prairie region.

Monitoring and baseline of glyphosate-resistant sourgrass in the main soybean growing regions of Brazil

Frequent application of glyphosate for consecutive years has enhanced the selection pressure on sourgrass (Digitaria insularis) populations, which resulted in the development of glyphosate-resistant biotypes. Therefore, this work was developed with the objective of monitoring sourgrass resistance to glyphosate, develop a baseline of sourgrass susceptibility to this molecule and, consequently, identify the discriminatory dose between resistant and susceptible populations. This work was divided into three steps. The first step consisted of identifying and sorting sourgrass resistant and susceptible biotypes among 30 samples. In the second step, glyphosate baseline was elaborated considering exclusively the glyphosate-susceptible biotypes, which allowed the definition of a discriminatory dose. At the end, the third step, monitoring of glyphosate-resistant biotypes was achieved, considering five growing seasons (2016 – 2020) and 809 samples of sourgrass populations, collected throughout 12 states of Brazil. Glyphosate baseline was elaborated to sourgrass and ideal discriminatory rate was identified as 960 g ha-1. Glyphosate-resistant populations of sourgrass were found in all soybean growing regions sampled. Among 809 populations, 25.96% were considered resistant to glyphosate. The states with the highest frequency of glyphosate-resistant populations were Rio Grande do Sul, Mato Grosso do Sul, Bahia, Mato Grosso and Paraná.

Transpiration Responses of Herbicide-Resistant and -Susceptible Palmer Amaranth (Amaranthus palmeri (S.) Wats.) to Progressively Drying Soil

Drought events are predicted to increase in the future. Evaluating the response of herbicide-resistant and -susceptible weed ecotypes to progressive drought can provide insights into whether resistance traits affect the fitness of resistant weed populations. Two experiments were conducted in the greenhouse between January and May 2021 to evaluate drought tolerance differences between Palmer amaranth accessions resistant to S-metolachlor or glyphosate and their susceptible counterparts. The accessions used were S-metolachlor-resistant (17TUN-A), a susceptible standard (09CRW-A), and glyphosate-resistant (22–165 EPSPS copies) and glyphosate-susceptible (3–10 EPSPS copies) plants from accession 16CRW-D. Daily transpiration of each plant was measured. The daily transpiration rate was converted to normalized transpiration ratio (NTR) using a double-normalization procedure. The daily soil water content was expressed as a fraction of transpirable soil water (FTSW). The threshold FTSW (FTSWcr), after which NTR decreases linearly, was estimated using a two-segment linear regression analysis. The data showed differences between S-metolachlor-resistant and -susceptible accessions (p ≤ 0.05). The FTSW remaining in the soil at the breakpoint for the S-metolachlor-susceptible accession (09CRW-A) was 0.17 ± 0.007. The FTSW remaining in the soil at the breakpoint for the S-metolachlor-resistant accession (17TUN-A) was 0.23 ± 0.004. The FTSW remaining in the soil at the breakpoint for the glyphosate-resistant and glyphosate-susceptible plants (16CRW-D) was 0.25 ± 0.007 and 0.25 ± 0.008, respectively. Although the mechanism endowing resistance to S-metolachlor might have contributed to increased drought tolerance, follow-up experiments are needed in order to verify this finding. Increased EPSPS copy numbers did not improve the drought tolerance of Palmer amaranth. As droughts are predicted to increase in frequency and severity, these results suggest that S-metolachlor-resistant and glyphosate-resistant Palmer amaranth populations will not be at a competitive disadvantage compared to susceptible genotypes. Alternative and diverse management strategies will be required for effective Palmer amaranth control, regardless of herbicide resistance status.

Glyphosate resistance in junglerice (Echinochloa colona) and alternative herbicide options for its effective control

AbstractControl of glyphosate-resistant (GR) junglerice is a challenging task in eastern Australia. There is limited information on the efficacy and reliability of alternate herbicides for GR populations of junglerice, especially when targeting large plants and when temperatures are high. A series of experiments were conducted to confirm the level of glyphosate resistance in three populations of junglerice and to evaluate the efficacy of alternate herbicides for the control of GR junglerice populations. The LD50 of glyphosate of B17/7, B17/34, and B17/35 populations was found to be 298, 2,260, and 1,715 g ae ha–1, respectively, suggesting that populations B17/34 and B17/35 were highly resistant to glyphosate. Glyphosate efficacy was reduced at high-temperature (35 C day/25 C night) compared with low-temperature conditions (25 C day/15 C night), suggesting that control of susceptible populations may also be reduced if glyphosate is sprayed under hot conditions. Preemergence herbicides dimethenamid-P (1,000 g ai ha–1) and pendimethalin (1,500 g ai ha–1) provided 100% control of GR populations (B17/34 and 17/35). Postemergence herbicides, such as clethodim (60 or 90 g ai ha–1), glufosinate (750 g ai ha–1), haloxyfop (52 or 78 g ai ha–1), and paraquat (400 or 600 g ai ha–1), applied at the four-leaf stage provided 100% control of GR populations. For larger junglerice plants (eight-leaf stage), postemergence applications of paraquat (400 or 600 g ai ha–1) provided greater weed control than clethodim, glufosinate, and haloxyfop. A mixture of either glufosinate or haloxyfop with glyphosate provided poor control of GR junglerice populations compared with application of glufosinate or haloxyfop applied alone. Efficacy of glufosinate and haloxyfop for the control of GR populations decreased when applied in the sequential spray after glyphosate application. This study identified alternative herbicide options for GR junglerice populations that can be used in herbicide rotation programs for sustainable weed management.

Inheritance and mechanism of glyphosate resistance in annual bluegrass (Poa annua L.).

In initial screening, glyphosate was ineffective in controlling five Poa annua populations. These populations were tested for resistance, and studies undertaken to determine resistance mechanisms and inheritance pattern. Dose-response studies conducted at 16/12°C and 27/20°C on the five putative resistant populations showed low-level resistance (1.4- to 2.5-fold) to glyphosate. Shikimic acid accumulation in response to glyphosate confirmed differences among the populations, with greater shikimic acid accumulation in the susceptible population. The EPSPS gene copy number was 0.5- to 5.2-fold greater in one resistant population (HT) than in the susceptible (S) population, but not in the others. EPSPS gene expression was five- to tenfold higher in HT compared with the susceptible population. Target site mutations, differences in glyphosate absorption or translocation or altered expression of aldo-keto reductase (AKR) were not identified in any of the resistant populations. Crosses were successful between one resistant population and the susceptible population (P262-16♂ ✕ S♀) and inheritance of glyphosate resistance appears to be controlled by a single, nuclear dominant gene in this population. Our study identified EPSPS gene amplification in a South Australian glyphosate-resistant P. annua population (HT). This mechanism of resistance was not identified in the other four glyphosate-resistant populations, and other common mechanisms were excluded. Although the resistance mechanism in some P. annua populations remains unknown, inheritance studies with one population suggest the involvement of a single dominant gene. © 2021 Society of Chemical Industry.

Investigating the origins and evolution of a glyphosate-resistant weed invasion in South America.

The global invasion, and subsequent spread and evolution of weeds provides unique opportunities to address fundamental questions in evolutionary and invasion ecology. Amaranthus palmeri is a widespread glyphosate-resistant (GR) weed in the USA. Since 2015, GR populations of A. palmeri have been confirmed in South America, raising questions about introduction pathways and the importance of pre- vs. post-invasion evolution of GR traits. We used RAD-sequencing genotyping to characterize genetic structure of populations from Brazil, Argentina, Uruguay and the USA. We also quantified gene copy number of the glyphosate target, 5-enolpyruvyl-3-shikimate phosphate synthase (EPSPS), and the presence of an extrachromosomal circular DNA (eccDNA) replicon known to confer glyphosate resistance in USA populations. Populations in Brazil, Argentina and Uruguay were only weakly differentiated (pairwise FST ≤0.043) in comparison to USA populations (mean pairwise FST =0.161, range=0.068-0.258), suggesting a single major invasion event. However, elevated EPSPS copy number and the EPSPS replicon were identified in all populations from Brazil and Uruguay, but only in a single Argentinean population. These observations are consistent with independent in situ evolution of glyphosate resistance in Argentina, followed by some limited recent migration of the eccDNA-based mechanism from Brazil to Argentina. Taken together, our results are consistent with an initial introduction of A. palmeri into South America sometime before the 1980s, and local evolution of GR in Argentina, followed by a secondary invasion of GR A. palmeri with the unique eccDNA-based mechanism from the USA into Brazil and Uruguay during the 2010s.