Pollen-mediated gene flow from glyphosate-resistant common waterhemp (Amaranthus rudis Sauer): consequences for the dispersal of resistance genes

Genetic basis and origin of resistance to acetolactate synthase inhibitors in Amaranthus palmeri from Spain and Italy

BACKGROUND

Amaranthus palmeri is an aggressive annual weed native to the United States, which has become invasive in some European countries. Populations resistant to acetolactate synthase (ALS) inhibitors have been recorded in Spain and Italy, but the evolutionary origin of the resistance traits remains unknown. Bioassays were conducted to identify cross-resistance to ALS inhibitors and a haplotype-based genetic approach was used to elucidate the origin and distribution of resistance in both countries.

RESULTS

Amaranthus palmeri populations were resistant to thifensulfuron-methyl and imazamox, and the 574-Leu mutant ALS allele was found to be the main cause of resistance among them. In two Spanish populations, 376-Glu and 197-Thr mutant ALS alleles were also found. The haplotype analyses revealed the presence of two and four distinct 574-Leu mutant haplotypes in the Italian and Spanish populations, respectively. None was common to both countries, but some mutant haplotypes were shared between geographically close populations or between populations more than 100 km apart. Wide genetic diversity was found in two very close Spanish populations.

CONCLUSION

ALS-resistant A. palmeri populations were introduced to Italy and Spain from outside Europe. Populations from both countries have different evolutionary histories and originate from independent introduction events. ALS resistance then spread over short and long distances by seed dispersal. The higher number and genetic diversity among mutant haplotypes from the Spanish populations indicated recurrent invasions. The implementation of control tactics to limit seed dispersal and the establishment of A. palmeri is recommended in both countries. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Investigation of resistance mechanisms to fomesafen in Ipomoea nil from China

Recently, the herbicide fomesafen has frequently failed to control the troublesome weed Ipomoea nil in soybean fields in Liaoning Province, China. Hence, we collected 10 suspected resistant populations and evaluated their sensitivity to fomesafen. The results revealed various degrees of Ipomoea nil resistance to fomesafen, with a resistance index of 2.88 to 22.43; the highest value occurred in the LN3 population. Therefore, the mechanisms of the resistance in LN3 to fomesafen were explored. After fomesafen treatment, the expression levels of InPPX1 and InPPX2 genes were 4.19- and 9.29-fold higher, respectively, in LN3 than those in the susceptible (LN1) population. However, mutations and copy number variations were not detected between the two populations. Additionally, malathion pretreatment reduced the dose necessary to halve the growth rate of LN3 by 58%. Liquid chromatography with tandem mass spectrometry demonstrated that metabolism of fomesafen was significantly suppressed by malathion. Moreover, LN3 displayed increased reactive oxygen species scavenging capacity, which was represented by higher superoxide dismutase and peroxidase activities after fomesafen application than those in LN1. An orthogonal partial least squares-discriminant analysis revealed that the high resistance in LN3 could be attributed mainly to enhanced metabolism. Fortunately, the fomesafen-resistant I. nil remained sensitive to 2,4-D-ethylhexylester and bentazon, providing methods for its control.

Long-Distance Pollen Dispersal in Urban Green Roof and Ground-Level Habitats

Long-distance pollen dispersal is critical for gene flow in plant populations, yet pollen dispersal patterns in urban habitats such as green roofs have not been extensively studied. Pollen dispersal patterns typically are assessed either by fitting non-linear models to the relationship between the degree of pollen dispersal and distance to the pollen source (i.e., curve fitting), or by fitting probability density functions (PDFs) to pollen dispersal probability histograms (i.e., PDF fitting). Studies using curve fitting typically report exponential decay patterns in pollen dispersal. However, PDF fitting typically produces more fat-tailed distributions, suggesting the exponential decay may not be the best fitting model. Because the two approaches may yield conflicting results, we used both approaches to examine pollen dispersal patterns in the wind-pollinated Amaranthus tuberculatus and the insect-pollinated Solanum lycopersicum at two green roof and two ground-level sites in the New York (NY, United States) metropolitan area. For the curve fitting analyses, the exponential decay and inverse power curves provided good fits to pollen dispersal patterns across both green roof and ground-level sites for both species. Similar patterns were observed with the PDF fitting analyses, where the exponential or inverse Gaussian were the top PDF at most sites for both species. While the curve fitting results are consistent with other studies, the results differ from most studies using PDF fitting, where long-distance pollen dispersal is more common than we observed. These results highlight the need for further research to compare curve and PDF fitting for predicting pollen dispersal patterns. And, critically, while long-distance pollen dispersal may be an important component of overall pollen dispersal for A. tuberculatus and S. lycopersicum in both urban green roof and ground-level sites, our results suggest it potentially may occur to a lesser extent compared with plants in less-urban areas.

Pollen-mediated transfer of herbicide resistance between johnsongrass (Sorghum halepense) biotypes

Johnsongrass (Sorghum halepense) is a troublesome weed in row crop production in the United States. Herbicide resistance is a growing concern in this species, with resistance to ACCase-, ALS-, and EPSPS-inhibitors already reported. Pollen-mediated gene flow (PMGF) is capable of spreading herbicide resistance, but the extent of PMGF has not yet been studied in johnsongrass. Field experiments were conducted in a Nelder-wheel design to quantify the distance and frequency of PMGF from ALS-inhibitor-resistant (AR) to -susceptible (AS) johnsongrass across three environments (summer 2018, fall 2018, and fall 2019). The AR biotype (pollen donor) was established at the center of the wheel (5-m diameter), and a naturally occurring johnsongrass (AS) infestation was utilized as the pollen recipient, in eight directions and at nine distances (5, 10, 15, 20, 25, 35, 40, 45, and 50 m) within each direction. Seeds collected from the AS plants in each distance and direction were screened for survival to the ALS-inhibitor herbicide nicosulfuron (Accent Q) at 95 g ai ha⁻¹ under greenhouse conditions. The survivors (i.e. hybrids) were further confirmed based on the presence of the Trp₅₇₄Leu mutation. At the closest distance of 5 m, PMGF was 9.6–16.2% across the directions and environments, which progressively declined to 0.8–1.2% at 50 m. The exponential decay model predicted 50% reduction in PMGF at 2.2 m and 90% reduction at 5.8 m from the pollen donor block. Results demonstrate that herbicide resistance can spread between adjacent field populations of johnsongrass through PMGF, which necessitates sound monitoring and management.

Metabolic Pathways for S-Metolachlor Detoxification Differ Between Tolerant Corn and Multiple-Resistant Waterhemp

Herbicide resistance in weeds can be conferred by target-site and/or non-target-site mechanisms, such as rapid metabolic detoxification. Resistance to the very-long-chain fatty acid-inhibiting herbicide, S-metolachlor, in multiple herbicide-resistant populations (CHR and SIR) of waterhemp (Amaranthus tuberculatus) is conferred by rapid metabolism compared with sensitive populations. However, enzymatic pathways for S-metolachlor metabolism in waterhemp are unknown. Enzyme assays using S-metolachlor were developed to determine the specific activities of glutathione S-transferases (GSTs) and cytochrome P450 monooxygenases (P450s) from CHR and SIR seedlings to compare with tolerant corn and sensitive waterhemp (WUS). GST activities were greater (∼2-fold) in CHR and SIR compared to WUS but much less than corn. In contrast, P450s in microsomal extracts from CHR and SIR formed O-demethylated S-metolachlor, and their NADPH-dependent specific activities were greater (>20-fold) than corn or WUS. Metabolite profiles of S-metolachlor generated via untargeted and targeted liquid chromatography-mass spectrometry from CHR and SIR differed from WUS, with greater relative abundances of O-demethylated S-metolachlor and O-demethylated S-metolachlor-glutathione conjugates formed by CHR and SIR. In summary, our results demonstrate that S-metolachlor metabolism in resistant waterhemp involves Phase I and Phase II metabolic activities acting in concert, but the initial O-demethylation reaction confers resistance.

Population structure and evolution of resistance to acetolactate synthase (ALS)-inhibitors in Amaranthus tuberculatus in Italy

BACKGROUND

Before 2010, Amaranthus tuberculatus (Moq.) J. D. Sauer was barely known to farmers and stakeholders in Italy. Since then, several populations resistant to acetolactate synthase (ALS)-inhibiting herbicides have been collected. In most populations, a known target site resistance-endowing mutation was found, a Trp to Leu substitution at position 574 of the ALS gene, but it was unclear whether they had evolved resistance independently or not. The aims of the work were (i) to elucidate the population structure of Italian ALS-resistant A. tuberculatus populations, and (ii) to analyze the ALS haplotypes of the various populations to determine whether resistance arose multiple times independently.

RESULTS

In order to determine the population structure of eight A. tuberculatus populations, eight previously described microsatellite loci were used. Two ancestors were found: three populations derived from one, and five from the other. In the 4-kb ALS region of the genome, including the 2-kb coding region, 389 single nucleotide polymorphisms were found. In silico haplotype estimation was used to reconstruct the sequence of three distinct haplotypes carrying the Trp574Leu mutation. In addition, no mutation was found in 83% of plants of a single population.

CONCLUSIONS

(i) Resistance must have arisen independently at least three times; (ii) at least one population was already resistant to ALS inhibitors when introduced in Italy; (iii) a single haplotype with a Trp574Leu mutation was shared among six populations, probably because of broad seed dispersal; and (iv) one population likely evolved nontarget site ALS inhibitors resistance. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Pollen dispersal patterns differ among sites for a wind-pollinated species and an insect-pollinated species

PREMISE

Pollen dispersal, the main component of overall plant gene flow, generally decreases with increasing distance from the pollen source, but the pattern of this relationship may differ among sites. Although site-based differences in pollen dispersal may lead to over- or underestimation of gene flow, no studies have investigated pollen dispersal patterns among differing urban site types, despite the incongruent range of habitats in urban areas.

METHODS

We used paternity assignment to assess pollen dispersal patterns in a wind-pollinated species (waterhemp; Amaranthus tuberculatus) and in an insect-pollinated species (tomato; Solanum lycopersicum) in experimental arrays at four disparate sites (two roof-level sites, two ground-level sites) in the New York (New York, USA) metropolitan area.

RESULTS

The number of seeds or fruits, a proxy for the number of flowers pollinated, decreased with increasing distance from the pollen donors at all sites for both species. However, the mean number of Amaranthus tuberculatusseeds produced at a given distance differed two-fold among sites, while the slope of the relationship between Solanum lycopersicumfruit production and distance differed by a factor of four among sites.

CONCLUSIONS

Pollen dispersal patterns may differ substantially among sites, both in the amount of pollen dispersed at a given distance and in the proportional decrease in pollen dispersal with increasing distance, and these effects may act independently. Accordingly, the capacity of plant species to adapt to climate change and other selection pressures may be different from predictions based on pollen dispersal patterns at a single location.

Evolution of EPSPS double mutation imparting glyphosate resistance in wild poinsettia (Euphorbia heterophylla L.)

The evolution of glyphosate resistance (GR) in weeds is an increasing problem. Glyphosate has been used intensively on wild poinsettia (Euphorbia heterophylla L.) populations for at least 20 years in GR crops within South America. We investigated the GR mechanisms in a wild poinsettia population from a soybean field in southern Brazil. The GR population required higher glyphosate doses to achieve 50% control (LD50) and 50% dry mass reduction (MR50) compared to a glyphosate susceptible (GS) population. The ratio between the LD50 and MR50 of GR and GS resulted in resistance factors (RF) of 6.9-fold and 6.1-fold, respectively. Shikimate accumulated 6.7 times more in GS than in GR when leaf-discs were incubated with increasing glyphosate concentrations. No differences were found between GR and GS regarding non-target-site mechanisms. Neither population metabolized glyphosate to significant levels following treatment with 850 g ha-1 glyphosate. Similar levels of 14C-glyphosate uptake and translocation were observed between the two populations. No differences in EPSPS expression were found between GS and GR. Two target site mutations were found in all EPSPS alleles of homozygous resistant plants: Thr102Ile + Pro106Thr (TIPT-mutation). Heterozygous individuals harbored both alleles, wild-type and TIPT. Half of GR individuals were heterozygous, suggesting that resistance is still evolving in the population. A genotyping assay was developed based on the Pro106Thr mutation, demonstrating high efficiency to identify homozygous, heterozygous or wild-type EPSPS sequences across different plants. This is the first report of glyphosate-resistant wild-poinsettia harboring an EPSPS double mutation (TIPT) in the same plant.

Metabolic resistance to S-metolachlor in two waterhemp (Amaranthus tuberculatus) populations from Illinois, USA

BACKGROUND

Two waterhemp (Amaranthus tuberculatus) populations from Illinois demonstrating multiple-resistance to acetolactate synthase (ALS)-, 4-hydroxyphenylpyruvate dioxygenase, and photosystem II (PSII)-inhibiting herbicides (designated CHR and SIR) also displayed reduced sensitivity to very-long-chain fatty acid-inhibiting herbicides, including S-metolachlor. We hypothesized that a physiological mechanism, such as enhanced metabolism, could be responsible for the reduced efficacy of S-metolachlor.

RESULTS

Metabolism experiments indicated that resistant populations degraded S-metolachlor more rapidly than sensitive populations and equally as rapidly as corn 2-24 h after treatment (HAT). Resistant waterhemp and corn metabolized 90% (DT₉₀ ) of absorbed S-metolachlor in less than 3.2 h whereas DT₉₀ values for sensitive waterhemp exceeded 6 h. The glutathione S-transferase inhibitor 4-chloro-7-nitrobenzofurazon and cytochrome P450-inhibitor malathion decreased the amount of S-metolachlor metabolized in resistant waterhemp at 4 HAT but not in sensitive waterhemp or corn, and altered the abundance of certain metabolites in resistant waterhemp.

CONCLUSION

Results from this research demonstrate that resistance to S-metolachlor in these waterhemp populations is due to enhanced herbicide metabolism relative to sensitive populations. In addition, our results indicate that resistant waterhemp might utilize metabolic pathway(s) more intricate than either sensitive waterhemp or corn based on their metabolite profiles. © 2020 Society of Chemical Industry.

Combining rare alleles and grouped pollen donors to assign paternity in pollen dispersal studies

PREMISE

Pollen dispersal plays a critical role in gene flow of seed plants. Most often, pollen dispersal is measured using paternity assignment. However, this approach can be time-consuming because it typically entails genotyping all pollen donors, receptors, and offspring at several molecular markers.

METHODS

We developed a faster, simpler protocol to track paternity, using pollen receptors and grouped pollen donors that possess rare alleles. We tested this approach using wind-pollinated Amaranthus tuberculatus and insect-pollinated Solanum lycopersicum. After screening potential markers for rare alleles, we grew both species in experimental arrays under field conditions.

RESULTS

All tested A. tuberculatus seeds and 97% of S. lycopersicum fruits could be assigned to the grouped pollen donors using each of two markers. From these results, we could infer paternity of untested offspring and assess pollen dispersal patterns in each array.

DISCUSSION

By combining rare alleles and grouped pollen donors, we could assess pollen dispersal for both species and across all arrays after genotyping a small number of pollen donors and a representative subset of offspring. While directly applicable to A. tuberculatus and S. lycopersicum, this approach could be used in other species to assess pollen dispersal under field conditions.

Herbicide drift exposure leads to reduced herbicide sensitivity in Amaranthus spp

While the introduction of herbicide tolerant crops provided growers new options to manage weeds, the widespread adoption of these herbicides increased the risk for herbicide spray drift to surrounding vegetation. The impact of herbicide drift in sensitive crops is extensively investigated, whereas scarce information is available on the consequences of herbicide drift in non-target plants. Weeds are often abundant in field margins and ditches surrounding agricultural landscapes. Repeated herbicide drift exposure to weeds could be detrimental to long-term management as numerous weeds evolved herbicide resistance following recurrent-selection with low herbicide rates. The objective of this study was to evaluate if glyphosate, 2,4-D, and dicamba spray drift could select Amaranthus spp. biotypes with reduced herbicide sensitivity. Palmer amaranth and waterhemp populations were recurrently exposed to herbicide drift in a wind tunnel study over two generations. Seeds from survival plants were used for the subsequent rounds of herbicide drift exposure. Progenies were subjected to herbicide dose-response studies following drift selection. Herbicide drift exposure rapidly selected for Amaranthus spp. biotypes with reduced herbicide sensitivity over two generations. Weed management programs should consider strategies to mitigate near-field spray drift and suppress the establishment of resistance-prone weeds on field borders and ditches in agricultural landscapes.

Rapid metabolism increases the level of 2,4-D resistance at high temperature in common waterhemp (Amaranthus tuberculatus)

Common waterhemp emerges throughout the crop growing season in the Midwestern United States, and as a result, the seedlings are exposed to a wide range of temperature regimes. Typically, 2,4-D is used in the Midwest to control winter annual broad-leaf weeds before planting soybean and in an early post-emergence application in corn and sorghum; however, the evolution of 2,4-D-resistant common waterhemp in several Midwestern states may limit the use of 2.4-D for controlling this problem weed. Moreover, temperature is one of the crucial factors affecting weed control efficacy of 2,4-D. This research investigated the effect of temperature on efficacy of 2,4-D to control 2,4-D susceptible (WHS) and -resistant (WHR) common waterhemp. Do se-response of WHS and WHR to 2,4-D was assessed at two temperature regimes, high (HT; 34/20 °C, d/n) and low (LT; 24/10 °C, d/n). Whole plant dose response study indicated an increased level of 2,4-D resistance in WHR at HT compared to LT. Additional investigation of the physiological mechanism of this response indicated that both WHS and WHR common waterhemp plants rapidly metabolized ¹⁴C 2,4-D at HT compared to LT. In conclusion, a rapid metabolism of 2,4-D conferred increased level of resistance to 2,4-D in WHR at HT. Therefore, application of 2,4-D when temperatures are cooler can improve control of 2,4-D resistant common waterhemp.

Response of Amaranthus spp. following exposure to sublethal herbicide rates via spray particle drift

The adverse consequences of herbicide drift towards sensitive crops have been extensively reported in the literature. However, little to no information is available on the consequences of herbicide drift onto weed species inhabiting boundaries of agricultural fields. Exposure to herbicide drift could be detrimental to long-term weed management as several weed species have evolved herbicide-resistance after recurrent selection with sublethal herbicide rates This study investigated the deposition of glyphosate, 2,4-D, and dicamba spray particle drift from applications with two different nozzles in a low speed wind tunnel, and their impact on growth and development of Amaranthus spp. Herbicide drift resulted in biomass reduction or complete plant mortality. Inflection points (distance to 50% biomass reduction) for Amaranthus tuberculatus were 7.7, 4.0, and 4.1 m downwind distance for glyphosate, 2,4-D, and dicamba applications with the flat-fan nozzle, respectively, whereas these values corresponded to 2.8, 2.5, and 1.9 m for applications with the air-inclusion nozzle. Inflection points for Amaranthus palmeri biomass reduction were 16.3, 10.9, and 11.5 m for glyphosate, 2,4-D, and dicamba applications with the flat-fan nozzle, respectively, whereas these values corresponded to 7.6, 5.4, and 5.4 m for applications with the air-inclusion nozzle. Plants were more sensitive to glyphosate at higher exposure rates than other herbicides, whereas plants were more sensitive to 2,4-D and dicamba at lower exposure rates compared to glyphosate. Applications with the flat-fan nozzle resulted in 32.3 and 11.5% drift of the applied rate at 1.0 and 3.0 m downwind, respectively, whereas the air-inclusion nozzle decreased the dose exposure in the same distances (11.4 and 2.7%, respectively). Herbicide drift towards field boundaries was influenced by nozzle design and exposed weeds to herbicide rates previously reported to select for herbicide-resistant biotypes.

Interspecific and intraspecific transference of metabolism-based mesotrione resistance in dioecious weedy Amaranthus

Pollen-mediated gene flow (PMGF) might play an important role in dispersing herbicide resistance alleles in dioecious weedy Amaranthus species. Field experiments in a concentric donor-receptor design were conducted to quantify two sets of PMGF studies, an interspecific (Amaranthus tuberculatus × Amaranthus palmeri) and an intraspecific (A. tuberculatus × A. tuberculatus). In both studies, PMGF was evaluated using a resistant A. tuberculatus phenotype with enhanced mesotrione detoxification via P450 enzymes as a source of resistance alleles. For interspecific hybridization, more than 104 000 putative hybrid seedlings were screened with three markers, one phenotypic and two molecular. The two molecular markers used, including 2-bp polymorphisms in the internal transcribed spacer region, distinguished A. palmeri, A. tuberculatus and their hybrids. Results showed that 0.1% hybridization between A. tuberculatus × A. palmeri occurred under field research conditions. For intraspecific hybridization, 22 582 seedlings were screened to assess the frequency of gene flow. The frequency of gene flow (F_GF ) varied with distance, direction and year of the study. The farthest distance for 90% reduction of F_GF was at 69 m in 2015 however, after averaging across directions it was 13.1 and 26.1 m in 2014 and 2015, respectively. This study highlights the transfer of metabolism-based mesotrione resistance from A. tuberculatus to A. palmeri under field research conditions. The results presented here might aid in the rapid detection of A. palmeri among other Amaranthus species and show that PMFG could be expediting the increase of herbicide resistance in A. palmeri and A. tuberculatus across US crop production areas.

Confronting herbicide resistance with cooperative management

BACKGROUND

Resistance of pathogens and pests to antibiotics and pesticides worldwide is rapidly reaching critical levels. The common-pool-resource nature of this problem (i.e. whereby the susceptibility to treatment of target organisms is a shared resource) has been largely overlooked. Using herbicide-resistant weeds as a model system, we developed a discrete-time landscape-scale simulation to investigate how aggregating herbicide management strategies at different spatial scales from individual farms to larger cooperative structures affects the evolution of glyphosate resistance in common waterhemp (Amaranthus tuberculatus).

RESULTS

Our findings indicate that high-efficacy herbicide management strategies practiced at the farm scale are insufficient to slow resistance evolution in A. tuberculatus. When best practices were aggregated at large spatial scales, resistance evolution was hindered; conversely, when poor management practices were aggregated, resistance was exacerbated. Tank mixture-based strategies were more effective than rotation-based strategies in most circumstances, while applying glyphosate alone resulted in the poorest outcomes.

CONCLUSIONS

Our findings highlight the importance of landscape-scale cooperative management for confronting common-pool-resource resistance problems in weeds and other analogous systems. © 2018 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

The power and potential of genomics in weed biology and management

There have been previous calls for, and efforts focused on, realizing the power and potential of weed genomics for better understanding of weeds. Sustained advances in genome sequencing and assembly technologies now make it possible for individual research groups to generate reference genomes for multiple weed species at reasonable costs. Here, we present the outcomes from several meetings, discussions, and workshops focused on establishing an International Weed Genomics Consortium (IWGC) for a coordinated international effort in weed genomics. We review the 'state of the art' in genomics and weed genomics, including technologies, applications, and on-going weed genome projects. We also report the outcomes from a workshop and a global survey of the weed science community to identify priority species, key biological questions, and weed management applications that can be addressed through greater availability of, and access to, genomic resources. Major focus areas include the evolution of herbicide resistance and weedy traits, the development of molecular diagnostics, and the identification of novel targets and approaches for weed management. There is increasing interest in, and need for, weed genomics, and the establishment of the IWGC will provide the necessary global platform for communication and coordination of weed genomics research. © 2018 Society of Chemical Industry.

Distribution of glyphosate-resistant Amaranthus spp. in Nebraska

BACKGROUND

Palmer amaranth (Amaranthus palmeri S. Wats.), common waterhemp (Amaranthus tuberculatus var. rudis), and redroot pigweed (Amaranthus retroflexus L.) are major weeds occurring in fields throughout Nebraska with recurrent grower complaints regarding control with glyphosate. The objective of this study was to investigate the frequency and distribution of glyphosate-resistant Palmer amaranth, common waterhemp, and redroot pigweed populations in Nebraska. The study also aimed to investigate how agronomic practices influence the occurrence of glyphosate resistance in the three Amaranthus species.

RESULTS

Glyphosate resistance was widespread in common waterhemp (81% of the screened populations), few Palmer amaranth populations were glyphosate-resistant (6% of the screened populations), whereas no glyphosate-resistant redroot pigweed populations were identified in Nebraska. Weed species, geographic region within the state, and current crop were the most important factors predicting the occurrence of glyphosate resistance in fields infested with Amaranthus species in Nebraska.

CONCLUSION

The intensive glyphosate selection pressure exerted in soybean (Glycine max) fields in eastern Nebraska is one of the major factors causing widespread occurrence of glyphosate resistance in common waterhemp in the state. The relatively low frequency of glyphosate-resistant Palmer amaranth in the state highlights the importance of the application timing and the adoption of multiple modes of action in weed management practices to delay the evolution of glyphosate resistance. © 2017 Society of Chemical Industry.

Reversing resistance to tembotrione in an Amaranthus tuberculatus (var. rudis) population from Nebraska, USA with cytochrome P450 inhibitors

BACKGROUND

A population of Amaranthus tuberculatus (var. rudis) was confirmed resistant to 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibitor herbicides (mesotrione, tembotrione, and topramezone) in a seed corn/soybean rotation in Nebraska. Further investigation confirmed a non-target-site resistance mechanism in this population. The main objective of this study was to explore the role of cytochrome P450 inhibitors in restoring the efficacy of HPPD-inhibitor herbicides on the HPPD-inhibitor resistant A. tuberculatus population from Nebraska, USA (HPPD-R).

RESULTS

Enhanced metabolism via cytochrome P450 enzymes is the mechanism of resistance in HPPD-R. Amitrole partially restored the activity of mesotrione, whereas malathion, amitrole, and piperonyl butoxide restored the activity of tembotrione and topramezone in HPPD-R. Although corn was injured through malathion followed by mesotrione application a week after treatment, the injury was transient, and the crop recovered.

CONCLUSION

The use of cytochrome P450 inhibitors with tembotrione may provide a new way of controlling HPPD-inhibitor resistant A. tuberculatus, but further research is needed to identify the cytochrome P450 candidate gene(s) conferring metabolism-based resistance. The results presented here aid to gain an insight into non-target-site resistance weed management strategies. © 2017 Society of Chemical Industry.

Overview of glyphosate-resistant weeds worldwide

Glyphosate is the most widely used and successful herbicide discovered to date, but its utility is now threatened by the occurrence of several glyphosate-resistant weed species. Glyphosate resistance first appeared in Lolium rigidum in an apple orchard in Australia in 1996, ironically the year that the first glyphosate-resistant crop (soybean) was introduced in the USA. Thirty-eight weed species have now evolved resistance to glyphosate, distributed across 37 countries and in 34 different crops and six non-crop situations. Although glyphosate-resistant weeds have been identified in orchards, vineyards, plantations, cereals, fallow and non-crop situations, it is the glyphosate-resistant weeds in glyphosate-resistant crop systems that dominate the area infested and growing economic impact. Glyphosate-resistant weeds present the greatest threat to sustained weed control in major agronomic crops because this herbicide is used to control weeds with resistance to herbicides with other sites of action, and no new herbicide sites of action have been introduced for over 30 years. Industry has responded by developing herbicide resistance traits in major crops that allow existing herbicides to be used in a new way. However, over reliance on these traits will result in multiple-resistance in weeds. Weed control in major crops is at a precarious point, where we must maintain the utility of the herbicides we have until we can transition to new weed management technologies. © 2017 Society of Chemical Industry.

Population Genomics of Herbicide Resistance: Adaptation via Evolutionary Rescue

The evolution of herbicide resistance in weed populations is a highly replicated example of adaptation surmounting the race against extinction, but the factors determining its rate and nature remain poorly understood. Here, we explore theory and empirical evidence for the importance of population genetic parameters-including effective population size, dominance, mutational target size, and gene flow-in influencing the probability and mode of herbicide resistance adaptation and its variation across species. We compiled data on the number of resistance mutations across populations for 79 herbicide-resistant species. Our findings are consistent with theoretical predictions that self-fertilization reduces resistance adaptation from standing variation within populations, but increases independent adaptation across populations. Furthermore, we provide evidence for a ploidy-mating system interaction that may reflect trade-offs in polyploids between increased effective population size and greater masking of beneficial mutations. We highlight the power of population genomic approaches to provide insights into the evolutionary dynamics of herbicide resistance with important implications for understanding the limits of adaptation.

Spray particle drift mitigation using field corn (Zea mays L.) as a drift barrier

BACKGROUND

Herbicide particle drift reduces application efficacy and can cause severe impacts on nearby vegetation depending on the herbicide mode of action, exposure level, and tolerance to the herbicide. A particle drift mitigation effort placing windbreaks or barriers on the field boundaries to reduce off-target movement of spray particles has been utilized in the past. The objective of this research was to evaluate the effectiveness of field corn (Zea mays L.) at different heights as a particle drift barrier.

RESULTS

Applications with a non-air inclusion flat fan nozzle (ER11004) resulted in greater particle drift when compared with an air inclusion nozzle (TTI11004). Eight rows of corn were used as barriers (0.91, 1.22, and 1.98 m height) which reduced the particle drift for both nozzles, especially at shorter downwind distances. Applications with the ER11004 nozzle without corn barriers had 1% of the applied rate (D99 ) predicted to deposit at 14.8 m downwind, whereas this distance was reduced (up to 7-fold) when applications were performed with corn barriers. The combination of corn drift barriers and nozzle selection (TTI11004) provided satisfactory particle drift reduction when the D99 estimates were compared with those for applications with the ER11004 nozzle without corn barriers (up to 10-fold difference).

CONCLUSION

The corn drift barriers were effective in reducing particle drift from applications with the ER11004 and the TTI11004 nozzles (Fine and Ultra Coarse spray classifications, respectively). The corn drift barrier had appropriate porosity and width as the airborne spray was captured within its canopy instead of deflecting up and over it. © 2018 Society of Chemical Industry.

Inheritance of Mesotrione Resistance in an Amaranthus tuberculatus (var. rudis) Population from Nebraska, USA

A population of Amaranthus tuberculatus (var. rudis) evolved resistance to 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor herbicides (mesotrione, tembotrione, and topramezone) in Nebraska. The level of resistance was the highest to mesotrione, and the mechanism of resistance in this population is metabolism-based likely via cytochrome P450 enzymes. The increasing number of weeds resistant to herbicides warrants studies on the ecology and evolutionary factors contributing for resistance evolution, including inheritance of resistance traits. In this study, we investigated the genetic control of mesotrione resistance in an A. tuberculatus population from Nebraska, USA. Results showed that reciprocal crosses in the F1 families exhibited nuclear inheritance, which allows pollen movement carrying herbicide resistance alleles. The mode of inheritance varied from incomplete recessive to incomplete dominance depending upon the F1 family. Observed segregation patterns for the majority of the F2 and back-cross susceptible (BC/S) families did not fit to a single major gene model. Therefore, multiple genes are likely to confer metabolism-based mesotrione resistance in this A. tuberculatus population from Nebraska. The results of this study aid to understand the genetics and inheritance of a non-target-site based mesotrione resistant A. tuberculatus population from Nebraska, USA.

Modeling pollen-mediated gene flow from glyphosate-resistant to -susceptible giant ragweed (Ambrosia trifida L.) under field conditions

A field experiment was conducted to quantify pollen mediated gene flow (PMGF) from glyphosate-resistant (GR) to glyphosate-susceptible (GS) giant ragweed under simulated field conditions using glyphosate resistance as a selective marker. Field experiments were conducted in a concentric design with the GR giant ragweed pollen source planted in the center and GS giant ragweed pollen receptors surrounding the center in eight directional blocks at specified distances (between 0.1 and 35 m in cardinal and ordinal directions; and additional 50 m for ordinal directions). Seeds of GS giant ragweed were harvested from the pollen receptor blocks and a total of 100,938 giant ragweed plants were screened with glyphosate applied at 2,520 g ae ha-1 and 16,813 plants confirmed resistant. The frequency of PMGF was fit to a double exponential decay model selected by information-theoretic criteria. The highest frequency of gene flow (0.43 to 0.60) was observed at ≤0.5 m from the pollen source and reduced rapidly with increasing distances; however, gene flow (0.03 to 0.04) was detected up to 50 m. The correlation between PMGF and wind parameters was inconsistent in magnitude, direction, and years.