Evolution Of Herbicide Resistance Research Articles

Molecular mechanisms of adaptive evolution revealed by global selection for glyphosate resistance.

The human-directed, global selection for glyphosate resistance in weeds has revealed a fascinating diversity of evolved resistance mechanisms, including herbicide sequestration in the vacuole, a rapid cell death response, nucleotide polymorphisms in the herbicide target (5-enolpyruvylshikimate-3-phosphate synthase, EPSPS) and increased gene copy number of EPSPS. For this latter mechanism, two distinct molecular genetic mechanisms have been observed, a tandem duplication mechanism and a large extrachromosomal circular DNA (eccDNA) that is tethered to the chromosomes and passed to gametes at meiosis. These divergent mechanisms have a range of consequences for the spread, fitness, and inheritance of resistance traits, and, particularly in the case of the eccDNA, demonstrate how evolved herbicide resistance can generate new insights into plant adaptation to contemporary environmental stress.

Weed management and herbicide resistant weeds: a case study from wheat growing areas of Pakistan

Evolution of herbicide resistance in weeds is among the serious challenges that agriculture face today. In Pakistan herbicide use over last 6 decades has enabled earlier planting of short-season crops and improved weed control in wheat as well as other cropping systems. This has greatly favored the use of more and more herbicides. Conversely, this exercise increased the evolution of resistant biotypes. Wheat being Pakistan's number one staple crop faces daunting weeds problem. In order to understand the status of herbicide resistant weeds in wheat fields, survey was conducted for two consecutive years (2015-2017) in different regions of Pakistan. A questionnaire was designed to collect data from three hundred farmers randomly selected in all four provinces of Pakistan on herbicides used in wheat, history of crop cultivation regimes and weed species that have seized to respond to field rates of herbicides. Seed of suspected biotypes were collected from the survey sites and preserved in our gene pool for the confirmation of the resistance in the laboratory by bioassay and molecular level in future. Farmers (96%) perceived that Phalaris minor Retz. was the most important annual weed of wheat crop followed by Avena fatua L. (94%). Customarily, three aryloxyphenoxy propionate herbicides, clodinafop-propargyl, fenoxaprop-P-ethyl and diclofop-methyl were used to control different grass weeds such as P. minor, A. sativa L. and A. fatua. In farmers' opinion, high yield of wheat was associated with the continuous use of these herbicides. Resistance-suspicious weeds were reported from all studied areas (Punjab, KPK, Sindh, and Baluchistan provinces) in fields with the herbicide use history of five to seven years or more. This study suggests that for long-term avoidance of herbicide resistance, in our wheat growing systems we will require embracing practices that decrease selection pressures favoring resistant weeds. Achieving these goals requires an understanding of the evolution and dynamics of resistant populations. It is recommended that for the effective management of resistant populations of P. minor and A. fatua, farmers' education on ecologically sustainable weed management is critically important. This study will provide a basis for examining the questions that are relevant to understanding herbicide resistance evolution in Pakistan and that may help determine appropriate weed management strategies.

Evolutionary and ecological insights from herbicide-resistant weeds: what have we learned about plant adaptation, and what is left to uncover?

The evolution of herbicide resistance in crop weeds presents one of the greatest challenges to agriculture and the production of food. Herbicide resistance has been studied for more than 60yr, in the large part by researchers seeking to design effective weed control programs. As an outcome of this work, various unique questions in plant adaptation have been addressed. Here, I collate recent research on the herbicide-resistant problem in light of key questions and themes in evolution and ecology. I highlight discoveries made on herbicide-resistant weeds in three broad areas - the genetic basis of adaptation, evolutionary constraints, experimental evolution - and similarly discuss questions left to be answered. I then develop how one would use herbicide-resistance evolution as a model for studying eco-evolutionary dynamics within a community context. My overall goals are to highlight important findings in the weed science literature that are relevant to themes in plant adaptation and to stimulate the use of herbicide-resistant plants as models for addressing key questions within ecology and evolution.

Control of glyphosate-resistant waterhemp with preemergence herbicides in glyphosate- and dicamba-resistant soybean

Glyphosate-resistant (GR) waterhemp was first discovered in Ontario, Canada, in 2014. In Ontario, GR waterhemp interference in previous studies reduced soybean yield up to 73%. Tank-mixes of herbicides with multiple modes-of-action are important for delaying the evolution of herbicide resistance. The objective of this study was to evaluate the efficacy of pyroxasulfone (150 g a.i. ha−1), S-metolachlor/metribuzin (1943 g a.i. ha−1), pyroxasulfone/sulfentrazone (300 g a.i. ha−1), and pyroxasulfone/flumioxazin (240 g a.i. ha−1) applied preemergence (PRE) with and without the addition of glyphosate/dicamba (1800 g a.e. ha−1) for the control of GR waterhemp in soybean. At 8 wk after treatment application (WAA), glyphosate/dicamba applied PRE controlled GR waterhemp 45%. Pyroxasulfone, S-metolachlor/metribuzin, pyroxasulfone/sulfentrazone, and pyroxasulfone/flumioxazin applied PRE controlled GR waterhemp 79%, 87%, 91%, and 95%, respectively. At 2, 4, 8, and 12 WAA, the addition of glyphosate/dicamba to the aforementioned PRE herbicides did not improve GR waterhemp control. There was no increase in GR waterhemp control with the addition glyphosate/dicamba; however, multiple herbicide modes-of-action should be utilized to reduce the selection intensity for herbicide-resistant weeds.

A global perspective on the biology, impact and management of Chenopodium album and Chenopodium murale: two troublesome agricultural and environmental weeds.

Chenopodium album and C. murale are cosmopolitan, annual weed species of notable economic importance. Their unique biological features, including high reproductive capacity, seed dormancy, high persistence in the soil seed bank, the ability to germinate and grow under a wide range of environmental conditions and abiotic stress tolerance, help these species to infest diverse cropping systems. C. album and C. murale grow tall and absorb nutrients very efficiently. Both these species are allelopathic in nature and, thus, suppress the germination and growth of native vegetation and/or crop plants. These weed species infest many agronomic and horticultural crops and may cause > 90% loss in crop yields. C. album is more problematic than C. murale as the former is more widespread and infests more number of crops, and it also acts as an alternate host of several crop pests. Different cultural and mechanical methods have been used to control these weed species with varying degrees of success depending upon the cropping systems and weed infestation levels. Similarly, allelopathy and biological control have also shown some potential, especially in controlling C. album. Several herbicides have been successfully used to control these species, but the evolution of wide-scale herbicide resistance in C. album has limited the efficacy of chemical control. However, the use of alternative herbicides in rotation and the integration of chemicals and biologically based control methods may provide a sustainable control of C. album and C. murale.

Control of herbicide resistant Phalaris minor by pyroxasulfone in wheat

Evolution of multiple herbicide resistance in Phalaris minor in wheat has resulted from the repeated use of herbicides with a similar mode of action which could threaten the sustainability of the rice-wheat cropping system in North-West India. In this context, field studies were conducted at Research Farm, Department of Agronomy, Punjab Agricultural University, Ludhiana during 2011–12, 2012–13 and 2013–14 in randomized block design replicated four times and at farmer's field during 2016–17 and 2017–18 with the objective to evaluate the efficacy of herbicide with alternative mechanism of action for the control of resistant P. minor in wheat. The treatments included pyroxasulfone 85, 102, 127.5 g/ha, pendimethalin 750 g/ha as pre-emergence (PE), sulfosulfuron 25 g/ha, clodinafop 60 g/ha as postemergence (PoE) and unsprayed control. However, the treatments during 2013–14 included pyroxasulfone 85, 102, 127.5 g/ha, pendimethalin 750 g/ha and unsprayed control with pendimethalin 1125 g/ha as PE at farmers’ field instead of pendimethalin 750 g/ha. The results indicated that pyroxasulfone at 127.5 g/ha recorded effective control of P. minor and its biomass and gave the highest wheat grain yield (4.87, 4.80 and 5.43 t/ha) during 2011–12, 2012–13 and 2013–14, respectively At farmer's field, pyroxasulfone 127.5 g/ha has been found effective against the resistant populations of Phalaris with 5.37 and 5.42 t/ha grain yield. The current study demonstrated that PE application of pyroxasulfone at 127.5 g/ha would be a suitable option for the control of resistant populations of Phalaris in wheat in Punjab.

Cropping practices influence incidence of herbicide resistance in annual ryegrass (Lolium rigidum) in Australia

Herbicide resistance is a common occurrence in southern Australia. The evolution of herbicide resistance is influenced by the selection pressure placed on the weed species controlled by that herbicide. Results from resistance screening of ~4500 annual ryegrass (Lolium rigidum Gaud.) samples were entered in a GIS database, together with several agricultural parameters used in the Australian Bureau of Statistics Agricultural Surveys. This allowed a study of the associations between mode of action of resistance, geographic distribution of resistance across southern Australia, and farming practices employed in particular regions. Cultivation was negatively associated with resistances in acetyl-CoA carboxylase (ACCase)-inhibiting cyclohexanedione and acetolactate synthase (ALS)-inhibiting herbicides. Higher proportions of wheat sown were associated with higher incidences of resistance. ACCase-inhibiting aryloxyphenoxypropionate and cyclohexanedione and ALS-inhibiting resistances were higher in those shires where soils were predominantly acidic. This study demonstrates the association between farm practice and the evolution of herbicide resistance. The analysis provides reinforcement to the principle of rotating chemical modes of action with non-chemical weed control measures to minimise the risk of herbicide resistance evolution in any farming system.

Herbicide-Resistant Kochia (Bassia scoparia) in North America: A Review

AbstractKochia [Bassia scoparia(L.) A. J. Scott] is a problematic annual broadleaf weed species in the North American Great Plains.Bassia scopariainherits unique biological characteristics that contribute to its propensity to evolve herbicide resistance. Evolution of glyphosate resistance inB. scopariahas become a serious threat to the major cropping systems and soil conservation practices in the region.Bassia scopariapopulations with resistance to four different herbicide sites of action are a concern for growers. The widespread occurrence of multiple herbicide–resistant (HR)B. scopariaacross the North American Great Plains has renewed research efforts to devise integrated weed management strategies beyond herbicide use. In this review, we aim to compile and document the growing body of literature on HRB. scopariawith emphasis on herbicide-resistance evolutionary dynamics, distribution, mechanisms of evolved resistance, agronomic impacts, and current/future weed management technologies. We focused on ecologically based, non-herbicidal strategies such as diverse crop rotations comprising winter cereals and perennial forages, enhanced crop competition, cover crops, harvest weed seed control (HWSC), and tillage to manage HRB. scopariaseedbanks. Remote sensing using hyperspectral imaging and other sensor-based technologies would be valuable for early detection and rapid response and site-specific herbicide resistance management. We propose research priorities based on an improved understanding of the biology, genetic diversity, and plasticity of this weed that will aid in preserving existing herbicide resources and designing sustainable, integrated HRB. scopariamitigation plans.

Fitness costs associated with multiple resistance to dicamba and atrazine in Chenopodium album.

Detrimental pleiotropic effects of resistance mutation(s) were observed for multiple-resistant phenotypes (resistant to both atrazine and dicamba). The multiple-resistant phenotypes had lower growth rates and less capacity for vegetative growth compared to the phenotypes only resistant to atrazine. The fitness costs that are conferred by herbicide resistance alleles can affect the rate of herbicide resistance evolution within populations. We evaluated the direct fitness costs involved with multiple resistance to dicamba and atrazine (R1 and R2) in Chenopodium album by comparing the performance of multiple-resistant phenotypes to those phenotypes that were only resistant to atrazine (S1 and S2). The R1 and R2 phenotypes were consistently shorter and produced less dry matter than the S1 and S2 phenotypes. The R1 and R2 phenotypes were shown to have lower relative growth rates (RGR) and net assimilation rates (NAR) than the S1 and S2 phenotypes at an early stage of growth. However, there was no significant difference in RGR between the R1 and R2 and, S1 and S2 phenotypes at a later stage of growth, though the R1 and R2 phenotypes still had a lower NAR at this later stage. Further investigations using a neighbouring crop competition approach showed that the R1 and R2 phenotypes were weaker competitors, and exhibited significantly less capacity for vegetative growth compared to the S1 and S2 phenotypes during competition. Overall, the results of this study revealed multiple- resistance to atrazine and dicamba endowed a significant fitness penalty to C. album, and it is possible that the frequency of multiple-resistant individuals would gradually decline once selection pressure from herbicides was discontinued.

Interaction of PS II‐ and HPPD‐Inhibiting Herbicides for Control of Palmer Amaranth Resistant to Both Herbicide Sites of Action

Core Ideas Additive interactions with photosytem II‐ and HPPD‐inhibitors tank‐mixtures applied pre‐emergence. Synergistic interactions with photosytem II‐ and HPPD‐inhibitors tank‐mixtures applied post‐emergence. Palmer amaranth resistant to photosytem II‐ and HPPD‐inhibitors applied pre‐emergence or post‐emergence. Palmer amaranth (Amaranthus palmeri S. Watson) resistant to post‐emergence (POST) applications of photosystem (PS) II‐ and 4‐hydroxyphenylpyruvate dioxygenase (HPPD)‐inhibiting herbicides has been confirmed in a continuous corn (Zea mays L.) seed production field in Nebraska. Field studies were conducted in 2014 through 2016 near Shickley in Fillmore County, Nebraska, to determine the response of Palmer amaranth to PS II‐ and HPPD‐inhibiting herbicides applied pre‐emergence (PRE) and to their tank‐mixtures applied PRE or POST. Results from the dose‐response study indicated that atrazine at 3757 and 17,920 g a.i. ha−1 applied PRE provided 25 and 45% Palmer amaranth control, respectively, at 28 d after treatment (DAT). Mesotrione and isoxaflutole doses required for 90% control were 1030 and 1140 g ha−1, respectively. To determine the interactions of mesotrione, tembotrione, or topramezone tank‐mixed with atrazine applied POST, Colby’s equation was used to calculate the expected Palmer amaranth control achieved with those tank‐mixtures compared to a nontreated control. Based on Colby’s equation, additive interactions occurred for Palmer amaranth control, density reduction, and biomass reduction when atrazine tank‐mixed with mesotrione or isoxaflutole was applied PRE at all rate combinations. However, synergistic interactions occurred when mesotrione or topramezone tank‐mixed with atrazine was applied POST. The results indicated that Palmer amaranth from Nebraska is resistant to both PS II‐ and HPPD‐inhibiting herbicides applied PRE or POST alone; however, they can be applied POST in a tank‐mixture with different site of action herbicides for effective control and to reduce the risk of herbicide resistance evolution.

Weed Control and Selectivity of Pethoxamid Alone and in Mixture as a Delayed Preemergence Application to Rice

AbstractThe evolution of herbicide resistance is making it extremely difficult for US rice producers to use chemical control on weed species such as barnyardgrass and red rice. To combat herbicide resistance, it is imperative that alternative herbicide sites of action (SOAs) be incorporated into rice whenever possible. There are currently no very-long-chain fatty acid–inhibiting herbicides (WSSA Group 15) labeled for use in US rice; however, pethoxamid is one such herbicide currently under development. If appropriate rice tolerance and weed control can be established, pethoxamid would represent a unique herbicide SOA for use in US rice. We conducted field trials near Stuttgart, AR, in 2015 and near Colt and Lonoke, AR, in 2016 to assess selectivity of pethoxamid and weed control alone and in combination with other herbicides as a delayed preemergence (DPRE) application in drill-seeded rice. Pethoxamid was applied at 0, 420, or 560 g ai ha–1 alone and in combination with clomazone, imazethapyr, pendimethalin, and quinclorac. Minimal rice injury occurred with any treatment assessed. A reduction in rice shoot density and plant height compared to the nontreated control followed the use of pethoxamid; however, no decrease in yield resulted. The highest levels of barnyardgrass control followed the use of imazethapyr at 91% and quinclorac at 89% regardless of the presence of pethoxamid near Lonoke; however, pethoxamid applied at both rates in combination with clomazone and quinclorac increased barnyardgrass control compared to clomazone and quinclorac applied alone. Near Colt, barnyardgrass control of 92% and 96% resulted from pethoxamid alone, averaged over the high and low rates. Based on these data, rice can tolerate pethoxamid when applied DPRE, and adequate levels of barnyardgrass control can be achieved at the rates evaluated within a program; hence, pethoxamid appears to be a viable option for use in rice to allow for increased rotation of herbicide SOAs to combat herbicide-resistant and difficult-to-control weeds.

Competitiveness of Herbicide-Resistant Waterhemp (Amaranthus tuberculatus) with Soybean

AbstractWaterhemp [Amaranthus tuberculatus(Moq.) J. D. Sauer] is a troublesome weed occurring in cropping systems throughout the U.S. Midwest with an ability to rapidly evolve herbicide resistance that could be associated with competitive disadvantages. Little research has investigated the competitiveness of differentA. tuberculatuspopulations under similar environmental conditions. The objectives of this study were to evaluate: (1) the interspecific competitiveness of three herbicide-resistantA. tuberculatuspopulations (2,4-D and atrazine resistant [2A-R], glyphosate and protoporphyrinogen oxidase [PPO]-inhibitor resistant [GP-R], and 2,4-D, atrazine, glyphosate, and PPO-inhibitor susceptible [2AGP-S]) with soybean [Glycine max(L.) Merr.]; and (2) the density-dependent response of eachA. tuberculatuspopulation within a constant soybean population in a greenhouse environment.Amaranthus tuberculatuscompetitiveness with soybean was evaluated across five target weed densities of 0, 2, 4, 8, and 16 plants pot−1(equivalent to 0, 20, 40, 80, and 160 plants m−2) with 3 soybean plants pot−1(equivalent to 300,000 plants ha−1). At the R1 soybean harvest time, no difference in soybean biomass was observed acrossA. tuberculatuspopulations. AtA. tuberculatusdensities <8 plants pot−1, the 2AGP-S population had the greatest biomass and stem diameter per plant. At the R7 harvest time, the 2AGP-S population caused the greatest loss in soybean biomass and number of pods compared with the other populations at densities of <16 plants pot−1. The 2AGP-S population had greater early-season biomass accumulation and stem diameter compared with the otherA. tuberculatuspopulations, which resulted in greater late-season reduction in soybean biomass and number of pods. This research indicates there may be evidence of interspecific competitive fitness cost associated with the evolution of 2,4-D, atrazine, glyphosate, and PPO-inhibitor resistance inA. tuberculatus. Focus should be placed on effectively using cultural weed management practices to enhance crop competitiveness, especially early in the season, to increase suppression of herbicide-resistantA. tuberculatus.

The P450 gene CYP749A16 is required for tolerance to the sulfonylurea herbicide trifloxysulfuron sodium in cotton (Gossypium hirsutum L.)

BackgroundWeed management is critical to global crop production and is complicated by rapidly evolving herbicide resistance in weeds. New sources of herbicide resistance are needed for crop plants so that applied herbicides can be rotated or combined to thwart the evolution of resistant weeds. The diverse family of cytochrome P450 proteins has been suggested to be a source of detoxifying herbicide metabolism in both weed and crop plants, and greater understanding of these genes will offer avenues for crop improvement and novel weed management practices.ResultsHere, we report the identification of CYP749A16 (Gh_D10G1401) which is responsible for the natural tolerance exhibited by most cotton, Gossypium hirsutum L., cultivars to the herbicide trifloxysulfuron sodium (TFS, CGA 362622, commercial formulation Envoke). A 1-bp frameshift insertion in the third exon of CYP749A16 results in the loss of tolerance to TFS. The DNA marker designed from this insertion perfectly co-segregated with the phenotype in 2145 F2 progeny of a cross between the sensitive cultivar Paymaster HS26 and tolerant cultivar Stoneville 474, and in 550 recombinant inbred lines of a multi-parent advanced generation inter-cross population. Marker analysis of 382 additional cotton cultivars identified twelve cultivars containing the 1-bp frameshift insertion. The marker genotypes matched perfectly with phenotypes in 188 plants from the selected twelve cultivars. Virus-induced gene silencing of CYP749A16 generated sensitivity in the tolerant cotton cultivar Stoneville 474.ConclusionsCYP749A16 located on chromosome D10 is required for TFS herbicide tolerance in cotton. This finding should add to the repertoire of tools available to farmers and breeders for the advancement of agricultural productivity.

Herbicide hormesis can act as a driver of resistance evolution in weeds - PSII-target site resistance in Chenopodium album L. as a case study.

Herbicide hormesis may play a role in the evolution of weed resistance by increasing resistance selection. A standard herbicide rate may be subtoxic to resistant plants and make them more fit than untreated plants. If this increase in fitness is ultimately expressed in reproductive traits, resistance genes can accumulate more rapidly and exacerbate resistance evolution by magnifying the selection differential between resistant and sensitive plants. The hypothesis of hormetically enhanced reproductive fitness was studied for a photosystem II (PSII) target-site resistant (TSR) biotype of Chenopodium album exposed to the triazinone metamitron in comparison with its wild-type. Both biotypes showed an initial hormetic growth increase at different doses leading to fitness enhancements of between 19% and 61% above untreated plants. However, hormetic effects only resulted in higher fitness at maturity in resistant plants with a maximum stimulation in seed yield of 45% above untreated plants. Applying realistic metamitron rates, reproductive fitness of resistant plants was increased by 15-32%. Agronomically relevant doses of metamitron induced considerable hormesis in a PSII-TSR C. album genotype leading to enhanced relative fitness through reproductive maturity. This increase in relative fitness suggests an impact on resistance selection and can compensate for the oft-reported fitness costs of the mutation studied. Field rates of herbicides can, thus, not only select for resistant plants, but also enhance their reproductive fitness. The finding that herbicide hormesis can be eco-evolutionary important may have important implications for understanding the evolution of herbicide resistance in weeds. © 2018 Society of Chemical Industry.

First Case of Conyza canadensis from Hungary with Multiple Resistance to Glyphosate and Flazasulfuron

Conyza canadensis is a species invading large areas throughout the world, mainly due to its ability to evolve herbicide resistance. In Hungary, extensive areas have been infested by this species due to the difficulty in controlling it with glyphosate. To determine whether poor control was a result of misapplication or glyphosate resistance, eight suspected glyphosate-resistant C. canadensis populations from different Hungarian regions were studied. In whole-plant dose-response assays with glyphosate, the LD50 and GR50 values (survival and fresh weight reduction at 50% relative to the untreated control, respectively) indicated that resistance was confirmed in five of the eight populations (H-5 population being the most resistant). Additionally, the shikimic acid accumulation tests corroborated the results observed in the dose–response assays. 11 alternative herbicides from six different modes of action (MOA) were applied at field doses as control alternatives on populations H-5 and H-6 (both in the same regions). The H-5 population showed an unexpected resistance to flazasulfuron (ALS-inhibitor). The ALS enzyme activity studies indicated that the I50 for H-5 with flazasulfuron was 63.3 times higher compared to its correspondent susceptible population (H-6). Therefore, the H-5 population exhibited multiple-resistance to flazasulfuron and glyphosate, being the first case reported in Europe for these two MOA.

Exploring the Herbicidal and Hormetic Potential of Allelopathic Crops Against Fenoxaprop-Resistant Phalaris minor

ABSTRACT: Recent increases in the development of herbicide resistance in Phalaris minor worldwide demand alternative non-chemical strategies to control this weed. A series of experiments were conducted under laboratory and greenhouse conditions to explore the herbicidal potential of four allelopathic crops, including maize, rice, sorghum and sunflower, at different concentrations of aqueous extracts (2.5% and 5%), residues (1%, 2% and 4%) and mulches (4, 8, and 12 ton ha-1) against fenoxaprop-resistant P. minor. Aqueous extracts, residues and mulches provided 86-100%, 73-100% and 16-40% control of this resistant weed biotype, respectively. The dry biomass reduction due to aqueous extracts, residues and mulches was 48-100%, 48-100% and 20-54%, respectively. Mulches also caused 17-41% reduction in the seed production potential of P. minor. Lower concentrations of allelochemicals showed hormesis (positive effect) against some emergence and growth traits of P. minor. The phytotoxic chemicals of these four crops have a strong herbicidal potential against herbicide-resistant P. minor, and can be used as an organic alternative to control herbicide resistant P. minor, thus ensuring a sustainable wheat production.

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.

Inheritance of 4-hydroxyphenylpyruvate dioxygenase inhibitor herbicide resistance in an Amaranthus tuberculatus population from Iowa, USA

Waterhemp (Amaranthus tuberculatus (Moq.) J.D. Sauer) is a weed prevalent in the Midwest United States and can cause yield losses up to 74% in maize (Zea mays L.) and 56% in soybean (Glycine max (L.) Merr.). An important adaptive trait commonly found in waterhemp is the ability to evolve herbicide resistance and waterhemp populations have evolved resistance to six herbicide sites of action. In 2011, two waterhemp populations were discovered resistant to p-hydroxyphenylpyruvate-dioxygenase (HPPD, EC 1.13.11.27) inhibitor herbicides. We reciprocally crossed a known HPPD-resistant waterhemp population with a known HPPD-susceptible waterhemp population and then intermated the F1 families to established a pseudo-F2 generation. We challenged the parent, F1 and pseudo-F2 generations against four HPPD-inhibiting herbicide rates (mesotrione). Our results suggest the HPPD-resistance trait is polygenic. Furthermore, the number of genes involved with the herbicide resistance increase at higher herbicide rates. These data indicated at least one dominant allele at each major locus is required to confer HPPD herbicide resistance in waterhemp. Using different waterhemp populations and methodologies, this study confirms the reported “complex” HPPD resistance inheritance while providing new information in the response of HPPD-resistant waterhemp to HPPD herbicides.

Stress-induced evolution of herbicide resistance and related pleiotropic effects.

Herbicide-resistant weeds, especially those with resistance to multiple herbicides, represent a growing worldwide threat to agriculture and food security. Natural selection for resistant genotypes may act on standing genetic variation, or on a genetic and physiological background that is fundamentally altered because of stress responses to sublethal herbicide exposure. Stress-induced changes include DNA mutations, epigenetic alterations, transcriptional remodeling, and protein modifications, all of which can lead to herbicide resistance and a wide range of pleiotropic effects. Resistance selected in this manner is termed systemic acquired herbicide resistance, and the associated pleiotropic effects are manifested as a suite of constitutive transcriptional and post-translational changes related to biotic and abiotic stress adaptation, representing the evolutionary signature of selection. This phenotype is being investigated in two multiple herbicide-resistant populations of the hexaploid, self-pollinating weedy monocot Avena fatua that display such changes as well as constitutive reductions in certain heat shock proteins and their transcripts, which are well known as global regulators of diverse stress adaptation pathways. Herbicide-resistant populations of most weedy plant species exhibit pleiotropic effects, and their association with resistance genes presents a fertile area of investigation. This review proposes that more detailed studies of resistant A. fatua and other species through the lens of plant evolution under stress will inform improved resistant weed prevention and management strategies. © 2018 Society of Chemical Industry.

Herbicide resistance evolution can be tamed by diversity in irrigated Australian cotton: a multi‐species, multi‐herbicide modelling approach

Because herbicide resistance evolves in very large populations over periods of many years, modelling is an important tool for investigating the dynamics of the problem. The Diversity model tracks the simultaneous evolution of resistance to multiple herbicides, using multiple genetic pathways, in several weed species at once. Tracking multiple species and simultaneous resistances is an important development in resistance modelling. We used the Diversity model to test weed management strategies for new cropping cotton varieties with multiple herbicide tolerances ('triple-stacked' varieties), in an Australian context. The diversity required for long-term control of three key weeds in Australian cotton goes beyond using three herbicides, especially where there is already a substantial background of existing resistance to one or more of these herbicides. Assuming some glyphosate resistance is already present, simulations showed that glyphosate-resistant summer grass populations reach 20 000 seeds m-2 within 12 years using the triple-stack herbicides (glyphosate, glufosinate and dicamba) and a minimum of other tactics. Adding three pre-emergent modes of action plus cultivation to the system effectively controls glyphosate-resistant grasses for over 30 years. In conditions where resistance genes are as frequent as 1 in 100, however, highly fecund weeds such as Conyza bonariensis are hard to control beyond 15 years even with very highly diverse management. Stacked herbicide tolerances in new crop varieties offers potential for increased herbicide diversity, but existing glyphosate-resistant weed populations need substantial extra management beyond what a glyphosate/glufosinate/dicamba resistance stack provides. More diverse systems can provide robust management over 30 years in the absence of very high levels of background resistance to other herbicides. © 2018 Society of Chemical Industry.