Multiple Herbicide Resistance Research Articles

Molecular and physiological characterization of six-way resistance in an Amaranthus tuberculatus var. rudis biotype from Missouri.

Previous research reported the first case of six-way herbicide resistance in a common waterhemp (Amaranthus tuberculatus var. rudis) biotype from Missouri, USA designated MO-Ren. This study investigated the mechanisms of multiple-resistance in the MO-Ren biotype to herbicides from six site-of-action (SOA) groups, i.e. synthetic auxins, 5-enolypyruvyl-shikimate-3-phosphate synthase (EPSPS)-, protoporphyrinogen oxidase (PPO)-, acetolactate synthase (ALS)-, photosystem II (PSII)-, and 4-hydroxyphenyl-pyruvate-dioxygenase (HPPD)-inhibitors. Genomic DNA sequencing confirmed the presence of known mutations associated with ALS- or PPO-inhibiting herbicide resistance: the Trp-574-Leu amino acid substitution in the ALS enzyme and the codon deletion corresponding to the ΔG210 in the PPX2 enzyme. No target-site point mutations associated with resistance to PSII- and EPSPS-inhibitors were detected. Quantitative polymerase chain reaction (qPCR) indicated that MO-Ren plants contained five-fold more copies of the EPSPS gene than susceptible plants. Malathion in combination with 2,4-D (2,4-dichlorophenoxyacetic acid), mesotrione, and chlorimuron POST enhanced the activity of these herbicides indicating that metabolism due to cytochrome P450 monooxygenase activity was involved in herbicide resistance. 4-Chloro-7-nitrobenzofurazan (NBD-Cl), a glutathione-S-transferase (GST)-inhibitor, in combination with atrazine did not reduce the biomass accumulation. Reduced absorption or translocation of 2,4-D did not contribute to resistance. However, the resistant biotype metabolized 2,4-D, seven- to nine-fold faster than the susceptible. Target-site point mutations, gene amplification, and elevated rates of metabolism contribute to six-way resistance in the MO-Ren biotype, suggesting both target site and non-target site mechanisms contribute to multiple herbicide resistance in this Amaranthus tuberculatus biotype. © 2018 Society of Chemical Industry.

Changes in the proteome of the problem weed blackgrass correlating with multiple-herbicide resistance.

SummaryHerbicide resistance in grass weeds is now one of the greatest threats to sustainable cereal production in Northern Europe. Multiple‐herbicide resistance (MHR), a poorly understood multigenic and quantitative trait, is particularly problematic as it provides tolerance to most classes of chemistries currently used for post‐emergence weed control. Using a combination of transcriptomics and proteomics, the evolution of MHR in populations of the weed blackgrass (Alopecurus myosuroides) has been investigated. While over 4500 genes showed perturbation in their expression in MHR versus herbicide sensitive (HS) plants, only a small group of proteins showed >2‐fold changes in abundance, with a mere eight proteins consistently associated with this class of resistance. Of the eight, orthologues of three of these proteins are also known to be associated with multiple drug resistance (MDR) in humans, suggesting a cross‐phyla conservation in evolved tolerance to chemical agents. Proteomics revealed that MHR could be classified into three sub‐types based on the association with resistance to herbicides with differing modes of action (MoA), being either global, specific to diverse chemistries acting on one MoA, or herbicide specific. Furthermore, the proteome of MHR plants were distinct from that of HS plants exposed to a range of biotic (insect feeding, plant–microbe interaction) and abiotic (N‐limitation, osmotic, heat, herbicide safening) challenges commonly encountered in the field. It was concluded that MHR in blackgrass is a uniquely evolving trait(s), associated with changes in the proteome that are distinct from responses to conventional plant stresses, but sharing common features with MDR in humans.

Multiple Herbicide–Resistant Junglerice (Echinochloa colona): Identification of Genes Potentially Involved in Resistance through Differential Gene Expression Analysis

AbstractHerbicide resistance, and in particular multiple-herbicide resistance, poses an ever-increasing threat to food security. A biotype of junglerice [Echinochloa colona (L.) Link] with resistance to four herbicides, imazamox, fenoxaprop-P-ethyl, quinclorac, and propanil, each representing a different mechanism of action, was identified in Sunflower County, MS. Dose responses were performed on the resistant biotype and a biotype sensitive to all four herbicides to determine the level of resistance. Application of a cytochrome P450 inhibitor, malathion, with the herbicides imazamox and quinclorac resulted in increased susceptibility in the resistant biotype. Differential gene expression analysis of resistant and sensitive plants revealed that 170 transcripts were upregulated in resistant plants relative to sensitive plants and 160 transcripts were upregulated in sensitive plants. In addition, 507 transcripts were only expressed in resistant plants and 562 only in sensitive plants. A subset of these transcripts were investigated further using quantitative PCR (qPCR) to compare gene expression in resistant plants with expression in additional sensitive biotypes. The qPCR analysis identified two transcripts, a kinase and a glutathione S-transferase that were significantly upregulated in resistant plants compared with the sensitive plants. A third transcript, encoding an F-box protein, was downregulated in the resistant plants relative to the sensitive plants. As no cytochrome P450s were differentially expressed between the resistant and sensitive plants, a single-nucleotide polymorphism analysis was performed, revealing several nonsynonymous point mutations of interest. These candidate genes will require further study to elucidate the resistance mechanisms present in the resistant biotype.

Investigations of 2,4-D and Multiple Herbicide Resistance in a Missouri Waterhemp (Amaranthus tuberculatus) Population

AbstractResearch was conducted from 2015 to 2017 to investigate the potential for 2,4-D and multiple herbicide resistance in a waterhemp [Amaranthus tuberculatus(Moq.) J. D. Sauer] population from Missouri (designated MO-Ren). In the field, visual control of the MO-Ren population with 0.56 to 4.48 kg 2,4-D ha−1ranged from 26% to 77% in 2015 and from 15% to 55% in 2016. The MO-Ren population was highly resistant to chlorimuron, with visual control never exceeding 7% either year. Estimates of the 2,4-D dose required to provide 50% visual control (I50) of the MO-Ren population were 1.44 kg ha−1compared with only 0.47 kg 2,4-D ha−1for the susceptible population. Based on comparisons to a susceptible population in dose–response experiments, the MO-Ren population was approximately 3-fold resistant to 2,4-D, and 7-, 7-, 22-, and 14-fold resistant to atrazine, fomesafen, glyphosate, and mesotrione, respectively. Dicamba and glufosinate were the only two herbicides that provided effective control of the MO-Ren population in these experiments. Examinations of multiple herbicide resistance at the individual plant level revealed that 16% of the plants of the MO-Ren population contained genes stacked for six-way herbicide resistance, and only 1% of plants were classified as resistant to a single herbicide (glyphosate). Results from these experiments confirm that the MO-RenA. tuberculatuspopulation is resistant to 2,4-D, atrazine, chlorimuron, fomesafen, glyphosate, and mesotrione, making this population the third 2,4-D–resistantA. tuberculatuspopulation identified in the United States, and the first population resistant to six different herbicidal modes of action.

Multiple resistance to glyphosate, paraquat and ACCase-inhibiting herbicides in Italian ryegrass populations from California: confirmation and mechanisms of resistance.

Glyphosate, paraquat and acetyl CoA carboxylase (ACCase)-inhibiting herbicides are widely used in California annual and perennial cropping systems. Recently, glyphosate, paraquat, and ACCase- and acetolactate synthase (ALS)-inhibitor resistance was confirmed in several Italian ryegrass populations from the Central Valley of California. This research characterized the possible mechanisms of resistance. Multiple-resistant populations (MR1, MR2) are resistant to several herbicides from at least three modes of action. Dose-response experiments revealed that the MR1 population was 45.9-, 122.7- and 20.5-fold, and the MR2 population was 24.8-, 93.9- and 4.0-fold less susceptible to glyphosate, sethoxydim and paraquat, respectively, than the susceptible (Sus) population. Accumulation of shikimate in Sus plants was significantly greater than in MR plants 32 h after light pretreatments. Glyphosate resistance in MR plants was at least partially due to Pro106-to-Ala and Pro106-to-Thr substitutions at site 106 of 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). EPSPS gene copy number and expression level were similar in plants from the Sus and MR populations. An Ile1781-to-Leu substitution in ACCase gene of MR plants conferred a high level of resistance to sethoxydim and cross-resistance to other ACCase-inhibitors. Radiolabeled herbicide studies and phosphorimaging indicated that MR plants had restricted translocation of 14 C-paraquat to untreated leaves compared to Sus plants. This study shows that multiple herbicide resistance in Italian ryegrass populations in California, USA, is due to both target-site and non-target-site resistance mechanisms. © 2017 Society of Chemical Industry.

Non‐target site resistance to flucarbazone, imazamethabenz and pinoxaden is controlled by three linked genes in Avena fatua

Summary Extensive herbicide usage has led to the evolution of resistant weed populations that cause substantial crop yield losses and increase production costs. The multiple herbicide-resistant (MHR) Avena fatua populations utilised in this study are resistant to members of all selective herbicide families, across five modes of action, available for A. fatua control in US small grain production, and thus pose significant agronomic and economic threats. Resistance to acetolactate synthase and acetyl-CoA carboxylase inhibitors is not conferred by known target site mutations, indicating that non-target site resistance (NTSR) mechanisms are involved. Understanding the inheritance of NTS MHR is of upmost importance for continued agricultural productivity in the face of the rapid increase in resistant weed populations worldwide. As few studies have examined the inheritance of NTSR in autogamous weeds, we investigated the inheritance and genetic control of NTSR in the highly autogamous, allohexaploid species A. fatua. We found that NTSR in MHRA. fatua is controlled by three separate, closely-linked nuclear genes for flucarbazone-sodium, imazamethabenz-methyl and pinoxaden. The single-gene NTSR inheritance patterns reported here contrast with other examples in allogamous species and illustrate the diversity of evolutionary responses to strong selection.

Constitutive redox and phosphoproteome changes in multiple herbicide resistant Avena fatua L. are similar to those of systemic acquired resistance and systemic acquired acclimation

Plants are routinely confronted with numerous biotic and abiotic stressors, and in response have evolved highly effective strategies of systemic acquired resistance (SAR) and systemic acquired acclimation (SAA), respectively. A much more evolutionarily recent abiotic stress is the application of herbicides to control weedy plants, and their intensive use has selected for resistant weed populations that cause substantial crop yield losses and increase production costs. Non-target site resistance (NTSR) to herbicides is rapidly increasing worldwide and is associated with alterations in generalized stress defense networks. This work investigated protein post-translational modifications associated with NTSR in multiple herbicide resistant (MHR) Avena fatua, and their commonalities with those of SAR and SAA. We used proteomic, biochemical, and immunological approaches to compare constitutive protein profiles in MHR and herbicide susceptible (HS) A. fatua populations. Phosphoproteome and redox proteome surveys showed that post-translational modifications of proteins with functions in core cellular processes were reduced in MHR plants, while those involved in xenobiotic and stress response, reactive oxygen species detoxification and redox maintenance, heat shock response, and intracellular signaling were elevated in MHR as compared to HS plants. More specifically, MHR plants contained constitutively elevated levels of three protein kinases including the lectin S-receptor-like serine/threonine-protein kinase LecRK2, a well-characterized component of SAR. Analyses of superoxide dismutase enzyme activity and protein levels did not reveal constitutive differences between MHR and HS plants. The overall results support the idea that herbicide stress is perceived similarly to other abiotic stresses, and that A. fatua NTSR shares analogous features with SAR and SAA. We speculate that MHR A. fatua’s previous exposure to sublethal herbicide doses, as well as earlier evolution under a diversity of abiotic and biotic stressors, has led to a heightened state of stress preparedness that includes NTSR to a number of unrelated herbicides.

Impact of Automated Guidance for Mechanical Control of Herbicide Resistant Weeds in Corn

The use of herbicide weed control has been an integral part of farm management for several decades due to being an efficient and cost-effective alternative to mechanical weed control management. However, repeated use of broad spectrum herbicides has resulted in herbicide resistance in several weed species (Norsworthy et al., 2012). Although the indiscriminate use of herbicides has been linked to the quick and widespread adoption of herbicide resistant crop species (Fernandez-Cornejo et al., 2014), research indicates that herbicide resistance predated the introduction of bio-tech crops by several decades (WSSA, 2016). By the time that USDA began tracking the adoption of biotech soybean production in 2000, over half of US acreage were planted to herbicide-tolerant varieties and reached over 90% within 7 years (USDA NASS) (Figure 1). By 2013, 90% of corn and soybean acreage were planted to bio-tech cultivars including herbicide-tolerate only and stacked genes (Figure 1). Currently, 470 unique cases of herbicide resistance have been documented (Heap, 2016). Multiple herbicide-resistant weed species causes additional concern due to reduced herbicide options and increased weed control costs. Multiple herbicide resistance has been confirmed in economically important weeds including Palmer amaranth (Amaranthus palmeri) (Nandula et al. 2012), waterhemp (Amaranthus tuberculatus Sauer) (Bell et al., 2013), horseweed (Conyza canadensis L. Cronq.) (Davis et al. 2009), rigid ryegrass (Lolium rigidum Gaudim) (Owen et al. 2014), and kochia (Kochia scoparia (L.) Schrad.) (Foes et al. 1999) (see Heap, 2016, for more details on herbicide resistance weeds).

Intensive herbicide use has selected for constitutively elevated levels of stress-responsive mRNAs and proteins in multiple herbicide-resistant Avena fatua L.

Intensive use of herbicides has led to the evolution of two multiple herbicide-resistant (MHR) Avena fatua (wild oat) populations in Montana that are resistant to members of all selective herbicide families available for A. fatua control in US small grain crops. We used transcriptome and proteome surveys to compare constitutive changes in MHR and herbicide-susceptible (HS) plants associated with non-target site resistance. Compared to HS plants, MHR plants contained constitutively elevated levels of differentially expressed genes (DEGs) with functions in xenobiotic catabolism, stress response, redox maintenance and transcriptional regulation that are similar to abiotic stress-tolerant phenotypes. Proteome comparisons identified similarly elevated proteins including biosynthetic and multifunctional enzymes in MHR plants. Of 25 DEGs validated by RT-qPCR assay, differential regulation of 21 co-segregated with flucarbazone-sodium herbicide resistance in F3 families, and a subset of 10 of these were induced or repressed in herbicide-treated HS plants. Although the individual and collective contributions of these DEGs and proteins to MHR remain to be determined, our results support the idea that intensive herbicide use has selected for MHR populations with altered, constitutively regulated patterns of gene expression that are similar to those in abiotic stress-tolerant plants. © 2017 Society of Chemical Industry.

Current status in herbicide resistance in Lolium rigidum in winter cereal fields in Spain: Evolution of resistance 12 years after

Lolium rigidum Gaud. is the most prevalent and damaging grass weed of winter cereals in Spain. L. rigidum infestations are frequently treated with herbicides and, consequently, populations have evolved resistance. In 2012–2013 a random survey was conducted across cereal cropping areas of the Castilla-León and Cataluña regions to establish the distribution and frequency of herbicide resistance in L. rigidum populations to chlortoluron (Photosystem II inhibitor), chlorsulfuron (Acetolactate synthase inhibitor) and diclofop-methyl (Acetyl CoA Carboxylase inhibitor), commonly used herbicides for L. rigidum control in Spain. The results of this survey were compared with the results of a previous survey conducted in 2000–02. Resistance to PSII and ALS-inhibiting herbicides was common: 51% and 92% of the accessions collected from Castilla-León and Cataluña respectively were resistant to chlortoluron, while 75% of accessions from both regions were resistant to chlorsulfuron. Resistance to ACCase was more widespread in Cataluña, where 83% of accessions were classified as resistant, than in Castilla-León where 74% of the populations were still classified as susceptible to diclofop-methyl. These results show that resistance levels to all three herbicides had increased in Castilla-León since 2000 and to chlortoluron and chlorsulfuron in Cataluña. The accessions were also treated with the double dose (2X) of each herbicide. The percentage of L. rigidum that now exhibits multiple herbicide resistance has increased considerably, especially in Cataluña where 75% of the accessions were resistant to multiple herbicides, therefore herbicide sustainability and resistance management present a great challenge.

Mechanism of resistance to mesotrione in an Amaranthus tuberculatus population from Nebraska, USA.

Amaranthus tuberculatus is a troublesome weed in corn and soybean production systems in Midwestern USA, due in part to its ability to evolve multiple resistance to key herbicides including 4-hydroxyphenylpyruvate dioxygenase (HPPD). Here we have investigated the mechanism of resistance to mesotrione, an important chemical for managing broadleaf weeds in corn, in a multiple herbicide resistant population (NEB) from Nebraska. NEB showed a 2.4-fold and 45-fold resistance increase to mesotrione compared to a standard sensitive population (SEN) in pre-emergence and post-emergence dose-response pot tests, respectively. Sequencing of the whole HPPD gene from 12 each of sensitive and resistant plants did not detect any target-site mutations that could be associated with post-emergence resistance to mesotrione in NEB. Resistance was not due to HPPD gene duplication or over-expression before or after herbicide treatment, as revealed by qPCR. Additionally, no difference in mesotrione uptake was detected between NEB and SEN. In contrast, higher levels of mesotrione metabolism via 4-hydroxylation of the dione ring were observed in NEB compared to the sensitive population. Overall, the NEB population was characterised by lower levels of parent mesotrione exported to other parts of the plant, either as a consequence of metabolism in the treated leaves and/or impaired translocation of the herbicide. This study demonstrates another case of non-target-site based resistance to an important class of herbicides in an A. tuberculatus population. The knowledge generated here will help design strategies for managing multiple herbicide resistance in this problematic weed species.

Proteomic and biochemical assays of glutathione-related proteins in susceptible and multiple herbicide resistant Avena fatua L.

Extensive herbicide usage has led to the evolution of resistant weed populations that cause substantial crop yield losses and increase production costs. The multiple herbicide resistant (MHR) Avena fatua L. populations utilized in this study are resistant to members of all selective herbicide families, across five modes of action, available for A. fatua control in U.S. small grain production, and thus pose significant agronomic and economic threats. Resistance to ALS and ACCase inhibitors is not conferred by target site mutations, indicating that non-target site resistance mechanisms are involved. To investigate the potential involvement of glutathione-related enzymes in the MHR phenotype, we used a combination of proteomic, biochemical, and immunological approaches to compare their constitutive activities in herbicide susceptible (HS1 and HS2) and MHR (MHR3 and MHR4) A. fatua plants. Proteomic analysis identified three tau and one phi glutathione S-transferases (GSTs) present at higher levels in MHR compared to HS plants, while immunoassays revealed elevated levels of lambda, phi, and tau GSTs. GST specific activity towards 1-chloro-2,4-dinitrobenzene was 1.2-fold higher in MHR4 than in HS1 plants and 1.3- and 1.2-fold higher in MHR3 than in HS1 and HS2 plants, respectively. However, GST specific activities towards fenoxaprop-P-ethyl and imazamethabenz-methyl were not different between untreated MHR and HS plants. Dehydroascorbate reductase specific activity was 1.4-fold higher in MHR than HS plants. Pretreatment with the GST inhibitor NBD-Cl did not affect MHR sensitivity to fenoxaprop-P-ethyl application, while the herbicide safener and GST inducer mefenpyr reduced the efficacy of low doses of fenoxaprop-P-ethyl on MHR4 but not MHR3 plants. Mefenpyr treatment also partially reduced the efficacy of thiencarbazone-methyl or mesosulfuron-methyl on MHR3 or MHR4 plants, respectively. Overall, the GSTs described here are not directly involved in enhanced rates of fenoxaprop-P-ethyl or imazamethabenz-methyl metabolism in MHR A. fatua. Instead, we propose that the constitutively elevated GST proteins and related enzymes in MHR plants are representative of a larger, more global suite of abiotic stress-related changes.

Growth stage of Phalaris minor Retz. and wheat determines weed control and crop tolerance of four post-emergence herbicides

Phalaris minor Retz. has evolved multiple herbicide resistance in wheat growing areas in northwestern India. An understanding of the effect of growth stage on herbicide tolerance of wheat and control of P. minor will help in selecting the most appropriate herbicide for different situations. The weed control and crop safety of four commonly used wheat herbicides (sulfosulfuron, pinoxaden, fenoxaprop plus metribuzin and mesosulfuron plus iodosulfuron), each applied at four different wheat growth stages was investigated in field studies for two years. P. minor plants were at 1, 2-3, 3-4 and 7-8 leaf stages when the herbicides were applied at Zadok 12-Z12, Z13, Z21 and Z23 stages of wheat, respectively. Sulfosulfuron application at Z12 and Z13 wheat stages (before first irrigation), provided >80% control of P. minor and produced wheat grain yield (4.5-4.7 t/ha) similar to the weed-free check (4.9 t/ha) in both years. Pinoxaden, fenoxaprop plus metribuzin and mesosulfuron plus iodosulfuron application at Z12 and Z13 wheat stages recorded significantly lower wheat grain yield (3.62-3.95 t/ha) due to poor weed control, crop toxicity or both. All the four herbicides were equally effective on P. minor when applied at Z21 wheat stage. At Z23 wheat stage, pinoxaden gave >90% control of P. minor and the highest wheat grain yield (4.82 t/ha). The results are expected to allow changes in the current recommendation of the timing of post-emergence herbicides for the management of P. minor in wheat.

Glyphosate-Resistant Common Ragweed (Ambrosia artemisiifolia) in Nebraska: Confirmation and Response to Postemergence Corn and Soybean Herbicides

Common ragweed is an important broadleaf weed in agronomic crops in the northcentral United States. A common ragweed biotype in glyphosate-resistant (GR) soybean production field in southeast Nebraska was not controlled after sequential applications of glyphosate at the labeled rate. The objectives of this study were to confirm GR common ragweed in Nebraska by quantifying the level of resistance in greenhouse and field whole-plant dose-response studies and to evaluate the response of the putative GR common ragweed to POST corn and soybean herbicides. Greenhouse whole-plant dose-response studies confirmed 7- and 19-fold resistance to glyphosate compared to the known glyphosate-susceptible (GS) biotype based on biomass reduction and control estimates, respectively. Field dose-response studies conducted in 2015 and 2016 at the putative GR common ragweed research site suggested that glyphosate doses equivalent to 15- and 40-times the labeled rate (1,260 gaeha–1) were required for 90% control and biomass reduction, respectively. Response of GR common ragweed to POST soybean herbicides in greenhouse studies indicated ≥89% control with acifluorfen, fomesafen, fomesafen plus glyphosate, glyphosate plus dicamba or 2,4-D choline, glufosinate, imazamox plus acifluorfen, and lactofen. POST corn herbicides, including 2,4-D, bromoxynil, diflufenzopyr plus dicamba, glufosinate, halosulfuron-methyl plus dicamba, mesotrione plus atrazine, and tembotrione provided ≥87% control, indicating that POST herbicides with distinct modes of action are available in corn and soybean for effective control of GR common ragweed. Results also suggested a reduced efficacy of the acetolactate synthase (ALS)-inhibiting herbicides tested in this study for control of GR and GS biotypes, indicating further research is needed to determine whether this biotype has evolved multiple herbicide resistance.

Herbicide resistance in weeds and its management

Repeated use of herbicides with similar modes of action for weed control in wheat has resulted in evolution of multiple herbicide resistance in Phalaris minor, which could threaten the sustainability of the rice-wheat cropping system in north-western India. Herbicide resistance could also become a problem in direct seeded rice and soybean crops, which rely heavily on acetolactate synthase (ALS) and acetyl coenzyme A carboxylase (ACCase) inhibiting herbicides. As discovery of new herbicide modes of action has slowed dramatically, all effort should be made to increase the effective life of existing herbicides and make weed management cost-effective and efficient. Early detection of resistance can facilitate timely adoption of alternative tactics and minimise the financial impact on farmers. Structured surveys of resistance affected areas can provide required information for site-specific recommendations as expression of herbicide resistance tends to be highly site-specific. Greater emphasis is needed on weed management systems based on sound knowledge of weed ecology. Refinement is needed in seeding machinery for high residues systems, improvements in equipment for inter-row cultivation and application technology of herbicides. Farmer-participatory research and adoption of improved agronomy could delay evolution of resistance in weeds and enhance the sustainability of Indian cropping systems. This paper has reviewed and discussed herbicide resistance in weeds in Indian context.

Target-Site Resistances to ALS and PPO Inhibitors Are Linked in Waterhemp (Amaranthus tuberculatus)

It is generally expected that, in the case of multiple herbicide resistance, different resistance mechanisms within a weed will follow Mendel’s law of independent assortment. Research was conducted to investigate anecdotal observations suggesting that target site–based resistances to inhibitors of acetolactate synthase (ALS) and protoporphyrinogen oxidase (PPO) did not follow independent assortment in common waterhemp. Cosegregation of the two resistances was observed in backcross lines (population sensitive to both herbicides as recurrent parent). Specifically, whereas 52% of backcross plants were resistant to a PPO inhibitor, this percentage increased to 92% when the backcross plants were preselected for resistance to an ALS inhibitor. Molecular marker analysis confirmed that the corresponding genes (ALSandPPX2) were genetically linked. When data from all plants analyzed were pooled, the genetic distance between the two genes was calculated to be 7.5 cM. The two genes were found to be about 195 kb apart in the recently published grain amaranth genome, explaining the observed genetic linkage. There is likely enough recombination that occurs between the linked genes to prevent the linkage from having significant implications in terms of resistance evolution. Nevertheless, documentation of the happenstance linkage between target-site genes for resistance to ALS and PPO inhibitors in waterhemp is a reminder that one should not assume distinct resistance mechanism will independently assort.

Phorate can reverse P450 metabolism-based herbicide resistance in Lolium rigidum.

Organophosphate insecticides can inhibit specific cytochrome P450 enzymes involved in metabolic herbicide resistance mechanisms, leading to synergistic interactions between the insecticide and the herbicide. In this study we report synergistic versus antagonistic interactions between the organophosphate insecticide phorate and five different herbicides observed in a population of multiple herbicide-resistant Lolium rigidum. Phorate synergised with three different herbicide modes of action, enhancing the activity of the ALS inhibitor chlorsulfuron (60% LD50 reduction), the VLCFAE inhibitor pyroxasulfone (45% LD50 reduction) and the mitosis inhibitor trifluralin (70% LD50 reduction). Conversely, phorate antagonised the two thiocarbamate herbicides prosulfocarb and triallate with a 12-fold LD50 increase. We report the selective reversal of P450-mediated metabolic multiple resistance to chlorsulfuron and trifluralin in the grass weed L. rigidum by synergistic interaction with the insecticide phorate, and discuss the putative mechanistic basis. This research should encourage diversity in herbicide use patterns for weed control as part of a long-term integrated management effort to reduce the risk of selection of metabolism-based multiple herbicide resistance in L. rigidum. © 2016 Society of Chemical Industry.

Multiple Herbicide Resistance in Lolium multiflorum and Identification of Conserved Regulatory Elements of Herbicide Resistance Genes.

Herbicide resistance is a ubiquitous challenge to herbicide sustainability and a looming threat to control weeds in crops. Recently four genes were found constituently over-expressed in herbicide resistant individuals of Lolium rigidum, a close relative of Lolium multiflorum. These include two cytochrome P450s, one nitronate monooxygenase and one glycosyl-transferase. Higher expressions of these four herbicide metabolism related (HMR) genes were also observed after herbicides exposure in the gene expression databases, indicating them as reliable markers. In order to get an overview of herbicidal resistance status of L. multiflorum L, 19 field populations were collected. Among these populations, four populations were found to be resistant to acetolactate synthase (ALS) inhibitors while three exhibited resistance to acetyl-CoA carboxylase (ACCase) inhibitors in our initial screening and dose response study. The genotyping showed the presence of mutations Trp-574-Leu and Ile-2041-Asn in ALS and ACCase, respectively, and qPCR experiments revealed the enhanced expression of HMR genes in individuals of certain resistant populations. Moreover, co-expression networks and promoter analyses of HMR genes in O. sativa and A. thaliana resulted in the identification of a cis-regulatory motif and zinc finger transcription factors. The identified transcription factors were highly expressed similar to HMR genes in response to xenobiotics whereas the identified motif is known to play a vital role in coping with environmental stresses and maintaining genome stability. Overall, our findings provide an important step forward toward a better understanding of metabolism-based herbicide resistance that can be utilized to devise novel strategies of weed management.

Germination Ecology of Herbicide-Resistant Population of Littleseed Canarygrass from North–Western India

ABSTRACTLittleseed canarygrass (Phalaris minor Retz.) is a most dominant weed of wheat (Triticum aestivum L.) in the north-western states of India. Little information is available on germination of littleseed canarygrass and wheat under similar ecological conditions. The present study was conducted under controlled conditions to study germination ecology of littleseed canarygrass population possessing multiple-herbicide resistance, along with wheat. Germination of littleseed canarygrass and wheat was independent of light. Optimum temperature for germination of littleseed canarygrass and wheat was 20°C. Germination of both species was completely inhibited at osmotic potentials lower than −0.4 MPa. Seeds of littleseed canarygrass and wheat were able to germinate even at NaCl concentration of 200 mM, with complete inhibition at 300 mM. Germination of both species was favored in a pH range of 5–8. Moisture, salinity, and pH stresses significantly reduced the speed of germination, with a concomitant increase in mean germination time, thus exposing seedlings to longer periods of less favorable conditions. This information should help in predicting potential distribution of littleseed canarygrass to new areas and also assist in formulating effective weed-management strategies for the future.

Multiple-Herbicide Resistance Is Widespread in Roadside Palmer Amaranth Populations.

Herbicide-resistant Palmer amaranth is a widespread issue in row-crop production in the Midsouthern US. Palmer amaranth is commonly found on roadside habitats in this region, but little is known on the degree of herbicide resistance in these populations. Herbicide resistance in roadside Palmer amaranth populations can represent the spread of an adaptive trait across a selective landscape. A large-scale survey was carried out in the Mississippi Delta region of eastern Arkansas to document the level of resistance in roadside Palmer amaranth populations to pyrithiobac and glyphosate, two important herbicides with broad history of use in the region. A total of 215 Palmer amaranth populations collected across 500 random survey sites were used in the evaluations. About 89 and 73% of the surveyed populations showed >90% survival to pyrithiobac and glyphosate, respectively. Further, only 3% of the populations were completely susceptible to glyphosate, while none of the populations was completely controlled by pyrithiobac. Among the 215 populations evaluated, 209 populations showed multiple resistance to both pyrithiobac and glyphosate at varying degrees. Dose-response assays confirmed the presence of high levels of herbicide resistance in the five selected populations (≥ 25-fold compared to a susceptible standard). Results demonstrate the prevalence of multiple-herbicide resistance in roadside Palmer amaranth populations in this region. Growers should be vigilant of Palmer amaranth infestation in roadsides adjacent to their fields and implement appropriate control measures to prevent likely spread of herbicide resistance into their fields.