The development and commercialisation of sunflower varieties tolerant to acetolactate synthase (ALS)-inhibiting herbicides some 20 years ago provided farmers with an alternative method for the cost-effective control of Orobanche cumana. In 2020, however, two independent sunflower broomrape populations from Drama (GR-DRA) and Orestiada (GR-ORE), Greece, were reported to be heavily infested with O. cumana after application of the ALS-inhibiting herbicide imazamox. Here we have investigated the race of GR-DRA and GR-ORE and determined the basis of resistance to imazamox in the two Greek O. cumana samples. Using a set of five diagnostic sunflower varieties characterised by different resistant genes with respect to O. cumana infestation, we have clearly established that the GR-ORE and GR-DRA populations belong to the invasive broomrape races G and G+, respectively. Live underground tubercles and emerged shoots were identified at the recommended field rate of imazamox for GR-DRA and GR-ORE but not for two other standard sensitive populations in a whole plant dose response test using two different herbicide-tolerant sunflower hybrids as hosts. Sequencing of the ALS gene identified an alanine 205 to aspartate mutation in all GR-ORE samples. Most GR-DRA tubercles were characterised by a second serine 653 to asparagine ALS mutation whilst a few GR-DRA individuals contained the A205D mutation. Mutations at ALS codons 205 and 653 are known to impact on the binding and efficacy of imazamox and other imidazolinone herbicides. The knowledge generated here will be important for tracking and managing broomrape resistance to ALS-inhibiting herbicides in sunflower growing regions.

BACKGROUND: Japanese brome (Bromus japonicus Thumb.) is one of the problematic annual weeds in winter wheat (Triticum aestivum L.) and is generally controlled by acetolactate synthase (ALS) inhibitors. Repeated use of the ALS inhibitor propoxycarbazone-Na resulted in the evolution of resistance to this herbicide in three B. japonicus populations, i.e., R1, R2, and R3 in Kansas (KS). However, the level of resistance and mechanism conferring resistance in these populations is unknown. The objectives of this research were to (i) evaluate the level of resistance to propoxycarbazone-Na in R1, R2, and R3 in comparison with a known susceptible population (S1), (ii) investigate the mechanism of resistance involved in conferring ALS-inhibitor resistance, and (iii) investigate the cross-resistance to other ALS inhibitors.
RESULTS: Dose-response (0 to 16x; x = 44 g ai ha-1 of propoxycarbazone-Na) assay indicated 167, 125, and 667-fold resistance in R1, R2 and R3 populations, respectively, compared to S1 population. ALS gene sequencing confirmed the mutations resulting in amino acid substitutions, i.e., Pro-197-Thr (R3, R1)/Ser (R2, R1) bestowing resistance to these ALS inhibitors. Such amino acid substitutions also showed differential cross-resistance to sulfosulfuron, mesosulfuron-methyl, pyroxsulam, and imazamox among resistant populations. Pretreatment with malathion (a cytochrome P450 enzyme-inhibitor) followed by imazamox treatment suggested cross-resistance to this herbicide possibly via metabolism only in R3 population.
CONCLUSIONS: Overall, these results confirm the first case of target-site based resistance to ALS inhibitors in B. japonicus in the US, highlighting the need for exploring herbicides with alternative modes of action to enhance weed control in winter wheat. © 2024 Society of Chemical Industry.

BACKGROUND: Digitaria sanguinalis has been identified as a species at high risk of evolving herbicide resistance, but thus far, there are no records of resistance to glyphosate. This weed is one of the most common weeds of summer crops in extensive cropping areas in Argentina. It shows an extended period of seedling emergence with several overlapping cohorts during spring and summer, and is commonly controlled with glyphosate. However, a D. sanguinalis population was implicated as a putative glyphosate-resistant biotype based on poor control at recommended glyphosate doses.
RESULTS: The field-collected D. sanguinalis population (Dgs R) from the Rolling Pampas has evolved glyphosate resistance. Differences in plant survival and shikimate levels after field-recommended and higher glyphosate doses were evident between Dgs R and the known susceptible (Dgs S) population; the resistance index was 5.1. No evidence of differential glyphosate absorption, translocation, metabolism or basal EPSPS activity was found between Dgs S and Dgs R populations; however, a novel EPSPS Pro-106-His point substitution is probably the primary glyphosate resistance-endowing mechanism. EPSPS in vitro enzymatic activity demonstrated that an 80-fold higher concentration of glyphosate is required in Dgs R to achieve similar EPSPS activity inhibition to that in the Dgs S population.
CONCLUSIONS: This study reports the first global case of glyphosate resistance in D. sanguinalis. This unlikely yet novel transversion at the second position of the EPSPS 106 codon demonstrates the intensity of glyphosate pressure in selecting unexpected glyphosate resistance alleles if they retain EPSPS functionality. © 2022 Society of Chemical Industry.

Avena fatua and A. ludoviciana (commonly known as wild oats) are the most problematic winter grass species in fallows and winter crops in the northeast region of Australia. A series of experiments were conducted to evaluate the performance of glyphosate and alternative post-emergence herbicides on A. fatua and A. ludoviciana. This study reports the world\'s first glyphosate-resistant (GR) biotypes of A. fatua and A. ludoviciana. The glyphosate dose required to kill 50% of the plants (LD50) and to reduce 50% of the biomass (GR50) for the GR biotype of A. fatua was 556 g a.e./ha and 351 g a.e./ha, respectively. These values for A. ludoviciana were 848 g a.e./ha and 289 g a.e./ha. Regardless of the growth stage (3-4 or 6-7 leaf stages), clethodim (120 g a.i./ha), haloxyfop (78 g a.i./ha), pinoxaden (20 g a.i./ha), and propaquizafop (30 g a.i./ha) were the best alternative herbicide options for the control of A. fatua and A. ludoviciana. The efficacy of butroxydim (45 g a.i./ha), clodinafop (120 g a.i./ha), imazamox + imazapyr (36 g a.i./ha), and paraquat (600 g a.i./ha) reduced at the advanced growth stage. Glufosinate (750 g a.i./ha), flamprop (225 g a.i./ha), and pyroxsulam + halauxifen (20 g a.i./ha) did not provide effective control of Avena species. This study identified alternative herbicide options to manage GR biotypes of A. fatua and A. ludoviciana.

The occurrence of multiple herbicide resistant weeds has increased considerably in glyphosate-resistant soybean fields in Brazil; however, the mechanisms governing this resistance have not been studied. In its study, the target-site and nontarget-site mechanisms were characterized in an Eleusine indica population (R-15) with multiple resistance to the acetyl-CoA carboxylase (ACCase) inhibitors, glyphosate, imazamox, and paraquat. Absorption and translocation rates of 14C-diclofop-methyl14C-imazamox and 14C-glyphosate of the R-15 population were similar to those of a susceptible (S-15) population; however, the R-15 population translocated ∼38% less 14C-paraquat to the rest of plant and roots than the S-15 population. Furthermore, the R-15 plants metabolized (by P450 cytochrome) 55% and 88% more diclofop-methyl (conjugate) and imazamox (imazamox-OH and conjugate), respectively, than the S-15 plants. In addition, the Pro-106-Ser mutation was found in the EPSPS gene of this population. This report describes the first characterization of the resistance mechanisms in a multiple herbicide resistant weed from Brazil.

BACKGROUND: Weedy sunflower (Helianthus annuus L.) is a troublesome weed in row-crop production fields in South Texas. Populations with suspected resistance to glyphosate were evaluated with 1X and 4X rates (X = 868 g ae ha-1 ) of the herbicide, followed by a dose-response assay of the most resistant population. Molecular studies were conducted to determine if target-site mechanisms were responsible for resistance in these populations. Additionally, field experiments were conducted at two locations (Somerville and Granger, TX) to evaluate the effectiveness of different tank-mix combinations in controlling naturally infesting glyphosate-resistant (GR) weedy sunflower populations in GR corn.
RESULTS: In a study conducted in the growth chamber, seven of the 11 tested populations survived up to the 4X rate of glyphosate. The most-resistant population (TX15-11) was 29-fold more resistant to glyphosate, compared to the susceptible standard. In resistant populations, 5-21 more copies of the EPSPS gene were observed compared to the susceptible standard. In the field studies, tank-mix applications of glyphosate + halosulfuron-methyl, glyphosate + prosulfuron, glyphosate + a premix of halosulfuron-methyl and dicamba or glyphosate + a premix of diflufenzopyr and dicamba effectively controlled GR weedy sunflower populations.
CONCLUSIONS: Glyphosate-resistance was observed in 81% of the putative resistant weedy sunflower populations tested in this study. Resistance in these populations was conferred primarily by amplification of the EPSPS gene. Effective control of GR weedy sunflower can be achieved by tank-mixes tested in the current study, which provides acceptable levels of crop safety. © 2020 Society of Chemical Industry.

BACKGROUND: It is well known that exposure to mild stress can precondition organisms to better tolerate subsequent stress exposure in the same or future generations. Since herbicide hormesis also represents a moderate stress to exposed plants, a transgenerational priming is likely but not proven. Especially in herbicide-resistant weeds showing enhanced reproductive fitness after regular herbicide treatments, the ability to induce resilient offspring phenotypes via hormesis may hasten the evolution of herbicide resistance in weeds. This hypothesis was studied for the triazinone metamitron in an F1 offspring generation of PSII target-site resistant (TSR) plants of Chenopodium album propagated after parental conditioning with various metamitron doses.
RESULTS: In two independent dose-response greenhouse trials, there was a positive correlation between the strength of the stimulatory response during parental preconditioning and the magnitude of transgenerational changes in herbicide sensitivity and hormesis expression. Parental conditioning at subhormetic and toxic concentrations lead to less resilient offspring, while conditioning doses that induced a pronounced hormetic effect in F0 plants had a sensitivity-reducing and hormesis-promoting effect on the offspring. The observed reduction in sensitivity in F1 plants compared to unconditioned F1 plants was up to 2.2-fold.
CONCLUSIONS: This study demonstrates that hormetic herbicide treatments have the ability to prime weeds for enhanced tolerance to subsequent treatments in the next generation. Effects proved dose sensitive and may act in concert with other stimulatory adaptations in plant populations. This is relevant for weed control and herbicide resistance evolution, but also for herbicide side-effects that go beyond the exposed area. © 2020 The Author. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Mutations that confer herbicide resistance are a primary concern for herbicide-based chemical control of invasive plants and are often under-characterized structurally and functionally. As the outcome of selection pressure, resistance mutations usually result from repeated long-term applications of herbicides with the same mode of action and are discovered through extensive field trials. Here we used acetohydroxyacid synthase (AHAS) of Kochia scoparia (KsAHAS) as an example to demonstrate that, given the sequence of a target protein, the impact of genetic mutations on ligand binding could be evaluated and resistance mutations could be identified using a biophysics-based computational approach. Briefly, the 3D structures of wild-type (WT) and mutated KsAHAS-herbicide complexes were constructed by homology modeling, docking and molecular dynamics simulation. The resistance profile of two AHAS-inhibiting herbicides, tribenuron methyl and thifensulfuron methyl, was obtained by estimating their binding affinity with 29 KsAHAS (1 WT and 28 mutated) using 6 molecular mechanical (MM) and 18 hybrid quantum mechanical/molecular mechanical (QM/MM) methods in combination with three structure sampling strategies. By comparing predicted resistance with experimentally determined resistance in the 29 biotypes of K. scoparia field populations, we identified the best method (i.e., MM-PBSA with single structure) out of all tested methods for the herbicide-KsAHAS system, which exhibited the highest accuracy (up to 100%) in discerning mutations conferring resistance or susceptibility to the two AHAS inhibitors. Our results suggest that the in silico approach has the potential to be widely adopted for assessing mutation-endowed herbicide resistance on a case-by-case basis.

BACKGROUND: 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.
RESULTS: 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%.
CONCLUSIONS: 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.

BACKGROUND: A field-evolved herbicide-resistant weed population can represent a heterogeneous composite of subpopulations that differ in their susceptibility and responsiveness to herbicide hormesis. Variable hormesis responsiveness can result in selection for and against certain subpopulations under low herbicide doses, and this has the potential to contribute to the evolution of resistance. The relevance of this hypothesis at practical field rates was studied for two field-collected acetyl-coenzyme A carboxylase (ACCase) target-site resistant (TSR) biotypes of Alopecurus myosuroides Huds. (haplotype Leu1781) exposed to three ACCase inhibitors. Herbicide dose responses were evaluated at the population level and at different subpopulation levels after the dissection of individual plants by herbicide selection and genotyping.
RESULTS: The practical field rates of fenoxaprop-P were lower than the observed hormetic doses in the resistant subpopulation, whereas the field rates of clodinafop and cycloxydim stimulated the shoot biomass in different resistant subpopulations by 21-38% above that of the control. Because variable dose levels induced hormesis in the different subpopulations, the practical field rates showed a significant potential to selectively enhance parts of a resistant field population, but did not impact or adversely affect other parts of the population.
CONCLUSIONS: As a consequence of population heterogeneity, herbicide hormesis may impact resistance evolution in weeds at realistic use rates via the selective promotion of individual genotypes. However, the practical relevance of this phenomenon may be influenced by many factors, such as the herbicidal active ingredient used, as indicated in this study. © 2018 Society of Chemical Industry.