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Background: We have previously demonstrated that an aldo‐keto reductase (AKR) from Echinochloa colona (EcAKR4‐1) can metabolize glyphosate and confers glyphosate resistance. This study aims to investigate if the EcAKR4‐1 orthologs from Lolium rigidum also play a role in glyphosate resistance in non‐target‐site based, glyphosate‐resistant (R) L. rigidum populations from Western Australia. Results: The full‐length L. rigidum AKR gene (LrAKR4C10) orthologous to EcAKR4‐1, together with a distinct LrAKR1, were cloned from plants of a glyphosate‐susceptible (S) (VLR1) and three glyphosate R L. rigidum populations (WALR50, WALR60 and WALR70). Reverse transcription quantitative polymerase chain reaction (RT‐qPCR) results showed that basal expression levels of the two LrAKR genes did not differ between the R and S populations, but their expression was significantly induced by glyphosate (up to 4.3‐fold) or 2,4‐D treatment (up to 3.4‐fold) in R populations. Escherichia coli cells transformed respectively with LrAKR4C10 and LrAKR1 were more tolerant to glyphosate. Rice (Oryza sativa) seedlings overexpressing each of the two LrAKR gene survived glyphosate rates that were lethal to the green fluorescence protein (GFP) control plants. Structural modeling predicts a similar way of glyphosate binding and detoxification by LrAKR4C10 and EcAKR4‐1, but an alternative way of glyphosate binding by LrAKR1. Relatively lower capacity of the two LrAKRs in conferring glyphosate resistance than the known EcAKR4‐1 was discussed in relation to structural interaction. Conclusion: Glyphosate‐induced higher expression of the two LrAKR genes in L. rigidum populations contributes to a moderate level of glyphosate resistance likely through enhanced glyphosate metabolism. The herbicide 2,4‐D can also induce the LrAKR expression, indicating the potential antagonistic effect of 2,4‐D to glyphosate. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry. [ABSTRACT FROM AUTHOR]

BACKGROUND: Eleusine indica (L.) Gaertn. (goosegrass) is a major weed in global cropping systems. It has evolved resistance to glyphosate due to single Pro‐106‐Ser (P106S) or double Thr‐102‐Ile + Pro‐106‐Ser (TIPS) EPSPS target site mutations. Here, experiments were conducted to evaluate the single effect of soybean competition and its combined effect with a glyphosate field dose (1080 g ae ha−1) on the growth and fitness of plants carrying these glyphosate resistance endowing target site mutations. RESULTS: TIPS E. indica plants are highly glyphosate‐resistant but the double mutation endows a substantial fitness cost. The TIPS fitness penalty increased under the effect of soybean competition resulting in a cost of 95%, 95% and 96% in terms of, respectively, vegetative growth, seed mass and seed number investment. Glyphosate treatment of these glyphosate‐resistant TIPS plants showed an increase in growth relative to those without glyphosate. Conversely, for the P106S moderate glyphosate resistance mutation, glyphosate treatment alone reduced survival rate, vegetative growth, aboveground biomass (34%), seed mass (48%) and number (52%) of P106S plants relative to the glyphosate nontreated plants. However, under the combined effects of both soybean competition and the field‐recommended glyphosate dose, vegetative growth, aboveground biomass, seed mass and number of P106S and TIPS plants were substantially limited (by ≤99%). CONCLUSION: The ecological environment imposed by intense competition from a soybean crop sets a significant constraint for the landscape‐level increase of both the E. indica single and double glyphosate resistance mutations in the agroecosystem and highlights the key role of crop competition in limiting the population growth of weeds, whether they are herbicide‐resistant or susceptible. © 2022 Society of Chemical Industry. [ABSTRACT FROM AUTHOR]

Concurrent natural evolution of glyphosate resistance single‐ and double‐point EPSPS mutations in weed species provides an opportunity for the estimation of resistance fitness benefits and prediction of equilibrium resistance frequencies in environments under glyphosate selection. Assessment of glyphosate resistance benefit was conducted for the most commonly identified single Pro‐106‐Ser and less‐frequent double TIPS mutations in the EPSPS gene evolved in the global damaging weed Eleusine indica. Under glyphosate selection at the field dose, plants with the single Pro‐106‐Ser mutation at homozygous state (P106S‐rr) showed reduced survival and compromised vegetative growth and fecundity compared with TIPS plants. Whereas both homozygous (TIPS‐RR) and compound heterozygous (TIPS‐Rr) plants with the double TIPS resistance mutation displayed similar survival rates when exposed to glyphosate, a significantly higher fecundity in the currency of seed number was observed in TIPS‐Rr than TIPS‐RR plants. The highest plant fitness benefit was associated with the heterozygous TIPS‐Rr mutation, whereas plants with the homozygous Pro‐106‐Ser and TIPS mutations exhibited, respectively, 31% and 39% of the fitness benefit revealed by the TIPS‐Rr plants. Populations are predicted to reach stable allelic and genotypic frequencies after 20 years of glyphosate selection at which the WT allele is lost and the stable genotypic polymorphism is comprised by 2% of heterozygous TIPS‐Rr, 52% of homozygous TIPS‐RR and 46% of homozygous P106S‐rr. The high inbreeding nature of E. indica is responsible for the expected frequency decrease in the fittest TIPS‐Rr in favour of the homozygous TIPS‐RR and P106S‐rr. Mutated alleles associated with the glyphosate resistance EPSPS single EPSPS Pro‐106‐Ser and double TIPS mutations confer contrasting fitness benefits to E. indica under glyphosate treatment and therefore are expected to exhibit contrasting evolution rates in cropping systems under recurrent glyphosate selection. [ABSTRACT FROM AUTHOR]

Glyphosate is the most widely used herbicide in world agriculture and for general vegetation control in a wide range of situations. Global and often intensive glyphosate selection of very large weedy plant populations has resulted in widespread glyphosate resistance evolution in populations of many weed species. Here, working with a glyphosate-resistant (GR) Echinochloa colona population that evolved in a Western Australia agricultural field, we identified an ATP-binding cassette (ABC) transporter (EcABCC8) that is consistently up-regulated in GR plants. When expressed in transgenic rice, this EcABCC8 transporter endowed glyphosate resistance. Equally, rice, maize, and soybean overexpressing the EcABCC8 ortholog genes were made resistant to glyphosate. Conversely, CRISPR/Cas9-mediated knockout of the EcABCC8 ortholog gene OsABCC8 increased rice susceptibility to glyphosate. Subcellular localization analysis and quantification of glyphosate cellular levels in treated ABCC8 transgenic rice plants and isolated leaf protoplasts as well as structural modeling support that EcABCC8 is likely a plasma membrane-localized transporter extruding cytoplasmic glyphosate to the apoplast, lowering the cellular glyphosate level. This is a report of a membrane transporter effluxing glyphosate in a GR plant species, and its function is likely conserved in crop plant species. [ABSTRACT FROM AUTHOR]

Summary: We reviewed the literature to understand the effects of glyphosate resistance on plant fitness at the molecular, biochemical and physiological levels. A number of correlations between enzyme characteristics and glyphosate resistance imply the existence of a plant fitness cost associated with resistance‐conferring mutations in the glyphosate target enzyme, 5‐enolpyruvylshikimate‐3‐phosphate synthase (EPSPS). These biochemical changes result in a tradeoff between the glyphosate resistance of the EPSPS enzyme and its catalytic activity. Mutations that endow the highest resistance are more likely to decrease catalytic activity by reducing the affinity of EPSPS for its natural substrate, and/or slowing the velocity of the enzyme reaction, and are thus very likely to endow a substantial plant fitness cost. Prediction of fitness costs associated with EPSPS gene amplification and overexpression can be more problematic. The validity of cost prediction based on the theory of evolution of gene expression and resource allocation has been cast into doubt by contradictory experimental evidence. Further research providing insights into the role of the EPSPS cassette in weed adaptation, and estimations of the energy budget involved in EPSPS amplification and overexpression are required to understand and predict the biochemical and physiological bases of the fitness cost of glyphosate resistance. [ABSTRACT FROM AUTHOR]

An Echinochloa colona population from Western Australia has evolved resistance to glyphosate. This current study investigates the physiological, molecular and biochemical basis of glyphosate resistance in this population. To minimise genetic differences the susceptible (S) and resistance (R) phenotypes were isolated from within this resistant population. The S phenotype was found to accumulate significantly more shikimate in the leaf tissue than the R phenotype following glyphosate treatment. Target-site EPSPS gene sequencing revealed no resistance mutations and EPSPS gene expression was similar between the S and R phenotypes. Thus, glyphosate resistance in this population is unlikely target-site based. Similarly, there were no significant differences between the S and R phenotypes in glyphosate leaf uptake and translocation at the whole plant level. It is also unlikely that the resistance is associated with glyphosate metabolism as no major glyphosate metabolites were detected in leaf tissue of the S and R phenotypes. Despite much effort the exact glyphosate resistance mechanisms in this R population remain unclear, and novel resistance mechanisms are to be determined. [ABSTRACT FROM AUTHOR]

BACKGROUND This study confirms and characterises glyphosate resistance in two polyploid Echinochloa colona populations from north-eastern Australia. RESULTS Glyphosate dose response revealed that the two resistant populations were marginally (up to twofold) resistant to glyphosate. Resistant plants did not differ in non-target-site foliar uptake and translocation of 14C-glyphosate, but contained the known target-site 5-enolpyruvylshikimate-3-phosphate synthase ( EPSPS) mutation Pro-106-Thr and/or Pro-106-Leu. Although plants carrying either a single or two EPSPS mutations were glyphosate resistant relative to the susceptible population, they were still controlled at the field rate of glyphosate (450 g a.e. ha−1) when treated under warm conditions (25/20 °C). However, when treated in hot conditions (35/30 °C), most mutant resistant plants (68%) can survive the field rate, and an increase (2.5-fold) in glyphosate LD50 was found for both the R and S populations. CONCLUSIONS This study shows that one or two EPSPS Pro-106 mutations are insufficient to confer field-rate glyphosate resistance in polyploidy E. colona at mild temperatures. However, control of these mutant plants at the glyphosate field rate is poor at high temperatures, probably owing to reduced glyphosate efficacy. Therefore, glyphosate should be applied during relatively mild (warm) temperature periods in the summer growing season to improve E. colona control. © 2015 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

BACKGROUND: A population of Echinochloa colona infesting agricultural fields in the northern region of Western Australia evolved glyphosate resistance after 10 years of glyphosate selection. This study identified two phenotypic (susceptible S versus resistant R) lines from within a segregating glyphosate-resistant population. Estimation of survival, growth and reproductive rates of the phenotypes in response to glyphosate selection helped to characterise the level of resistance, fitness and the selection intensity for glyphosate in this species. RESULTS: Estimations of LD50 (lethal dose) and GR50 (growth rate) showed an eightfold glyphosate resistance in this population. The resistant index based on the estimation of seed number (SYn50) showed a 13-fold resistance. As a result of linear combination of plant survival and fecundity rates, plant fitness values of 0.2 and 0.8 were estimated for the S and R phenotypeswhen exposed to the low dose of 270 g glyphosate ha-1. At the recommended dose of 540 g glyphosate ha-1, fitness significantly decreased (fivefold) in S plants but remained markedly similar (0.7) in plants of the R phenotype. Thus, the calculated selection intensity (SI) at 540 g glyphosate ha-1 was much greater (SI = 17) than at 270 g glyphosate ha-1 (SI = 4). CONCLUSIONS: The assessment of plant survival and fecundity in response to glyphosate selection in the S and R phenotypes allowed a greater accuracy in the estimation of population fitness of both phenotypes and thus of glyphosate selection intensity in E. colona. The estimation of seed number or mass of phenotypes under herbicide selection is a true ecological measure of resistance with implications for herbicide resistance evolution. [ABSTRACT FROM AUTHOR]

Glyphosate is the most important and widely used herbicide in world agriculture. Intensive glyphosate selection has resulted in the widespread evolution of glyphosate-resistant weed populations, threatening the sustainability of this valuable once-in-a-century agrochemical. Field-evolved glyphosate resistance due to known resistance mechanisms is generally low to modest. Here, working with a highly glyphosate-resistant Eleusine indica population, we identified a double amino acid substitution (T102I + P106S [TIPS]) in the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene in glyphosate-resistant individuals. This TIPS mutation recreates the biotechnology-engineered commercial first generation glyphosate-tolerant EPSPS in corn (Zea mays) and now in other crops. In E. indica, the naturally evolved TIPS mutants are highly (more than 180-fold) resistant to glyphosate compared with the wild type and more resistant (more than 32-fold) than the previously known P106S mutants. The E. indica TIPS EPSPS showed very high-level (2,647-fold) in vitro resistance to glyphosate relative to the wild type and is more resistant (600-fold) than the P106S variant. The evolution of the TIPS mutation in crop fields under glyphosate selection is likely a sequential event, with the P106S mutation being selected first and fixed, followed by the T102I mutation to create the highly resistant TIPS EPSPS. The sequential evolution of the TIPS mutation endowing high-level glyphosate resistance is an important mechanism by which plants adapt to intense herbicide selection and a dramatic example of evolution in action. [ABSTRACT FROM AUTHOR]

BACKGROUND In Australia, glyphosate has been used routinely to control wild radish ( Raphanus raphanistrum L.) for the past 40 years. This study focuses on two field-evolved glyphosate-resistant populations of wild radish collected from the grainbelt of Western Australia. RESULTS Two wild radish biotypes were confirmed to be glyphosate resistant by comparing R/S of two suspected populations. Based on R/S from dose-response curves, the R1 and R2 populations were 2.3 and 3.2 times more resistant to glyphosate respectively. Dose response on glyphosate-selected progeny (>1080 g ha−1) demonstrated that the glyphosate resistance mechanism was heritable. When compared with the pooled mortality results of three known susceptible populations ( S1, S2 and S3), the R1 and R2 subpopulations were 3.4-fold and 4.5-fold more resistant at the LD50 level respectively. Both populations were found to have multiple resistance to the phytoene desaturase inhibitor; diflufenican, the synthetic auxin; 2,4-D and the ALS inhibitors; chlorsulfuron, sulfometuron-methyl, imazethapyr and metosulam. CONCLUSIONS This is the first report confirming glyphosate resistance evolution in wild radish and serves to re-emphasise the importance of diverse weed control strategies. Proactive and integrated measures for resistance management need to be developed to diversify control measures away from glyphosate and advance the use of non-herbicidal techniques. © 2014 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

Amplification of the EPSPS gene has been previously identified as the glyphosate resistance mechanism in many populations of Amaranthus palmeri, a major weed pest in US agriculture. Here, we evaluate the effects of EPSPS gene amplification on both the level of glyphosate resistance and fitness cost of resistance. A. palmeri individuals resistant to glyphosate by expressing a wide range of EPSPS gene copy numbers were evaluated under competitive conditions in the presence or absence of glyphosate. Survival rates to glyphosate and fitness traits of plants under intra-specific competition were assessed. Plants with higher amplification of the EPSPS gene (53-fold) showed high levels of glyphosate resistance, whereas less amplification of the EPSPS gene (21-fold) endowed a lower level of glyphosate resistance. Without glyphosate but under competitive conditions, plants exhibiting up to 76-fold EPSPS gene amplification exhibited similar height, and biomass allocation to vegetative and reproductive organs, compared to glyphosate susceptible A. palmeri plants with no amplification of the EPSPS gene. Both the additive effects of EPSPS gene amplification on the level of glyphosate resistance and the lack of associated fitness costs are key factors contributing to EPSPS gene amplification as a widespread and important glyphosate resistance mechanism likely to become much more evident in weed plant species. [ABSTRACT FROM AUTHOR]

Background Glyphosate resistance in populations of the C4 perennial Sorghum halepense (Johnsongrass) and C3 annual Lolium rigidum (rigid ryegrass) has evolved and been documented in many cropping areas around the globe. In S. halepense and in the majority of reported cases in L. rigidum the glyphosate resistance trait has been associated with a mechanism that reduces glyphosate translocation within plants. Here, the significant decrease in the glyphosate resistance level when resistant plants of S. halepense and L. rigidum are grown at suboptimal cool temperature conditions is reported. Copyright © 2012 Society of Chemical Industry Results Lowering temperature from 30 to 19 °C in S. halepense and from 19 to 8 °C in L. rigidum significantly reduced both plant survival and above-ground biomass produced by glyphosate-resistant plants. Thus, glyphosate resistance parameters significantly decreased when glyphosate-treated resistant plants of both species were grown under non-optimal temperature conditions. The results suggest that the resistance mechanism against glyphosate damage is less efficient at optimal growing temperatures. Conclusion It is possible to increase the control of glyphosate-resistant S. halepense and L. rigidum populations by treatment with glyphosate during growing conditions at suboptimal low temperatures. Conversely, glyphosate failure will continue to occur on glyphosate-resistant populations treated during periods of higher temperatures.© 2012 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

This study investigates mechanisms of multiple resistance to glyphosate, acetyl-coenzyme A carboxylase (ACCase) and acetolactate synthase (ALS)-inhibiting herbicides in two Lolium rigidum populations from Australia. When treated with glyphosate, susceptible (S) plants accumulated 4- to 6-fold more shikimic acid than resistant (R) plants. The resistant plants did not have the known glyphosate resistance endowing mutation of 5-enolpyruvylshikimate-3 phosphate synthase (EPSPS) at Pro-106, nor was there over-expression of EPSPS in either of the R populations. However, [14C]-glyphosate translocation experiments showed that the R plants in both populations have altered glyphosate translocation patterns compared to the S plants. The R plants showed much less glyphosate translocation to untreated young leaves, but more to the treated leaf tip, than did the S plants. Sequencing of the carboxyl transferase domain of the plastidic ACCase gene revealed no resistance endowing amino acid substitutions in the two R populations, and the ALS in vitro inhibition assay demonstrated herbicide-sensitive ALS in the ALS R population (WALR70). By using the cytochrome P450 inhibitor malathion and amitrole with ALS and ACCase herbicides, respectively, we showed that malathion reverses chlorsulfuron resistance and amitrole reverses diclofop resistance in the R population examined. Therefore, we conclude that multiple glyphosate, ACCase and ALS herbicide resistance in the two R populations is due to the presence of distinct non-target site based resistance mechanisms for each herbicide. Glyphosate resistance is due to reduced rates of glyphosate translocation, and resistance to ACCase and ALS herbicides is likely due to enhanced herbicide metabolism involving different cytochrome P450 enzymes. [ABSTRACT FROM AUTHOR]

The article presents a discussion on glyphosate-resistant weeds. Glyphosate is a herbicide used around the world to control weeds since 1996. With the increase in the planting of glyphosate-resistant crops in the U. S. came the growth of glyphosate-resistant weeds including Ambrosia artemissifolia L. and Amaranthus palmeri S Watson. Glyphosate-resistant weeds were also found in Argentina and Brazil. The article mentions that glyphosate-resistant weeds are expected to evolve with continued planting of glyphosate-resistant crops. It also discusses the importance of diversifying weed management for the continued use of glyphosate as herbicide.

This article reports on glyphosate, a highly effective broad-spectrum herbicide. It discusses that glyphosate has become the dominant herbicide worldwide because it is very toxicologically and environmentally safe. Glyphosate is the only herbicide that targets 5-enolpyruvyl-shikimate-3-phosphate synthase and it translocates well. Its cost has dropped dramatically after glyphosate became a generic compound. However, the use of this herbicide is being threatened by the evolution of glyphosate-resistant weeds.

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Glyphosate is the world’s most widely used herbicide. A potential substitute for glyphosate in some use patterns is the herbicide paraquat. Following many years of successful use, neither glyphosate nor paraquat could control a biotype of the widespread annual ryegrass ( Lolium rigidum), and here the world’s first case of multiple resistance to glyphosate and paraquat is confirmed. Dose–response experiments established that the glyphosate rate causing 50% mortality (LD50) for the resistant (R) biotype is 14 times greater than for the susceptible (S) biotype. Similarly, the paraquat LD50 for the R biotype is 32 times greater than for the S biotype. Thus, based on the LD50 R/S ratio, this R biotype of L. rigidum is 14-fold resistant to glyphosate and 32-fold resistant to paraquat. This R biotype also has evolved resistance to the acetyl-coenzyme A carboxylase (ACCase) inhibiting herbicides. The mechanism of paraquat resistance in this biotype was determined as restricted paraquat translocation. Resistance to ACCase-inhibiting herbicides was determined as due to an insensitive ACCase. Two mechanisms endowing glyphosate resistance were established: firstly, a point mutation in the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene, resulting in an amino acid substitution of proline to alanine at position 106; secondly, reduced glyphosate translocation was found in this R biotype, indicating a co-occurrence of two distinct glyphosate resistance mechanisms within the R population. In total, this R biotype displays at least four co-existing resistance mechanisms, endowing multiple resistance to glyphosate, paraquat and ACCase herbicides. This alarming case in the history of herbicide resistance evolution represents a serious challenge for the sustainable use of the precious agrochemical resources such as glyphosate and paraquat. [ABSTRACT FROM AUTHOR]

In this article the author discusses the role of gene amplification in glyphosate-resistant weed prevention that affects food production. The author states that the excessive reliance on glyphosate alongside with the prevalence of transgenic glyphosate-resistant crops resulted to the resistance of weeds on the herbicide. Moreover, he highlights the fact that gene amplification is a revolutionary response to herbicide selection pressure as well as the need to use glyphosate with more diversity.

The article discusses various reports published within the issue, including one on the evolution of glyphosate resistance and another one glyphosate-resistant crops.

Abstract: Multi-species RIM, a bio-economic model that simulates the population dynamics of two annual weed species over a 20-year period, was used to investigate the value of transgenic glyphosate-resistant canola in the management of herbicide resistant annual ryegrass and wild radish in a Western Australian dryland farming system. The perceived advantage of growing this crop is the potential to control post-emergent weeds with the broad-spectrum herbicide glyphosate, and without the yield penalty evident in triazine-resistant canola. We found that the economic value of glyphosate-resistant canola is consistently higher than that of the commonly grown triazine-resistant canola. However, the benefits of glyphosate-resistant canola would need to be weighed up against potential risks to marketability (due to consumer resistance) and risks of increased weed resistance to glyphosate (due to increased selection pressure). [Copyright &y& Elsevier]