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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., (© 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.)

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., (© 2022 Society of Chemical Industry.)

Background: Lolium rigidum is the most important weed in Australian agriculture and pre-emergence dinitroaniline herbicides (e.g., trifluralin) are widely and persistently used for Lolium control. Consequently, evolution of resistance to dinitroaniline herbicides has been increasingly reported. Resistance-endowing target-site α-tubulin gene mutations are identified with varying frequency. This study investigated the putative fitness cost associated with the common resistance mutation Val-202-Phe and the rare resistance mutation Arg-243-Met causing helical plant growth., Results: Results showed a deleterious effect of Arg-243-Met on fitness when plants are homozygous for this mutation. This was evidenced as high plant mortality, severely diminished root and aboveground vegetative growth (lower relative growth rate), and very poor fecundity compared with the wild-type, which led to a nearly lethal fitness cost of >99.9% in competition with a wheat crop. A fitness penalty in vegetative growth was evident, but to a much lesser extent, in plants heterozygous for the Arg-243-Met mutation. By contrast, plants possessing the Val-202-Phe mutation exhibited a fitness advantage in vegetative and reproductive growth., Conclusion: The α-tubulin mutations Arg-243-Met and Val-202-Phe have contrasting effects on fitness. These results help understand the absence of plants homozygous for the Arg-243-Met mutation and the high frequency of plants carrying the Val-202-Phe mutation in dinitroaniline-resistant L. rigidum populations. The α-tubulin Arg-243-Met mutation can have an exceptional fitness cost with nearly lethal effects on resistant L. rigidum plants. © 2022 Society of Chemical Industry., (© 2022 Society of Chemical Industry.)

Background: Trifluralin is widely used in Australia as one of the important pre-emergence herbicides to control annual ryegrass (Lolium rigidum Gaud.) populations. Trifluralin resistance evolution and mechanisms have been identified in some ryegrass populations., Results: In this study, 21 putative resistant field survey populations from Western Australian were screened with trifluralin, and 90% (19 of 21) contained individuals surviving 480 g ha -1 trifluralin treatment. Twelve populations contained individuals possessing the known α-tubulin resistance mutations at Val-202, Thr-239 and Arg-243 in TUA4 (alpha-tubulin 4 n), plus multiple potential resistance mutations in TUA4 pending genetic confirmation. Three populations had only individuals carrying newly identified (but uncharacterized) mutations in TUA3/TUA4. Radioactive work found that six populations evolved metabolic resistance to trifluralin, and at least four of them also possessed the known and/or putative target-site mutations., Conclusion: These results confirm that a high incidence of resistance to the dinitroaniline herbicide (trifluralin) is present, and target-site tubulin mutations make a major contribution to resistance in these annual ryegrass populations. Co-evolution of both target-site and non-target-site resistance to per-emergence herbicides warrants diverse management tactics., (© 2021 Society of Chemical Industry.)

Background: Tubulin, the target site of dinitroaniline herbicides, is encoded by small gene families in plants. To better characterize the mechanisms of target-site resistance to dinitroaniline herbicides in the globally important weedy species Lolium rigidum, attempts were made to amplify and sequence α-tubulin transcripts., Results: Four α-tubulin isoforms (TUA1, TUA2, TUA3 and TUA4) were identified in L. rigidum. Variations in the number and sequence of transcripts encoding these α-tubulin proteins were found in individuals from the two L. rigidum populations examined. Within and among populations, differences in the 5'- and 3'-untranslated regions of cDNA in TUA3 and TUA4 were identified. Furthermore, a novel double mutation, Arg-390-Cys+Asp-442-Glu, in the TUA3 transcript was identified and has the potential to confer dinitroaniline resistance., Conclusion: This research reveals the complexity of the α-tubulin gene family in individuals/populations of the cross-pollinated weedy species L. rigidum, and highlights the need for better understanding of the molecular architecture of tubulin gene families for detecting resistance point mutations. Although TUA4 is a commonly expressed α-tubulin isoform containing most frequently reported resistance mutations, other mutant tubulin isoforms may also have a role in conferring dinitroaniline resistance., (© 2020 Society of Chemical Industry.)

Background: Barnyardgrass (Echinochloa spp.) is a global weed in rice fields. Quinclorac is commonly used to control barnyardgrass. However, due to persistent use, quinclorac resistance has evolved. We obtained quinclorac-susceptible (QS) and -resistant (QR1, QR2) lines from the progeny of a single resistant E. crus-pavonis for a resistance mechanism study., Results: Line QR1 exhibited resistance to high quinclorac rates (up to 6400 g ha -1 ), whereas line QR2 exhibited a resistance/susceptibility segregation ratio of 3:1 at the field or lower rates (400, 100 g ha -1 ). Intriguingly, a lower level of 14 C-quinclorac metabolism and hence a higher level of 14 C-quinclorac translocation was observed in QR1 than QS plants. The basal expression levels of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) and ACC oxidase 2 (ACO2) genes did not differ significantly between the QR1 and QS lines. However, more expression of ACS and ACO genes was induced by quinclorac treatment in QS than in QR1. Basal levels of β-cyanoalanine synthase (β-CAS) gene expression were similar in QS and QR1 plants, but a greater level of down-regulation was detected in QS than in QR1 plants after quinclorac treatment., Conclusion: These results indicate QR plants are less responsive to quinclorac than QS plants in terms of up-regulating quinclorac metabolism and ethylene synthesis. Resistance in this E. crus-pavonis line is likely controlled by a single major gene, involving possibly an alteration in auxin signal perception/transduction to the ethylene biosynthesis pathway. The β-CAS is unlikely to play a major role in quinclorac resistance in this particular population., (© 2020 Society of Chemical Industry.)

Background: Resistance to the dinitroaniline herbicide trifluralin in Lolium rigidum (annual ryegrass) often is mediated by the enhanced capacity to metabolize the herbicide to less toxic polar conjugates and/or by functionally recessive target-site mutations in α-tubulin., Results: In two L. rigidum populations possessing enhanced trifluralin metabolism, resistance was largely reversed by recurrent selection with the thiocarbamate herbicide prosulfocarb (i.e. plant survival was two- to >20-fold lower). Their ability to metabolize trifluralin was significantly decreased (by ≈2.3-fold) following recurrent prosulfocarb selection, to levels comparable to those observed in susceptible plants or when trifluralin metabolism was inhibited by treatment with the insecticide phorate., Conclusions: This study provides evidence that trait(s) enabling efficient trifluralin metabolism in L. rigidum are purged from the population under prosulfocarb recurrent selection. The level of trifluralin metabolism in vitro and its inhibition caused by phorate action on trifluralin-metabolizing enzyme(s) is equivalent to the effect produced by prosulfocarb selection. The hypothetical link between the two phenomena is that the putative monooxygenase(s) conferring trifluralin metabolic resistance also mediate the activation of prosulfocarb to its toxic sulfoxide. Thus, we speculate that survival to prosulfocarb via a lack of metabolic herbicide activation, and survival to trifluralin conferred by enhanced herbicide metabolism, are mutually exclusive. These findings not only open up a new research direction in terms of the interaction between different herbicide resistance mechanisms in L. rigidum, but also offer strategies for immediate management of the population dynamics of metabolism-based resistance in the field. © 2020 Society of Chemical Industry., (© 2020 Society of Chemical Industry.)

Background: The photosystem II (PSII)-inhibiting herbicides are important for Australian farmers to control Lolium rigidum Gaud. and other weed species in trazine tolerant (TT)-canola fields. A L. rigidum population (R) collected from a TT-canola field from Western Australia showed multiple resistance to PSII, acetyl-coenzyme A carboxylase (ACCase) and acetolactate synthase (ALS) inhibitors. The mechanisms of multiple resistance in this R population were determined., Results: The R population showed a low-level (about 3.0-fold) resistance to the PSII-inhibiting herbicides metribuzin and atrazine. Sequencing of the psbA gene revealed no differences between the R and susceptible (S) sequences. Furthermore, [ 14 C]-metribuzin experiments found no significant difference in metribuzin foliar uptake and translocation between the R and S plants. However, [ 14 C]-metribuzin metabolism in R plants was 2.3-fold greater than in S plants. The cytochrome P450 monooxygenase inhibitor piperonyl butoxide (PBO) enhanced plant mortality response to metribuzin and atrazine in both R and S populations. In addition, multiple resistance to ALS and ACCase inhibitors are due to known resistance mutations in ALS and ACCase genes., Conclusion: The results demonstrate that enhanced metribuzin metabolism likely involving cytochrome P450 monooxygenase contributes to metribuzin resistance in Lolium rigidum. This is the first report of metabolic resistance to the PSII-inhibiting herbicide metribuzin in Australian Lolium rigidum. © 2020 Society of Chemical Industry., (© 2020 Society of Chemical Industry.)

Background: Multiple-herbicide resistance in Lolium rigidum and other weed species is increasingly exerting pressure on herbicide discovery research for solutions against resistance-prone weeds. In this study we investigate: (i) the responses of L. rigidum populations and wheat to the new herbicide cinmethylin in comparison with other pre-emergence herbicides, (ii) the effect of seed burial depths on cinmethylin efficacy and crop selectivity, and (iii) the basis of cinmethylin selectivity in wheat., Results: Cinmethylin at 400 g ha -1 controls herbicide-susceptible and multiple-resistant L. rigidum, with a reduction of >85% in plant emergence and 90% in aboveground biomass. Cinmethylin provides effective control of a large number of field populations of L. rigidum with evident resistance to trifluralin. When the wheat seed is buried ≥1 cm below the cinmethylin-treated soil surface, the emergence of crop seedlings is not different from the untreated control. The organophosphate insecticide phorate synergizes cinmethylin toxicity in wheat, with an LD 50 of 682 g ha -1 in the absence of phorate versus 109 g ha -1 in the presence of phorate (84% reduction). The synergistic effect of phorate with cinmethylin on herbicide-susceptible L. rigidum appears smaller (a 44% reduction in the LD 50 of cinmethylin)., Conclusions: Cinmethylin is effective in controlling multiple-resistant L. rigidum and appears safe for wheat when the seed is separated at depth from the herbicide applied to the soil surface. The basis of this metabolism-based selectivity is likely regulated by cytochrome P450 monooxygenases. © 2020 Society of Chemical Industry., (© 2020 Society of Chemical Industry.)

Background: Diflufenican resistance has been reported in wild radish populations since 1998, but the resistance mechanisms have not been investigated. Recently, we identified a wild radish population (H2/10) from the Western Australian grain belt that is resistant (R) to the phytoene desaturase (PDS)-inhibiting herbicide diflufenican., Results: Dose-response results showed this R population is 4.9-fold more resistant than the susceptible (S) population based on the LD 50 R/S ratio. In addition, the R population also exhibits cross-resistance to the PDS-inhibiting herbicide fluridone. The cytochrome P450 inhibitor malathion reversed diflufenican resistance and partially reversed fluridone resistance in the R population. The full coding sequences of the PDS gene were cloned from the S and R plants and there are natural variations in the PDS gene transcripts/alleles with no correlation to resistance. In addition, the R plants had a level of PDS gene expression that is not significantly different from the S plants., Conclusion: These results demonstrated that diflufenican resistance in this R wild radish population is likely due to non-target-site based enhanced herbicide metabolism involving cytochrome P450s. © 2019 Society of Chemical Industry., (© 2019 Society of Chemical Industry.)

Background: Relatively new herbicides that target 4-hydroxyphenylpyruvate dioxygenase (HPPD) are now available for use on the world's great grain crops (rice, wheat, corn and soybean) and for other uses. With widespread and persistent use of HPPD-inhibiting herbicides, the evolution of HPPD-inhibiting herbicide resistant weeds is inevitable. Currently, resistance to HPPD-inhibiting herbicides is known in two weed species, waterhemp and Palmer amaranth. Here, we report a HPPD-inhibiting herbicide resistant wild radish population from the Western Australia grain belt. This population was not selected with HPPD-inhibiting herbicides, rather it evolved resistance to earlier used herbicides with different modes of action and exhibits cross-resistance to HPPD-inhibiting herbicides., Results: Dose-response experiments showed the resistant (R) population exhibits 4 to 6.5-fold resistance to the HPPD-inhibiting herbicides mesotrione, tembotrione and isoxaflutole, compared to the susceptible (S) population. This resistance is not target-site based as cloning of full coding sequences of the HPPD genes from S and R plants did not reveal resistance-endowing single nucleotide polymorphisms. The HPPD gene expression levels are similar in S and R plants. In addition, no differences in [ 14 C]-mesotrione uptake and translocation were observed in the S and R plants. However, the time required for R plants to metabolise 50% [ 14 C]-mesotrione is 7.7-fold faster than for the S plants., Conclusion: We confirm resistance to HPPD-inhibiting herbicides exists in a population of the economically damaging global weed wild radish. The resistance in this population is due to a non-target-site based enhanced rate of herbicide metabolism. © 2019 Society of Chemical Industry., (© 2019 Society of Chemical Industry.)

Background: Weed resistance to foliar herbicides has dramatically increased worldwide in the last two decades. As a consequence, current practices of weed management have changed, with an increased adoption of soil-applied herbicides to restore control of herbicide-resistant weeds. We foresee metabolism-based resistance and cross-resistance to soil-applied herbicides as a potential global consequence to the increased and widespread adoption of new and old soil-applied herbicides. Thus, the aim of this study is to use computer simulation modelling to quantify and rank the risk of weeds evolving resistance to soil-applied herbicides under different usage strategies (single herbicide use, rotations and mixtures) and population genetic hypotheses., Results: Simulations indicate that without rotation it takes twice as long to select for resistance to a particular soil-applied herbicide - trifluralin - than to any other herbicide option considered. Relative to trifluralin-only use, simple herbicide rotation patterns have no effect in delaying resistance, whereas more complex rotation patterns can delay resistance two- or three-fold. Herbicide mixtures further delay resistance up to six-fold in comparison to single use or simple herbicide rotations., Conclusion: By computer modelling simulations we demonstrate that mixtures maximize herbicide effectiveness and the selection heterogeneity of soil-applied herbicides, and delay herbicide resistance evolution in weedy plants. Our study is consistent with previous state-of-art scientific evidence (i.e. epidemiological and modelling studies across different systems and pests) and extension efforts (i.e. 'rotate herbicide mixtures') to provide insight to manage the selection and evolution of weed resistance. © 2019 Society of Chemical Industry., (© 2019 Society of Chemical Industry.)

Background: A Lolium rigidum population collected from Western Australia was previously reported as highly resistant to dinitroaniline herbicides mainly due to a Val-202-Phe substitution in the target site α-tubulin protein. To further determine the contribution of the 202 mutation to resistance, two sub-populations, respectively comprising the 202 mutant and wild-type (WT) individuals, were isolated from within the same resistant population and subject to dinitroaniline herbicide doses. A rice transgenic study was conducted to demonstrate whether the amino acid substitution at the 202 residue confers resistance. In addition, as indicated in the phenotyping and genotyping study, non-target enhanced trifluralin metabolism was further examined in the same population., Results: The 202 mutants were more resistant than the wild-type plants. Rice calli transformed with the L. rigidum mutant α-tubulin gene (Val-202-Phe) were more resistant to dinitroaniline herbicides relative to calli transformed with the wild-type gene. Also, enhanced trifluralin metabolism was detected in the 202 mutants in comparison to the susceptible seedlings., Conlcusion: Both target-site Val-202-Phe α-tubulin mutation and non-target-site enhanced trifluralin metabolism co-exist in this dinitroaniline-resistant L. rigidum population. © 2019 Society of Chemical Industry., (© 2019 Society of Chemical Industry.)

Background: Wild radish (Raphanus raphanistrum) is a globally important weed of crops. Two atrazine-resistant wild radish populations (R1 and R2), collected from the Western Australia grain belt, were investigated for resistance to photosystem II (PSII) herbicides., Results: Sequencing of the full-length psbA gene revealed the well-known Ser264-Gly substitution in population R1, whereas population R2 displayed a novel Phe274-Val substitution. Herbicide dose-response studies confirmed that the population with the Ser264-Gly mutation exhibited high-level resistance to atrazine, but super-sensitivity to bromoxynil. Plants possessing the novel Phe274-Val mutation exhibited a modest level of resistance to atrazine, metribuzin and diuron, and were bromoxynil susceptible. Structural modelling of the mutant D1 proteins predicts that the Ser264-Gly mutation endows atrazine resistance by abolishing H-bonds, but confers bromoxynil super-sensitivity by enhancing hydrogen bonding. The Phe274-Val substitution provides resistance to atrazine and diuron by indirectly affecting H-bond formation between the Ser264 residue and the herbicides., Conclusion: The results demonstrate that the Phe274-Val mutation is likely responsible for resistance to PSII-inhibiting triazine and urea herbicides. To our knowledge, this is the first evidence of the psbA Phe274-Val mutation in wild radish conferring resistance to PSII herbicides. © 2018 Society of Chemical Industry., (© 2018 Society of Chemical Industry.)

Background: The pre-emergence dinitroaniline herbicides (such as trifluralin and pendimethalin) are vital to Australian no-till farming systems. A Lolium rigidum population collected from the Western Australian grain belt with a 12-year trifluralin use history was characterised for resistance to dinitroaniline, acetyl CoA carboxylase (ACCase)- and acetolactate synthase (ALS)-inhibiting herbicides. Target-site resistance mechanisms were investigated., Results: This L. rigidum population exhibited 32-fold resistance to trifluralin, as compared with the susceptible population. It also displayed 12- to 30-fold cross-resistance to other dinitroaniline herbicides (pendimethalin, ethalfluralin and oryzalin). In addition, this population showed multiple resistance to commonly used post-emergence ACCase- and ALS-inhibiting herbicides. Two target-site α-tubulin gene mutations (Val-202-Phe and Thr-239-Ile) previously documented in other dinitroaniline-resistant weed species were identified, and some known target-site mutations in ACCase (Ile-1781-Leu, Asp-2078-Gly and Cys-2088-Arg) and ALS (Pro-197-Gln/Ser) were found in the same population. An agar-based Petri dish screening method was established for the rapid diagnosis of resistance to dinitroaniline herbicides., Conclusion: Evolution of target-site resistance to both pre- and post-emergence herbicides was confirmed in a single L. rigidum population. The α-tubulin mutations Val-202-Phe and Thr-239-Ile, documented here for the first time in L. rigidum, are likely to be responsible for dinitroaniline resistance in this population. Early detection of dinitroaniline herbicide resistance and integrated weed management strategies are needed to maintain the effectiveness of dinitroaniline herbicides. © 2017 Society of Chemical Industry., (© 2017 Society of Chemical Industry.)

Background: An Eleusine indica population has evolved resistance to glufosinate, a major post-emergence herbicide of global agriculture. This population was analysed for target-site (glutamine synthetase) and non-target-site (glufosinate uptake, translocation and metabolism) resistance mechanisms., Results: Glutamine synthetase (GS) activity extracted from susceptible (S) and resistant (R*) plants was equally sensitive to glufosinate inhibition, with IC 50 values of 0.85 mm and 0.99 mm, respectively. The extractable GS activity was also similar in S and R* samples. Foliar uptake of [ 14 C]-glufosinate did not differ in S and R* plants, nor did glufosinate net uptake in leaf discs. Translocation of [ 14 C]-glufosinate into untreated shoots and roots was also similar in both populations, with 44% to 47% of the herbicide translocated out from the treated leaf 24 h after treatment. The HPLC and LC-MS analysis of glufosinate metabolism revealed no major metabolites in S or R* leaf tissue., Conclusions: Glufosinate resistance in this resistant population is not due to an insensitive GS, or increased activity, or altered glufosinate uptake and translocation, or enhanced glufosinate metabolism. Thus, target-site resistance is likely excluded and the exact resistance mechanism(s) remain to be determined. © 2017 Society of Chemical Industry., (© 2017 Society of Chemical Industry.)

Background: Across several agricultural systems the evolution of herbicide resistance has occurred more rapidly to post-emergence than pre-emergence herbicides; however, the reasons for this are not clear. We used a new simulation model to investigate whether interactions between differences in order of application and weed cohorts affected could explain this historically observed difference between the herbicide groups., Results: A 10 year delay in resistance evolution was predicted for a shorter-acting residual pre-emergence (cf. post-emergence), when all other parameters were identical. Differences in order of application between pre- and post-emergence herbicides had minimal effect on rates of resistance evolution when similar weed cohorts were affected., Conclusion: This modelling suggested that the historically observed lower levels of resistance to pre-emergence herbicides are most likely to be due to the smaller number of weed cohorts affected by many pre-emergence herbicides. The lower number of weed cohorts affected by pre-emergence herbicides necessitated the use of additional, effective control measures, thereby reducing resistance evolution. This study highlights the advantages of applying multiple control measures to each weed cohort. © 2016 Society of Chemical Industry., (© 2016 Society of Chemical Industry.)

Background: 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., Results: Phorate synergised with three different herbicide modes of action, enhancing the activity of the ALS inhibitor chlorsulfuron (60% LD 50 reduction), the VLCFAE inhibitor pyroxasulfone (45% LD 50 reduction) and the mitosis inhibitor trifluralin (70% LD 50 reduction). Conversely, phorate antagonised the two thiocarbamate herbicides prosulfocarb and triallate with a 12-fold LD 50 increase., Conclusion: 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., (© 2016 Society of Chemical Industry.)

Background: Rice is a major field crop of paramount importance for global food security. However, the increased adoption of more profitable and resource-efficient direct-seeded rice (DSR) systems has contributed to greater weed infestations, including weedy rice, which has become a severe problem in several Asian regions. In this study we have developed a conceptually novel method to protect rice plants at high doses of clomazone and triallate., Results: The insecticide phorate applied to rice seeds provided a substantial level of protection against the herbicides clomazone or triallate. A quantity of 15 kg phorate ha -1 significantly increased the LD 50 values, which were more than twofold greater than for rice plants treated only with clomazone. A quantity of 20 kg phorate ha -1 in combination with 2000 g triallate ha -1 safened rice plants (80% survival) with LD 50 >3.4-fold greater than in phorate-untreated rice. Weed control efficacy was not lowered by the presence of phorate-treated rice seeds., Conclusion: Weedy rice is one of the most damaging global weeds and a major threat to DSR systems. In this study we have developed a proof-of-concept method to allow selective weedy rice control in rice crops. We call for herbicide discovery programmes and research to identify candidate safener and herbicide combinations to achieve selective herbicide control of weedy rice and alleviate weed infestations in global rice crops. © 2016 Society of Chemical Industry., (© 2016 Society of Chemical Industry.)

Background: When used at effective doses, weed resistance to auxinic herbicides has been slow to evolve when compared with other modes of action. Here we report the evolutionary response of a herbicide-susceptible population of wild radish (Raphanus raphanistrum L.) and confirm that sublethal doses of 2,4-dichlorophenoxyacetic acid (2,4-D) amine can lead to the rapid evolution of 2,4-D resistance and cross-resistance to acetolactate synthase (ALS)-inhibiting herbicides., Results: Following four generations of 2,4-D selection, the progeny of a herbicide-susceptible wild radish population evolved 2,4-D resistance, increasing the LD 50 from 16 to 138 g ha -1 . Along with 2,4-D resistance, cross-resistance to the ALS-inhibiting herbicides metosulam (4.0-fold) and chlorsulfuron (4.5-fold) was evident. Pretreatment of the 2,4-D-selected population with the cytochrome P450 inhibitor malathion restored chlorsulfuron to full efficacy, indicating that cross-resistance to chlorsulfuron was likely due to P450-catalysed enhanced rates of herbicide metabolism., Conclusion: This study is the first to confirm the rapid evolution of auxinic herbicide resistance through the use of low doses of 2,4-D and serves as a reminder that 2,4-D must always be used at highly effective doses. With the introduction of transgenic auxinic-herbicide-resistant crops in the Americas, there will be a marked increase in auxinic herbicide use and therefore the risk of resistance evolution. Auxinic herbicides should be used only at effective doses and with diversity if resistance is to remain a minimal issue. © 2016 Society of Chemical Industry., (© 2016 Society of Chemical Industry.)

Background: Weeds can be a greater constraint to crop production than animal pests and pathogens. Pre-emergence herbicides are crucial in many cropping systems to control weeds that have evolved resistance to selective post-emergence herbicides. In this study we assessed the potential to evolve resistance to the pre-emergence herbicides prosulfocarb + S-metolachlor or pyroxasulfone in 50 individual field Lolium rigidum populations collected in a random survey in Western Australia prior to commercialisation of these pre-emergence herbicides., Results: This study shows for the first time that in randomly collected L. rigidum field populations the selection with either prosulfocarb + S-metolachlor or pyroxasulfone can result in concomitant evolution of resistance to both prosulfocarb + S-metolachlor and pyroxasulfone after three generations., Conclusions: In the major weed L. rigidum, traits conferring resistance to new herbicides can be present before herbicide commercialisation. Proactive and multidisciplinary research (evolutionary ecology, modelling and molecular biology) is required to detect and analyse resistant populations before they can appear in the field. Several studies show that evolved cross-resistance in weeds is complex and often unpredictable. Thus, long-term management of cross-resistant weeds must be achieved through heterogeneity of selection by effective chemical, cultural and physical weed control strategies that can delay herbicide resistance evolution. © 2016 Society of Chemical Industry., (© 2016 Society of Chemical Industry.)

Background: When applied at the correct plant stage and dose, herbicides are highly toxic to plants. At reduced, low herbicide doses (below the recommended dose) plants can survive and display continuous and quantitative variation in dose-survival responses. Recurrent (directional) selection studies can reveal whether such a phenotypic variation in plant survival response to low herbicide dose is heritable and leads to herbicide resistance. In a common experimental garden study, we have subjected a susceptible population of self-pollinated hexaploid Avena fatua to low-dose recurrent selection with the ACCase-inhibiting herbicide diclofop-methyl for three consecutive generations., Results: Significant differences in response to low-dose diclofop-methyl selection were observed between the selected progenies and parent plants, with a twofold diclofop-methyl resistance and cross-resistance to ALS-inhibiting herbicides. Thus, the capacity of self-pollinated A. fatua to respond to low-dose herbicide selection is marginal, and it is much lower than in cross-pollinated L. rigidum. Lolium rigidum in the same experiment evolved 40-fold diclofop-methyl resistance by progressive enrichment of quantitative resistance-endowing traits., Conclusion: Cross-pollination rate, genetic variation and ploidy levels are identified as possible drivers affecting the contrasting capacity of Avena versus Lolium plants to respond to herbicide selection and the subsequent likelihood of resistance evolution at low herbicide dose usage., (© 2015 Society of Chemical Industry.)

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 (14) C-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.)

Background: Lolium rigidum populations in Australia and globally have demonstrated rapid and widespread evolution of resistance to acetyl coenzyme A carboxylase (ACCase)-inhibiting and acetolactate synthase (ALS)-inhibiting herbicides. Thirty-three resistant L. rigidum populations, randomly collected from crop fields in a most recent resistance survey, were analysed for non-target-site diclofop metabolism and all known target-site ACCase gene resistance-endowing mutations., Results: The HPLC profile of [(14) C]-diclofop-methyl in vivo metabolism revealed that 79% of these resistant L. rigidum populations showed enhanced capacity for diclofop acid metabolism (metabolic resistance). ACCase gene sequencing identified that 91% of the populations contain plants with ACCase resistance mutation(s). Importantly, 70% of the populations exhibit both non-target-site metabolic resistance and target-site ACCase mutations., Conclusions: This work demonstrates that metabolic herbicide resistance is commonly occurring in L. rigidum, and coevolution of both metabolic resistance and target-site resistance is an evolutionary reality. Metabolic herbicide resistance can potentially endow resistance to many herbicides and poses a threat to herbicide sustainability and thus crop production, calling for major research and management efforts., (© 2015 Society of Chemical Industry.)

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 LD(50) (lethal dose) and GR(50) (growth rate) showed an eightfold glyphosate resistance in this population. The resistant index based on the estimation of seed number (SY(n50)) 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 phenotypes when 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., (© 2015 Society of Chemical Industry.)

Background: Lolium rigidum (annual ryegrass) is a widespread annual crop weed that has evolved high levels of resistance to selective herbicides. Anecdotal evidence suggests that intensive cropping also leads to higher seed dormancy in L. rigidum. This was quantified by measuring dormancy levels in L. rigidum populations collected from paired sites (one with nil to low cropping intensity, the other intensively cropped) located throughout the Western Australian grain belt., Results: Populations from non-cropped fields or those with low cropping intensity showed higher and faster germination than populations from fields with a medium- or high-intensity cropping regime. Resistance to selective herbicides was also higher in the medium- and high-intensity cropping fields than in the low-intensity cropping fields., Conclusion: High-intensity cropping systems are likely to impose greater selection pressures for seed dormancy and selective herbicide resistance, because late-emerging seedlings avoid preplanting weed control practices (tillage and non-selective herbicide application) but are exposed to selective in-crop herbicides., (© 2014 Society of Chemical Industry.)

Herbicide resistance continues to escalate in weed populations infesting global wheat (Triticum aestivum L.) crops, threatening grain production and thereby food supply. Conservation wheat production systems are reliant on the use of efficient herbicides providing low-cost, selective weed control in intensive cropping systems. The resistance-driven loss of herbicide resources combined with limited potential for new herbicide molecules means greater emphasis must be placed on preserving existing herbicides. For more than two decades, since the initial recognition of the dramatic consequences of herbicide resistance, the challenge of introducing additional weed control strategies into herbicide-based weed management programmes has been formidable. Throughout this period, herbicide resistance has expanded unabated across the world's wheat production regions. However, in Australia, where herbicide resources have become desperately depleted, the adoption of harvest weed seed control is evidence, at last, of a successful approach to sustainable weed management in wheat production systems. Growers routinely including strategies to target weed seeds during crop harvest, as part of herbicide-based weed management programmes, are now realising significant weed control and crop production benefits. When combined with an attitude of zero weed tolerance, there is evidence of a sustainable weed control future for wheat production systems. The hard-learned lessons of Australian growers can now be viewed by global wheat producers as an example of how to stop the continual loss of herbicide resources in productive cropping systems., (© 2013 Society of Chemical Industry.)

Acetohydroxyacid synthase (AHAS) inhibitor herbicides currently comprise the largest site-of-action group (with 54 active ingredients across five chemical groups) and have been widely used in world agriculture since they were first introduced in 1982. Resistance evolution in weeds to AHAS inhibitors has been rapid and identified in populations of many weed species. Often, evolved resistance is associated with point mutations in the target AHAS gene; however non-target-site enhanced herbicide metabolism occurs as well. Many AHAS gene resistance mutations can occur and be rapidly enriched owing to a high initial resistance gene frequency, simple and dominant genetic inheritance and lack of major fitness cost of the resistance alleles. Major advances in the elucidation of the crystal structure of the AHAS (Arabidopsis thaliana) catalytic subunit in complex with various AHAS inhibitor herbicides have greatly improved current understanding of the detailed molecular interactions between AHAS, cofactors and herbicides. Compared with target-site resistance, non-target-site resistance to AHAS inhibitor herbicides is less studied and hence less understood. In a few well-studied cases, non-target-site resistance is due to enhanced rates of herbicide metabolism (metabolic resistance), mimicking that occurring in tolerant crop species and often involving cytochrome P450 monooxygenases. However, the specific herbicide-metabolising, resistance-endowing genes are yet to be identified in resistant weed species. The current state of mechanistic understanding of AHAS inhibitor herbicide resistance is reviewed, and outstanding research issues are outlined., (© 2013 Society of Chemical Industry.)

Background: Problem weeds in agriculture, such as Lolium rigidum Gaud., owe some of their success to their large and dormant seed banks, which permit germination throughout a crop-growing season. Dormant weed seed banks could be greatly depleted by application of a chemical that stimulates early-season germination and then kills the young seedlings. Fluridone, a phytoene desaturase-inhibiting herbicide that can also break seed dormancy, was assessed for its efficacy in this regard., Results: The germination of fluridone-treated Lolium rigidum seeds was stimulated on soils with low organic matter, and almost 100% seedling mortality was observed, while the treatment was only moderately effective on a high-organic-matter potting mix. Seedlings from wheat, canola, common bean and chickpea seeds sown on fluridone-treated sandy loam were bleached and did not survive, but lupins and field peas grew normally., Conclusion: This proof-of-concept study with fluridone suggests that it may be possible to design safe and effective molecules that act as germination stimulants plus herbicides in a range of crop and soil types: a potentially novel way of utilising herbicides to stimulate seed bank germination and a valuable addition to an integrated weed management system., (© 2014 Society of Chemical Industry.)

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.)

Background: Plants can rapidly evolve resistance to herbicide in response to repeated selection. This study focuses on cross-resistance patterns observed in Lolium rigidum following pyroxasulfone recurrent selection., Results: The parental MR (multiresistant) population following four generations of pyroxasulfone recurrent selection evolved cross-resistance to prosulfocarb and triallate. At the recommended label rate of prosulfocarb or triallate (2000 g ha(-1) ), the progeny selected four times with pyroxasulfone (MR4) displayed 58 and 35% plant survival respectively. One additional cycle of prosulfocarb selection increased the resistance level to both prosulfocarb and triallate in the population MR4-P1. Prosulfocarb resistance is yet to be reported in L. rigidum field populations., Conclusions: This study suggests that L. rigidum plants can rapidly evolve cross-resistance to several wheat-selective herbicides under recurrent selection of a single mode of action. Weed populations displaying broad-spectrum cross-resistance to several herbicide modes of action are increasing in frequency in intensive world agriculture. Proactive and integrated measures for resistance management need to be developed globally on appropriate herbicide use in crop rotations., (© 2013 Society of Chemical Industry.)

Background: The auxinic herbicide 2,4-D amine is known, in vitro, as a cytochrome P450 inducer. The current study uses 2,4-D pre-treatment, at the whole plant level, to study mechanism(s) of non-target site based herbicide resistance to the ACCase-inhibiting herbicide diclofop-methyl in Lolium rigidum., Results: The 2,4-D pre-treatment caused up to 10-fold shift in LD50 and GR50 in dose-response to subsequently applied diclofop-methyl in a herbicide susceptible L. rigidum population. Foliar uptake and translocation of (14) C-diclofop-methyl did not differ in 2,4-D pre-treated versus untreated plants. HPLC analysis revealed that de-esterification of diclofop-methyl to toxic diclofop acid was similar, but further metabolism of diclofop acid to non-toxic metabolites was significantly (1.8-fold) faster in 2,4-D pre-treated than untreated plants. HPLC profile of major polar metabolites was similar when L. rigidum and diclofop-methyl tolerant wheat were compared, but wheat metabolised diclofop acid three-fold faster than L. rigidum. In addition, 2,4-D pre-treatment also induced cross-protection against the ALS-inhibiting herbicide chlorsulfuron, and the known P450 inhibitor malathion can reverse this effect., Conclusions: Protection against diclofop-methyl provided by 2,4-D pre-treatment in susceptible L. rigidum is associated with higher rates of herbicide metabolism, mirroring that identified in field-evolved, non-target site-based diclofop-methyl resistant populations. 2,4-D may induce higher level expression of herbicide-metabolising genes hence providing protection, and therefore, this 2,4-D induction system can be used, in combination with other genomic approaches, to assist isolating cytochrome P450 and other genes that are involved in herbicide metabolism and endow herbicide resistance in L. rigidum., (© 2013 Society of Chemical Industry.)

Background: Gene mutations that endow herbicide resistance may cause pleiotropic effects on plant ecology and physiology. This paper reports on the effect of a number of known and novel target-site resistance mutations of the ALS gene (Ala-122-Tyr, Pro-197-Ser, Asp-376-Glu or Trp-574-Leu) on vegetative growth traits of the weed Raphanus raphanistrum., Results: The results from a series of experiments have indicated that none of these ALS resistance mutations imposes negative pleiotropic effects on relative growth rate (RGR), photosynthesis and resource-competitive ability in R. raphanistrum plants. The absence of pleiotropic effects on plant growth occurs in spite of increased (Ala-122-Tyr, Pro-197-Ser, Asp-376-Glu) and decreased (Trp-574-Leu) extractable ALS activity., Conclusion: The absence of detrimental pleiotropic effects on plant growth associated with the ALS target-site resistance mutations reported here is a contributing factor in resistance alleles being at relatively high frequencies in ALS-herbicide-unselected R. raphanistrum populations., (© 2012 Society of Chemical Industry.)

Background: Glyphosate resistance in populations of the C(4) perennial Sorghum halepense (Johnsongrass) and C(3) 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., 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 sub-optimal [corrected] 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, (Copyright © 2012 Society of Chemical Industry.)

Background: Wild radish, a problem weed worldwide, is a severe dicotyledonous weed in crops. In Australia, sustained reliance on ALS-inhibiting herbicides to control this species has led to the evolution of many resistant populations endowed by any of several ALS mutations. The molecular basis of ALS-inhibiting herbicide resistance in a novel resistant population was studied., Results: ALS gene sequencing revealed a previously unreported substitution of Tyr for Ala at amino acid position 122 in resistant individuals of a wild radish population (WARR30). A purified subpopulation individually homozygous for the Ala-122-Tyr mutation was generated and characterised in terms of its response to the different chemical classes of ALS-inhibiting herbicides. Whole-plant dose-response studies showed that the purified subpopulation was highly resistant to chlorsulfuron, metosulam and imazamox, with LD₅₀ or GR₅₀ R/S ratio of > 1024, > 512 and > 137 respectively. The resistance to imazypyr was found to be relatively moderate (but still substantial), with LD₅₀ and GR₅₀ R/S ratios of > 16 and > 7.8 respectively. In vitro ALS activity assays showed that Ala-122-Tyr ALS was highly resistant to all tested ALS-inhibiting herbicides., Conclusion: The molecular basis of ALS-inhibiting herbicide resistance in wild radish population WARR30 was identified to be due to an Ala-122-Tyr mutation in the ALS gene. This is the first report of an amino acid substitution at Ala-122 in the plant ALS that confers high-level and broad-spectrum resistance to ALS-inhibiting herbicides, a remarkable contrast to the known mutation Ala-122-Thr endowing resistance to imidazolinone herbicide., (Copyright © 2012 Society of Chemical Industry.)

Background: Bromus rigidus is a common weed species that has increased in cropping fields owing to limited control options. During a random field survey in Western Australia, six B. rigidus populations that had survived in-crop weed control programmes were collected. The study aimed to determine the resistance profile of these six populations., Results: Based on dose-response studies, all six B. rigidus populations had a low-level resistance to sulfosulfuron and sulfometuron (both sulfonylurea herbicides) while remaining susceptible to herbicides with other modes of action. ALS in vitro activity assays revealed no differences in enzyme sensitivity between susceptible and resistant populations, while the use of malathion (a cytochrome P450 inhibitor) in combination with sulfosulfuron caused the resistant populations to behave like the susceptible population., Conclusion: This study established that these six B. rigidus populations have a low-level resistance to the ALS-inhibiting sulfonylurea herbicides, but are able to be controlled by other herbicide modes of action. The low-level, malathion-reversible resistance, together with a sensitive ALS, strongly suggest that a non-target-site enhanced metabolism is the mechanism of resistance., (Copyright © 2012 Society of Chemical Industry.)

Background: Hordeum populations are becoming increasingly difficult to control in cropping fields. Two herbicide-resistant H. leporinum populations were identified during a random crop survey after herbicides were applied. The study aimed to determine the herbicide resistance profile of these H. leporinum biotypes to a range of herbicides used for their control., Results: Based on dose-response studies, one H. leporinum population was very highly resistant to sulfosulfuron and sulfometuron (both sulfonylurea herbicides) and also displayed low-level resistance to imazamox (an imidazolinone herbicide). Reduced sensitivity of the ALS enzyme was identified with in vitro activity assays. Gene sequence analysis revealed a proline-to-threonine substitution at amino acid position 197 of ALS, which is likely to be the molecular basis for resistance in this population. Herbicide screening also revealed a different H. leporinum population with resistance to the bipyridyl herbicide paraquat., Conclusion: This study established the first cases of (1) sulfonylurea-to-imidazolinone cross-resistance and (2) field-evolved paraquat resistance in a Hordeum species in Western Australia., (Copyright © 2012 Society of Chemical Industry.)

Background: In a large cropping area of northern Argentina, Sorghum halepense (Johnsongrass) has evolved towards glyphosate resistance. This study aimed to determine the molecular and biochemical basis conferring glyphosate resistance in this species. Experiments were conducted to assess target EPSPS gene sequences and (14)C-glyphosate leaf absorption and translocation to meristematic tissues., Results: Individuals of all resistant (R) accessions exhibited significantly less glyphosate translocation to root (11% versus 29%) and stem (9% versus 26%) meristems when compared with susceptible (S) plants. A notably higher proportion of the applied glyphosate remained in the treated leaves of R plants (63%) than in the treated leaves of S plants (27%). In addition, individuals of S. halepense accession R(2) consistently showed lower glyphosate absorption rates in both adaxial (10-20%) and abaxial (20-25%) leaf surfaces compared with S plants. No glyphosate resistance endowing mutations in the EPSPS gene at Pro-101-106 residues were found in any of the evaluated R accessions., Conclusion: The results of the present investigation indicate that reduced glyphosate translocation to meristems is the primary mechanism endowing glyphosate resistance in S. halepense from cropping fields in Argentina. To a lesser extent, reduced glyphosate leaf uptake has also been shown to be involved in glyphosate-resistant S. halepense., (Copyright © 2011 Society of Chemical Industry.)

Background: In the important grass weed Lolium rigidum (Gaud.), resistance to ALS-inhibiting herbicides has evolved widely in Australia. The authors have previously characterised the biochemical basis of ALS herbicide resistance in a number of L. rigidum biotypes and established that resistance can be due to a resistant ALS and/or enhanced herbicide metabolism. The purpose of this study was to identify specific resistance-endowing ALS gene mutation(s) in four resistant populations and to develop PCR-based molecular markers., Results: Six resistance-conferring ALS mutations were identified: Pro-197-Ala, Pro-197-Arg, Pro-197-Gln, Pro-197-Leu, Pro-197-Ser and Trp-574-Leu. All six mutations were found in one population (WLR1). Each Pro-197 mutation conferred resistance to the sulfonylurea (SU) herbicide sulfometuron, whereas the Trp-574-Leu mutation conferred resistance to both sulfometuron and the imidazolinone (IMS) herbicide imazapyr. A derived cleaved amplified polymorphic sequences (dCAPS) marker was developed for detecting resistance mutations at Pro-197. Furthermore, cleaved amplified polymorphic sequences (CAPS) markers were developed for detecting each of the six mutant resistant alleles. Using these markers, the authors revealed diverse ALS-resistant alleles and genotypes in these populations and related them directly to phenotypic resistance to ALS-inhibiting herbicides., Conclusion: This study established the existence of a diversity of ALS gene mutations endowing resistance in L. rigidum populations: 1-6 different mutations were found within single populations. At field herbicide rates, resistance profiles were determined more by the specific mutation than by whether plants were homo- or heterozygous for the mutation.

Since its commercial introduction in 1974, glyphosate [N-(phosphonomethyl)glycine] has become the dominant herbicide worldwide. There are several reasons for its success. Glyphosate is a highly effective broad-spectrum herbicide, yet it is very toxicologically and environmentally safe. Glyphosate translocates well, and its action is slow enough to take advantage of this. Glyphosate is the only herbicide that targets 5-enolpyruvyl-shikimate-3-phosphate synthase (EPSPS), so there are no competing herbicide analogs or classes. Since glyphosate became a generic compound, its cost has dropped dramatically. Perhaps the most important aspect of the success of glyphosate has been the introduction of transgenic, glyphosate-resistant crops in 1996. Almost 90% of all transgenic crops grown worldwide are glyphosate resistant, and the adoption of these crops is increasing at a steady pace. Glyphosate/glyphosate-resistant crop weed management offers significant environmental and other benefits over the technologies that it replaces. The use of this virtually ideal herbicide is now being threatened by the evolution of glyphosate-resistant weeds. Adoption of resistance management practices will be required to maintain the benefits of glyphosate technologies for future generations., (Copyright (c) 2008 Society of Chemical Industry.)

Glyphosate is the world's most important herbicide, with many uses that deliver effective and sustained control of a wide spectrum of unwanted (weedy) plant species. Until recently there were relatively few reports of weedy plant species evolving resistance to glyphosate. Since 1996, the advent and subsequent high adoption of transgenic glyphosate-resistant crops in the Americas has meant unprecedented and often exclusive use of glyphosate for weed control over very large areas. Consequently, in regions of the USA where transgenic glyphosate-resistant crops dominate, there are now evolved glyphosate-resistant populations of the economically damaging weed species Ambrosia artemissifolia L., Ambrosia trifida L., Amaranthus palmeri S Watson, Amaranthus rudis JD Sauer, Amaranthus tuberculatus (Moq) JD Sauer and various Conyza and Lolium spp. Likewise, in areas of transgenic glyphosate-resistant crops in Argentina and Brazil, there are now evolved glyphosate-resistant populations of Sorghum halepense (L.) Pers and Euphorbia heterophylla L. respectively. As transgenic glyphosate-resistant crops will remain very popular with producers, it is anticipated that glyphosate-resistant biotypes of other prominent weed species will evolve over the next few years. Therefore, evolved glyphosate-resistant weeds are a major risk for the continued success of glyphosate and transgenic glyphosate-resistant crops. However, glyphosate-resistant weeds are not yet a problem in many parts of the world, and lessons can be learnt and actions taken to achieve glyphosate sustainability. A major lesson is that maintenance of diversity in weed management systems is crucial for glyphosate to be sustainable. Glyphosate is essential for present and future world food production, and action to secure its sustainability for future generations is a global imperative., (Copyright (c) 2008 Society of Chemical Industry.)

Background: The acetolactate synthase (ALS)-inhibiting herbicide sulfosulfuron is registered in Australia for the selective control of Hordeum leporinum Link. in wheat crops. This herbicide failed to control H. leporinum on two farms in Western Australia on its first use. This study aimed to determine the level of resistance of three H. leporinum biotypes, identify the biochemical and molecular basis and develop molecular markers for diagnostic analysis of the resistance., Results: Dose-response studies revealed very high level (>340-fold) resistance to the sulfonylurea herbicides sulfosulfuron and sulfometuron. In vitro ALS assays revealed that resistance was due to reduced sensitivity of the ALS enzyme to herbicide inhibition. This altered ALS sensitivity in the resistant biotypes was found to be due to a mutation in the ALS gene resulting in amino acid proline to serine substitution at position 197. In addition, two- to threefold higher ALS activities were consistently found in the resistant biotypes, compared with the known susceptible biotype. Two cleaved amplified polymorphic sequence (CAPS) markers were developed for diagnostic testing of the resistant populations., Conclusion: This study established the first documented case of evolved ALS inhibitor resistance in H. leporinum and revealed that the molecular basis of resistance is due to a Pro to Ser mutation in the ALS gene., (2007 Society of Chemical Industry)