505 ¸ois ber 2011 CatonaHotTin 4 2011 Society of Chemical Industry c With some modes of action the delay between 5 , ‘sometimes I think a vacuum is a hell of a lot better than the Correspondence to: Stephen O Duke, United StatesAgricultural Department Research of Agriculture, Service, Natural ProductsBox Utilization 8048, University, Research MS 38677, Unit, USA. PO E-mail: [email protected] AgriculturalResearchService,NaturalProductsUtilizationResearchUnit,United States Department of Agriculture, University of Mississippi, Oxford, MS, USA Since the advent of synthetic in the mid-twentieth ∗ this question. Theespoused views by Gerwick in expressed his recent are short review. quite similar to those 2 WHYARE NEW NEEDED MODESAccording OF to the ACTION sixteenth-century physician and writer Franc Rabelais, ‘Nature abhorswith a the ecological vacuum’. vacuum caused This bywhere highly any weed is efficient that herbicides, can certainly inhabit theformed the unoccupied by ecological a niche case highly efficientof herbicide has reduced the competition. marked advantage However,Williams as wrote the in playwright one Tennessee Roof of Big Daddy’sstuff that lines nature replaces in it with’. Inweeds the case occupying of the herbicide-resistant ecologicalfarmers vacuum would agree created with by Big Daddy’s herbicides, sentiments. century, weeds have readily evolved to fill theby ecovacuums formed herbicides. introduction of the herbicideonly and 3 evolved resistance orwhereas has 4 taken years with [e.g. others acetolactate synthase the (ALS) evolution inhibitors], of resistance has taken reported 137 new herbicide 4 ∗ : 505–512 www.soci.org 68 2012; ; herbicide; mode of action; resistance

2011 Society of Chemical Industry c active ingredients introduced frommodes 1980 of action to of 2009. herbicidesof They introduced some before all 1990. older had The herbicidesof uses and action new have herbicides sometimesherbicide been with safeners, expanded old a through modes fungicides. strategy Gerwick’s analysis the of unavailable 2009 use US, Japanese and for of PCTapplications patent found insecticides that the total and composition of matter patentsinsecticides for (57), fungicides (78) and herbicidesthat (51) much. did not However, differ thewere patents not obvious in from the which chemical structure the were quiteten modes different: insecticides, of action tenherbicides fungicides launched or scheduled and for launch only during 2010–2012target two all old target herbicides. sites (aminocyclopyrachlor, New anpyroxasulfone, auxinic molecule; a very-long-chainbicyclopyrone, fatty a acid hydroxyphenylpyruvate synthesis dioxygenaseindaziflam, inhibitor; inhibitor; a cell wall biosynthesislagging inhibitor). behind in Why introducing are new herbicides modes ofthe action, prospects and for what a are major new modethe of action future? being This introduced in review sets out the author’s thoughts regarding Pest Manag Sci 1 INTRODUCTION No new major herbicide mode ofcommercial action herbicide has active been ingredient introduced in in the last a this 20 drought, years. a Before new mode of action wasevery introduced 3 approximately years, leading tomodes current of use action of (Table 1). approximately The 20 samefungicides, known is for not true which of insecticides major and receptor modes insecticides of and action fungicides such suchattack as as the the ryanodine Qol strobilurins that bindingwithin site the past of two decades. respiration Gerwick have been introduced Abstract Herbicides with new modes ofYet action are no badly major needed to newreasons manage mode for the this. of evolution New of action potential resistancehave products has of may reduced weeds been have to the introduced remained existing dormant herbicides. market toGR owing for the crops to market a led concerns that place to newalso glyphosate-resistant significantly for herbicide. (GR) due diminished crops about to The company herbicide 20 capture years. consolidationsherbicide discovery of and molecular There efforts. target the a are Some sites availability may large probably of of haveare more fraction several the already inhibitors generic been of that reduced discovered. herbicides. are herbicide the However, Another highly target discovery problemdiscovery effective herbicide sites at might (e.g. research that market killing be are gene was plants, by not that knockout suggests utilized, the glyphosatesites that for methods) best coming this which with are from is there these mostly not approaches. true. not Inhowever, Results summary, public, the of there relative but modern are magnitude several methods there of reasons of each is forherbicide-resistant is target a no unclear. weeds, site long The especially evidence dry economic GR period stimulus of weeds, for to goodtoo may new the distant herbicide herbicide result herbicides future. industry target in with caused sites; one by new or the target more evolution of new modes of action becoming available in the not Keywords: appeared in recent years? Stephen O Duke Why have no new herbicide modes of action (wileyonlinelibrary.com) DOI 10.1002/ps.2333 Perspective Received: 2 September 2011 Revised: 21 September 2011 Accepted article published: 12 October 2011 Published online in Wiley Online Library: 22 Decem 14 Since 7 : 505–512 However, 68 11,12 2012; Target site Pest Manag Sci Alteration of more than one codon, GR soybeans were introduced. 2 13 Adoption of this technology provided 10 Very long-chain fatty acid synthase Protoporphyrinogen oxidase Tubulin Acetyl-CoA carboxylase D-1 of PSII 7,8- Other sites related to microtubule organization Accepts protons from PSI Membrane disruptors Serine/threonine protein phosphatases Phytoene desaturase Cellulose synthase Hydroxyphenylpyruvate dioxygenase ABCB proteins Deoxyxylulose-5-phosphate synthase Glutamine synthetase F-box proteins EPSPS Acetolactate synthase 12 17 13 11 6 9 7 15 5 10 14 8 16 19 2 18 1 4 3 Many of the farmers who adopted this technology 8,9 From a biological standpoint, glyphosate-resistant (GR) crops Natural site-of-action mutations can give a more resistant form herbicides with differentfarmers modes have run of out of action. cost effectiveeffective and/or In herbicide even options. technologically some situations, have made the situationfirst worse. GR weed In was 1996, reported, the same year that the then, the adoption of GR soybean, maize,beet cotton, has alfalfa been and sugar rapid, nowthan accounting for 1 about billion 85% accumulated ofworldwide. hectares the more of transgenic crops grown as was done byversion site-directed mutagenesis of to EPSPS, produce provides the a GA21 much higher level of resistance. the extensive selectionglyphosate pressure resistance. exacerbated the evolution of of EPSPS, but a onea codon high change level in of the gene resistance. does not provide several environmental and toxicological benefits. used it yearfor after weed year, mangement. with glyphosate as the only herbicide The numbers of the target sites are given in bold 6 1–3 www.soci.org SO Duke 2011 Society of Chemical Industry c Even in these cases, there were usually effective 6 Modes of action of currently used commercial herbicides. others sulfonylamino-carbonyl-triazolinones There are now many examples of multiple resistance, as well as Before the 1990s, farmers could rely on new mechanisms of Cellulose synthesis Nitriles, isoxaben, alylazines and flupoxam Folate synthesis Lipid synthesis Aryloxyphenoxypropionates, cyclohexanediones and pinoxaden Acetamides, oxyacetamides, chloroacetamides, tetrazolinines and others Mitosis Dinitroanilines, benzamides, , phosphoroamidates and DCPA Carbamates Carotenoid synthesis Trifluroheterocyclic compounds such as fluridone and norflurazon Triketones, isoxazones and benzoylpyrazoles Photosynthesis Triazines, , nitriles and many others Bipyridyliums transport inhibition Naptalam and diflufenzopyr-sodium Porphyrin synthesis Diphenyl ethers, phenylpyrazoles, thiadiazoles, pyrimidinediones and others Auxin receptors carboxylic acids, benzoates, phenylcarboxylic acids, quinolinecarboxylic acids and Protein phosphatase Uncoupler Dinitrophenols Herbicide or herbicide class Sulfonylureas, imidazolinones, triazolpyrimidines, pyrmidinyl(thio)benzoates and Amino acid Glyphosate Table 1. fields with mixtures of weed species with resistance to different herbicides that were not degraded by these weed biotypes. wileyonlinelibrary.com/journal/ps Thus, applying a herbicide withwould generally a eliminate different the mechanism herbicide-resistant weeds. of action action being introducedresistance every could few usuallymultiple be years. resistance, easily based Thus, on countered, site-of-action site-of-actionherbicide even target mutations in sites of within several cases theof of cross-resistance same based species. on There metabolic were degradationwith of cases herbicides different modes ofor action more genes owing for toxenobiotics. a involved single in gene metabolic or alteration two of decades [e.g. glyphosate, the inhibitor of 5-enolpyruvylshikimate- 3-phosphate synthase (EPSPS)]. The lag time between introduction and the first reportedfunction case of of many evolvedpressure, existing resistance factors, genetic is diversity including asite. and The complex mechanism the weed of nature resistance is of species, usually an the altered target selection target site.

506 507 27 25 The 23 5 per year . 5

∼ Issued US herbicide patents per year per patents herbicide US Issued 4 500 400 300 200 100 0 wileyonlinelibrary.com/journal/ps Also, during the past two decades, 24 2 per year after 2001) (Fig. 1). ∼ were thought by many to be controllable with ca 2/year 26 US herbicide patents might be eventually introduced, companies reduced 28 Introduction of glyphosate- resistant crops 24 The reduction in the price of glyphosate after the patent expired Before the evolution of GR weeds began to threaten this The second economic factor that has slowed mode-of-action < exacerbated the situation. Weed species shifts to those withto a glyphosate low level of natural resistance and rice 3 THECHANGINGECONOMICSOFHERBICIDEDISCOVERY The fact that there havefor been over two no decades new is partly herbicide due to modesThe three of major first economic action factors. of these isreliance the on adoption glyphosate alone of by GR farmers crops.combinations who had There of previously several was used herbicides massive for weedGR management crops. before This led touse the substantially devaluation of decreased other in herbicides, cotton, as soybean their and maize. their discovery effortsfor for insecticides new and fungicides. herbicides Thisdecrease in in can herbicide patents be in relation less seen than 5 in to yearswere after the GR introduced, those soybeans dramatic which was followednumber by of a herbicide active clear ingredients reduction introduced in ( the the patents for manyfurther other devaluation of herbicides the herbicide have market by expired, generic herbicides. leading to technology, companies involved in pesticide discoveryreason had to good think thatfor the decades. utility At of thisof GR time, GR crops the weeds would general remainevolution would of view high be very was a low that levels very evolution of minor resistance, if problem, it with happened very at all. slow price of seed withthan the that glyphosate of conventional resistance seed, gene(s) so aof was significant weed higher portion control of was the shifted cost fromadded herbicides to to a the ‘technology price fee’ cost of of superior seeds. weed Even management was withcrops. generally the reduced by technology GR fee, the before 2001 and only discovery involves the consolidation of the pesticide discovery higher rates ofmarket glyphosate. for three Considering major crops a and the devalued possibility that herbicide GR wheat Year or 5,20 andGR 15,16 19 ca 5.5/year 2011 Society of Chemical Industry A. tuberculatis c concluded that Weeds with these 21 . can be expected. 17,18 Cumulative AI introductions et al population. In addition Of 18 fields sampled, all Tranel 21 5 HPPD was the last new target site individuals with resistance to ALS, A. tuberculatis 1975 1980 1985 1990 1995 2000 2005 2010 2015 0 22

80 60 40 20

160 140 120 100 A. tuberculatis is only one of several weed species with introductions AI herbicide new Cumulative : 505–512 68 2012; A. tuberculatis US herbicide patents and new active ingredient introductions over the past 30 years. Redrawn from Gerwick. Amaranthus tuberculatus There are now fields in the mid-western United States that In addition to the problem with this species within these fields, New modes of herbicide action www.soci.org have protoporphyrinogen oxidase inhibitorsin (PPO) every and possible glyphosate combination. there is an urgent need formanagement new paradigm. herbicide As options efficacy or of athe herbicides new evolution is weed of lost herbicide owing resistance, to herbicidesof with action new are modes neededolder more herbicides than with ever. unique modes Furthermore, ofthe some action of marketplace are the being in lost from of some countries or infactors. states some (e.g. European the countries) banning owing to regulatory Figure 1. Pest Manag Sci However, such double mutationsoccurred of in EPSPS nature. The have most apparentlymechanisms resistant not weed of biotypes have resistance other based on gene amplification two mechanisms of action have beenplaces highly where problematic GR in crops some have beenyears,suchasGRcottoninthesoutheasternUnitedStates grown continuously for several sequestration of glyphosate in the vacuole. fields withALS glyphosate inhibitor resistance resistance,modes and (10 of action four out withto had the of resistance these to 18) target-sitephotosystem all II also resistances, (PSII)-inhibiting three non-target-sitespecies herbicides had in is resistance some also of to population found these in fields. with One this resistance fieldall had to four an allthis resistances four field. were herbicide Thisfarmers’ level found classes, options for of effective and and in economical multiple weedFurthermore, management. resistance some some greatly populations individuals of reduceshave this within the evolved species resistance in to the(HPPD)-inhibiting hydroxyphenylpyruvate same herbicides. dioxygenase area soybeans in the mid-southern and mid-western United States. introduced for herbicides. Stackingother of herbicide resistances HPPD of resistance with the there are other weedresistances. species with their own arrays of herbicide resistance to multiple herbicides. : 505–512 68 2012; 35 inhibition is needed Nevertheless, when the values of the six enzymes i 32 in vivo Pest Manag Sci describe a number of natural in vitro K 34 This is partly because ALS is a low-abundance . 1 36 The best ALS inhibitor herbicides are effective at − but the most ideal herbicide thus far introduced, does not fit these chemical parameters. 36 31 32 Desirable properties of a herbicide target site and a recent review 33 . lethality properties compatible with good uptake and translocation Good inhibitors with similar Low-use-rate herbicides are more desirable than those with Clearly, there are many more potential target sites for herbicides One criterion for a mode of action is that the target site not Table 2. Critical for plant survival No similar target in non-target organisms, particularlySubject mammals to irreversible inhibition Inhibition of a relatively small percentage of the target causes Inhibition causes accumulation of a toxin No alternative isozymes or pathways around theInhibitor target that binds compounds with physicochemical of the branched-chainbut amino only acid inhibitors pathwaycommercialized. of have acetolactate been synthease found, (ALS) have been higher use rates, andrate the of target a site herbicide. stronglyinexpensive, However, influences if the the a use use herbicide rateglyphosate is is may safe, used be effective at and lessmost relatively perfect important. high herbicide For rates, yet example, discovered. but it is perhaps the lessthan1gha glyphosate, than are currently beinget used. al The companion paper by Dayan phytotoxin target sites that areherbicides. In different a to few those cases, these ofcommercialherbicides.However,asignificantfractionofthetarget compounds commercial are as active as some sites of natural phytotoxinstoxicity are and even suspect general cytotoxicity. in Furthermore, many terms ofexcellent these of natural mammalian phytotoxins areeconomical too structurally and/or complex lack to therequired be for sufficient proper uptake, translocation physicochemical and/or environmental properties stability. be one that is presentcell. in For relatively example, great it abundancedark in might reactions the of be photosynthesis plant thought would be thatsites, but good the these herbicide enzymes target enzymes are found of atto high the concentrations process in order the largeFor amount example, of ribulose-1,5-bisphosphate carbon(RuBisCo) dioxide carboxylase is fixed oxygenase the by most plants. abundantfact protein that in it green plant ishave cells. even a larger The amounts rather of inefficientthis it. Inhibiting enzyme only in requires a small target theper fraction plants of plant hectare. would to A take numberassimilation, kilograms including of of RuBisCo, inhibitors herbicide have ofherbicides, but been the none studied has enzymes been as commercialized. of potential carbon are exceptions to almost anyapart criterion from the that essential one might – be that thefor formulated, molecular target plant is survival. essential For example,been chemical proposed descriptor for criteria have a predicting herbicide, whether a compound might be enzyme that isinactivation rate by ALS limiting inhibitoreffectiveness. herbicides in Still, also 60–80% the contributes inhibition to is pathway.enzyme their required that for The precedes lethality. The ALS, irreversible ketol-acid reductoisomerasepresent (KARI), in is higher amounts, and 95% to kill a plant. www.soci.org SO Duke 2011 Society of Chemical Industry c Year This is partly due to the fact 30 not only has the cost of getting a 29 4 Examination of the changes in the number 29 1970 1975 1980 1985 1990 1995 2000 2005

Attrition of companies involved in herbicide discovery. Drawn 0

50 40 30 20 10

discovery research discovery Number of companies doing herbicide doing companies of Number Lastly, the cost of discovery, development and meeting To paraphrase Gerwick, regulatory requirements hassynthetic increased. pesticide The to cost the2000 market of increased to bringing from $184 $246 a million million in in 2008. of members andto membership which affiliations herbicide of discoveryconsolidation scientific of scientists this industry societies belong has resulted reveals inin tremendous that attrition the the numbereven of before scientists GR engagedphilosophies crops in between were this introduced. different typein Different companies, of cultures which different research, and countries thosebetween sites within companies the same company maximized operated invention. The contribution of and this diversity to discovery cultural is somethingbe that differences cannot quantified. The reductionfew in remaining diversity research between groupsstagnation the has in probably the relatively growth contributed in numbers to of herbicide the modes of action. wileyonlinelibrary.com/journal/ps From what is knowncommercial of herbicide the active ingredients, target there sitesto is and this no modes question. easy answer One oftarget can action site list of (Table desirable 2). properties However, of there is a no herbicide rule of thumb, as there 4 WHATSITE? MAKES A GOOD HERBICIDE TARGET industry over the pastherbicide 50 modes years. The ofthere 20 action or were so were manyresearch currently discovered companies utilized compared at involved withgraphically a in in the Fig. time 2. herbicide past when discovery 20 years. This can be seen Figure 2. from data provided by Appleby. that many moreviable compounds active must ingredient than be in themay evaluated past. have All to been the low-hanging picked discover fruit already.toxicological a Furthermore, and after environmental hurdles discovery, that the mustincreasingly be higher cleared and, are thus, more costly.partially This increased fueled cost the has mergerhave of also made pesticide it attractive companies. to introduce Theseold familiar compounds costs modes with ofpredictable action than for for which anmode the entirely of results action new and of an chemical unpredictable testing class toxicity profile. with are a more new new to market risen, butmarket getting has the been new of less product value on owingand the to generic glyphosate-resistant herbicides. crops

508 509 49 45 wileyonlinelibrary.com/journal/ps 2,4-D Cinmethylin 43 Examples of studies to determine the modes of action of 39 metabolomics Going from the most fundamental to phenotypic re- There are several difficulties with this approach. First, the effort Other approaches to mode-of-action discovery include gene GenomicsGenomicsGenomics 2,4-D Flufenacet and 48 47 46 GenomicsGenomics Glyphosate Cinidon-ethyl, tribenuron-methyl, 44 Table 3. phytotoxins by ‘omics’ approaches Approach(es)MetabolomicsMetabolomicsMetabolomics AscaulitoxinPhysionomics and Pyrenophorol Several herbicides Phytotoxin Reference 40 42 41 sponses, ‘omics’ approaches include transcriptomics, proteomics, metabolomics, physionomics andthese phenomics. approaches have The been used lastwere for ushered in many two with years. the introduction of The of first powerfuladvances genomics three in tools, proteomics methods andties powerful for analytic quantitative capabili- analysis ofextracts. hundreds Table of 3 metabolites provides inproaches plant to examples determine new of modes papers ofcompany action. data using Herbicide discovery are these generally ap- formation not is published, only a so hint the ofgenerally what published has not in- been resulted done. Such in studiesaction. discovery have However, of even a if published astudy new eliminates new mode known mode target of of sites and actionthe provides is actual clues target as not site to found, is. what the and cost infor generating transcriptomics through a metabolomics.times robust are Dose problems, database in and that comparing sampling isdifferent two modes compounds high, with of very especially action,PPO for inhibitor) example and one another thatdifficult. that acts All fast acts lethal (e.g. phytotoxins slowly eventually a and (e.g. cause glyphosate), tertiary many is secondary effectsresearchers that use can sublethal beindicators doses of confounding. the and mode of Therefore, action hope soon after most with that treatment. Compounds multiple target there sites will are also clear provide confounding results. expression manipulation, such as overexpression, and reducingeliminating or expression via knockout or gene silencing methods. Several potential herbicide target sites have beenantisense identified or using RNAi technologiesare (Table needed 4). to get Calibrational realistic methods answers,of as the herbicides target-site never molecules. These inhibit methods all to can knock be out used gene partially expression. Findingis a just new potential the target first site may step. be more Discovery daunting than of finding the a site. Indeed, good in spite herbicide of the for that site Using this strategy, a(both library commercial of and experimental) response with known profiles modesis to of first action compounds generated. Then, the responsean unknown profile mode of to action a is produced compound and with comparedof with those known mode ofprofile actions. of any If of the a known modes new ofit action, compound it has can does be a assumed not that new mode fitclues. of the action, for which the profile might provide 2011 Society of Chemical Industry c creens. At the time 36 15,16 ProtoIX is very toxic in the the limitation for obtaining Toxic intermediates do not in vitro s 37 36 38 screening. This approach has been In some cases, the lack of knowledge inhibitors are discovered. 2 but that are poorly or not translocated are in vitro : 505–512 in vitro 68 in vitro 2012; More recently, ‘omics’ approaches have been used to determine As stated by Wittenbach and Abell, A desirable feature of a target site is that inhibition of a relatively Another approach has been to pick a new molecular target -ketobutryrate, may play a role in the toxicity of acetolactate New modes of herbicide action www.soci.org the mode of action of phytotoxins with unknown modes of action. small part of the pool of theoxidase enzyme is (PPO) lethal. Protoporphyrinogen has thissubstrate moving property. to parts of Inhibition the cell of whereproduct, it PPO protoporphyrin is oxidized IX to leads (ProtoIX). the PPO to the accumulate when other enzymessynthesis of branched-chain are amino inhibited. acid Thisinhibitors may of be the anotheramino enzymes acid reason pathway prior why are not good to good herbicides. ALS in the branched-chain site without knowingefficient inhibitors its with potential for herbicides and to find may be due to morebut than the one target lack site of forherbicides interest the that in same have herbicide, exploring relatively little themajor market cause modes of share of this is knowledge action probably gap. the of old facilitated by evaluating vast combinatorialcompounds chemistry using libraries of high-throughput that this strategy was first implemented,companies most pesticide discovery werethis affiliated method with forindustry pharmaceutical companies has used discovery. this approach that successfully. The There isclaim no pharmaceutical were published that any using commercialdiscovered herbicide in mode of thisdiscovery efforts action with way. this has approach been did Thisthe not have pharmaceutical may the discovery resources endeavors of be thatmethod used because on it. unvalidated To the use targeteven this sites when herbicide excellent may often be unsuccessful, presence of light and molecular oxygen,Other causing enzymes rapid of cell the death. porphyrintargets, pathway presumably are because not the good unique herbicide accumulationphotodyamic of a porphyrin highly intermediatecellular (ProtoIX) compartment in does a notThere vulnerable occur is some when evidence theyα that are accumulation inhibited. ofsynthase a (ALS)-inhibiting herbicides. toxic , Pest Manag Sci 5 CURRENTDISCOVERY APPROACHES TO HERBICIDE The traditional methodto of discover discovering a adetermine its mode mode herbicidal of action of using or physiologicalapproaches. and action biochemical phytotoxic The was mode compoundintroduced of before and 1980 action then werewas of marketed. The understood modes relatively of before action of the fewdiscovered these herbicides by herbicide were public herbicides often sector scientists. Thereherbicides are that still have several unknown older modesarsenical herbicide of MSMA). action (e.g. the organic number of molecules of EPSPSamplification, is the amount significantly of increased glyphosate bybecomes needed gene too for high effective to use be used in the field. new herbicide modes of action is not due to failuresinhibitors in finding of good essential plantof enzymes, but finding rather sensitive, to lethalgenerally the those difficulty that target translocate to sites. target sites Themeristematic that tissue. are best Thus, essential to compounds herbicides that are are such good target sites inhibitors of not good herbicides. . , Proc et al . Weed [Online]. :470–478 : 505–512 64 68 Pest Manag Sci Comprehensive J Environ Qual 2012; , ed. by Nandula VK. Amaranthus palmeri Pest Manag Sci [Online]. Herbicide Resis- Herbicide Resistance in Plants, (Silver Jubilee Edition): VII–IX :5793–5798 (2011). :86–95 (2011). :346–357 (2009). 59 99 Pest Manag Sci Agrow 12 4 , ed. by Powles SB and Holtum JAM. CRC :1029–1034 (2010). , 2nd edition, ed. by Moo-Young M. Elsevier, AgBioForum 107 J Agric Food Chem Pestic Biochem Physiol , Benchmark study on glyphosate-resistant cropping Global Status of Commercialized Biotech/GM Crops: 2010 :108–117 (2009). Herbicide-resistant crops likely to be introduced within the International Survey of Herbicide Resistant Weeds. et al 57 Glyphosate Resistance in Crops and Weeds :771–780 (2011). :1633–1658 (2006). Gene amplification confers resistance in Natl Acad Sci USA (2008). molecular basis of evolved glyphosatein resistance in weed species, Wiley, Singapore, pp. 119–140 (2010). Sci DL, systems in the United States.and Part effect on 4: weed Weed populations management and67 soil practices seedbanks. impacts of glyphosate-resistant35 crops: a review. acute mammalian toxicity: simulatingglyphosate-resistant an weeds in environmental the USA. effect of ISAAA Brief 42–2010, ISAAA, Ithaca, NY (2010). management. Biotechnology, Vol. 4 Amsterdam, The Netherands, pp. 23–35 (2011). on thetance Mode Actioncom/Publications/WorldofHerbicidesMap/tabid/354/Default.aspx of Committee.[15 November Action 2011]. Available: 2010. http://www.hracglobal. plant cell cycle by theinhibitor. herbicide endothall, a protein phosphatase Available: http://www.weedscience.org/In.asp [16 August 2011]. resistance and multiple resistance, in Biology and Biochemistry Press, Boca Raton, FL, pp. 243–261 (1994). and in the future. and contemplating the future. (2010). The World of Herbicides, according to HRAC Classification Table 5. next 5 years. Data are from Gewick CropMaizeMaizeMaize 2,4-D/aryloxyphenoxys Herbicide(s)Soybean ALS inhibitors Auxin mimicSoybean and ACCase ALS inhibitorsSoybean 2,4-D/glufosinate ModeSoybean of action Isoxaflutole or target site Soybean Glufosinate/isoxaflutole ALS Cotton Auxin GS/HPPD mimic/GS ALS Cotton Dicamba/glufosinate Auxin mimic 2,4-D HPPD Auxin mimic/GS HPPD Auxin mimic 9 Duke SO, Comparing conventional- and biotechnology-based pest 8JamesC, 1 Duke SO and Dayan FE, Bioactivity of herbicides, in 6 Hall LM,Holtum JAMandPowles SB,Mechanisms responsible for cross 7 Duke SO and Powles SB, Glyphosate-resistant crops and weeds: now 4 Gerwick BC, Thirty years of herbicide discovery: surveying5HeapI, the past 2 3 Tresch S, Schmotz J and Grossmann K, Probing mode of action in 13 Perez-Jones A and Mallory-Smith C, Biochemical mechanisms and 14 Green JM, Evolution of glyphosate-resistant crop15 technology. Gaines TA, Zhang W, Wang D, Bukun B, Chisholm ST, Shaner DL, 10 Wilson RG, Young BG, Matthews JL, Weller SC, Johnson WG, Jordan 11 Cerdeira AL and Duke SO, The current12 status Gardner and JG environmental and Nelson GC, Herbicides, glyphosate resistance and ACKNOWLEDGEMENTS The author thanks Dr Dale ShanerDr for Cliff Gerwick his for helpful providing suggestions the and data for Fig. 1. REFERENCES If 5 www.soci.org SO Duke 2011 Society of Chemical Industry c ıve to believe that a ¨ However, in every case, the utility of some of these 4,67 21 - 55 β Examples of herbicide target sites identified by antisense or Multipleapproachestoamode-of-actiondiscoveryareprobably Thus, a better solution to such weed problems would be AldolaseBiotin synthaseCarbonic anhydraseChlorophyll synthaseCoproporphyringen oxidaseCystathionine Dehydroquinate dehydrase/shikimate dehydrogenaseFerredoxin : NADP reductaseFlavanone 3-hydroxylaseGeranylgeranyl reductase 56 Glutamine-semialdehyde amino transferaseMg protoporphyrin monomethylester cyclasePectin esteraseSphingolipid 4-hydroxylaseThioredoxinTransketolase 54 52 Uroporphyrinogen 51 decarboxylase 53 60 50 61 57 58 59 63 66 62 64 65 Table 4. RNAi methods Target site Reference wileyonlinelibrary.com/journal/ps 6 FUTUREAND DEVELOPMENTS CONCLUSIONS The spectre ofresistant increased weeds has evolution apparently caused and theto agrochemical spread industry increase of glyphosate- investmentinterest has in not yet herbicidepatents. resulted in discovery. A significant number This increases ofstacked in new renewed herbicide resistances herbicide-resistant to crops, morelaunched some in than with the one near herbicide, future (Table are 5). due to be more productive than reliance on a single method.target After a site putative has beenengineering identified, a the plant ultimate with proofsite. a of resistant this form site of is the putative target many potential target sites foundherbicides by have gene been silencing introduced methods, with no any of these target sites. herbicides with new modes ofevolved. action Although for which there no is resistance noof has clear new timeline for modes theeconomics, introduction of there action, is consideringintroduced the little eventually. need doubt It and that would thus such the be products na will be new transgenic crops is questionable. pipeline of herbicides withrange new solution modes to of weed action managementguarding will the problems be long-term without utility a farmers of long- suchonly part products of by a using diverse them array of as avoid weed evolution management of tools resistance. to delay or used properly, they willmitigate give evolved farmers resistances. new However,of tools if the to the future, past prevent it is and use would a these be predictor products optimistic to asstrategy. predict part In that of places farmers a where will modes multiple robust of action resistance has resistance already to occurred, management several of these the herbicides to be usedto with these which crops have there modes are of action already examples of evolved resistance.

510 511 , , , , , et al et al et al et al et al Funct Plant J :747–758 23 ¨ ofgren R, :1687–1692 (2006). Plant J H nuclear magnetic 54 1 :191–205 (2009). Pestic Biochem Physiol :695–704. 2 DOI: 10.1002/ps.2319. . :398–413 (2006). :847–856 (2003). 120 , ed. by Stewart CN. Blackwell 59 142 :6355–6363 (2008). Plant Genome 56 :1155–1167 (2006). Pest Manag Sci wileyonlinelibrary.com/journal/ps Plant Phyiol J Agric Food Chem 62 :4–17 (2005). Plant Physiol Lemna pausicostata Pest Manag Sci 5 . )-pyrenophorol using :3712–3720 (1995). − , A visible marker for antisense mRNA expression 14 )-( R et al ,16 S Pest Manag Sci J Agric Food Chem EMBO J , Effects of the aglycone of ascaulitoxin on amino ,13 R , Identification of inhibitors of biotin synthase expression or :41–50 (2011). et al ,8 :147–157 (1998). Weed and Invasive Plant Genomics S -lyase in plant growth and development. N, acid metabolism in 100 the investigation(5 of the mode of action of the phytotoxin resonance fingerprinting. ferrodoxin : NADP reductase inhibitors. US Patent 6,124242 (2000). Identification of inhibitors of flavanone 3-hydroxylase expression or activity in plants for screening of20050100879 herbicides. A1 US 20050512 Patent (2005). Application activity of geranylgeranyland reductase tocopherol leads to andin loss to of partially transgenic geranylgeranyl geranylgeranylated reductase. tobacco chlorophyll plants expressing antisense RNA for Transcriptome response tosoybean. glyphosate in sensitive and resistant Clarification of pathway-specificion inhibition by cyclotron Fourierphenotyping studies. transform resonance/mass spectrometry-based metabolic gene expressiondichlorophenoxyacetic acid, patterns cinidon-ethyl andin tribenuron-methyl wheat. induced by the herbicidesIntegrative Genomics 2,4- herbicide treatmentArabidopsis thaliana phenocopies the fiddlehead mutant in application of 2,4-dichlorophenoxyacetic acid in Arabidopsis. photosystem-II-interfering herbicides atrazinethe soybean and transcriptome. bentazon on Ehrhardt T, On the mode of action of5-benzyloxymethyl-1,2-isoxazolines: the herbicides cinmethylin putative and inhibitorstyrosine aminotransferase. of plant in Publishing, Singapore, pp. 221–247 (2009). in plastid aldolase activity inhibitsof photosynthesis, alters sugars the and levels starch,14 and inhibits growth of potato plants. et al activity in plants for screening of20050101485 herbicides. A1 US 20050512 Patent (2005). Application Identification of carbonic anhydrasefor as plant an growth essential and plant a target2004248152 enzyme for A1 20041209 herbicides. (2004). US Patent Application Fluorescent assays forchlorophyll identifying synthase useful inhibitors as2005130118 of (2005). herbicides. plant US Patent or Application fungal oxidase level by antisense RNA synthesis leadsexpression to deregulated of gene plastidsystems. proteins and affects the oxidative defense Transgenic plants revealβ the indispensible role of cystathionine Willmitzer L, (2000). dratase/shikimate dehydrogenases for use as herbicide targetsincreasing and plant dry weight yield. US Patent 200330145348 (2003). Hajirezaei M, Herbicidal compositions and processes based on ¨ ofgren R, Axelsen KB, Kannangara CG, Schutte I, Pohlenze HD, 40 Duke SO, Evidente A, Fiore M, Rimando AM, Vurro M, Christiansen 41 Aliferis KA and Chrysayi-Tokousbalides M, Metabonomic strategy for 58 Davis K, Zayed A, Ascenzi R, Boyes D, Hoffman N, Gorlach J, 59 Tanaka R, Oster U, Kruse E, Rudiger W and Grimm B, Reduced 44 Zhu J, Patzoldt WL, Shealy RT, Vodkin LO, Clough SK and Tranel PJ, 42 Oikawa A, Nakamura Y, Ogura T, Kumura A, Suzuki H, Sakurai N, 45 Pasquer F, Oschner U, Zarn J and Keller B, Common and distinct 47 Lechelt-Kunze C, Meissner RR, Drewes M and Tietjen K, Flufenacet 55 Maimann S, Wagner C, Kreft O, Zeh M, Willmitzer L, H 46 Raghavan C,Ong EK,Dalling MJandStevenson TW,Effectofherbicidal 48 Zhu J, Patzoldt WL, Radwan O, Tranel PJ and Clough SJ, Effects of 43 Grossmann K, Hutzler J, Tresch S, Christiansen N, Looser R and 49 Duke SO, Baerson SR and Gressel J, Genomics and weeds: a synthesis, 50 Haake V, Zrenner R, Sonnewald U and Stitt M, A moderate decrease 51 Hoffman N, Gorlach J, Boyes D, Zayed A, Woessner J,52 Hamilton C, Davis K, Zayed A, Ascenzi R, Smith M, Kurnik53 BS, Aslamkhan A, Boyes Guo L, Mulpur D, R, Hoffman N, Kjemtrup S, Kearney J, 54 Kruse E, Mock HP and Grimm B, Reduction of coproporphyrinogen 60 H 56 Freund A, Sonnewald U and Ding L,57 Plant Wagner dehydroquinate A, dehy- Rohl R, Grossmann K, Schmidt RM, Sonnewald U and . , , - Pest DOI: et al Agents . Glyphosate Pest Manag :5886–5889 :2206–2213 :1215–1221 Vol. 2 2011 Society of Chemical Industry ) populations . 59 44 67 Pest Manag Sci Agrobacterium c :189–202 (2003). J Agric Food Chem , ed. by Singh BK. 45 Chem Eng News Pest Manag Sci Crop Sci conferring sulfometuron :281–289 (2010). :189–198 (1997). :2209–2222 (1989). ) from Illinois, United States. 57 11 Pest Manag Sci Physiology of Herbicide Action Ecol Econom Conyza Canadensis 207 J Agric Food Chem ). :762–768 (2005). , ed. by Nandula VK. Wiley, Singapore, PNMR. Weed Sci Plant Amino Acids 53 31 : a call for new options. Weed Technol :1038–1064 (2011). 3 Weed Sci J Gen Microbiol :971–985 (2003). :258–261 (2011). :319–325 (2008). :345–348 (2010). : 505–512 :900–909 (2007). 62 67 66 64 Salmonella typhimurium 68 21 Haye’s Handbook of Pesticide Toxicology :346–353 (2008). Toxins Amaranthus tuberculatus Amaranthus palmeri . Academic Press/Elsevier, San Diego, CA, pp. 1733–1751 64 2012; et al :3–16 (2001). , Resistance to HPPD-inhibiting herbicides in a population of 57 :320–326 (2000). :5808–5812 (2011). (16):13–17. methyl sensitivity. (2010). mutations of 3rdEdition,ed.byKrieger RI,Doull J,HodgsonRoss E,Maiback J, H,Reiter L, classifies pathways affectedneural by network bioactive classificationPhytochemistry compounds. of NMR Artificial spectra of plant extracts. Prentice Hall, Englewood Cliffs, NJ, 441 pp. (1993). isoleucine biosynthesis, in Marcel Dekker, New York, NY, pp. 385–416 (1999). herbicides, in higher yield and herbicide resistance in rice by (2011). resistant palmer amaranthResistance in Crops in and Weeds thepp. 195–212 United (2010). States, in soil-related factors thatglyphosate-resistant horseweed affect ( the presence and prediction of Glyphosate-induced weed shift inrotation of glyphosate-resistant glyphosate-resistant corn, corn sugarbeet, and orWeed spring Technol wheat. a from Indiana soybean fields. Amaranthus tuberculatus other herbicides and on56 resistance management. glyphosate resistance. 59 et al waterhemp ( mediated multiple gene transformation. (2004). Canada – a sequel. of glyphosate-resistant soybeans. Manag Sci Pest Manag Sci Mechanism of resistance ofamaranth ( evolved glyphosate-resistant Palmer (2011). sequestration: the horseweed glyphosatePest Manag resistance Sci mechanism. Sammons RD, Glyphosate-resistant horseweedglyphosate: made low-temperature resistant suppression to ofsequestration glyphosate revealed vacuolar by 89 rule of 5 for pharmaceuticalsSci apply to agrochemicals? Pest Manag Sci 10.1002/ps.2332. phytotoxins. products approach to herbicide discovery. 38 LaRossa RA and Van Dyk TK, Leaky pantohenate and thiamin 39 Ott K-H, Aranibar N, Singh B and Stockton GW, Metabonomics 36 Wittenbach VA and Abell LM,37 Inhibitors of Dayan valine, FE leucine, and and Duke SO, Protoporphyinogen oxidase-inhibiting 35 Devine MD, Duke SO and Fedtke C, 28 Cao MX, Huang JQ, Wei ZM, Yao QH, Wan CZ and Lu JA, Engineering 19 Culpepper AS, Webster TM, Sosnoskie LM and York AC, Glyphosate- 20 Davis VM, Gibson KD, Mock VA and Johnson WG, In-field and 27 Wilson RG, Miller SD, Westra P, Kniss AR, Stahlman PW, Wicks GW, 21 Tranel PJ, Riggins CW, Bell MS and Hager AG, Herbicide resistances in 26 Shaner DL, The impact of glyphosate-tolerant crops on the use of 25 Bradshaw LD, Padgette SR, Kimball SL and Wells BH, Perspectives on 22 Hauseman NE, Singh S, Tranel PJ, Riechers DE, Kaudun SS, Polge ND, 29 Appleby AP, A history of weed control in the United States and 24 Giannessi LP, Economic impacts of glyphosate-resistant crops. 23 Nelson DS and Bullock GC, Simulating a relative environmental effect 16 Gaines TA, Shaner DL, Ward SM, Leach JE, Preston C and Westra P, 17 Ge X, d’Avignon DA, Ackerman JJH and Sammons RD, Rapid vacuolar 18 Ge X, d’Avignon DA, Ackerman JJH, Duncan B, Spaur MB and 30 Bomgardner MM, Germinating pesticides. 31 Tice CM, Selecting the right compounds for screening: does Lipinski’s 32 Duke SO and Powles SB, Glyphosate: a once in a century herbicide. 34 Duke SO and Dayan FE, Modes of action of microbially-produced 33 Dayan FE, Owens DK and Duke SO, Rationale for a natural New modes of herbicide action www.soci.org Pest Manag Sci , et al : 505–512 68 :100–104 (2011). 2012; :535–551 (2001). 22 13 Plant Cell Pest Manag Sci :1101–1112 (1997). Outlooks Pest Manag 113 Plant Physiol by antisense RNAinvolved expression in affects tetrapyrole biosynthesis activities andnecrosis. of leads other to light-dependent enzymes need for diversification. Transgenic Arabidopsis expressing antisense thioredoxinherbicide gene screening. for US Patent Application 2003113786 (2003). A small decreasetobacco of transformants plastid hasand phenylpropanoid transketolase dramatic metatolism. activity effects on in photosynthesis antisense 66 Mock HPandGrimm B,Reductionofuroporphyrinogendecarboxylase 67 Green JM, Outlook on weed management in herbicide-resistant crops: 64 Kurnik BS, Davis K, Azyed A, Ascenzi R, Harper A, Boyes D, 65 Henkes S, Sonnewald U, Badur R, Flachmann R and Stitt M, , , et al et al www.soci.org SO Duke Plant Cell 2011 Society of Chemical Industry c Proc Natl Acad Sci :1229–1241 (2010). 51 :1726–1730 (1994). 91 Identification of inhibitors of pectinin esterase Arabidopsis expression or for activity screening20050112715 of A1 200500526 herbicides. (2005). US Patent Application protoporphyrin monomethylester cyclase deficiency and effects on tetrapyrrole metabolism in different light conditions. USA Physiol Identification of sphingolipid 4-hydroxylase as anfor essential enzyme plant growth and a target204265836 for (2005). herbicides. US Patent Application in plants: inhibition ofsemialdehyde aminotransferase chlorophyll antisense synthesis gene. with a glutamate-1- 63 Todd MD, Guo L, Davis K, Zayed A, Kjemtrup S, Boyes D, 62 Zayed A, Ascenzi R, Boyes D, Mulpuri R, Hoffman N, David K, 61 Enrico P, Rothbart M, Oelze M-L, Shalygo N, Deitz K-J and Grimm B, Mg wileyonlinelibrary.com/journal/ps

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