Editing Herbicide (section)

==Mechanism of action==
[[File:Classification of herbicides according to mechanism of action%2C chemical family%2C and herbicide group.pdf|thumb|Herbicides classified by their mechanism of action]]

Herbicides interfere with the biochemical machinery that supports plant growth. Herbicides often mimic natural [[plant hormone]]s, enzyme substrates, and [[Cofactor (biochemistry)|cofactor]]s. They interfere with the metabolism in the target plants. Herbicides are often classified according to their site of action because as a general rule, herbicides within the same site of action class produce similar symptoms on susceptible plants. Classification based on the site of action of the herbicide is preferable as herbicide resistance management can be handled more effectively.<ref name=Vats /> Classification by [[mechanism of action]] (MOA) indicates the first enzyme, protein, or biochemical step affected in the plant following application:

* '''ACCase inhibitors''': [[Acetyl coenzyme A carboxylase]] (ACCase) is part of the first step of [[lipid synthesis]].<ref name=ACCase>{{cite journal |doi=10.1080/09670874.2013.821212 |title=ACCase inhibitor herbicides – selectivity, weed resistance and fitness cost: A review |date=2013 |last1=Kukorelli |first1=Gábor |last2=Reisinger |first2=Péter |last3=Pinke |first3=Gyula |journal=International Journal of Pest Management |volume=59 |issue=3 |pages=165–173 |s2cid=83922917 }}</ref> Thus, ACCase inhibitors affect cell membrane production in the [[meristems]] of the grass plant. The ACCases of grasses are sensitive to these herbicides, whereas the ACCases of [[dicot]] plants are not.
* '''ALS inhibitors''': [[Acetolactate synthase]] (ALS; also known as acetohydroxyacid synthase, or AHAS) is part of the first step in the synthesis of the branched-chain amino acids ([[valine]], [[leucine]], and [[isoleucine]]). These herbicides slowly starve affected plants of these [[amino acid]]s, which eventually leads to the inhibition of [[DNA synthesis]]. They affect grasses and dicots alike. The ALS inhibitor family includes various [[Sulfonylurea#Agricultural uses|sulfonylureas]] (SUs) (such as [[flazasulfuron]] and [[metsulfuron-methyl]]), [[Imazaquin#Imidazolinone herbicides|imidazolinone]]s (IMIs), [[:Category:Triazolopyrimidines|triazolopyrimidine]]s (TPs), [[pyrimidinyl oxybenzoate]]s (POBs), and [[sulfonylamino carbonyl triazolinone]]s (SCTs). The ALS biological pathway exists only in plants and microorganisms (but not animals), thus making the ALS-inhibitors among the safest herbicides.<ref>{{cite journal | vauthors = Zhou Q, Liu W, Zhang Y, Liu KK | title = Action Mechanisms of Acetolactate Synthase-Inhibiting Herbicides | journal = Pesticide Biochemistry and Physiology | volume = 89 | issue = 2 | year = 2007 | pages = 89–96 | doi = 10.1016/j.pestbp.2007.04.004| bibcode = 2007PBioP..89...89Z }}</ref>
* '''EPSPS inhibitors''': [[EPSP synthase|Enolpyruvylshikimate 3-phosphate synthase enzyme]] (EPSPS) is used in the synthesis of the amino acids [[tryptophan]], [[phenylalanine]] and [[tyrosine]]. They affect grasses and dicots alike. [[Glyphosate]] (Roundup) is a systemic EPSPS inhibitor inactivated by soil contact.<ref name= EPSPS/>
* {{anchor|Auxin|Auxins}}'''[[Auxin]]-like herbicides''': The discovery of synthetic auxins inaugurated the era of organic herbicides. They were discovered in the 1940s after a long study of the plant growth regulator auxin. Synthetic auxins mimic this plant hormone in some way. They have several points of action on the cell membrane, and are effective in the control of dicot plants. [[2,4-D]], [[2,4,5-T]], and [[Aminopyralid]] are examples of synthetic auxin herbicides.
* '''[[Photosystem II]] inhibitors''' reduce electron flow from water to [[NADP|NADP<sup>+</sup>]] at the photochemical step in [[photosynthesis]]. They bind to the Qb site on the [[D1 protein]], and prevent quinone from binding to this site. Therefore, this group of compounds causes electrons to accumulate on [[chlorophyll]] molecules. As a consequence, [[oxidation]] reactions in excess of those normally tolerated by the cell occur, killing the plant. The [[triazine]] herbicides (including [[simazine]], [[cyanazine]], [[atrazine]]) and urea derivatives ([[diuron]]) are photosystem II inhibitors.<ref name="stryer">{{cite book |last = Stryer |first = Lubert |title = Biochemistry, 4th Edition |publisher = W.H. Freeman and Company |year = 1995 |page = 670 |isbn = 978-0-7167-2009-6}}</ref> Other members of this class are chlorbromuron, pyrazon, isoproturon, [[bromacil]], and terbacil.
* '''Photosystem I inhibitors''' steal electrons from [[ferredoxin]]s, specifically the normal pathway through [[Ferredoxin|FeS]] to Fdx to NADP<sup>+</sup>, leading to direct discharge of electrons on oxygen. As a result, [[reactive oxygen species]] are produced and oxidation reactions in excess of those normally tolerated by the cell occur, leading to plant death. [[Bipyridinium]] herbicides (such as [[diquat]] and [[paraquat]]) inhibit the FeS to Fdx step of that chain, while [[diphenyl ether]] herbicides (such as [[nitrofen]], [[nitrofluorfen]], and [[acifluorfen]]) inhibit the Fdx to NADP<sup>+</sup> step.<ref name="stryer"/>
* [[P-hydroxyphenylpyruvate dioxygenase inhibitor|'''HPPD inhibitors''']] inhibit [[4-Hydroxyphenylpyruvate dioxygenase|4-hydroxyphenylpyruvate dioxygenase]], which are involved in [[tyrosine]] breakdown.<ref name=Moran>{{cite journal | author = Moran GR | date = Jan 2005 | title = 4-Hydroxyphenylpyruvate dioxygenase | url = http://alchemy.chem.uwm.edu/classes/chem603/Handouts/ABBHPPDreview.pdf | journal = Arch Biochem Biophys | volume = 433 | issue = 1 | pages = 117–28 | doi = 10.1016/j.abb.2004.08.015 | pmid = 15581571 | url-status = dead | archive-url = https://web.archive.org/web/20140303175955/http://alchemy.chem.uwm.edu/classes/chem603/Handouts/ABBHPPDreview.pdf | archive-date = 2014-03-03 }}</ref> Tyrosine breakdown products are used by plants to make [[carotenoids]], which protect chlorophyll in plants from being destroyed by sunlight. If this happens, the plants turn white due to complete loss of chlorophyll, and the plants die.<ref>{{cite book |editor-first=Wolfgang |editor-last=Krämer |title= Modern crop protection compounds |date=2012 |publisher= Wiley-VCH-Verl. |location= Weinheim |isbn= 978-3-527-32965-6 |pages= 197–276 |edition= 2nd, rev. and enl.}}</ref><ref>{{Cite journal |doi= 10.1564/20feb09 |title= New HPPD-Inhibitors – A Proven Mode of Action as a New Hope to Solve Current Weed Problems |journal= Outlooks on Pest Management |volume= 20 |pages= 27–30 |year= 2009 |last= Van Almsick |first= A. |issue= 1 }}</ref> [[Mesotrione]] and [[sulcotrione]] are herbicides in this class; a drug, [[nitisinone]], was discovered in the course of developing this class of herbicides.<ref>{{Cite journal | pmid = 9728330| year = 1998| last1 = Lock| first1 = E. A.| title = From toxicological problem to therapeutic use: The discovery of the mode of action of 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC), its toxicology and development as a drug| journal = Journal of Inherited Metabolic Disease| volume = 21| issue = 5| pages = 498–506| last2 = Ellis| first2 = M. K.| last3 = Gaskin| first3 = P| last4 = Robinson| first4 = M| last5 = Auton| first5 = T. R.| last6 = Provan| first6 = W. M.| last7 = Smith| first7 = L. L.| last8 = Prisbylla| first8 = M. P.| last9 = Mutter| first9 = L. C.| last10 = Lee| first10 = D. L.| doi=10.1023/A:1005458703363| s2cid = 6717818}}</ref>

Complementary to mechanism-based classifications, herbicides are often classified according to their chemical structures or motifs. Similar structural types work in similar ways. For example, aryloxphenoxypropionates herbicides ([[diclofop]] [[chlorazifop]], [[fluazifop]]) appear to all act as ACCase inhibitors.<ref name=ACCase/> The so-called cyclohexanedione herbicides, which are used against grasses, include the following commercial products [[cycloxydim]], [[clethodim]], [[tralkoxydim]], [[butroxydim]], [[sethoxydim]], [[profoxydim]], and [[mesotrione]].<ref>{{cite journal|title=Cyclohexane-1,3-dione Oxime Ether Grass-Specific Herbicides and the Discovery of Butroxydim|author=Keith G. Watson|journal=Aust. J. Chem.|year=2011|volume= 64|issue=4|pages=367–372|doi=10.1071/CH10366}}</ref> Knowing about herbicide chemical family grouping serves as a short-term strategy for managing resistance to site of action.<ref name="10.4141/P05-193">{{cite journal|last1=Beckie|first1=H. J.|last2=Harker|first2=L. M.|last3=Hall|first3=S. I.|title=A decade of herbicide-resistant crops in Canada|journal=Canadian Journal of Plant Science|volume=86|issue=4|pages=1243–1264|date=2006|doi=10.4141/P05-193|display-authors=etal|doi-access=free|hdl=10388/9421|hdl-access=free}}</ref> The [[phenoxy herbicides| phenoxyacetic acid]] mimic the natural [[auxin]] [[indoleacetic acid]] (IAA). This family includes [[MCPA]], [[2,4-D]], and [[2,4,5-T]], [[picloram]], [[dicamba]], [[clopyralid]], and [[triclopyr]].

===WSSA and HRAC classification===
Using the Weed Science Society of America (WSSA) and [[HRAC classification|herbicide Resistance and World Grains (HRAC) system]]s, herbicides are classified by mode of action.<ref name=Shaner>{{cite book|last1=Shaner|first1=D. L.|last2=Leonard|first2=P.|editor1-last=Powles|editor1-first=S. B.|editor2-last=Shaner|editor2-first=D. L.|title=Herbicide Resistance and World Grains|date=2001|publisher=CRC Press, Boca Raton, FL.|pages=279–294|isbn=9781420039085|chapter=Regulatory aspects of resistance management for herbicides and other crop protection products}}</ref> Eventually the Herbicide Resistance Action Committee (HRAC)<ref>{{Cite web|url=https://hracglobal.com/|title=PROTECTING CROP YIELDS AND QUALITY WORLDWIDE|website=Herbicide Resistance Action Committee}}</ref> and the [[Weed Science Society of America]] (WSSA)<ref>{{Cite web|url=https://wssa.net/|title=Weed Science Society of America|accessdate=4 April 2023}}</ref> developed a classification system.<ref name=Retzinger>{{cite journal|last1=Retzinger|first1=E. J. Jr.|last2=Mallory-Smith|first2=C.|title=Classification of herbicides by site of action for weed resistance management strategies|journal=Weed Technology|date=1997|volume=11|issue=2|pages=384–393|doi=10.1017/S0890037X00043116|s2cid=251572710 }}</ref><ref name=Schmidt>{{cite book|last1=Schmidt|first1=R. R.|title=1997 Brighton crop protection conference: weeds|date=1997|publisher=Proceedings of an international conference, Brighton, UK, 17–20 November 1997, British Crop Protection Council|pages=1133–1140|chapter=HRAC classification of herbicides according to mode of action}}</ref> Groups in the WSSA and the HRAC systems are designated by numbers and letters, inform users awareness of herbicide mode of action and provide more accurate recommendations for resistance management.<ref name=Mallory-Smith>{{cite journal|last1=Mallory-Smith|first1=C.|title=Impact of labeling herbicides by site of action: A University view|journal=Weed Technology|date=1999|volume=13|issue=3|page=662|doi=10.1017/S0890037X00046376|s2cid=89106416 }}</ref>