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Inhibitors of Fatty Acid Synthesis and Elongation

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 Mechanism of Herbicide Selectivity:  Monocots and dicot plastidic ACCase enzymes are fundamentally different, and these differences create the basis for herbicide selectivity.  The multidomain form of ACCase found only in graminaceous monocots is the only one completely inhibited by the ACCase-inhibiting herbicides, which allows them to specifically control certain weedy members of the Poacea, mostly in dicot crops.  Further subtle differences between the ACCase enzymes from small grain crops (wheat and barley) and grassy weeds allow the use of selective herbicides like diclofop to control wild oats in these crops.
    Much of our knowledge about plant ACCases derives from studies of their herbicide inhibitors, their mechanism of toxicity, and the mechanisms of evolved resistance.   For example, investigations on the sensitivity of ACCases from certain grass families have provided detailed information on the different ACCase isoforms and their relative contributions to lipid biosynthesis.  The ability of herbicides to specifically inhibit certain isoforms indicates that the plastidic form of the enzyme provides most of the lipids for membranes and cellular metabolism in the Poacea, as well as other plant families. 


Dicots are naturally tolerant to ACCase inhibitors because their ACCase enzymes do not bind the herbicides.

ACCase Inhbitors:  Two herbicide families with very different chemical structures, the aryloxyphenoxy propionates and the cyclohexanediones, inhibit ACCase, the first enzyme of fatty acid synthesis.  These herbicides are collectively known as graminicides, or sometimes ‘grass killers.’ 

Aryloxyphenoxy propionate herbicides: The aryloxyphenoxy propionate herbicides are informally termed ‘fops,’ reflecting the suffix on most of their chemical names.  Fops were first developed in Japan and later in Germany, and diclofop-methyl was the first one commercially marketed in 1979.  These herbicides are lethal to many grasses, with the exception of cultivated oats, wheat, red fescue, and annual bluegrass.  Since most fops have very low water solubility, they are formulated and sold as esters.  Once absorbed into plant tissues, the esters are rapidly cleaved to produce the free acids, which are the herbicidal form of the molecules.   The links to photos of herbicide injury symptoms were graciously provided by Dr. Jack Dekker, Iowa State University.


   Aryloxyphenoxy propionate Family (‘fops’)
 Base Structure and Examples   R” = ester or butyl ester group for all.
R’ = 6-chloro-2-quinoxalinyl for quizalofop-p;
R’ = 4-[5-(trifluoromethyl)]-2-pyridinyl for fluazifop-p;
R’ = 2,4-dichlorophenoxy for diclofop-methyl.
Diclofop-methyl = Hoelon.
Fenoxaprop-ethyl = Acclaim, Horizon, Whip, or Fusion.
Fluazifop-P-butyl =    Fusilade 2000 or Fusilade DX.
Haloxyfop-methyl = Verdict.
Quizalofop-P-ethyl = Assure II.
 Field Use Applied early post-emergence in many crops to control perennial and annual grassy weeds; diclofop may be used PRE or PPI in winter wheat for ryegrass or cheat control; soil half-lives range from 15 to 60 days
 Selectivity Most grasses are sensitive, while broadleaves are tolerant due to tolerant ACCase; wheat is tolerant to diclofop because of metabolism
 Symptoms  Rapid growth cessation, browning of new leaf sheaths, death of shoot meristems, red or orange coloring of older leaves, and general plant necrosis within 1 to 3 weeks
and Translocation
Quickly absorbed by foliage and slowly translocated in symplast to meristems; diclofop also absorbed by roots
Plant Metabolism Applied esters are absorbed and rapidly converted to free acids, which are the toxic forms
 Soil Activity Metabolized relatively quickly by soil microbes

Cyclohexanedione herbicides:  The cyclohexanedione herbicides are chemically quite different from the fops and yet also inhibit ACCase.  These herbicides are termed ‘dims,’ reflecting the suffix on their chemical names.  Sethoxydim (Poast) was the first dim commercially developed in the early 1980’s, by BASF.   Unlike the fops, sethoxydim is very labile in the environment and in plants; however, many of the metabolites are phytotoxic and so herbicidal activity is usually retained.

  Cyclohexanedione Family (‘dims’)
Base Structure and Examples   R’ = OC2H5 and R” = (CH2)2CH3 for sethoxydim.
R’ = OCH2CH-CHCl and R” = CH2CH3 for clethodim.
Clethodim =  Select.
Sethoxydim =  Poast, Poast Plus, Vantage.
 Field Use Applied early post-emergence in many crops to control perennial and annual grassy weeds; soil half-lives range from 3 to 5 days
 Selectivity Most grasses are sensitive while broadleaves and some grasses (centipede grass and fine fescue) are tolerant due to a tolerant ACCase
 Symptoms Rapid growth cessation, browning of new leaf sheaths, death of shoot meristems, red or orange coloring of older leaves, and general plant necrosis within 1 to 3 weeks
Absorption and Translocation Quickly absorbed by foliage and roots; translocated in both symplast and apoplast and accumulates in meristems
 Plant Metabolism Metabolized relatively quickly in most plant species
Soil Activity Subject to photodegradation and soil hydrolysis; metabolized quickly by soil microbes

Resistance to ACCase Inhibitors

As is true for all herbicides, consistent or near-continuous use of these herbicides has selected for resistant individuals, which soon dominate local weed populations (Herbicide Resistance Overview).  In most cases, resistance to ACCase inhibitors is due to point mutation(s) in the gene encoding the target enzyme, which reduces its binding affinity for the herbicide.  Naturally herbicide-sensitive plant species contain an isoleucine at position 1769 of their ACCase amino acid sequences, while this amino acid is changed to a leucine in resistant biotypes.  This amino acid substitution has been confirmed for resistant wild oat and green foxtail, although there is some question of whether this substitution confers resistance to both aryloxyphenoxy propionate and cyclohexanedione herbicides. 
    Some resistant biotypes have been reported with enhanced levels of herbicide metabolism.  For example, Australian resistant biotypes of annual ryegrass (Lolium rigidum) are resistant to ACCase inhibitors and at least six other herbicide families.  Only two studies have compared the relative fitness of resistant and susceptible biotypes, and in both cases there appear to be no fitness penalties associated with the amino acid substitution at position 1769. 


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