Chapter 4
Why Are Imidazolinones Such Potent Herbicides Dale L. Shaner and Bijay K. Singh
Downloaded by CORNELL UNIV on June 18, 2012 | http://pubs.acs.org Publication Date: May 14, 1998 | doi: 10.1021/bk-1998-0686.ch004
Cyanamid Agricultural Research Center, American Cyanamid Company, P.O. Box 400, Princeton, NJ 08543-0400
The imidazolinones control weeds at 32 to 125 g/ha, making them some of the most potent herbicides on the market. They kill plants by inhibiting acetohydroxyacid synthase (AHAS), the first common enzyme in the branched chain amino acid biosynthetic pathway. Mode of action studies suggest that factors in addition to the potency at the target site are important in determining herbicidal activity. These factors include the absorption and translocation properties of the imidazolinones, localization of AHAS, the developmental regulation of the branched chain amino acid biosynthetic pathway, and in vivo inhibition of AHAS by imidazolinones. These results illustrate the importance of taking a holistic view of herbicide activity rather than focusing solely on the site of action of the herbicide. In 1986 American Cyanamid Company introduced imazaquin, the first imidazolinone herbicide, for broad spectrum weed control in soybeans (/). The synthesis effort that led to the imidazolinones started with a compound mat killed several important perennial weeds at 4 kg/ha (2, 5). Analog synthesis work on this lead compound led to a compound which had GA-like activity. From this chemistry, additional synthesis work yielded a promising herbicide, particularly on perennial weeds. Further work in this area of chemistry eventually led to the synthesis of the first imidazolinone (4). Currently, six different imidazolinones are used as commercial herbicides in various cropping and non-cropping situations (Figure 1). The use rate of the common herbicides in cropping situation (e.g. corn and soybean) is in the range of 32-125 g/ha which makes them among the most potent herbicides on the market. In this paper, we have attempted to provide the possible reasons for such high potency of the imidazolinone herbicides. Absorption of Imidazolinones One of the best features of the imidazolinone herbicides is the flexibility in the way they may be applied in the field. These herbicides are active when applied to the foliage of plants or to the soil because they are rapidly taken up by the snoots and the roots. The imidazolinones are weak acids with a pKa between 3.6 and 5.3, and these compounds are taken up by weak acid ion trapping (5, 6). After foliar application, uptake of imidazolinones occurs rapidly and reaches maximum level within a few hours after treatment. Thus, these compounds have good rain fastness provided the ©1998 American Chemical Society
In Synthesis and Chemistry of Agrochemicals V; Baker, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
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24 rainfall occurs 6 hours or more after application. Addition of non-ionic surfactants and urea ammonium nitrate greatly increase the penetration of the imidazolinone herbicides into leaves. Rapid absorption of these compounds is evident from one study where 61% of the applied C-imazaquin was absorbed by Xanthium strumarium foliage in 3 days (7). 14
Downloaded by CORNELL UNIV on June 18, 2012 | http://pubs.acs.org Publication Date: May 14, 1998 | doi: 10.1021/bk-1998-0686.ch004
O
O
IMAZAQUIN
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AC 263222
IMAZAMOX
Figure 1. The commercial imidazolinone herbicides. Imidazolinones are absorbed by roots by a passive process that is dependent on the pH of the external solution and the lipophilicity of the herbicide. Study of the structure-activity relationship between the physico-chemical parameters of 5'substituted pyridine imidazolinones and absorption by roots of corn and sunflower showed that the lipophilicity of the analogs was the predominant factor affecting root absorption of these imidazolinone (8,9). Uptake increased as lipophilicity increased. Similar to foliage, roots also readily absorb imidazolinones. For example, 66% of the applied imazaquin was rapidly taken up by sunflower roots (10). Translocation of Imidazolinones Imidazolinones are rapidly translocated from the leaves and the roots to the other parts of the plants which indicates that these compounds can move through both xylem and phloem. Translocationfromthe roots to the shoots takes place via xylem and is correlated with the lipophilicity of the compound (8, 9). For example, imazaquin is translocated much more effectively than imazethapyr which, in turn, is translocated better than imazapyr. Translocation of imidazolinones from the leaves to other parts of the plants occurs through the phloem. Transport via phloem varies with the chemical structure, however multiple physical-chemical factors, which includes the pKa of the imidazolinone as well as its lipophilicity, determine the phloem mobility of the imidazolinones. Imazapyr is highly phloem mobile, with greater than 60% of the absorbed herbicide moving out of the treated leaf of perennial grasses and maize (10). The differences in the mobility of the imidazolinones in different species may also be related to the pathway of metabolism in the symplast.
In Synthesis and Chemistry of Agrochemicals V; Baker, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
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Downloaded by CORNELL UNIV on June 18, 2012 | http://pubs.acs.org Publication Date: May 14, 1998 | doi: 10.1021/bk-1998-0686.ch004
Inhibition of AHAS by imidazolinones There are four main lines of evidence which prove that the site of action of imidazolinones is acetohydroxyacid synthase (AHAS), the first common enzyme in the branched chain amino acid biosynthetic pathway (Figure 2). First, an imidazolinone treatment caused a simultaneous increase in the total free amino acid levels in the treated tissue and a corresponding decrease in the soluble protein levels (77, 12). Further analysis revealed that the levels of most of the amino acids increased after treatment with the exception of the branched chain amino acids (12). Second, growth inhibition by an imidazolinone was prevented when the growth medium was supplemented with the branched chain amino acids (77, 72). Third, imidazolinones were in vitro inhibitors of AHAS extracted from plants (75). Fourth and the most definitive result was the demonstration that imidazolinone resistant plants, selected in the presence of herbicidal concentrations of an imidazolinone, contained an altered AHAS that was no longer inhibited by the imidazolinones (1416). Further analysis revealed that a single amino acid change in the AHAS protein was responsible for resistance to the imidazolinones (77, 18).
Threonine
Pyruvate
X
2-Ketobutyrate
AHAS
Acetolactate
Acetohydroxybutyrate
Dihydroxyisovalerate
Dihydroxymethylvalerat
Ketoisocaproate
