Target sites herbicides

The evolution of herbicide-resistant weeds due to repeated use of herbicides with the same site of action has become a challenge in modem agriculture. A different target-site herbicide can aid in the management of herbicide resistance by... 

P. Bitger and G. Sandmaim, eds.. Target Sites of Herbicide Mction, CRC Press, Boca Raton, Ela., 1989. 

The molecular target site of triketone herbicides is the enzyme -hydroxyphenylpyruvate dioxygenase (HPPD). Inhibition of this enzyme disrupts the biosynthesis of carotenoids and causes a bleaching (loss of chlorophyll) effect on the foliage similar to that observed with inhibitors ofphytoene desaturase (e.g. norflurazon). However, the mechanism of action of HPPD inhibitors is different. Inhibtion of HPPD stops the synthesis of homogen tisate (HGA), which is a key precursor of the 8 different tocochromanols (tocopherols and tocotrienols) and prenyl quinones. In the absence of prenylquinone plastoquinone, phytoene desaturase activity is interrupted. The bleaching of the green tissues ensues as if these compounds inhibited phytoene desaturase. 

Proherbicides. Thio- and dithiocarbamates probably require metabolic activation prior to exerting their herbicidal effects. Sulfoxide metabolites of the -alkyl thiocarbamates are generally more potent herbicides than the parent compounds ( -5). The herbicidal action of these sulfoxides probably results from their carbamylating action for thiols, although the specific target site or receptor is not defined (23, 24). It is conceivable that the -chloroallyl thiocarbamate herbicides may act in the same way, since their sulfoxides are also potent carbamylating agents... 

In summary, triazine resistance in weeds is most commonly due to a target site alteration that confers a very high level of resistance to. y-triazinc herbicides. Although a Ser264 to Gly mutation in the D1 protein is most common, additional alterations have been identified that confer resistance to triazines and other classes of PS II inhibitors. Enhanced herbicide metabolism plays a major role in conferring resistance in only a few weed biotypes. In these biotypes, the pattern of resistance may be broader, with some cross-resistance to av-trazinones, uracils, heterocyclic ureas and phenyl ureas. The level and pattern of resistance to various herbicides in these biotypes depend, presumably, on the activity and specificity of the enzyme(s) responsible for the enhanced herbicide metabolism. 

Devine, M.D. and C.V. Eberlein (1997). Physiological, biochemical and molecular aspects of herbicide resistance based on altered target sites, pp. 295-348. In Roe, R.M. J.D. Burton, and R.J. Kuhr, eds., Herbicide Activity Toxicology, Biochemistry and Molecular Biology. Amsterdam, The Netherlands I. O. S. Press, Inc. 

Shortly after the introduction of the triazine herbicides, it was confirmed that their target site in the photosystem II (PS II) complex was in the thylakoid membranes. Triazines displace plastoquinone at the QB-binding site on the D1 protein, thereby blocking electron flow from QA to QB. This in turn inhibits NADPH2 and ATP synthesis, preventing C02 fixation. 

Target site cross-resistance, in which a change at the site of action of one herbicide also confers resistance to herbicides from a different class (e.g., selection by triazine-resistant D1 protein that is also less sensitive to triazinones). 

Diebold et al. (2003) concluded that multiple resistance in a Powell amaranth biotype in Ontario was due to the presence of altered target sites for triazine and imidazolinone herbicides. 

Hall et al. (1998) reported that an ALS-resistant biotype of false cleavers was cross-resistant to a broad range of ALS inhibitors, as well as to an auxin-type herbicide, quinclorac, which had never before been applied to these fields. A similar case of quinclorac multiple resistance in smooth crabgrass has been reported in California when plants were previously treated with ACCase herbicides. Data suggest a target site-based mechanism of resistance involving the accumulation of cyanide derived from stimulated ACC synthesis, which is a precursor of ethylene (Abdallah et al., 2004). 

Richter, J. and S.B. Powles (1993). Pollen expression of herbicide target site resistance genes in annual ryegrass (Lolium rigidum). Plant Physiol., 102 1037-1041. 

In relation with resistance of weeds to herbicides, Duke et al. (2000) mentioned that new mechanisms of action for herbicides are highly desirable to fight evolution of resistance in weeds, to create or exploit unique market niches, and to cope with new regulatory legislation. Comparison of the known molecular target sites of synthetic herbicides and natural phytotoxins reveals that there is little redundancy. Comparatively little effort has been expended on determination of the sites of action of phytotoxins from natural sources, suggesting that intensive study of these molecules will reveal many more novel mechanisms of action. These authors gave some examples of natural products that inhibit unexploited steps in the amino acid, nucleic acid, and other biosynthetic pathways AAL-toxin, hydantocidin, and various plant-derived terpenoids. 

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