Metabolic detoxification, herbicides

Nontarget-site resistance is caused by mechanisms that reduce the amount of herbicidally active compound reaching the target site. An important mechanism is enhanced metabolic detoxification of the herbicide in the weed, with the effect that only insufficient amounts of herbicidally active substance will reach the target site. Furthermore, reduced uptake and translocation or sequestration of the herbicide may lead to insufficient herbicide transport to the target site. 

Cross-resistance means that a single resistance mechanism causes resistance to several herbicides. The term target-site cross-resistance is used when these herbicides bind to the same target site, whereas nontarget-site cross-resistance is due to a single nontarget-site mechanism (e.g., enhanced metabolic detoxification) that entails resistance across herbicides with different modes of action. 

GSTs in plants were first studied because of their ability to detoxify herbicides (30, 16). GST-based metabolism imparts herbicide tolerance in several plant species especially to the sulfonylureas, aryloxyphenoxypropionates, triazinone sulfoxides, and thiocarbamates herbicide families that are susceptible to GSH conjugation (16,31-35). There is a positive correlation of both GSH levels and the activity of specific GST enzymes wifti the rate of herbicide conjugation and detoxification (36-39). 

Figure L Pathways for the metabolism of herbicides in plant cells. Herbicides may need de-esterification mediated by esterases in the cell wall prior to transport into the cell. Once inside the cell, detoxification followed by conjugation and transport into the vacuole is the main pathway.

In crop protection as well, understanding plant metabolism is of paramount importance to increase selectivity and to address resistance of chemical compounds. Moreover, dissipation of a compound in the aquatic ecosystem is very similar to the excretion phenomena of the bodies. An extensive amount of evidence has been accumulated to support the involvement of CYPs in the metabolism and detoxification of herbicides, fungicides and insecticides. The understanding of their biotransformations at the molecular level may be extremely helpful for herbicide- or insecticide-synergistic development. 

A-Dealkylation Montgomery et al. (1969) studied the further metabolism of hydroxysimazine in plants. They demonstrated that the primary metabolite was the result of dealkylation to produce 2-amino-4-ethylamino-6-hydroxy-.y-triazine (GS-17792). There was also chromatographic evidence for a second dealkylation step that possibly produced ammeline, 2,4-diamino-6-hydroxy-.v-triazine (GS-17791) andammelide, 2-armno-4,6-dihydroxy-y-triazine (G-35713). The authors concluded that the dealkylation of these herbicides appears to be an important pathway of detoxification. 

In other weed biotypes, resistance to triazine herbicides is likely conferred by rapid metabolism of the herbicides to inactive compounds. A chlorotoluron-resistant biotype of blackgrass (slender foxtail) was cross-resistant to various other groups of herbicides, including triazines (Kemp et al., 1990). The mechanism of chlorotoluron resistance was Cyt P450-based enhanced oxidative metabolism through /V-demethylation and ring-methyl hydroxylation (Moss and Cussans, 1991). Consequently, it is likely that resistance to triazines in this blackgrass biotype is also due to enhanced herbicide detoxification. 

In some triazine-resistant species where resistance is due to more rapid metabolism of the herbicide, the weeds develop resistance gradually and may be only slightly resistant. This is especially true with some of the monocot or grass weeds that are already partially inherently resistant to atrazine (Thompson et al. 1971 Gressel et al., 1982, 1983). DePrado et al. (1995) found that fall panicum has the capacity for rapid detoxification, which is slightly greater in plants from fields that have been repeatedly treated with atrazine. 

Adapted species may have developed, however, strategies which enable them to survive allelopathic attacks. One of those strategies certainly includes detoxification of absorbed allelochemicals by constitutive or inducible pathways. Metabolization and detoxification are known reactions in a number of crops upon application of diverse synthetic herbicides.38 Enhanced herbicide detoxification is an important factor in the development of nontarget-site cross-resistance and multiple resistance. It is reasonable to expect comparable strategies in plants that are relatively resistant to allelochemicals such as DIBOA, DIMBOA, and their derivatives. Especially in ecosystems where co-existing species have to be adapted to each other, detoxification of absorbed allelochemicals may play a crucial role under defined circumstances. 

In certain situations it is possible to overcome herbicide metabolism-based resistance by adding an ingredient that will block detoxification of the herbicide in the resistant weed. One example is with propanil-resistant Echinochloa colona in rice in Latin America. The addition of piperophos, an organophosphate insecticide that inhibits the aryl acylamidase activity that confers resistance on the weed biotype [11]. This combination, based on an undo standing of the resistance mechanism, has beat approved for use on resistant... 

A direct relationship has been observed between the ability of some plant species to form 0-glucosldes of herbicides and resistance of those species to the herbicides however. In most cases a phase I reaction proceeds glucose conjugation and It Is not known whether the phase I reaction or conjugation results In herbicide detoxification. Chlorpropham is metabolized in plants by ring-hydroxylation and subsequent conjugation with glucose (Equation 3). In vitro, the... 

Triazine Resistance We attempted to answer the previous four questions using data and examples derived from the study of the best documented case of herbicide resistance, triazine resistance. Two kinds of mechanisms may be responsible for this triazine resistance first is the presence of detoxification metabolic pathways, as seen in corn (11). This also may occur in weed populations, especially Panicoideae, but a low heritability makes its study complex. The second mechanism of triazine resistance is the loss of herbicide binding at the level of the chloroplast. 

DETOXIFICATION OF HERBICIDES. Almost all herbicides are metabolized to some extent by higher plants. The selective use of herbicides is often possible because the rates and the mechanisms by which plants detoxify herbicides differ... 

Figure 9.7 Schematic representation of herbicide detoxification in a plant cell. Both XH and XZ are herbicides entering the oxidation-glucose conjugation pathway and the glutathione-dependent pathway, respectively. XOH is the hydroxylated herbicide XOGlc, XGS and Xcys are the glucose, GSH and Cysteine conjugates respectively. [Reproduced with permission of the authors from K. Kreuz and E. Martinoia, Herbicide Metabolism in Plants Integrated Pathways of Detoxication, in G. T. Brooks and T. R. Roberts, Eds., Pesticide Chemistry and Bioscience, The Food-Environment Challenge, Serial Pub. No. 233, The Royal Society of Chemistry, Cambridge, 1999, pp. 277-287.]...

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