Resistance metabolic detoxification

Our studies indicate that rapid metabolic detoxification of linear furanocoumarins is an effective resistance mechanism for K polyxenes against the toxic effects of these compounds. It has been postulated that the adaptation of some plants to produce angular furanocoumarins was in response to the reduced effectiveness of the linear furanocoumarins as deterrents for herbivores such as polyxenes (22). Such may Indeed be true, but our studies on the comparative detoxification of linear and angular furanocoumarins suggest that, at best, the presence of angular furanocoumarins in plants confers only a tenuous margin of relative "safety" against polyxenes. 

This characteristic, however, is not universally found in all triazine-resistant weeds. Gray et al. (1995a, b) found that velvetleaf resistance to atrazine in Wisconsin was not associated with a reduction in fitness, productivity, or intraspecific competitive ability. This triazine-resistant species found in Maryland and Wisconsin does not have D1 level resistance in the chloroplasts, but instead has a more rapid metabolic detoxification of triazines in these biotypes. The extent of the rapid metabolic resistance in other velvetleaf-resistant biotypes is unknown. 

Ivie, G.W., Bull, D.L., Beier, R.C., Pryor, N.W., and Oertli, E.H., Metabolic detoxification Mechanism of insect resistance to plant psoralens, Science, 221, 374,1983. 

The challenge here is to find suitable inhibitors at the enzyme level that retain this activity at the whole-plant level. In addition, this approach is only feasible when resistance is conferred by target site mutations if resistance were conferred also by metabolic detoxification an additional set of screens against detoxifying enzymes would have to be included. 

Two factors are most commonly associated with the development of pesticide resistance in arthropods enhanced metabolic detoxification and/or decreased target site sensitivity. Other factors that sometimes come into play are reduction in the rate at which a toxicant is absorbed into the body, or in the rate at which it is translocated to the site of action. In rare cases, there may be behavioral adaptations which allow the arthropod to minimize contact with the toxicant (25-22.) ... 

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. 

Nontai et-site Resistance by Enhanced Metabolic Detoxification... 

There are multiple potential mechanisms for avoidance of autotoxicity, including resistance at the molecular target site, metabolic detoxification, sequestration, and exudation. The last three mechanisms all keep the active compound from reaching the molecular target site. 

Yu SJ (1983) Induction of detoxifying enzymes by allelochemicals and host plants in the fall armyworm. Pestic Biochem Physiol 19 330-336 Yu SJ (1984) Interactions of allelochemicals with detoxification enzymes of insecticide-susceptible and resistant fall armyworms. Pestic Biochem Physiol 22 60-68 Yu SJ (1986) Consequences of induced foreign compound-metabolizing enzymes in insects. In Brattsten LB, Ahmad S (eds) Molecular aspects of insect-plant associations. Plenum, New York, pp 153-174... 

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. 

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. 

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