Herbicide detoxification

Herbicide Detoxification Herbicide Selectivity in Crops and Herbicide Resistance in Weeds... 

Salleh MA, JM Pemberton (1993) Cloning of a DNA region of a Pseudomonas plasmid that codes for detoxification of the herbicide paraquat. Curr Microbiol 27 63-67. 

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. 

The hydroxylation of the ring moiety of 2,4-D similarly converts the parent herbicide to a non-toxic product. Microorganisms may bring about such a detoxification when they hydroxylate the ring in the 4-position, a process that leads to a migration of the chlorine to give 2,5-dichloro-4-hydroxyphenoxy-acetic acid. 

Stalker, D.M., McBride, K.E., and MalyJ, L.D. Herbicide resistance in transgenic plants expressing a bacterial detoxification gene. Science (Washington, DC), 242(4877) 419-423, 1988. 

Clearly, if a plant is able to metabolise a herbicide more quickly than the herbicide can accumulate at the site of action within the plant, then that plant will be tolerant of that herbicide. This is detoxification. 

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. 

Fedtke, C. (1983). Leaf peroxisomes deaminate as-triazinone herbicides Method of detoxification by tolerant plants. Naturwissenschaften, 70 199-200. 

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. 

Shimabukuro, R.H., G.L. Lamoureux, and D.S. Frear. (1978). Glutathione conjugation A mechanism for herbicide detoxification and selectivity in plants, pp. 133-149. In Pallos, F.M. and J.E. Casida, eds., Chemistry and Action of Herbicide Antidotes. New York Academic Press Inc. 

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. 

Christopher, J.T., S.B. Powles, J.A.M. Holtum, and D.R. Liljegren (1991). Cross-resistance to herbicides in annual ryegrass (Lolium rigi-dum) II Chlorsulfuron resistance involves a wheat-like detoxification system. Plant Physiol., 100 1036-1043. 

Indirect effects can be substantially more far-reaching (in an ecosystem perspective) than direct chemical effects. Most herbicides used in the forest are not likely to directly affect many organisms in any ecosystem because few herbicides used in forestry are so inherently toxic or so widely distributed that the avoidance or detoxification mechanisms of all organisms would be overwhelmed. On the other hand, severe deleterious direct effects on only a few key organisms or ecosystem processes can have far-reaching (indirect) effects for many other components of the ecosystem. 

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. 

Approximately 300 organic pesticide chemicals are being marketed in more than 10,000 different formulations. Last year over 750 million pounds were used in the United States. Insecticides account for nearly half this amount, but herbicides will far surpass them in tonnage within a few years. The chlorinated hydrocarbon insecticides have attracted attention because of their so-called persistence However, unlike some of their inorganic predecessors, organic pesticides are decomposed in the environment by biological and physicochemical processes which influence the amounts that will be found in the environment. Their mere presence in the environment does not necessarily jeopardize the public health. The amounts present, their toxicities, and the rate of detoxification and decomposition must be considered to assess their significance in the environment... 

Phytoalexins are an Important component of the plant disease defense reaction called the hypersensitive response. Successful pathogens have evolved methods for dealing with plant phytoalexins. Including suppressors of their production, detoxification of the phytoalexins and In some cases avoiding elaboration of substances, called elicitors, that would otherwise Initiate the defense reaction. Elicitors obtained from pathogens are of considerable utility for study of various aspects of plant biology because of their interaction with the products of plant disease resistance genes. Substantial information has been obtained on how elicitors are perceived by plant cells and how they function, but much remains to be done. Finally, elicitors may prove of value for the economic production of exotic plant secondary metabolites and as specific herbicides. 

Yu, S.J., Induction of detoxification enzymes by triazine herbicides in the fall army worm, Spodoptem frugiperda (J. E. Smith), Pestic. Biochem. Physiol., 80,113, 2004. 

The use of herbicide rotation to avoid resistance will not be successful in cases where resistance is conferred by non-specific detoxification mechanisms that act on herbicides with different modes of action. In this case, selection for weeds resistant to members of two or more mode-of-action groups can and does occur [9]. Therefore, alternating or rotating amongst herbicides from different mode-of-action groups does necessarily delay resistance development. Clearly, there is no simple herbicide rotation solution to resistance avoidance. The tremendous genetic diversity in some seed populations allows the evolution of resistance, with the resistance mechanism simply reflecting the nature of the selection pressure that was applied. 

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