Monooxygenases herbicide resistance

Resistance to herbicides has been associated with a high capacity to metabolise these compounds. N-Demethylation, a first step in the deactivation of the phenyl-ureas, is thought to involve a cytochrome P-450 dependent oxygenase [16]. Some inhibitors of this reaction, including triazole derivatives, act synergistically with phenylurea herbicides by reducing their metabolism [7, 29]. The normal function of these monooxygenases is unknown, but could be one or more of the processes discussed above. There is also evidence that cytochrome P-450 activity can be induced in plants treated with some xenobiotics [25] and this may represent a natural defense mechanism. 

As the grass weeds in Table 2 evolved resistance to all the wheat selective herbicides, irrespective of their site of action, the most likely hypothesis is that they evolved the same biochemical detoxification mechanism as wheat, i.e. evolved a biochemical mimicry . This is supported by evidence that compounds that suppress the herbicide degradation in wheat also suppress their degradation. Little is known about the monooxygenases of wheat, and less is known about those in weeds, and presently not too much can be said. Still, it is clear from the data in Table 2 that biochemical mimicries need not be absolute. The weeds evolved spectra of cross resistances that are broader than the resistance in wheat. 

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