DETOXIFICATION AND ACTIVATION OF AGROCHEMICALS IN PLANTS

Plants are able to metabolize agrochemicals and other foreign compounds by a variety of mechanisms and with extraordinary species diversity. Minor structural alterations of these compounds can bring about dramatic and unpredictable changes in the routes of their metabolism. The enzymes responsible for this exist in multiple forms which renders prediction of herbicide metabolism and, therefore, of selectivity, difficult. In some notable instances, pesticides are activated by plant metabolism. In the main, however, mechanisms such as hydroxylation, dealkylation and glutathione conjugation bring about detoxification and form the basis of herbicide selectivity. The properties of oxygenating and conjugating enzymes in plants are highlighted, with emphasis on the evident narrow substrate specificities, species differences and physiological roles. The molecular cloning of the genes specifying these enzymes will permit a much better definition of these mechanisms and will illuminate the natural roles of the enzymes involved. The prospects for utilizing recombinant enzymes as tools for the rational design of new selective herbicides are discussed. Herbicide safeners can protect certain crops from herbicide injury by promoting herbicide metabolism. The precise mechanisms of safener action and the reasons for their specificity are attracting much interest but are at present obscure. Natural variation in detoxification abilities of weed populations has allowed the field selection of some biotypes resistant to repeatedly used herbicides. By analogy, the introduction of microbial detoxification genes into major crops through genetic transformation has created new herbicide resistant crops which will enhance the flexibility of herbicide usage.