DIFFERENTIAL INDUCTION OF CYTOCHROME P450-MEDIATED TRIASULFURON METABOLISM BY NAPHTHALIC ANHYDRIDE AND TRIASULFURON

Cytochrome P450 monooxygenases play paramount roles in the detoxificat ion of herbicides as well as in the synthesis of lignins, flavonoids, and phenolic acids. Biochemical analysis of triasulfuron metabolism in maize (Zea mays) seedlings has demonstrated that the P450(s) responsi ble for detoxification of this herbicide is induced by naphthalic anhy dride (NA), a plant safener, and by triasulfuron, the herbicide itself . Induction studies conducted with seedlings of different ages suggest that two separate response pathways modulate this P-450 activity. Ind uction by NA is independent of the developmental age of the seedlings up to 6.5 d; induction by triasulfuron is tightly modulated with respe ct to developmental age in that triasulfuron metabolism can be induced by triasulfuron in young (2.5 d) but not older (6.5 d) seedlings. Ind uction by NA administered in combination with triasulfuron synergistic ally enhances triasulfuron metabolism in younger seedlings to levels s ubstantially above that obtained with either herbicide or safener trea tment alone. In older seedlings, NA plus triasulfuron treatment induce s triasulfuron metabolism to only the level of NA treatment alone, ind icating again that the induction cascade responding to triasulfuron is nonfunctional in later development. MnCl2 studies indicate that the t riasulfuron insensitivity of older seedlings does not result from a ge neral limitation in the inducibility of this P-450 detoxification syst em but rather from specific limitations in the triasulfuron-response p athway.