Involvement of cytochrome P‐450 enzyme activity in the selectivity and safening action of pyrazosulfuron‐ethyl

Abstract

To investigate the selectivity and safening action of the sulfonylurea herbicide pyrazosulfuron-ethyl (PSE), pyrazosulfuron-ethyl O-demethylase (PSEOD) activity involving oxidative metabolism by cytochrome P-450 was studied in rice (Oryza sativa L cv Nipponbare) and Cyperus serotinus Rottb. Cytochrome P-450-dependent activity was demonstrated by the use of the inducers 1,8-naphthalic anhydride and ethanol, the herbicides PSE, bensulfuron-methyl, dimepiperate and dymron, or the inhibitor piperonyl butoxide (PBO). Growth inhibition in C serotinus seedlings was more severe than that in rice seedlings. O-Dealkylation activities of PSE were induced differently in rice and in C serotinus, with distinctly higher activity in rice seedlings. The induced PSEOD activities were slightly inhibited by PBO in rice seedlings, whereas they were strongly inhibited in C serotinus seedlings. Dimepiperate and dymron were effective safeners of rice against PSE treatment. Treatments with herbicide alone resulted in less induction of PSEOD activity compared with combined treatments of the herbicide and safener. PSEOD activity in rice seedlings induced with herbicide alone was strongly inhibited by PBO, whereas it was weakly inhibited in rice seedlings induced with combinations of PSE and two safeners. These results suggest that O-demethylation by cytochrome P-450 enzymes may be involved in the metabolism of PSE and may contribute to its selectivity and safening action. Furthermore, these results suggest the existence of a multiple form of cytochrome P-450 in plants.

Plant cytochrome P450-mediated herbicide metabolism

“…In addition to the compounds listed in Table 1, P450s have been implicated in the metabolism of several herbicides by in vivo studies in which the changes in herbicidal activity and/or the rate of herbicide metabolism were measured in response to various P450 inhibitors. Unconfirmed by in vitro data, the involvement of P450s are suggested by in vivo results in the metabolism of chlortoluron in wheat (Canivenc et al 1989;Mougin et al 1992), corn and cotton (Cole and Owen 1987); bentazon in soybean (Sterling and Balke 1990); pyrazosulfuron ethyl in rice (Yun et al 2001); clodinafop in wheat, barley and corn (Kreuz et al 1991); phenoxaprop-p-ethyl in barley (Romano et al 1993); clomazone in cotton (Ferhatoglu et al 2005); thiazopyr in several weedy species (Feng et al 1995) and EPTC in some cereals (Barta and Dutka 1991). (Additional examples are discussed under ''P450-Endowed Herbicide Resistance in Weeds'').…”

Physiological basis for isoxadifen-ethyl induction of nicosulfuron detoxification in maize hybrids

“…Previous research has found that cloquintocet-mexyl antagonized the effects of malathion or ABT on topramezone, but the implications of this are not clear. It is possible that cloquintocet-mexyl increased topramezone metabolism through mechanisms not catalyzed by P450 monooxygenases [39,40]. We speculated that isoxadifen-ethyl increased plant metabolism of nicosulfuron through non-P450-catalyzed routes or through P450 monooxygenases not inhibited by PBO, ABT, and malathion.…”

“…We speculated that isoxadifen-ethyl increased plant metabolism of nicosulfuron through non-P450-catalyzed routes or through P450 monooxygenases not inhibited by PBO, ABT, and malathion. Yun et al [40] evaluated the effects of the P450 monoxygenase inhibitor PBO alone and in combination with safeners on the activity of microsomal pyrazosulfuron-ethyl O-demethylase, which metabolizes pyrazosulfuron-ethyl in rice ( Oryza sativa L.). They found that PBO inhibited 63% of pyrazosulfuron-ethyl O-demethylase activity when applied in combination with the herbicide, but only 19% inhibition was achieved when PBO was applied in combination with the herbicide and the safener.…”

“…The researchers suggested that this was because the herbicide and safeners induce different P450 monooxygenases. Previous research indicates that ABT inhibits P450 monooxygenases responsible for aryl ring hydroxylation but not those responsible for N-demethylation [40]. …”

Intoxication and Physiological Aspects of Forage Plants and Weeds Submitted to Clomazone Atmospheric Waste

“…The selectivity of some plants to clomazone may also be related to the action site of the cytochrome P-450 mono-oxygenase enzyme, which is involved in resistance metabolites of various herbicides (Yun et al, 2001). This enzyme normally acts with the function of detoxifying, and it may responsible for the oxidation of the clomazone molecule, making it more toxic to plants (Yun et al, 2005).…”