Resistance of a soybean cell line to oxyfluorfen by overproduction of mitochondrial protoporphyrinogen oxidase

The diphenyl ether herbicide oxyfluorfen (2-chloro-4-trifluoromethylphenyl 3-ethoxy-4-nitrophenyl ether) inhibits protoporphyrinogen oxidase (Protox) which catalyzes the oxidation of protoporphyrinogen IX (Protogen) to protop orphyrin IX (Proto IX), the last step of the common pathway to chlorophyll and haeme biosynthesis. We have selected an oxyfluorfen-resistant soybean c ell line by stepwise selection methods, and the resistance mechanism has be en investigated. No growth inhibition was observed in resistant cells at a concentration of 10(-7) M oxyfluorfen, a concentration at which normal cell s did not survive. While the degree of inhibition of total extractable Prot ox by oxyfluorfen was the same in both cell types, the enzyme activity in t he mitochondrial fraction from non-treated resistant cells was about nine-f old higher than that from normal cells. Northern analysis of mitochondrial Protox revealed that the concentration of mitochondrial Protox mRNA was muc h higher in resistant cells than that in normal cells. There were no differ ences in the absorption and metabolic breakdown of oxyfluorfen. The growth of resistant cells was also insensitive to oxadiazon [5-tert-butyl-3-(2,4-d ichloro-5-isopropoxyphenyl)- 1,3,4-oxadiazol-2- (3H)-one], the other chemic al class of Protox inhibitor. Therefore, the resistance of the selected soy bean cell line to oxyfluorfen is probably mainly due to the overproduction of mitochondrial Protox. (C) 2001 Society of Chemical Industry.