CHARACTERISTICS OF A METAZACHLOR-RESISTANT SCENEDESMUS-ACUTUS CELL-LINE

A metazachlor-resistant strain of the unicellular green alga Scenedesm us acutus (''Mz-l'') is compared with the wild-type to understand chlo roacetamide tolerance mechanisms in the Mz-l mutant. Presently the res istant cells grow with 5 mu M metazachlor as fast as the wild-type wit hout herbicide. Mz-l cells look somewhat similar to cells treated with herbicide. They are ovoid and single, while those of the wild-type ar e lens-shaped and grow mostly in colonies (coenobia) of eight cells. R estriction fragment-length polymorphism studies confirmed that the Mz- l line derived from an axenic wild-type and not from another species t hat might have overgrown the sensitive culture. In long-term experimen ts (1 to 2 days), metazachlor (2 mu M) inhibited fatty acid desaturati on in the wild-type lipids but not in those from the resistant line. F atty acid profiles of Mz-l and wild-type were similar in the absence o f herbicide. In short-term experiments (few hours), acetate uptake int o wild-type cells was slower than into Mz-l cells; furthermore, it was decreased by metazachlor in the wild-type while the herbicide had no effect on uptake in the mutant. Incorporation of [C-14]oleic acid into a fraction not solubilized by saponification of the cells was highly sensitive to metazachlor in the wild-type but could not be inhibited i n the resistant cells even after applying a 1000-fold higher herbicide concentration, thus indicating a close relationship between oleic aci d incorporation into that fraction and herbicidal activity. HPLC analy sis of the radioactive fatty acids showed a rapid desaturation of exog enous [C-14]oleic acid in the wild cells, while it was nearly absent i n Mz-l cells. The findings give evidence that tolerance might be linke d with oleic acid metabolism which in turn might alter uptake properti es of the cells, possibly via changes in the membrane lipids. (C) 1995 Academic Press, Inc.