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

A metazachlor-resistant strain of the unicellular green alga Scenedesmus acutus (''Mz-l'') is compared with the wild-type to understand chloroacetamide tolerance mechanisms in the Mz-l mutant. Presently the resistant cells grow with 5 mu M metazachlor as fast as the wild-type without herbicide. Mz-l cells look somewhat similar to cells treated with herbicide. They are ovoid and single, while those of the wild-type are lens-shaped and grow mostly in colonies (coenobia) of eight cells. Restriction fragment-length polymorphism studies confirmed that the Mz-l line derived from an axenic wild-type and not from another species that might have overgrown the sensitive culture. In long-term experiments (1 to 2 days), metazachlor (2 mu M) inhibited fatty acid desaturation in the wild-type lipids but not in those from the resistant line. Fatty acid profiles of Mz-l and wild-type were similar in the absence of herbicide. In short-term experiments (few hours), acetate uptake into 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 in the resistant cells even after applying a 1000-fold higher herbicide concentration, thus indicating a close relationship between oleic acid incorporation into that fraction and herbicidal activity. HPLC analysis of the radioactive fatty acids showed a rapid desaturation of exogenous [C-14]oleic acid in the wild cells, while it was nearly absent in Mz-l cells. The findings give evidence that tolerance might be linked with oleic acid metabolism which in turn might alter uptake properties of the cells, possibly via changes in the membrane lipids. (C) 1995 Academic Press, Inc.