Characteristics of a Metazachlor-Resistant Scenedesmus acutus Cell Line

Abstract

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 μM 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 μM) 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 [14C]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 [14C]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.