A glyphosate-resistant 5-enolpyruvilshikimate-3-phosphate synthase confers tolerance to a maize cell line

Among a few cell lines of maize (Zea mays L., cv. Black Mexican Sweet (BMS)) tested, one showed a natural, remarkable tolerance to glyphosate at concentrations as high as 10 mM. Two activities of 5-enol-pyruvyl-shikimate-3-phosphate (EPSP) synthase, the target enzyme of the herbicide, were separated in the tolerant culture by anion-exchange chromatography. One peak of activity was not significantly inhibited by glyphosate even at millimolar concentrations. While the glyphosate-sensitive isoform persisted throughout the growth cycle of the culture, the glyphosate-resistant EPSP synthase increased only after the onset of exponential growth and declined in the stationary phase. Tolerance to the herbicide was accompanied by a reduced affinity of the enzyme for the substrate phospho-enol-pyruvic acid