Herbicidal effects of geldanamycin and nigericin, antibiotics from Streptomyces hygroscopicus

Geldanamycin (2) and nigericin (1) (primarily the Na+ salt) are responsible for the phytotoxicity of a Streptomyces hygroscopicus strain found to be highly active in our screens for producers of herbicidal antibiotics. After extraction from the cells, the compounds were purified with column and thin layer layer chromatography on silica gel, bioassayed for inhibition of garden cress (Lepidium sativum L.) radicle elongation, and identified with ms, ir, nmr, and co-chromatography with authentic standards. Both caused 50% reduction in garden cress radicle growth at concentrations of 1-2 ppm and nearly complete inhibitions at 3-4 ppm. Symptoms of toxicity differed markedly for the two compounds. Geldanamycin at high concentrations caused radicles to turn brown and disintegrate, whereas, nigericin did not cause visible necrosis. Geldanamycin is structurally similar to the herbimycins, which are also produced by S. hygroscopicus and have been reported to have herbicidal activity. The phytotoxicity of geldanamycin and nigericin has stimulated investigation of their potential for use as natural product herbicides.