Phytotoxicity of microbial antibiotics helvolic and fusidic acids

The phytotoxicity of high purity helvolic acid and a related compound, fusidic acid was assessed using multiple growth and biochemical endpoints performed on tissues of several plant species. Bioassay tests for phytotoxicity of helvolic (0.25 mM) and fusidic (0.50 mM) acids were conducted on weed and crop species including: germination/growth (seeds/seedlings) of hemp sesbania [Sesbania exalta (Raf.) Rydb.], sicklepod (Cassia obtusifolia L.), mung bean (Vigna radiata L.), wheat (Triticum aestivum L.), and cucumber (Cucumis sativa L.; leaf disk electrolyte leakage of corn (Zea mays L.) and giant ragweed (Ambrosia trifida L.) leaf tissues; and greening of etiolated tissues of hemp sesbania, sicklepod, wheat (Triticum aestivum L.), mung bean, and sorghum (Sorghum vulgare L.) tissues. Fusidic acid reduced the chlorophyll accumulation in hemp sesbania (95%), wheat (70%), mung bean (60%), sorghum (60%) and sicklepod (48%); helvolic acid at this low concentration had only weak effects, except in mung bean. Neither compound affected the germination at the concentrations used, but both inhibited the seedling growth of all test species except sicklepod and helvolic acid-treated hemp sesbania. Both chemicals increased the electrolyte leakage in corn and giant ragweed leaf disks after 48-h exposure. Overall these natural products have broad-range phytotoxicity and may affect the plants directly or indirectly via antibiotic effects on soil/rhizosphere organisms.