Veterinary medicines, which reach the soil mostly through the application of contaminated manures, can affect beneficial soil microorganisms, such as nitrogen-fixing rhizobia bacteria, which engage in important symbiotic associations with plants. The anthelmintic compounds albendazole and fenbendazole were tested for toxicity against the nitrogen-fixing rhizobium Mesorhizobium loti and its symbiotic relationship with the model legume Lotus japonicus. The effects of anthelmintics on rhizobial growth were tested in vitro in bacterial cultures, while their effects on the establishment of the symbiotic relationship were tested by phenotypic and molecular analyses of inoculated plants. None of the two compounds showed phytotoxicity. Albendazole and fenbendazole inhibited the growth of rhizobia at concentrations often (0.75 mg kg-1) or rarely (2.25 mg kg-1) found in agricultural soils. Albendazole reduced the number of infection threads formed in roots and downregulated key symbiosis-related genes, indicating inhibition of symbiosis at early stages, unlike fenbendazole, which showed no effects. However, nodulation and nitrogenase activity were not significantly affected by albendazole at advanced stages of the symbiosis, suggesting a recovery possibly associated with the progressive dissipation of albendazole. Our results suggest different toxicity of albendazole and fenbendazole to the symbiotic nitrogen fixation system that may have serious implications for soil health.
Gorgia et al. (Thu,) studied this question.