ROS Involves the Fungicidal Actions of Thymol against Spores of Aspergillus flavus via the Induction of Nitric Oxide

Aspergillus flavus is a well-known pathogenic fungus for both crops and human beings. The acquisition of resistance to azoles by A. flavus is leading to more failures occurring in the prevention of infection by A. flavus. In this study, we found that thymol, one of the major chemical constituents of the essential oil of Monarda punctate, had efficient fungicidal activity against A. flavus and led to sporular lysis. Further studies indicated that thymol treatment induced the generation of both ROS and NO in spores, whereas NO accumulation was far later than ROS accumulation in response to thymol. By blocking ROS production with the inhibitors of NADPH oxidase, NO generation was also significantly inhibited in the presence of thymol, which indicated that ROS induced NO generation in A. flavus in response to thymol treatment. Moreover, the removal of either ROS or NO attenuated lysis and death of spores exposed to thymol. The addition of SNP (exogenous NO donor) eliminated the protective effects of the inhibitors of NADPH oxidase on thymol-induced lysis and death of spores. Taken together, it could be concluded that ROS is involved in spore death induced by thymol via the induction of NO.