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Mesotrione (2‐[4‐(methylsulfonyl)‐2‐nitrobenzoyl]‐cylohexane‐1,3‐dione) is an herbicide brought to the market 15 years ago and commonly used on maize cultures. This review summarizes the results obtained from worldwide scientific studies on its efficiency, effects, and fate in environment, in order to bring first overall conclusions about its safety of use. The comprehensive review of the available scientific literature reveals that mesotrione applied as recommended ensures the desired effects on sensitive crops, mainly in post‐emergence application, while exhibiting favorable toxicological, and environmental profile. According to the studies realized so far, mesotrione presents no significant risks to humans and other non‐target organisms or to the environment. Its degradation by soil microorganisms is fast, ensuring a low possibility to retrieve mesotrione in ground water. Its terminal residues in soil, maize, grain, and maize straw are below the limit of quantification. All these results suggest that mesotrione applied as directed ensures the respect of current environmental standards and food safety. Future prospects should focus on its associated pesticides when applied in mixtures on field, and on the detection, quantification, and risk assessment of its transformation products in order to exclude any toxicological synergistic effects.

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Mesotrione is a selective herbicide belonging to the triketone family, commonly used on maize cultures since 2003. A mesotrione-transforming Bacillus megaterium Mes11 strain isolated from an agricultural soil was used as a model to identify the key enzymes initiating the biotransformation of this herbicide. Two enzymes (called NfrA1 and NfrA2/YcnD) were identified, and functionally and structurally characterized. Both belong to the NfsA FRP family of the nitro-FMN reductase superfamily (type I oxygen-insensitive nitroreductase) and show optimal pH and temperature of 6-6.5 and 23-25°C, respectively. Both undergo a Ping Pong Bi Bi mechanism, with NADPH and NADPH/NADH as cofactors for NfrA1 and NfrA2/YcnD, respectively. It is interesting that both can also reduce various nitro compounds including pesticides, antibiotics, one prodrug and 4-methylsulfonyl-2-nitrobenzoic acid, one of the mesotrione metabolites retrieved from the environment. The present study constitutes the first identification of mesotrione-transforming enzymes. These enzymes (or their corresponding genes) could be used as biomarkers to predict the capacity of ecosystems to transform mesotrione and assess their contamination by both the parent molecule and/or the metabolites.

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