Interactions between pesticides and microorganisms : The case of biodegradation of synthetic β-triketone herbicides.

Agricultural use of pesticides ensures a higher crop quality and production but it is also one of the major sources of diffuse pollution in the environment. Microbial degradation is considered as an important dissipation process limiting the accumulation of pesticides in the environment. In this context, two bacterial strains able to degrade sulcotrione, a β-triketone herbicide, were isolated from an agricultural soil previously exposed to this herbicide. The two isolates were identified using 16S rRNA gene sequencing as Pseudomonas sp.1OP and Bradyrhizobium sp.SR1. Their capacity to degrade sulcotrione was estimated and 2-Chloro-4-mesylbenzoic acid, one of its main metabolites, was clearly detected. Their ability to degrade other β-triketone herbicides was tested showing that only Bradyrhizobium sp.SR1 was able to completely degrade mesotrione and producing already known metabolites. Microbial toxicity of sulcotrione and mesotrione and their related metabolites in bacterial cultures was estimated by monitoring 4-hydroxyphenylpyruvate dioxygenase (HPPD) enzyme inhibition. Our results indicate that triketone herbicides toxicity linked to HPPD inhibition was due to parent molecules and not to the formed metabolites. Attempts were done to identify the genetic localization of sulcotrione degradation in Pseudomonas sp.1OP and Bradyrhizobium sp.SR1. Plasmid profiles of both strains revealed the presence of large plasmids (>12 kb and >50 kb, respectively). Curing experiments showed that Pseudomonas sp.1OP lost its ability to degrade sulcotrione under non-selective conditions, therefore degradation capacity may be attributed to the presence of this plasmid. On the contrary, under the same conditions, Bradyrhizobium sp.SR1 plasmid was not cured and the sulcotrione-degrading ability of the strain was maintained. Furthermore, a 14 000 Tn5 mutant library was constructed using a Tn5 mutagenesis approach conducted on Bradyrhizobium sp. SR1. Among this library, two mutants affected in their biodegradation capacity were identified. Full sequencing of SR1 and Tn5 mutants is ongoing to identify possible degrading gene candidates