Assessing the effects of β-triketone herbicides on HPPD from environmental bacteria using a combination of in silico and microbiological approaches

4-hydroxyphenylpyruvate dioxygenase (HPPD) is the molecular target of beta-triketone herbicides in plants. This enzyme, involved in the tyrosine pathway, is also present in a wide range of living organisms, including microorganisms. Previous studies, focusing on a few strains and using high herbicide concentrations, showed that beta-triketones are able to inhibit microbial HPPD. Here, we measured the effect of agronomical doses of beta-triketone herbicides on soil bacterial strains. The HPPD activity of six bacterial strains was tested with 1x or 10x the recommended field dose of the herbicide sulcotrione. The selected strains were tested with 0.01x to 15x the recommended field dose of sulcotrione, mesotrione, and tembotrione. Molecular docking was also used to measure and model the binding mode of the three herbicides with the different bacterial HPPD. Our results show that responses to herbicides are strain-dependent with Pseudomonas fluorescens F113 HPPD activity not inhibited by any of the herbicide tested, when all three beta-triketone herbicides inhibited HPPD in Bacillus cereus ATCC14579 and Shewanella oneidensis MR-1. These responses are also molecule-dependent with tembotrione harboring the strongest inhibitory effect. Molecular docking also reveals different binding potentials. This is the first time that the inhibitory effect of beta-triketone herbicides is tested on environmental strains at agronomical doses, showing a potential effect of these molecules on the HPPD enzymatic activity of non-target microorganisms. The whole-cell assay developed in this study, coupled with molecular docking analysis, appears as an interesting way to have a first idea of the effect of herbicides on microbial communities, prior to setting up microcosm or even field experiments. This methodology could then largely be applied to other family of pesticides also targeting an enzyme present in microorganisms.

Automated summary

This study investigated the effects of agronomical doses of beta-triketone herbicides on soil bacterial strains, and found that different strains exhibited varying responses to the herbicides and different molecules had varying inhibitory effects. Molecular docking analysis revealed different binding potentials between the herbicides and bacterial HPPD. The results suggest that these herbicides may impact the HPPD enzymatic activity of non-target microorganisms.