Mechanisms of the enantioselective effects of phenoxyalkanoic acid herbicides DCPP and MCPP

Phenoxyalkanoic acids (PAAs), synthetic indole-3-acetic acid (IAA) auxin mimics, are widely used as herbicides. Many PAAs are chiral molecules and show strong enantioselectivity in their herbicidal activity; however, there is a lack of understanding of mechanisms driving enantioselectivity. This study aimed to obtain a mechanistic understanding of PAA enantioselectivity using dichlorprop and mecoprop as model PAA compounds. Molecular docking, in vitro ³H-IAA binding assay, and surface plasmon resonance analysis showed that the R enantiomer was preferentially combined with TIR1-IAA7 (Transport Inhibitor Response1- Auxin-Responsive Protein IAA7) than the S enantiomer. In vivo tracking using ¹⁴C-PAAs showed a greater absorption of the R enantiomer by Arabidopsis thaliana, and further comparatively enhanced translocation of the R enantiomer to the nucleus where the auxin co-receptor is located. These observations imply that TIR1-IAA7 is a prior target for DCPP and MCPP, and that PAA enantioselectivity occurs because the R enantiomer has a stronger binding affinity for TIR1-IAA7 as well as a greater plant absorption and translocation capability than the S enantiomer. The improved understanding of PAA enantioselectivity is of great significance, as the knowledge may be used to design "green" molecules, such as R enantiomer enriched products, leading to improved plant management and environmental sustainability.