Selective targeting of the TIR1-IAA7 co-receptor by synthetic auxin herbicides

Auxin herbicides exert herbicidal effects by over-inducing auxin response in susceptible plants. This study investigated the molecular interactions between TIR1-IAA7 and distinct chemical classes of auxin herbicides. MCPA (2-methyl-4-chlorophenoxy acetic acid), chloramben (3-Amino-2,5-dichlorobenzoic acid), quinclorac (3,7-dichloro-8-quinolinecarboxylic acid), and aminocyclopyrachlor (6-amino-5-chloro-2-cyclopropylpyrimidine-4-carboxylic acid) exhibited relatively lower docking energies among all the 8 candidate auxin herbicides in the molecular docking simulation and were selected as representatives. In vitro 3 H-IAA-binding and Surface Plasmon Resonance experiments indicated MCPA and chloramben showed stronger affinities for TIR1-IAA7, further corroborated by mutant resistance assays. Despite their weak affinity for TIR1-IAA7, quinclorac and aminocyclopyrachlor are likely to interact with an alternative AFB protein in combination with IAA7, thereby disrupting auxin signaling and down-regulate photosynthetic gene expression. These findings demonstrate that different herbicide classes exhibit distinct co-receptor preferences. Such target specificities provide a structural basis for the rational design of novel auxinic herbicides.