Enantioselective phytotoxicity of the herbicide imazethapyr and its effect on rice physiology and gene transcription.

Imazethapyr (IM) is a chiral herbicide and a widely used racemic mixture. This report investigated the enantioselectivity between R- and S-IM in rice and explored its causative mechanism at the physiological and molecular levels. The results suggested that R-IM inhibited acetolactate synthase (ALS) activity to a greater extent than S-IM, which reduced the synthesis of branched-chain amino acids (BCAAs). Additionally, most other amino acids showed enantioselectivity. On the cellular level, R-IM showed stronger toxicity against protoplasts than S-IM. A gene transcription profile analysis showed that gene transcripts in many metabolic pathways, including amino acid metabolism, photosynthesis, starch and sugar metabolism, and the tricarboxylic acid cycle displayed enantioselectivity between the IM enantiomers. R-IM regulated more genes more strongly than S-IM. This study suggested that R-IM has stronger toxicity against plants than S-IM; this toxicity is caused not only by changing targeted enzyme activity and amino acid synthesis, but also by affecting gene transcription in other metabolic pathways directly or indirectly in an enantioselective manner.