(3R,7S)-12-Hydroxy- jasmonoyl-L-isoleucine is the genuine bioactive stereoisomer of a jasmonate metabolite in Arabidopsis thaliana.

The oxylipin plant hormone JA-Ile is widely recognized as a plant defense hormone against pathogens and chewing insects. The metabolism of JA-Ile into 12-OH-JA-Ile and 12-COOH-JA-Ile is the central mechanism for the inactivation of JA signaling. Recently, 12-OH-JA-Ile was reported to function as a ligand for the JA-Ile co-receptor COI1-JAZ. However, in previous studies, the "12-OH-JA-Ile" used was a mixture of four stereoisomers, the naturally occurring cis-isomer (3R,7S)-12-OH-JA-Ile and the trans-isomer (3R,7R)-12-OH-JA-Ile and the unnatural cis-isomer (3S,7R)-12-OH-JA-Ile and the trans-isomer (3S,7S)-12-OH-JA-Ile. Thus, the genuine bioactive form of 12-OH-JA-Ile has not yet been identified. In this study, we prepared pure stereoisomers of 12-OH-JA-Ile and identified (3R,7S)-12-OH-JA-Ile, as the naturally occurring bioactive form of 12-OH-JA-Ile and found that it binds to COI1-JAZ9 as effectively as (3R,7S)-JA-Ile. In addition, we revealed that the unnatural trans-isomer (3S,7S)-12-OH-JA-L-Ile functions as another bioactive isomer. The pure (3R,7S)-12-OH-JA-Ile causes partial JA-responsive gene expression without affecting the expressions of JAZ8/10, which are involved in the negative feedback regulation of JA-signaling. Thus, (3R,7S)-12-OH-JA-Ile could cause weak and sustainable expression of certain JA-responsive genes until the catabolism of (3R,7S)-12-OH-JA-Ile into (3R,7S)-12-COOH-JA-Ile and (3R,7S)-12-OH-JA occurs. The use of chemically pure (3R,7S)-12-OH-JA-Ile confirmed the genuine biological activities of "12-OH-JA-Ile" by excluding the possible effects of other stereoisomers. A chemical supply of pure (3R,7S)-12-OH-JA-Ile with an exact bioactivity profile will enable further detailed studies of the unique role of 12-OH-JA-Ile in planta.