Jasmonates: Structural Requirements for Lipid-Derived Signals Active in Plant Stress Responses and Development

Jasmonates are lipid-derived signals that mediate plant stress responses and development processes. Enzymes participating in biosynthesis of jasmonic acid (JA) (1, 2) and components of JA signaling have been extensively characterized by biochemical and molecular-genetic tools. Mutants of Arabidopsis and tomato have helped to define the pathway for synthesis of jasmonoyl-isoleucine (JA-Ile), the active form of JA, and to identify the F-box protein COI1 as central regulatory unit, However, details of the molecular mechanism of JA signaling have only recently been unraveled by the discovery of JAZ proteins that function in transcriptional repression. The emerging picture of JA perception and signaling cascade implies the SCFCOI1 complex operating as E3 ubiquitin ligase that upon binding of JA-Ile targets JAZ repressors for degradation by the 26S-proteasome pathway, thereby allowing the transcription factor MYC2 to activate gene expression. The fact that only one particular stereoisomer, (+)-7-iso-JA-L-Ile (4), shows high biological activity suggests that epimerization between active and inactive diastereomers could be a mechanism for turning JA signaling on or off. The recent demonstration that COI1 directly binds (+)-7-iso-JA-L-Ile (4) and thus functions as)A receptor revealed that formation of the ternary complex COI1-JA-Ile-JAZ is an ordered process. The pronounced differences in biological activity of)A stereoisomers also imply strict stereospecific control of product formation along the JA bio-synthetic pathway. The pathway of JA biosynthesis has been unraveled, and most of the participating enzymes are well-characterized. For key enzymes of)A bio-synthesis the crystal structures have been established, allowing insight into the mechanisms of catalysis and modes of substrate binding that lead to formation of stereospecific products.